Method for forming a metal matrix composite structure
A method for friction stir forming a metal matrix composite (MMC) structure ( 76 ). The method includes the step of providing a substrate ( 12 ) comprising a metallic material and securing a preformed MMC layer ( 14, 16 ) comprising an MMC material in a position overlying at least a portion of the substrate ( 12 ). The method further includes the step of friction stirring the preformed MMC layer ( 14, 16 ) with a friction stirring tool ( 50 ) which includes a rotating probe ( 56 ), including locating the probe ( 56 ) at a stirring depth at which the probe ( 56 ) extends through the preformed MMC layer ( 14, 16 ) into a portion of the substrate ( 12 ) and passing the tool ( 50 ) through the preformed MMC layer ( 14 ) at the stirring depth to friction stir the preformed MMC layer ( 14, 16 ) and integrate the preformed MMC layer ( 14, 16 ) with the substrate ( 12 ).
1. A method for friction stir forming a metal matrix composite (MMC) structure, the method including the steps of:
a) providing a substrate comprising a metallic material and securing a preformed MMC layer comprising an MMC material in a position overlying at least a portion of the substrate; and
b) friction stirring the preformed MMC layer with a friction stirring tool which includes a rotating probe, including locating the probe at a stirring depth at which the probe extends through the preformed MMC layer into a portion of the substrate and passing the rotating probe through the preformed MMC layer at the stirring depth to friction stir the preformed MMC layer and integrate the preformed MMC layer with the substrate;
wherein the rotating probe is passed through the preformed MMC layer along an overlapping stir path and wherein the stir path is configured to relocate defects to an edge of the MMC structure to facilitate subsequent removal.
2. A method according to claim 1 , wherein the stirring tool stirs substantially all of the MMC material in the preformed MMC layer.
3. A method according to claim 1 , wherein the preformed MMC layer comprises a rigid sheet.
4. A method according to claim 1 , wherein the preformed MMC layer comprises a metal/ceramic MMC material and wherein the preformed MMC layer has a ceramic volume fraction of at least 20%.
5. A method according to claim 1 , wherein the preformed MMC layer is secured within a recess of the substrate.
6. A method according to claim 1 , wherein securing the preformed MMC layer to the substrate comprises one or more initial passes of the friction stirring tool.
7. A method according to claim 1 , wherein the friction stirring tool includes a rotating shoulder in contact with a surface of the preformed MMC layer and wherein the probe and shoulder are integrally formed in a tool insert and wherein the friction stirring tool further includes a holder for receiving the insert and wherein the insert and holder are configured to minimise heat transfer from the insert to the holder.
8. A method according to claim 1 , wherein the friction stirring tool includes a rotating shoulder in contact with a surface of the preformed MMC layer and wherein the shoulder is circular and a ratio of the shoulder diameter to a root probe diameter is approximately between 3:1 and 5:1.
9. A method according to claim 1 , wherein the substrate is removably secured to a support plate and wherein the support plate is invertible after completion of the friction stirring performed using a first side of the support plate to permit a subsequent friction stirring operation performed using a second and opposite side of the support plate.
10. A method according to claim 1 , wherein the MMC layer comprises a plurality of preformed MMC segments.
11. A method according to claim 10 , wherein each preformed MMC segment comprises a rigid MMC sheet.
12. A method according to claim 10 , wherein the preformed MMC segments are stirred one-by-one according to a predetermined stirring sequence.
13. A method according to claim 10 , wherein the MMC layer has a planar configuration and the plurality of MMC segments is arranged in an approximately co-planar arrangement to collectively form the MMC layer.
14. A method according to claim 1 , wherein the stir path includes a series of overlapping stir passes which conclude at or adjacent an edge of the structure.
15. A method according to claim 5 , wherein the MMC layer corresponds in shape to the recess.
16. A method according to claim 5 , wherein the MMC layer comprises a plurality of MMC segments which collectively form a shape corresponding to the substrate recess.
17. A method according to claim 7 , wherein the tool insert is formed of an insert material and the holder is formed of a holder material having a lower thermal conductivity than the inset material.
18. A method according to claim 7 , wherein the stirring tool includes a void within the holder which surrounds at least a portion of the insert.
19. A method according to claim 1 , wherein the probe is frustoconical and has a tapering angle of between 10° to 20°.