循环超声冲击对MB8镁合金对接接头疲劳性能影响

发布时间：2018-02-26 11:11

本文关键词： MB8镁合金对接接头循环超声冲击疲劳性能晶粒细化残余应力　出处：《华东交通大学》2015年硕士论文　论文类型：学位论文

【摘要】：疲劳断裂是引起焊接结构失效的主要原因，占到结构失效的70%~90%。镁合金具有高强度比、高阻尼、易切削加工性能以及易回收再利用等一系列独特的优点。提高镁合金焊接接头的疲劳强度及改善其疲劳性能，能提高镁合金焊接结构的可靠性及适用性，从而对推广镁合金结构的应用具有深远的意义。本次研究中，通过循环超声冲击处理MB8镁合金焊接接头的焊趾及其附近区域，比较了超声冲击前后对接接头的疲劳性能，并分析了超声冲击MB8镁合金焊接接头表面的纳米化机理。试验结果表明：（1）循环超声冲击试样S-N方程：lgN=21.13-8.67Δσ，焊态试样的S-N方程：lgN=28.17-13.82Δσ，超声冲击态试样S-N曲线方程：lgN=22.04-9.27Δσ；在试验基数为2×106下，冲击态的疲劳强度为49.87MPa，比焊态试样的疲劳强度38.24MPa大约提高了32%。循环超声冲击的疲劳强度为51.33MPa，比超声冲击的疲劳强度大约提高了3%。（2）通过SEM发现冲击前后的试样断口均没有疲劳辉纹。瞬断区存在大量的撕裂棱，同时存在解理小刻面、解理台阶。在断口扩展区都出现了大量的二次裂纹，说明超声冲击并不能改变MB8镁合金焊接接头的断裂方式，都为解理断裂。（3）超声冲击后，在MB8镁合金焊接接头区域的残余拉应力都转变为残余压应力。疲劳试验后，焊接接头区域的残余应力都有很大程度的释放。（4）超声冲击后，MB8接头焊趾区域表面的晶粒细化现象很明显。在相同的冲击时间下，随着超声冲击电流的增大，，MB8焊趾区域表层所获得的晶粒尺寸逐渐减小。（5）超声冲击细化MB8镁合金晶粒主要分为以下几个过程：超声冲击初期，焊趾区域发生了严重的塑性变形，位错缠结及位错墙逐渐形成。随后位错缠结及位错墙逐步转变为小角度的亚晶结构，把原始的粗晶粒分割为多个亚晶结构。随着变形量的增加，亚晶在晶界处发生动态再结晶，亚晶最终会转变为纳米晶。（6）超声冲击后试样的表面产生了一层塑性变形层，该层的晶粒得到细化。随着冲击时间的增加冲击表面的耐磨损性、耐腐蚀性能以及材料表面的显微硬度均有不同程度的提高。冲击电流为1.2A，冲击时间为6min的试样比没有击的试样，耐磨损性提高了49.1%、耐腐蚀性提高了52.2%、表面硬度提高了103%。
[Abstract]:Fatigue fracture is the main cause of failure of welded structures, accounting for 70% of structural failure. Magnesium alloys have high strength ratio and high damping. A series of unique advantages, such as easy cutting performance and easy recycling and reuse, can improve the fatigue strength and fatigue properties of the welded joints of magnesium alloys, which can improve the reliability and applicability of the welded structures of magnesium alloys. In this study, the weld toe of MB8 magnesium alloy welded joint and its adjacent area were treated by cyclic ultrasonic impact, and the fatigue properties of butt joint before and after ultrasonic impact were compared. The nanocrystalline mechanism of ultrasonic impact on the surface of MB8 magnesium alloy welded joint is analyzed. The experimental results show that:. (1) the S-N equation of circulating ultrasonic shock specimen is: 1: lgN = 21.13-8.67 螖蟽, the S-N equation of welded specimen is: lgN = 28.17-13.82 螖蟽, and the S-N curve equation of ultrasonic impact state sample is 1: lgN equation 22.04-9.27 螖蟽, the test base is 2 脳 10 ~ 6. The fatigue strength of the impact state is 49.87 MPA, which is about 38.24 MPA higher than that of the welded specimen, and the fatigue strength of the cyclic ultrasonic impact is 51.33 MPA, which is about 3 times higher than that of the ultrasonic impact. (2) it was found by SEM that there were no fatigue ripples on the fracture surface of the specimen before and after impact. There were a large number of tearing edges, cleavage surface and cleavage steps in the transient fracture zone. A large number of secondary cracks appeared in the fracture propagation zone. The results show that ultrasonic impact can not change the fracture mode of MB8 magnesium alloy welded joint, which is cleavage fracture. 3) after ultrasonic impact, the residual tensile stress in MB8 magnesium alloy welded joint is transformed into residual compressive stress, and after fatigue test, the residual stress in welded joint area is released to a great extent. At the same impact time, the grain size of the weld toe surface decreases with the increase of ultrasonic impact current. 5) Ultrasonic impact refining of MB8 magnesium alloy grain is divided into the following processes: in the initial stage of ultrasonic impact, serious plastic deformation occurs in the weld toe area. Dislocation entanglement and dislocation wall are gradually formed. Then dislocation entanglement and dislocation wall are gradually transformed into subcrystalline structures at small angles, and the original coarse grains are divided into multiple subcrystalline structures. With the increase of deformation amount, dynamic recrystallization occurs at grain boundaries. Subcrystals will eventually be transformed into nanocrystals. (6) A plastic deformation layer was formed on the surface of the specimen after ultrasonic impact, and the grain of the layer was refined. With the increase of impact time, the wear resistance of the impact surface was increased. The corrosion resistance and microhardness of the material surface were improved in varying degrees, the impact current was 1.2 A, the impact time was 6 min, the wear resistance was increased by 49.1%, the corrosion resistance was increased by 52.2%, and the surface hardness was increased by 103%.
【学位授予单位】：华东交通大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG407

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