MAGNESIUM-CONTAINING, ALUMINUM-BASED ALLOY FOR THIN-WALL CASTINGS

Abstract

An aluminum-magnesium alloy is disclosed which provides superior properties for casting in steel dies and good ductility for forming castings of complex shapes, including thin-wall portions. The aluminum-based alloy contains, in weight percent, about 2-15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5 percent manganese, 0.05 to 0.2% titanium, and a minimal content of iron. In its molten state this aluminum-magnesium-chromium alloy can be pushed into the molding cavities of iron-based dies in a high pressure die casting procedure and conform to complexly-shaped die surfaces with thin cavity portions without dissolving appreciable amounts of iron or experiencing die soldering on the die surfaces. The resulting castings display good strength and ductility and can be further enhanced by an artificial aging process after solution heat treatment.

Claims

1. An aluminum-based die-casting alloy consisting essentially, by weight, of 2 to 15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5 percent manganese, 0.05 to 0.2 percent titanium, less than 0.2 percent iron, up to about 0.5 percent other elements, and the balance aluminum.

2. An aluminum-based die-casting alloy consisting essentially, by weight, of 2 to 15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5 percent manganese, 0.05 to 0.2 percent titanium, less than 0.2 percent iron, up to about 0.5 percent other elements, and the balance aluminum; a cast specimen of the alloy, without heat treatment of the cast specimen, having a tensile strength of at least 250 MPa, an ultimate tensile strength of 280 MPa, and a total elongation of at least 15 percent, each determined on a test specimen at 25 C.

3. An aluminum-based die-casting alloy consisting essentially, by weight, of 5 to 9 percent magnesium, 0.25 to 0.35 percent chromium, 0.15 to 0.35% manganese, 1.0 to 3.0 percent silicon, 0.05 to 0.1 percent titanium, less than 0.15 percent iron, less than 0.01 percent copper, less than 0.01 percent zinc, less than 0.003 percent phosphorus, less than 0.03% strontium, and the balance aluminum.

4. A method of die-casting an article using an aluminum-based alloy, the article having at least one thin-wall section with a thickness of three millimeters or less, the article being cast in a mold cavity of a die formed of an iron-based alloy; the method comprising: injecting a mold-filling volume of a molten aluminum-based alloy into the mold cavity of the iron-based alloy die, the cavity having surfaces formed by separable, facing mold members and a portion of the cavity defining each thin-wall section and maintaining a predetermined pressure on the molten aluminum-based alloy to force the molten alloy into full conformance with the surfaces of the cavity, the molten aluminum-base alloy having a composition consisting essentially, by weight, of 2 to 15 percent magnesium, 0.2 to 3 percent silicon, 0.05 to 0.5 percent chromium, 0.05 to 0.5% manganese, less than 0.2 percent iron, up to about 0.5 percent other elements, and the balance aluminum; and cooling the molten aluminum-based alloy to form the solid shape of the article and removing the solid die-cast article shape.

5. A method of die-casting an article as stated in claim 4 in which the cast molten aluminum-based alloy consists essentially, by weight of 5 to 9 percent magnesium, less than 0.15 percent iron, 0.25 to 0.35 percent chromium, 0.15 to 0.35% manganese, less than 0.01 percent copper, 2.0 to 3.0 percent silicon, less than 0.01 percent zinc, 0.05 to 0.1 percent titanium, less than 0.003 percent phosphorus, less than 0.01 percent strontium, and the balance aluminum.

6. A method of die-casting an article as stated in claim 4 in which the temperature of the volume of molten aluminum-based alloy injected into the mold cavity is in the range of 670 C. to 730 C.

7. A method of die casting an article as stated in claim 4 in which the facing mold members are maintained at a temperature in the range of 180 C. to 230 C. during the formation of the cast article.

8. A method of die-casting an article as stated in claim 4 in which the facing mold members are maintained at a temperature in the range of 180 C. to 230 C. during the formation of the cast article and the cast article is removed from the mold members at a cast article temperature in the range of 180 C. to 230 C.

9. A method of die-casting an article as stated in claim 4 in which the die-cast article shape is removed from the mold and, before further cooling, re-heated in a precipitation hardening process.

10. A method of die-casting an article as stated in claim 4 in which the die-cast article shape is (i) removed from the mold and re-heated to a temperature in the range of 480 C. to 540 C. for a period of two to six hours to promote solution of separate phases in the microstructure of the cast article, (ii) rapidly cooled to a temperature below 100 C., and (iii) then reheated to a temperature in the range of 180 C. to 250 C. for a period to produce precipitation hardening in the cast article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 is a perspective view of a die cast shock tower.

[0018] FIG. 2 is a simplified view of a die casting machine, shown in perspective, partial cutaway view.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0019] Thin-wall (up to 3 millimeters thick) aluminum alloy castings are potentially attractive for structural components of a vehicle body. Such thin wall castings commonly enable replacing a multi-component welded assembly with a single casting and so, in addition to mass savings, also simplify the body assembly process. Representative applications include shock towers, such as the example shown at FIG. 1, which are intended to secure, in a vehicle suspension system employing McPherson struts, an end of a shock absorber and spring combination to a vehicle body. A cast shock tower may replace as many as eight individual stamped and assembled components with a single casting.

[0020] Other examples of thin-walled structural castings include front body hinge pillars, rear shock caps and rear rails, all of which have demonstrated comparable component consolidation. Most thin-wall structural castings such as shock tower 1 shown at FIG. 1, in addition to being of generally thin-wall construction overall as illustrated best at features 2, incorporate a plurality of localized stiffening features. These localized stiffening features are commonly thin-walled ribs, not necessarily of uniform geometry along their length, extending outwardly or inwardly from the primary casting surface, for example the outwardly-extending ribs 3 shown incorporated into shock tower 1. Some castings, such as shock tower 1, may be generally compact, but others of the exemplary structures listed above may extend, in at least one dimension, over several hundreds of millimeters. Thus, thin wall casting presents significant challenges in completely filling the die, including the stiffening ribs or other localized features, with molten metal before local solidification prematurely chokes off introduction of molten metal to the die prior to filling the die. Successful practice of such thin-wall die casting therefore requires that the molten metal have high fluidity as well as requiring rapid introduction of the molten metal into the die.

[0021] Preferably the aluminum alloy selected for such castings will develop the physical properties required of the cast part, primarily strength and ductility, when the part is in its as-fabricated or as-cast condition. But, the selected aluminum alloy should also be responsive to a subsequent heat treatment to develop enhanced properties if required. To achieve production volumes suitable for vehicle production, such parts may be die cast, and because of the thin wall design, should be high pressure die cast to rapidly fill the die and avoid shutting off the flow of molten metal due to premature solidification of the inrushing casting charge or shot. A suitable alloy composition, described in detail below, when limited in thickness to less than 3 millimeters thick, may develop yield strengths in excess of 200 MPa at ambient temperature or about 25 C. This thickness generally corresponds to the overall thickness of aluminum alloy based structural castings, such as those listed above and also to the thickness of stiffening ribs or other local features incorporated into the casting.

[0022] The shock tower 1 of FIG. 1 is shown ready for assembly into a vehicle. But, in addition to a cavity representative of the intended cast part geometry, a die casting die will incorporate additional features intended to enable and facilitate the making of a sound casting. Without limitation, these additional features may include a gating system, or gate, comprising an entry point for introduction of molten metal, a runner system, or runners, for distributing the molten metal within the die cavity, and overflow features to ensure complete die filling and to accommodate oxidation products or any excess die lubricant. Any solidified metal contained or retained within these die features will be incorporated into the as-cast body removed from the die and will be appended to the intended cast part such as shock tower 1 in FIG. 1. Thus, the as-cast body removed from the die will be cut, trimmed, machined or otherwise processed so that the casting or cast part, ready for assembly into a vehicle, may be separated from these appended features and removed from the as-cast body.

[0023] The strength and stiffness of modern vehicle bodies result from the cooperative interaction and deflection of all the parts comprising the body structure. Thus, these body parts must be securely joined together. While welding has traditionally been employed in bodies of all-steel construction, aluminum parts may also be joined using mechanical fasteners such as self piercing rivets (SPR). Making an effective load-transmitting joint using SPR requires that the structure receiving the rivet have sufficient ductility to deform without cracking or fracturing. This requirement may typically be met if the material of the receiving structure exhibits a total elongation, that is, the elongation to fracture, sustained by a test specimen or coupon with a two inch (50.8 mm) gage length, of no less than about 10%. Vehicle body assembly is commonly performed at ambient temperature, or about 25 C., thus, to enable flexibility in the choice of joining methods, thin wall die cast structures should exhibit tensile total elongations of at least 10% and preferably greater than 10% at ambient temperature.

[0024] A simplified representation of a die casting machine 10, which may be used for aluminum alloy die castings, is shown in perspective, partial cutaway view at FIG. 2. The die casting machine 10 comprises three basic sections, an injection unit 12 to enable the rapid introduction of a predetermined volume of molten metal into a die, a die assembly 14 which receives the molten metal, and a clamping unit 16 to maintain die portions 18, 20 tightly pressed together as the die is filled and to separate the die portions for removal of the solidified as-cast body.

[0025] In operation, a predetermined volume of molten metal 22 at a temperature in the range of about 670 C. to 730 C., here shown for purposes of illustration only as contained in ladle 23, is introduced into injection unit 12 ahead of piston 24. Piston 24 is then rapidly advanced in the direction of arrow 26 by some suitable means, here as by the combined action of hydraulic cylinder 28 and piston 30. The advancement of piston 24 propels the molten metal charge 22 through a suitable system of gates and runners (not illustrated in detail) into the die assembly 14. Die assembly 14 comprises mating die portions 18 and 20 which contact along parting line 32. Mating die portions 18, 20 comprise complementary and opposing die cavities, which, when the die portions are brought together, enclose a die volume 34 whose boundaries are shaped to correspond to the outer surfaces of the intended cast part. The respective portions of die portions 18, 20 have been cut away to more clearly illustrate die volume 34 and the specific contributions to die volume 34 from die portions 18, 20 are not shown. However, die volume 34 must be suitably oriented and positioned with respect to parting line 32 to enable removal of the cast part from the die. It will be appreciated that the die volume 34 shown in the FIG. 2 has a very simple shape, and is intended only to illustrate the die casting process and is not representative of the highly complex part shapes, described previously, which may be fabricated by such die casting processes. Die portions 18, 20 may also comprise passages 36 for circulation of cooling fluid to maintain the temperature of the die assembly 14 within a predetermined temperature range, here between 180 C. and 230 C. Clamping unit 16, usually hydraulically operated (hydraulics not shown), is operatively connected to die portion 18 and enables die portion 18 to be maintained in tight engagement with die portion 20 to resist the injection pressure as molten metal shot 22 is injected into the die assembly. Clamping unit 16 is also intended to retract die portion 18 from engagement with die portion 20 when the molten metal has solidified to enable removal of the as-cast body. For reasons detailed below, the as-cast body does not usually readily disengage from the die cavity, and pressure, applied to localized portions of the as-cast body, is required to separate the as-cast body from the die mold cavity. This localized pressure is applied by one or more ejector pins (not shown) which advance outwardly from die portion 20 to eject the as-cast body. The temperature of the solidified as-cast body will typically be in the temperature range of 180 C. to 260 C. when it is removed from die members 18, 20.

[0026] Those of skill in the die casting arts will recognize that the die casting of aluminum alloys, and particularly the casting of thin wall aluminum castings, requires some modifications to the simplified representation shown in FIG. 2 and the correspondingly simplified description provided above. For example, molten aluminum is highly reactive with air and so all handling and transfer of molten aluminum should be conducted under vacuum or a protective atmosphere. Also, the die cavity and associated gate and runner system will preferably be evacuated and maintained under vacuum, both to facilitate die filling and also to minimize, and preferably eliminate, any entrainment or entrapment of air within the casting. Other die features, such as squeeze pins, intended to eliminate local shrinkage and porosity, may also be incorporated into the die portions. These and other features not explicitly recited but known to those of skill in the die casting arts are intended to be comprehended by the above discussion.

[0027] Die portions are commonly fabricated of hardened steel. At common aluminum alloy die casting temperatures, iron is soluble in pure, molten, aluminum in amounts of up to about 1.5 wt %. This, coupled with the opportunity for die erosion during the rapid metal flow as the aluminum alloy is propelled into the die may result in some dissolution of the die material. On solidifying, the aluminum alloy may then locally weld, or solder, to the die surfaces, creating problems in ejecting the as-cast body from the die cavity. Again, the as-cast body will comprise the intended part or casting and any appended features originating from metal retained in gates, runners and overflow features. If severe, the welding of the part to the die may require applying high ejection pressures to remove the part from the die, potentially resulting in damage to the casting. Damage to the casting is particularly probable if the casting has a thin wall which, under excessive ejector pressure, may be subjected to stresses in excess of its yield strength. It will be appreciated that the casting will generally be ejected at a temperature appreciably above ambient temperature, at between about 180 C. to 260 C. in the above example, so that its yield strength will be reduced relative to its ambient temperature yield strength.

[0028] Aluminum alloys in which magnesium is a major alloying element offer attractive combinations of ambient temperature strength and ductility in the as-cast condition, and so are appealing candidates for thin wall, cast components such as described and illustrated above. However, aluminum-magnesium based alloys are not widely used for die casting or squeeze casting because of their propensity for die soldering.

[0029] One approach which may be used to reduce die soldering is to employ aluminum alloys of higher iron content so that the ability of the cast alloy to absorb and dissolve additional iron from the die surfaces is limited. However iron, which forms ductility-reducing, complex intermetallic compounds, is generally considered an undesirable impurity in aluminum alloys, rather than an alloying element, and is generally maintained at 0.25 wt. % and below, significantly less than the maximum solubility of about 1.5 wt. %. Some die-soldering benefit may be achieved in alloys containing iron contents of about 0.2 wt. % when coupled with manganese additions of up to 0.8 wt. %, but these alloys may be more difficult to cast, have some susceptibility to stress corrosion cracking and exhibit properties which vary considerably with the thickness of the cast part. As an example, one manganese-containing alloy may contain about 5 wt. % magnesium, about 2 wt. % silicon, about 0.6 wt. % manganese with iron limited to 0.2 wt. % and balance substantially aluminum. In castings of this composition, with wall thicknesses of less than 2 millimeters, the reported as-cast yield strength is greater than 220 MPa with an as-cast, tensile, total elongation of 10-15%. For castings of like composition with wall thicknesses ranging from 2 to 4 millimeters, the as-cast yield strength ranges from 160 MPa to 220 MPa and the as-cast, tensile, total elongation is from 12% to 18%.

[0030] Welding or soldering may be also reduced or minimized by applying lubricants or parting agents to the die. But such parting agents must be applied to all portion of the die which encounter the molten aluminum alloy after every part is removed and prior to injection of the next shot and may, in some formulations build up on the die with repeated used requiring that the dies be cleaned of residue after some number of casings have been made.

[0031] An alternative aluminum-magnesium based die casting alloy has been determined to offer significant advantages over alternative compositions particularly in castings 3 millimeters or less in thickness. Particular advantages include: the tendency for die soldering is reduced; good castability is maintained so that thin and medium wall castings may readily be achieved; superior mechanical properties are obtained in thin wall castings; and, good corrosion resistance is exhibited in wet and marine environments. Suitable compositions (all in weight percent) which develop such properties include 2 to 15% magnesium, 0.2 to 3% silicon, 0.05 to 0.5% chromium, 0.05 to 0.5% manganese, less than 0.2% iron, 0.05 to 0.2% titanium, other metal elements (as impurities) less than 0.5%, and the balance aluminum. Cast billets of such alloys have demonstrated, at ambient temperature, tensile yield strength values of 250 MPa and higher, total elongation values of 15% and higher, and ultimate strengths of 280 MPa. Such properties are obtained in as-cast sections of up to 3 millimeters in thickness.

[0032] Preferred compositions of the alloys are, by weight, 5.0 to 9.0% magnesium, 0.25 to 0.35% chromium, 0.15 to 0.35% manganese, 1.0 to 3.0% silicon, 0.05 to 0.15% titanium, less than 0.15% iron, less than 0.01% copper, less than 0.003% phosphorus, less than 0.03% strontium, and the balance substantially all aluminum. Although such theory is not relied on, the contributions of the various alloying elements are believed to be: magnesium provides solution strengthening of the alloy; chromium and manganese both enhance the microstructure of a cast alloy article and are found to effectively reduce the solubility of iron in the alloy, reducing reactivity of the alloy with a casting die and unwanted die-soldering; iron is limited to avoid inter-metallic phases which reduce the ductility of cast products; titanium serves as a grain-refiner to improve ductility of cast products and reduce hot crack formation in cast products; silicon forms an Mg.sub.2Si eutectic phase that inhibits formation of the beta-phase (Al.sub.3Mg.sub.2) which tends to precipitate at grain boundaries and promote stress corrosion cracking.

[0033] Although it is preferred, for avoidance of process complexity, that the above-described alloys exhibit high strength and good ductility in the as-cast condition, the mechanical properties, particularly the strength properties may be enhanced with a subsequent heat treatment to promote precipitation hardening. A suitable heat treatment, appropriate to the compositions described above, involves subjecting the casting to a solutionizing treatment consisting of heating the casting to a temperature between about 480 C. to about 540 C. and holding the casting at this temperature for between 2 and 6 hours. This heat treatment will dissolve at least some of the separated phases in the microstructure of the cast article so that the elemental constituents are taken into solution to render a more homogeneous composition and a solutionized microstructure. At the conclusion of the solutionizing treatment the casting should be rapidly cooled to a temperature of between 50 C. and 70 C., either by water quenching or by forced air cooling to freeze in the solutionized microstructure. The casting may then be precipitation hardened by heating to a temperature of between 180 C. and 250 C. and maintaining the casting at this temperature for between 3 to 8 hours to enable precipitation of the elements in solution, followed by air cooling to ambient temperature. Following this heat treatment process the cast article may have a yield strength of 280 MPa, an ultimate tensile strength of 320 MPa, and 7 to 10% total elongation.

[0034] Thus, these chromium-containing, aluminum-base alloys are found to prevent both dissolution of iron and soldering of the casting die, and to retain fluidity sufficient for the fabrication of spatially-extensive thin-wall die castings. These chromium-containing aluminum-base alloys are thus suited for forming complex-shape castings with thin sections and developing, in a cast article, a combination of good strength and ductility coupled with good resistance to corrosion in wet and marine environments. Further, the surfaces of the cast article may be readily finished or polished for decorative purposes.