US3578511A - Solid metal molding - Google Patents

Abstract

THE SUPERPLASTIC EUTECTOID ALLOY OF ZINC AND ALUMINUM IS DEFORMED IN A DIE HAVING A TEMPERATURE SUBSTANTIALLY IN EXCESS OF THE CRITICAL TEMPERATURE LIMIT FOR THE ALLOY WITHOUT DETERMENTAL EFFECT. AFTER FORMING, THE TEMPERATURE OF THE RESULTING PART EXCEEDS THE CRITICAL LIMIT AND TO THUS ENABLE THE PART TO BE HANDLED WITHOUT DISTORTION. IN ADDITION, THE PART TEMPERATURE AFTER FORMING IS SUFFICIENTLY HIGH TO PERMIT IMMEDIATE HEAT TREATMENT FOR ROOM TEMPERATURE STRUCTURAL PROPERTY ENHANCEMENT AS PART OF THE FORMING CYCLE SIMPLY BY CONTROLLING THE COOLING RATE OF THE PART.

Description

y 11, 1971 D. L. mp -ML 3,578511 SOLID METAL MOLDING File d Dec. 13, 1968 3 Sheets-Sheet 1 INVENTORS.

I DANIEL l.v MEHl DELBERI 1 WILSON RICHARD J. YOUNG ATTORNEY.

May 11,1971 L. MEHL, ETAL SOLID METAL MOLDING Filed Dec. v 13, 1968 3 Sheets-Sheet 2 uoum T u nF I --532F 5o0-- I a h? i 10o JV 9'0 8'0 1'0 WT. PERCENTAGE zmc r Zn-Al ALLOY FIG. s &5

May 11, 971 0. I... MEHL. ETAL SOLID METAL MOLDING 3 Sheets-Sheet 5 Filed Dec. 13, 1968 United States Patent l 3,578,511 SOLID METAL MOLDING Daniel L. Mehl, Lexington, Ky., Delbert T. Wilson,

Austin, Tex., and Richard J. Young, Lexington, Ky.,

assignors to International Business Machines Corporation, Armonk, NY.

Filed Dec. 13, 1968, Ser. No. 783,675 Int. Cl. C22f 1 I16 US. Cl. 148-115 8 Claims ABSTRACT OF THE DISCLOSURE The superplastic eutectoid alloy of zinc and aluminum is deformed in a die having a temperature substantially in excess of the critical temperature limit for the alloy Without detrimental effect. After forming, the temperature of the resulting part exceeds the critical limit and to thus enable the part to be handled without distortion. In addition, the part temperature after forming is sufficiently high to permit immediate heat treatment for room temperature structural property enhancement as part of the forming cycle simply by controlling the cooling rate of the part.

DISCLOSURE OF THE INVENTION Cross-references US. Pat. 3,340,101 describes a process for forming materials conditioned to exhibit a significant strain rate sensitivity at elevated temperature. The ability of the zinc-aluminum eutectoid and other socalled superplastic materials to be severely deformed by anomalously low applied stress has been employed to advantage in other processes as described for example in US. patent application Ser. No. 653,396, entitled Injection Molding of Solid Metal, filed July 14, 1967, by L. Hymes and D. L. Mehl; US. patent application Ser. No. 689,823, entitled Machine Assembly, filed Dec. 12, ,1967 by L. Hymes; and US. patent application Ser. No. 744,843, entitled Method of Molding Vertical Bosses, filed July 15, 1968, by D. T. Wilson and R. J. Young.

The anomalous superplastic or low flow stress behavior in metal is usually encountered in association with a gross retransformation of a metallurgical structure from an artificially constrained and hence unstable or metastable condition. Most commonly, gross metallurgical structural transformations the induced by heat energy and occur at specific energy levels or temperatures. A com mon example of a typical so-called superplastic material is the 78% zinc-22% aluminum eutectoid which is prepared for superplastic behavior by quenching from a uniform temperature of about 600 degrees F. At the time of forming, conditioning of the material is completed by raising its temperature to between about 520 F. and the eutectoid invariant or 532 F. for the forming process.

The temperature of the eutectoid invariant has been demonstrated as a critical upper limit for obtaining superplastic behavior. Properly prepared material will lose its ability to exhibit the anomolous superplastic behavior if it is heated above its critical limit. Historically, steps have been taken to insure that the material being formed not exceed the eutectoid invariant. These steps principally have involved the use of sophisticated temperature controls on the dies and metal handling equipment to maintain a temperature as close as practically possible to the eutectoid invariant but always to the lower side. We have discovered that on a dynamic basis, superplastic materials which have solid phases above their critical forming temperature will be relatively tolerant of localized applied super-critical temperatures, due to the absorption of heat Patented May 11, 1971 energy by the process of metallurgical structure transformation occurring at the critical temperature. We have further discovered that this tolerance can be employed to great advantage by intelligent selection of molding speed, die temperature, die designs and die materials as guided by well known principles of heat transfer.

Accordingly, it has been an object of our invention to decrease the overall forming cycle time for superplastic materials.

A further object of our invention has been to provide a molding process for superplasticity material wherein the superplastic behavior is substantially eliminated immediately following complete part formation to enhance the handle-ability of the formed part.

Another important object of our invention has been to provide a molding process for superplastic materials and particularly the zinc-aluminum eutectoid wherein the process of molding is combined with a heat treatment process for enhancement of room temperature properties whereby the overall molding and heat treatment cycle time is reduced.

A further important object of our invention has been to provide a process for molding superplastic metals wherein accurate control of metal temperature at the level of maximum formability is obtainable by reliance on the dynamics of a predictable heat transfer situation rather than on the accuracy of an elaborate temperature control mechanism.

An additional object of our invention has been to improve the dimensional stability of parts formed by our process by obtaining a more uniform structure of the part before it is removed from the shaping die.

Our process involves the primary steps of:

(1) A prepared body of potentially superplastic metal is provided of a composition known to have a wholly solid phase above its critical superplastic temperature.

(2) A precision mold cavity or die is provided preferably of a material having a significantly loiwer coeflicient of heat conductivity than the selected superplastic metal.

(3) The mold or die is heated to a temperature substantially in excess of the critical forming temperature for the selected superplastic metal taking into consideration such principles of heat transfer as relative surface area to mass of the various mold configurations, the severity of deformation, particularly in small parts, the existence of an actual melting limit and a desired final part temperature.

(4) Having thus selected a material, designed a mold and heated the mold to a predetermined temperature, the body of superplastic metal is placed in the mold where it can be heated from the die if desired or immediately deformed. The temperature of the body of material rises as the body receives heat from the mold. This temperature rise is arrested locally at the temperature of phase transformation due to the absorption of heat required to effect the transformation. Formation should be completed by the time that any full section of the superplastic material has been completely transformed.

(5) Preferably, the part thus formed is further heated by the die to a temperature definitely above the critical superplastic forming temperature to assu", complete transformation of the metallurgical structure and the resulting increased strength necessary to enable its immediate removal from the die. In the case of the zinc-aluminum eutectoid, the additional heating prepares the formed material for a slow cool equilibrium phase transformation and grain growth and creep characteristics at room temperature.

These and other objects, features and advantages of our invention will be apparent to those skilled in the art from the following description of a specific application of our process wherein reference is made to the accompanying drawings, of which:

FIG. 1 is a partially broken away perspective view of a molding die suitable for performance of our process.

FIG. 2 is a front cross-sectional view of a portion of the die shown in FIG. 1 and including a part in place as molded therein.

FIG. 3 is a typical phase diagram describing a eutectoid phase phenomena of the type that exists in the zinc-aluminum eutectoid.

Referring now to FIGS. 1 and 2, there is shown a mold, die or similar shaping member forming a cavity 11 that is complementary to the configuration of the part P desired to be formed. An eject pin or button 12 is provided for assisting removal of the part P after it is formed. Heating means such as commercially available electrical resistance heaters 13 are embedded in the body of die 10 and are separated from the cavity 11 by some thickness 14 of die material. If the die body 10 is made of steel, for example, and it is desired to mold the zinc-aluminum eutectoid, the thickness 14 will determine the rate at which heat can be transferred from the heaters 13 to the body P being molded. Inasmuch as heat transfers nearly four times faster in the zinc-aluminum than in steel, the thickness 14 can effectively behave as a control on the rate of temperature rise of the outer surface of the body P. Heaters 13 are connected to a suitable power source (not shown) through cable 15 and are maintained at a predetermined elevated temperature within relatively wide temperature limits.

A top plate or cover 16 is provided for enclosing the die cavity 11 and includes an inlet opening 17 through which a blank or body of prepared stock superplastic metal P can be inserted. While the cover plate 16 is not shown as being heated, it may be desirable to include heaters similar to 13 in the top plate, particularly where large parts are being formed. In addition, top plate 16 can be made to include an insulating material particularly where a long forming process is involved.

In operation, top plate 16 is clamped or otherwise forceably held to the die body 10 and the body of superplastic material P is placed in the die through inlet opening 17. Preferably, body P is preheated to a temperature close to the forming temperature. For example, in the case of the zinc-aluminum having a critical forming temperature limit of approximately 532 degrees F., the body P may be preheated to within approximately 2% of the forming temperature, eg (taken on an absolute scale) between 500 and 520 degrees F., to minimize the requirements for heat transfer in the die itself. A plunger or piston 18 is then closed down upon the body P and deforms the body by compression into intimate contact with the cavity 11. The forming time is ideally selected such that complete formation just precedes complete transformation of the last to be formed portions of the part P. The part P is left in the die cavity 11 for a short period of time (in the case of the zinc-aluminum this period can be as short as five seconds for moderately small parts) to assure complete transformation throughout the body. The top cover 16 is then removed from the die and the part P which is above the critical temperature can be ejected by force exerted upwardly against eject pin 12. In the case of the zincalumin-um eutectoid, the part as ejected can be slow cooled directly either in the air or under controlled conditions in a heat treat furnace to increase grain size and to permit equilibrium phase transformation of the alloy.

The characteristics of a material suitable for use as the blank or stock P are illustrated by reference to a portion of the zinc-aluminum phase diagram, FIG. 3, which shows a typical equilibrium phase relationship known as a eutectoid. This specific eutectoid has a critical phase transformation at the eutectoid invariant of 532 F. and has a wholly solid phase a up to temperatures as high as about 800 F. The a phase has strength characteristics like those of conventional metals Whereas the 4 strength of this material just below the eutectoid invariant, when a properly preconditioned state, is anomalously low.

The heat of phase transformation for this material is demonstrated by an experiment as illustrated in FIGS. 4 and 5. Cylindrical slugs S of quenched 78% zinc-22% aluminum were placed between heating platens 21 and 22 as shown in FIG. 4. Steel stops 23 were also placed between the platens 21 and 22 to eliminate significant deformation of the slug S. A thermocouple 24 was located in the center of the slug S and the temperature change was recorded for a period of time during which the slug was elevated from below its superplastic critical temperature to above that temperature. Platen temperature was varied between 550 and 650 F. for dilferent tests and characteristic curves like curve S shown in FIG 5, were recorded in each instance. We noted that a level area or temperature arrest S is a significant part of this temperature rise curve. The following data indicates the platen temperature for a slug having a cylindrical conmagnitude of this temperature arrest relative to the figuration with a diameter of two inches and a vertical height of two inches.

Duration of thermal arrest, secs.

Platen temperature degrees F ahrenheit:

EXAMPLE A typical part, as shown in FIG. 6, was repeatedly formed in accordance with our invention. The part shown in FIG. 6 has an overall diameter 31 of 2.520 in., a peripheral rim width 32 of .690 in., a hub depth 33 of 1% in., a tooth height 34 of .055 in. and an overall weight of 177 grms. The blank of stock metal from which it was formed was a disk having a diameter of 2.36 in., a depth or thickness of .483 in. and a center bore of .343 in.

The blank is preconditioned by homogenization at 600 F. for one hour followed by a water quench with agitation. Low temperature recalescence is permitted.

The blank is preheated at the time of forming to 500 E, which requires a period of about 2 minutes. A die mold of AISI Type H13 and A181 C10l8 steels weighing 350 lbs. is heated by heaters totalling 6000 watts capacity and separated from this die cavity by 1% in. at the closest point, to an initial temperature of between 610-630 F. These heaters supplement the 29,300 watts available in press platens employed to clamp the die components together.

The blank disk was placed in the die which closes in a period of 5 seconds. Forming occurs under a load that varies to a maximum of 60,000 lbs. during a period of 4 to 7 seconds. The part thus formed is left in the die under pressure for an additional 10 seconds to assure a temperature throughout in excess of 532 F. The cycle is completed by opening the die (approximately five seconds) and ejecting the part (three to five seconds). During this eight to ten seconds, approximately 50% of the part remains in intimate contact with the 610630 F. lower die section, thus subjecting the part to additional heat treatment. The part is air cooled to complete the combined forming and heat treating process.

Our invention can also be applied to sheet forming techniques as illustrated in FIG. 7. A die or shaping member 40 like that described in aforesaid US. Pat. 3,340,101 is heated to above the critical temperature of blank sheet metal B by heaters 41. A vacuum is applied to plenum 42 to create a fluid pressure across the opposed principle surfaces B and B of sheet B thereby deforming it into conformity with the die 40. After forming, the resulting part can be removed from the die 40 with little danger of distortion.

Having thus described the concepts of our invention, some typical applications thereof and a specific performed example, we define the invention sought to be patented by the following claims:

1. The method of molding solid metal of a metallurgical composition and state characterized by conditionability to a state of anomalously low strength within a temperature range at and below a critical temperature, and the existence of a wholly solid phase having ordinary strength at temperatures above said critical temperature, comprising the steps of:

providing a shaping member defining a configuration to be molded,

providing a body of said metal at a temperature below said critical temperature,

heating said shaping member to a temperature substantially in excess of said critical temperature, and deforming said metal body against said shaping member to form a part having said configuration.

2. The method of molding as defined in claim 1 wherein said metal is of a eutectoid alloy composition.

3. The method as defined in claim 2 wherein said conditionable metal comprises, by weight, essentially 78% Zinc and 22% aluminum.

4. The method of molding metal as defined in claim 1 wherein said metal body is provided at a temperature that is within approximately 2% of said critical temperature.

5. The method of molding metal as defined in claims 1, 2, 3 or 4 comprising the further step of separating said part from said shaping member while said part is at a temperature in excess of said critical temperature.

6. The method as defined in claim 3 comprising the further steps of:

removing said part from said shaping member while said part is at a temperature in excess of said critical temperature, and

slow cooling said part to below said critical temperature at a rate sutficiently slow so as to permit substantially equilibrium phase transformation of the metallurgical structure.

7. The process as defined in claims 1, 2, 3, 4, 5 or 6 wherein said shaping member comprises a substantially enclosed cavity and said deforming is accomplished by application of compressive stress to said metal body.

8. The process as defined in claims 1, 2, 3, 4, 5 or 6 wherein said metal body is provided in a form having two opposed principal surfaces and said deforming step is accomplished by the application of a fluid pressure across said principal surfaces.

References Cited UNITED STATES PATENTS 1,993,942 3/1935 Novotny 264 -328 2,814,101 11/1957 Prough et a1. 72342 3,340,101 9/1967 Fields, Jr., et al l48l1.5 3,420,717 1/1969 Fields, Jr. et al l4811.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner my UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,578 5 1 Dated ay 1 i i971 Mehi, Daniel L,; Wilson, Delbert T.; Young, Inventor(s) Richard J It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, i ine 2i deiete [magnitude of this temperature arrest reiative to the] insert "figuration with a diameter of two inches and a verticai Column 4, line 22 delete [figuration with a diameter of two inches and a verticai] insert "magnitude of this temperature arres t reiati ve to the" Column 4, iine 27 delete [650-600] insert "650-660" Signed and sealed this 13th day of February 1973.

(SEAL) /\t test EDWARD M.I LETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

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