CA1129624A

CA1129624A - Process of shaping a metal alloy product

Info

Publication number: CA1129624A
Application number: CA332,032A
Authority: CA
Inventors: Malachi P. Kenney
Original assignee: ITT Industries Inc
Current assignee: ITT Inc
Priority date: 1978-07-25
Filing date: 1979-07-18
Publication date: 1982-08-17
Also published as: IT1122314B; DE2929845C2; JPS585748B2; JPS5519498A; FR2433993B1; GB2026363B; FI792254A; DK311879A; NL7905471A; ES482797A1; BE877874A; GB2026363A; DE2929845A1; CH639300A5; FR2433993A1; IT7924620A0

Abstract

M. P. Kenney 1 PROCESS OF SHAPING A METAL
ALLOY PRODUCT
Abstract of the Disclosure:
A process for shaping a metal alloy in which a semi-solid metal alloy charge is shaped under pressure in a closed die cavity. The metal alloy contains discrete degenerate dendritic primary solid particles, in a concentration from about 30 to 90% by volume based upon the volume of the alloy, suspended homogeneously in a secondary liquid phase. The process is characterized by low pressures and very rapid shaping and solidification times. The process produces com-plex, close tolerance, high quality metal alloy parts.

Description

M. P. Kenney This invention relates to a process of forming a shaped metal alloy product and more particularly to a process for producing complex, close tolerance metal alloy parts by press forging.

Shaped metal alloy parts are produced from wrought alloys by forging techniques to obtain optimum physical pro-perties. Where the part has a relatively complex shape, it must normally be formed by utilizing casting alloys, usually at the sacrifice of physical properties. It would be desir-able to u~ilize alloys providing the characteristics of wrought products in a forming process capable of producing complex shapes.

There has recently been developed certain alloys having a microstructure such that they may be cast from a liquid-solid mixture rather than a liquid and thus solidi-ied from a lower temperature than conventional casting alloys.
Such alloys and their preparation are disclosed, for example, in U. S~ patent 3,948,650 which issued on April 6, 1976 and U. S. patent 3-,954,455 which issued on May 4, 1976. As there disclosed, the partially solidified metal alloys, in the form of slurries, can be shaped into alloy parts by a variety o~ metal forming processes, including die casting, permanent mold casting, closed die forging, hot pressing and other known techniques.

A primary object of the present invention is to provide a procsss for producing metal alloy parts having the complex shapes normally characteristic of cast alloys with properties approximating those of parts produced from wrought alloys.

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1~2~3~i2'-1 An additional object of this invention is to produce complex, close tolerance metal alloy parts by a low pressure press forging process having the economics of casting techniques.
It is still an additional object of this invention to provide a process for producing such metal alloy parts a~ high production rates.
The foregoing and other objects of the invention are achieved by press forging a semi-solid metal alloy cllarge in a closed die cavity while controlling certain critical process variables. The invention involves rela-tively low pressures, extremely short forming times and, in its preferred form, very high solids fraction alloy compositions. Specifically, a semi-solid alloy charge is shaped in a die cavity under pressure for a time of less than about one second, the die cavity having been preheated to a temperature of from about 100 to 450C. The liquid phase of the shaped alloy is then solidified in the die cavity at a pressure of from about 500 to, preferably, 5000 psig in a time of less than one minute.
Thus, the invention provides a process for shaping a metal alloy in which a semi-solid metal alloy charge is shaped in a closed die cavity, said metal alloy containing discrete degenerate dendritic primary solid particles in a concentration from 70 to 90% by volume based upon the volume of said alloy, said primary solid particles being derived from the alloy and being suspended homogeneously in a secondary liquid phase, said secondary liquid phase being derived from the alloy and having a lower melting point than said primary solid particles, said process comprising shaping said metal alloy charge under pressure in said die cavity in a time of less than about one second, said die cavity having been preheated to a temperature of from about 100 to 450C., and solidifying the liquid phase of said shaped alloy in the die cavity at a pressure in excess of about 500 psig in a time of less than one minute.
The invention will be better understood from the following descrip-tion taken in connection with the accompanying drawing in which Figure 1 is a vertical cross-sectional view of dies in closed position in a press suitable for use in the invention;

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~1296~

Figure 2 is an elevational view of an automobile wheel produced in the press of Figure l; and Figure 3 is a plan view of the wheel shown in Figure 2.

- 3a -1~29624 ~. P. Renney The metal charge or preform used in the process of the invention is semi-solid -- a part liquid and part solid mixture. The solid particles, between 30% and 90% of the total volllme, are rounded in shape and are normally between S about ~0 and 200 microns in diameter. This is the result of a prior treatment of the metal in which the metal is melted and then during freezing, is vigorously stirredO This breaks up the grain formation into the generall~y rounded particles.
The resulting metal composition is characterized by discrete degenerate dendritic primary solid particles suspended homo-geneously in a secondary phase having a lower melting point than the primary particles. Both the primary and secondary phases are derived from the metal alloy which has been vigor-ously agitated during freezing. The pxocess and the result-ing alloy are more fully disclo~ed in the aforesaid U.S.patents 3,948,650 and 3,954,455, reference to which should be made for a more complete description thereof.

~ he generally rounded nature of the discrete de-generate dendritic particles permits the solid particles to flow in a viscous fashion in a continuous liquid matrix.
This permits the relatively low pressure forming of the part. The pressures used in the process range from about 25 to 5000 psig which permits the forming of parts às large as a full sized ~14") automobile wheel to ~e formed in a 250 ton press as compared to a 1200 ton die casting machine or an 8000 ton press used for conventional forging.

~ he largely solid nature of the charge, which ranges from 30 to 90~, but preferably over 70~, by volume solids, permits very rapid solidification with a minimum of liquid/

~9~2~

, M. P. Kenney solid shrinkage. This, in turn, permits forming parts wi~hout large "feeding reservoirs" or risers and allows very short re-sidence in the dies. The latter point is vital to the high production rates attainable with this process, e.g. a realis-tic rate of 240 automobile wheels an hour or 500 small parts an hour may readily be sustained.

The rapid sdlidification means that nearly all sec-tions of the part, of equal section thickness, will solidify at the same time and thus may be e~ected very rapidly, and usually in less than 4 seconds after forming for high con-ductivity alloys such as aluminum and copper. For ferrous alloys or for parts of relatively large crossection, solidi-fication time may extend to 15 to 20 seconds, but in any event, will always be less than a minute and usually sub-stantially less. The rapid ejection releases the part from many of the constraints of the solid state contraction which normally occurs with decreasing temperature. Such contraction can progress to the point at which binding on the dies causes high stresses and resulting hot tears or cracks in the shaped paxt.

Products produced in accordance with the invention possess many of the properties of a forging, but may contain the comple~ shapes and shape tolerances typical of a casting.
The products may be produced using nominally wrought compos-ition alloys having the levels of tensile strength, fatigue strength, ductility and corrosion resistance comparable to forged or wrought products produced from these alloys. ~lore-over, the process is capable of producing relatively large -- 5 ~

~ ~2g62~

M. P. I~enney parts. Automobile wheels, for example, have been prepared having many of the characteristics of forged wheels, utiliz-ing considerably simplified pressing equipment in a consider-ably more efficient manner than conventionally forged wheels.

In the process of the invention, a preform is heated until 10-70~ of its volume becomes liquid. As indicated above, the preform or charge has previously been produced by vigorous agitation of a liquid-solid mixture of the selected alloy which was then rapidly cooled. The temperature to which the preform is heated is between the liquidus and solidus temper-ature for the particular alloy and will vary from heat to heat within a given alloy system depending on the particular chemistry.
Since there is no specific temperature at which the metal will form properly, the viscosity as measured by the resistance to pene~ration of a probe into the semi-solid, may ~e used as an indicator of the % liquid present in the mixture. Generally the range of 5 psig to 15 psig will be used, the exact pressure ~eing selected to suit the conditions of the part to be formed.
It is possible to avoid cooling and reheating of the preform by using as the charge the vigorously agitated slurry directly - i.e. before it is cooled to form a billet or preform.

Low pressures may be used to shape the preheated billet providing no significant additional solidification occurs during the shaping step. Thus, in order to insure the use of low pressures, a shaping time in the die cavity of less than one second is required. The die cavity i5 preiheated to a temperature of from lO0 to ~50C., depending primarily upon part configuration, in order to prevent significant solidi-fication during the forming or shaping step. If temperatures are too high, there is a tendency for adhesion of ~129~2-", M. P. Kenney the preform to the die, known as die soldering, to occur.
During the forming stroke, the pressure raises from zero to the pressure used for solidification. By the end of the forming stroke, the pressure has accordingly risen from about 25 to 5000 psig, usually 500 to 2500 psig, and solidi-fication of the liquid phase begins. Thus, the pressure gradually rises during the shaping stroke and remains at a peak of from ~5 to 5000 psig during solidification. The applied pressure enhances heat transfsr from the metal alloy to the die and feeds solidification shrinkage. If the pressure is too low, porosity may be at an ~macceptable level or complex molds may fill incompletely. Pressures above 5000 psig may be used, but they are not necessary. Moreover, higher pressures may create a venting problem. It is desirable to form the part at as low a pressure as possible for reasons of process economy, simplicity of pressing equipment and for die life. Residence time in the die cavity, subsequent to the shaping step, should be short enoush, under one minute and pre-ferably less than 4 seconds, to avoid hot cracking of the shaped part from thermal contraction stresses but long enough to complete solidification of the liquid phase of the alloy.
Specific times will depend on part thickness. The tendency for hot cracking to occur is a function of alloy composition, fraction solids percent, die temperature and part configuration.
Within the ranges of forming and solidification times herein set forth, times should, of course, be kept as short as possible to maximize part-making productivity. As is apparent from the foregoing discusslon, times, pressures, temperatures and allov solid fraction are a combination of critical variables which together function to achieve the significant process economies and product improvements herein set forth.

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- M. P. Renney The shaping process of the invention may be carried out, for example in a 150-250 ton hydraulic ~ress equipped with dies or molds of the type illustrated in FIG. 1 of the drawing. The specific die set there shown is contoured to S produce a relatively large complex shape, in this case a highly styled automobile wheel. The die set comprises a movable top die or ram 1, two side dies 2 and 3 and bottom die 4. The dies are shown in closed position, the alloy metal 5 having been shaped into the contour of an automobile wheel.

Another feature of the invention involves the manner in which the dies are vented. The length and diameter of venting channels must be of adequate size to provide ample venting. On the other hand, tha channels must normally be sufficiently narrow and long to avoid spraying the molten metal to the exterior of the die~s. Venting channels of con-ventional size, of a diameter used for example in die casting, have proven too narrow to eliminate air pockets in the present press forming process. It has been found, however, that the high solids fraction present during the pressing cycle of the present invention permits wider and shorter ventiny channels to be used. The result is not only the absence of air pockets in the shaped product, but fewer limitations on die design, the latter because less area is needed to achieve adequate venting. Four such vents, 6, 7, 8 and 9, are shown in cross-section in FIG. 1. It will be seen from FIG. 1 that the shapingoperation actually involves a concurrent forward extrusion of semi-solid metal into the narrow channels opening into vents 6 and 7, a backward extrusion of semi-solid metal into the channels leading to vents ~ and 9 and a forging stroke against the central portion of the metal in the press. Reference 1~2962~
M. P. Kenney herein to "complex" shapes is intended to identify parts which require such concurrent forward and backward extrusion com-bined with a forging step in the process herein set forth.

The following example is illustrative of the practice of the invention. Unless otherwise indicated, all parts are by weight.

Example An 18 pound billet of 6061 wrought aluminum alloy was cast, substantially as set forth in U.S. patent 3,948,650, from a serni-solid slurry containing approxi~ately 50% by vol-ume degenerate dendrites. The billet, approximately six inches in diameter, had the following composition:

Si Cr Mn Fe Mg Ti Cu B Al 0.63 0.06 0.06 0.22 0.90 0.012 0.24 0.002 Balance lS The billet, contained in a stainless steel canister, was placed within a resistance furnace set at a temperature of 677C. This 1temperature, approximately 28C. above the liquidus temperature of the alloy, was sufficient to induce partial melting of the alloy without creating significant variations in fraction liquid within the billet. At a tem-perature of 632C., corresponding to a ~raction solid of approximately 0.80, as detected by the penetration of a weighted probe, the billet in its canister was transferred to the closed bottom half of a cast iron die set, of the type shown in FIG. 1, maintained at 315C. and ejected from the canister to the bottom of the die. The die set was coated with a graphite base lubricant. The top die, also maintained ~lf~.9~2~
M. P. Kenney 1 with a surface temperature of approximately 315C., was then closed at a speed of 20 inches per second, resulting in a preform shaping time of about 0.2 seconds, the die reaching a maximum pressure of 210Q psig such that the cavity so formed was filled with alloy. After a holding tlme under pressure of

2.4 seconds, during which the liquid phase of the part solidi-fied, the die set was opened and the shaped part extracted.

The shaped part, an aluminum wheel, was sectioned and specimens for mechanical property measurement were taken.
Room temperature properties were measured. Ultimate tensile streng~h was 47,000 psi, yield strength was 43,000 psi and elongation in a 1" gauge length was 7~. Minimum specifications for closed die forgings of 6061 aluminum alloys as set forth in ~luminum Standards and Data 197~, Fifth Edition, 1976 are 38,000 psi ultimate tensile strength, 35,000 psi yield strength and 7~ elongation. Representative minimum specifications of an automobile manufacturer for cast aluminum wheels are 31,000 ultimate tensile strength, 16,500 yield strength and 7~ elonga-tion.

Unlike wrought products whose properties are direct-ional, the products of the invention are isotropic - ~heir properties are equal in all directions. The metallurgical structure of the wheel of the example consisted of randomly oriented, equiaxed grain structure without the "texture"
associated with wrought components having similar properties~

A finished wheel generally identified by the numeral 10 produced in accordance with the invention is shown in ele-vation in FIGS. 2 and 3. The plan view of FIG. 3 shows the ~129~4 M. P. Kenney wheel as viewed from the direction of ~le bottom die in FIG. 1.
The wheel contains a plurality of roughly rectangular contours 11 around the periphery, each of the contours containing a punched or machined hole 12 therethrough. A hub area 13 con-tains four cored and tapped holes 14 and four larger punchedor machined holes 15. A wheel configuration of this complexity is normally produced by permanent mold or die casting techniques and is accordingly limited in its properties to the relatively inferior properties of cast alloys. Material properties are thus a limiting factor on wheel weight. Lower properties must be compensated by greater bulk in a cast wheel. Moreover, larger crossections are normally necessary in casting because of limitations inherent in casting techniques - it is difficult to fill a permanent mold with thin sections. Thus, the wheels of the invention have the very important capa~ility of being lighter in weight than comparable wheels of the prior art.

Representative alloys useful in the press forging ; process are, in addition to aluminum alloys, ferrous alloys such as the stainless steels, tool steels, low alloy steels and irons and copper alloys of the type normally used in castings and forgings.

It will be recognized that, within the scope of the process parameters set forth herein, many variations may be made in order to accommodate the geometry or the specific property objectives of the component being formed. Changes in alloy chemistry, temperature, speed and pressure of the press and duration of dwell may influence grain structure, avoid shrin~age defects and provide properties to specific portions of the component. ~oreover, the process may be ~12962~

M. P. Kenney used for producing a variety of shaped metal parts other than wheels including, for example, hand tools, valve and pump bodies and parts, propellers and impellers, automotive and appliance parts and electrical and marine components.
It is intended in the claims which follow to cover all variations which fall within the scope of the invention.

HJH:tlm

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for shaping a metal alloy in which a semi-solid metal alloy charge is shaped in a closed die cavity, said metal alloy containing discrete degenerate dendritic primary solid particles in a concentration from 70 to 90% by volume based upon the volume of said alloy, said primary solid particles being derived from the alloy and being suspended homogeneously in a secondary liquid phase, said secondary liquid phase being derived from the alloy and having a lower melting point than said primary solid particles, said process comprising shaping said metal alloy charge under pressure in said die cavity in a time of less than about one second, said die cavity having been preheated to a temperature of from about 100 to 450°C., and solidifying the liquid phase of said shaped alloy in the die cavity at a pressure in excess of about 500 psig in a time of less than one minute.

2. The process of claim 1 in which the said metal alloy is solidified at a pressure not greater than about 5000 psig.

3. The process of claim 1 in which said metal alloy is solidified at a pressure of from 500 to 2500 psig.

4. The process of claim 1 in which the die cavity is maintained at a temperature of from 200 to 300°C.

5. The process of claim 1 in which the metal alloy is shaped under pressure in the die cavity in a time of from 0.1 to 0.5 seconds.

6. The process of claim 1 in which the solidification of the liquid phase of the shaped alloy under pressure in the die cavity occurs in a time of less than 4 seconds.

7. The process of claim 1 in which the alloy is an aluminum alloy.

8. The process of claim 1 in which the alloy is a copper alloy.

9. The process of claim 1 in which the alloy is a ferrous alloy.

10. The process of claim 1 in which said shaping process produces to close tolerances a metal alloy of complex configuration.

11. The process of claim 1 in which said die cavity is vented to the atmosphere through a pluarlity of spaced channels extending from the die cavity to the atmosphere, said channels being of a size sufficient to exhaust any air entrapped in the die cavity during the pressing stage.