| As a type of solid state welding method, Friction Stir Welding(FSW) and Friction Stir processing(FSP) technology have been applied to the study and application of aluminum alloys, magnesium alloys and other light alloys immediately after their invention. 7075 aluminum alloy, as a high-strength aluminum alloy which can be strengthened by thermal treatment, has been widely used in aerospace, aviation and automotive industries. Currently, there are the plenty of related researches on welding and processing of 7075 aluminum alloy and it was found that defect- free welds can be obtained respectively with a larger scope of process parameters variation. However, it was noticed that most studies has only considered the effect of single process parameter on microstructure and property o f 7075 aluminum alloy while lacked of the quantitative study with a larger process parameters variation. The present work is to analyze systematically microstructures and properties of 7075 aluminum alloy under a larger variation of friction stir process parameters by optical microscopy, scanning electron microscopy, the Backscattered electron channel phase contrast(ECC) and micro- hardness and other methods. The results have both theoretical and practical significance for the study of friction stir welding of 7075 aluminum alloy.The tests of as-rolled 7075 aluminum alloy by friction stir processing showed that high quality such as good appearance and internal defect- free process structure can be obtained as the rotation speed change from 400 rpm to 1200 rpm while welding speed range from 60 mm/min to 100mm/min. Studies revealed that, under various process parameters, variation of grain size at the centerline of process area from the plate surface to plate bottom has following regularities: when the welding speed keep constant, the area of process deformed zone and thermo- mechanical affected zone(TMAZ) and the grain size in the center of weld nugget zone(WZ) increases as the rotation speed increases, this phenomena is obvious especially at the plate bottom; w hen the rotation speed keep constant, the quicker the welding speed, the larger area of process deformed zone, and the smaller the grain size in the center of WZ; when the linear speed(rotation speed / welding speed) are the same, the area of process defor med zone is found to be almost the same while the grain size in the center of WZ increases slightly as the rotation speed increases.The micro- hardness distribution in processing area shows that the hardness values of at the most layers are not symmetrica l around the processing center line under various process parameters. The lowest hardness values is discovered to locate at the interface of TMAZ/HAZ of the advancing side. The hardness distribution at the layers of 1.0, 1.5, 2.0 2.5, 3.0 and 3.5mm is W-shaped while 0.5mm layer hardness distribution is V-shaped. The hardness of WZ is significantly lower than that of the base metal, and the hardness values at processing center line gradually decreases from the surface to the bottom of plate. There are smaller equiaxed recrystallization grains with high hardness values at the top of WZ.Studies have shown that when the welding speed is determined, the hardness values of WZ and TMAZ increases with increasing rotation speed, especially, the hardness values of WZ was found to increases significantly under the condition of the 1200 rpm, the differences between the lowest hardness value in TMAZ/HAZ and hardness value of WZ becomes obvious. When the rotation speed is determined, the average and the maximum hardness values of WZ reduce constantly with the increase of welding speed. When the ratio of rotation speed to welding speed are the same, the average hardness of WZ increases slightly with the increase of rotation speed, however, the variation trend of overall hardness is not significant.The grain size and micro-hardness values under various process parameters shows a decreasing trend from the plate surface to the bottom, which indicates that precipitate morphology has a significant influence on the mechanical performance of processing area because the effect of second phase strengthening is bigger than that grain refinement strengthening.The backscattered electron channeling contrast(ECC) contrast processing area of hardness distribution map and metallographic microstructure shows that weld nugget zone is composed of fine equiaxed grains, and from the bottom to the top of the grains became bigger, precipitates from grain boundary decreased gradually, intracrystalline precipitated phase gradually increased, hardness distribution by the bottom plate to the plate surface increases gradually.Research shows that when the welding speed is determined, the hardness values of weld nugget zone is increased with the increasing rotation speed. The increasing hardness is related to smaller weld zone grain and the increasing of the number of second phase particles in the particle. When the rotation speed is determined, the hardness values of weld nugget zone is reduced and the grain size of weld nugget zone is reduced and with the increase of welding speed, the reducing hardness is related to intracrystalline precipitated phase gradually increasing. When the line speed(rotational speed / welding speed) are the same, the average hardness of weld nugget zone increased slightly with the increase of rotation speed, intracrystalline precipitated phase gradually increased, and the size of second phase is smaller,all of that shows the size and distribution of the second phase in the microstructure have great influence on the performance of friction stir processing. |
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