Patent Publication Number: US-2005126246-A1

Title: Solid shapes extrusion

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of U.S. Provisional Application No. 60/529,442, filed Dec. 12, 2003, incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      This invention relates to extrusions such as aluminum extrusions and more particularly it relates to a method of extruding to produce extrusions having improved flatness and straightness.  
      In extruding conventional parts from aluminum there is often great difficulty in producing an extrusion having dimensional stability. For example, if the extrusion has an L-shaped cross section with one leg thinner than the other, i.e., high aspect ratio, then it is necessary to vary the bearing surface of the extrusion depending on the leg. That is, if the L-shaped extrusion has one leg approximately 0.35 inch thick and a second leg 0.050 inch thick, then the die has to be designed to slow down the extrusion speed of the 0.35 inch thick leg to the speed of the 0.050 inch thick leg, which can be in the order of seven times slower extrusion speed. Thus, the die must be designed to provide the 0.350 inch leg or section with a long bearing surface to slow it down to the extrusion speed of the thin or 0.050 inch section which is required to have a short bearing surface. Bearing surface is the distance the metal travels in friction contact with the die opening walls. Slowing extrusion speed translates to a loss in productivity.  
      In addition to the loss of productivity, the L-shaped extrusion can have dimensional stability problems. That is, even if the two legs extrude straight, the two sections will have a different extrusion factor which results in problems. For example, the extrusion factor can create dimensional stability problems after the thermal treatment or during the straightening process. Thus, the conventional extruding process has problems of dimensional change after the thermal treatments. These problems include flatness, angularity, straightness and twist of the extrusion. Further, conventional extruding can result in a large amount of straightening time. In addition, there can be inconsistent dimensions within the same part resulting from the manual rework required. Also, there can be numerous set-up changes required for machining. Thus, it will be seen that there is a great need for an improved extruding process which avoids these problems.  
      Different extrusions and methods therefor are disclosed in the patents. For example, U.S. Pat. No. 6,029,353 discloses a multi-void structural member alterable to form separate yet integral strut members extending from the original member. The defined voids are joined by webs which are removed from any or all of the defined voids to form the integral strut members of a desired configuration. The defined voids may be joined to others or separated to an individual extending strut. The multi-void member or any of the integral strut members formed therefrom are formed to a desired configuration by conventional methods to provide an integral structural member of a desired configuration.  
      U.S. Pat. No. 4,991,047 discloses tape guides for a magnetic tape cassette comprising a half of cylindrical tube each adapted to fit securely over a support post within the housing of the cassette. The method of manufacturing the guides consists of forming thin-walled portions as grooves on at least the inner or outer side of a tube, with or without inwardly protruding ribs, which facilitate the division and provide a tight fit of the half piece to the post, and then dividing the tube along the plane into two halves. The tube is made of a metal, particularly brass or stainless steel, or plastics and formed by drawing or other irregular-shape forming technique, with polishing, plating, and/or precision polishing before division.  
      U.S. Pat. No. 1,211,193 discloses a forging machine for making hollow bodies, including a die, an automatic shear operating at the feeding-in side of the die, a combined positioning and ejecting plunger cooperating with the die, the plunger operating as a stationary anvil at one stage and as an ejector at a later stage of the operation, a movable locking element for temporarily holding the plunger rigidly in position while functioning as an anvil, and a piercing tool operating in opposition to the plunger.  
      U.S. Pat. No. 1,545,364 discloses a method of producing a zinc nail which consists in extruding, through a die opening, a portion of the metal of a blank to form the shank, the unextruded portion of the blank forming a head, guiding the extruded shank to prevent undue distortion thereof out of the plane of the die opening during the extruding action, and then returning the shank through the die opening.  
      U.S. Pat. No. 4,102,172 discloses a method of producing caster supporting frames having a first end portion with a hollow space therethrough for receiving therein an upright connector rod, and a second end portion for supporting a horizontal axle of a caster wheel or wheels. The method comprises the steps of shaping a bar-like rod or uniform cross-sectional configuration to have along one side thereof a first through hole extending longitudinally therethrough and along the other side thereof a second through hole extending longitudinally therethrough parallel to the first through hole; and cutting the bar-like rod transverse to the longitudinal length thereof into a number of pieces of a predetermined length to obtain the caster supporting frames, whereby the first through hole forms the hollow space of the first end portion and the second end portion is also hollow therethrough.  
      U.S. Pat. No. 4,203,311 discloses heat transfer elements made by forming a plurality of tubes extending in side-by-side relation with connector means holding adjacent tubes together, cutting and outwardly projecting surface portions of the tubes to form fins, and then severing the connector means to divide the plurality of tubes into individual heat transfer elements.  
      U.S. Pat. No. 4,546,634 discloses methods and apparatus for initiating extrusion of elongated hollow product, more particularly cylindrical tubular product, by increasing the space between the end of a mandrel and an opposed extrusion die during extrusion start-up and thereafter gradually decreasing the space to the desired width for extrusion of the product to enhance extrusion initiation.  
      U.S. Pat. No. 4,862,728 discloses an extrusion die having a die aperture which is negatively tapered essentially throughout its length at an angle of at least 1° such that any friction stress between the die lands and metal flowing through them is negligible, the length of the lands being not more than 2 mm so that fouling does not significantly take place thereon during extrusion. Faster extrusion speeds can be achieved, particularly when extruding aluminium alloy having a shear strength of from 1.2 to 4.0 Kg/mm 2  at 500° C.  
      U.S. Pat. No. 5,749,135 discloses a method for producing a U shaped seat back frame from a single extruded blank. The blank&#39;s cross section is nearly circular, but with a pair of short, flattened off crests that create a pair of concentric arcs, in cross section. Over a length of each end of the blank, a rectangular cross section, solid mandrel is inserted between the crests and the arcs are flattened against the mandrel. The arcs are flattened into wider side walls, while the crests move apart, without deformation, to create narrower and stiffer walls of a rectangular cross section. Lastly, the U blank with flattened ends is bent into a U shape, creating an upper beam with a nearly circular cross section, and legs with truly flat and rectangular lower ends.  
      The subject invention discloses an improved extrusion process which can produce, for example, two finished parts from an extrusion, the process having improved productivity while avoiding problems and disadvantages of the conventional extruding process.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide an improved extrusion process.  
      It is another object of the invention to provide an improved extrusion process which results in flat, straight extrusion members free from twisting.  
      It is yet another object of the invention to provide a method for extruding aluminum alloys having L-shaped cross-sectional configurations, for example, which are free from warping and twisting and which can eliminate the straightening process.  
      These and other objects of the invention will become apparent from reading the specification and claims and an inspection of the figures attached hereto.  
      In accordance with these objects, there is provided a method of forming a generally L-shaped extrusion from a body of aluminum alloy, the extrusion having good flatness and straightness without the use of a straightening step. The method comprises the steps of providing a body of a precipitation hardenable aluminum alloy and providing an extrusion press having a die for extruding a hollow-shaped tube having a generally rectangular-shaped cross section. The body is extruded to provide the rectangular-shaped tube which is then precipitation hardened to increase its strength. After precipitation hardening, the tube is machined in the longitudinal direction to separate it into two L-shaped members having good flatness and straightness.  
      While the tube has been disclosed and machined to form two L-shaped members, it will be appreciated that the tube may be machined or cut to form U-shaped members. Further, while the body has been extruded to form a tube having a rectangular cross section, other shapes can be extruded such as square, parallelogram, triangular, circular, cross-sectional shapes or even more complex shapes are contemplated. Such other shapes can be machined to provide the desired shaped member having good flatness and straightness. While a rectangular extrusion has been disclosed for machining into L- or U-shaped members, it will be appreciated that the method can be used for forming other shaped members from extrusions.  
      Thus, a method is also disclosed for producing a shaped member from a body of aluminum alloy, the member having good flatness and straightness without the use of a straightening step, the method comprises the steps of providing a body of an aluminum alloy and providing an extrusion press having a die for extruding an extrusion having at least one hollow tubular member. The method further comprises extruding the body to provide the extrusion and thermally treating the extrusion to increase its strength. Then, the extrusion is machined in a longitudinal direction to separate it into at least one shaped member having good flatness and straightness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view of an L-shaped member in accordance with the invention.  
       FIG. 2  is a cross-sectional view of a rectangular-shaped extrusion in accordance with the invention for forming into the L-shaped member.  
       FIG. 3  is a cross-sectional view of a parallelogram-shaped extrusion in accordance with the invention.  
       FIG. 4  is a cross-sectional view of a shaped member formed from the extruded parallelogram.  
       FIG. 5  is a cross-sectional view illustrating a U-shaped member which may be machined or cut from the rectangular-shaped extrusion of  FIG. 2 .  
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT  
      Referring now to  FIG. 1  there is illustrated a cross section of an extrusion member which is difficult to extrude because of the high aspect ratio between leg  4  and leg  6 . Prior methods of forming such L-shaped member included machining such shape from plate which is labor intensive and generates substantial scrap in the form of chip. The machining method is also undesirable because of the lowered corrosion resistance when the member is formed from aluminum alloys. Another method includes extruding a member having a low aspect ratio and using chemical milling to obtain the desired thickness in the legs. However, as will be appreciated, this is an expensive process which is very time consuming and generates substantial aluminum scrap.  
      Referring again to  FIG. 1 , a typical thickness for leg  4  can be in the range of 0.05 inch to about 0.1 inch although the thickness can vary substantially depending on the application. Leg  6  can range in thickness from greater than 0.70 inch to about less than 0.25 inch. The radius of transition zone  7  is about 0.19 inch in the embodiment shown. As noted earlier, if member  2  is extruded and precipitation hardened to improve strength, flatness and straightness deteriorate and substantial twisting is experienced, requiring considerable time and effort for straightening purposes. Further, because of the high aspect ratio between legs  4  and  6 , substantial reduction in extruding rate is required. By comparison, the process in accordance with the invention can produce net shape members with extremely tight tolerances. Further, the use of the present invention can reduce scrap generation from about 10% to 1 to 2%. The members produced are substantially free from twisting or warping while maintaining good flatness and straightness even after thermal treatments to improve strength. In addition, aluminum alloy members produced in accordance with the invention have a more uniform grain structure throughout the cross section, resulting in more consistent mechanical and physical properties with reduced levels of residual stress. The subject inventive process eliminates the need for the straightening operation after the thermal treatment and stress relief.  
      For purposes of producing members such as illustrated in  FIG. 1  having good flatness and straightness, a rectangular extrusion is produced as shown in  FIG. 2 . It has been discovered that if an extrusion is produced having a rectangular cross section, such extrusion maintains very tight tolerances on angularity between leg  4  and leg  6  with substantially no variation along the length after extrusion or after thermal treatments and stress relief. Thus, the step of straightening is eliminated. After the rectangular-shaped extrusion of  FIG. 2  has been thermally treated, it is then subjected to a machining or cutting operation to separate the rectangular-shaped extrusion of  FIG. 2  into members having the cross-section shape of  FIG. 1 . After machining, member  2  of  FIG. 1  maintains its tolerances as noted. The angle between leg  4  and leg  6  can be maintained, for example, between ±0.5°.  
      It should be noted that the rectangular-shaped extrusion of  FIG. 2  is comprised of two opposed sides  6   a  and  6   b  and two legs or sides  4   a  and  4   b.  When legs  4   a  and  4   b  of extrusion of  FIG. 2  is machined or cut at points  8  and  10 , then the L-shaped member of  FIG. 1  is obtained.  
      The L-shaped member when fabricated from AA7000 series alloys such as AA7150 is useful in aircraft applications for “fastening wing skins to stringers”, for example, and of course must meet very tight tolerances.  
      It will be appreciated that other shapes may be fabricated from the rectangular hollow extrusion of  FIG. 2 . For example, a U-shaped member  20  ( FIG. 5 ) may be machined or cut from the hollow rectangular-shaped extrusion of  FIG. 2 . The U-shaped member is shown in  FIG. 5  having two legs  22  and  24 . Two U-shaped members may be obtained from the rectangular extrusion of  FIG. 2 , if desired.  
      It will be noted in  FIG. 2  that the angle between leg  4   b  and  6   b  is about 90° which can be maintained along the length of the member with precision that can be maintained at less than ±0.5° The invention can be applied to other shapes such as parallelograms as shown in  FIG. 3 . That is, hollow-shaped extrusion  30  shown in  FIG. 3  has parallel sides  32  and  34  with side  32  forming a large angle with side  34  of about 104°. The extrusion is thermally treated as noted earlier to increase strength and machined or cut to provide a member  40  having legs  32   a  and  34   a  as shown in cross section in  FIG. 4 .  
      The process of the invention is particularly suitable for producing members having high aspect ratios. Thus, for purposes of the invention, the aspect ratio can range from 1:1 to 14:1 with preferred aspect ratios being in the range of 7:1 to 14:1. Aspect ratio is the ratio of the thickness of leg  4  to leg  6  ( FIG. 1 ).  
      Any alloy may be used in the process of the invention. In aluminum alloys, the invention is particularly suitable for the precipitation hardenable alloys, although the process may be applied to any aluminum alloy. Thus, the invention may be applied to AA (Aluminum Association) 2xxx, 6xxx and 7xxx series aluminum alloys. In the AA 7xxx series alloys, the invention has application to AA7050, AA7150, AA7075 and AA7175, for example. In the AA2xxx series alloys, the invention has particular application to AA2014, AA2024, and AA2224, for example. The chemical compositions of these alloys are set forth in a publication by The Aluminum Association dated January 2001, entitled “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys”, incorporated herein by reference as if specifically set forth. All ranges set forth herein are intended to include all the numbers within the ranges.  
      Prior to extruding, the body of material may be heated to 800° to 850° F., preferably 800° to 820° F. for alloys such as 7000 series alloy.  
      Any dies may be used that extrude billet to produce the desired extrusion. Further, the dies may be designed to produce net shape extrusions having variation in angles of less than ±3° and preferably less than ±1°. Dies that are particularly useful include porthole or bridge dies. Such dies produce seamed hollow shapes or seamed tubes. The seam results from a weld that occurs under high temperature and pressure where the metal was separated by a bridge in the die. The metal then welds together before entering the die opening. The die should be designed to locate the weld so that it does not compromise the properties of the extrusion.  
      When the alloy used is an AA7xxx alloy, it may be subject to different thermal treatments depending on the properties desired in the extrusion or member separated from the extrusion. For example, the extrusion may be subject to a single step thermal treatment to achieve high or peak strength, referred to as T6 type tempers. However, such tempers in the AA7xxx series alloys are susceptible to stress corrosion cracking. T6 tempers are obtained by aging at a temperature range of 175° to 325° F. for 3 to 30 hours. Another single step aging process includes aging at 300° to 325° F. for 5 to 30 hours to produce an overaged condition referred to as a T7 temper. This condition improves stress corrosion cracking but can decrease strength. For AA7xxx series alloys, the extrusion may be thermally treated to a T73 (9 hrs @245° F. followed by 12 hrs @ 300° F.) or T76 (16 hrs @250° F. followed by 8 hrs @ 345° F.) condition.  
      To improve strength and corrosion resistance, the extrusion may be subject to a three-step aging process before separating or machining the L-shaped member from the extrusion. The aging steps or phases include a low-high-low aging sequence. In the first or low aging step, the extrusion is subject to a temperature for a period of time which precipitation hardens the extrusion to a point at or near peak strength. This can be effected by subjecting the extrusion to precipitation hardening in a temperature range of about 150° to 325° F. typically for a time between 2 to 30 hours. Then, the extrusion is subject to a second treatment to improve corrosion resistance. The second treatment for 7xxx alloys includes subjecting the extrusion to a temperature range of 325° to 500° F. for 5 minutes to about 3 hours, for example. In the third step, the extrusion is subject to another strengthening step. The third thermal treatment includes subjecting the extrusion to a temperature of 175° to 325° F. for about 2 to 30 hours,  
      After the thermal treatments, the extrusion is machined or cut to provide the desired shape. As noted, extrusion shown in  FIG. 2  is cut at points  8  and  10  to provide the L-shaped member shown in  FIG. 1 , for example.  
      It will be appreciated that any number of shapes may be extruded, aged and then machined to provide the desired member and such is contemplated within the purview of the invention. That is, the rectangular-shaped extrusion in  FIG. 2  is provided as example of the invention. Other examples of cross-sectional shapes that may be formed in accordance with the invention are T-shaped, H-shaped, and X-shaped. Such shapes can be formed by machining the appropriate hollow extrusion.  
      When the invention is applied to rectangular-shaped extrusion, e.g.,  FIG. 2 , it has the advantage doubling production when L-shaped extrusions are desired. Or, it the L-shaped extrusion requires further machining, this may be performed prior to separation from the rectangular-shaped extrusion.  
      The following example is still further illustrative of the invention.  
     EXAMPLE  
      A billet (7 inch diameter) of AA7075 alloy was heated to 820° F. and extruded at a rate of about 3 ft/min into an extrusion shape 27 feet long, shown in  FIG. 2 .  
      The thick portion  6   b  of the extrusion had a thickness 0.25 inch and the thin portion had a thickness of 0.05 inch. The angle “C” between the thick portion and the thin portion was designed for 90°. The extrusion was thermally treated for T73 511 to enhance its strength properties. After the extrusion was cooled to room temperature, angles were measured at about one foot intervals along the finished length for four different extrusions. It will be seen from Table 1 that these all measured 90°. The extrusion was machined to provide the L-shaped member shown in  FIG. 1  and the angles were measured again. It will be seen from Table 2 that the angle remained at 90°. Thus, it will be seen that the present invention can provide an extrusion member having good flatness and straightness with freedom from twisting. That is, when the present invention is used for extrusion, the straightening step can be omitted.  
                       TABLE 1                                      Angle Before Separating                                     Sample No.   Front   Middle   Back                       1   90°   90°   90°           2   90°   90°   90°           3   90°   90°   90°           4   90°   90°   90°                      
 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
               
               
                   
                 Angle After Separating 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Sample No. 
                 Front 
                 Middle 
                 Back 
               
               
                   
                   
               
               
                   
                 1 
                 90° 
                 90° 
                 90° 
               
               
                   
                 2 
                 90° 
                 90° 
                 90° 
               
               
                   
                 3 
                 90° 
                 90° 
                 90° 
               
               
                   
                 4 
                 90° 
                 90° 
                 90° 
               
               
                   
                   
               
            
           
         
       
     
      Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.