US2410837A - Cast ingot - Google Patents

Description

Nov. 12, 1946.

A. T. PETERS CAST. I'NGOT Filed April 17, 1944 4 Sheets-Sheet 1 INVENTOR; Adolph 7.'Pe fcr.5

Mgw v I ATTOEMEKS Nov. 12, 1946.

A. T. PETERS 2,410,837

CAST INGOT Filed April 17, 1944 4 Sheets-Sheet 2 INVEN TOR. Ado/pf? 7i Pefens M ,4 M4 ATTORNEYS Nov. 12, 1946.

A. T. PETERS CAST INGOT 4 Sheets-Sheei; 5

Filed April 17,1944

IN VEN TOR. Adolph 7'. Ps/ers ATTORNEYS I NOV. 12, 19.46. V PETERS 7 2,410,837

CAST INGOT Filed April 17, 1944 4 Sheets-Sheet 4 Patented Nov. 12, 1946 UNITED STATESTPATENT orrics Adolph T. Peters,

Dow Chemical poration of Michigan Application April 17, 1944, Serial No. 531,313

7 Claims.

The invention relates to cast metal. It more particularly concerns improved cast ingot and a method of and means for casting the same of alloyed m gn sium, the liquidus of which passes through a wide range of temperature as it solidifies.

Wide freezing range magnesium alloys, particularly those used for structural purposes, generally solidify in two principal phases, a solid solution which constitutes the major portion of the solid metal and an intermetallic compound formed between magnesium and the principal alloying constituent, comprising the balance. Attempts to cast such alloys in ingot form have heretofore yielded a product lacking in uniformity, of composition as regards the average concentration of alloying constituents from point to point throughout the ingot and having an undesirable metallographic structure particularly as regards the particle size and distribution of the second phase throughout the solid solution phase and the nature of the distribution ofthe dissolved constituent in the same solid solution phase. The maldistribution of the alloying metal and particle size of the second phase is responsible. in considerable measure, for the undesirable variations in physical properties developed in wrought products formed from conventional ingots of alloyed magnesium. w

A particular object is to provide an alloyed magnesium ingot yielding superior wrought metal on being mechanically deformed thereinto as by forging, rolling, and extrusion.

Another object is to provide a method of casting ingot of the aforesaid character in a continuous length,

Another object is to provide an apparatus suitable for casting wide freezing range metal in a continuous length.

Other objects and advantages will appear as the description proceeds.

The term freezing range, as used herein and in the appended claims, means the difference between the temperature of the molten alloy when solid metal first appears on cooling the same and the' temperature of the alloy when it has just entirely solidified from the molten state under equilibrium conditions. The term liquidus refers to the curve relating the temperature and composition of the liquid portion of the alloy as it is cooled through the freezing range in contact with the solid portion of the alloy thereby deposited from the liquid; and the term solidus refers to the curve relating the temperature and composition of the solid metal so deposited. The lower Midland, Mich., assignor to The Company, Midland, Mich., a cor- 2 temperature of the freezing range so defined is the melting point of the lowest melting eutectic if such exists.- Molten magnesium alloy having a temperature above the maximum of the freezing range contains superheat and is referred to herein as superheated."

My invention is predicated upon the discovery that by cooling the upper end of the preformed ingot of alloyed magnesium, the alloy having a wide freezing range, positioned adjacent to the larger end of a shallow mold, the inside tapering inwardly from the bottom; adding to the mold molten superheated alloyed magnesium having a wide freezing range while preventing loss of heat therefrom through the mold wall so as to main tain a pool of liquid metal therein turning to slush at the bottom thereof in contact with the cooled ingot as heat from the slush is removed substantially wholly by conduction through the ingot, slush being thereby molded to the form of the ingot and then frozen thereto to increase its length, the unfrozen portion of the slush acting to seal the mold against escape of liquid metal between the frozen slush and the mold wall; lowering the so lengthened ingot at a rate equivalent to the rate at which such increase in length occurs and in alignment with the mold so as to withdraw from the mold slush as it is molded, sound ingot is obtained having a unique metallographic struc-' ture and substantial uniformity of composition throughout its length and breadth adapted for.

mechanical deformation into wrought products possessing superior strength properties.

The invention then consists in the ingot, method, and apparatus hereinafter fully described and particularly pointed out in the claims, the annexed drawings and the following description setting forth certain means and mode of carrying out the invention.

In the said annexed drawings: Fig.- 1 is a side elevation partly in section of an embodiment of the ingot casting apparatus showing ingot being cast from slush metal in a metal holder or mold.

Fig. 2 is a similar view showin a portion of startin its Fig. 4 is a similar view of. a ortion of Fig. 1' showing a short section of ingot formed on the dummy block.

Fig. 5 isa similar but enlarged fragmentary through a pipe.

view of a portion of Fig. l showlng in detail the transformation of liquid metal into slush, molding it to the form of ingot, and freezing the same into solid ingot in continuous sequence./

Fig. 6 is a photomicrograph, at 100 ,diameters, showing the metallographic structure of Dowmetal J-l cylindrical 13-inch diameter ingot, taken at a point 1 inch from the cylindrical surface in a plane perpendicular to the axis, the ingot being cast in accordance with the invention.

Fig. 7 is a similar photomicrograph to Fig. taken 3 inches from the cylindrical surface of the ingot.

Fig. 8 is a similar photomicrograph to Fig. 6, taken on the axis of the ingot.

Fig. 9 is a similar photomicrograph to Fig. 6 but more deeply etched, revealing coring of the solid solution phase, magnification 200 diameters.

In the several figures like numerals refer to like parts.

Referring to the drawings, numeral i is a vertically disposed metal holder or mold, open at the top and bottom, for molten metal and slush formed thereof. The inside wall of the mold slopes inwardly from the bottom Ii which is the larger end of the mold and opens downwardly. Th metal holder is provided with means to preclude loss of heat, such as a covering of thermal insulation l2. If desired, a thin metal ring 28 is attached close to the bottom of the mold to hold the insulation in place. The rim i3 is attached to the top of the mold i0 and rests upon a support ll carried on legs l and i8. Means for imparting vibration to the mold. such as an air operated vibrator I1, is shown mounted on the beam l8 carried by supports it and 20, the lower end of which are attached to diametrically opposite points 21 and 22, respectively, on the rim i3.

Means are provided for bringing molten metal to the metal holder such as the tilting ladl 23 carried by ball 24 which is shown in Fig. 1 resting upon the hook 25, although other means may be used, such as a syphon or liquid metal pump, delivering molten metal to the holder or mold A cooling water launder 21, such as one having a right triangular cross-section, is arranged adjacent to the bottom of the metal holder. Extending around the inner circumferential face 28 of the launder ar parallel rows of uniformly spaced small holes 28, drilled perpendicularly to the face 28 and thereby forming iets directed at an angle, such as 45, to the vertical. The launder is provided with a cooling water connection 20 to a valved water supply ll.

Guide wheels 82 and 3; are arranged so that their treads 3! and 35. respectively, are in vertical alignment with the inner circumference of the larger end ll of the metal holder; Driving wheel 38 has a tread 31 in vertical alignment with a diametrically opposite point on the inner circumference of the metal holder from that of the treads of guide wheels 32 and 33. The drivme wheel 36 is rotated by worm gear it operated by worm inion 39 driven through reduction gear 40 by motor 4i provided with speed regulation controls l2.

In startin the process, a-dummy block I; of suitable length and having the cross-sectional form of the ingot to be cast, is positioned against the treads ll and 35 of the guitk wheels 32 and 33 and. the tread 31 of the driving wheel 3', with which it frictionally mes, so that the concave head 44 engages the bottom end I I of the liquid metal and slush holder II to form a bottom closure therefor, .as shown particularly in Fig. 2. The dummy block is preferably made by casting a length or ingot in the usual manner or otherwise of a magnesium alloy having a similar composition to that to be cast, although other metals may be used.

Cooling water is directed against the dummy block by opening the valve in the cooling water supply line 2| thereby providing a curtain of water 45 from the Jets 2! drenching the vertical surface of the block. In so drenching the surface of the ingot, it is desirable that the water he directed downwardly at an angle to the vertical, as with the apparatus provided so that thewater is uniformly distributed and directed away from contact with the bottom ii of the metal holder l0.

While the dummy bloclcis thus cooled molten magnesium alloy to be cast into ingot and contraining a sufiicient amount of superheat is introduced into the metal holder in from the top,

as by ladling in the manner indicated in Fig. l,

to a depth approaching the brim ll of the metal holder Ill. The average temperatur of the wall of the metal holder or mold is maintained above the temperature of the liquidus of the alloy in the mold by the superheat of the liquid metal introduced thereinto, loss 0! heat from the mold being precluded by insulation l2. or by the use of an external heater around the mold (not shown), so as to prevent the formation of solid alloy particles by abstraction of latent heat through the mold wall. i0 is insulated, as with the insulation 12, or otherwise made proof against substantial heat loss, heat from the liquid metal (i. e. superheat of the liquid and latent heat) immediately begins to be conducted away substantially wholly through the head 44 into the cooled dummy block 43. The effect of such heat transfer is to bring about crystallization of solid metal within the liquidto form therewith a bed of slush l1 of solid metal particles mixed with liquid metal supported by the wall of the mold and the top of the dummy block as shown in Fig. 3.

The slush bed thus formed is of gradually decreasing density (i. e. ratio of volume of solid metal particles to that of liquid) toward the supernatant liquid metal 48. The slush adjacent to the top of the dummy block becomes increasingly dense as the liquid therewith cools until it all freezes. I have found that as this slush becomes increasingly dense, it passes through a stage in which it is moldable and use is made of this property in my process by shaping or molding the slush to the cross-sectional form of the ingot desired before final freezing occurs. This shaping is accomplished adjacent to the big inwardly tapered end I l where the slush approaches its maximumdensity and takes the cross-sectional form of the opening of the mold before final freezing of the entrained liquid alloy occurs.

The slush thus shaped is lowered from the mold by lowering the dummy block thereby bringing a further quantity of slush into molding position. while the molded slush is being thus lowered, the liquid therein freezes forming of the slush a solid body of metal having the cross-section of the ingot desired. A portion of ingot ll thus producedis shown in Fig. 4. resting on the top H of the dummy block -43, the line of demarkation between the frozen and unfrozen slush being in- 76 dicated at I. The unfrolen slush also acts to Since the metal holder liquid pressure head. on the slush. Completesolidification of the metal into ingot is thus brought about below or just inside the bottom of the mold or holder ID, the temperature of the wall thereof being maintained by the liquid metal introduced above the lowest temperature reached on the liquidus curve when the liquid metal is all just frozen, usually the eutectic melting point.

In carrying out the method in continuous manner, after forming a body of slush "as described, the dummy block 43 i lowered continuously at a rate such that only the relatively dense molded slush emerges from the metal holder, the balance of the slush slumping downwardly, forming a seal while liquid metal is added, preferably at a substantially constant rate, so as to maintain its level and the inventory of slush therein, at a substantially constant shallow depth. Too rapid lowering of the slush through the big end II is revealed bya loss of the sealing action, while, if

the lowering is too slow, the lower portion of the slush tends to freeze to a completely solid mass within the metal holder, hindering lowering. Regulation of the rate of lowering of the dummy block, and the ingot metal which'follows it, as the operation proceeds, as shown in Fig. 3, is accomplished by adjusting the speed of the motor 4| with controls 42. The speed of the driving wheel 36 is thereby adjusted to the proper value. The proper speed varies primarily with the alloy composition, initial temperature (extent of superheating) of the liquid metal, diameter of the cast ingot, and temperature and volume of cooling water and is readily ascertained by trial. As an illustration, in casting 12-inch diameter ingot of Dowmetal O (nominally composed of 8.5 per cent aluminum, 0.2 per cent manganese, and 0.5 per cent zinc, the balance being magnesium), using a pouring temperature of 1260 F. (i. e. temperature at which the metal is introduced into the metal holder) in a frusto-conical metal holder tapering inwardly from the bottom at the rate of A, inch per foot, and having a depth of 8 inches, formed of inch boilerplate surrounded by 1 inch thick asbestos thermal insulation, and using about 20 gallons per minute of cooling water at 45 F., a suitable rate of lowering is between about 2.0 and about 2.7 inches per minute, about 2.5 inches being preferred.

As the operation proceeds, cohesion between the solid metal particles and the ingot is largely relied upon to move the slush downwardly through the metal holder in, which molds the slush as it passes therethrough, however, some of the metal particles in the slush may tend to stick to the mold surface, necessitating the provision of means to dislodge them as by the use of a vibrator (air hammer), such as that shown as I! in Fig. 1, which vibrates or preferably imparts a continuous series of sharp blows to the metal holder. I have foun that by making the total depth of the slush and supernatant liquid metal between about 5 inches and 9 inches and preferably about 7 inches, using a metal holder tapering inwardly from the bottom at a rate between about inch and inch per foot, continuous downward movement of the slush is readily obtained using vibrations at the rate of per second and an amplitude of not over about 0.07 inch or a series of sharp blows at one second intervals moving the mold rapidly a distance not over about 0.07 inch. In such operation, the depth of the slush bed 6 may be from about 1 to 2 inches and the height of the upper edge of the solidified ingot 50 may be as high as 1 inch insideithe bottom of the metal holder or as low as 4 inch below the same, this height being controlled by suitably regulating the rate of lowering the formed ingot as aforesaid. In Fig. 5, the molded slush emerging below the mold as the slush slumps therein is indicated at 5| adjacent to the frozen-top edge 52 of the ingot, as when the process isoperated so. that final freezing of the ingot occurs wholly outside or below the mold.

meet 50 formed in this manner may be increased in length as much as desired. In Fig. 1, the ingot is shown increased in length to below the guide wheel 33 with the dummy block still attached. This may be cut off, as with a traveling saw (not shown), and the ingot cut in a similar manner into convenient lengths, if desired, as ingot is produced.

The method maybe practiced with any 01 the magnesium-base alloys, the liquidus curveof which extends over a substantial range of temperature and composition as the alloy solidifies.

This is the case with the alloys of magnesium usually used for the manufacture of extrusions, forgings, rolled and drawn products, which have a freezing range of morethan Fahrenheit degrees. Such alloys usually contain aluminum, as the principal solid solution and intermetallic compound forming alloying ingredient, in amount between about 3 per cent and 10 per cent, although, in some cases, the aluminum content may be as high as 12 per cent or more. Zinc also forms solid solutions and sometimes intermetallic compounds with magnesium and may be present in the alloys in amount between about 0.5 and 3 per cent or more, either as the sole alloying agent or in combination with other alloying constituents, particularly aluminum. In the magnesium-base alloys containing both aluminum and zinc as alloying constituents in the aforesaid amounts, the solid solution contains most of the aluminum and substantially, if not all, the zinc, the balance of the aluminum and zinc, if any, forming. intermetallic compound. Manganese in small amount, e. g. 0.1 to 0.3 per cent, is generally added to the magnesium alloys to enhance corrosion resistance and it does not substantially afiect the freezing range. Examples of such alloys containing aluminum, zinc, and manganese are given in the ac-- companying table, together with the freezing tem- I perature, that is, the temperature at which solid particlesbegin to deposit and form slush as the molten alloy is slowly cooled,,as under equilibrium conditions. Temperatures whlchare 125 to 250 Fahrenheit degrees or more below the freezing temperature are usually required to completely solidify these alloys.

Table I Nominal composition, percent Freezing Dowmeial alloy lamp 4] Mn Zn Mg rs and rs-i 3.0 0.3 1.0 Balance 1, ice Jand J-l 0.6 0.2 1.0 do. 1,145 0 and O-l 8. 5 0. 2 0. 5 .do... 1,130

By thus manipulatin the slush so as to mold it as it is formed, substantially wholly abstracting its heat through the ingot into which it solidifies, and regulating the rate of lowering of the slush so that the solidification occurs adjacent the bottom of the mold, preferably substantially wholly below the-mold, a unique metallographic structure results in the ingot. This structure is characterized by (1) substantially uniform but small sized masses of cored solid solution phase of one or more alloying constituents in magnesium and (2) a second phase which is generally an intermetallic compound of magnesium and the principal alloying ingredient (e. g. MgnAlu in magnesium-base alloys containing aluminum as the principal alloying metal) in exceedingly small particles having a maximum width of 0.0006 inch distributed through the solid solution phase. In Figs. 6, 7, and 8, the intermetallic compound is shown as small light particles having widths of up to about 0.0004 inch surrounded by a dark line of demarkation, the remaining, light background being the cored solid solution phase. In Fig. 9, the background of cored solid solution phase is accentuated by the deeper etch. This reveals the aluminum concentration gradient in the cored solid solution phase, the aluminum concentration being greatest adjacent the particles of the intermetallic comp und.

{is regards uniformity of composition, typical analysis of representative samples from scalped 12-inch diameter Dowmetal J-l ingot cast, in accordance with the invention, and sampled at the center line, and at 3 inches, and 6 inches therefrom in a plane perpendicular to the axis, show an aluminum concentration of 6.5 per cent, 6.6 per cent, and 6.6 per cent, respectively. Similar samples taken from adjacent sections of the ingot showed similar values for the aluminum concentration. The variations in the aluminum concentration in ingot similarity cast in other magnesium-base alloys do not exceed the foregoing and all such variations are less than about 2 per cent of the total amount of alloying ingredient from place to place-throughout the ingot, the size of the sample being large enough, e. g. to 10 grams, to be representative. In comparison with this, similar sampling of conventional 12-inch diameter ingot of the same alloy cast in 30inch lengths gave aluminum concentrations varying both from the center toward the outside and from top to bottom, the variations from top to bottom, respectively, being from 6.0 per cent to 6.8 per cent along the axis, 6.4 per cent to 6.8 per cent 3 inches from the axis, and 6.6 per cent to 6.7 per cent 6 inches from the axis. As regards the uniformity of distribution of manganese, that of the ingot of thc invention showed no significant variation from center to outside and lengthwise of the ingot, while the concentration in conventional ingot varied from a minimum of 0.16 per cent to a maximum of 0.33 per cent.

By virtue of the unique metaliographic structure, especially the uniform distribution and smallness of the particle size of the second phase, even spacing and sharp concentration gradient of the coring oi the solid solution phase, and uniformity of composition, the ingot has its principal utility as stock material for making masses of cored solid solution, was extruded into- 'long. Tensile strength properties of specimens cut from the extruded bar at 3 /2 feet, 8 /2 feet, and 15 feet from the end first issuing from the die, measured in the direction parallel to the direction of extrusion are tabulated in Table II.

Table II Longitudinal strength in thousands of pounds Temple, per square inch lest distdainoe mm s Per cent Com ressicn Yield 1 Ultimate elongation I At 0.2 per cent deviation from the modulus line.

i In 2 inches.

For comparison, the properties of similar test pieces cut from the same, size bar extruded similarly but from conventionally cast ingot are tabulated in Table III.

I and -see Table II.

While the invention has been illustrated with the use of a mold having a circular cross-section (frusto-conical) other shapes may be used such as those of rectangular cross-section or those formed of combinations of plane and curved walls, as an oblong section with rounded sides. In all such molds the inside is made to slope inwardly from the bottom with a substantial taper, such as /8 inch to inch per foot, as aforesaid, the taper exceeding the slope required merely for relieving frictional resistance as employed in conventional molds.

Among the advantages or the method are that oxidation of the ingot surface, which occurs between the ingot and mold wall in conventional continuous casting, is obviated as the mold is so filled with metal below the liquid line that air cannot attack the metal surface adjacent to the mold. Also, the interior of the mold wall below the liquid level is not subject to severe abrasion since substantially only either liquid or slush is present in the mold. In addition, the ingot does'not sufl'er from the defects which result from contact with the wall of the mold common in conventional continuous casting. For this reason, also, sticking of the ingot to the mold is obviated.

Iclaim:

1. The method of casting ingot of alloyed magnesium having a wide freezing range which comprises continuously supplying tbe molten alloy to a vertically disposed'mold, the inside of said mold sloping inwardly from the bottom, while continuously lowering ingot from below the mold; drenching the sides of the ingot with a coolant so as to carry away heat from the metal supplied to the mold thereby to form a bed of slush therein of liquid metal mixed with solid metal particles, the lower portion of the bed adJacent to the upper end of the ingot freezing thereto and increasing its length; and regulating the rate of lowering of the ingot so that the unfrozen portion of the slush in contact with the ingot is maintained adjacent to the bottom of the mold forming a seal with the mold preventing leakage of liquid metal therefrom.

2. The method of casting ingot of alloyed magnesium having a wide freezing range which comprises continuously supplying the molten alloy to a vertically disposed thermally insulated mold at a rate such that a substantially constant level of liquid metal is established therein, the inside wall of said mold sloping inwardly from the bottom, while lowering ingot from below and in line with the mold; drenching with a coolant the sides of the ingot as it is lowered so as to carry away heat from the metal supplied to the mold thereby to form a bed of slush therein of liquid metal mixed with solid metal particles, the portion adjacent to the upper end of the ingot freezing thereto and increasing its length, the unfrozen portion forming a seal between the mold and the frozen slush preventing escape of liquid metal from the mold; and regulatingthe rate of lowering of the ingot so that the freezing of the slush onto the end of the ingot occurs substantially whollybelow the mold while maintaining the aforesaid seal.

3. The method of casting ingot of alloyed magnesium having a freezing range of more than 125 Fahrenheit degrees which comprises continuously supplying the molten alloy to a vertically disposed mold, the inside wall of said mold sloping inwardly from the bottom at a rate of about $4; to inch per foot while continuously lowering ingot from below the mold; drenching the sides of the ingot with a coolant so as to carry away heat from the metalsupplied to the mold thereby to form a body ofslush therein of liquid metal mixed with solid metal particles, the lower portion adjacent to the upper end of the ingot feezing thereto and'increasing its length; and

a 10 5. In a method of casting ingot of alloyed magnesium in a continuous length, the steps which consist in cooling a, section of the precast ingot near oneend thereof; positioning the cooled end adjacent to the bottom of a, vertically disposed mold insulated against substantial heat loss and havinginsides sloping inwardly from the bottom; introducing molten magnesium alloy into the mold so as to maintain a, pool of liquid metal at a substantially constant level therein, thereby cooling the alloy by transferring'heat to the cooled end of the ingot and forming a body of slush of solid metal particles mixed with liquid metal beneath the pool, the slush adjacent to the ingot conforming to the shape of the mold and freezing onto the end of the ingot to form an extension thereof, and the unfrozen slush forming a seal preventing the liquid metal from escaping from the mold between it and the extension of the end of the ingot; and lowering the ingot at a rate sufficient to compensate for the extension thus produced so as to maintain its position just below the mold without breaking the seal formed by the slush.

6. In a method of casting ingot of alloyed magnesium in a continuous length, the steps which consist in cooling 9. section of the precast ingot cooled and of the ingot and forming a body of regulating the rate of lowering of the ingot so that the freezing of the slush onto the end of the ingot occurs substantially wholly below the mold, the unfrozen portion of the slush forming a seal between the mold and the frozen slush thereby to prevent escape of liquid metal from the mold.

4. The method of casting ingot of alloyed magnesium having a freezing range of more than 125 Fahrenheit degrees which comprises continuously supplying the molten alloy to a, vertically disposed thermally insulated mold, the inside wall of said mold sloping inwardly from the bottom at the rate of from to Apinch per foot, while continuously lowering ingot from below the mold; vibrating the mold; drenching the sides of slush of solid metal particlesmixed with liq d metal beneath the pool, the slush adjacent to the ingot conforming to the shape of the mold and freezing onto the end. of the ingot to form an extension thereof, and the unfrozen slush forming a seal preventing the liquid metal from escaping from the mold between it and the extension of the end of the ingot; lowering the ingot at a rate sumcient to compensate for the extension thus produced so as to maintain its position just below the mold without breaking the seal formed by the slush; and vibrating the mold so as to dislodge'alloy particles tending to stick thereto.

'7. The method of continuously casting ingot of alloyed magnesium having a wide freezing range which comprises maintaining the precast end of the ingot directly below and adjacent to the bottom of a short vertically disposed mold, the inside wall of said mold sloping inwardly from the bottom; continuously supplying the molten alloy to .the mold so as to form a pool therein over the end of the ingot; applying a coolant to the sides of the ingot so as to carry away heat from the liquid metal in the mold thereby to form a body of slush of liquid metal mixed with solid metal particles beneath the pool, the lower portion of the slush adjacent to the precast end of the ingot freezing thereto and increasing its length; continuously lowering the ingot while supplying the molten metal to the mold and vibrating the mold; and regulating the lowering rate of the in ot so that the freezing of slush thereonto occurs substantially wholly below the mold, the unfrozen portion of the slush forming a seal between the mold and the frozen slush thereby to prevent escape of liquid metal from

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