红外热像法表征AZ31B镁合金电子束焊接接头疲劳行为及断裂机理的研究

发布时间：2018-03-20 19:48

本文选题：镁合金　切入点：电子束焊　出处：《太原理工大学》2015年硕士论文　论文类型：学位论文

【摘要】：镁合金因其轻质高强且易于回收等特性，在“陆海空天”等领域作为结构材料将具有广泛的应用前景。但其广泛的应用离不开焊接技术的支持。而焊接接头的力学性能，尤其是疲劳问题成为制约其安全应用的瓶颈。因此，研究镁合金焊接接头的疲劳行为及其断裂机理将具有重要的理论意义和应用价值。针对镁合金化学性质活泼、易氧化等特点，本文采用真空电子束焊接方法，结合红外热像法，重点研究了镁合金电子束焊接接头在静载拉伸、疲劳裂纹萌生及扩展过程中试件表面温度的演变规律及产热机制，快速预测了焊接接头的疲劳极限和疲劳裂纹失稳扩展门槛值，，结合断口形貌探讨了其疲劳断裂机理。研究结果表明：镁合金及其电子束焊接接头在拉伸试验过程中，试件表面温度呈现4个阶段变化规律：初始温度下降阶段（热弹性效应占主导）；温度缓慢升高阶段（非弹性效应占主导）；温度急剧上升阶段（加工硬化现象）以及最后自然冷却阶段。镁合金电子束焊接接头在疲劳试验过程中，温度演变规律呈5个阶段：初始快速升温阶段（非弹性效应占主导地位）；温度下降阶段（热传导效应占据主导地位）；温度平稳阶段（非弹性效应和热传导效应达到平衡）；温度急剧上升阶段（疲劳裂纹萌生）和最后温度自然冷却阶段。镁合金电子束焊接接头在疲劳裂纹扩展试验过程中，温度演变规律呈4个阶段：初始快速升温阶段（裂纹处于快速扩展阶段，da/dN=9.05×10-14(ΔK)9.13）；温度平衡阶段（裂纹进入稳定扩展阶段，da/dN=1.01×10-7(ΔK)2.49）；温度急剧上升阶段（裂纹快速处于失稳扩展阶段，da/dN=3.38×10-16(ΔK)10.17）以及最后温度自然冷却阶段。红外热像法预测得到镁合金焊接接头的疲劳极限和疲劳裂纹失稳扩展门槛值分别为83MPa和17.73MPa m1/2，并同传统试验结果相比，二者均具有较高的吻合度。红外热像法能够快速、实时及全场地对镁合金电子束焊接接头在静载拉伸试验、疲劳试验以及疲劳裂纹扩展试验过程中的损伤演变进行监测。试件表面颜色和热点区域的变化可作为评定试件损伤演变的有效工具。镁合金电子束焊接接头疲劳裂纹扩展过程历经三个阶段：初期快速扩展阶段，断口呈现以解理为特征的脆性断裂机制；稳定扩展阶段，断口呈现以解理和撕裂棱为特征的混合断裂机制；失稳扩展阶段，断口呈现以撕裂棱为主的断裂机制。
[Abstract]:Magnesium alloys are widely used as structural materials in the field of "land, sea and sky" because of their light weight and high strength and easy to be recovered. However, their wide application can not be separated from the support of welding technology, and the mechanical properties of welded joints. Especially the fatigue problem has become the bottleneck of its safety application. Therefore, it is of great theoretical significance and practical value to study the fatigue behavior and fracture mechanism of magnesium alloy welded joints. Aiming at the characteristics of chemical activity and easy oxidation of magnesium alloy, vacuum electron beam welding (VEBW) method and infrared thermal imaging (IR) method are used to study the static tensile properties of magnesium alloy welded joints by electron beam welding. In the process of fatigue crack initiation and propagation, the evolution law of surface temperature and heat generation mechanism of the specimen are studied. The fatigue limit and fatigue crack instability propagation threshold of welded joint are predicted quickly. The fatigue fracture mechanism is discussed in combination with fracture morphology. The results show that during the tensile test of magnesium alloy and its electron beam welded joint, The surface temperature of the specimen shows four stages of variation: the initial temperature decreases (thermoelastic effect dominates), the temperature rises slowly (inelastic effect dominates), and the temperature rises sharply (work hardening phenomenon). And the final natural cooling stage. During fatigue test of magnesium alloy electron beam welded joint, There are five stages of temperature evolution: initial rapid heating stage (inelastic effect dominates; temperature drop stage (heat conduction effect dominates); temperature stable stage (inelastic effect and heat conduction effect reach). At the equilibrium stage, the temperature rises sharply (fatigue crack initiation) and the final temperature is naturally cooled. During the fatigue crack growth test of magnesium alloy electron beam welded joint, There are four stages of temperature evolution: initial rapid heating stage (crack is in rapid growth stage) 9.05 脳 10 ~ (-14) (螖 K ~ (9.13)); temperature equilibrium stage (crack entering stable growth stage) 1.01 脳 10 ~ (-7) (螖 K ~ (2.49)); temperature rising stage (crack is fast in the stage of unstable growth). Da / DN = 3.38 脳 10 ~ (-16) (螖 K ~ (10) 10.17) and the natural cooling stage of the final temperature. The fatigue limit and fatigue crack instability propagation threshold of magnesium alloy welded joints predicted by infrared thermal imaging method were 83 MPA and 17.73 MPA m ~ (1 / 2), respectively. Compared with the traditional test results, the fatigue limit and fatigue crack instability propagation threshold of magnesium alloy welded joints were higher than those of conventional test results. The infrared thermal imaging method can be used to test the tensile properties of magnesium alloy welded joints by electron beam welding in real time and in real time under static load. The damage evolution during fatigue test and fatigue crack growth test is monitored. The change of surface color and hot spot can be used as an effective tool to evaluate the damage evolution of specimen. The fatigue crack propagation process of magnesium alloy electron beam welded joint goes through three stages: the initial rapid growth stage, the brittle fracture mechanism characterized by cleavage, the stable growth stage, The fracture surface presents a mixed fracture mechanism characterized by cleavage and tearing edge, while at the stage of instability extension, the fracture surface presents a fracture mechanism dominated by tearing edge.
【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG407

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