Patent Publication Number: US-6338189-B1

Title: Method and apparatus for expansion forming a workpiece using an external deformable supporting fixture

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to methods and apparatuses for expansion forming and, more specifically, to a method and apparatus for expansion forming a workpiece using an external deformable supporting fixture. 
     BACKGROUND OF THE INVENTION 
     Oxide-dispersion-strengthened (ODS) high temperature alloys offer combinations of high-temperature strength, oxidation resistance, and hot corrosion resistance that can not be obtained from other alloys. For example, INCONEL alloy MA 754 has been considered as one of the ODS alloys with greater potential in the next generation advanced gas-turbine hot section components such as turbine vanes and combustor liners. In such applications, the ODS alloys are typically used to fabricate LAMILLOY sheets. As is known in the art, LAMILLOY is a multilayered porous material designed for cooled airframe and propulsion system components. It features a labyrinth of holes and passages in a laminated assembly. LAMILLOY is produced by photochemical machining an array of pedestals and holes in two or more layers of sheet material and subsequently diffusion bonding the layers into the laminated sheet configuration. 
     One particular use of MA 754 LAMILLOY material is in the construction of a combustor outer liner  10  for a gas turbine engine, as illustrated in FIG.  1 . The combustor liner  10  is formed by roll forming two LAMILLOY sheets into half cylinders (or conic sections)  12  and  14 , brazing the half cylinders  12  and  14  together at two lap joints  16  and  18 , and then expansion forming the brazed detail  10  into the required design dimension, as indicated by the dashed line  20  in FIG.  2 . 
     The MA754 LAMILLOY material  12  and  14  is preferably joined by brazing lap joints  16  and  18  rather than welding the two half cylinders  12  and  14  together at butt joints, because welding processes result in melting of the base material. Welding is unacceptable in MA754 LAMILLOY material due to agglomeration of the oxide dispersoids in the melted region resulting in drastic reductions in high temperature strength and environmental resistance. 
     The joining of two MA754 LAMILLOY sheets is accomplished by brazing, which is performed at a temperature below the melting point of the base material. The brazing process requires a lap joint (overlapping ends of the material to be brazed) instead of the butt joint normally used in welding. Therefore, the two half-cylinders  12 ,  14  are configured with the two lap joints  16 ,  18  which are brazed to form a permanent joint. 
     As shown in FIG. 2, the combustor liner  10  initially has a flat-sided configuration; however, the design requirements call for the sides to exhibit the curved configuration indicated by the dashed lines  20 . In order to achieve the configuration  20 , the workpiece  10  is placed onto a radial expander  22  (see FIG.  3 ), which will exert radial forces upon the workpiece  10  until it assumes the configuration  20 . As is known in the art, the expander  22  includes a plurality of jaws  24  which may be moved in a radial direction under, hydraulic pressure. 
     Referring to FIG. 4A, a top plan view of the lap joint  16 ,  18  is shown. When the expander  22  is operated, a biaxial tension state of stresses is produced in the directions shown by the arrows in FIG.  4 C. Because the half cylinders  12 ,  14  lie in different planes in the area of the lap joint  16 ,  18 , the braze joint plane  26  rotates through an angle  28  (see FIG.  4 B), which can be as much as 30 degrees relative to the stress axis. 
     This joint rotation results in loading the braze joints  16 ,  18  in combination of shear, bending and direct tensile stresses. Stress analysis confirms that high stress concentrations exist in the LAMILLOY near the lap joints  16 ,  18  during expansion forming. As an illustrative example, a combustor liner  10  was formed from two halves  12 ,  14  of rolled MA 754 LAMILLOY. Two brazed lap joints  16 ,  18  in the liner  10 , featured overlap dimensions of 0.250 inch wide and 6.750 inches in length. During expansion forming of this combustor liner  10 , the LAMILLOY material failed in a ductile manner due to strain localization near one of the brazed lap joints  16 ,  18  with less than 2% bulk diametral expansion. The design deformation objective for this application was 4.5% diametral expansion. 
     A need exists for a method and apparatus that will allow expansion forming of a workpiece by minimizing the concentration of stresses in the workpiece, particularly stresses related to joint rotation out of the plane of the stress axis. The present invention is directed toward meeting this need. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to an external cylindrical or conic section shaped fixture which is tightly fit around the same shaped workpiece prior to expansion forming to a final dimension. During expansion forming, the external fixture is also deformed and is therefore designed for one-time use. The external cylindrical or conic section shaped fixture of the present invention is particularly useful for expansion forming of workpieces which contain brazed lap joints. The external fixture restrains out-of-plane joint rotation and, reduces the stress concentrations existing near the lap-joints. Additionally, the external fixture provides additional load carrying capability to allow better load distribution during expansion forming. No change is required to the expansion forming machinery or the inner expander dies (jaws) with use of the present invention. Because the external fixture fits tightly over the workpiece, applying a compressive force when the inner expander jaws expand during the forming process, the bending moment on the brazed lap joints is significantly reduced. 
     In one form of the invention, a method for expansion forming a workpiece is disclosed, comprising the steps of a) providing an expansion forming device; b) providing an external fixture; c) fitting the external fixture around the workpiece; d) fitting the workpiece and external fixture assembly around the expansion forming device, such that the workpiece is positioned between the expansion forming device and the external fixture; e) expanding the expansion forming device such that the workpiece and the external fixture are deformed; and f) removing the external fixture from the workpiece after step (e). 
     in another form of the invention, a method for expansion forming a workpiece is disclosed, comprising the steps of a) providing an expansion forming device; b) providing a fixture; c) coupling the workpiece to the expansion forming device; d) coupling the fixture to the workpiece; e) expanding the expansion forming device such that both the workpiece and the fixture are deformed; f) removing the fixture from the workpiece after step (e). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a cylindrical workpiece having two brazed lap joints. 
     FIG. 2 is a side elevation view of the workpiece of FIG. 1, illustrating the dimensions to which the workpiece is to be expanded. 
     FIG. 3 is a plan view of the workpiece of FIG. 1 being formed with a radial expander. 
     FIGS. 4A-C illustrate the out-of-plane rotation which occurs at a lap joint during expansion forming. 
     FIG. 5 is a perspective view of a preferred embodiment external fixture of the present invention. 
     FIG. 6 is a top plan view of a weld butt joint of the fixture of FIG.  5 . 
     FIG. 7 is a top plan view of the workpiece of FIG.  1  and the fixture of FIG. 5 in use with a radial expander. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device, and further applications of the principles of the invention as illustrated therein are herein contemplated as would normally occur to one skilled in the art to which the invention relates. 
     In order to restrict joint bending and out-of-plane rotation experienced by the brazed lap joints  16 ,  18  during the prior art expansion forming process, the present invention provides an external fixturing cylinder or conic section  30 , as illustrated in FIG.  5 . In a preferred embodiment, the external fixture  30  is made from L605 nickel-based alloy and is formed from two half cylinders or conic sections  32  and  34  that are butt welded at their joints. The preferred embodiment material for the fixture  30  is 3-4 times stronger at room temperature than the LAMILLOY sheet from which the combustor liner  10  is fabricated. Typically, the material forming the fixture  30  is approximately 0.100 inch thick. Because L605 is a highly weldable material, the external fixture is fabricated by butt welding the half cylinders  32 ,  34  at  36 ,  38 . As shown in FIG. 6, the butt welds  36 ,  38  allow the material of the half cylinders  32 ,  34  to form a continuous, true cylinder. 
     As illustrated in FIG. 7, after the external fixture  30  is first fit around the combustor liner  10 , the combustor liner  10  is then placed upon the radial expander  22 . The fixture  30  preferably has substantially the same inner diameter as the outer diameter of the combustor liner  10 . The fixture  30  is therefore preferably heated (for example, to 500° F.) in order to slightly expand its diameter so that it may be fit over the combustor liner  10 . Upon cooling to room temperature, the fixture  30  shrinks to form a tight fit over the combustor liner  10 . 
     Prior to fitting the fixture  30  over the combustor liner  10 , the outer surface of the combustor liner  10  and/or the inner surface of the fixture  30  are preferably coated with a layer of anti-bonding material in order to prevent bonding between the combustor liner  10  and the fixture  30  during either the expansion forming process or in-process annealing and to facilitate fixture  30  removal after forming. In the preferred embodiment, a fine ceramic powder, such as alumina powder in liquid form, is used for this purpose and is available under the brand STOP-OFF. 
     The combustor liner  10 /fixture  30  combination then undergoes several expansion forming steps at room temperature. Each forming cycle is followed by a high temperature annealing process to relieve stresses within the materials. After each annealing, further expansion forming is performed at room temperature, followed by another annealing step, etc. 
     Because the weld joints in the fixture  30  are butt joints, they do not experience any out-of-plane rotation during the expansion forming process. Furthermore, because the fixture  30  applies compressive stresses to the combustor liner  10  during the expansion forming process, the brazed lap joints  16 ,  18  of the combustor liner  10  are held in-plane between the expansion forming jaws  24  and the fixture  30 , which opposes any out-of-plate rotation of these joints  16 ,  18 . This significantly reduces the stresses existing near the lap joints  16 ,  18 . Furthermore, the fixture  30  provides for more uniform load distribution around the combustor liner  10  during the forming process. 
     Once the combustor liner  10  has been expansion formed to its final design dimensions, the fixture  30  is cut off and removed therefrom. Because the fixture  30  deforms (yields) during the expansion forming process and is cut off of the combustor liner  10  once expansion forming has been completed, the fixture  30  is not a multiple-use fixture. 
     It will be appreciated by those having ordinary skill in the art that the external fixture  30  of the present invention provides support and strength to a somewhat fragile workpiece during the expansion forming process and is particularly useful in preventing out-of-plane rotation of lap joints in the workpiece during the expansion forming process. The fixture  30  is relatively inexpensive to manufacture and use, and greatly reduces the failure rate when expansion forming workpieces having difficult geometries. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. For example, the materials specified herein for the construction of the workpiece and the fixture are only a preferred embodiment. Those having ordinary skill in the art will recognize that any materials having acceptable physical properties may be used. Furthermore, the present invention will find application with a wide variety of workpieces, and is not limited solely to use with the preferred embodiment combustor liner.