无针搅拌摩擦点焊材料流动行为及其与接头宏观形貌, 晶粒特征的相关性

The materials flow behavior during friction stir welding joints is very important for optimizing the welding process. At present,there is no unified understanding of the flow behavior during probeless friction stir spot welding. 1.8 mm thick sheets of the AA2198-T8 Al-Li alloy are lap-welded with probeless friction stir spot welding by changing the rotation speed and dwell time. The tracer material used in the present study is Ni with 0.02 mm in thickness,and the tool is machined the involute groove on the shoulder surface. The materials flow behavior and its correlation with macroscopic morphology and grain characteristics of joints are investigated with a three-dimensional X-ray tomography and scanning electron microscopy. The experimental results showed that materials softened by friction heat and extrusion will flow radially inwards and flow downwards with a spiral pattern,forming the stir zone(SZ). With the increase of dwell time,the tendency of the materials in the SZ to flow upwards and outwards,causing the interface on the outer edge of the SZ to warp and forming the Hook defect. With the increase of rotation speed and dwell time,the materials of the SZ flow downwards,upwards and outwards more intensely,thus the depth and diameter of the SZ increase and the size of grains decrease;also the softening degree of the bottom sheet materials is strengthened,so the more materials of the outer edge of the stir zone flowed upwards and outwards,causing the Hook to be more warped. The results provide a theoretical basis for understanding the flow behavior during probeless friction stir spot welding materials and optimizing the welding process.