【作者】 李娜；

【导师】 王希靖；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2012， 硕士

【摘要】 搅拌摩擦焊（friction stir welding, FSW）是英国焊接研究所（TWI）于1991年针对铝合金开发的一种先进的固相连接技术。与传统的焊接方法相比具有优质、高效、低耗、变形小、无污染、成本低等特点。搅拌摩擦焊作为一种新型的固态焊接技术，还存在着许多问题需要更深入广泛地研究，如材料的流动、温度场、接头的残余应力等。目前国内外许多学者通过试验手段及数值模拟方法对搅拌摩擦焊接热过程及焊接残余应力进行了许多研究，但针对铝合金薄板失稳变形的研究较少，因此通过试验与数值模拟相结合的方法来研究铝合金薄板搅拌摩擦焊接头的残余应力和失稳变形规律，对在大型铝合金薄板结构制造中推广和应用搅拌摩擦焊技术具有重要工程应用意义。本文采用盲孔法对3mm LY12铝合金薄板在不同焊接参数下进行搅拌摩擦焊对接后的残余应力进行了测量和分析。为了消除孔边塑性变形对测量精度的影响，建立了三维有限元法模拟了应变释放系数的测试试验，使用基于孔边形状改变比能S的A、B系数修正法对盲孔释放系数进行修正。结果表明：接头残余应力呈不对称分布，前进侧应力高于返回侧；纵向残余应力总是高于横向残余应力，但是横向残余应力并非在零附近，而是有一定的大小，约为焊前母材强度的12.7%；在一定范围内，搅拌摩擦焊接接头残余应力随旋转速度先增大后减小，随下压量和焊接速度的增加而增加。其次建立了三维有限元模型并分析搅拌摩擦焊过程中温度场的分布。在温度场模型中施加边界条件，确定搅拌摩擦焊热源模型，得到整个焊接过程中的热循环曲线，最高温度值均在各自的高塑性温度区间。最后采用间接热耦合法把在温度场中得到的热载荷施加到构件模型中，获得了薄板焊后的残余应力及焊接热弹塑性失稳变形分布特征。同时研究了焊接速度、旋转速度等焊接参数对温度分布及残余应力分布的影响。模拟结果与相关实验吻合良好。残余应力计算分析表明，LY12铝合金薄板搅拌摩擦焊对接接头的纵向残余应力数值较大，接近材料的屈服强度，与试验测量值有较大差异。但基于残余应力计算的固有应变法和间接热耦合法得到的残余应力结果，与用盲孔法的试验测量结果趋势相同，同时相对变形量吻合较好，误差控制在20％以内。失稳变形分析结果表明铝合金薄板搅拌摩擦焊后存在较大的残余应力，明显高于测量值，但残余应力的数值分析结果与试验测量值具有相似的应力分布趋势，说明间接热耦合模型预测铝合金薄板搅拌摩擦焊接残余应力是基本可行的，并且采用固有应变法与热力耦合相结合的方式预测焊接变形和残余应力比只用热力耦合方法更节省时间。更多还原

【Abstract】 Friction Stir Welding which is an advanced Connection technology wasdeveloped by British Welding Institute (TWI) in1991for the aluminum alloy.Compared with traditional welding methods, it has characteristic of high-quality, highefficiency, low consumption, small deformation, no pollution, low cost.and so on.As a new solid joint technique, there is also a great deal of problems for frictionstir welding,such as the flow of material, temperature field and residual stress, need tobe studied widely. Most scholars at home and abroad researched thermal process andthe residual stress of the FSW by means of experiment and numerical simulation.Butonly a little research was about aluminum alloy sheets buckling deformation. So it isnecessary to integrate the numerical simulation and experiment to analysis theresidual stress and buckling deformation in the aluminum sheet FSW joints. Thenanalyze the feasibility ofthe prediction model.This paper investigates the residual stresses for friction stir welded3mmLY12alloy sheets in different parameters by using the blind-hole technique. To release theinfluence on the measuring accuracy of high welding residual stress when the holedrilling method is used, finite element modle of three dimensions was created tosimulate the test of the strain release coefficients. The strain release coefficientsA andB was corrected based on the energy parameter S which was obtained by the FEManalysis results. The results showed that longitudinal residual stresses areasymmetrically distributed at different sides of the weld center and it is high at theadvancing side and relatively low at the retreating side; like the longitudinal residualstresses, the transverse residual stresses are not at zero but about12.7%of base metal.The residual stresses will become larger at fist and have some decrease then with therotation rate and become more and more larger with the pressed-in and welding speed.On the other hand, a three-dimensional finite element model was established andthe temperature distribution in the FSW process was analyzed.Through imposing theboundary conditions in the temperature model，the heat source model of FSW wasdefined and the thermal cycle curve of the whole welding process was obtained.Theresults showed that the maximum temperature values were all in their high plastictemperature range.At last,the heat load obtained in the temperature distribution wasapplied to thecomponent model through indirect thermal coupled method.Theresistantstress and the thermal elastic-plastic buckling deformation of welded sheets were obtained. At the same time, the effects of welding speed, rotating speed and thereduced altitude on temperature and residual stress distributions were investigated.Theresults correlate well with corresponding experiments.The results of calculatingresistant stress showed that the longitudinal resistant stress of LY12joints were nearlyequal to the material yield strength. The difference between calculating andexperiment was large.But after calculating by inherent strain method and indirect heatcoupled methods which were based on the resistant stress, the result trend was assame as the experiment. At the same time, the relative deformation is anatomized welland errors were alllimited below20％.The results of buckling deformation showedthat there was high resistant stress in the aluminum sheets after FSW: which wasobviously higher than the experimental results.The numerical analysis results ofresistant stress and the experimental results showed the similar stress distributingtrend.This proved that it was feasible to prediet the resistant stress in FSW aluminumsheets through indirect heat coupled model and the inherent strain method is moretimesaver than the coupled thermal-mechanical method.更多还原