Microstructure Evolution And Mechanical Properties During Friction Stir Processing Of Solution Treated7075Alloy

The high-strength7075aluminum alloy is hard to weld by conventional fusionwelding what has been widely researched by Friction Stir Welding and Friction StirProcessing（FSW/FSP）.However,all these studies are focused on the deposited weld. Itis difficult to understand development of microstructure during FSW/FSP. Based on theapplication of consideration, FSW/FSP researches of7075aluminum alloy have beenmainly focused on age-treated7075aluminum alloy, a relative lack of researches onsolution treated7075alloy. In these paper,through stop-action FSP，combined withmicro-hardness experiments，tensile testing at ambient temperature and SEM-ECCexperiments,studied on microstructure and property evolution during FSP of solutiontreated7075alloy, and SEM-EBSD experiments were used to study grain development,dynamic recrystallization mechanism and micro-texture evolution during FSP ofsolution treated7075alloy.①Micro-hardness and microstructure on the longitudinal-section (RD-TDsection)of stop-action processing area was studied.The results show that: There was alow hardness value region similar to‘N’shape on the edge area of the shoulder, insidethe‘N’shape region, hardness value in the WNZ of the advancing side was higher thanthe retreating side. At the forepart,the advancing side and the retreating side of thehole,with the increasing distance from the hole edge,the amounts of subgrain graduallydecreased, the amounts and size of precipitates gradually increased. Formed therecrystal grains beind the edge of hole, with the increasing distance from the holeedge,the size of recrystal grains just a little increased, the amounts of precipitatesgradually increased.②Micro-hardness and microstructure on cross-section（ND-TD section）ofdifferent locations behind the center of hole was studied. The results show that: Faraway the the center of hole, the hardness profiles on cross-section had largerfluctuation,softening zone was more apparent,the hardness value of the WNZ andsoftening zone became more and more low. Hardness in the center of the WNZgradually increased from bottom to surface. During the weld cooling process,in theWNZ,intragranular fine precipitates growed up under the influention oftemperature,simultaneously,coarser precipitates formed at grain boundary,in thesoftening zone,intragranular fine precipitates gradually coarsened under effect of thermal cycle.③SEM-ECC experiments and tensile testing at ambient temperature from bottomto top in the WNZ show that:The sizes of fine equiaxed grains increased gradually frombottom to top in the WNZ. The amounts of intergruanlar fine precipitates graduallydecreased from bottom to top while the amounts of intragranular coarser precipitatesgradually increased from bottom to top. Tensile strength and elongation increasedgradually from bottom to top in the WNZ, combined with micro-hardness distribution inthe WNZ. Demonstrate that dispersion strengthening mechanism is main effect factorsfor hardness and strength.④EBSD experiments on the longitudinal-section of stop-action processing areashow that:Initial grains gradually developed around the edge of hole under the effect ofthe tool, finally, evolved into fine equiaxed grains. The TMAZ of the advancing sideand the retreating side all happened different degree dynamic recovery and dynamicrecrystallization, the only difference is acting force pattern of tool. Dynamicrecrystallization mechanism during FSP of solution treated7075alloy, are geometricdynamic recrystallization and continuous dynamic recrystallization. Micro-textureresearch shows that typical rolling texture of initial grain under the effect of the tool,continuously turned to shear direction and ultimately evolved intostrong B/B components of shear textures.