| Friction stir processing (FSP) is a novel severe plastic deformation (SPD) technique and has been successfully used in preparing metallic materials with fine-grained structure. Compared with other SPD technologies, FSP can complete a program in a shorter time with less pre-treatment. FSP is an environmental friendly technology. There are remarkable evolutions of microstructure in FSP, but microstructure evolution mechanism during FSP is still not fully understood. So the study on microstructure evolution mechanism and material's mechanical behavior characteristics in FSP is of great significance.In this paper, AZ31 magnesium alloy with 4mm thickness was processed by FSP. Defect-free materials could be obtained under a range of rotation speed (500-1500 rpm) and travel speed (50-500 mm/min). The whole processing area was composed of stired zone (SZ), thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and base material (BM). Fine equiaxed grains were obtained in SZ, and the mean grain size increased with the rotation speed increasing or travel speed decreasing. The influence of rotation speed "ω" (rpm) and travel speed "υ" (mm/min) on grain size in SZ was studied. The relationship between grain size and Zener Hollomon parameter "Z" was examined, and it was found that in a range of smaller "Z",the grain size decreased with the increasing of "Z", while in a bigger "Z" range, grain size did not change significantly as "Z" value increased.Intense (0002) basal texture was formed on the normal plane in the AZ31 magnesium alloy after FSP. The angle between basal texture and normal direction of the plate changed with different processing parameters.The fine-grain materials prepared by FSP were heat-treated at 673K for 1 hour. Abnormal grain growth phenomenon was found in the materials processed under lower rotation speed (500-900rpm), meanwhile, other materials showed better microstructure stability. The stability of specimens processed under 1500rpm and 60mm/min was examined by annealing over a range of temperature 673-723K for 1 hour, meanwhile, the mean grain size was increased from-14μm to-19.6μm.Micro-hardness measurement showed that the hardness of stir zone was higher than that of the other zones. Microhardness value and grain size fit Hall-Petch relationship. The tensile strength of fine grain materials was lower than that of the base material, but most specimens' elongation increased. Superplastic deformation behavior of FSP AZ31 magnesium alloy was investigated at temperatures ranging from 573K to 723K and strain rates ranging from 5×10-4s-1 to 1×10-2s-1 The result indicated that FSP material exhibited excellent superplasticity at elevated temperature. The maximum elongation of 1050% was obtained at 723K and a strain rate of 5×10-4s-1. During superplastic deformation, two competitive mechanisms, i.e. dynamic grain growth and dynamic recrystallization were operated in the microstructure evolution. Grain boundary sliding (GBS) was the primary mechanism of the superplastic deformation. Cavities interlinkage and coarse grain were the main reasons lead specimens to fail at superplastic deformation.A newly designed FSP experiment performed under water was studied. Defect-free joints could be obtained above the rotation speed 800rpm at a constant welding speed 60mm/min. The grain size in stir zone processed submerge was finer than that in the air under the same processing parameter. The influence of process submerge on mechanical properties was analyzed. The further study was forecasted. |
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