【作者】 鄢东洋；

【导师】 吴爱萍；

【作者基本信息】 清华大学 ， 材料科学与工程， 2010， 博士

【摘要】 搅拌摩擦焊是一项先进的固相焊接技术,在航空航天等精密制造领域有广阔的应用前景。但目前该方法在热及力学过程方面的研究并不完善,严重影响了人们科学和准确地认识和应用这项先进技术。基于此,本文深入研究搅拌头和被焊材料的相互作用,建立了计算焊接生热功率的生热方程,理清了热载荷和搅拌头机械载荷对接头残余应力和变形的影响,实现了工程结构焊后残余应力和变形的高效预测。本研究具有基础研究和生产指导两方面的重要意义。设计和开展不同工艺参数条件下的铝合金搅拌摩擦焊试验,通过测量搅拌头扭矩和焊接温度场的变化,系统分析了转速、焊速和下压力对搅拌摩擦焊生热功率的影响规律,发现搅拌摩擦焊能始终保持固相过程的根本原因在于其生热和温度具有反馈平衡作用。以搅拌摩擦焊中的功热转换为基础,结合工艺参数对生热功率的影响规律,建立了能体现固相焊接过程并能全面反映转速、下压力和焊速影响的生热方程,实现了直接依据工艺条件计算焊接热输入量、预测焊接温度场、预测焊接所需主轴扭矩等目标。针对铝合金薄板在搅拌摩擦焊后残余应力和变形与电弧焊结果显著不同的现象,利用数值模拟工具深入探索搅拌头机械载荷对被焊结构瞬态应力应变的影响规律和机制。发现搅拌头下压力具有增加搅拌区材料受力状态中的静水压力成分、增大试板下表面和垫板之间的摩擦力、使搅拌区内靠近试板上表面的材料产生面内附加膨胀等作用,是铝合金薄板搅拌摩擦焊后残余变形与电弧焊结果相反的根本原因,同时对残余应力和变形的减小也有贡献,而搅拌头扭矩主要导致焊接残余应力产生不对称的分布特征,对残余变形影响较小。根据搅拌头与被焊材料之间相互作用的研究结果,建立了合理的搅拌摩擦焊应力和变形数值分析模型,为提高残余应力和变形数值预测的效率,开发了以温度为控制变量、并且温度载荷分段添加的温度函数法。对于大型薄壁结构件的焊接残余应力和变形,温度函数法计算结果与移动热源法计算结果的差别在15%以内,但计算时间可以减少到移动热源法的10%以内。利用温度函数法实现了铝合金薄壁结构搅拌摩擦焊残余变形的高效预测。更多还原

【Abstract】 Friction stir welding (FSW) is an advanced solid state joining technology, which has great potential of application in many fields like aerospace industry. The thermo-mechanical effect of FSW on welded structure becomes more and more important in industry application. Meanwhile the thermo-mechanical effect of FSW is different from most arc welding processes for its lower temperature, lower residual stress and small distortion. Therefore, the interaction between stir tool and the material being welded, which is the most important character of FSW, was investigated in this dissertation. First, an equation was established to calculate the heat generation. Then, the effects of thermal load and stir tool’s mechanical loads on welding residual stress and distortion of joint were distinguished. Finally, a high efficiency numerical simulation method was developed to predict the residual stress and distortion of engineering structures after FSW. The research in this dissertation could give powerful supports in some aspects of foundational research and application of FSW.FSW experiments on aluminium alloy sheets with different rotation speed, travel speed and down force were carried out, and the torques of stir tool and the welding temperature fields were also measured during welding process. Based on these measured results, the influence of rotation speed, travel speed and down force on heat generation was discussed. The results of experiment and analysis showed that there was a reactive-balanceable effect between heat generation and welding temperature in FSW, which resulted in the solid state welding process. Based on the transformation from mechanical energy to heat energy and the influence of welding parameters on heat generation in FSW process, an equation was established to calculate the heat generation using only welding parameters. The equation successfully represented the solid state welding process and the influence of rotation speed, travel speed and down force on heat generation in FSW. With this heat generation equation, the welding temperature field and the torque of stir tool during FSW process can be predicted through numerical simulation.The residual stress and distortion of aluminium alloy sheet after FSW are obviously different from those of the arc fusion weld. Their distortion directions are opposite, and the value of residual stress and distortion in FSW is lower than those in arc welding. According to this phenomenon, the trends and mechanisms of the stir tool’s mechanical loads affecting on the transient stress and strain of welded structure were analyzed with numerical simulation. The investigation finds out that the down force of stir tool have the functions of increasing the components of hydrostatic stress in the stress state of the material in stir zone, increasing the friction between back plate and bottom surface of welded sheet, and bringing additional expansion in longitudinal and transversal direction of the sheet to the material in the upper part of stir zone besides the thermal expansion. These functions of down force are the essential reasons for the abnormal distortion phenomenon on the friction stir welded aluminium alloy sheet compared with the conventional arc welding. The torque of stir tool results in the asymmetrical distribution of residual stress in FSW, but it has little influence on residual distortion.According to the investigation of the interaction between stir tool and material being welded, a reasonable numerical simulation model on FSW was established to analyze the residual stress and distortion. In order to enhance the efficiency of the numerical calculation, a new welding numerical simulation technique called temperature function method was developed, in which the controlling variable was welding temperature, and this temperature was added into the model in subsection form. For the prediction of welding residual stress and distortion of large structure, the error between computed results from the temperature function method and the moving heat source method was no more than 15%, while the time cost in the temperature function method is less than 10% of that in the moving heat source method. At last, the temperature function method was applied to predict the residual stress and distortion of aluminium alloy thin-walled structure after FSW.更多还原