Patent Publication Number: US-2019177818-A1

Title: Magnesium-containing, aluminum-based alloy for thin-wall castings

Description:
TECHNICAL FIELD 
     Aluminum-based alloys containing 2 to 15 weight percent magnesium, 0.2 to 3 weight percent silicon and small amounts of chromium, manganese, and titanium are useful in high pressure die casting of thin wall products such as automotive vehicle body structural castings. The alloy provides a good combination of strength and ductility and resists the solution of iron from die-soldering during processing, such as high pressure die casting of the articles. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 9,352,388, titled “Integration of One Piece Door Inner Panel with Impact Beam”, issued May 31, 2016, and assigned to the assignee of the subject invention, describes a die casting process for manufacturing an inner door panel for an automotive vehicle side door that includes an integral impact beam. The co-inventors in that patent include a co-inventor of the subject invention. The &#39;388 patent reflects the need to make light weight articles of manufacture using light weight metal alloys and polymers. 
     However, there is a continuing need to provide high strength, ductile alloys and high volume production processes for forming thin-wall articles of manufacture such as body structural castings for automotive vehicles and other thin wall section (up to about two to five millimeters in thickness) articles of manufacture. A variety of aluminum-base alloys have been considered and used in applications such as forming automotive vehicle structural castings, for example, shock absorber towers. But the complex shapes of the cast structures has required the use of steel dies to define a complex three-dimensional die cavity, to receive a charged volume of molten aluminum-based alloy and to sustain a substantial pressure exerted on the alloy as it assumes the shape of the complex cavity surfaces and solidifies. 
     Molten aluminum alloys have tended to dissolve iron from the die surfaces, which both alters the composition of the alloy and dramatically reduces the life of the die. Further, the iron dissolved in the newly formed article adversely affects its ductility in its intended use. 
     Thus, there remains a need for improvements in compositions of aluminum-based alloys and processes for forming such high performance thin-wall cast articles without processing problems or issues. 
     SUMMARY OF THE INVENTION 
     A magnesium-containing, aluminum-based casting alloy is provided with superior castability, strength, and ductility. It is particularly useful in making thin-wall castings with extended two-dimensional shapes. For example, the alloy is useful in repeatedly making castings characterized by substantial surface regions with thicknesses of about three millimeters or less. In the examples of body panels and body structures for automotive vehicles, the complexly shaped members often have dimensions of fifty centimeters to one hundred fifty centimeters in length and width, but with substantial regions in which they are only a few millimeters in thickness. 
     In order to repeatedly form such castings to a solid cast shape of closely specified dimensions, molten charges of the aluminum alloy are injected under suitable pressure into a mold cavity or die cavity (or cavities) defined by facing, tightly-closed complementary steel die members or die members of other ferrous metal alloy composition. The mold cavity is heated to a temperature range to accommodate the molten aluminum alloy and to solidify and form the desired casting shape. The aluminum-based casting alloy must be heated to a temperature at which it is in a suitably fluid molten state and then forced into a vented die cavity so that the liquid aluminum alloy fully fills each thin section of the die cavity and assumes the shape defined by each surface of the die cavity. The molten alloy must not chemically react with the ferrous surface or adhere or solder to it. The steel die is then cooled to solidify the aluminum alloy shape and the hot solid cast shape must be removed from the die cavity without significantly altering the composition of the alloy or the die cavity surfaces. The subject aluminum-based alloy compositions have been devised in order to repeatedly accomplish the casting process and repeatedly produce complexly shaped cast articles with desired physical properties. 
     The composition of the aluminum-based alloy (in weight percent) is 2 to 15% magnesium, 0.2 to 3% silicon, 0.05 to 0.5% chromium, 0.05 to 0.5% manganese, 0.05 to 0.2% titanium, less than 0.2% iron, other metal elements less than 0.5%, and the balance substantially all aluminum. Cast billets of such alloys under as-cast condition have demonstrated tensile yield strength values of 250 MPa and higher, tensile total elongation values at fracture of 15% and higher, and ultimate strength values of 280 MPa. Such properties are often obtained without requiring heat treatment of a newly formed casting. But, if desired or necessary, properties of the cast articles can be enhanced by a further precipitation-hardening process step. Melts of these alloy compositions provide good castability properties and are useful in forming thin-wall castings and medium-wall castings. 
     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. Magnesium provides solution strengthening of the alloy. The chromium and manganese contents enhance the microstructure of a cast alloy and are found to effectively reduce the solubility of iron in the alloy, reducing reactivity of the molten alloy with a casting die and unwanted die-soldering. The iron content of the alloy is limited to avoid inter-metallic phases which reduce ductility of a cast product. The presence of titanium serves as a grain-refiner to improve castability and ductility of cast products and reduce hot crack formation in cast products. And the silicon content forms a Mg 2 Si eutectic phase with lamellar morphology that contributes to the ductility of the cast products. 
     The subject aluminum-base alloys with their chromium content are found to prevent both dissolution of iron and soldering of the casting die, and to retain fluidity, in high-pressure die-casting and squeeze-casting processes, and like pressurized casting processes, to form complexly-shaped cast aluminum-magnesium alloy products and with thin wall sections in the castings. Cast products of the subject compositions display a combination of good strength and ductility. They also provide good resistance to corrosion in wet and marine environments. And surfaces of resulting castings are readily finished or polished for decorative purposes. 
     As an example, the subject magnesium-containing, aluminum-based alloy may be used in a pressurized casting step such as high-pressure die-casting to make castings of complex shapes such as automotive vehicle body shock towers (for containing a shock absorber). Shock towers are load-bearing shapes with deep-rib structural members. But the practice of the invention is not limited to making shock towers. The subject alloy may be used in making other complexly shaped articles with integral thin structural portions for both vehicle applications and non-vehicle applications. 
     In the forming of such high performance, thin-wall structural castings, the subject alloy may be used in a high pressure die-casting step. The die casting step may be performed by pushing a predetermined volume of the subject molten aluminum-magnesium alloy, at a temperature in the range of about 670° C. to 730° C., into a heated die cavity, defined by separable, facing, complementary die members, carried in a suitable press structure, for forming the casting shape. Preferably, the die members are maintained at a predetermined temperature in the range of 180° C. to 230° C. for the liquid metal charging step. A predetermined intensification pressure is maintained on the molten alloy as it is cooled and solidified in the die cavity. Depending on the complexity and thinness of the casting, it may also be desirable to use a squeeze pin in the casting apparatus to minimize or eliminate local shrinkage of the cast aluminum alloy. Since porosity may form in the cast metal due to air-entrapment, it may also be preferred to employ a vacuum/valve system to remove air and eliminate porosity in the thin wall structural castings. 
     After the structural casting is solidified and cooled to the die temperature, it is ejected from its die. In many applications, the thin-wall as-cast structure has the desired physical and metallurgical properties for its intended application. The cast structure may be simply subjected to any finishing processing (like machining) for removal of casting runners or the like, and assembled or used with other prepared members. 
     If it is necessary to further enhance the performance or properties of the casting, it may be subjected to a heat-treatment step (after its removal from the casting die) to promote solution of separate phases and subsequent precipitation hardening by a suitable artificial aging process as follows. The as-cast structural shape of the subject aluminum-based alloy is heated (suitably in air) to a temperature of about 480° C. to about 540° C. for a predetermined period of two to six hours to obtain suitable dissolution of at least some separated phases in the microstructure of the cast article and render a more homogeneous composition and a solutionized solid microstructure. The heated die-cast shape is then rapidly cooled to about 50° C. to 70° C. (preferably below 100° C.) using water or compressed air to quench the solutionized microstructure. After such cooling, the cast part is reheated in a furnace, pre-set at 180° C. to 250° C., for a predetermined period of three to eight hours to form a desired precipitation-hardened microstructure. The thus-aged casting is cooled to an ambient temperature, or about 25° C., and is ready for any finish machining, or the like, and subsequent assembly with other body structures. 
     Such forming of the aluminum-magnesium alloy can produce a thin-wall structural casting of complex shape that requires minimal machining to obtain a specified finish shape. The subject aluminum-magnesium alloy can be used in forming many thin-wall die cast articles for automotive vehicles and for other applications. The cast parts may, for example, be of complex three-dimensional shapes with thicknesses in the range of two to five millimeters over their entire cross-sections or over a major portion of their cross-sections. Examples of other thin-wall vehicle body castings include, for example, front body hinge pillar castings, front shock tower castings, front torque box castings, cap structures for rear shock absorbers, tunnel structures for the passenger compartments, and rear rail structures. 
     Other practices and advantages of the invention will be apparent from further illustrative examples presented below in this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a die cast shock tower. 
         FIG. 2  is a simplified view of a die casting machine, shown in perspective, partial cutaway view. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 . 
     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. 
     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. 
     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. 
     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%. 
     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. 
     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. 
     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 2 Si eutectic phase that inhibits formation of the beta-phase (Al 3 Mg 2 ) which tends to precipitate at grain boundaries and promote stress corrosion cracking. 
     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. 
     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.