搅拌针转速对厚板镁合金SSFSW焊缝组织及性能的影响

Effect of pin rotational speed on microstructure and properties of SSFSW weld for thick-plate magnesium alloy

对9 mm厚板AZ31B镁合金进行静止轴肩搅拌摩擦焊(stationary shoulder friction stir welding,SSFSW)工艺试验,探讨搅拌针转速(500~1000 r/min)对焊缝组织及力学性能的影响规律.结果表明,在给定焊接速度80 mm/min下,搅拌针转速在600~800 r/min范围可获得表面光滑、无内部缺陷的对接焊缝,当转速为1000 r/min时焊缝表面出现不连续凹坑但内部仍无缺陷.随着转速增加,晶粒尺寸由(11.11±1.68)μm增加到(18.95±1.83)μm;在700 r/min时焊核区晶粒尺寸沿板厚差异最小.焊缝中间硬度分布具有不均匀性且随转速增加而减小,最大差异为10.97 HV,最低硬度47 HV位于前进侧的热力影响区与焊核区界面处.在700 r/min下接头力学性能最佳,强度系数为90.2%、对应断后伸长率为母材69.3%.随着转速增加,断裂模式由韧-脆混合断裂转变为剪切-韧性混合断裂.

The thick-plates of AZ31B magnesium alloy with 9 mm thickness were joined successfully by stationary shoulder friction stir welding(SSFSW)to explore the influence of stirring needle speed(500–1000 r/min)on the microstructure and mechanical properties of weld.The results show that the butt weld with smooth surface and no internal defects can be obtained at the rotation speed of 600−800 r/min under the given welding speed of 80 mm/min.When the rotation speed is 1000 r/min,discontinuous pits appear on the surface but there are still no defects in the weld.With the increase of rotating speed,the grain size increase from(11.11±1.68)μm to(18.95±1.83)μm.At 700 r/min,the grain size difference in the nugget zone is the smallest along the plate thickness.The inhomogeneity of hardness distribution in the WNZ decreases with the increase of rotational speed.The maximum difference of hardness in the middle of the plate thickness is 10.97 HV,and the minimum hardness is 47 HV at the interface between the heat affected zone and the nugget zone of the advancing side.The joint has the best mechanical properties at 700 r/min,the strength coefficient is 90.2%and the corresponding elongation is 69.3%of the BM.With the increase of rotational speed,the fracture mode changes from ductile-brittle mixed fracture to shear-ductile mixed fracture.