US1777658A - Method of forming magnesium ingots for working - Google Patents

Description

Oct. 7, 1930. T. D. STAY El' AL 1,777,658

METHOD OF FORMING MAGNESIUM INGOTS FOR WORKING Filed June 4. 1929 IIII IIII

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- IIII llll Patented Oct. 7, 1930 'UNITED STATES PATENT orrlce THnaoN L. STAY, or CLEVELAND, AND WILLIAM IIoLzHAUEa, or LAxnwooD, omo, AssIcNons To AMERICAN MAGNESIUM CORPORATION, or NIAGARA rALLs, NEW YORK, A ConroaATIoN or NEW Yom:

METHOD 0F .'FOBMING MAGNESIUM NGOTS FOB. WORKING Application 'led June 4, 1929. Serial No. 368,378.-

lThe invention relates to the formation ofV nesium alloy objects from ingots has been` attended by numerous difficulties due largely to the unsatisfactory character of ingots.

Not onlyis it desirable to yavoid piping, whichV necessitates cropping and corresponding scrap loss, but the ingots should be free from porosity, which, as is well known ma cause defects in the finished article. In a dition,

in the case of magnesium base alloys, liqua tion and segregation should be avoided because of their prejudicial eect upon the working properties ofthe ingots and upon articles produced from them. These considerations apply to vmost metals and alloys, but it is more difficult to produce satisfactory ingots of magnesium and magnesium base alloys because when molten they are highly reactive to moisture, oxidizing agents and the constituents of the atmosphere. because of the low Aspecific gravity of magnesium, the ingots are more likely to contain larger amounts of oxide and other non-metallic impurities, and to be more porous than other less readily oxidizable metals and alloys.

Furthermore', in producing metal ingots for working, it is desirable to rovide a grain structure throughout which est meets the requirements of the particular manner of working to' which they are to be subjected, and such as will result in good physical roperties of the worked metal. A. particular difficulty in the fabrication of ingots of magnesium and magnesium base alloys refsides in their tendency to surface and corner cracking upon beingworked, this beingyin general more pronounced than in most other metals. As far as we are aware, the grain structure of magnesium ingots has not heretofore been controlled to avoid these difficulties.

It lhas heretofore been observed that metal ingots of improved character in some respects can be produced by progressively cooling the molten metal from the -bottoms of the ingot Also,

molds upwardly, either by gradually lowermg the molds into bodies of water, or by spraying water upon the molds, beginning at the bottom and advancing the spray upwardly, or moving the molds downwardl through the spray. As compared with natural cooling, this practice results in the formation of ingots in which piping is materlally reduced, the tendency to liquation and segregation is lessened, and porosity due to gas occlusion and to' shrinkage is dimin-' ished. VvVhile these procedures improve the grainvstructure, nevertheless the ingots have grains of irregular size and dendrite forms extending upwardly and inwardly from the outer faces to the centersof the ingots. Such a structure does not Wholly overcome corner and surface cracking, and it does not lend itself to attainment of the best physical properties in the worked metal.

rlhe object of our invention is to provide a method of forming magnesium and magneslum base alloy ingots for working, the ingots having such grain structure and such freedom from segregation, liquation, and

porosity, that the worked metal has high and uniform physical characteristics and does not develop surface cracksand corner cracks While being worked.' f

Our invention is predicated. on our discovery that its stated object is attained by progressively, gradually and rapidly solidifying a body of molten magnesium or magnesium base alloy in an ingotmold from its bottom to its top. While applying heat to the unsolidified portion of the metal about its progressively rising plane of solidification.

In the practice of our invention as applied to the formation of magnesium or magnesium base alloy ingots for working; an ingot mold of suitable form is supplied with molten metal at a temperature best suited to the formation of the desired grain structure of the solidified ingot, the metal being poured into the mold in a careful manner so as to preclude any large amount of! splashing of the metal and thereby avoid consequent formation of oxide and nitride occlusions and the development ofother prejudicial characteristics. The metal in the mold 'is then progressively and gradually solidified from the bottom to the top of the mold while heat is applied to the body of the'metal above its rogressively rising lane of solidification. Buch solidiication ofp the metal may be accomplished by gradually lowering the mold through a burner or series of burners which apply heat to its w'alls to keep the upper portion of the metal molten and at its desired temperature, and through a spray or sprays of Water applied to the mold below the burners to progressively cool the metal substantially in that portion of the mold lying in the horizontal plane passing through the spray. In a similar manner, the mold may be gradually lowered through a burner or burners into a body of water below the burner, the body of water being used instead of spraying water upon the mold. As a further alternative, the mold may remain stationary within a series of alternating Water sprays and burners which are individually controlled to cause the metal to progressively solidify in a gradually rising general plane while being maintained molten above such plane. While gas or other Huid fuel burners are preferably used for heating the upper portion of the mold and its contained metal, it will be understood that such heating may be effected electrically or otherwise.

Apparatus which may be and has been used in the practice of the invention in the several ways just explained is illustrated in the accompanying drawings, of which Fig. 1 is a vertical central sectional view of a mold passing downwardly through a heater and a water spray; Fig. 2 a similar view of a mold passing downwardly through a heater and into a body of water; and Fig. 3 a similar view of a stationary mold surrounded by alternately arranged burners and sprays.

The apparatus shown in Fig. 1 comprises a vertically disposed shaft 1 of refractory material suitably supported by posts 2 and provided at its bottom with a gas or other liuid fuel burner 3 preferably extending on all sides of the shaft. Below burner 3 there is a spray pipe l which also preferably eX-V tends on all sides of the shaft, or in other words, completely surrounds the mold. ln

the course of being lowered throughthe superposed heater and cooler, there is an ingot mold here shown as consisting of a tubular side wall member which is preferably square in cross section, and ak removable bottom 6 attached to the side wall member by straps 7, the lower'edge of the wall being suitably luted in a groove formed in the upper face of the bottom 6. The top of the mold is provided with lugs 8 engaged by hooks 9 attached to a cable 10 which is paid out at a predetermined rate by means of any of the various well known mechanisms for doing this. As the mold is lowered through cooling spray 4, the metal in the vmold solidifies in a general plane lying substantially in that of the spray, and the metal above this plane of solidication is maintained in its molten condition by heat from burner 3, the flames of which rise between the wall of the mold and shaft 1.

The apparatus shown in Fig. 2 is similar to that of Fig. l, the water spray 4 of Fig. 1 being displaced by a vessel l5 provided with a body of water, which, if desired, may be circulated and ke t at a definite horizontal level by means o inlet and outlet pipes 16 and 17. The mold 18, shown in Fig. 2 as being lowered through the heater and into t-he cooler, is of the closed bottom integral type, and may be suspended and lowered in the Sallie manner as explained with reference to Fig. 1.

In the apparatus shown in Fig. 3, a mold 20 of the'same form as that shown in F i0. 1 is indicated as resting upon a base 22, an( as being surrounded by a series of water spray pipes 23, 23a, 23" and 23C, each connected to water supply line 24 from which flow of water to each of the several spray pipes is independently controlled by valves 25, 25a, 25b and 25C. Between adjacent water spray pipes there are burners 26, 26, and 2Gb, each connected to a fuel which flow of fuel is independently controlled by valves 28, 28il and 28.

In the operation of the apparatus of Fig. 3, a valve 25 is first opened to cause Water to spray from pipe 23 upon the lower portion of the mold, and burner 26 is lighted to maintain molten the metal in the upper portion of the mold. At the beginning of the cooling operation sprays 23, 23h and 23 are not used, but where necessary the upper burners 26a and 26b may be used, this being deter mined by a suitable pyronieter placed in the molten metal. As the plane of solidiication of the metal rises, burner 26 is turned off by closingvalve 28, and spray 23a is brought into play by opening valve 25", burner 26a being then lighted if not previously in use. This procedure is followed until the rising plane of solidification reaches the top of the metal in the mold. In the use of this apparatus heat is effectively abstracted from the molten lmetal through the lower solidified portion of it and through the lower cooled portion of the mold wall, and accordingly in some cases, depending upon the size of the ingot being solidified, it is unnecessary to use the upper sprays and burner or burners.

A substantial advantage in the use of the spray cooling apparatus of Figs. 1 and 3 is that the ingots may be formed without taper from end to end. When spray cooling is used, open bottom molds having parallel side walls as shown may be employed without liability of water obtaining access to the molds at their bottom joints. By removing the bottoms the side walls of the molds may be supply pipe 27 from stripped from the solidified ingots, either by I stri from the ingot. Whenthe metal is coo ed by lowering the molds into the body of water in the manner illustrated in Fig. 2 it is usually necessa to use closed bottom ingot molds, which, in order to assure the removal of ingots from them, must be tapered outwardly from their bottoms to their tops, as illustrated.

From the foregoing description of the construction and operation lof the apparatus which maybe, and which in point of fact has been, used in the practice of our invention and in the attainment of its object, it will be noted that the gradually rising plane of solidilication of the metal is not a geometricall true plane, this term being used in the speci cation and in the claims to describe and define a solidification stratum which is flat as compared' to the prior somewhat conical solidification strata incident to the solidification of metal' proceeding from the side walls as well as the bottom of a mold. The chilling of the mold at and' Ibelow the point of application of a. cooling medium'being soy rapid, and the mold being maintained at an elevated temperature above the plane of aplication of the cooling medium, the metal 1n the mold above such lane d oes not chill upon the upper side wal of the mold, and accordingly upwardly and inwardly extending elongate dendrite crystallization 'of the metal is precluded. e

In the practice of each of the several described ways of progressively cooling the metal various advantageous grain structures may be produced. By maintaining that portion of the metal above the rising plane of solidication at a temperature materially higher than its 'melting point, we have found that the grain structure is of an elongated form extending parallel to the vertical axis of the ingot, which is in the direction of working of metal by rolling. By maintainy ing, as is preferred, the unsolidified portion of the metal at a temperature slightly above its melting point, there results an ingot having a ne equiaxed grain structure substantially uniform from end' to end, and which may be forged -or otherwise worked without developing corner and surface cracks. It Vis inherent in both of these procedures that the metal solidies simultaneously throughout its substantially horizontal rising plane of solidification, as distinguished from solidifying progressively from the wall of a mold towards the center of the metal in it,such simultaneous solidiication being due to the continual and uniform extraction of heat by and from the solidifying metal which is beneath the molten metal. Also ingots formed by both. of these procedures have substantially uniform compositions throughout.

and porosity are largely eliminate As a specific example of the practice of our invention in the formation of ingots havin have ound that in forming ingots 8 x 8 x 24 inches from magnesium of commercial V uritv, and bythe use of the apparatus of ig. 1, e equiaxed grain structure, coupled with freedom from porosity and piping, are produced by lowering the ingot mold at a rate of about y1 of an inch per minute while spraying on it from 15 to 20 gallons of water per minute, 'and `while maintaining the unline equiaxed grain structures, we

solidified metal at a temperature of about 'I 1250 F., the mold having been -filled'witli metal at an initial temperature of about 13009 F. In like manner there may be produced ingots of magnesium base alloys,

which, in addition to having the foregoingcharacteristics, are substantially free from segregation and liquation.

The benefits of this invention arise in part from the provision of ingots of magnesium or magnesium base alloys, in which (piping which are also of improved quality due to the opportunity given the ingots to scavenge themselves on non-metallic impurities. Furand thermore, in the case of magnesium base y alloys, segregation and liquation are substantially eliminated. A marked advantage of these ingots arises from their greatly improved grain structure resulting in improved Working characteristics by the elimination of surface and corner cracking. The supe-v rior qualities of metal worked from these ingots becomes apparent by comparison of the physical properties of the worked metal with those of metal of the same composition.

worked from ingots having the coarser grain structures and less desirable ingot character? istics consequent upon the formation of ingots according to the prior practice. Specifically, the tensile strength and elongation of the worked metal are superior to those of the same metal made from ingots formed cordng to prior practice. Y

Because the invention is applicable to both magnesium and magnesium .base alloys, the term magnesium is used in the claims to collectively define both.

Accordin to the provisions of the patent statutes, we ave explained the principle and mode of operation of our invention, .and have given specific directions concerning the manner of practicing it. However, we desire to have it understood that, within the scope of the appended claims, the invention ,may be practiced otherwise than as specifically described, and with the use of apparatus other than that illustrated.

We claim as our invention:

1. The method of forming in a mold from a molten body of magnesium an ingot forV position throughout comprising solidi ing said body in and simultaneouslyfhrougliout a substantially horizontal plariue rising gradually and progressively from the bottom to the top ofthe body of metal, and maintaining in its molten condition the portion of the body of metal above itssaid rising plane of solidiication.

2. The method of forming in a mold from a molten body of magnesium an ingot for Working having-a fine equiaxed grain structure and of substantially uniform composition throughout, comprising solidifying said body in and simultaneously throughout a substantially horizontal plane rising gradually and progressively from the bottom to the top of the body of metal, and maintaining at a. temperature slightly above its freezing point the portion of the body of metal immediately a ove its said rising plane of solidilication.

3. The method of forming in a mold from a molten body of magnesium an ingot for working having a grain structure of elongated form extending parallel to the vertical axis of the ingot as formed and of substantially uniform composition throughout, comprising solidifying said body in and simultaneously throughout a substantially horizontal plane rising gradually and progressively from the bottom to the top of the body of metal7 and maintaining at a temperature materially higher than its freezing point the portion of the body of metal above its said rising plane of solidication.

In testimony whereof, we hereunto our names.

f THERON D. STAY.

WILLIAM HOLZHAUER.

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