静轴肩FSW铝合金6061-T4板的T形接头组织特征及动静载特性研究

Microstructure Characteristics,Static and Fatigue Properties of Stationary Shoulder Friction Stir Welded T-joint of AA6061-T4

利用自主开发的静止轴肩焊接工具及工装,在不同的焊接参数下均获得到了外观成形良好、无焊接缺陷的T形接头,并对接头内部成形、显微组织、硬度、静载强度和疲劳性能及疲劳失效机制进行了研究。研究结果表明铝合金静止轴肩搅拌摩擦焊T性接头内部质量良好,无缺陷,接头表面存在表面超细晶区,且焊核区内部由于流动状态不同导致组织及硬度存在一定差异,T形接头底板及筋板硬度较母材有不同程度的降低,底板和筋板拉伸试验均断裂于接头热影响区,在优化的参数下底板和筋板方向的接头系数均高于0.7,在2×106疲劳寿命下的特征疲劳强度可达101.0 MPa,远高于IIW建议的设计准则。焊接速度对接头疲劳性能及疲劳失效机制影响较大,疲劳裂纹萌生于T形接头底板和筋板过渡处,低焊速时断裂机制为穿晶断裂,高焊速时由于晶界稳定性差,断口呈现穿晶断裂和晶间断裂的混合形貌。完成了铝合金T形接头的无缺陷焊接及组织性能研究,为铝合金静止轴肩搅拌摩擦焊T形接头在新型轻量化航天器密封舱结构中的应用提供了技术支撑。

T-joints of AA6061-T4 were fabricated by self-made stationary shoulder friction stir welding tool. Defect-free T-joints with good appearance were obtained under different welding parameters. The weld formation,microstructure,micro-hardness,static performance and fatigue properties of the T-joint were evaluated. The weld surface was characterized by super fine equiaxed grains.Microstructure and hardness distribution in weld nugget vary due to the material flow condition. The hardness in the skin and stringer decreased at different levels compared to that of the base material. T-joints failed at the heat affected zone during the tensile tests in both skin and stringer direction. Using optimized welding parameter,the characterized fatigue strength was 101 MPa at the cycle of 2×106. Fatigue crack initiated at the corner of skin and stringer,the failure mechanism in fatigue tests was transgranular fracture for T-joints at low welding speed and a mixture of intergranular and transgranular fracture for T-joints at high welding speed. The study of microstructures and mechanical properties of defect-free T-joint provides basic technical support for the manufacturing of the next-generation light-weight spacecraft structure.