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

Materials Flow Behavior during Probeless Friction Stir Spot Welding and Its Correlation with Macroscopic Morphology and Grain Characteristics of Joints

搅拌摩擦焊接头材料流动行为是优化焊接工艺的根本所在,目前关于无针搅拌摩擦点焊流动行为尚未形成统一的认识。以0.02 mm镍箔为示踪材料,采用轴肩端面具有渐开线凹槽的无针搅拌头,改变旋转速度和焊接时间进行1.8 mm厚2198-T8铝锂合金搭接搅拌摩擦点焊试验,借助微焦点锥束三维CT设备、扫描电镜等测试手段,研究材料流动行为及其对接头宏观形貌、晶粒特征的影响。结果表明,轴肩下方的金属在轴肩挤压和摩擦热作用下先软化,以螺旋形向下向内流动形成搅拌区;随着焊接时间的延长,搅拌区金属向上和向外流动增强,致使搅拌区外缘界面翘曲,形成Hook缺陷。随旋转速度或焊接时间增大,搅拌区金属向下和向上向外流动加剧,焊核的深度和直径增大、晶粒更细小;下板金属软化程度加强,搅拌区外缘下板更多的塑化金属向上向外流动,致使Hook更翘曲。研究结果为深入了解无针搅拌摩擦点焊材料流动行为和优化焊接工艺提供了理论基础。

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.