Patent Publication Number: US-2006000094-A1

Title: Forged aluminum vehicle wheel and associated method of manufacture and alloy

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an improved forged aluminum vehicle wheel and a method of making such a wheel out of a low copper bearing 7000 series aluminum alloy.  
      2. Description of the Prior Art  
      It has been known to employ aluminum alloys in the manufacture of vehicle wheels. U.S. Pat. No. 6,315,367 discloses a cast aluminum truck wheel which was said to eliminate the welding of a separately manufactured wheel disc and wheel rim.  
      It has long been known to employ aluminum alloy 6061 in the commercial manufacture of truck wheels. It has been known to create forged aluminum alloy wheels made from alloy 6061 by subjecting the forging to solution heat treatment followed by a water quench and artificial aging. It has also been known to suggest the use of casting alloy 356 in making wheels. See generally U.S. Pat. No. 4,316,637.  
      U.S. Pat. No. 4,345,360 discloses an extrudable aluminum alloy which is extruded, cut, deformed to the desired shape and welded such as by cold pressure welding.  
      The use of aluminum wheels on commercial vehicles was suggested in U.S. Pat. No. 5,026,122. See also the two-piece wheel disclosure of U.S. Pat. No. 5,740,609.  
      The use of aluminum alloy 6061 as well as 5454 and A356 was disclosed in U.S. Pat. No. 5,441,334. See also U.S. Pat. No. 5,210,948 which disclosed an aluminum alloy 6013 wheel for a track-laying vehicle (e.g., tank).  
      The use of copper bearing aluminum alloy 7075 has been disclosed for use in the generally C-shaped tread member on a non-pneumatic tire wheel combination. See U.S. Pat. No. 4,558,727.  
      Published U.S. patent application 2002/0003373 discloses the use of copper bearing aluminum alloys 7050 and 7075 in creating cold forged wheels and spun light alloy rims. The rim is said to be made of a 5000 series aluminum alloy which is said to have strength and light weight similar to a cold forged alloy rim.  
      U.S. Pat. No. 4,490,189 discloses stamping or forging of 2000, 6000 or 7000 series aluminum alloys, but does not relate to the vehicle wheels and focuses on certain sequences of thermal treatments.  
      In spite of the aforegoing disclosures, there remains a very real and substantial need for wheels having improved properties.  
     SUMMARY OF THE INVENTION  
      The present invention has met the hereinbefore described needs.  
      One embodiment of the method of making a forged aluminum vehicle wheel includes forging a low copper 7000 series aluminum alloy ingot into a wheel workpiece, initially cooling the forged wheel workpiece, machining the workpiece and surface-treating the workpiece.  
      The forged aluminum vehicle wheel made with a low copper bearing 7000 series alloy is characterized by improved fatigue properties as compared with 6061 wheels.  
      The invention includes a unique alloy suitable for use in the wheel.  
      It is an object of the present invention to provide a method of making a forged aluminum wheel and the resultant wheel which will have desired strength and decreased weight with similar fatigue life.  
      It is another object of the present invention to provide a method of making a forged aluminum vehicle wheel and the resultant wheel which exhibits improved fatigue properties.  
      It is a further object of the present invention to provide a method of making a forged aluminum vehicle heel which minimizes the post-forging distortion due to residual stresses established through prior art post-forming water quenching.  
      It is a further object of the present invention to provide modified thermal treatments which facilitate reduction in undesired distortion of the wheel.  
      It is another object of the invention to provide a method of making a forged low copper bearing 7000 series aluminum wheel which requires less post forming machining.  
      It is a further object of the invention to provide an alloy which may be employed in the method and wheel.  
      It is a further object of the present invention to provide such a method and associated forged aluminum wheel which will have increased strength.  
      These and other objects of the invention will be more fully understood from the following detailed description of the invention on reference to the illustrations appended hereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a flow diagram showing a first embodiment of the method of the present invention.  
       FIG. 2  is a flow diagram of another embodiment of the methods of the present invention.  
       FIG. 3  is a flow diagram of another embodiment of the methods of the present invention.  
       FIG. 4  is a flow diagram of another embodiment of the methods of the present invention.  
       FIG. 5  is a flow diagram of another embodiment of the methods of the present invention.  
       FIG. 6  is a flow diagram of another embodiment of the methods of the present invention.  
       FIG. 7  is an illustration of a plot of load vs. cycles to fatigue crack initiation comparing aluminum forged one-piece wheels made of 6061 alloy with aluminum forged one-piece wheels made of low copper bearing 7000 series aluminum alloy of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      As employed herein, the term “vehicle” means a motorized wheeled vehicle having pneumatic tires and being structured to ride on and off roads and shall expressly include, but not be limited to, automobiles, Class 1-8 trucks, sport utility vehicles, vans, pick-up trucks, buses, recreational vehicles, motorcycles, construction vehicles, trailers structured to be pulled by a motorized tractor, boat trailers and campers.  
      As employed herein, the term “wheel workpiece” means a workpiece which alone or when joined to one or more components will become a vehicle wheel.  
      As employed herein, the tern “low copper” means either (1) no copper or (2) copper being present in an amount under the recited ceiling.  
      All references to percentages herein unless expressly indicated to the contrary shall refer to weight percent.  
      The present invention is a forged aluminum vehicle wheel made from a low copper bearing 7000 series aluminum alloy and the method of making the same.  
      The method of the present invention involves making a forged aluminum vehicle wheel from a low copper bearing 7000 series aluminum alloy ingot into a wheel workpiece. Forging is followed by initial cooling at the rate of less than 30° F./sec and preferably less than 15° F./sec and most preferably about 0.1° F./sec to 4° F./sec followed by machining the wheel workpiece and surface treatment.  
      A preferred low copper bearing 7000 series aluminum alloy has a composition on a weight basis of about 3 to 7% zinc, about 0.5 to 2% magnesium, less than 1% manganese, less than 0.3% chromium, about 0.05 to 0.3% zirconium, less than 0.2% titanium, less than 0.2% copper and as impurities up to 0.4% iron and up to 0.35% silicon with the balance being aluminum.  
      Another preferred low copper bearing 7000 series aluminum alloy has a composition on a weight basis of about 3.6 to 4.6% zinc, about 0.7 to 1.5% magnesium, less than 0.7% manganese; less than 0.2% chromium, about 0.05 to 0.18% zirconium, less than 0.1% titanium, less than 0.1% copper and as impurities a total of iron and silicon up to about 0.5% with the balance being aluminum.  
      The most preferred low copper bearing 7000 series aluminum alloy will have the following composition on a weight basis: about 3.6 to 4.6% zinc; about 0.7 to 1.5% magnesium; about 0.1 to 0.5% manganese; about 0.06 to 0.2% chromium; about 0.05 to 0.18% zirconium; about 0.02 to 0.06% titanium; less than 0.1% copper, and as impurities a total of iron and silicon up to about 0.5% with the balance being aluminum. Copper may be present in about 0.02 to 0.1 weight percent.  
      One of the advantages of the present invention is that it does not require the traditional solution heat treatment followed by water quenching which tended to build up internal stresses in the wheel thereby contributing to distortion, the use of more material and the requirement of additional machining.  
      In the present invention, the initial post-forming cooling may be in the form of an air quench and may be through still or forced (e.g., fan or compressed) air. This initial cooling preferably cools the wheel workpiece to at least 400° F. and may cool it to ambient temperature. Natural aging may be effected for several days and preferable about one hour to seven days followed by artificial aging. The artificial aging may be effected over a period of about one to eight hours at a first temperature of about 200 to 250° F. and subsequently at about 290 to 360° F. for about two to sixteen hours.  
      Among the preferred alloys for use in the present invention are 7004, 7005, 7108, 7003, 7008, 7108A, 7018, 7019, 7019A, 7020 and 7021 aluminum alloys.  
      The low copper bearing 7000 series aluminum alloy temper employed in making the wrought wheel of the present invention preferably is T5 temper. If desired, other tempers such as T1, T6 and T7 may be used.  
      It will be appreciated that as well known to those skilled in the art, the wheel workpiece can be so structured as to become a one-piece wheel having an integrally formed wheel disc and surrounding rim. It can also be a separate disc and surrounding rim which will be joined to as by at least one securement selected from the group consisting of welding and mechanical fasteners to create a wheel structure. The low copper bearing 7000 series aluminum alloy of the present invention is readily weldable for this purpose.  
      After completion of artificial aging which effects the desired strengthening of the wheel or wheel workpiece, the wheel workpiece is machined, may be surface pre-stressed to impart a compressive residual stress state to enhance fatigue performance and consistency, is surface-finished and typically provided with a protective coating.  
      The protective coating may be a curable coating which may be cured at an elevated temperature which also may be a step of the artificial aging. Curing may, for example, be effected at about 300° F. to 380° F. for about ten to eighty minutes. The protective coating provides corrosion resistance.  
      Referring to  FIG. 1 , a first embodiment of the method of the present invention will be considered. An aluminum ingot  2  in the low copper bearing 7000 series aluminum alloy is subjected to forging  6  at about 200 to 1000° F., for example, to produce a wheel workpiece which is subsequently cooled by a forced air quench  10  which preferably reduces the temperature to about 100 to 400° F. after which the aluminum wheel workpiece is subjected to natural aging  12  which in the example illustrated may be for about one hour to seven days. Subsequently, the workpiece is subjected to artificial aging  14  which in the preferred approach is a two-step process with the first step  14 A being at about 200 to 250° F. for about one to eight hours and followed by a second step  14 B at about 290 to 360° F. for about two to sixteen hours with the total time of both artificial aging steps being about three to twenty-four hours. The wheel workpiece is then machined  20  to achieve the desired shape and is surface-finished  24  to achieve the desired appearance, for example, polished, textured or satin-finished. The wheel workpiece is then given a protective coating which, for example, may include surface preparation  28 A, cleaning and/or etching and/or anodizing, drying  28 B at about room temperature to 250° F., polymeric coating  28 C, for example, with an acrylic coating and curing the coating  28 D at about 300 to 380° F. for about ten to eighty minutes.  
      Turning now to another embodiment of the method of the present invention and referring to  FIG. 2 , there is shown an aluminum ingot  32  in the low copper bearing 7000 series aluminum alloy that is subjected to forging  36  at about 200 to 1000° F. to produce a wheel workpiece which is subsequently cooled by a forced air quench  40  which preferably reduces the temperature to about 100 to 400° F. after which the aluminum wheel workpiece is subjected to machining  42  to achieve the desired shape. Subsequently, the workpiece is subjected to artificial aging  44  which in the preferred approach is a tvo-step process with the first step  44 A being at about 200 to 250° F. for about one to eight hours and followed by a second step  44 B at about 290 to 360° F. for about two to sixteen hours with the total time of both artificial aging steps being about three to twenty-four hours. The wheel workpiece is then surface-finished  54  to achieve the desired appearance, for example, polished, textured or satin-finished. The wheel workpiece is then given a protective coating, within process steps including, for example, surface preparation  58 A, cleaning and/or etching and/or anodizing, drying  58 B at about room temperature to 250° F., polymeric coating  58 C, for example, with acrylic coating and curing the coating  58 D at about 300 to 380° F. for about ten to eighty minutes.  
      Turning now to another embodiment of the method of the present invention and referring to  FIG. 3 , there is shown an aluminum ingot  62  in the low copper bearing 7000 series aluminum alloy that is subjected to forging  66  at about 200 to 1000° F. to produce a wheel workpiece which is subsequently cooled by a forced air quench  70  which preferably reduces the temperature to about 100 to 400° F. after which the aluminum wheel workpiece is subjected to natural aging  72  which in the example illustrated may be for about one hour to seven days. Subsequently, the workpiece is subjected to artificial aging  74  which in the preferred approach is a two-step process with the first step  74 A being at about 200 to 250° F. for about one to eight hours and followed by a second step  74 B at about 290 to 360° F. for about two to sixteen hours with the total time of both artificial aging steps being about three to twenty-four hours. The wheel workpiece is then machined and pre-stressed  80  to achieve the desired shape and compressive residual stress state in the surface, and then is surface-finished  84  to achieve the desired appearance, for example, polished, textured or satin-finished. The wheel workpiece is then give a protective coating, with process steps comprising: surface preparation  88 A, for example, cleaning and/or etching and/or anodizing, drying  88 B at about room temperature to 250° F., polymeric coating  88 C. for example, with an acrylic coating and curing the coating  88 D at about 300 to 380° F. for about ten to eight minutes.  
      Turning now to another embodiment of the method of the present invention and referring to  FIG. 4 , there is shown an aluminum ingot  102  in the low copper bearing 7000 series aluminum alloy that is subjected to forging  106  at about 200 to 1000° F. to produce a wheel workpiece which is subsequently cooled by a forced air quench  110  which preferably reduces the temperature to about 100 to 400° F. after which the aluminum wheel workpiece is machined and pre-stressed  112  to achieve the desired shape and compressive residual stress state in the surface. Subsequently, the workpiece is subjected to artificial aging  114  which in the preferred approach is a two-step process with the first step  114 A being at about 200 to 250° F. for about one to eight hours and followed by a second step  114 B at about 290 to 360° F. for about two to sixteen hours with the total time of both artificial aging steps being about three to twenty-four hours. The wheel workpiece is then surface-finished  124  to achieve the desired appearance, for example, polished, textured or satin-finished. The wheel workpiece is then given a protective coating, with process steps comprising, for example, surface preparation  128 A, cleaning and/or etching and/or anodizing, drying  128 B at about room temperature to 250° F., polymeric coating  128 C, for example, with an acrylic coating and curing the coating  128 D at about 300 to 380° F. for about ten to eighty minutes.  
      Turning now to another embodiment of the method of the present invention and referring to  FIG. 5 , there is shown an aluminum ingot  132  in the low copper bearing 7000 series aluminum alloy that is subjected to forging  136  at about 200 to 1000° F. to produce a wheel workpiece which is subsequently cooled by a forced air quench  140  which preferably reduces the temperature to about 100 to 400° F. after which the aluminum wheel workpiece is machined  142  to achieve the desired shape. The wheel workpiece is then surface-finished  154  to achieve the desired appearance, for example, polished, textured or satin-finished. The wheel workpiece is then given a protective coating, with process steps comprising, for example, surface preparation  158 A, for example, cleaning and/or etching and/or anodizing, drying  158 B at about room temperature to 250° F., polymeric coating  158 C, for example, with an acrylic coating and curing the coating  158 D at about 300 to 380° F. for about ten to eighty minutes. This embodiment of the method of the present invention illustrates forged aluminum wheel made with a low copper bearing 7000 series aluminum alloy in the T1 temper.  
      Turning now to another embodiment of the method of the present invention and referring to  FIG. 6 , there is shown an aluminum ingot  162  in the low copper bearing 7000 series aluminum alloy that is subjected to forging  166  at about 200 to 1000° F. to produce a wheel workpiece which is subsequently cooled by a forced air quench  170  which preferably reduces the temperature to about 100 to 400° F. after which the aluminum wheel workpiece is machined and pre-stressed  172  to achieve the desired shape and compressive residual stress state in the surface. The wheel workpiece is then surface-finished  184  to achieve the desired appearance, for example, polished, textured or satin-finished. The wheel workpiece is then given a protective coating, with process steps comprising, for example, surface preparation  188 A, cleaning and/or etching and/or anodizing, drying  188 B at about room temperature to 250° F., polymeric coating  188 C, for example, with an acrylic coating and curing the coating  188 D at about 300 to 380° F. for about ten to eighty minutes. This embodiment of the method of the present invention shows the forged and pre-stressed aluminum wheel made with a low copper bearing 7000 series aluminum alloy in the T 1  temper.  
      It will be appreciated that as well known to those skilled in the art, the forged wheel workpiece may be one-piece forged or one-piece forged and spun, may be multiple-pieces welded and/or mechanically fastened, for example, with a stamped plate disc and rolled sheet rim, may be post-form quenched or cooled with still air, forced air, mist, spray or any other suitable means or media for achieving the cooling rate sought. It may be produced with or without a surface compressive residual stress state induced by pre-stressing. It may be produced with or without a surface finish. It may be produced with or without a protective coating, may have a polymeric protective coating such as, for example, an acrylic, polyester or other suitable material, a painted coating, a chrome plate coating or any other coating for the purpose of appearance and/or corrosion resistance. If desired, both a surface finish and a protective coating may be employed.  
      It will be appreciated that as well known to those skilled in the art, the wheel workpiece may be thermally treated to optimize combinations of natural and artificial aging in response to structure section thickness. One example of such optimization can include utilizing longer heat-up rates to the first step artificial aging temperature in lieu of natural aging time at ambient room temperature. It will also be appreciated that as well known to those skilled in the art, the artificial aging may be tailored somewhat to effect resultant material properties such as strength and corrosion resistance. Examples of such tailoring that are well known to those skilled in the art include coupling the degree to which equipment ramp-up temperatures (i.e., relatively slow heat-up rates) may be employed in conjunction with shorter hold times at temperature, and the preferred second step aging practice can be purposefully ramped up directly from the first step or there may be a purposeful and distinct time/temperature interruption between first and second steps. Another variation in artificial aging could reverse first and second steps. It is also generally known that ramping Lip to and/or down from a given or target treatment temperature, in itself, can produce aging effect which can, and often need to be, taken into account by integrating such ramping conditions and their aging effects into the total aging treatment. For instance, in a programmable air furnace, the temperature can be gradually progressively raised to temperature levels over a suitable length of time, even with no true hold time, to effect a suitable aging treatment, after which the metal can then be cooled to room temperature. This more continuous, aging regime with ramping and its corresponding integration of multiple temperature aging effects makes artificial aging possible in one continuous thermal treatment in a single, programmable furnace. For purposes of convenience and ease of understanding, however, preferred embodiments of this invention have been described in more detail as if each step was distinct from the other.  
      Referring now in greater detail to  FIG. 7 , there is shown the results of tests performed comparing the standard 6061 alloy with the low copper bearing 7000 series aluminum alloy as employed in making the forged aluminum vehicle wheels of the present invention. The test is a full wheel component rotary test with induced bending and is considered a meaningful test for this wheel design.  FIG. 7  is a plot of load in Newton-meters (N-m) versus the logarithmic representation of cycles to fatigue crack initiation. In connection with the 6061 alloy, crack initiation for the 6061 at a load of approximately 5039 N-m occurred after an average of about 90,000 cycles as represented on the logarithmic scale, with subsequent crack initiations occurring at approximately 4344 N-m after an average of about 180,000 cycles and at approximately 3649 N-m after an average of about 670,000 cycles. In order to compare these fatigue test results with those of the present invention, it is seen that comparing plots of the low copper bearing 7000 series aluminum alloy fatigue testing within the 6061 tests, very substantial fatigue properties improvements were achieved by the present invention. In connection with the low copper bearing 7000 series aluminum alloy of the present invention, the crack initiation at a load of approximately 5039 N-m occurred after about 750,000 cycles as represented on the logarithmic scale, with subsequent crack initiations occurring at approximately 4344 N-m after an average of about 1,3000,000 cycles and at approximately 3649 N-m after about 2,500,000 cycles. Table I shows the test data.  
                           TABLE I                       ALLOY   LOAD   CYCLES   CRACK INITIATION                                                6061-T6   5039   100,000   90,000           5039   90,000           5039   70,000           4344   200,000   180,000           4344   200,000           4344   150,000           3649   823,000   670,000           3649   646,359           3649   546,000       7XXX-T5   5039   750,000   750,000           4344   1,500,000   1,300,000           4344   1,400,000           4344   1,050,000           3649   2,500,000   2,500,000                  
 
      The presently preferred low copper bearing 7000 series aluminum alloys for use in present invention are 7004, 7005, 7108, 7003, 7008, 7108A, 7018, 7019, 7019A, 7020 and 7021 with T5, T1, T6 and T7 being the preferred tempers. When T6 or T7 is selected, it is preferred that air cooling be employed after solution heat treatment.  
      Another advantage of the present invention is the increased strength of the low copper bearing 7000 series aluminum alloy permits the wheel securing studs to be placed in greater tension. This increases the fatigue strength of the wheel. With existing generally employed studs and nuts, the present invention facilitates the use of nut torque of about 480 to 500 ft.-lb. and even higher torque with studs and nuts of greater strength.  
      While the wheels of the present invention may particularly advantageously be employed in Class 1 through 8 trucks as well as automobiles, advantageous use may be made in other vehicles as well.  
      Whereas particular embodiments of the invention have been described herein for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as set forth in the appended claims.