采用传统搅拌摩擦焊和冷源辅助搅拌摩擦焊对3 mm厚的AZ31B镁合金进行焊接. 利用电子背散射衍射、透射电子显微镜和静拉伸试验研究焊缝区的微观组织对力学性能的影响. 结果表明，液态二氧化碳不仅降低焊接峰值温度，还提高焊后冷却速度. 焊缝峰值温度的降低为激活{10-12}孪生行为创造了有利条件. {10-12}孪晶可降低基面织构的强度，也可进一步分割晶粒，起到细化晶粒的作用. 焊后冷却速度的提高使焊接过程中产生的大量位错保留在晶粒内部. 因此冷源辅助搅拌摩擦焊缝表现为具有大量{10-12}孪晶和位错的细晶结构. 在拉伸过程中，细晶强化和位错强化为主要强化机制. 孪晶界面可有效吸收和分解变形时产生的位错，从而协调应变和减小应力集中，使焊缝具有合理的应变硬化行为和强塑性匹配.
3-mm-thick AZ31B magnesium alloy plates were successfully joined by conventional friction stir welding and cold source-assisted friction stir welding. The effect of microstructure on mechanical properties of the welds were investigated by electron backscatter diffraction, transmission electron microscopy, and tensile tests. The obtained results showed that the welding peak temperature was significantly reduced, and the cooling rate after welding also remarkably increased due to liquid CO2 cooling. The decreased welding peak temperature created favorable condition for the activation of {10-12} twinning behavior. The {10-12} twins reduced the basal texture intensity, and further refined the grain size. Because of the enhanced cooling rate, massive dislocations which generated during the welding process were maintained in the grain interior. Therefore, the weld obtained by cold source-assisted friction stir welding exhibited a fine-grained structure with massive {10-12} twins and dislocations. During the tensile tests, the main strengthening mechanisms were attributed to grain boundary strengthening and dislocation strengthening. The twin boundaries can effectively coordinate strain and reduce stress concentration by absorbing and decomposing dislocations which produced in plastic deformation. Therefore, the weld showed reasonable strain hardening behavior and a good matching of strength and ductility.
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