US2983972A - Metal casting system - Google Patents

Description

y 1961 G. E. MORITZ 2,983,972

METAL CASTING SYSTEM Original Filed April 4, 1958 7 Sheets-Sheet 1 f 54 mum METAL HEIGHT 64 ABOVE EFFECTIVE N mow 20MB 50 6 FIGJa. FIGJb.

INVENTOR GUNTHER E. MORITZ BY W ATTORNEYS y 1961 G. E. MORITZ 2,983,972

METAL CASTING SYSTEM Original Filed April 4, 1958 7 Sheets-Sheet 2 INVENTOR GUNTH 'R E. MORITZ BY adw fiww A'ITORNEY5 May 16, 1961 G. E. MORITZ METAL CASTING SYSTEM Original Filed April 4, 1958 FIG. 7.

7 Sheets-Sheet 3 GUNTHER E. MORITZ BY y ATTORNEYS y 1961 G. E. MoRrrz 2,983,972

METAL CASTING SYSTEM Original Filed April 4, 1958 7 Sheets-Sheet 4 FIG. 14.

FIGJb'.

INVENTOR GUNTH E R E. MORITZ ATTORNEYS Original Filed April 4, 1958 7 Sheets-Sheet 5 D?) METHOD EFFECTIVE MOLD LENGTH l l 2" v; 2

Z mukmisa JJmO EFEOZUO FIGJZ .00 l

Z mm m 1" V M 6 m E m (K R m E u H m 5 T N x ,0 U I\ D G 4 m- "M 31 m .w H 8 26 f N m e P O R D I" .f k 0 252. owmmm wzxiomo EFFECTIVE MOLD LENGTHJN.

BY %M/ r %K ATTORNEYS y 1961 G. E. MORITZ 2,983,972

METAL CASTING SYSTEM Original Filed April 4, 1958 7 Sheets-Sheet 6 E .7" 3 .6- g .5- s- Mg 3 .3- '5 .2- Ill 0 .l I a 2- I l I III 2 3 DISTANCE mom INGOT cam R 5 5 g h 9 .4 8 .3 8 l- .2 2 Ill 0 ,l E CL 5 i I III 0 I 2 3 Dc MOLD LENGTHSM DISTANCE mom INGOT CENTER 'oRoPPm SPEED 5lN/MlN. INVENTOR +C l V2 'N- GUNTHER E. MORITZ DROPPING P550 5 lN/MlN. Q 00 MOLD, ENGTH H/filN,

DROPPING s sso 2 n IMI A cc mow,|.eue1'u m, QZ

DROPPINGSPEED 4IN/MIN. BY

ATTORNEYS May 16, 1961 G. E. MORITZ METAL CASTING SYSTEM Original Filed April 4, 1958 CRATER DEPTH 7 Sheets-Sheet 7 6 "F IL 5 1 i o I E T g 4 g 3 J A m T m 0. I!) 2 \J u r o I I! O L o l 2 3 4 o EFFECTIVE mow LENGTHJN.

i. I l o c- 1 L 2 a 4 5 6 DROPPING SPEEDJN/MIN. MENTOR FIG.22.

GUNTHER E. MORITZ BY w;

ATTORNEYS United States Patent METAL CASTING SYSTEM Gunther -E. Moritz, Henrico County, Va., asslgnor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Continuation of application Ser. No. 726,546, Apr. 4, 1958. This application Nov. 17, 1960, Ser. No. 70,076

14 Claims. (Cl. 22-572.)

This invention relates to an apparatus and process for the continuous casting of ingots of aluminum, aluminum alloys, and other metals. It is a particular object of this invention to provide a novel apparatus and highly versatile process for continuous casting of aluminum under controlled cooling conditions which include the formation of a meniscus-shaped or annular radially curved surface on the embryo ingot beneath the surface of the molten metal.

The purpose of ingot casting is, of course, to produce ingots of uniform composition, metal structure and strength, and of desired size. At the same time, it is necessary that the process and apparatus be safe and readily controlled and rapid and economical in its operation. Since various types of ingots, having different mechanical properties and internal structural characteristics are desired for different uses and applications, flexibility of casting conditions which can be used with a given mold is of great importance.

To better understand this invention, it is of importance to consider conventional casting methods, and the problems encountered in those techniques. The following discussion is especially directed to aluminum metals, as this invention is of particular advantage when used therewith. In this application, the term aluminum metals is intended to mean aluminum metal and alloys containing at least 50% aluminum.

Background of the invention The casting of aluminum and aluminum alloys was originally carried out by what is called the tilt-mold or cold-mold technique. mold is used and at the start of the casting is nearly horizontal. Metal is poured into the mold, and as pouring continues, the mold is tilted back to a substantially vertical position, the object being to reduce the amount of agitation of the metal in the mold and decrease the amount of entrapped oxide skin. The metal is allowed to slowly solidify in the mold which is then opened and the ingot removed. From a quality standpoint, this method of casting leaves much to be desired. The structure of the metal is unfavorable for most uses and sheet production was always plagued with blisters and other defects hereinafter described until better ingots were produced.

The method now in greatest use for continuous casting of aluminum ingots is the direct chilling or DC casting method disclosed by Ennor in US. Patent 2,301,027, issued November 3, 1942. This method employs a mold in the form of an open-ended sleeve or shell having a cylindrical or other straight bore therethrough, with a surrounding sprayer which directs cooling water against the mold and also directly against the embryo ingot as it is withdrawn from the mold. The operation of the DC process forms a solidified surface around the ingot while it is within the mold, and then exposes that surface to the cooling water spray beneath the mold. It

,has been found in practice that successful operation of a In this method, a book-type.

DC mold requires skillful control of several operating conditions'within a narrow range in order to produce an acceptable ingot.

When using the Ennor DC mold, a close relationship between the diameter of the ingot and the depth of the pool of molten metal, as illustrated in Figure 3 of the patent, must be maintained. This means that in the DC process, it is exceedingly important to maintain the level of the molten metal within a narrow range in order to obtain a proper ingot. For this reason, the DC process requires special valving and control means regulating the temperature and rate of flow of molten metal into the mold, the quantity of cooling water sprayed on the mold and ingot, and the dropping speed or rate of removal of the embryo ingot from the mold.

Subsequent efiorts to avoid the difficulties of the DC process include those disclosed by Smart in US. Patent 2,367,148, issued January 9, 1945, and by Smart et al. in US. Patent 2,740,177, issued April 3, 1956. In these patents direct spray cooling of the ingot is not employed and a considerably longer graphite mold is used. Both of these processes are difficult to operate mechanically due to frictional problems and the criticality of axial alignment of the mold and the dropping ingot.

The present invention represents an improvement over the Ennor-type DC process, and also represents a new concept in both apparatus and method of metal casting, This invention provides a solution to many problems in the casting of aluminum and other metals which cannot be solved either with the DC process and mold, or the subsequent efforts to improve that basic technique.

The following background discussion of continuous casting of metal ingots may aid in understanding the invention.

Problems in metal casting The phenomena which take place during the casting largely determine the characteristics of the metal ingots, the economic feasibility of their production, and the uses for which they are suitable. Among these phenomena and characteristics are cold-shutting, segregation, liquation, bleeding, heat removal and transverse temperature gradation, crater depth, splitting or cracking, and micro structure.

(a) Splitting 0r cracking-One of the major. limitations of the DC process has always been the tendency for the ingot, particularly large ingots cast from highstrength alloys, to split during and after casting. During the freezing and cooling of a DC ingot, internal stresses are developed since the outer shell of the ingot is rapidly chilled, and contracts, while molten metal is still present in the center of the same cross-section. When the tensile strength of the ingot is exceeded, a split or crack will develop. The actual splitting may take place during cooling, but it not infrequently happens that ingots which have been stored for even several weeks will suddenly, without warning, split or crack with great violence. It will be appreciated that cracked ingots are generally unsatisfactory products and have to be scrapped, but in addition, due to the great forces which are released, an ingot which has latent splitting tendencies can produce extremely serious damage and injury.

Splitting manifests itself in two distinct patterns according to whether it occurs in a high strength or medium strength alloy. With high strength alloys, the split will generally be along a diametric plane of the axis of the ingot and will extend longitudinally for considerable distances or even all the way through the ingot. With me: dium strength alloys, the splitting will characteristically take the form of a series of star-shaped cracks originating from one point at or near the center of the ingot. The star-shaped cracks may not extend the entire length of the ingot and may occur at either end or only at the middle where they cannot be readily detected.

The causes of splitting and cracking are not completely understood but appear to be related primarily to associate excessive crater depth during casting, with consequent high transverse temperature gradations across the cooling ingot, and to be associated with other defects such as segregation and liquation.

' (b) Segregatin.-This term generally describes the non-uniformity of distribution of alloy constituents in the ingot, particularly over a cross-sectional portion of the ingot and is observed to a substantial degree in larger ingots produced by the DC process. It is obviously frequently desirable to have an ingot of uniform crosssectional composition, for otherwise the desired alloy characteristics will not be obtained throughout an article manufactured from the ingot by either extrusion, drawing, or rolling techniques. While the casual phenomena are not well understood, segregation may result from displacement of unsolidified eutectic during the solidificatron stages of the ingot formation. That is, the nonuniform cross-sectional temperature gradation in the ingot will apparently lead to varying non-uniform solidification of the alloy composition. Segregation may be one of four types, direct, inverse, gravity, or liquation. Most major alloying elements show inverse segregation in a tilt-mold or a DC ingot, i.e., the greatest concentration of constituents at the surface and the least at the center. The presence and degree of segregation is effected by the rate of casting, i.e., speed of withdrawal of the ingot from the DC mold, possibly because of the changes introduced in the internal isothermal lines in the cooling ingot. An increased speed of withdrawal of the ingot will increase transverse segregation.

(c) Liquati0n.-This term is applied to the exuding of metal of high alloy constituents content out onto the circumferential ingot surface of varying thickness. This structure may again result from eutectoidal effects in the freezing and partial remelting of the outer area of the molten metal in contact with the cold-mold surface, and from the hydrostatic pressure of the molten metal, but theoretical explanations are by no means firmly established. It will be appreciated the liquation introduces an aggravated form of the disadvantages accompanying segregation and makes it necessary to scalp the ingot surface, an operation which may cost as much as casting the ingot itself and which also leads to a net loss of metal recovery. As pointed out in an article by W. T. Ennor in Metals Handbook, 1948 edition, published by American Society for Metals, at page 769, liquation and cold shutting are major disadvantages of direct chill method of casting.

(d) Bleeding-This phenomenon seems to be associated with liquation and, in a sense, may be regarded as symptomatic thereof. Sometimes the term liquation is so used as to include bleeding phenomena. As the name implies, bleeding describes a flow of molten metal along an already solidified outer surface of the ingot, appearing at times like wax flowing down a candle. It seems to be the result of a localized remelting through the initially thin wall of the ingot, caused by the heat of the hot molten metal in the inside of the ingot, probably under influence of pressure from the hydrostatic head of molten metal above the ingot. While a visual surface defect, with the disadvantages of non-uniform cross-sectional composition and the necessity of scalping, the presence of bleeding also indicates that serious irregularities are taking place inside the ingot during the casting opera tion and a poor ingot structure may be expected. It will be appreciated that bleeding is more likely with higher casting speeds due to deeper crater formation and/or steeper (longitudinally) isothermal lines. On the other hand, the effect can also be seen with relatively slow casting speeds as well. I l

(2) Heat removal and transverse temperature gradation.-In the embryo ingot, the direction of the cooling forces and behavior will result in an isothermal line structure in the cooling ingot. Steep isothermal lines will rather clearly result in large temperature gradations from the center to the surface of the ingot. The type and rate of cooling applied to the ingot will be the principal factor controlling its internal temperature variations, particularly the direction of heat removal. With heat removal predominantly horizontal, i.e., the cooling is from the outside in, various structural defects will be observed depending on the cooling rate. Internal stresses and strains can be introduced and segregation, liquation, and bleeding can be emphasized. The eifects are, however, generally unpredictable. In tilt-mold casting, a very deep crater and very steep isothermal lines will be observed due to the direction of heat removal. In DC casting, crater form and the aspect of the isothermal lines will be influenced by the rate of ingot withdrawal which factor is obviously closely related to the rate of cooling, and the amount and area of the water sprayed on the ingot and mold. Generally speaking, slower withdrawal rates will lead to shallower craters, but the use of slow casting speeds, slow enough to significantly reduce the crater depth, is limited by other defects in the case of DC casting, as explained below, in connection with cold-shutting. and the predominant horizontal direction of heat removal may still leave fairly deep isothermals.

(f) Crater depth-The above discussion has already mentioned this aspect of casting. The crater depth is a term which is applied to the distance between the lowest point of the existence of molten metal Within the ingot and the point at which the solidification of the metal begins to take place. For most applications, the ideal condition in cooling the ingot would be from the bottom to the top. Such a cooling would result in a substantially flat crater. So long as the cooling is from the outside to the inside, the temperature at the center of the forming ingot will be higher than at the surface and a crater of some form will. be obtained. To an extent, the depth of the crater will be related to the steepness of the isothermal lines. It is also clear that a deep crater will generally exist along with rather thin walls of solidified aluminum in the upper portions of the forming ingot, a condition which is susceptible to bleeding and liquation phenomena. High casting speeds will lead to deep craters Whereas crater depth will generally be reduced, given equivalent cooling conditions, by slower casting speeds.

(g) C0ld-shutting.-This term applies to a surface behavior which occurs in DC casting. Since the inner wall of the mold shell is at a temperature below the freezing point of the metal, when the molten metal comes in contact with this chilled wall it will begin to solidify. At low dropping rates the solidification tends to swiftly rise into the meniscus at the molten surface adjacent the point of contact. When this occurs, the liquid metal will tend to flow outwardly over the frozen interface to again contact the chilled wall of shell. The metal will now again tend to solidify into the new meniscus and the same steps will repeat themselves. This lateral to and fro movement of the metal surface in the mold, at the point where solidification takes place, results in an ingot having a wrinkled or corrugated surface, i.e., cold-shuts. These corrugations may be as deep as to A of an inch and generally must be removed by scalping the surface of the ingot. The waste and expense involved clearly indicates that the elimination of cold-shutting is a major problem in the metal casting, particularly aluminum casting industry.

Generally speaking, cold-shutting is most prominent at slow casting speeds, i.e., slow rates of the continuous removal of the ingot from the DC mold. It may be avoided, then, by using high casting speeds, but only with the introduction of other serious defects and problems.

(h) Structure.-The structure of an aluminum ingot refers to both grain structure and dendritic cell structure n within the ingot, as well as its porosity and microporosity. Part of these structures is, of course, of a much more microscopic nature than segregation, liquation, etc. They can be detected and seen by using an optical microscope after a cross-sectional slice of the ingot has been suitably etched, as with phosphoric acid.

Porosity has always been a definite point to consider in ingot production. It can result from lack of feeding during solidification or from gas dissolved in the metal or occluded in non-metallics such as skin entrapped'during casting. The DC ingot has been superior to tilt-mold ingots because of its relatively small amount of macro and microporosity; however, variations in quality do exist which can be traced to the small amount of microporosity still remaining. More rapid rates of solidification tend to minimize this defect.

Coarse grain structures will typically result from tiltmold techniques and indicate that the ingot is unsuitable for many applications where high tensile strengths and low porosity are desired. The DC ingot, in general, has a quite small and uniform grain size, particularly if grain refiners are used. The smallest grain is, however, about ten times the average dendritic cell diameter, and it is desirable to reduce this grain size still further to control the cell size. Although various techniques have been developed for the reduction of grain size by the addition of a grain refiner, the dendritic cell size is not, however, affected by these methods. Consequently, it is important in some cases to have a reduced dendritic cell size as well as grain size resulting from the casting method per se.

With the above considerations in mind, it will be seen that in typical DC casting techniques, changing conditions which will eliminate one defect will merely lead to the introduction and occurrence of some other defect. For instance, increasing the speed of ingot withdrawal, i.e., the dropping speed in the DC mold, will tend to eliminate cold-shutting; however, splitting, bleeding and liquation will then tend to take place, and the structure of the ingot will be adversely affected by the deeper crater, higher transverse temperature gradation, and large dendritic cell size andpossibly increased porosity due to the slower rate of cooling and solidification. Conversely, decreasing the dropping speed, to eliminate bleeding and to obtain a flatter crater and to improve the microstructure, will result in cold shuts on the surface of the ingot and require an expensive 'scalping operation. Of course, slower dropping speeds decrease the production rate, and are uneconomical. Thus, it is apparent that in order to successfully cast high quality aluminum alloys in a DC mold, it is necessary'to carefully select and balance the casting conditions within a relatively narrower range to obtain a compromise of the desired properties in the ingot. The control of such conditions must be maintained by the human operator, and consequently an uncontrollable variable is introduced into the overall casting system. 7

In moving from low strength alloys to high strength alloys, and from small to large diameter ingots, the permissible range of conditions which will not form, in the one extreme, cold shuts, and, in the other extreme, bleeding, becomes increasingly narrow. Eventually, at a certain point, the casting of larger ingots of medium strength and high strength alloys without the formation of either cold shuts (particularly at low speeds) or bleeding or cracks (particularly at high speeds) is substantially impossible. For instance, with the DC process, certain high strength alloys cast into ingots which alternately or simultaneously show cold-shutting and bleeding on the surface, and thus neither defect can be eliminated by varying conditions.

One of the most critical factors in controlling the casting of an ingot in the DC mold is the height of the molten metal surface in the mold. This must be very carefully controlled at the level which has been empirically found to provide optimum ingot characteristics. The reason iii) for this narrow control will be understood from the nature of the cooling which takes place in a DC mold. Since the entire mold is cold, the initial formation of a solidified skin on the ingot will take place at, or at least very close to, the molten metal surface meniscus. Thereafter, this solidified skin will move down in the mold, as the ingot is withdrawn, and will undergo, with further cooling, further solidification and contact slightly away from the mold.

From these events, in the DC process, one can see that the crater depths, for instance, will be very greatly atfected by the level of the ingot in the mold. By the same token, it is immediately evident that close control must be maintained over the level of the metal or the metal head. In actual operation of the DC mold rather complex valving devices, all of which must be preheated before casting, are necessary to closely control the rate of pouring of the molten metal into the mold, since this is the most direct method by which the operator can maintain a given metal head.

Should the metal head momentarily drop in the mold, there will be less molten metal present. This means, as a corollary, that there will be less heat present in the upper portions of the ingot. In effect, a further cooling factor has been introduced. This factor results, of course, in significant and substantial changes in the nature of the ingot being cast during this period. When the reduced head is observed, the operator of the mold will, therefore, endeavor to increase the flow of molten metal into the mold to raise this head again to the optimum point.

Conversely, if the head of metal in the mold should momentarily rise above the desired point, an increased amount of molten metal, over that desired, will be present momentarily in the mold. Here, the greater amount of heat is present due to a greater amount of molten metal at the higher temperature. In effect, this will act as if the cooling had been decreased. The result is obvious: a deeper crater will form, the isothermal lines will be displaced, and again the internal and external structure of the ingot will be seriously affected.

Even if 'an automatic control such as a floating baffle is used, the problem still exists of maintaining the additional apparatus clean and in operating order.

When it is considered that for obvious economical reasons, it is desirable to cast more than one ingot at the same time, and utilize a given mold pit and elevator mechanism for several ingots at once, and that each mold will necessarily have an individual supply of molten aluminum controlled by a separate valving device with its individual peculiarities it becomes apparent that a very skillful operator is required to produce acceptable ingots by means of the DC process.

The apparatus and process of the present invention substantially overcome the defects and disadvantages present in the DC process just described. The operation of this invention is not dependent upon close control of the metal head and, in fact, complex valving devices for controlling the molten metal flow arecompletely unnecessary. The level of the molten metal head may be at any point in the mold above the cooling contact point, as will be seen hereinafter, and the location of cooling and freezing is not dependent upon the height of such head. A great variation of casting speeds and eifective mold lengths is possible and a far greater range of varieties of ingots according to diameters and internal structures may be produced. Ingots substantially completely free of splitting, liquation and structural defects can be obtained and cold-shutting can be eliminated while still producing ingots having desirable microstructure. It is also possible to produce crack-free ingots having a desirable microstructure which are at the same time substantially free of undesirable segregation or non-uniform distribution of alloy constituents. It is even possible to produce ingots having the characteristics of a tilt-mold ingot, a type of 7 ingot which it is impossible to produce by means of the apparatus and method of the DC process.

These advantages are achieved by using a novel method of controlling the zone of cooling of the metal in the mold, and the present invention may be described as a controlled cooling or CC casting technique.

In this invention, the mold has a distinctly different cross-section design than those previously used, and to aid in the understanding of the invention, reference will be made to the accompanying drawings:

In Figure 1, there is generally illustrated in cross section one type of mold in operation according to the invention;

Figures 1a and lb are enlarged illustrations in cross section of portions of Figure 1;

Figure 1c shows an illustration in cross section similar to Figures 1a and 1b, but of a mold according to a further and desirable embodiment of this invention;

Figures 2 to 9 show, in cross-section, some of the various alternative designs for molds within the scope of this invention;

Figure 10 is an isometric view of one embodiment of the invention where a plurality of ingots are cast simultaneously;

Figures 11 to 13 show some of the various mold shapes in plan view which can be used in this invention;

Figures 14 and 15 are photomicrographs of cross-sectional areas of ingots showing the dendritic cell structure of ingots produced by the present invention;

Figure 16 is a photomicrograph of the cross-sectional area of an ingot produced by the DC process; and

Figures 17 to 22 are graphs showing the relationship in the present invention of various casting conditions to each other, and the properties of the ingot, as will be described hereinafter.

Referring now to Figures 1, 1a, and 1b, the controlled cooling mold 50 comprises an annular shell 52 fitted with a liner 60. Annular recess 54 is adapted to receive an annular boss, or similar mounting means, on a suitable support, not shown. The inner shell wall 56 adjoins angular liner surface 62 of the mold liner at point 72. Wall 56 is vertical, i.e. cylindrical, or may be gradually tapered outwardly or inwardly to the bottom or exit end 76 of the shell 52. The outer wall 58 of the lower portion 86 of the mold shell is preferably tapered toward the mold axis, as shown, for best cooling effects. Angular liner surface 62 terminates at point 74 and the liner wall 64 is substantially perpendicular up to the upper end 66. A cooling ring 80 is arranged circumferentially around the mold 50 to provide a water spray 82, directed against the lower portion 86 of shell 52. This is accomplished simply by providing perforations at a suitable angular disposition in the ring 80 and supplying water under pressure from a source (not shown).

It is important to recognize that shell 52 is constructed of a good heat conducting material and will generally be made of metal or graphite or similar materials; when casting aluminum, shell 52 is preferably made of aluminum. On the other hand, liner '60 is constructed of a material which has properties of high heat resistance and inertness as respects the molten metal being cast and is desirably an insulator since liner wall 64 must be hot surface, as will be pointed out hereinafter. It should be dimensionally stable and capable of being cast or machined to provide a strong reasonably hard surface. An example of an especially suitable insulating material is Marinite, a composition of asbestos and an inorganic binder. Non-insulating materials such as graphite may, of course, be used provided that they are backed with a heat insulator to insulate liner 69 from shell 52.

The surface 92 of a pool of molten metal 90 is shown at a level near the level of the top 66 of liner 60. The molten metal 90 will be in continuous contact with inner wall 64 and angular surface 62 below the molten metal surface 92 down to contact point 96. As shown, a submerged free meniscus 98, shown somewhat enlarged for clarity of illustration, is formed between contact points 96 and 100, and, as shown, the metal is not in contact with either of surfaces 62 or 56, and an annular meniscus cavity 102 is formed. Such a cavity is not absolutely necessary, however, as will be pointed out hereinafter.

Referring to the events taking place in the metal, 104 indicates generally the line of freezing, above which is the molten metal pool and below which the embryo ingot 106 begins to form. It will be seen that line 104 intersects meniscus 98 intermediate contact points 96 and 100. Actually, the portion 108 of ingot 106 below line 104 is in a somewhat mushy or pasty state and dotted solidification line 110 marks the boundary between the mushy state and the true solid ingot portions where fluid flow is substantially no longer evident. It will be noted in the drawing that the embryo ingot 106 is in contact with cylindrical inner shell wall 56 for a distance below point 100, but as further solidification takes place, the embryo ingot will undergo a thermal contraction and the surface 112 will no longer contact the shell. A small gap 114 will be present.

Realizing that during the process of casting molten metal is continuously added to pool 90 and embryo ingot 106 is continuously downwardly withdrawn from mold 59. there will be a constant downward flow of metal along liner wall 64, angular surface 62 and cylindrical shell wall 56. As a small increment or point mass of the metal flows along sloping surface 62, it will reach a contact point 96. Due to the removal of heat through contact of the lower portions of the ingot below point with cooled wall 56. somewhere below contact point 96, the temperature of the metal at the surface will be cooled below the freezing temperature. Under this influence, and the surface tension of the metal, a flexible and partly semi-solid film will mark the outer boundary of meniscus 98. As previously indicated, when this surface intersects freezing line 104, the portions of the metal on the immediate interior of the meniscus 98 will pass into the mushy state which is semisolid but still possesses some flow characteristics. During this period, i.e., between the intersection of meniscus 98 and freezing line 104 and contact point 100, the characteristic fragile skin observed in metal casting is formed and it is important that this skin not be ruptured or torn during casting. This tearing is avoided in the present invention. Due to the further cooling forces exerted through shell 52 by water spray 82, and further cooling in an upward direction from the lower portions of the ingot, which are also subjected to Water spray 82, the solidification line is reached. As previously stated, thermal contraction takes place and the metal is no longer in contact with shell surface 56.

In the practice of controlled cooling casting, as contrasted to the DC method, the length of contact of the ingot surface 112 with cylindrical shell wall 56 may be varied considerably. As shown in Figures 1a and 1b, this zone of contact is very short, but this represents one method of operation. The rate at which the ingot is withdrawn from the mold will in part determine the amount of heat removed from the embryo ingot through shell 52, i.e., in the horizontal direction. With fast dropping speeds, the zone of contact of ingot surface 112 and inner shell Wall 56 will be rather longer than that illustrated since the time for a given increment of metal within the mold will be much less. Consequently, the temperature of the ingot will not be lowered sufficiently to the point where contraction begins to take place. Under these conditions, this zone of contact may be similar to that which is normally observed in a DC mold. On the other hand, if the dropping speed is considerably decreased, this zone of contact will be much shorter due to the greater time then available for heat removal while the ingot remains in the mold. In this case, the contact zone will be much shorter than that which could be obtained in a DC mold because in the latter, the slower dropping speed would lead to problems such as cold-shutting and possible sticking in the mold. As a further method of operation of a controlled cooling mold, the dropping speed can be slowed down tothe point where freezing line 104 and solidification line 110 will be much higher in the embryo ingot than as shown. Of course, freezing line 104 will still be maintained below angular liner surface 62 so that freezing into the liner does not occur. However, when the freezing and solidification lines move upwardly, using the slow speeds, the thermal contraction of the embryo ingot will commence at a point above point 100. The result which is then obtained is that the surface of the embryo ingot never makes contact with inner shell wall 56 but will take the form indicated by dotted line 112a. Thus, the surface of the embryo ingot is free from the mold shell surface and, of course, tearing or rupturing of the fragile skin cannot take place. In this method, very smooth surfaced ingots are obtained which could not possibly be made if a DC mold were used.

In any of these three methods of operation, i.e. a long or short contact zone or the free surface method, as a result of the formation of the free submerged meniscus 98 intermediate the hot sloping liner surface 62 and the cold shell wall 56, and the disposition of line 104, there will be formed a continuous smooth ingot surface 112. As previously discussed, the phenomenon of cold-shutting takes place due to a repeated cooling of theembryo ingot into the meniscus, followed by flow of the metal over the solid metal again to the mold surface, and the repeti tion of these events. This to and fro movement of the freezing interface of the molten metal does not occur when operating by this invention in the mold just described because angular surface 96 blocks any tendency for the molten metal to flow repeatedly over the freezing interface in the meniscus.

The figures just described also illustrate a further feature of the invention which is completely foregin to the DC apparatus and methods. Due to the location of cooling shell wall 56 and hot inclined surface 62, and the angular disposition therebetween, the juncture of freezing line 104 with the outer surface of the embryo ingot 106 is fixed in the region between points 96 and 100 regardless of the height of the molten metal surface 92 relative to line 104. That is, regardless of whether the surface of molten metal pool 90 is at the position shown or is at, for instance, line 92a, the freezing line will still intersect the free submerged meniscus 98. This should be contrasted with the DC mold where the distance between the exit end of the mold and the freezing line is completely determined by the upper surface level of the molten metal pool, i.e. the metal head.

Thus, it will be seen that the present invention provides a mold wherein the line of pouring of the molten metal is completely unrelated to the effective mold length, or the corresponding metal head in the DC process, provided, of course, that the level of the molten metal is at least above contact point 96, it will be immediately evident that this destruction of a previously critical relationship between the height of the surface 92 of the molten pool and exit 76 of mold 50 is of immediate advantage in eliminating the previously required complicated valving devices for close control of the operator of the level of the metal. In addition, since freezing occurs well below the surface of metal, skimming devices to prevent occlusion of the surface oxide skin are not necessary.

In Figure 1c, an alternative embodiment of the invention is illustrated. In this mold, shell 52 is provided with an annular shell protuberance 120 so that liner 60 rests on shelf 122. Shell surface 124 is inclined radially outward below point 130 to point 126, marking the upper extent of the cylindrical shell wall 128. It will be seen that inclined liner surface 62 is a hot surface, but that 10 cylindrical shell wall 128 and inclined surface 124 are cold.

In again considering the events taking place in the metal as it moves downwardly through the mold 50 in this embodiment, it is important to observe that here two submerged free menisci are formed 136 and 138. Meniscus 136 is formed at the angular juncture between contact points 140 and 142 on hot liner surface 62 and cool inclined shell surface 124, respectively. Meniscus 138 is formed between contact points 144 and 146 on the cool angular shell surface 124 and cylindrical shell wall 128, respectively. Meniscus cavities 150 and 152 are formed in these respective regions.

It is believed that as a result of providing these two angular regions in the mold, and the temperature changes in each region, freezing line 104 will generally intercept the embryo ingot surface at a point in meniscus 136, and solidification line 110 will generally tend to intercept the embryo ingot surface in meniscus 138. Thus, the loci of the two significant events which take place at the surface of the ingot have been restricted, respectively, to two narrow regions, and very great control over the casting of the ingot has been achieved substantially independent of the rate of flow of molten metal into the mold and the height of the surface thereof above the exit end 76, Figure 1.

It will be appreciated that the extent of contact of ingot surface 112 with shell wall 128, can vary in the same manner in the embodiment of Figure 1c as just discussed with respect to the mold of Figures 1a and 1b. That is, the zone of contact shown in Figure 1c is quite short, but it is also within the scope of this invention to operate at faster casting speeds so that this contact zone is considerably longer. In addition, free surface casting is also possible with the tapered mold embodiment of Figure 10, by either of two techniques. When casting in a given mold, if the casting speed is decreased, the removal of heat will become sufliciently higher, for a given point mass of metal, so that freezing line 104 and solidification line 110 will tend to move upwards along the menisci 138 and 136. When the speed is sufficiently reduced, the thermal contraction of the metal will begin at a point above 146 and the embryo ingot surface will appear as indicated by dash line 112a. In this method, angular liner surface 62 and tapered shell surface 124 will be in contact with the metal but the ingot does not touch the cylindrical wall 128. This technique can be carried to a further degree so that the solidification and thermal contraction will begin to take place in the region of meniscus 136. Here the surface of the embryo ingot Will take the form indicated by dash line 112b. In this instance, neither the tapered shell wall 124 nor the cylindrical wall 128 will be in contact with the metal and a completely free surface casting will be achieved. It will be appreciated that tearing and rupturing of the initially formed fragile skin will not take place when using such free surface casting methods, and ingots having extremely smooth surfaces can be obtained. Coldshutting is, of course, completely avoided.

It will also be evident that with slower casting speeds of this sort, a shallow crater will be obtained and the internal structure of the ingot will also be very much improved.

In a further method of operation according to this invention, ingots can be obtained which substantially have the structural characteristics of tilt-mold ingots. This type of ingot cannot be produced in a DC mold. In the present invention, if a very long mold issued, and direct chilling of the ingot itself is substantially avoided, the heat removal vector will be substantially horizontal. A relatively strong skin can be formed on the surface of the embryo ingot before it contacts the cylindrical cold shell wall 56, over a wide range of dropping speeds. This method will produce deep craters in the ingot and a cooling behavior similar to that which occurs in a tiltmold. The resulting ingot is especially suitable for subsequent bright anodizing and deep-drawing applications; the former because of the larger constituent size, and the latter due to the non-directional or isotropic mechanical properties.

Referring again to Figure 1, the important linear dimensions of the molds in the present invention are indicated as the crater depth and the effective mold length. While these terms may have some application to distances in the DC mold, they are of greater significance in the present invention. As used herein, the effective mold length is the distance between the exit end 76 of the mold and point 72, in the embodiments shown in Figures 1a and lb, and point 130 in the embodiment shown in Figure 10. In other words, this is the distance between the point where the ingot leaves the mold and the point where solidification first takes place. This distance is somewhat analogous to the metal head in the DC process. The crater depth refers to the distance between this point of first solidification at the surface of the ingot and the lowermost point where molten metal exists in the center of the ingot. The liquid metal height (not to be confused with the DC metal head) is the height of the surface 92 of the molten pool of metal 90 above the point of first solidification at the surface of the embryo ingot. Whereas in a DC mold, the liquid metal head is very rigorously dependent upon the variables of the rate of supply of liquid metal to the mold, and dropping speed, in the mold of the present invention, the effective mold length is instead fixed by the design of the mold. That is, as just defined, the effective mold length has nothing to do with the level of the metal above the point of first solidification but rather relates to the length of the inner cold shell wall below the point of first solidification, which is fixed at the juncture point of the angularly disposed hot liner and cool shell surfaces.

For given ingot diameters and alloys, the liquid metal head in the DC process can only be varied to a limited degree without running into the casting difiiculties described previously. In the present invention, widely varying effective mold lengths can be used by employing different molds having different lengths of exposed inner surfaces below the juncture with the annular hot surface of the liner, thus producing widely varying structures and properties in the ingots.

In Figures 2 through 9, some of the additional types of molds according to this invention are illustrated, only one side of the symmetrical mold being shown.

Referring to Figure 2, shell 52 is provided with a liner 60, constructed of an insulating material, and which also has an angular surface 62. Intermediate shell 122 and liner 6% a graphite insert 160 having a tapered surface 162 is shown. It will be appreciated that since graphite is a heat conductor, the cooling water spray applied to the outer wall 58 of liner 52 will also cool tapered surface 162 to a temperature below the temperature of angular liner surface 62, but this mold will reduce radial or horizontal heat removal. A mold having this construction will operate similarly to the embodiment illustrated in Figure 10, with a meniscus forming in each or" the regions of angularly disposed surfaces, or the free surface molding technique can be used.

In Figure 3, still another type of construction is illustrated. In this embodiment, shell 52 is provided with an insulating liner portion 60 arranged above an annular insulating block 166 and an annular graphite insert 160 is fitted between liner 60 and block 166, as shown. This figure illustrates the point that it is not necessary that the hot angular surface 62 be constructed of an insulating material. This surface will remain a hot surface in this type of mold since it is insulated from the cold shell. One of the advantages of being able to use this embodiment is the fact that graphite has desirable surface characteristics as respects the flow of metal therealong.

This techniques of providing an annular graphite liner &

at the interior portions of the mold, but insulated from the shell 52, may, of course, be generally used in other embodiments as illustrated in Figure 4. In this case. 164 is an annular liner of graphite which is insulated from shell 52 by an annular insulating insert 166, as shown. This mold will again have a hot inclined surface 62 adjoining cold tapered shell wall 124.

Still another mold design is shown in Figure 5. In this instance, insulating liner 60 may be provided with an angular surface 62, but this surface does not directly adjoin tapered wall 124. Instead, a step 168 is provided. The operation of this mold will be generally similar to the mold of Figure 10, with the meniscus cavity formed adjacent step 168. Alternatively, step 168 can be extended along broken line 168a and the cylindrical liner surface 64 will be similarly extended along broken line 6411. In this instance, the meniscus will form between surfaces 168, or 168a, and tapered wall 124. It is possible that a mold formed according to the design illustrated in Figure 10 might tend to have a profile similar to that shown in Figure 5 in actual operation due to the different thermal contraction, and expansion characteristics of the material forming shell 52 and liner 60. That is, a small step such as illustrated at 168 in Figure 5 might actually be formed in the mold in Figure 1c when liner 60 is heated and shell 52 is cooled. Of course, if this possibility is to be avoided, it is a simple matter to so design the actual dimensions of the shell and the liner of Figure 10 so that they will take the form illustrated under the increased temperatures. Alternatively, depending on the construction materials used, thermal expansion might result in shelf 122 extending axially inwardly of point so that tapered wall 124 would not be contiguous with angular liner surface 62. This could be avoided, if desired, by the design of Figure 5 when such expansion could be accommodated by step 168.

Still another form of mold is shown in Figure 6. In this case, the shell 52 has an extended portion having a tapered surface 172 and shelf 122 is extended by angular surface 174 upwardly part way along inclined liner surface 62. With reference to Figure 10, it will be seen that the design of Figure 6 is essentially modification of the design of Figure 1c. One of the advantageous features of Figure 6 is that even if the respective materials forming liner 60 and shell 52 undergo different dimensional changes due to thermal contraction or expansion, the angle between tapered wall 172 and angular surface 62 will remain substantially the same. It will. be appreciated that the mold of Figure 6 may be used in the same manner as the mold shown in Figure 10.

As shown in Figure 7, still a different type of mold is provided wherein curved surfaces in the freezing region are employed. Shell 52 has a small protuberance 120 providing shelf 122 on which liner 60 can rest similar to the constructions previously shown. However, instead of the tapered surface 124 shown in Figure 1c, a curved wall is provided. This curved wall terminates at point 182 and the inner profile of the mold is extended upwardly therefrom by angular liner surface 184. Curved wall 180 will be continuous with lower cylindrical wall 128 of the shell. In this mold, substantially the same course of freezing and solidification will take place as shown in Figures 1 and 10 with the formation ofa meniscus in the region of point 182 at the juncture of curved wall 180 and hot liner surface 182. However, the meniscus cavity 152 (shown in Figure 1c) is not present. Instead an annular curved meniscus-like metal surface will be formed on the embryo ingot adjacent and substantially in contact with curved shell wall 180. Coldshutting is still avoided and a wide variety of casting conditions can be used. In some situations, where increased lateral cooling is desired, this would be advantageous. As would also be possible with the other designs, the liner may also have a curved surface 186 where it meets perpendicular wall 64, or the form indicated by dash lines 188 and 190,.surface 188 being horizontal, or surface 188 could be radially inclined like surface 62 in Figure 1.

Figure 8 illustrates a mold design which has the advantage of greatly decreasing the rate of horizontal heat removal. In this embodiment, liner 60 may have a radially inclined surface such as 62. The important feature of this design is the provision of the series of steps 192, 194, and 196. As shown, the molten metal will form a meniscus 200 between the hot liner surface and in contact with the cold shell stepped wall. However, contact of the surface of the embryo metal ingot with the shell is limited to the series of points 192a, 194a, and 1960. This mold design can be considered as a modification of that shown in Figure 7, with portions of curved surface 180 cut away. It will be seen that points 192a, 194a, and 196a lie on a radially inclined but discontinuous curved shell surface. The thus limited area of contact will minimize the horizontal heat removal and thereby permit the use of even slower droppingspeeds. The

direction of cooling will consequently be primarily vertical by heat conduction through the ingot to the area below the mold where the ingot is in direct contact with the water spray. By using this mold, ingots having superior surfaces can be obtained. 1

A further advantage of the stepped shell wall resides in the improved lubrication which can be obtained. In all of the molds shown, it is important to have a lubricant on the surface of the mold shell wall to minimize friction between the embryo ingot and the shell surface and to avoid sticking possibilities. Generally a graphite grease lubricant is used, but as the ingot is withdrawn from the mold, in the region where it makes contact with the shell wall, the lubricant will remain throughout each .drop, in special cases as for extremely long ingots, it is advantageous to insure sufficient lubrication. The mold embodiment of Figure 8- will act so as to hold or retain the lubricant in the annular grooves 192b, 194b, and 196b, and consequently there will tend to be a continuous supply of lubricant to the points 192a, 194a, and 1960:, where the embryo ingot is in contact with the stepped shell wall. It will be appreciated that the size of the grooves will be rather small, for instance, several thousandths of an inch. Of course, the free surface casting technique may also be used in this embodiment.

It will be appreciated that if the grooves are filled in with some inert material, a surface such as 180 shown in Figure 7 can be obtained, but that decreased horizontal heat removal in accordance with the operation of the step mold of Figure 8 will result if the inert material is a less eflicient heat conductor than the material forming shell 52. The most convenient inert material to use in a solid grease, which can be applied so as to fully occupy cavities 192b, 194b, 196b, and which will continuously provide a lubricated embryo ingot surface.

It is also possible to introduce lubricant continuously on to the surface of the meniscus in each of the molds illustrated by allowing a material such as peanut oil to flow down a passageway between shell 52 and liner 60 and wick out through the porous insulating material to the meniscus forming region. To facilitate this technique, an annular recess such as 68 (see Figure la) may be provided at the top of the mold to act as a supply reservoir for the lubricant.

Figure 9 shows another variation in mold design according to this invention. Mold shell 52 may be substantially similar to shell 52 shown in Figure 10, that is, with a radially inclined tapered wall 124 and a substanstan tially cylindrical wall 128. Liner 60 rests on shelf 122. In this case, however, the liner surface slopes downwardly in a direction to the axis of the mold to form sloping surface 202. The molten metal 90 will flow some distance into recess 204 and will form a free meniscus 206 making tangential contact with surfaces 202 and 124 below point 200. The angle between sloping surface 202 and tapered wall 124 may conveniently be about 90,

but other anglescan also be used, either acute or obtuse,

or that the free surface method of casting could also be employed in which case direct contact between the solidified metal 106 and the shell surface would be absent.

Other mold designs in keeping with the basic principle of this invention will be apparent to one skilled in this art from the foregoing description. It will be noted that in each of the molds shown, there are means for providing an annular curved metal surface on the embryo ingot of a meniscus shape, and that this curved surface is at least partially in contact with at least one radially inclined surface in the interior of the mold in the region where solidification of the metal commences. In embodiments corresponding to that shown in Figure l, the angle formed between inclined liner surface 62 and shell surface 56 should be substantially greater than for instance upwards of about but somewhat less than As indicated in the discussion of Figure 5, the liner surface 62 may be substantially horizontal where a tapered or radially inclined shell wall 124, or radially inclined curved shell wall (as in Figure 7 or as modified in Figure 8) is provided. On the other hand, in embodiments such as that shown in Figure 9, the angle between the hot liner surface and the tapered shell wall may be 90 or greater or less than 90.

Figure 10 shows an isometric, partially diagrammatic, view of one type of arrangement which can be used to cast a plurality of ingots simultaneously. A table 220, which may be made of a heat resistant insulating material such as asbestos, is designed to support any desired number of molds 50 comprising shells 52 fitted with liners 60 and cooling rings, not shown. Each mold has a channel 222 conforming generally to the cross-sectional shape of transverse feeding trenches 224 in the table. Feeding trenches 224 communicate with supply canal 226. The molten metal will be poured into canal 226 by means not shown but which may simply comprise a tiltable cauldron in which the metal is melted, and the liquid metal will flow through trenches 224 and channel 222 into the molds 50.

Ingots 106 will be continuously withdrawn by lowering the elevator, schematically shown as 230.

In operation, the rate of flow of the metal into canal 226 is adjusted so that trenches 224 will be at least partially filled with the liquid metal, and of course each of molds 50 will have a metal head at substantially the same height, and naturally overflowing of the metal onto the surface of table 220 will be avoided. No other valving means or control criteria are necessary and since freezing of the metal takes place well below the surface of the molten pool, skimming devices, etc., need not be used. The simplicity of this equipment will be evident by comparison with the characteristic apparatus employed with the DC mold as illustrated in the Ennor patent. Of course, this invention is not limited to any particular method of feeding the metal to the mold.

While Figure 10 has shown a table for casting cylindrical ingots, it will be appreciated that other crosssectional ingots may also be cast, such as those shown in the plan views of Figures 11, 12, and 13. In these figures, 64 again indicates the substantial perpendicular surface of the liner and dotted lines 232 indicate the juncture between the shell and the liner. The distance between 64 and 232 will be varied, as desired, a thicker 'te'ri'al by providing heating means in the liner. For instance, a mold could be formed having the design shown in Figure 4, ie an annular graphite ring separated from the shell by an insulating insert, and the graphite could be heated by various means such as electrical resistance.

The invention is also not limited to the casting of solid ingots. For instance, a hollow ingot may be cast by using a mold formed by revolution of the mold element of Figure 1 about an axis such as that indicated by line 240 (see Figure 1). In this case, the inner casting surfaces of the mold would be modified so that shell wall 56 would taper inwardly towards the tubular axis of the cylinder to accommodate the thermal contraction of the embryo ingot taking place below solidification line 104. That is, when casting such tubular ingots, as the ingot solidifies, the internal radius thereof will become slightly smaller due to this thermal contraction. This invention has particular advantage in casting such tubular ingots because of the wide range of dropping speeds which can be employed without sacrifice of surface and structural characteristics. For instance, in the casting of solid ingots by the DC process, closely controlled conditions must be maintained to avoid cold-shutting and bleeding. If a tubular ingot is to be made by the DC method, the difiiculties of obtaining good surfaces on both the inside and the outside of the ingot are substantially increased. It will be apparent that the cooling effects on the internal radius of the embryo ingot will necessarily be considerably different than the cooling forces on the outside of the ingot for the same dropping speeds. Since even slight changes in the effective cooling rate in DC casting can lead to either cold-shutting or bleeding, with some alloys and ingot sizes, it is practically impossible to obtain a suitable product. In the present invention, however, such criticality in dropping speed is not present and consequently even though the cooling rates on the inside and on the outside of the tubular ingot will be different, good surfaces can still be obtained over a wide range of dropping speeds and effective mold lengths and various microstructural characteristics are also possible without sacrifice of the desired surface properties.

To illustrate the variety of ingot structures which can be obtained from this invention with either a tubular or solid ingot, reference is made to Figures 14, 15, and 16. These figures are photomicrographs of cross-sectional slices of ingots which have been etched with phosphoric acid to indicate the dendritic cell diameter. Each of the figures has been magnified two hundred times. The ingot of Figure 14 was cast from 6063 aluminum alloy in a mold of the design shown in Figure having an effective mold length of one-half inch and at a dropping speed of six inches per minute. It will be seen that a very fine microstructure was obtained and the average dendritic cell diameter in this ingot was .00134 inch. The photomicrograph of Figure was made from an ingot cast from 6063 aluminum alloy in a mold having the design shown in Figure 1c with an effective mold length of ten inches and at a dropping speed of five inches per minute. It will be seen that the dentritic cell diameter is much larger and has an average diameter of about 0.00255 inch.

These two figures should be compared with the photomicrograph of Figure 16 which shows the microstructure of a six-inch diameter DC ingot of 6063 aluminum alloy cast with a five-inch metal head at a dropping speed of five inches per minute. In this case, the average dendritic cell diameter is approximately 0.00189 inch. It should be noted that the ingot slices photographed in Figures 14 and 15 were etched to a much higher degree than that illustrated in Figure 16 to better delineate the cell structure and that the thickness of the black areas cannot be strictly compared. Cell diameter is, however, the average distance between such black areas. It will be seen that this invention can be used to produce ingots having cell diameters much smaller or much larger than the cell diameters obtained from the DC process since in the latter there is relatively little, if any variation possible from the structure shown in Figure 16. It is significant that, while the structure of Figure 16 can be obtained equally well from the present process, neither the structure of Figure 14 nor the structure of Figure 15 (which corresponds very closely to the structure of a tilt-mold ingot) can be obtained from the DC method.

In Figure 17, the variation of dendritic cell size diameter with dropping speed for various elfective mold lengths is illustrated. The points in this graph were obtained in casting six-inch diameter 7075 alloy ingots in a mold having a design according to Figure 10. It will 'benoted that the dendritic cell size is primarily infiuenced by the effective mold length and this figure illustrates the variations in internal structure which can be achieved by the present invention. The legend DC Method indicates the dendritic cell diameter which is obtained in the DC casting process. Of course, as previously discussed, larger dendritic cell sizes can be obtained than those illustrated in this figure (for instance, as illustrated in Figure 15).

Figures 19 and 20 represent plots of the distribution of alloy constituents over a transverse area in the ingot. As indicated in the legend for the graph, various mold lengths and dropping speeds are used. The 0 point on the abscissa is taken at the center of the ingot. It will be noted that the DC. ingot, indicated by the solid circle, shows the inverse segregation usually found in ingots of this kind. By contrast, the ingot cast according to the present invention in a mold having a threeinch effective mold length showed slight direct segregation, and ingots cast in molds having effective mold lengths of 1 /2 inches and /2 inch showed practically no segregation.

In Figure 18, there is illustrated the range of conditions which have been used to cast eight-inch diameter ingots of 6063 aluminum alloy. In this figure, the plus signs represent conditions where ingots superior to the DC. ingots are obtained. The circles below curve e indicate casting conditions where the freezing line in the embryo ingot tended to rise up into the hot angular liner surface. The plus-minus signs indicate conditions where there was a tendency for the ingots to show splitting or cracks. The most suitable casting conditions using this mold and alloy are then located in the region between curves e and 1, but the location of such curves is not necessarily limitative of the invention. If a heated liner or molds with reduced radial heat removal characteristics, such as the designs shown in Figures 2 and 8 are used, curve e would tend to be lower. The position of curve f is influenced by the quality of the metal used, within the specified limits of the alloy composition. Of course, entirely different curves are obtained for other alloys. In each instance, however, it is a simple matter to determine the permissible limits of operation for a given mold with a given alloy and changes in mold design to provide a greater variation in casting conditions, if desired, will be apparent to one skilled in the art from the description of this invention.

For instance, and by way of further illustration of the invention, a mold was made in accordance with the design shown in Figure 1c having an effective mold length of 4 /2 inches and was used to cast ingots of 6063 aluminum alloy. In this mold, the angle between angular liner surface 62 and tapered wall 124 was and the angle which tapered wall 124 formed withthe perpendicular cylindrical shell wall 128 was 15. The diameter of the mold was 5% inches.

An ingot was cast at a dropping speed of six inches per minute. The surface of the molten metal was allowed to remain at approximately the surface of the mold but with no special care taken in this aspect. The ingot was 17 freeof cold-shutting and showed no evidence of splitting defects.

The wide variety of casting conditions which can be used in this invention is also illustrated by the graph of Figure 21. In this graph, dropping speed, in inches per minute, is plotted against effective mold length, in inches, in a series of castings with 7075 aluminum alloy. The plus signs in the graph indicate conditions which led to ingots superior to those obtained from the DC casting technique. The minus signs indicate conditions under which ingots better than those obtained from the DC method were not formed. Curve a then represents the approximate boundary of casting speeds which can be used with 7075 alloy for the indicated elfective mold lengths when using an insulating liner and the angle between the angular liner surface and the tapered shell wall is 135. 7

In the region below curve a, the dropping speed was so low that the freezing line moved up into the angular liner surface. The approximate practical range of oper- 'ating conditions with a DC mold for casting a six-inch diameter ingot of 7075 alloy is indicated by circle b. The far greater flexibility of the controlled cooling method of this invention is evident by comparing circle b with the region of permissible dropping speeds above curve a. It is also important to observe that far slower casting speeds can be used with the present mold, with the advantage of improved microstructural characteristics. On the other hand, if the desired ingot need be no better than the DC ingot with respect to its microstructure, similar casting speeds and effective mold lengths can be used, and, here, the invention will still produce a superior ingot due to the better surface obtained. In each instance, of course, at a given dropping speed, i.e. rate of production, the present invention permits variation in the effective mold length so that ingots having (littering internal structures may be made, as desired.

The graph of Figure 22 is a plot of crater depth against dropping speed per minute for casting at effective mold lengths of /2 inch, 1 /2 inches, and 3 inches, obtained from the same castings plotted in Figure 21. Figure 22 thus indicates the variety of internal structures which would result, since crater depth is roughly related to transverse temperature gradations which, as discussed above, strongly affect dendritic cell size, constituent size and segregation. In Figure 22, the practical range of variations possible in the DC method are shown by circle 0.

In another embodiment of this invention, a mold having the design of Figure with a fourteen-inch diameter efiective mold length of three inches, and an angle of 135 between the tapered shell wall and the angular liner surface was used to cast ingots of 7075 aluminum alloy of a dropping speed of 1 /2 inches per minute with a length of drop of 110 inches. Here, again, with this larger ingot and using a high strength alloy, an ingot was obtained having a substantially smooth surface, free from evidence of cold-shutting and bleeding. Furthermore, no splitting was observed with this ingot.

Using 6063 aluminum alloy, a mold having the same design as that just described, but with an effective mold length of five inches, was used to form a series of ingots at various dropping speeds from 1 /3 inches to 2% inches per minute. All of the ingots were free from cracks, coldshuts, bleeding, and all of these ingots were superior to the ingots produced by the DC process in structural characteristics. Of course, diiferent structures were present in these ingots due to the dilferent casting rates.

Ingots of 6063 aluminum alloy having eight-inch diameter and superior to ingots produced by the DC method have also been obtained using a mold of the design of Figure 10 having an effective mold length of 1 /2 inches and a dropping speed of four inches per minute and wherein tapered wall 124 formed an angle of 14 from the vertical and inclined liner surface 62 formed an angle design of Figure 1, having an effective mold length of 1 /2 inches and wherein inclined liner surface 62 formed an angle of 45 with the vertical, was used to cast similar ingots at a dropping speed of 3 /2 inches per minute. Another mold of the design shown in Figure 5, the liner .surface being along line 168a and tapered wall 124 forming an angle of 14 with the vertical and having an effective mold length of 1 /2 inches was used to cast similar ingots at dropping speeds of 2% inches per minute, 3 inches per minute, 3 /2 inches per minute, and 4 inches per minute, none of the ingots showing any evidence of cold-shutting, bleeding, or splitting. A mold of the same shape but with an eifective mold length of one inch produced similar ingots at a dropping speed of four inches per minute. Another mold had the design shown in Figure 1 wherein inclined liner surface 62 formed an angle of 69 with the mold axis and an effective mold length of 1 /2 inches, produced excellent ingots at a dropping speed of four inches per minute.

Of course, each of these molds could be used at different dropping speeds and with different mold lengths to produce ingots of different structural characteristics and of different alloy compositions. I The description of this invention has been primarily related to the casting of aluminum metals. It is apparent, however, that the instant casting method and molds may be used for other metals, including various light metals and also steels, copper, and brass. In each instance, substantially the same casting phenomena will be observed and while some mold designs may be preferred for some metals and not preferred for other metals, it is within the skill of the art to make appropriate selection of the particular mold and method suitable for the particular case.

It will, of course, be evident that this invention can be employed in connection with a wide range of casting techniques. For example, the direct chilling of the ingot as it leaves the mold, which was employed in the examples given above, can in some instances be modified. It might be omitted entirely, but this would ordinarily be disadvantageous. It is also possible to vary the crosssectional shape of the mold opening in various ways in order to vary the cross-sectional shape of the ingot, or for other purposes, such as the use of longitudinal grooves to control the area of the chilled surface in contact with the ingot. Similarly, it will be apparent to those skilled in the art that the invention is applicable to casting in horizontal and other directions as well as vertically.

While present preferred embodiments of the invention, and methods of practicing the same, have been illustrated and described, it will be recognized that the inventionis not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.

I claim:

1. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough, the effective mold length being substantially less than the maximum interior dimension of said passageway normal to the mold axis; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; at least one of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said conductive Wall surface being non-convergent; the protrusion ofsaid heatpf 45 with the vertical. In another case, a mold of the insulating wall, radially inwardly from the intersection of said surfaces, being sufiicient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

2. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heatconductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; said conductive wall surface being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufficient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

3. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; said conductive wall surface being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said insulating wall surface being approximately perpendicular to the mold axis; the protrusion of said heatinsulating wall, radially inwardly from the intersection of said surfaces, being sufficient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging'from the mold, whereby the cooled heat-conductive 20 wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

4. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; each of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a substantial angle to the portion of the mold axis surrounded thereby; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufiicient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; said heat-conductive wall having a second interior surface adjacent the exit end of said mold passageway, said second conductive wall surface being substantially parallel to the mold axis; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

5. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; each of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a substantial angle to the portion of the mold axis surrounded thereby, and said conductive wall surface being divergent at least about 15; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufficient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one'end to the insulating wall surface and at the other end to the conductive wall surface; and means'to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery 'of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidificationfrcm exf 21 tending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fedthrough the mold.

6. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said moldhaving a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; each of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a substantial angle to the portion of the mold axis surrounded thereby; and a graphite element which forms said conductive wall surface; the protrusion of said heatinsulating wall, radially inwardly from the intersection of said surfaces, being suflicient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout-a wide range of rates of casting metal as it is fed through the mold.

7. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heatconductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; said insulating wall surface being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said conductive wall surface being nonconvergent; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufficient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heatconductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold. V

8. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the ,entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passagewayi said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heatconductive annular wall which surrounds the portion of said passageway through which themetal subsequently passes; each of said walls having an'annularLnterior sur face, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway, said included angle being no more than about 150; said insulating wall surface being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said conductive wall surface being non-convergent; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufficient that a meniscus of metal is formed which is positively confined by the barrier of said'surfaces and which describes an arc of less than tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold. V

9. Apparatus for the continuous casting of aluminous metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heatconductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; at least one of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a sub,- stantial angle to that portion of the mold axis surrounded by the surface; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufficient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; said conductive wall surface being irregular and including a plurality of steps successively diverging toward the exit end of the mold passageway, whereby the area of contact between the metal and said'wall is limited and the radial withdrawal of heat from the metal is thereby minimized; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooledheat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while. the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

10. A plurality of devices for the continuous vertical casting of aluminous metal, and conjoint feeding means therefor; each of said devices comprising: an annular mold having a vertical passageway therethrough, 'the effective mold length being substantially less than the 15 maximum interior dimension of ,said passageway nor mal to the mold axis; means for feeding molten metal into the .entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway; at least one of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said conductive wall surface being non-convergent; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sulficient that a meniscus of metal is formed which is positively confined by the barrier of said surfaces and which describes an arc of less than 90, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold; and said feeding means comprising: means to supply a single main stream of molten metal; means to regulate the rate of flow of the main stream; and means to form separate streams leading from said main stream and into the respective entrance ends of each mold passageway, so disposed that the molten metal in each mold entrance has a common level with the surface ends of each mold passageway, so disposed that the molten metal in each mold entrance has a common level with the surface of the molten metal in the main stream and all of the separate streams leading therefrom, whereby said single regulating means is effective to regulate flow to all of said molds.

11. Apparatus for the continuous casting of metal, comprising: an annular mold having a passageway therethrou-gh, the effective mold length being substantially less than the maximum interior dimension of said passageway normal to the mold axis; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface,- said surfaces intersecting and including therebetween an angle substantially less than 180, interiorly of the mold passageway; at least one of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said conductive surface being non-convergent; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being suflicient and the angularity of said surfaces being such that a meniscus of metal is formed which is positively confined by the barrier of said surfaces, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heat-conductive wall surface in contact with one end of the meniscus ab ptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

12. Apparatus for the continuous casting of metal, comprising: an annular mold having a passageway therethrough, the effective mold length being substantially less than the maximum interior dimension of said passageway normal to the mold axis; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an angle substantially less than 180, interiorly of the mold passageway; said insulating wall surface being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said conductive surface being non-convergent; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufficient and the angularity of said surfaces being such that a meniscus of metal is formed which is positively confined by the barrier of said surfaces, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby the cooled heatconductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

13. Apparatus for the continuous casting of metal, comprising: an annular mold having a passageway therethrough; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heatconductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an angle substantially less than 180, interiorly of the mold passageway; said conductive surface being divergent toward the exit end of the mold passageway at a substantial angle to that portion of the mold axis surrounded by the surface, and said included angle being less than said angle of divergence)"; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being sufiicient and the angularity of said surfaces being such that a meniscus of metal is formed which is positively confined by the barrier of said surfaces, tangent at one end to the insulating wall surface and at the other end to the conductive wall surface; and means to cool the heatconductive wall surface in contact with one end of the meniscus abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the other end of the meniscus prevents solidification from extending to said other end of the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

14. Apparatus for the continuous casting of metal, comprising: an annular mold having a passageway therethrough, the effective mold length being substantially less than the maximum interior dimension of said passageway normal to the mold axis; means for feeding molten metal into the entrance end of said passageway; means for withdrawing solidified metal from the exit end of said passageway; said mold having a heat-insulating annular wall which surrounds a portion of said passageway and a heat-conductive annular wall which surrounds the portion of said passageway through which the metal subsequently passes; each of said walls having an annular interior surface, said surfaces intersecting and including therebetween an obtuse angle interiorly of the mold passageway, at least one of said intersecting wall surfaces being divergent toward the exit end of the mold passageway at a substantial angle to that portion 26 of the mold axis surrounded by the surface, and said conductive wall surface being non-convergent; the protrusion of said heat-insulating wall, radially inwardly from the intersection of said surfaces, being suflicient and the angularity of said surfaces being such that a meniscus of metal is formed which is positively confined by the barrier of said surface and tangent at one end to the insulating wall surface; and means to cool the heat-conductive wall and apply cooling fluid directly against the solidified metal emerging from the mold, whereby said cooling means abruptly initiates chilling and consequent solidification of the outer periphery of molten metal advancing through the mold while the heat of molten metal against the insulating wall surface at the end of the 15 meniscus prevents solidification from extending beyond the meniscus, throughout a wide range of rates of casting metal as it is fed through the mold.

No references cited.

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