Aluminum alloy for forming an axisymmetric article

Abstract

A vehicle wheel, or other axisymmetric shaped article, is formed of an aluminum-based alloy by a combination of a liquid forging step of a pre-form shape of the wheel and a subsequent solid-state flow forming step to complete the specified shape of the wheel. An aluminum-based alloy, containing specified amounts of zinc, silicon, and magnesium is devised for use in the forming process. The composition of the aluminum-based alloy is devised to facilitate the performance of each forming step of the article and the mechanical properties of the final shaped product.

Claims

1. A method of forming an integral wheel structure, the integral wheel structure comprising a hub having an axis of rotation, a plurality of equal length spokes extending radially outwardly from the hub, in a plane that is perpendicular to the axis of rotation of the hub, to a circumferential rim at the radially outer ends of the spokes, the circumferential rim having a predetermined width extending transverse to the plane of the spokes and having a thickness and shape for sealing engagement with a flexible pneumatic tire to be placed on the rim for use of the wheel structure, the integral wheel structure being formed of an aluminum-based alloy by the method comprising: injecting a mold-filling volume of a molten aluminum-based alloy into a mold cavity, the cavity having surfaces formed by separable, facing mold members 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 3.0-4.0% zinc, 1.8-2.5% silicon, 1.3-2.0% magnesium, 0.2-0.5% manganese, 0.15-0.2% titanium, 0.05% maximum copper, 0.1% maximum iron, and the balance aluminum, the surfaces of the cavity substantially defining predetermined shapes of the hub and spokes in the integral wheel structure to be formed, and a pre-form shape of the circumferential rim that has a larger thickness and shorter width than the desired shape of the rim in the integral wheel structure to be formed; cooling the molten aluminum-based alloy to form a solid pre-form wheel shape and removing the solid pre-form wheel shape from the facing mold members; and applying a mechanical forming force to the pre-form shape of the circumferential rim of the solid pre-form wheel shape at a predetermined temperature above ambient temperature to obtain solid-state flow of the aluminum-based alloy metal and to form the circumferential rim into its intended shape in the integral wheel structure.

2. A method of forming an integral wheel structure as stated in claim 1 in which the temperature of the volume of molten aluminum-based alloy injected into the mold cavity is in the range of 650 C. to 750 C.

3. A method of forming an integral wheel structure as stated in claim 1 in which the facing mold members are maintained at a temperature in the range of 180 C. to 260 C. during the formation of the solid pre-form wheel shape.

4. A method of forming an integral wheel structure as stated in claim 1 in which the facing mold members are maintained at a temperature in the range of 180 C. to 260 C. during the formation of the solid pre-form wheel shape and the solid pre-form wheel shape is removed from the mold members at a temperature in the range of 180 C. to 260 C.

5. A method of forming an integral wheel structure as stated in claim 1 in which the rim portion of the solid pre-form wheel shape removed from the mold cavity has a yield strength of at least 139 MPa and a tensile strength of at least 249 MPa and an elongation of two percent or higher, each determined on a test specimen at 25 C.

6. A method of forming an integral wheel structure as stated in claim 1 in which the solid pre-form wheel shape is subjected to a softening heat-treatment at 450 C. to 500 C. in air before solid state flow forming of the rim-forming portion of the solid pre-form wheel shape.

7. A method of forming an integral wheel structure as stated in claim 6 in which the softened solid pre-form wheel shape has a yield strength of at least 150 MPa and a tensile strength of at least 287 MPa and an elongation of at least twenty one percent, each determined on a test specimen at 25 C.

8. A method of forming an integral wheel structure as stated in claim 1 in which at least the rim-forming portion of the solid pre-form wheel shape is heated to a temperature in the range of 250 C. to 370 C. and the forming force is applied to the heated rim portion of the solid pre-form wheel shape.

9. A method of forming an integral wheel structure as stated in claim 1 in which, following the forming of the rim of the integral wheel structure, the integral wheel structure is subjected to hardening heat-treatment in air at a temperature in the range of 480 C. to 530 C. and is thereafter quenched in water.

10. A method of forming an integral wheel structure as stated in claim 9 in which integral wheel structure has a yield strength of at least 300 MPa and a tensile strength of at least 350 MPa and an elongation of at least ten percent, each determined on a test specimen at 25 C.

11. A method of forming an integral wheel structure, the integral wheel structure comprising a hub having an axis of rotation, a plurality of equal length spokes extending radially outwardly from the hub, in a plane that is perpendicular to the axis of rotation of the hub, to a circumferential rim at the radially outer ends of the spokes, the circumferential rim having a predetermined width, extending transverse to the plane of the spokes, and having a thickness and shape for sealing engagement with a flexible pneumatic tire to be placed on the rim for use of the integral wheel structure, the integral wheel structure being formed of an aluminum-based alloy by the method comprising: injecting a mold-filling volume of a molten aluminum-based alloy into a cavity of a mold, the mold being maintained at a temperature in the range of 180 C. to 260 C., the molten aluminum-based alloy being at a temperature in the range of 650 C. to 750 C., 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 3.0-4.0% zinc, 1.8-2.5% silicon, 1.3-2.0% magnesium, 0.2-0.5% manganese, 0.15-0.2% titanium, 0.05% maximum copper, 0.1% maximum iron, and the balance aluminum, the surfaces of the cavity substantially defining predetermined shapes of the hub and spokes and a pre-form shape of the circumferential rim that has a larger thickness and shorter width than the desired shape of the rim in the integral wheel structure to be formed; cooling the molten aluminum-based alloy to form a solid wheel pre-form shape and removing the solid wheel pre-form shape from the facing mold members; heating the solid wheel pre-form shape in air to soften the solid wheel pre-form shape for shaping of the circumferential rim of the wheel; applying a mechanical forming force to the pre-form shape of the circumferential rim of the wheel at a predetermined temperature above ambient temperature to obtain solid-state flow of the aluminum-based alloy metal and to form the circumferential rim into its intended shape in the integral wheel structure.

12. A method of forming an integral wheel structure as stated in claim 11 in which the facing mold members are maintained at a temperature in the range of 180 C. to 260 C. during the formation of the solid wheel pre-form shape and the solid wheel pre-form shape is removed from the mold members at a temperature in the range of 180 C. to 260 C.

13. A method of forming an integral wheel structure as stated in claim 11 in which the solid wheel pre-form shape is subjected to a softening heat-treatment at 450 C. to 500 C. in air and then cooled in air before forming of the rim-forming portion of the solid wheel pre-form shape.

14. A method of forming an integral wheel structure as stated in claim 11 in which at least the rim-forming portion of the solid pre-form wheel shape is heated to a temperature in the range of 250 C. to 370 C. and the forming force is applied to the heated rim portion of the solid wheel pre-form shape.

15. A method of forming an integral wheel structure as stated in claim 11 in which at least the pre-form shape of the circumferential rim is heated to a temperature in the range of 250 C. to 370 C., the wheel is rotated, and forming rollers are pressed against surfaces of the circumferential rim to progressively deform it into its intended rim shape for the integral wheel structure.

16. A method of forming an integral wheel structure as stated in claim 11 in which, following the forming of the rim of the integral wheel structure, the integral wheel structure is subjected to hardening heat-treatment in air at a temperature in the range of 480 C. to 530 C. and is thereafter quenched in water.

17. A method of forming an integral article of manufacture having an axisymmetrical shape, the integral article comprising a center portion having a central axis, a body structure extending radially outwardly from the center portion, in a plane that is perpendicular to the central axis of the center portion, to a circumferential perimeter, and an axisymmetric member, axisymmetric with respect to the axis of the center portion, extending a predetermined length outwardly from and transverse to the radial plane of the body structure, the axisymmetric member having a thickness and shape, the integral article being formed of an aluminum-based alloy by the method comprising: injecting a volume of a molten aluminum-based alloy into a mold cavity and filling the mold cavity, the mold cavity having surfaces formed by separable, facing mold members 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 3.0-4.0% zinc, 1.8-2.5% silicon, 1.3-2.0% magnesium, 0.2-0.5% manganese, 0.15-0.2% titanium, 0.05% maximum copper, 0.1% maximum iron, and the balance aluminum, the surfaces of the mold cavity substantially defining predetermined shapes of the center portion and body structure, and a pre-form shape of the axisymmetric member, the pre-form shape having a larger thickness and shorter length than the desired shape of the axisymmetric member in the integral article; cooling the molten aluminum-based alloy to form a solid article pre-form shape and removing the solid article pre-form shape from the facing mold members; and applying a mechanical forming force to the pre-form shape of the axisymmetric member of the article at a predetermined temperature above ambient temperature to obtain progressive incremental solid-state flow of the aluminum-based alloy metal to form the axisymmetric member into its intended final shape in the formed integral article.

18. A method of forming an integral article as stated in claim 17 in which the temperature of the volume of molten aluminum-based alloy injected into the mold cavity is in the range of 650 C. to 750 C.

19. A method of forming an integral article as stated in claim 17 in which the facing mold members are maintained at a temperature in the range of 180 C. to 260 C. during the formation of the solid article pre-form shape.

20. A method of forming an integral article as stated in claim 17 in which at least the axisymmetric member portion of the article shape is heated to a temperature in the range of 250 C. to 370 C. and the forming force is applied to the heated axisymmetric member portion of the solid article pre-form shape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a fragmentary, side, cross-sectional view of a liquid forged preform of a vehicle wheel.

(2) FIG. 2 is a front and cross-sectional view of a vertically-actuated press with upper and lower mold pieces with mating cavities for liquid forging of a precursor wheel structure. Molten aluminum alloy is delivered from the right side of the illustrated press, through the lower mold member into the wheel shape cavity defined by the press-closed mold members. Pressure is applied to the metal in the closed molds by a plunger acting upwardly through the shot sleeve.

(3) FIG. 3 is a side cross sectional illustration of a portion of the liquid forged precursor shape of the wheel with an inner roller working on the inside of its preformed rim structure and with an outer roller applying flow forming pressure to the outer side of the rim structure to cooperatively incrementally flow the solid-state aluminum alloy material into its final rim shape.

(4) FIG. 4 is a side cross-sectional view of a portion of the vehicle wheel with the rim portion flow formed to its intended shape.

DESCRIPTION OF PREFERRED EMBODIMENTS

(5) FIG. 1 is a diametrical cross-section of the upper half of a liquid forged, pre-formed, integral wheel structure 10 of an aluminum-based alloy, which is formulated to be processed in accordance with this invention to form wheels for an automotive vehicle. The liquid forged preformed wheel structure 10 comprises a central hub 12, several radial spokes 14, and a precursor or pre-formed rim structure 16. The respective members of the pre-formed wheel structure 10 are integrally formed of an LF-FF aluminum-based alloy. The central hub 12 is shaped to define an axis of rotation of the wheel. Several radially extending, equal length spokes 14 (for example, five to eight spokes) lie in a plane transverse to the axis of rotation of hub 12. One end of each of the spokes is integrally formed with the outer surface of the hub 12. The radially outward end of each of the spokes is integrally connected with a circumferential pre-formed rim structure 16. In this illustration, circumferential pre-formed rim structure 16 extends axially, in one direction, from the plane of the spokes 14. In the final shape of the wheel, the radially outward surface 18 and radially inward surface 20 of pre-formed rim structure 16 will be re-sized and re-shaped to a final shape to sealingly engage a pneumatic tire in the use of the wheel. But in this liquid forged, pre-form structure of wheel 10, the pre-formed rim structure 16 has not attained its final shape. The final shape and the physical properties of the rim are important in the final wheel structure. It is preferred to form a precursor shape of the rim portion by liquid forging and to subsequently heat-treat the pre-form wheel shape 10 to soften the pre-formed rim structure 16 portion and to flow form and subsequently heat-treat the precursor shape of the pre-formed rim structure portion to its final shape and physical properties.

(6) The pre-form structure of wheel 10 is formed by liquid forging using an aluminum-based alloy that facilitates the forming of the pre-form wheel structure 10 and the subsequent solid-state flow forming of its pre-form rim portion 16 to a final rim shape (like rim 116 in FIG. 4). The elemental composition of the aluminum-based alloy, by weight, is 3.0-4.0% zinc, 1.8-2.5% silicon, 1.3-2.0% magnesium, 0.2-0.5% manganese, 0.15-0.2% titanium, 0.05% maximum copper, 0.1% maximum iron, 0.05% maximum other individual elements or materials, 0.15% maximum total other elements or materials, and the balance aluminum. Again, the family of aluminum-based alloys having such compositions is referred to in this specification as LF-FF aluminum alloys.

(7) FIG. 2 illustrates a front, cross sectional view of equipment and tooling for the making of the liquid forged pre-form structure of wheel 10. In FIG. 2, vertically-actuated press 50 comprises lower platen 52 which carries four vertical columns 54 (the front two columns being visible in FIG. 2). Press 50 further comprises an upper platen 56 which is movable, by means not illustrated, on vertical columns 54. A lower mold section 58 is affixed to lower platen 52 and a complementary upper mold section 60 is attached by connectors 62 to upper platen 56.

(8) Complementary and facing cavity portions for the liquid forging of pre-form wheel structure 10 are formed in lower mold section 58 and upper mold section 60. When the mold sections 58, 60 are closed, their complementary cavities define cavity 64 for the liquid forging of pre-form wheel shape 10. Mold sections 58, 60 contain suitable passages (not illustrated) for the flow of a liquid oil-base heating medium which is managed to maintain both of the mold sections at a temperature in the range of 180 C. to 260 C. The liquid oil-base is circulated from outside of mold sections 58, 60, and its temperature is closely controlled. A specific heating medium temperature is predetermined for a specific article shape and LF-FF aluminum alloy and preferably the temperature of the mold members 58, 60 is maintained within 5 C. of this predetermined temperature (in the range of 180 C. to 260 C.) by control of the heating oil temperature. Upper mold section 60 may also contain ejector pins (not shown) for removal of the liquid forged wheel or other article from the mold members when the article has solidified and been cooled to about the mold temperature.

(9) A molten LF-FF aluminum alloy is prepared under a suitably protective atmosphere at a suitable predetermined temperature in the range of about 650 C. to 750 C. A more specific or narrow temperature range for the molten alloy may be determined for each liquid forging job depending on the specific alloy composition, the shape of the mold cavity 64 and the mass of the liquid forged wheel structure 10 to be formed. A volume of the molten LF-FF alloy is transferred from a holding vessel through a suitably enclosed melt transfer passage 66 into shot sleeve 68. Access to the shot sleeve 68 is than closed and piston (or punch) 70 is actuated to force the charge of molten aluminum alloy from shot sleeve 68 into cavity 64 formed by closed molds 58, 60. Cavity 64 may be suitably vented, if necessary, to enable liquid aluminum alloy to fully fill the cavity 64 and engage all surfaces of the cavity. Piston 70 may be mechanically or hydraulically actuated by means not shown, and is capable of exerting a suitable predetermined liquid forging force (or pressure) on the molten aluminum LF-FF alloy that is pushed into cavity 64. A forging force of about 3,000 tons is typically applied by piston 70 in this liquid forging step. This application of liquid forging force is maintained until the pre-form wheel shape has solidified for opening of the mold members 58, 60 and removal of the liquid forged pre-form wheel 10 from them. The temperature of the solidified wheel shape or other articles will typically be in the temperature range of 180 C. to 260 C. when it is removed from mold members 58, 60.

(10) In the development of a specific LF-FF aluminum alloy and liquid forging process for a specific preform wheel shape (e.g., like wheel shape 10 in FIG. 1) it will be desirable to determine the physical properties of the liquid forged pre-form wheel structure. This may be done, for example, by cooling a liquid forged wheel sample to an ambient temperature to determine mechanical properties of the pre-formed wheel specimen. In general, it is preferred that a test specimen from at least the rim portion of the liquid forged wheel pre-form shape be removed and that its yield strength and tensile strength be determined at about room temperature (for example, 18 C.-28 C.). Preferably, both the outer rim portion and inner rim portion of the solid wheel shape removed from the mold cavity has a yield strength of at least 139 MPa and a tensile strength of at least 249 MPa and an elongation of two percent or higher, each determined on a test specimen at 25 C.

(11) At this stage of the processing, the wheel preform shape 10 (except for the pre-form rim structure portion 16) is substantially formed to its final shape and dimensions. In the event that any finish machining is required, it may be performed at this time, or subsequently, after the rim portion has been re-shaped by flow forming.

(12) The pre-form rim structure portion 16 of the liquid forged wheel pre-form 10 shape now requires thinning and lengthening. This may be accomplished, for example, by simultaneously exerting roller pressure on both the outer radial surface 18 and the inner radial surface 20 of the pre-form rim structure 16 as the wheel is rotated, a spinning flow forming process, to reach a final (or near to final) shape as depicted at rim portion 116 in FIG. 4. As stated, flow forming forces will be applied to its radially outer surface 18 and radially inward surface 20. Before the solid-state flow forming of pre-form rim structure 16 is started, it is preferred to subject the preformed wheel 10, or at least pre-formed rim portion 16, to a softening heat-treatment. This is accomplished by heating the pre-formed wheel structure 10 in air to a temperature in the range of about 450 C. to about 500 C., and upon reaching this temperature, re-cooling the pre-formed wheel structure in ambient air.

(13) Following the softening heat-treatment process step, it may be desirable to determine the mechanical properties of at least the heat-treated rim portion of the pre-form wheel shape. In general, it is preferred that a test specimen obtained from at least the rim portion of the liquid forged wheel pre-form shape be removed and its yield strength and tensile strength be determined. Preferably, the rim portion of the solid wheel shape removed from the mold cavity has a yield strength of at least 152 MPa and a tensile strength of at least 287 MPa and an elongation of twenty one percent or higher, each determined on a test specimen at 25 C. The rim portion 16 of the pre-form wheel structure 10 is now much more formable and is ready for the flow forming step to deform the solid-state rim to its final shape and dimensions.

(14) The pre-form wheel structure 10, or at least rim portion 16, is heated in air to a temperature in the range of 300 C. to 420 C. The preheated wheel structure 10 with pre-formed rim portion 16 is then suitably supported for rotation in an ambient workplace environment and spun and subjected to solid-state flow forming of the rim to obtain a finished rim shape as illustrated, for example, as rim structure 116 in FIG. 4. The heated pre-form wheel structure 110 (referring to FIG. 3) may be placed on a suitable support, such as on a spindle (not illustrated), that engages the wheel 110 at its hollow hub 112 for spinning at a predetermined rate of rotations per minute. Then, a first roller 122, serving as a mandrel, may be pressed against the preheated inner rim surface 120 and a second working roller 124 pressed against the outer rim surface 118 with sufficient force and relative movement along the surfaces 118, 120 of the rim 116 to reshape the rim structure from the pre-form rim shape 16 of FIG. 1 to final (or near-final) rim shape 116 of FIG. 4. The rollers 122, 124 are pressed into and moved along the rim surfaces at predetermined rates of advancement of, for example, millimeters of penetration per rotation of the wheel. In FIG. 3, a portion 116 has been re-shaped, and a portion 16 remains to be re-shaped. This reshaping of original pre-form rim shape 16 is performed around the entire circumference of the original liquid forged pre-form rim 16, preferably starting from the spoke side of the pre-form rim 16, so as to reduce the thickness of the preformed rim, refine its shape, and to increase is axial length to obtain the finished shape of rim 116 of wheel 110 as illustrated in FIG. 4.

(15) In the wheel spinning method of flow forming the heated rim of the wheel, the wheel may, for example, be rotated at a rate of 1000 to 1400 rotations per minute, and the rollers may be fed into the rim surfaces at a rate of, for example, 0.25 to 0.80 millimeters per each rotation of the wheel. The solid-state flow forming of pre-form rim structure 16 (FIG. 1) to finished (or near finished) rim structure 116 (FIG. 4) may be accomplished using any suitable supporting and shaping tools for carefully incrementally spinning, and elongating and thinning the pre-form rim shape 16.

(16) In this example, the spokes 114 and hub 112 of wheel 110 are not further shaped by flow forming. Their final shape, or near-to-final shape, is attained in the liquid forging step.

(17) Following the solid-state flow-forming of the preform wheel structure 10 to obtain wheel structure 110, it is preferred to subject the wheel structure 110 to a T6 type hardening heat-treatment to impart the desired strength and hardness properties to the LF-FF aluminum alloy wheel structure. Further, any final trimming or machining of the wheel may be performed. In general, it is believed that the use of the specified LF-FF aluminum alloy permits suitable shaping of the wheel by the combination of the liquid forging step and the solid-state flow forming step and little, if any, machining and removal of aluminum alloy metal will be required.

(18) The formed wheel structure 110 is heated in air in a suitable furnace, or the like, to a temperature in the range of 480 C. to 530 C. for a period of, for example, fifteen to thirty-five minutes. The wheel 110 is then quenched into warm water at, e.g., about 70 C. The wheel, thus formed of the LF-FF aluminum-based alloy, has a yield strength of at least about 300 MPa and a tensile strength of at least about 350 MPa and an elongation of at least about 10 percent, measured at 25 C. These properties compare very favorably with wheels that are formed by the forging (and re-forging) of AA6061 workpieces.

(19) Thus, the use of the LF-FF aluminum-based alloy in combination with a suitable liquid forging step and a solid-state flow forming step is capable of producing light weight, high strength wheels for automotive vehicles. The use of the LF-FF aluminum-based alloy and the processing steps may also be beneficially used to make other axisymmetric articles having a projecting structure with a shape that is difficult to form by liquid forging in a mold cavity in which the other portions of the integral article can be formed in substantially their finished shapes and sizes.

(20) Intermediate heat-treatment steps enhance the processing of the alloy and the properties of the intermediate and final products. The required number of manufacturing steps is minimized. And the specified LF-FF aluminum alloy lends itself to the forming of wheels and other axisymmetric articles with exceptional physical properties and durability.