Patent Application
20110076419
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.
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.
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:
With reference first to
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
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
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
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
With reference now to
As best shown in
With reference now to
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.
