TC4等离子微束焊接接头微振疲劳失效分析

发布时间：2018-06-14 23:42

本文选题：共振疲劳试验 + 失效分析　；参考：《中国民航大学》2015年硕士论文

【摘要】：本文基于一阶恒幅共振试验,对TC4钛合金板材及其微束等离子焊接接头的共振疲劳性能进行了研究。首先探寻了试样尖端振幅与失效前振动周次之间的关系;通过替代试验将振幅与应变联系起来,并通过Abaqus有限元仿真修正了替代数值;绘制了TC4钛合金板材共振疲劳S-N曲线,该S-N曲线的寿命周次区间贯穿低周、高周、超高周三个阶段。然后对TC4钛合金共振疲劳断口形貌进行了分析;统计了不同应力级别与试样裂纹源总数之间的关系;通过扫描电子显微照片中疲劳辉纹的尺寸计算了不同应力下裂纹稳定扩展阶段的扩展速率;宏观统计了裂纹快速扩展阶段的扩展速率;对裂纹扩展速率试样的断口成因和相应机理进行了解释。之后完成对微束等离子弧焊接接头的显微组织观察、力学性能测试、显微硬度测试和共振疲劳性能研究;得到了应力级别与焊接接头裂纹源位置之间的关系;统计了焊接接头不同位置寿命与同振幅板材寿命之间的关系。最后观察了接头试样的疲劳断口,分析了不同失效部位的失效机理;探讨了试样表面机械损伤造成的凹陷缺陷、链状焊接气孔缺陷对裂纹萌生、扩展行为的影响;统计了两种缺陷造成试样寿命下降的比例。TC4钛合金共振疲劳试验结果表明,材料应力越高,寿命越低,在2×105周次之前,曲线下降明显,之后相对平缓,在振动周次超过107周次后依然会出现裂纹失效;其共振疲劳极限为422MPa,该数值与由TC4制造的叶片的共振疲劳极限基本相同。TC4钛合金板材共振疲劳断口形貌分析表明,材料在低周、高周、超高周情况下皆为脆性断裂,其中应力越高,断口越有延性特点,应力越低,断口越展现解理形貌;裂纹扩展遵循穿晶、沿晶两种机制;裂纹个数与应力级别基本呈负相关关系;在裂纹稳定扩展阶段,裂纹扩展速率与加载应力呈正相关;裂纹最初沿着剪切应力方向萌生,逐渐趋向于垂直于应力的方向扩展,当完全垂直时,裂纹扩展速率有明显变大现象。焊接接头共振疲劳试验结果表明,高应力下焊接接头失效位置为热影响区细晶区,该位置疲劳寿命相比于同振幅板材有所下降;低应力下接头失效位置为焊缝,焊缝的疲劳寿命高于同振幅板材。焊接接头振动疲劳断口形貌分析表明,无论在热影响区细晶区失效还是在焊缝失效,其失效形式皆为脆性,断口呈现解理形貌;对于裂纹稳定扩展阶段,焊接接头的裂纹扩展速率明显快于板材,试样寿命主要集中在裂纹萌生阶段;表面凹陷缺陷可以成长为裂纹源,导致接头疲劳寿命下降近一成;链状气孔缺陷没有成长为裂纹源,但是它们的存在改变了裂纹扩展速率和方向,使接头寿命下降近两成。
[Abstract]:Based on the first order constant amplitude resonance test, the resonance fatigue properties of TC4 titanium alloy sheet and its microbeam plasma welding joint are studied. The relationship between the amplitude of the specimen tip and the vibration cycle before failure is explored. The amplitude and strain are correlated with the strain through the substitution test, and the substitution value is corrected by Abaqus finite element simulation. The S-N curve of resonance fatigue of TC4 titanium alloy sheet is drawn. The life cycle of the S-N curve runs through three stages: low cycle, high cycle and super high cycle. Then, the morphology of resonance fatigue fracture of TC4 titanium alloy was analyzed, and the relationship between different stress levels and the total number of crack sources of TC4 titanium alloy was analyzed. Through the size of fatigue stripes in scanning electron microscopy (SEM), the propagation rate of crack stable growth stage under different stresses is calculated, and the propagation rate of crack rapid growth stage is calculated macroscopically. The fracture formation and the corresponding mechanism of crack growth rate specimen are explained. The microstructure, mechanical properties, microhardness and resonance fatigue properties of microbeam plasma arc welded joints were studied, and the relationship between the stress level and the location of the crack source of the welded joints was obtained. The relationship between the life of welded joint at different position and the life of plate with the same amplitude is calculated. Finally, the fatigue fracture of the joint specimen was observed, and the failure mechanism of different failure sites was analyzed, and the effect of the mechanical damage on the crack initiation and propagation behavior was discussed, as well as the effect of the chain welding porosity defect on the crack initiation and propagation behavior. The results of resonance fatigue test of TC4 titanium alloy show that the higher the stress is, the lower the life is, and the curve decreases obviously before 2 脳 10 ~ 5 weeks, and then relatively flat. The resonance fatigue limit is 422MPa, which is basically the same as the resonant fatigue limit of the blade fabricated by TC4. The results show that the material is in low cycle by the analysis of the resonance fatigue fracture morphology of the titanium alloy sheet, which is made of TC4, and the crack failure will occur after the vibration cycle exceeds 107 cycles, and the resonance fatigue limit of the material is 422MPa, which is similar to that of the blade fabricated by TC4. At high cycle and ultra-high cycle, the fracture surface is brittle fracture. The higher the stress, the more ductile the fracture surface, the lower the stress, the more cleavage appearance of fracture surface, the more transgranular and intergranular mechanism the crack propagation is, the higher the fracture surface is, the higher the stress is, the lower the stress is, the higher the fracture surface is. The number of cracks is negatively correlated with the stress level, the crack growth rate is positively correlated with the loading stress at the stage of stable crack growth, and the crack originates along the direction of shear stress and tends to propagate perpendicular to the direction of stress. When the crack growth rate is completely vertical, the crack growth rate increases obviously. The results of resonance fatigue test show that the failure position of welded joint under high stress is in the heat affected zone, and the fatigue life of this position is lower than that of the same amplitude plate, and the failure position of the joint under low stress is weld, and the fatigue life of the welded joint is lower than that of the plate with the same amplitude. The fatigue life of the weld is higher than that of the plate with the same amplitude. The analysis of the vibration fatigue fracture morphology of welded joint shows that the failure mode is brittle and cleavage in both the heat affected zone and the weld seam, and for the stable crack growth stage, the fracture is brittle and the fracture surface is cleavage. The crack growth rate of the welded joint is obviously faster than that of the plate, and the life of the specimen is mainly concentrated in the crack initiation stage, and the surface sunken defects can grow into the source of the crack, resulting in the fatigue life of the joint falling by nearly 10%. The chained pore defects did not grow into crack sources, but their existence changed the crack growth rate and direction and reduced the life of the joints by nearly 20%.
【学位授予单位】：中国民航大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG407

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