中厚板2219铝合金双轴肩搅拌摩擦焊焊接残余应力及微观组织演变

Residual Stress and Microstructure Evolution of Bobbin Tool Friction Stir Welded 2219 Aluminum Alloy Thick Plates

为了研究双轴肩搅拌摩擦焊缝内部焊接残余应力的大小及分布情况,本研究采用短波X射线衍射进行焊接件内部残余应力的无损检测分析;采用光学显微分析、显微硬度和电子背散射衍射(Electron Back-scattering Patterns,EBSD)对焊缝的前进侧和后退侧的母材、热影响区、热机械影响区和焊核区的组织结构演变进行了分析。金相观察结果显示双轴肩搅拌摩擦焊的接头组织在厚度方向上近似于对称分布,呈“腰鼓形”,焊核区与热机影响区的界面为近似双曲线,前进侧热机影响区的分界线更明显。EBSD扫描结果显示热影响区、热机械影响区均存在较强的形变组织;焊核区在剪切变形和焊接循环热的双重影响下发生了动态再结晶,主要为弱取向组织,小角度晶界含量较大。短波X射线衍射结果表明,双轴肩FSW焊接板内部板厚中心层,纵向方向残余应力均大于横向方向;沿着焊缝,拉应力较大区间位于距焊缝起始端150~250 mm的范围内,最大拉应力为244 MPa。

Welding residual stress has a very important influence on the fatigue strength,stress corrosion resistance,dimensional stability and service life of bobbin tool friction stir welds.In order to study the magnitude and distribution of the internal welding residual stress of bobbin tool friction stir welds,the short wave X-ray diffraction was used to conduct the nondestructive testing analysis of the internal residual stress of the weldments;the microstructure evolution of the base metal,heat affected zone,thermo mechanical affected zone and nugget zone at the forward and backward sides of the weld was analyzed by optical microscopy,microhardness and electron back-scattering patterns(EBSD).The metallographic observation results show that the joint structure of double shaft shoulder friction stir welding is approximately symmetrical in the thickness direction,showing a"waist drum"shape.The interface between the nugget zone and the thermo mechanical affected zone is approximately hyperbolic,and the demarcation line of the thermo mechanical affected zone at the forward side is more obvious.EBSD scanning results show that there are strong deformation structures in the heat affected zone and the thermo mechanical affected zone;dynamic recrystallization occurred in the weld nugget zone under the dual influence of shear deformation and welding cycle heat,which is mainly a weakly oriented structure with a large content of small angle grain boundaries.The results of short wave X-ray diffraction show that the longitudinal residual stresses in the central layer of the inner plate thickness of the double shoulder FSW welded plate are greater than those in the transverse direction;along the weld,the section with large tensile stress is 150~250 mm away from the starting end of the weld,and the maximum tensile stress is 244 MPa.