AZ31B镁合金TIG焊接接头的深冷强化机理

【作者】 赵菲；

【导师】 吴志生；

【作者基本信息】 太原科技大学 ， 材料加工工程， 2012， 博士

【摘要】 镁合金是节能环保型的金属结构材料,影响其广泛应用的因素之一是镁合金的焊接技术。镁合金焊接存在的问题之一是焊接接头的软化，其软化问题无论从焊接工艺方面采取措施还是在焊后采取热处理措施，解决效果皆存在一定的局限性。本文提出改善AZ31B镁合金TIG焊接接头的深冷强化方法，进行了镁合金TIG焊接工艺试验和焊接接头的深冷处理试验。研究了深冷处理参数对AZ31B镁合金TIG焊接接头组织和性能的影响，探讨了深冷处理提高AZ31B镁合金TIG焊接接头性能的机理，探索了改善镁合金焊接接头软化的新途径。主要研究结果如下：深冷处理可以改善AZ31B镁合金TIG焊接接头组织的力学性能。研究发现，当深冷处理在-185℃，保温12h时，焊接接头各区域的硬度值趋于均匀化。AZ31B镁合金TIG焊接接头抗拉强度、屈服强度随着保温时间的延长和深冷处理温度的降低而增大。深冷温度为-185℃，达到同样的强化目的所需保温时间有所缩短，在保温12h时抗拉强度和屈服强度都达到最大值，分别提高了16.1%和6.5%，但延伸率的提高有限。采用SEM观测了接头的拉伸断口形貌。深冷处理前样品为脆性断裂，深冷处理后局部区域表现出塑性断裂的特征，断口上具有与拉伸轴呈45°方向的斜断裂口，其断裂形式是剪切断裂，断口宏观平齐，无空洞存在，局部区域有撕裂刃和细小的韧窝，呈现出复合性断裂的特征。利用金相显微镜、X射线衍射仪（XRD）和透射电子显微镜（TEM）等手段观测了焊接接头的深冷前后的微观组织，应用位错理论分析了AZ31B镁合金TIG焊接接头深冷强化机理。研究结果表明，深冷处理使得AZ31B镁合金TIG焊接接头组织中形成亚晶结构，第二相Mg17Al12颗粒弥散析出，提高了基体的连续性，且第二相颗粒数量明显增加，使AZ31B镁合金TIG焊接接头的基体组织细化，从而强化基体组织；XRD分析表明，深冷处理后焊接接头组织中的某些晶粒的晶面取向发生了变化，产生了晶粒转动；利用TEM观测了深冷处理前后的AZ31B镁合金焊接接头的晶体结构，发现深冷处理可促使TIG焊接接头组织中的位错转化为位错环，使接头组织产生孪晶。采用BP神经网络仿真和预测了不同深冷处理工艺参数对焊接接头抗拉强度的影响，得出比较合理的深冷处理参数：温度为-135℃，保温时间为2.5h以上。BP神经网络对AZ31B镁合金TIG焊接接头抗拉强度的预测结果与深冷处理焊接接头实际测量结果比较一致。采用有限元模型研究了深冷处理过程中的镁合金焊接接头温度场和应力场的分布特点及影响规律，有限元分析结果与试验结果基本吻合。更多还原

【Abstract】 Magnesium alloy is the metal structural material for saving energy andenvironmental protection. The welding technology is one of the keytechnologies which decide the magnesium alloy will be widely used or not. Oneof the welding problems for magnesium alloy is the softening of welded joints.There are some limitations whether to take measures in welding procedure or totake heat treatment measures after welding. Deep cryogenic treatment is putforward to improve the strengthening of welded joints by TIG of AZ31Bmagnesium alloy，and welding test and deep cryogenic treatment test are alsotaken in this paper. The author studies the microstructure and mechanicalperformance of welded joints by TIG in the different parameters of deepcryogenic treatment, and investigates the mechanism to improve theperformance of the welded joints by TIG, and explores the new ways to improvethe softening of the welded joints. The major research results are as follows:Mechanical properties of welded joint by TIG of AZ31B magnesium alloywere improved by deep cryogenic treatment. The Vickers hardness value of thewelded joints for each region tends to be homogenizing after deep cryogenictreatment for4h at-185℃. The tensile strength and yield strength of the weldedjoint by TIG of AZ31B magnesium alloy improves as the holding time increasesand treatment temperature decreases. When the deep cryogenic temperature is-185℃, the holding time is shortened for achieving the same the purpose ofstrengthen. The tensile strength and yield strength reaches its maximum whenthe deep cryogenic treatment for12h at-185℃,16.1%and6.51%respectively,but elongation improves limited.The tensile fracture surface of the welded joints is observed by SEM. Thesample before cryogenic treatment breaks by way of brittle fracture, and thelocal area after cryogenic treatment shows the characteristics of ductile fracture.There is45°oblique fracture with the tensile axis direction on the fracturesurface. The form of fracture is shear fracture, with the fracture flush, and novoid exists. There is a torn edge and small dimple in the local area, showing the characteristics of the compound fracture.The microstructure before and after deep cryogenic treatment of weldedjoint by TIG is observed by using optical microscope, X-ray diffraction (XRD)，and transmission electron microscope (TEM). The cryogenic strengtheningmechanism of the welded joints of AZ31B magnesium alloy is analyzed by thedislocation theory. The subgrain structure was formed in the welded joint ofAZ31B by deep cryogenic treatment. The Mg17Al12particles precipitated, whichimproved the continuity. The number of second phase particles increased, whichrefined the microstructure of welded joint by TIG of AZ31B magnesium alloyand strengthened mechanism. The analysis by XRD shows that the orientation ofgrain crystal of welded joint of AZ31B changes, resulting in the grain rotationafter deep cryogenic treatment. During the deep cryogenic treatment, thedislocation loop and twins are formed by TEM in the welded joints.BP neural network was used to simulate and predict for the tensile strengthof welded joint by TIG of AZ31B magnesium alloy. The reasonable parametersof deep cryogenic treatment are as follows: temperature is-135℃, duration timeis more than150minutes. The predicting results of BP neural network arerelatively accord with the experimental test results.Based on the FEM analysis, the distribution and the law of the temperatureand stress field of the welded joints of magnesium alloy are studied during thecryogenic treatment process. These results are consistent with the resultsobtained from experiments of effect of welding.更多还原