Patent Publication Number: US-2011076419-A1

Title: Method for developing fine grained, thermally stable metallic material

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates generally to a method for developing a fine grained thermally stable metallic material. 
     II. Description of Related Art 
     It is necessary to use fine grained, thermally stable materials where the materials are subsequently used to form complex components through hot forming processes. Such fine grained materials are required since large grains appearing in the material may subject the material to fracture or other failures prematurely during subsequent hot forming processes. 
     Fine grained metallic materials are usually prepared by conventional severe plastic deformation processes, such as accumulative roll bonding, reciprocating extrusion, and equal channel angular extrusion. These processes could involve elaborate surface preparations for the workpieces, the necessity to preheat the workpiece, and the presence of large friction between the dies and the workpieces. As such, it is difficult to utilize these severe plastic deformation processes to prepare fine grained bulk materials. 
     It has also been previously known that subjecting the metallic material to friction stir welding results in a fine-grained microstructure in the weld zone. As such, friction stir processing has been developed using the friction stir welding tool to produce fine grained bulk materials. Friction stir welding can be regarded as a single pass of friction stir processing. As known, a friction stir welding tool consists of a tool shoulder and a hard pin of any profile. The hard pin has a pin length slightly less than the thickness of the material or blank to be welded or processed. During friction stir welding or friction stir processing, the hard pin is fully penetrated into the material or blank, and the tool shoulder is in close contact with the top surface of the material or blank. Along the thickness, the friction stir welded or friction stir processed material or blank is divided in sequence into the weld/bead surface region or so-called top layer (in contact with the tool shoulder), the stir zone, and the region underneath the welding tool or so-called bottom layer. 
     However, such fine grain microstructure within the metallic materials developed by friction stir welding or friction stir processing oftentimes undergoes abnormal grain growth when subsequently exposed to high forming temperatures. Abnormal grain growth is originated from the weld surface region, or the region underneath the welding tool, or both where the processed material retains the deformed or unrecrystallized microstructure. This remaining deformed or unrecrystallized microstructure stores excessive free energy, and is so unstable at high temperatures that a set of grains grow at a high rate and at the expense of their neighbors, resulting in the formation of a microstructure dominated by a few very large grains. Abnormal grain growth can propagate throughout the entire stir zone for a prolonged duration. The microstructural instability results in substantial degradation of deformability and other mechanical properties of the friction stir welded or friction stir processed materials at high temperatures. 
     Consequently, because of the occurrence of abnormal grain growth during exposure to high temperatures, the fine grained metallic materials or blanks prepared by friction stir welding or friction stir processing are oftentimes not suitable for subsequent hot forming operations. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention provides a method for developing a fine grained thermally stable metallic material which overcomes the above-mentioned disadvantages of the previously known methods. 
     As stated, at high temperatures, abnormal grain growth occurring in the friction stir welded or friction stir processed material is originated from the top surface layer, or the bottom surface layer, or both, and then propagates throughout the entire stir zone. Proper treatments are conducted to either of the surface layers, or both where abnormal grain growth is induced during the subsequent exposure of the processed material to high temperatures. To describe the concept of the present invention, both surface layers are chosen for proper treatments to eliminate the origin of abnormal grain growth. If only one surface layer induces abnormal grain growth during subsequent exposure of the friction stir welded or friction stir processed material to high temperature, then only that surface is required for proper treatments. 
     In brief, in one embodiment of the method of the present invention, friction stir processing is performed with the welding tool being plunged from one side of the metallic material or blank into the metallic material or blank until the entire metallic material is subjected to friction stir processing. Thereafter, the top and bottom layers of the friction stir processed metallic material are removed by any conventional means, such as grinding, to remove the material adjacent to the surfaces retaining deformed or unrecrystallized microstructure. After such removal, the remaining processed material is composed entirely of a substantially thermally stable fine grained microstructure. 
     Optionally, extraneous sheets of a compatible material are attached to both sides of the metallic material or blank to form a sheet stack. Friction stir processing is then performed on the sheet stack with the welding tool being plunged from one extraneous sheet through the metallic blank into the other extraneous sheet until the metal sheet stack is entirely friction stir processed. 
     Following the friction stir processing, the deformed or unrecrystallized microstructure resides entirely within the extraneous material sheets. The extraneous material is then removed from the metal blank by grinding or other conventional means. 
     In certain operations, such as welding a butt joint between two abutting metal blanks, extraneous sheets of compatible material are attached to the abutting blanks so that the compatible material extends along both the top and bottom surfaces of the abutting blanks along the seam. The friction stir welding is then performed along the seam with the welding tool being plunged from one extraneous sheet through the abutting blanks into the other extraneous sheet, thus welding the two blanks together. Afterwards, the compatible material is removed by grinding or other conventional means from both sides of the now conjoined metal blanks. 
     In an alternate form of the invention, a layer containing pinning particles, such as oxide powders, is deposited on both sides of the metal blank by any conventional means, such as laser powder deposition. Thereafter, friction stir processing is performed on the metal blank, thus intermixing the pinning particles into the top and bottom surface layers of the metal blank. In practice, the pinning particles impose resistance to the migration of the grain boundaries in the surface layers during exposure of the friction stir processed material to high temperatures, thus preventing the origination of abnormal grain growth from the surfaces of the blank and its propagation throughout the entire processed blank so that a fine grained thermally stable blank is obtained following friction stir processing. 
     Optionally, extraneous metal sheets containing a sufficient amount of pinning particles are attached to both sides of the metallic material or blank to form a sheet stack. Friction stir processing is then performed on the sheet stack with the welding tool being plunged from one extraneous sheet through the metallic blank into the other extraneous sheet until the metal sheet stack is entirely processed. Thereafter, a fine grained thermally stable blank is obtained following the friction stir processing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       A better understanding of the present invention will be had upon reference to the following detailed description when read in conjunction with the accompanying drawing, wherein like reference characters refer to like parts throughout the several views, and in which: 
         FIG. 1  is an elevational view illustrating one embodiment of the present invention; 
         FIG. 2  is an elevational view illustrating a still further embodiment of the present invention; 
         FIGS. 3A-3E  are end sectional views illustrating a still further embodiment of the present invention; 
         FIG. 4  is an elevational view illustrating a still further embodiment of the present invention; 
         FIG. 5  is a flowchart illustrating one embodiment of the present invention; 
         FIG. 6  is a view similar to  FIG. 4 , but illustrating a still further embodiment of the present invention; and 
         FIG. 7  is a flowchart of the embodiment shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
     With reference first to  FIG. 1 , a metallic material or blank  10  is illustrated which will subsequently be used in a hot forming process to form a final part. The blank  10  illustrated in  FIG. 1 , by way of example, is made of an aluminum alloy containing 3.0Mg, 0.4Fe, 0.4Si, and 0.5Mn by weight percentage. 
     In order to form the blank  10  into a blank having a fine grained thermally stable microstructure, a friction stir welding tool  12 , plunging from one side  14 , performs friction stir processing on the blank  10  using multiple and overlapping passes  16 . Although the friction stir processing creates a fine grained microstructure throughout the bank  10 , abnormal grain growth often takes place at the surface layers of the friction stir processed blank  10  and then propagates throughout the entire stir zone to release excessive free energy when the friction stir processed blank  10  is subsequently exposed to high temperatures. 
     With reference now to  FIG. 2 , in order to prevent the propagation of abnormal grain growth throughout the friction stir processed blank  10  during subsequent exposure to high temperatures, a sheet  20  of compatible material is attached across the upper surface  14  of the blank  10 , and a second sheet  22  of compatible material is attached to the opposite or bottom side  24  of the blank  10 . Although the sheets  20  and  22  of extraneous material may be the same material as the blank  10 , as used herein “compatible” means the absence of formation of large quantities of low melting-point intermetallic compounds between the extraneous sheets  20  and  22  and the blank  10 . 
     The sheets  20  and  22  of extraneous material are preferably 0.5-0.8 millimeters in thickness and are attached to the blank  10  to form a sheet stack, Friction stir processing is then performed on the metal sheet stack with the welding tool being plunged from the sheet  20  through the blank  10  into the sheet  22  using multiple and overlapping passes as shown in  FIG. 1  until the entire blank has been subjected to friction stir processing. 
     Following the friction stir processing of the blank  10  covered by the sheets  20  and  22  of extraneous material, any deformed or unrecrystallized microstructure is contained in the sheets  20  and  22  of extraneous material which undergoes abnormal grain growth during subsequent exposure to high temperatures, while the blank  10  is subjected only to the stir zone during the friction stir processing which has a thermally stable microstructure even at high temperatures, Consequently, by removal of the extraneous sheets  20  and  22  through any conventional means, such as grinding, a fine grained and thermally stable blank  10  remains. The blank  10  is then typically subjected to subsequent hot forming processes. 
     With reference now to  FIGS. 3A-3E , a modification to the present invention is illustrated. In  FIG. 3A  two metal sheets  30  and  32  are butted together along a butt line or seam  34  to be joined into a Tailor-Welded Blank. The concept of combining different materials or sheets into a welded blank is developed to enable the design engineers to tailor the blank so that the materials&#39; best properties are located precisely in the component where they are needed. It is desired to secure the two metal sheets  30  and  32  together along the seam  34  as well as develop a fine grained and thermally stable microstructure along the seam  34  using friction stir welding so that the welded blank is not subjected to premature failure around the weld region during subsequent hot forming processes. 
     In order to achieve this, a sheet  36  of extraneous material overlies the top of the seam  34  along the top of the blanks  30  and  32  while, similarly, a sheet  38  of extraneous material overlies the seam line  34  along the bottom of the blanks  30  and  32 . The sheets  36  and  38  of extraneous material are compatible with the blanks  30  and  32  and are attached to the blanks  30  and  32  in any conventional fashion. 
     As shown in  FIG. 3C , friction stir welding is performed along the seam  34  with the welding tool  40  being plunged from the sheet  36  through the blanks  30  and  32  into the sheet  38 , thus joining the blanks  30  and  32  together along the seam  34 . The resulting weld is illustrated in  FIG. 3D  in which a weld region  42  between the blanks  30  and  32  contains fine grained, thermally stable material while any deformed or unrecrystallized microstructure is contained in the sheets  36  and  38  of extraneous material. The sheets  36  and  38  containing deformed or unrecrystallized microstructure and inducing abnormal grain growth at later hot forming processes are then removed by any conventional means, such as grinding, from the blanks  30  and  32  as shown in  FIG. 3E . Consequently, only fine grained and thermally stable material remains in the weld zone  42  between the two blanks  30  and  32 . 
     With reference now to  FIGS. 4 and 5 , a still further embodiment of the present invention is shown in which a layer  50  containing pinning particles is deposited on one side  52  of the metallic blank  54 , and similarly a second layer  56  containing pinning particles is deposited on the opposite side  58  of the metallic blank  54 . Any conventional method, such as laser powder deposition, may be used to form the layers  50  and  56 . The layers  50  and  56  preferably comprise an oxide powder such as alumina or silica of submicron or nanometric size. 
     As best shown in  FIGS. 4 and 5 , after the step  60  of creating the layers  50  and  56  containing pinning particles, step  60  proceeds to step  62  in which friction stir processing is performed on the metal blank  54  covered by the layers  50  and  56 . Upon completion of the friction stir processing at step  62 , the friction stir processing intermixes the pinning particles into the surfaces  52  and  58  of the blank  54 . The pinning particles mainly located in the layers  50  and  56 , in turn, restrict the migration of the grain boundaries and therefore prevent abnormal grain growth in the layers  50  and  56 , and further prevent the propagation of abnormal grain growth into the blank  54  to be exposed to high temperatures so that the resulting blank  54  along with the layers  50  and  56  has a fine grained and thermally stable microstructure. 
     With reference now to  FIGS. 6 and 7 , a still further embodiment of the invention is shown in which sheets  70  and  76  of extraneous material and containing pinning particles are attached to the top surface  72  and bottom surface  78  of a metal blank  74  to form a metal sheet stack at step  80 . As before, the sheets  70  and  76  containing pinning particles must be compatible with the material of the blank  74 . 
     Friction stir processing is then performed at step  82  on the metal sheet stack comprising the sheets  70  and  76  containing pinning particles and the blank  74 . During the friction stir processing, the welding tool intermixes the pinning particles contained in the sheets  70  and  76  into the metal blank  74  along the surfaces  72  and  78  of the blank  74 . 
     Upon completion of the friction stir processing at step  82 , the pinning particles from the extraneous sheets  70  and  76  are intermixed with the blank  74  along its top surface  72  and its bottom surface  78 . These pinning particles mainly located in the extraneous sheets  70  and  76  restrict the migration of the grain boundaries and therefore prevent abnormal grain growth in the extraneous sheet  70  and  76 , and further prevent the propagation of abnormal grain growth into the blank  74  to be exposed to high temperatures so that the resulting joined metal sheet stack comprising the blank  74  and the sheets  70  and  76  is fine grained and thermally stable. 
     From the foregoing, it can be seen that the present invention provides a simple and yet effective method for forming metal blanks with fine grained and thermally stable microstructure and thus increased high temperature formability. Having described our invention, however, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.