AZ31B镁合金及其焊接接头的疲劳断裂机理

对AZ31B镁合金进行疲劳实验,在2×106循环次数下,母材、对接接头、横向十字接头和侧面连接接头的疲劳强度分别为66.72,39.00,24.38和24.40MPa。采用光学显微镜对裂纹扩展特征进行分析,结果表明,AZ31B母材的疲劳裂纹宏观扩展路径平滑,但微观观察发现疲劳裂纹扩展方向曲弯,有些裂纹分成两岔;裂纹尖端扩展均为沿晶扩展。焊接接头裂纹均在焊趾部位起裂,对接接头和横线十字接头的裂纹沿着热影响区扩展;侧面连接接头的裂纹起裂位于焊脚部位。采用扫描电子显微镜对疲劳断裂机理进行分析。疲劳断口由准解理或解理台阶组成,均为脆性断裂,断口中存在二次裂纹,对接接头中存在疲劳条纹,其间距约为5μm。

Fatigue fracture mechanism of AZ31B magnesium alloy and its welded joint

Fatigue test was carried out on AZ31B magnesium alloy. Under 2×10 6 cycle times, the fatigue strengths of base metal （BM）, butt joint （BJ）, transverse cross joint （TJ）, lateral connection joint （LJ） are 66.72, 39.00, 24.38 and 24.40 MPa, respectively. The crack propagation behavior of the alloy was analyzed by optical microscopy. The AZ31B magnesium alloy base metal has a smooth crack propagation macroscopic path. However, the microscopic path is twisted and some cracks have two forks, and the crack propagation is transgranular. The crack initiates in the weld toe and the crack propagates along the HAZ for the BJ and TJ; for the LJ crack initiates in the fillet weld leg. The fatigue fracture mechanisms were analyzed by SEM. The fatigue fracture surface consists of quasi-cleavage patterns or cleavage step and a brittle fracture occurs. Numerous secondary cracks are observed; some fatigue striations exist in butt joint and its size is about 5 μm.