Die casting of wrought aluminum alloys

Abstract

A method of making a wrought aluminum alloy component comprises die casting the wrought aluminum alloy in a die cavity to produce a die cast component and isostatically pressing the die cast component to promote closure of internal voids therein.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a method of making wrought aluminum alloy components by die casting the alloy to near net shape followed by isostatic pressing.

BACKGROUND OF THE INVENTION

[0002] There are two types or classes of aluminum alloys; namely, alloys that are wrought (or worked) into shape and alloys that are cast to shape. Approximately 75% of the aluminum produced in the US is in wrought product form. In the aerospace industry, nearly all of the aluminum used is in wrought form. For example, airplane wheels are made from precision forged wrought aluminum alloys, such wrought alloy 7050 (Al-6.2% Zn-2.3% Cu-2.3% Mg where %'s are weight %). Unlike castings, wrought products do not have to account for the alloy's ability to flow into thin mold sections. Thus, wrought products are afforded more flexibility in chemical composition, which can be used to provide increased alloy mechanical properties. In addition, forgings have essentially no porosity due to the redundant working of the alloy microstructure.

[0003] Because of the high strength-to-weight ratio of wrought aluminum alloys, they are excellent candidates for critical component applications in the aerospace and automotive industries. However, as a result of their poor castablity, wrought aluminum alloys are not traditionally cast into near net shape components. For example, wrought aluminum alloys exhibit poor flowability of the molten alloy into thin mold sections and poor hot tear resistance evidenced by unacceptable hot tear cracks in the cast product.

[0004] Manufacture of such wrought products to complex component shapes involves extensive fabrication costs resulting from the multiple fabricating (e.g. heating, forging and machining) operations that must be performed on the initial billet of wrought aluminum alloy.

[0005] There is a need for a method of making near net shape castings of wrought aluminum alloys that are suitable for use in applications in the aerospace, automotive and other industries.

[0006] It is an object of the present invention to provide a method of making wrought aluminum alloy components that satisfies this need.

SUMMARY OF THE INVENTION

[0007] The present invention provides a method for making wrought aluminum alloy components by die casting the alloy in a die cavity at subambient pressure to produce a near shape die cast component having an exterior surface that is amenable to subsequent isostatic pressing to promote closure of internal voids, such as for example internal microporosity and/or microcracking. The die cast component preferably is subsequently hot isostatically pressed to promote closure of internal voids.

[0008] In an illustrative embodiment of the invention, a wrought aluminum alloy is die cast under conditions to produce a near net shape die cast component that may exhibit internal voids that are not surface-connected to an extent that the die cast component can be subsequently hot isostatically pressed to promote closure of the internal voids. Particular illustrative preferred die cast conditions include a combination of a high vacuum during die casting, high injection (plunger) speeds, optional heating of the dies to superambient temperature, high intensification pressure on the initially molten wrought aluminum alloy after it is injected into the die cavity and for a sufficient time to allow the die cast component to completely solidify under pressure.

[0009] The die cast component then is hot isostatically pressed (HIP'ed) under conditions to promote closure of any internal voids, providing a near net shape die cast and HIP'ed component. Hot isostatic pressing typically is conducted in a gaseous atmosphere at a high enough temperature and for a long enough time to promote closure of internal voids in the die cast component and improve mechanical properties, such as yield strength, ultimate tensile strength and ductility, of the die cast and HIP'ed component.

[0010] Details of the present invention will become more readily apparent from the following detailed description taken with the following drawings.

DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic side elevation of a vacuum die casting machine for practicing an embodiment of the present invention with the shot sleeve, dies, and vacuum chamber shown broken away.

[0012] FIG. 2 is a plan view of one of the dies for die casting wrought aluminum alloy (7050) specimens.

DETAILED DESCRIPTION OF THE INVENTION

[0013] For purposes of illustration and not limitation, FIG. 1 shows a die casting machine that can be used to die cast a wrought aluminum alloy under conditions to produce a near net shape die cast component amenable to subsequent hot isostatic pressing in practice of the invention. The invention is practiced to die cast wrought aluminum alloys of the type described in the International Alloy Designations and Chemical Compositions Limits for Wrought Aluminum and Wrought Aluminum Alloys, Unified North American and International Registration Records of The Aluminum Association Incorporated, 900 19th Street, N.W., Washington, D.C., January, 2001. Wrought aluminum alloys are identified by four digit numerical designations (e.g. 2xxx, 3xxx, 5xxx, 6xxx, etc. series numbers) as opposed to casting aluminum alloys which are identified by three digit numerical designations. Wrought aluminum alloys which can be die cast, hot isostatically pressed, and precipitation heat treated by practice of the invention include all 2xxx alloys that include Al, Cu, Mg, 6xxx alloys that include Al, Mg, Si, and 7xxx alloys that include Al, Zn, Mg, Cu.

[0014] A die casting machine that can be used to practice the invention is described in U.S. Pat. No. 6,070,643, the teachings of which are incorporated herein by reference. The die casting machine comprises a base 10 which defines therein a reservoir 10a for hydraulic fluid that is used by hydraulic actuator 12 to open and close the fixed and movable die platens 14, 16. The platen 16 is disposed for movement on stationary tie bars or rods 18. A die clamping linkage mechanism 20 is connected to the movable die platen 16 in conventional manner to open/close the movable die 34 relative to fixed die 32 disposed on platen 14. For example, a conventional die casting machine available as 250 ton HPM #73-086 from HPM, Cleveland, Ohio, includes such a base 10, actuator 12, and die platens 14, 16 mounted on tie bars 18 and opened/closed by die clamping linkage mechanism 20 in the manner described. The die casting machine includes a gas accumulator 21 for rapid feeding of hydraulic fluid to the plunger mechanism.

[0015] The die casting machine also comprises a tubular, horizontal shot sleeve 24 that communicates to a die cavity 30 defined between the dies 32, 34 disposed on the respective die platens 14, 16. The die cavity 30 can be configured to form one or more cast components. The shot sleeve 24 has a discharge end section 24a that communicates with an entrance passage or gate 36 to the one or more die cavities 30 so as that molten wrought aluminum alloy can be pressure injected therein. The entrance passage or gate 36 can be machined in the stationary die 32 or the movable die 34, or both.

[0016] The discharge end section 24a of the shot sleeve 24 extends through a suitable passage 24b in the stationary platen 14 and die 32 as illustrated in FIG. 1.

[0017] The shot sleeve 24 extends through die 32 into a vacuum melting chamber 40 where the molten wrought aluminum alloy to be die cast is heated in a crucible 54 under relatively high vacuum conditions such as about 15 to about 50 microns. The vacuum chamber 40 is defined by a vacuum housing wall 42 that extends about and encompasses or surrounds the opposite charging end section of the shot sleeve 24 receiving the plunger 27 and the shot sleeve hydraulic actuator 25. The vacuum chamber 40 is evacuated by one or more conventional vacuum pumps P connected to the chamber 40 by a conduit 40a. The base 10 and the vacuum housing wall 42 rest on a concrete floor or other suitable support.

[0018] The chamber wall 42 is airtight sealed with the fixed platen 14 by peripheral airtight seal(s) 43 located therebetween so as to sealingly enclose the shot sleeve and a pair of side-by-side stationary, horizontal shot sleeve/plunger support members 44 (one shown) extending through chamber wall 42. Such shot sleeve/plunger support members are provided on the aforementioned conventional die casting machine (250 ton HPM #73-086).

[0019] A plunger 27 is disposed in the shot sleeve 24 for movement by plunger actuator 25 and plunger connector rod 27b between a start injection position located to the right of a melt entry or inlet opening 58 in shot sleeve 24 and an injection position proximate the die entrance gate 36. The melt inlet opening 58 communicates to a metal (e.g. steel) melt-receiving vessel 52 mounted adjacent the fixed platen 14 on the shot sleeve 24 by clamps such as screw clamps (not shown). The melt-receiving vessel 52 is disposed beneath crucible 54 to receive a charge of molten wrought aluminum alloy therefrom for die casting. The melt-contacting surfaces of dies 32, 34 and plunger tip 27a typically are coated with a graphite spray coating.

[0020] The crucible 54 can comprise a melting crucible in which a solid charge of the wrought aluminum alloy is melted in chamber 40. An induction coil 56 is provided about the crucible to heat the alloy to a desired alloy die casting temperature with some amount of superheat above the alloy melting point. Alternately, the crucible may be a melt holding crucible adapted to receive molten wrought aluminum alloy from a vacuum melting crucible (not shown) located outside the chamber 40.

[0021] If the crucible 54 is a melting crucible, it can comprise an induction skull crucible comprising copper segments in which a charge of solid wrought aluminum alloy to be die cast is charged via vacuum port 40b and melted by energization of induction coils 56 disposed about the crucible in conventional manner in the chamber 40. Known ceramic or refractory (e.g. graphite) lined crucibles 54 also can be used in practicing the present invention. The crucible 54 can be tilted by rotation about crucible trunnions T using a conventional hydraulic, electrical or other actuator (not shown) disposed outside the vacuum chamber 40 and connected to the crucible by suitable vacuum sealed linkage extending from the actuator to the crucible. The crucible 54 is tilted to pour the molten wrought aluminum alloy charge into the melt-receiving vessel 52, which is communicated to the shot sleeve 24 via opening 58 in the shot sleeve wall. The molten charge is introduced through opening 58 into the shot sleeve 24 in front of the plunger tip 27a.

[0022] The plunger 27 is moved from the start injection position to an injection position proximate the die entrance gate 36 by a conventional hydraulic actuator 25 that, for example, is provided on the aforementioned conventional die casting machine. Typical radial clearance between the shot sleeve 24 and the plunger tip 27a is in the range of about 0.0005 inch to 0.020 inch.

[0023] When dies 32, 34 are closed, the die cavity 30 defined therebetween is communicated to the vacuum chamber 40 via the shot sleeve 24 and can be evacuated through the shot sleeve. In particular, the stationary die 32 typically includes a series of grooves (one shown) on its inner face that face the opposing inner face of the movable die 34 when the dies are closed. The grooves encircle or extend about the die cavity 30 as well as gate 36 and a melt discharge opening communicated to the gate 36 and defined by shot sleeve end 24a. Each groove receives a respective resilient, reusable high temperature O-ring vacuum seal 60 for sealing in vacuum tight manner against the mating face of the movable die 34 when the dies are closed. Only one groove and seal 60 are shown for convenience. Alternately, the seal(s) 60 can be disposed in grooves on the mating face of the movable die 34, or on the mating faces of both dies 32, 34, so as to form a vacuum tight seal about and isolating the die cavity 30, gate 36, and shot sleeve end 24a from the ambient air atmosphere surrounding the exterior of the dies 32, 34 when closed. The vacuum seals 60 may comprises Viton material that can withstand temperatures as high as 400 degrees F. that may be present when the die cavity 30 is filled with molten alloy.

[0024] By use of vacuum seals 60, the die cavity 30 is isolated from the ambient air atmosphere when the dies 32, 34 are closed and enables the die cavity 30 to be evacuated through the shot sleeve 24 when the vacuum melting chamber 40 is evacuated to high vacuum levels of about 15 to about 50 microns employed in practice of the invention.

[0025] The dies 32, 34 optionally are maintained at a superambient temperature in a preferred range of about 200 to 400 degrees F. during die casting, although unheated dies may be used. For example, the dies 32, 34 are heated prior to injection of the molten alloy therein by one or more conventional electrical resistance rod heating elements (not shown) received in channels in the dies, by gas flame burners or any other conventional die heating means, or the dies can be self-heated as a result of prior injection of molten wrought aluminum alloy charge(s) in the die cavity 30. The dies 32, 34 may also be cooled by water cooling conduits (not shown) formed internally of the dies and through which cooling water is circulated to control die temperature in the preferred range. The shot sleeve 24 similarly can optionally be heated or cooled to control shot sleeve temperature within a desired range of about 200 to about 400 degrees F. using similar heating and cooling devices.

[0026] In die casting wrought aluminum alloys in practice of the invention, the dies 32, 34 and shot sleeve 24 are made of steel or other suitable material for purposes of illustration and not limitation, since other die and sleeve materials may be used.

[0027] In accordance with an embodiment of the invention, a wrought aluminum alloy is die cast under a combination of conditions to produce a near net shape die cast component that is amenable to subsequent hot isostatic pressing (HIP'ing) to promote closure of any internal voids. The die cast conditions typically produce a die cast component that includes internal voids, such as internal microporosity and/or microcracks (i.e. micro-hot tearing), that are not surface-connected in that the internal voids do not extend to or penetrate the exterior surface of the die cast component. That is, the exterior surface of the wrought aluminum alloy die cast component is substantially free of voids that extend from the interior to the exterior of the component such that the die cast component can be subjected to a subsequent HIP operation to promote closure of the internal voids.

[0028] Particular die cast conditions useful for die casting wrought aluminum alloys using the die cast machine described above and offered for purposes of illustrating but not limiting the invention include the combination of 1) a high vacuum of about 15 to about 50 microns in die cavity 30 and chamber 40, 2) optional heating of the dies 32, 34 to a superambient temperature of about 200 to 400 degrees F., 3) high injection (plunger 27) speeds greater than 30 inch/second, such as preferably 30 to 80 inches/second, and 4) high intensification pressure in the range of about 5,000 to about 20,000 psi on the wrought aluminum alloy after it is injected into the die cavity 30 and for a sufficient time to allow the die cast component to completely solidify through the component cross-section under such pressure in die cavity 30. These conditions can be selected and adjusted for a particular wrought aluminum alloy composition to achieve a HIP'able die cast component.

[0029] The die cast component then is hot isostatically pressed (HIP'ed) under conditions to promote closure of any internal voids. The exterior surface of the wrought aluminum alloy die cast component is substantially free of voids that extend from the interior to the exterior of the component such that the die cast component can be subjected to the HIP operation to promote closure of the internal voids. Hot isostatic pressing typically is conducted in an inert gas or other gas non-reactive with the wrought aluminum alloy. A typical gaseous atmosphere comprises argon. HIP'ing is conducted at a high enough temperature and for a long enough time to promote closure of internal voids, such as microporosity and microcracking (i.e. micro-hot tearing), in the die cast component and improve mechanical properties, such as yield strength, ultimate tensile strength and ductility, of the die cast and HIP'ed component. For purposes of illustration and not limitation, the HIP'ing step can be conducted at 850 to 1000 degrees F. and argon gas pressure of 15,000 psi for 2 hours for most wrought aluminum alloy die cast components. The HIP step can be conducted using conventional HIP'ing apparatus.

[0030] Prior to HIP'ing, the die cast component may be solution heat treated at an elevated temperature to reduce the amount of lower melting point eutectic phase(s) that may be present in the die cast microstructure and thereby homogenize the alloy microstructure and chemistry to avoid incipient melting during HIP'ing. Suitable solution heat treatment temperatures and times are well known for a wide variety of wrought aluminum alloys.

[0031] After HIP'ing, the wrought aluminum alloy die cast component may be heat treated to develop desired mechanical properties. For example, the die cast and HIP'ed wrought aluminum alloy component can be subjected to a conventional solution heat treatment followed by one or more lower temperature aging heat treatments to develop desired mechanical properties for the particular wrought aluminum alloy used. Illustrative of such heat treatments are the well known T6 and T7 heat treatments.

[0032] The following example is offered to further illustrate but not limit the invention.

EXAMPLE

[0033] Wrought aluminum alloy 7050 specimens were die cast using a die cast machine of the type described above where about 3 pounds of the alloy were induction melted in the crucible 54 in the vacuum chamber 40 (FIG. 1) at a vacuum level of 25 microns. The alloy was heated to a casting (pour) temperature of 1175 degrees F. to provide superheat relative to the alloy melting temperature of 1162 degrees F. The dies 32, 34 were heated to about 212 degrees F. The melted alloy was poured into the shot sleeve and injected into the die cavity at 30 inches/second plunger speed in a shot sleeve of 3 inches diameter. The plunger 27 continued to travel in the shot sleeve to apply intensification pressure at a maximum calculated hydrostatic pressure of 6,200 psi on the alloy in the die cavity for a time of 30 seconds such that the alloy was fully solidified through its cross-section while under intensification pressure. The dies then were then opened and the solidified die casting ejected using ejector pins. The die cavity had a configuration shown in FIG. 2 for one die with four elongated “finger” cavities C each measuring 13 inches long by 0.6 inch diameter and ejector pin holes H as shown. The ends of the “finger” cavities communicated to a “fillet” cavity F which received injected molten wrought aluminum alloy from the entrance gate. The 7050 die cast specimens produced in this manner exhibited some internal voids which included internal microporosity and microcracking (i.e. micro-hot tearing) due to solidification shrinkage. Hot tearing of the die cast specimens made pursuant to the invention was substantially reduced as compared to specimens made without the combination of high vacuum and intensification pressure and was confined to the interior of the die cast specimens as opposed to specimens that were not die cast with the combination of high vacuum and intensification pressure. That is, the internal voids (microporosity and microcracks) of specimens made pursuant to the invention did not connect to the exterior surface of the die casting such that the die casting could be HIP'ed in a subsequent step to promote closure of the internal porosity and microcracks. Specimens that were die cast without the combination of high vacuum and intensification pressure exhibited internal porosity and hot tears that could not be healed or closed by HIP'ing.

[0034] Chemical analysis of the die cast 7050 specimens indicated that there was a loss of volatile alloying elements, in particular Zn and Mg, at the high vacuum level of 25 microns used in the trials. This loss of volatile elements from the wrought aluminum alloy can be countered by increasing the initial concentrations of such volatile elements in the alloy charge to be melted. Alternately, the alloy can be melted in chamber 40 at a vacuum level insufficient to cause adverse loss of such volatile alloying elements from the charge. For example, the chamber 40 can be maintained at a vacuum level of 40 microns to reduce loss of Zn and Mg while the die cavity 30 can be maintained at a higher vacuum level (e.g. 10-25 microns). Such different vacuum levels in the chamber 40 and die cavity 30 can be achieved by modifying the above-described die casting machine to include a separate vacuum pumping system for the die cavity 30 to provide the higher vacuum in the die cavity and an isolation valve (not shown) located between the chamber 40 and die cavity 30 to isolate die cavity 30 from chamber 40 until the melted alloy is introduced into the shot sleeve 24 ready to be injected into the die cavity. The valve then is opened to permit injection of molten alloy from the shot sleeve into the die cavity.

[0035] While the invention has been described in terms of specific embodiments thereof, it is not intended to be thereto but rather only to the extent set forth in the following claims.

Claims

1. A method of making a wrought aluminum alloy component, comprising die casting the wrought aluminum alloy in a die cavity at subambient pressure to produce a die cast component with an exterior surface that permits isostatic pressing of said die cast component and isostatically pressing the die cast component to promote closure of internal voids therein.

2. The method of claim 1 wherein said alloy is die cast to produce said die cast component that includes an exterior surface substantially free of voids that extend from an interior of said die cast component.

3. The method of claim 1 wherein said alloy is die cast with a combination of conditions including a subambient pressure in said die cavity of about 15 to about 50 microns, injection speed greater than about 30 inches/second, and an intensification pressure in the range of about 5,000 to about 20,000 psi on the wrought aluminum alloy after it is injected into the die cavity.

4. The method of claim 3 including heating dies defining said die cavity in the range of about 200 to about 400 degrees F.

5. The method of claim 1 wherein said die cast component is hot isostatically pressed in a gaseous atmosphere.

6. The method of claim 1 including the further step of solution heat treating said die cast component before said isostatic pressing.

7. The method of claim 1 including the further step of solution heat treating and aging said die cast component after said isostatic pressing.

9. The method of claim 1 wherein said internal voids comprise internal porosity or cracking.

10. A method of die casting a wrought aluminum alloy component, comprising die casting the wrought aluminum alloy in a die cavity at subambient pressure in said die cavity of about 15 to about 50 microns, injection speed greater than about 30 inches/second, and an intensification pressure in the range of about 5,000 to about 20,000 psi on the wrought aluminum alloy after it is injected into the die cavity.

11. The method of claim 10 including heating dies defining said die cavity in the range of about 200 to about 400 degrees F.