US3568752A

US3568752A - Method for controlling the as-cast grain structure of solidified materials

Info

Publication number: US3568752A
Application number: US781375A
Authority: US
Inventors: Douglas C Williams
Original assignee: Ohio State University
Current assignee: Ohio State University
Priority date: 1968-12-05
Filing date: 1968-12-05
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09

Abstract

A process whereby the as-cast grain structure of metals and alloys is controlled by the application of duortical motion to a liquid metal or alloy as the liquid metal or alloy passes through the liquid-solid transition as the liquid cools.

Description

United States Patent [72] Inventor Douglas C. Williams Columbus, Ohio [21] Appl. No. 781,375 [22] Filed Dec. 5, 1968 [45] Patented Mar. 9, 1971 [73] Assignee The Ohio State University Columbus, Ohio [54] METHOD FOR CONTROLLING THE AS-CAST GRAIN STRUCTURE OF SOLIDIFIED MATERIALS 12 Claims, 7 Drawing Figs.

[52] US. Cl 164/47, 164/1 14 [51] Int. Cl B22d 13/06 [50] Field of Search 164/122, 114, 115, 1 16, 286, 289,47 (U.S.); 75/61 [5 6] References Cited UNITED STATES PATENTS 1,040,517 10/1912 Crawford 164/115 3,251,681 5/1966 Wakamatsu et a1. 75/61 Primary ExaminerJ. Spencer Overholser Assistant Examiner-R. Spencer Annear AttorneyAnthony D. Cennamo ABSTRACT: A process whereby the as-cast grain structure of metals and alloys is controlled by the application of duortical motion to a liquid metal or alloy as the liquid metal or alloy passes through the liquid-solid transition as the liquid cools.

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SHEET 1 OF 3 FIG. 3

INVENTOR. DOUGLAS C. WILLIAMS ATTORNEY PATENTED IIAR 9:911

SHEET 2 [IF 3 FIG. 4

"-III 1 IIIIIIIIIIIIIIIII IN VENTOR. DOUGLAS C. WILLIAMS ATTORNEY PATENTEDHAR 9|97| 3568752 sum 3 or a HEIGHT OF MOLTEN MATERIAL ECCENTRIC REVOLUTION NUMBER/MINUTE FIG. 6

IN VIL'NTOR. DOUGLAS C. WILLIAMS ATTORNEY METlimlil FUR @QNTRQLLTNG THE AS-CAST GllilAlN STRUQTURE 01F SOLTDHFEIED MATERHAEE BACKGROUND it is well known that the grain structure developed during the solidification of a metal or alloy generally determines the degree of chemical inhomogeneity, the strength, and the anisotropy of the metal or alloy in the as-cast state. The characteristics of the as-cast metal or alloy greatly influence the workability of the metals and alloys. Generally, where an as-cast structure is to be mechanically deformed, it is desirable to have a uniform grain structure within the as-cast geometry. Where the as-cast grain structure is uniform the as-cast geometry possesses isotropic mechanical properties thereby enhancing the predictability of the effects of any subsequent deforming process which is performed on the as-cast geometry.

The as-cast properties of a metal or alloy casting can be varied greatly by varying the conditions under which the metal or alloy is permitted to solidify. Methods by which metals and alloys are cast and solidified include static casting and centrifugal casting and casting involving agitation of the metal wherein the agitation is applied by some mechanical means.

The grain structure manifested by a static casting will, of course, reflect the characteristics inherent to the given alloy being cast. In addition, the as-cast structure can be effectively varied by controlling pouring temperature, mold temperature, the size of the casting, and other relevant considerations. However, speaking generally, a static casting will exhibit three distinct regions of grain structure: the chill region; the columnar region; and the usually. central equiaxed region. Immediately after liquid alloy is poured into the mold the liquid contacting the mold wall is cooled to below the liquidus temperature and solid particles are nucleated homogeneously and heterogen eously. Under normal conditions the amount of supercooling is appreciable and the nucleation occurs rapidly. The large number of nuclei thus formed produce the aforementioned layer of chili crystals which have random orientations. The number and size of the chill crystals vary with rate at which the alloy in the chill' region is cooled. Formation of nuclei ceases when there is more or less continuous solid layer adjacent to the mold wall. This is due to the fact that the mold wall and chill region reach a temperature approaching the freezing temperature thereby precluding further supercooling and nuclei formation. Further solidification then proceeds by columnar growth of crystals favorably oriented in the chill region inward toward the thermal center of the casting. The columnar crystals consist of those chill crystals which are most favorably oriented for rapid growth. Thus the columnar region shows preferred orientation such that the most rapid growth direction coincides with the direction of maximum temperature gradient, normal to the mold walls. Columnar growth continues until the liquid alloy in the volume between the columnar region becomes cooled to a temperature conducive to nucleation. When the temperature favoring nucleation is reached, virtually instantaneous nucleation occurs in the volume of molten metal adjacent to the columnar region. This produces an equiaxed grain structure in that region located about the thermal center of the casting; this is the central equiaxed region.

Observe, however, in a pure metal the columnar region extends virtually all the way to the thermal center of the casting. This is due to the fact that the central region does not become supercooled whereupon nucleation of the nuclei necessary to produce the central equiaxed grain structure occurs.

The prior art discloses that agitation of molten metals and alloys has been performed primarily by three different means: mechanical mixing as with a stirring rod; vibration means utilizing sonic and ultrasonic vibratory energy; and rotational means both with and without eccentricity. Other means of course are available such as stirring by magnetic means and other means but these means have not thus far played a significant role in heavy industrial applications. The aforementioned considerably. The mixing action causes the temperature of the molten alloy to become essentially uniform. This makes constitutional supercooling unlikely; that is, the temperature of no portion of the molten alloy is going to fall below the liquidus temperature. Vigorous mixing has been found to produce a fine grain structure, some degassing effect, and greater fluidity of molten metals and alloys. The mechanical mixing will produce the grain refinement only when the mixing action occurs when the molten metal or alloy is solidifying. The degassing effect and increased fluidity obtain even when the mechanical mixing is performed on the molten metal or alloy when it is superheated.

Another form of mechanical mixing is that which is a result of duortical mixing. Duortical is a compound word of Duo and Vortical whereby the compound word is intended to reflect the fact that duortical mixing is a result of the combination of the motion required to establish two vortices spinning in opposite directions. Duortical mixing contemplates an eccentric revolution of a vessel. The method is characterized by the high revolution number of a vessel and a changeable revolution direction. Successive vortex motions caused by the alternating right and left hand rotations of the vessel stir moltenmetal or alloy in all directions. Therefore, an excellent stirring effect is brought on the molten metal or alloy in the vessel. The prior art discloses that mechanical agitation through the use of duortical mixing has been applied to desulfurize pig iron and steel and to initiate graphite spheroidization in the production of nodular iron. in the processes for desulfurizing pig iron or steel and for graphite spheroidization, the normal additives are introduced into the molten alloy. The excellent mixing action of duortical mixing increases the speed with which the chemical reactions within the molten alloy proceed. This is due to the fact that the chemical reactions proceed, in general, by collision of particles such as atoms, molecules, or ions; the duortical mixing greatly increases the collision rate of these particles thereby increasing the reaction rate.

SUMMARY In the present invention duortical mixing is applied to a system of molten metal or alloy as the molten metal or alloy cools and passes through the liquid-solid phase change. The invention contemplates securing a container of a refractory nature suitable for retaining a molten metal or alloy to a table. The table to which the container is secured is connected to a driving force in such a manner that the turntable may be set in revolution at various predetermined speeds in either a clockwise or counterclockwise direction. A braking force is available to stop the table from rotating once it has been set in motion.

In the process of the present invention duortical mixing is achieved by revolving the turntable to which the refractory container holding molten metal or alloy is secured in a position eccentric to the center of rotation of the turntable. This revolution establishes a vorticallike movement of the molten metal or alloy within the refractory container as it rotates eccentrically. When the vorticallike movement of the molten metal or alloy is established the turntable is braked to a halt; eccentric revolution of the refractory container in the opposite direction is then initiated. The change in the direction of revolution creates a strong movement of the molten metal or alloy thereby effectively mixing the molten metal or alloy. The direction of eccentric rotation is alternately changed throughout the period during which the molten metal or alloy is solidifying or passing from the liquid to solid phase; this is duortical mixing. The application of duortical mixing to a mass of solidifying metal or alloy produces an as-cast solid object whose grain structure is completely equiaxed. The application of duortical mixing during the solidification completely eliminates the development of columnar growth.

Intermittent application of duortical mixing during the solidification produces a laminar as-cast structure consisting of alternate bands of equiaxed and columnar grains. To achieve this laminar structure, the duortical mixing is stopped for a predetermined period of time to initiate columnar growth and allow it to progress. After the predetermined time duortical mixing is resumed in order to prevent further columnar grain growth and to create a band or layer of equiaxed grains. The aforedescribed intermittent duortical mixing technique produces alternate bands of equiaxed grains and columnar grains until the casting is solidified to the thermal center.

OBJECTS The principal object of the present invention is to provide a method of casting whereby the as-cast grain structure and grain size of an alloy casting can be effectively regulated.

Another object of the present invention is to provide a method of casting whereby the as-cast grain structure of an alloy casting is entirely equiaxed.

Another object of the present invention is to provide a method of casting whereby the mechanical properties of the as-cast grain structure are isotropic in nature.

Another object of the present invention is to provide a method of casting whereby the necessity for heat treating prior to mechanical deformation processes is eliminated.

Another object of the present invention is to provide a method of casting whereby the cost of producing certain objects is decreased by virtue of the elimination of the step of heat treating.

Another object of the present invention is to provide a method of casting whereby the grain structure of an alloy casting is of a laminar nature where the various layers are alternately equiaxed and columnar.

A further object of the present invention is to provide a method of casting whereby an alloy casting having unique impact and other mechanical properties can be produced.

Still another object of the present invention is to provide a method of casting whereby the as-cast grain structure of an alloy casting can be used for decorative applications.

Other objects and features of the present invention will become apparent from reading the following detailed description when considered in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates a typical as-cast grain structure of an essentially cylindrical alloy ingot which has been statically cast and sectioned perpendicular to the longitudinal axis;

FIG. 2 schematically illustrates the as-cast grain structure of an essentially cylindrical alloy ingot which has been cast under conditions of continuous duortical mixing and sectioned perpendicular to the longitudinal axis;

FIG. 3 schematically illustrates the as-cast grain structure of an essentially cylindrical alloy ingot which has been cast under conditions of intermittent duortical mixing and sectioned perpendicular to the longitudinal axis;

FIG. 4 schematically illustrates a section through a plane of symmetry of the refractory container or mold used to contain the alloy to be case;

FIG. 5 schematically illustrates the apparatus necessary to create conditions of duortical mixing;

FIG. 6 illustrates the qualitative relation between the height of the molten alloy in an eccentrically revolving molding vessel and the eccentric revolution number; and,

FIG. '7 schematically illustrates the surface configuration of the Break Phenomenon in a molten alloy in an eccentrically rotating molding vessel upon reversal of rotation direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1, 2, and 3 illustrate the grain structures across a section perpendicular to the longitudinal axis of essentially identical alloy castings except that the conditions under which the alloys solidified were different for each of the aforementioned FIGS. FIG. 1 illustrates the typical grain structure perpendicular to the longitudinal axis of an essentially cylindrical ingot where the ingot is of an alloy composition which has solidified under static conditions. The periphery of the casting in FIG. 1 exhibits a grain structure which is equiaxed 1 in nature. This segment of the casting is known as the chill" zone. Colum nar grains 2 grow inward toward the thermal center of the 5 casting. The columnar grains 2 grew from those equiaxed grains 1 located in the chill zone which were located at the interface of the molten alloy and the solidified equiaxed grains in the chill zone. Further, the columnar grains are those grains located at the aforesaid interface whose longitudinal axes were oriented substantially perpendicular to the mold wall or a tangent thereto. Growth of the columnar grains 2 in a statically cast casting continues until the temperature of the molten alloy located about the thermal center drops below the liquidus temperature. Below the liquidus temperature nucleation occurs very rapidly giving rise to the centrally located equiaxed structure 3 which is analogous but often coarser to that equiaxed structure located in the chill zone 1.

FIG. 2 illustrates the grain structure obtainable when the duortical mixing casting process of the present invention is used to control the as-cast grain structure of the casting as the casting solidifies. The geometry of the casting illustrated in FIG. 1 is the same as that inFIG. 2; however, the as-cast grain structure of FIG. 2 is completely equiaxed 4 throughout the cross section. The equiaxed grain structure gives the object cast with the process of the present invention isotropic mechanical properties.

FIG. 3 illustrates a novel grain structure obtainable only with a variation of the process of the present invention. The as-cast grain structure is a laminar structure consisting of alter'nate layers of equiaxed grains 5, 7, 9, l1, and

columnar grains

6, 8, 10. The laminar structure is obtainable by intermittently applying the duortical mixing casting process as described hereinafter.

Referring generally to FIG. 5, the apparatus which may be used to effectively carry out the process of the present invention is illustrated. Shaft 12 is connected to a braking arrangement and power source which is capable of rotating the shaft 12 clockwise or counterclockwise. Shaft 12 is connected to a gear box 13 which transmits power to the turntable 14 thereby enabling the turntable to be driven at a predetermined number of revolutions per minute in the clockwise and counterclockwise directions. A molding vessel 15 is secured to the turntable 14. The molding vessel 15 can be clamped at a position away from the center of rotation of the turntable thereby enabling the molding vessel 15 to be rotated eccentrically.

FIG. 4 illustrates a typical molding vessel 15 in which the process of the present invention can be carried out. The molding vessel 15 is a steel vessel 16 which is lined with a refractory lining 17 such as molding sand or core sand. The bottom plate 18 of the molding vessel 15 is designed to facilitate clamping the molding vessel 15 to the turntable 14 which is illustrated in FIG. 5 and described hereinabove. Of course variations of the molding vessel materials can be used as required to establish desirable heat transfer rates as may be required for different alloys.

FIG. 6 shows the relation between the height of molten alloy in the cylindrical molding vessel 15 and its revolution number (revolutions per minute) when the molding vessel containing molten alloy is eccentrically rotated without turning the molding vessel 15 around its own axis. The height of the molten alloy in the molding vessel 15 grows higher with an increase of revolution number. The height of the molten alloy reaches a maximum point, decreases to a minimum point and then increases again after passing through the minimum point. The height of molten alloy plotted on the ordinate of the graph in n is graphically illustrated inFlG. 7 by distance All, the height the molten alloy above the height of the unagitated surface, H. The unique wave like the breaking of a wave crest, the so-ca led Break Phenomenon, appears at a revolution number a little higher than that of the maximum point.

The process of the present invention is used to create desirable as-cast grain structures in castings. In operation the process of the present invention contemplates the use of equipment illustrated in FIG. 5. As molten alloy is poured into the molding vessel the turntable M is caused to rotate by the driving force connected to shaft 12 and transmitted to the turntable lid through the gearbox 113. The molding vessel 15 is secured to the turntable 14 so as to cause the molding vessel 15 to rotate eccentrically. This causes the surface of the molten alloy to assume an essentially parabolic wave configuration like that described hereinabove in conjunction with FIGS. 6 and 7. l

Duortical mixing is initiated by alternately changing the direction of revolution when the revolution number is higher than the point where the Break Phenomenon appears or where the surface of the wave in the molten alloy is at a peak and the curved surface is analogous to a revolving parabolic surface. Circinate movement of the molten alloy and a large wave of molten alloy is created at the time'of the change in the direction of revolution thereby creating a strong movement of molten alloy and effectively mixing the alloy. The molding vessel 15 is alternately rotated eccentrically clockwise and counterclockwise throughout the solidification of the molten alloy.

lnitially,'as the molten alloy contacts. the molding vessel 15 the alloy adjacent to the mold wall 17 solidifies. This region adjacent to the mold wall 17 which solidifies is called the chill region. Fine equiaxed grains 1 are characteristic of the as-cast grain structure in the chill region of a casting as illustrated in Fifi. l. Under conditions of static casting or centrifugal casting a columnar grain structure forms from those grains in the chill region at the solid-liquid interface which are most favorably oriented, i.e., those grains whose longitudinal axis is perpendicular to the mold wall 17. In those alloys in which columnar structure is not common, relatively large grains typition. The angular velocity at which the molding vessel 15 must be eccentrically rotated to achieve a uniform intensity of duortical mixing is greater the smaller the diameter of the molding vessel 15. Hence, as the molten alloy solidifies, the effective diameter of the molding vessel l5 becomes smaller and the revolution number must be increased in order to agitate the remaining molten alloy with uniform vigor so as to obtain a equiaxed grain structure of substantially uniform as-cast grain size. However, even where a constant revolution number is used, an equiaxed structure results; even though the grain size is not completely uniform the desirable isotropic mechanical properties are still present in the object in the as-cast condition.

The process-of the present invention further contemplates the development of a laminar as-cast grain structure. The laminar grain structure consists of alternate layers of

equiaxed

5, 7, 8, ii, and columnar d, d, W grains as illustrated in FIG. 3. To achieve this novel structure, duortical mixing is intermittently applied to the system of solidifying molten alloy. The thickness of the respective layers of columnar and equiaxed grains can be controlled by correlating the solidification rate and the periods during which duortical mixing is applied to the system. More specifically, where a thick outer layer of equiaxed grains '5 is desired the duortical mixing is initiated at the time the molten alloy is poured into the molten vessel 15'. Duortical mixing is sustained for the period necessary to produce the layer of equiaxed grains 5 of the desired thickness. The duration of time that the duortical mixing must be applied to the system to achieve is the equiaxed layer of the desired thickness is predetermined. The duration of time is calculated by relating the cooling rate of the mold, properties of the given molten alloy, mass of the given 1 molten alloy and other relevant parameters.

To create a layer of columnar grains, the duortical mixing is stopped. The movement of the molten alloy is permitted to cease for a calculated predetermined time based on the solidification rate of the alloy. This promotes the conditions conducive to the formation of a columnar structure 6 by permitting the growth of those equiaxed grains at the solid-liquid interface which are preferentially oriented. The duortical mixing is resumed when the desired amount of grain growth has occurred as indicated by the passing of the predetermined time. The resumption of the duortical mixing once again creates conditions conducive to the creating of an equiaxed grain structure 7. The continued intermittent application of duortical mixing to the molten alloy system will produce addi tional alternate layers of equiaxed 5, 7, 9, 11 and

columnar

6, 8, l0 grain structure.

The process of this invention has been described hereinabove in terms of metal alloys. However, it is to be understood that the process of the present invention can be applied where the material is a pure metal or other material. The only requisite of the material is that the material exhibit different grain structures when it is under static conditions and conditions of duortical mixing. Hence, although certain and specific processes have been described, it is to be understood that modifications may be made thereto without departing from the true scope and spirit of the invention.

l. A method for creating a substantially equiaxed as-cast grain structure in a cast object by duortical mixing applied to a solidifying molten material comprising the steps of pouring molten material into a molding vessel, eccentrically rotating said molding vessel, braking said molding vessel to a halt for a substantially instantaneous time, eccentrically rotating said molding vessel in the direction opposite the preceding direction of eccentric rotation, and repeating the alternate eccentric rotations until such time as all the molten material has solidified.

2. A method as set forth in claim 1 wherein said step of eccentrically rotating said molding vessel further comprises maintaining the eccentric rotation until the resulting wave created in said solidifying molten material reaches a peak and the surface of said solidifying molten material attains a substantially parabolic shape.

3. A method as set forth in claim 1 wherein said step of eccentrically rotating said molding vessel in the direction opposite the preceding direction of eccentric rotation further comprises maintaining said eccentric rotation until the resulting wave created in said solidifying molten material reaches a peak and the surface of said solidifying molten material attains a substantially parabolic shape.

4. A method as set forth in claim ll wherein said molten material is a metal alloy.

5. A method as set forth in claim 1 wherein said molten material is a substantially pure metal.

6. A method for creating a substantially laminar equiaxed and columnar as-cast grain structure in a cast object by intermittent duortical mixing applied to a solidifying molten material comprising the steps of pouring molten material into a molding vessel, eccentrically rotating said molding vessel, braking said molding vessel to a bait for a substantially instantaneous time, eccentrically rotating said molding vessel in the direction opposite the preceding direction of eccentric rotation, repeating the alternate eccentric rotations, braking said molding vessel to a complete halt thereby permitting columnar grain growth, and thereafter resuming said alternate eccentric rotations and halting of said molding vessel thereby continuing intermittent duortical mixing until such time as all solidifying molten material has solidified.

7. A method as set forth in claim 6 wherein said step of eccentrically rotating said molding vessel further comprises maintaining said eccentric rotation until the resulting wave created in said solidifying molten material reaches a peak and the surface of said solidifying molten material attains a substantially parabolic shape.

8. A method as set forth in claim 6 wherein said step of eccentrically rotating said molding vessel in the direction opposite the preceding direction of eccentric rotation further comprises maintaining said eccentric rotation until the resulting wave created in the solidifying molten material reaches its peak and the surface of said solidifying molten material attains a substantially parabolic shape.

9. A method as set forth in claim 6 wherein said step of repeating the alternate eccentric rotations further comprises maintaining said alternate eccentric rotations until the desired thickness of equiaxed grains has solidified in the casting.

10. A method as set forth in claim 6 wherein said step of braking said molding vessel to a complete halt further comprises maintaining said molding vessel motionless until the desired thickness of columnar grains has solidified in the castmg.

11. A method as set forth in claim 6 wherein said molten material is a metal alloy.

12. A method as set forth in claim 6 wherein said molten material is a substantially pure metal.

Claims

1. A method for creating a substantially equiaxed as-cast grain structure in a cast object by duortical mixing applied to a solidifying molten material comprisiNg the steps of pouring molten material into a molding vessel, eccentrically rotating said molding vessel, braking said molding vessel to a halt for a substantially instantaneous time, eccentrically rotating said molding vessel in the direction opposite the preceding direction of eccentric rotation, and repeating the alternate eccentric rotations until such time as all the molten material has solidified.

4. A method as set forth in claim 1 wherein said molten material is a metal alloy.

6. A method for creating a substantially laminar equiaxed and columnar as-cast grain structure in a cast object by intermittent duortical mixing applied to a solidifying molten material comprising the steps of pouring molten material into a molding vessel, eccentrically rotating said molding vessel, braking said molding vessel to a halt for a substantially instantaneous time, eccentrically rotating said molding vessel in the direction opposite the preceding direction of eccentric rotation, repeating the alternate eccentric rotations, braking said molding vessel to a complete halt thereby permitting columnar grain growth, and thereafter resuming said alternate eccentric rotations and halting of said molding vessel thereby continuing intermittent duortical mixing until such time as all solidifying molten material has solidified.

10. A method as set forth in claim 6 wherein said step of braking said molding vessel to a complete halt further comprises maintaining said molding vessel motionless until the desired thickness of columnar grains has solidified in the casting.