| As the lightest metal structural material, magnesium alloy is promising for commercial applications in rail transit, aerospace, military, automobile, steamship and other fields. Welding technology is frequently involved in the manufacturing of magnesium alloys. However, it is very difficult to obtain high quality weld joint of magnesium alloy through traditional fusion welding processes(such as gas tungsten argon arc welding, plasma arc welding and laser welding). Such traditional processes easily induce holes, hot cracks, oxides, high residual stress and other defects, which highly impedes the large-scale commercial application of magnesium alloy. In return, friction stir welding(FSW) is a good choice for magnesium alloys as it avoids defects from traditional welding methods. FSW is a green solid-state joining technique, and hence, FSW is chosen in this study in order to produce welded Mg joints with good surface and high joint coefficient(more than 85%). The joint coefficient is defined as the ratio of tensile strength between the weld joint and the base material(BM). FSW technique was applied to weld both AZ31 plates with a thichness of 4-16 mm and Mg-Al-Sn plates with a thickness of 3 mm. The welding parameters were optimized. The effect of welding speed, rotation rate, plate thickness and post-weld heat treatment on the microstructures and mechanical properties of welded Mg alloy was studied. The relationship between microstructure and mechanical properties was clarified. The temperature field during FSW and residual stress of the welded joints were simulated. The conclusions are as follows:(1) The weld joints of AZ31 plates with a thickness of 4~10 mm exhibited good surface and a joint coefficient as high as 94 %, under the rotation rate of 1000~1600 r/min and welding speed of 120~360 mm/min.(2) Heat treatment was benifical to improve the mechanical properties of AZ31 magnesium alloy plates with a thickness of 16 mm after FSW. Before heat treatment, The joint coefficient of AZ31 magnesium alloy thick plates just reached 77%. Compared with the weld joints without heat treatment, post-heat treatment by 250 ℃ × 1 h improved the tensile strength, yield strength and elongation of welded joints by 12 %, 14 % and 34 %, respectively. The joint coefficient of AZ31 welded joint increased to 86 % after heat treatment.(3) The elastic property of Mg-5Al-x Sn(x=1,3) welded joints was higher than that of BM and the joint coefficient reached 91%。 After FSW, β-Mg17Al12 phases with poor thermal stability and low melting point dissolved into the α-Mg matrix, while Mg2 Sn phase with good thermal stability and high melting point remained in the nugget zone.(4) The simulation results of welding temperature were consistent with the experimental data. The simulation error of welding temperature was less than 6 %. Numerical simulation results showed that the maximum temperature of AZ31 magnesium alloy with a thickness of 10 mm during FSW increased from 471 ℃ to 578 ℃ with increasing the rotation rate from 800 r/min to 1600 r/min, The maximum temperature decreased from 578 ℃ to 454 ℃, as the welding speed increased from 120 mm/min to 360 mm/min. When the plate thickness increased from 4 mm to 16 mm, the maximum temperature increased from 442 to 570 ℃ ℃.(5) The results of simulation of residual stress after welding were in good agreement with the experimental data. The simulation error of residual stress was less than 8 %. The calculations showed that the longitudinal and transverse residual stress of AZ31 magnesium alloy with a thickness of 16 mm after FSW was 122 MPa and 115 MPa, respectively.The plates were welded at a rotation speed of 600 r/min, a welding speed of 72 mm/min, a tool tilt angle of 2.5 °and a shoulder plunge depth of 0.5 mm. |
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