CA1136679A - Automotive wheel - Google Patents

Abstract

M. P. Kenney 2 AUTOMOTIVE WHEEL

Abstract of the Disclosure:

A metal alloy automotive wheel produced as an inte-gral product to close tolerances by shaping under pressure a semi-solid metal alloy charge in a closed die cavity. The metal alloy charge contains discrete degenerate dendritic primary solid particles suspended homogeneously in a secondary liquid phase having a lower melting point than said primary solid particles. The wheel possesses properties approaching those of wrought products with the relatively complex con-figuration typical of castings.

Description

1136~79 This invention relates to a metal alloy automotive wheel and particularly to a metal alloy automotive wheel of complex configuration produced as an integral product by a press forging process.

Automotive wheels and particularly those referred to as "styled" wheels, are relatively complex shapes and accordingly must normally be produced by permanent mold cast-ing or die casting techniques. Such wheels are therefore limited by the relatively lower properties available from the casting alloys used in such forming processes. The wheels may be forged from wrought alloys, but with serious limitations on their geometry or configuration. The wheels may also be fabricated from multiple forged pieces which are welded together, but only certain alloys can be welded and the fabrication technique also imposes serious styling limitations. It would, therefore, ~e desirable to produce wheels of complex shape having the properties of wrought alloys.

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-fied 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. ~s there disclo~éd, the partially solidified metal alloys, in the form of slurries, can be shaped into alloy parts by a variety of metal forming processes, including die casting, permanent mold casting, closed die forging, hot pressing and other known techniques. ~ ~-11366~'9 It is a primary object of this invention to provide an automotive wheel having the complex shape normally characteristic of cast wheels with properties approxi-mating those of wheels produced from wrought alloys.

S It is an additional object of this invention to provide complex, shaped automotive wheels produced as an integral product to close tolerances by a low pres-sure press forging process.
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It is a more specific object of this invention to provide a highly styled aluminum automotive wheel com-bining excellent properties with relatively complex configuration.

The invention comprises an aluminum alloy automotive wheel of complex configuration produced as an integral product to close tolerances by shaping under pressure a semi-solid aluminum alloy charge in a closed die cavity, said aluminum alloy charge containing discrete degenerate dendritic primary solid particles suspended homogeneously in a secondary liquid phase having a lower melting point 20- than said primary solid particles, the properties of said automotive wheel being isotropic and the tensile pro-perties at least meeting the minimum specifications for forged aluminum alloys. Aluminum alloy wheels of the invention combine the complex configuration normally associated with a cast product with tensile properties which meet or surpass minimum specifications for forged aluminum alloys. The process of producing the wheels of this invention is the subject of my copending Canadian application No. 332,032, filed concurrently herewith.
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The invention will be better understood from the following description taken in connection with the accom-panying drawing in which FIG. 1 is a vertical crossectional view of dies in closed position in a press suitable for use in the invention;

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

The metal charge or preform used in preparing the wheels of the invention is semi-solid -- a part liquid and part solid mixture. The solid particles, between 30% and 90~ of the total volume, are rounded in shape and are nor-mally between about 20 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 stirred. This breaks up the grain formation into the generally rounded particles. The resulting metal composition is char-acterized by discrete degenerate dendritic primary solid particles suspended homogeneously 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 vigorously agitated during freezing. The pro-cess and the resulting alloy are more fully disclosed 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.

The generally rounded nature of the discrete de-generate dendritic particles permits the solid particles to il366~

flow in a viscous fashion in a continuous liquid matrix. This permits the relatively low pressure forming of the wheel. The pressures used in the process range from about 25 to 5000 psig which permits the forming of parts as large as a full sized (14") automobile wheel to be formed in a 250 ton press as compared to a 1200 ton die casting machine or an 8000 ton press used for conventional forging.

The 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/
solid shrinkage. This, in turn, permits forming wheels with-out large "feed reservoirs" or risers and allows very short residence 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.

The rapid solidification means that nearly all sec-tions of the wheel, of equal section thickness, will solidify at the same time and thus may be ejected very rapidly, and usually in less than 4 seconds after forming for high con-ductivity alloys such as aluminum. For ferrous alloys or for wheels of larger crossection, solidification time may extend to 15 to 20 seconds, but in any event, will always be less than a minute and usually substantially less. The rapid ejection releases the part from many of the constraints of the solid state contraction which normally occurs with de-creasing 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 ~heel.

1~36679 Wheels produced in accordance with the invention possess many of the properties of a forging, but contain the complex shapes and shape tolerances typical of a casting.
The wheels may be produced using nominally wrought aluminum or ferrous alloys having the levels of tensile strength, fatigue strength, ductility and corrosion resistance comparable to forged or wrought products produced from these alloys.
Automobile wheels have been prepared having many of the characteristics of forged wheels, utilizing considerably simplified pressing equipment in a considerably 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 chem-istry. Since there is no specific temperature at which themetal will form properly, the viscosity as measured by the resistance to penetration of a probe into the semi-solid, may be used as an indicator of the ~ liquid present in the mixture.
Generally the range of S psig to 15 psig will be used, the

2; exact pressure being selected to suit the conditions of the part to be formed. It is possible to avoid cooling and re-heating 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.

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113661~9 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 is preheated to a temperature of from 100 to 450C., 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 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 solidification of the liquid phase begins. Thus, the pressure gradually rises during the shaping stroke and remains at a peak of from 25 to 5000 psig during solidification. The applied pressure enhances heat transfer from the metal alloy to the die and feeds solidifica-tion shrinkage. If the pressure is too low, porosity may be at an unacceptable level or complex molds may fill incompletely.
Pressures above 5000 psig may be used, but they are not necess-ary. 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 enough, under one minute and preferably less than 4 seconds, to avoid hot cracking of the 1~366~9 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 configur-ation. Within the ranqes of forming and solidification times herein set forth, times should, of course, be kept as short as possible to maximize part-ma~ing productivity. ~s is apparent from the foregoing discussion, times, pressures, temperatures and alloy solid fraction are a combination of critical variables which together function to achieve the significant process economies and product improvements here-in set forth.

The shaping process may be carried out, for example in a 150-250 ton hydraulic press equipped with dies or molds of the type illustrated in FIG. 1 of the drawing. The specific die set there shown is contoured to produce 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 aspect of the process 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, the channels must normally be sufficiently narrow and long to avoid spraying the molten metal to the ex-terior of the dies. Venting channels of conventional size, of -- o --~13~67g 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 venting 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 crossection in FIG. 1. It will be seen from FIG. 1 that the shaping operation actually involves a concurrent forward extrusion of semi-solid metal into the narrow channels opening into vents Ç and 7, a backward extrusion of semi-solid metal into the channels leading to vents 8 and 9 and a forging stroke against the central portion of the metal in the press. Reference herein to "complex" shapes is intended to identify parts which require such concurrent forward and backward extrusion combined 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 semi-solid slurrv containing approximately 50% by vol-ume degenerate dendrites. The billet, approximately six inches in diameter, had the following composition:

Si Cr l~n Fe ~Ig Ti Cu B Al 0.63 0.06 0.06 0.22 0.90 0.012 0.24 0.002 3alance ~136679 The billet, contained in a stainless steel canister, was placed within a resistance furnace set at a temperature of 677C. This temperature, 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 temperature of 632C., corresponding to a fraction 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, main-tained 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 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 2100 psig such that the cavity so formed was filled with alloy. After a holding time under pressure of 2.4 seconds, during which the liquid phase of the part solidified, 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 strength was 47,000 psi, yield strength was 43,000 psi and elongation in a 1" gauge length was 7~. ~inimum specifica-tions for closed die forgings of 6061 aluminum alloys as setforth in Aluminum Standards and Data 1976, Fifth ~dition, 1976 are 38,000 psi ultimate tensile strength, 35,000 psi yield strength and 7% elongation. Representative minimum ~13667~

specifications of an automobile manufacturer for cast aluminum wheels are 31,000 ultimate tensile strength, 16,500 yield strength and 7% elongation.

Unlike wrought products whose properties are direct-ional, the products of the invention are isotropic - their 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 lO produced in accordance with the invention is shown in ele-vation in FIGS. 2 and 3. The plan view of FIG. 3 shows the wheel as viewed from the direction of the bottom die in FIG. 1.
The wheel contains a plurality of roughly rectangular contours ll 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 punched or machined holes 15. A wheel configuration of this complexity is normally produced by permanent mold or die casting tecnniques and is accordingly limited in its properties to the relatively inferior properties of cast alloys. Material properties are tnus a limiting factor on wneel weight. Lower properties must be compensated by greater bulk in a cast wheel. ~oreover, larger crossections are normally necessary in casting because of li~itations inherent in casting techniques - it is difficuit to fill a permanent mold with thin sections. Thus, tne ~heels of the invention nave the very important capabilit~,~ of being lighter in ~eight than comparable wheels of the prior art.

Claims (3)

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

1. An aluminum alloy automotive wheel of com-plex configuration produced as an integral product to close tolerances by shaping under pressure a semi-solid aluminum alloy charge in a closed die cavity, said aluminum alloy charge containing discrete degen-erate dendritic primary solid particles suspended homogenously in a secondary liquid phase having a lower melting point than said primary solid particles, the properties of said automotive wheel being isotropic and the tensile properties at least meeting the minimum specifications for forged aluminum alloys.

2. The automotive wheel of claim 1 in which the primary solid particles in the metal alloy charge are in a concentration of from 30 to 90% by volume based upon the volume of the alloy.

3. The automotive wheel of claim 2 in which the metal alloy charge is shaped in the die cavity in a time of less than about one second, the die cavity is at a temperature of from about 100 to 450°C. and solidification occurs in less than one minute at a pressure of from about 25 to 5000 psig.