| 其他摘要 | The 6 mm 6061-T651 and 8 mm 7075-T651 aluminum alloy rolled plates were friction stir welded, and the effects of welding process parameters, the welding tool, the initial butt surface, the post-weld heat treatment, the second-welding on the back of the weld on the joint microstructure, mechanical properties and fracture behavior were studied. This work has provided some beneficial references to the development and industrial application on friction stir welding.
Firstly, the compact non- flaw 6061-T651 welded joints were obtained with wider rotation rates (ω = 400-1600 rpm) and tool traverse speeds (ν = 100-400 mm/min). The samples at higher tool traverse speed of 400 mm/min exhibited higher tensile strength with 45o shear fracture, whereas lower tensile strength with nearly vertical fracture were observed for the sample welded at lower traverse speed of 100 mm/min. The fracture paths corresponded well with the lowest hardness distribution profiles in both joints. The heat indexes cannot be used as the parameters to evaluate the thermal input and mechanical properties.
Secondly, the 6061Al-T651 alloy plate friction stir welded with three different shoulder dimensions (16、20 and 24 mm) of the welding tools, and it also obtained the high grade welded joints. The results showed that the shoulder dimension had no effects of the mechanical properties, and they were destroyed with 45o shear fracture of HAZ in tensile experiments. Because they had the similar hardness distribution and values in HAZ, while the microstructures in HAZ move outside that result in the similar temperature grads moving. The HAZ were divided into three second-regions by microscopic appearance analysis. The hardness decreased greatly because most of the bulky phase β" and the needle-shape phase β’ in the region I near HAZ had dissolution in the form of solution strength, whereas the hardness increased because of the needle-shape phase β’ in the region III near base materials. The region II between region I and III had the lowest hardness because of the bulky phase β", which led to the fracture of the tensile tests.
Thirdly, the comparison between the butt and stir-in-plate 7075-T651 friction stir welded indicated that FSW of a single plate is a very effective approach to verify the effect of the initial butt surfaces, and the initial butt surfaces had close correlation to the formation of the lazy "S". The lazy "S" could be weakened, but could not be completely eliminated by clearing the butt surfaces before the FSW. The lazy "S" under as-welded condition did not show up in both welds and two welds exhibited similar tensile properties and fracture characteristics, and all fractured in HAZ. After post-weld T6-treatment, the lazy "S" appeared on the butt weld as "S" micro-crack and discontinuous cavities, which were verified to be associated with the oxide particles. This resulted in the reduced tensile strength and significantly deteriorated ductility with the fracture initiating and propagating along the lazy "S".
Finally, 7075-T651 aluminum alloy rolled plate was friction stir welded, and the effects of post-weld heat-treatment and second-welding on the back of the weld on the tensile properties were investigated in detail. The results show that the grains of the nugget did not grow abnormally at 470 oC; the grain began to grow abnormally at 480 oC; the bigger grain devoured the smaller ones and grew along the onion rings at 490 oC, and the abnormal grain occupy almost one half of the nugget. Advances along with the melting temperature, "S" the line is even more obvious. However, the tensile properties were greatly improved after second-welding on the back of the weld, because it eliminated the defects of the root, especially the both welded samples had the better properties than the one side welds after PWHT. |
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