【作者】 吕磊；

【导师】 宋奎晶；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2022， 硕士

【摘要】 随着各国逐渐重视碳排放问题,轻量化成为结构设计和加工的重要指导思想。异种金属复合结构在航空航天、交通运输领域应用和研究需求迫切。回填式搅拌摩擦点焊（RFSSW）是具有国际先进水平的连接技术,相比传统电阻点焊对于轻质铝、镁、钛合金等具有显著优势,但目前国内外关于铝/钛异种合金RFSSW工艺的基础研究数据较为有限。本文采用试验、表征、数值模拟和理论分析综合手段,探究不同工艺RFSSW焊点成形、组织演变、界面冶金行为以及接头力学性能,揭示焊接过程温度场、变形场和流场演变规律,阐明铝/钛RFSSW焊点成形、冶金连接机理及其与焊接工艺、材料属性之间的内在关联。首先,采用无螺纹搅拌头对2mm厚5A02铝合金和TC4钛合金进行RFSSW试验,制定了合适的工艺参数,探究焊接工艺对焊点成型、界面形态和力学性能的影响。结果表明,在搅拌头的高速旋转和挤压作用下,焊点表面只有少量飞边,成形良好,搅拌区塑性流变条带连续分布,无明显孔洞和Hook缺陷,界面实现冶金可靠连接。搅拌针作用区和搅拌套作用区晶粒形态不同,后者晶粒尺寸更细小,动态再结晶过程更剧烈。界面处形成厚度不足1μm的扩散区域。焊点拉剪失效载荷达到最高为6617N,达到铝合金母材强度的45%,呈现韧脆混合断裂模式。随焊接总时间延长,焊点失效载荷下降。其次,采用螺纹搅拌头探究不同焊接工艺对焊点成型、界面形态、动态再结晶组织和力学性能的影响。结果表明,随着搅拌头旋转速度增加、停留时间延长,搅拌针作用区晶粒尺寸更加细小,动态再结晶过程更加充分,位错密度逐渐降低。搅拌套作用区晶粒呈流线分布,并出现“洋葱环”结构,由大量细小的亚晶组成。搭接界面处出现厚度约为1μm的元素互扩散区域,并形成厚度约为0.3μm的Ti Al₃金属间化合物连续片层。在搅拌头旋转速度为2000rpm、停留0s时接头拉剪失效载荷达到最大值为7455N,达到铝合金母材强度51%。再次,基于ABAQUS建立了铝/钛回填式搅拌摩擦点焊热机耦合模型,并研究了不同工艺参数对焊接温度场、变形场和流场的影响,分析了焊接过程中各个物理场的演变和数值大小。最后,对界面元素扩散和冶金反应进行热力学、动力学分析。结果表明,Ti Al₃为铝/钛合金RFSSW接头最容易形成且最稳定的反应相,焊核区以下界面因位错、晶界、亚晶界增值效应,原子发生热力耦合超扩散,根据原子扩散层厚度计算其扩散系数,热力耦合扩散相比热激活扩散其扩散系数增大约2个数量级。更多还原

【Abstract】 As carbon emission is becoming the worldwide concern,the concept of lightweight has become an important guiding ideology of structural design and processing.There is an urgent need for the study on dissimilar composite structures in the fields of aerospace and transportation.Refill friction stir spot welding（RFSSW）is a cutting edge spot joining technology.Compared with traditional resistance spot welding,it has significant advantages and therefore is superior to conventional welding techniques to join light aluminum,magnesium and titanium alloys.However,at present,the basic research data on RFSSW processing of Al/Ti are relatively limited.This paper will explore the joint formation,microstructure evolution,interface metallurgical behavior and mechanical properties of RFSSW joints and the effect of processing parameters by means of experiment,characterization,numerical simulation and theoretical analysis.Evolution of temperature field,materials deformation field and flow field in the welding process will be clarified.Moreover,the formation and metallurgical connection mechanism of Al/Ti RFSSW joints and its internal relationship with welding processing and material properties are revealed.Firstly,2mm thick 5A02 aluminum alloy and TC4 titanium alloy was joined by RFSSW with non-threaded stirring tools.The process parameters were modified to explore the influence of welding process on the interface morphology and mechanical properties of RFSSW joint.Results show that the smooth joint surface was obtained,on which only a few amount of flash defect is formed under the action of high-speed rotation and extrusion of the tools.In the internal stirring zone,the plastic flow band was continuously distributed.There was no obvious void and Hook defects on the interface.Reliable interface connection was formed by good metallurgical reaction.The grain size and morphology in pin stir zone（P-SZ）and sleeve stir zone（S-SZ）are different.The grain size in the sleeve stir zone was smaller due to more complete recrystallization.The thickness of diffusion layer is less than 1μm at the interface.The maximum tensile shear failure load of the joint is up to 6617 N,up to 45%of the strength of aluminum alloy base material,which exhibits a hybrid ductile and brittle fracture mode.With the total welding time increasing,the tensile and shear fracture load of the RFSSW joint decreases.Secondly,RFSSW experiments with thread stirring tools are performed and effects of welding processing parameters on joint formation,dynamic recrystallization microstructure and mechanical properties are analyzed.Results show that with the increasing of rotation speed and dwell time,the grain size of the P-SZ was smaller,the dynamic recrystallization process was more complete,and the dislocation density decreases gradually.In the S-SZ,the grains show obvious streamline distribution and onion ring structure,which are composed of a large amount of fine sub-grains.The elemental interdiffusion zone with a thickness of about 1μm appears at the lap interface,and a thin continuous intermetallic compound layer of Ti Al₃,which has a thickness of about 0.3μm is formed.At the rotation speed of 2000 rpm,and the dwell time of 0 s,the tensile shear failure load of the joint reaches the maximum of 7455 N,up to 51%of the strength of aluminum alloy base material,.Based on ABAQUS software,thermo-mechanical coupled model for Al/Ti RFSSW is established and the influence of different process parameters on the welding process,including temperature field,material deformation field and flow field of the joint is explored.The evolution and numerical value of each physical field in the welding process are dynamically reproduced.Finally,thermodynamic and kinetic analysis of interfacial element diffusion and metallurgical reactions shows that Ti Al₃ is the most easily formed and stable reaction phase in the refill friction stir spot weld.Due to the increment effect of dislocation,grain boundary and sub grain boundary at the interface below the nugget zone,the atoms have thermal coupling hyperdiffusion.According to the thickness of atomic diffusion layer,it is significantly higher than the thermally activated diffusion about 2 orders.更多还原