【作者】 陈斌；

【导师】 杨新岐；

【作者基本信息】 天津大学 ， 材料加工工程， 2010， 硕士

【摘要】 本文对TC4钛合金高能束流(激光焊LBW和电子束焊EBW)搭接与叠焊接头的疲劳断裂行为进行了详细研究。首先应用ANSYS软件对焊接接头的应力集中进行了计算分析;其次对焊接接头微观组织、显微硬度、疲劳强度及寿命特性进行了试验研究,对疲劳断口上的焊接缺陷进行了观察与分析,探讨了焊接缺陷对疲劳行为的影响,对疲劳断口用扫描电镜(SEM)观察;最后焊接接头疲劳裂纹的萌生和扩展行为进行了分析讨论,探讨了焊接缺陷与疲劳强度的相互影响规律。有限元计算分析表明,焊接接头类型和搭接间隙是影响应力集中的关键因素,搭接接头应力集中为6.8~7.3明显高于叠焊接头的1.4~2.3,但搭接接头间隙的增大对应力集中影响较小;激光叠焊接头最大应力都出现在焊缝上板咬边缺陷处;而电子束焊最大应力出现在上板咬边或是下板凸高处。疲劳试验结果表明,TC4钛合金高能束流焊搭接接头疲劳断裂位置都出现在热影响区。通过对疲劳寿命进行线性拟合分析得出:激光焊对应N=2×106疲劳寿命下,搭接疲劳强度为41.3MPa,而电子束焊接接头所对应的疲劳强度为39.7MPa;激光叠焊焊接接头所对应的疲劳强度为210.3MPa,而电子束焊接接头所对应的疲劳强度184.1MPa,激光焊接接头具有更高的疲劳强度。对于搭接接头,影响其疲劳强度的主要因素为搭接间隙;对于激光叠焊焊接接头,随着咬边深度和焊缝根部凸高的增加,疲劳强度不断下降;对电子束焊表面进行重熔,使得其疲劳强度平均提高80%。疲劳断口分析表明,应力水平和接头形式对TC4钛合金焊接接头疲劳断裂特征的影响较大。随着应力水平的增加,断口疲劳源区和扩展区的光滑小刻面数量增多,裂纹扩展初始阶段层片状开裂的二次裂纹更加明显,扩展区变得粗糙,高度差增大;搭接接头存在多个裂纹源,裂纹扩展速度快,基本不产生二次裂纹。微观组织分析表明,原始TC4母材中细小的α+β等轴晶粒结构经焊后转变为α+β和α′马氏体交叉混合不均匀和粗化组织结构。激光焊缝中的网篮状α′马氏体组织要多于电子束重熔焊缝中的,而热影响区中情况刚好相反。激光焊上板与下板的焊缝和热影响区硬度均高于母材,而电子束焊上板的焊缝区的硬度值和母材相当,下板的焊缝和热影响区硬度均高于母材。研究结果为激光和电子束焊接在航空制造领域的应用提供重要参考依据。更多还原

【Abstract】 In this paper ,fatigue strength,hardness,fatigue fracture behavior,welded defects and the effect of welded defects on fatigue behavior of TC4 titanium alloy welded joints of high density beams welding (LBW—laser beam welding and EBM—electron beam welding ) were investigated in terms of experimental observations and microscopic characteristics ananlysis.The stress concentration of welded joints were calculated by ANSYS software.The mechanism of fatigue crack initiation and propagation were also researched by using SEM observated the fatigue fracture morphology. The mutual influence between welding defects and fatigue strength was investigated.Calculation results show that the form of welded joints and lap gap are main influence factors for the stress concentration coefficient. The stress concentration coefficient of lap joint is 6.8~7.3 much higher than 1.4~2.3 of overlapped joint. The stress concentration coefficient increase slowly with the increasing of lap joint gap.The maximum stress appears at the biting edges of above board of overlapped joints of LBW, but the maximum stress appears at the biting edges or convex edge of overlapped joints of EBW.Fatigue test results show that The TC4 titanium alloy lap joint of high density beam welding of fatigue fracture location appear in the HAZ. The fatigue life analysis with linear fitting showed that the fatigue strength of lap joints of LBW is 41.3MPa corresponds to N=2?106 fatigue life,while the fatigue strength of lap joints of EBW is 39.7 MPa; the fatigue strength of overlapped welded joints of LBW is 210.3MPa,while the fatigue strength of overlapped joints of EBW is 184.1 MPa.the fatigue strength of LBW is higher than that of the welded joints of EBW .For the lap joint,the lap gap is the main influence factors for the fatigue strength.For the stitch joint of LBW,the fatigue strength is decreasing with depth of undercut and high of welded root.The fatigue strength of the stitch joint of EBM increase 80% after surface remelting process.Fatigue fracture analysis showed that The stress level and form of welded joints have clear effect on fatigue fracture characteristics of TC4 titanium alloy welded joints of high density beams welding.The quantity of slick plane and secondary crack of samdwich increase and the fatigue crack propagation zone become rougher with the increasing of fatigue loading.For lap joints, there are several crack source and no secondary cracks and crack propagation speed is faster.The results of Microstructure analysis show that theα+βfine equiaxed grain structure of base metal transformed intoα+βandα’martensite rough and uneven cross-mixing structure of the weld metal after welding. FZ(fusion zone) of Joints of LBW haveα’martensite more than that of FZ of Joints of EBW,while the opposite phenomenon in the HAZ( heat affected zone).For the joints of LBW ,the hardness of FZ and HAZ is higher than that of base metal. The handness of above plate joints of EBW is equal, while the hardness of FZ and HAZ is higher than that of base metal on the lower plate.Research results provide important basis for the application of LBW and EBW in aerial manufacturing field.更多还原