Extrusion method for high strength heat treatable aluminum alloys
Sperry et al.  
 EXTRUSION METHOD FOR HIGH STRENGTH HEAT TREATABLE ALUMINUM ALLOYS Inventors: Philip R. Sperry, North Haven;  
 William C. Setzer, Hamden; Joseph Winter, New Haven; Michael J. Pryor, Woodbridge, all of Conn.  
 Swiss Aluminium Ltd., Chippis, Switzerland Filed: May 23, 1974 Appl. No.1 472,656  
 Assignee:  
 US. Cl 72/364, 72/700, 148/1 1.5 A Int. Cl. B21c 29/00 Field of Search 72/364, 700; 148/1 1.5 A  
 References Cited UNITED STATES PATENTS 7/1941 Fritzlen l48/l1.5 4/1962 Criner l48/ll.5  
 [451 Apr. 1, 1975 3,113,052 12/1963 Schneck 148/1 1.5 3,307,978 3/1967 Foerster... 148/] 1.5 2/1974 Kroger l48/ll.5  
 Primary ExaminerLowell A. Larson Attorney, Agent, or Firm-David A. Jackson; Robert H. Bachman [57] ABSTRACT A method for extruding high strength, heat treatable aluminum alloys is disclosed which comprises providing a homogenized cast alloy billet, conducting a first hot extrusion of said billet to a reduction in area of from 20 75%, conducting a second hot extrusion at the solutionizing temperature of the alloy, and quenching said billet after said second extrusion. The method of this invention is particularly useful for the processing of Alloys 2014, 2024 and 7075, and eliminates the need of conducting a separate solution heat treatment after the extrusion operation is completed.  
 9 Claims, No Drawings EXTRUSION METHOD FOR HIGH STRENGTH HEAT TREATABLE ALUMINUM ALLOYS BACKGROUND OF THE INVENTION This invention relates to a process for extruding and heat treating high strength aluminum alloys without the need of separate solution heat treatment after extrusion.  
  It has long been the practice to heat extrusions of heat-treatable aluminum alloys by a process known as solution heat treatment, to achieve the desired temper. Such treatment involves heating the extrusion to a temperature at which solution and diffusion of the heattreatable alloying constituents take place and produces, as nearly as practicable, a homogeneous solid solution. The extrusion is subsequently quenched (i.e., rapidly cooled) in order to prevent the hardening constituents from precipitating substantially from solid solution during the cooling period. Slow cooling, on the other hand, would permit these constituents to precipitate to a greater extent, so that the alloy would be in a partially annealed condition unsuitable for subsequent precipitation heat treatment. Solution heat treatment, including the quench, is considered a necessary preliminary to subsequent precipitation heat treatment to increase the mechanical properties of the alloy.  
  Various methods have been proposed in the art to eliminate the need for post-extrusion solutionizing of aluminum alloys, however, no method is known which has been particularly successful when employed with the high strength aluminum alloys, such as Alloys 2014, 2024 and 7075. These alloys possess certain characteristics which prevent the successful reduction of the solutionizing treatment, among them, a relatively small critical temperature range between solvus temperature and solidus temperature, quench sensitivity causing loss of strength and corrosion problems, and susceptibility to hot cracking when hot worked at solutionizing temperatures due to the inherent heterogeneous nature of the near-cast structure. Of the above characteristics, the first inheres in the specific composition of the alloy and cannot be changed, and the second is a function of the physical means available for effecting a quench, and may be overcome by the choice of a suitable system from among those that exist in the art. The third characteristic, however, has proved most difficult to overcome, and, it is the solution to the problem posed by this third characteristic with which the method of this invention is primarily concerned.  
 SUMMARY OF THE INVENTION The method of this invention relates to a process for extruding and heat treating high strength aluminum alloys, without the need of a separate solution heat treatment and quenching after extrusion, by the use of a prior hot extrusion preceding the final extrusion operation.  
  The method of this invention facilitates the successful extrusion of high strength heat treatable aluminum alloys at commercial tempers without the employment of an additional solutionizing treatment after final extrusion. The product obtained by the method of this invention can be hot worked at their solutionizing temperature ranges without breaking up, and can develop characteristic mechanical strength properties when quenched and aged.  
  Accordingly, it is a principal object of this invention to provide a method for the preparatioin of high strength aluminum alloys by extrusion which eliminates the need of separate solution heat treatment after extrusion.  
  It is a further object of this invention to provide a method as aforesaid which is both economical and time saving.  
  It is yet a further object of this invention to provide a method as aforesaid whereby extrusion can be conducted in solutionizing temperature ranges without causing break-up of the alloy during extrusion.  
  lt&#39;is yet a further object of this invention to provide a method as aforesaid by conducting a prior hot extrusion preceding the final extrusion of the alloy.  
  Other objects and advantages of the present invention will become apparent from the detailed description which follows.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, it has been found that the foregoing objects and advantages can be readily achieved, and that high strength aluminum alloys may be successfully extruded to commercially acceptable tempers.  
  The method of this invention comprises an extrusion operation for high strength heat-treatable aluminum alloys which eliminates the necessity of a postextrusion solution heat treatment by providing a hot extrusion of the cast billet to a reduction in area from 2075% preceding the final extrusion operation.  
  The method of this invention may be employed with a broad range of aluminum alloys, and is particularly useful with high strength heat treatable alloys, such as those designated by the Aluminum Association as Alloys 2014, 2024 and 7075. The compositions of these alloys are presented in Table I, below.  
 All of the values for the constituents set forth above are stated on a percentage part by weight basis, and represent maximum amounts unless given in a range.  
  It has been found in accordance with this invention that these high strength heat treatable alloys may be extruded at solutionizing temperatures without causing break-up of the extrusion surface, and without the need of either reduced extrusion speeds or a previous solution heat treatment.  
  Before commencing the practice of the method of this invention, a cast and homogenized billet of suitable diameter is prepared. The procedures of casting and homogenization are not critical and may be followed in accordance with conventional practice in the art.  
 The billet is then reheated to a temperature of from about 700850 F. This heating must be uniform so that all parts of the billet are within the stated temperature range upon completion.  
  Upon completion of the reheating step, the billet is then extruded to a smaller diameter to obtain a reduction in area of about -75%. Such a reduction range would correspond to extrusion ratios of from about 1.25:1 to 4:1. This prior&#39;hot working at a lower temperature than the conventional solutionizing extrusion step serves to break up and elongate the cast grain so that the alloy can better withstand the deformation at the subsequently applied high temperatures. Extrusion within the range of reductions set forth for the above would, at most, serve to reduce the diameter of the billet by about 50% and the resulting billet would remain large enough to undergo further extrustion.  
  Upon being partially extruded in the above manner, the billet is then cut to a usable length and reheated to a temperature in a range generally residing near the lower end of the solutionizing temperature range. The solutionizing temperature range varies with the particular alloy in question and thus, for example, the lower end of the temperature range would be 910 F for Alloy 2024, 925 F for Alloy 2014 and 860 F for Alloy 7075. The reheating range should be controlled to extend from an upper limit approximated by the solutionizing temperature ranges, above, to a temperature level roughly 30 F lower. This lower range is feasible because adiabatic heating during extrusion can cause some temperature rise to solutionizing temperatures and, in the event that solutionizing is not complete, the extrusion effect serves to provide extra strength to the alloy which compensates for the resulting loss of solutionizing.  
  The control of the reheating step is considered critical, since, as noted earlier, the alloys in question possess a relatively small temperature range between solvus and solidus. Further, reheating must be conducted for a time sufficient to dissolve substantial amounts of soluble elements present within the alloy. The ability to control both reheating temperature and residence time are dependent upon the method of heating employed. Two methods of billet heating are presently in general use. Induction heating affords a rapid rise in temperature, and the desired temperature control, but is usually not maintained at that temperature for a time sufficient to constitute an effective solutionizing treatment. A conveyor-type furnace, however, which is heated electrically or by gas flame, provides a longer heating time; however, the billet surface is usually directly exposed to the heat source elements, increasing the danger of overheating the surface with the result that partial melting may occur. This latter danger is especially great with high strength aluminum alloys having little spread between solvus and solidus temperatures. In order to provide adequate control and duration of reheating, a modified induction heating method may be employed which makes use of a controlled time-hold cycle and, in the alternative, combines induction heatup with a hold in a circulating air furnace with a separate plenum chamber to isolate the heat source from the &#39;billet metal.  
  Upon completion of reheating, the billet is then extruded in the conventional manner at a solutionizing temperature with the limitation, however, that the extrusion rate is controlled to prevent a rise in temperature during extrusion of more than about 20 F above the minimum solutionizing temperatures, described earlier. To this end, the temperature of the extrusion container or canister should only be permitted to rise higher than about 25 below billet temperature. The criticality of temperature control during extrusion is likewise necessitated by the temperature sensitivity of the high strength aluminum alloys involved. Temperature control in this instance is a function of the interrelationship between heat generated by the working process itself and heat conducted away by the extrusion tools and surroundings. Thus, extrusion speed must be regulated to prevent unacceptable temperature fluctuations and, as noted above, the container temperature should be well controlled. The actual temperature limits and the speed of extrusion, however, will depend on the individual extruded shape and the alloy involved and, therefore, cannot be rigidly defined herein.  
  Two other factors which contribute to successful extrusion are the extrusion ratio and extrusion speed. The extrusion ratio was determined with reference to a single hole die, and should not exceed about 30:1. In conjunction with this, the length-diameter ratio of the billets should not exceed 2521. The factor of extrusion speed is interrelated with the former two, in that extrusion speed is in part determined by magnitude of the extrusion ratio. Control of extrusion speed is important since excessive speed tends to cause break up of the extrusion as it exits from the die. In experiments dealing with the alloys relating to this invention, extrusion speeds of up to about 4 ft./min. were found to be acceptable, however, the process is not restricted thereto, provided the desired temperature limits are obtained and cracking does not occur.  
  The finally extruded shape must then be quenched, in accordance with conventional practice in the art. The specific nature of the quenching operations, however, is critical, for, as noted before, the alloys which are to be extruded by the method of this invention process quench sensitivity and are known to exhibit loss of strength and corrosion resistance. Thus, the quench must be started before the alloy has cooled enough to lose some of its solid solution, and must be rapid enough to assure than adequate mechanical strength and corrosion resistance are retained. In a preferred embodiment, quenching is conducted continuously with the extrusion moving through a water wall. However, because of the stop and start nature of the extrusion operation, it may be desirable to maintain the entire extruded length at the solutionizing temperature range while it is being extruded and cut off or removed from the die. This can be accomplished by the use of an insulating tunnel or some similar apparatus. After cutting, the entire extrusion may be quenched by immersion in a horizontal through or by passing it through water sprays and the like. As an alternative, a moving saw may be placed as close as possible to the die face in order to quickly sever the extrusion and allow it to pass through a quench system without stopping. The determination of specific quenching time will vary with the alloy involved and the attendant extrusion conditions and need not be further developed at this time.  
  Upon emerging from the quench, the extruded alloy may be subjected to conventional processing such as stretch or roll straightening, natural or artificial aging depending upon the final temper which is desired. Thus, for example, if it is desired to prepare Alloy 7075 in T-6 temper, the quench shape may be subjected to aging treatment which may be conducted at a temperature of about 250 F for about 24 hours. Such an aging treatment, however, is applicable to specific alloys for which particular temper is sought and may vary considerably in relation to the particular alloy treatment desired.  
  As an illustration of the method of this invention, one may, for example, prepare or otherwise obtain cast and homogenized billets of the aforementioned aluminum alloys, and expose them to a heat treatment in a gas fired furnace with controlled heat zones to raise their temperature to about 800 F, for a period of about 25-40 minutes. The heated billets may then be extruded on an extrusion press which employs a circular die of 8 inch diameter and an extrusion ratio of approximately 2.25:].  
  The extruded cylindrical shape emerging from such extrusion may then be cut to length to serve as billet for final extrusion. It is first reheated to a temperature level which approximates the solutionizing temperatures of the respective alloys, and subsequently extruded in a 3,000 ton extrusion press which employs a die having an extrusion ratio of 25:], at a surface speed of from about 23 ft./minute for alloys such as Alloy 2024, and 1-2 ft./minute for alloys such as Alloy 7075. Upon existing from the end of the extrusion press, the extrusion enters a water wall quenching through to effect the rapid cooling which must follow solutionizing.  
  The alloy products prepared by the above extrusion method are not prone to cracking and break-up during processing, and may be further processed to commercial tempers. Thus, for example, extrusions of Alloy 7075 may be treated at 250 F for 24 hours to achieve T-6 temper for that alloy. Extrusions of Alloy 2024, if left in the naturally aged condition, are equivalent to T-4 temper.  
  The above processing scheme is outlined for purposes of illustration only and should not be construed as limitative of the invention.  
  This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.  
 What is claimed is:  
  1. A method for extruding high strength, heat treatable aluminum alloys which comprises providing a homogenized cast alloy billet, conducting a first hot extrusion of said billet to a reduction in area of from 20-75%, conducting a second hot extrusion at the solutionizing temperature of the alloy, and quenching the resultant extruded shape after said second extrusion.  
  2. The method of claim 1 wherein said first extrusion provides a reduction in area of about 50%.  
  3. The method of claim 1 wherein the billet is reheated to within the range of about 700850 F prior to conducting said first extrusion.  
  4. The method of claim 1 wherein, after said first extrusion but before said second extrusion, said billet is reheated to a temperature residing within the lower end of the solutionizing temperature range of the alloy.  
  5. The method of claim 1 wherein said second extrusion is regulated to prevent a rise in the temperature of the billet during extrusion above the maximum permissible solutionizing temperature of the alloys.  
  6. The method of claim 5 wherein said regulation is achieved by control of extrusion speed and container temperature.  
  7. The method of claim 6 wherein container temperature is maintained at about 25 F below the temperature of the billet upon entering said container.  
  8. The method of claim 1 including the step of againg the shape after said quenching step is completed.  
  9. The method of claim 1 including the step of stretch and roll straightening the shape after said quenching step is cpmpleted.