AZ31镁合金搅拌摩擦焊焊接接头高周疲劳断裂行为研究

发布时间：2018-02-11 18:32

本文关键词： AZ31镁合金搅拌摩擦焊疲劳断裂前进侧双面搅拌摩擦焊　出处：《太原理工大学》2017年硕士论文　论文类型：学位论文

【摘要】：镁合金材料由于其本身优势和节约能源的优点,已经广泛应用于汽车、高铁以及航空航天等工业制造业中,这些产品结构往往需要焊接技术的支持。常规熔化焊方法焊接镁合金后,接头的力学性能和冶金性能较差;搅拌摩擦焊技术用于镁合金的焊接,其接头力学性能有了极大的改善,焊接过程也更加环保。然而,这些焊接结构在使用中往往承受疲劳载荷作用,很容易发生疲劳断裂事故。而疲劳失效是金属结构件中常见的一种严重破坏形式,所以改善金属结构件的疲劳性能对其使用性和安全性的提高有着重要的意义。红外热像法以其便捷、无损等优点可以用来表征试验过程中试样表面温度的变化特征,从而分析疲劳过程中产热温度与变形的关系,因而采用红外热像法对镁合金FSW接头疲劳断裂行为进行研究具有一定的实用价值。本试验对AZ31镁合金采用优化的FSW参数进行对接焊接,结合金相组织、显微硬度、EBSD、应力-应变曲线、扫描电子显微镜以及红外热像仪研究了镁合金搅拌摩擦焊焊接接头高周疲劳裂纹断裂行为以及红外热像温度特征,最终提出了改善AZ31镁合金FSW接头疲劳寿命的有效方法。试验结果表明,AZ31镁合金FSW焊接接头疲劳循环次数达到107时,疲劳极限为55.11 MPa,焊接接头的疲劳裂纹均沿着焊接接头前进侧(AS)发生疲劳断裂。采用红外热像仪获得FSW焊接过程温度场分布,发现AS摩擦产热高于RS。通过金相组织分析可以得出,热影响区(HAZ)存在大量粗大晶粒和少部分孪晶,热机影响区(TMAZ)相比于HAZ晶粒尺寸较小,并且存在部分等轴晶,表明该区域在FSW过程中存在不完全的动态再结晶现象,焊核区(NZ)所有晶粒都是细小的等轴晶粒,表明焊接过程NZ区域存在完全的动态再结晶现象。搅拌摩擦焊焊接接头显微硬度结果显示,AS的HAZ区域显微硬度最低,进一步证明了这一区域力学性能较差,为焊接接头软化区。在循环变形过程中,搅拌摩擦焊焊接接头AS应变大于后退侧(RS)应变。利用扫描电子显微镜(SEM)对疲劳断口微观形貌进行了研究,获得结论为:AZ31镁合金FSW接头疲劳断裂方式为脆性解理断裂,同时断口中可以观察到撕裂棱和二次裂纹的存在。红外热像仪检测结果表明,在循环载荷作用下AZ31镁合金试样表面存在五个区段的温度演化:温度极速上升、温度下降、温度水平、温度二次升高和温度陡降。AS侧的HAZ温度高于FSW接头其他区域,进而验证了此区域在相同循环载荷下应变最大。双面FSW接头疲劳极限比单面FSW接头提高了67%。双面焊焊缝两侧热输入相等,微观组织对称分布,避免了由单面焊在AS部位粗大晶粒弱化焊接接头的现象发生。相同循环载荷下,单面FSW接头的AS应变范围大约为2.5%-5.25%,RS应变范围大约为1.5%-3.75%。双面FSW接头某一侧应变范围大约为1.25%-3.75%,这与单面FSW接头的RS应变很接近,也说明了双面FSW有较高的抗疲劳强度。利用扫描电子显微镜(SEM)对疲劳断口微观形貌进行了研究,获得结论为:AZ31镁合金双面FSW接头疲劳裂纹萌生于AS的表面或者近表面,微观形貌为“人字型”花样,最终在另一侧焊缝的RS区域瞬断,其微观形貌特征为脆性解理形貌。
[Abstract]:The magnesium alloy material because of its own advantages and energy saving advantages, has been widely used in automobile, iron and aerospace industries in the manufacturing industry, the product structure often requires welding technology. The conventional fusion welding method of welding joints of magnesium alloy, mechanical and metallurgical properties of friction stir welding technology for the poor; the welding of magnesium alloy, the mechanical properties of the joints were greatly improved, the welding process is also more environmentally friendly. However, these are often subjected to fatigue load of welded structure is in use, it is easy to fatigue fracture accident. And fatigue failure of metal structures is a common form of serious damage, so improving the fatigue performance of metal structure one is of great significance to improve its usability and security. Infrared thermography with its advantages of convenient, nondestructive sample surface can be used to characterize the test process Temperature change characteristics, to analyze the relationship between the thermal fatigue process of middle temperature and deformation, and the use of infrared thermography to study the fatigue fracture behavior of FSW magnesium alloy joints has certain practical value. The experiment of AZ31 magnesium alloy by FSW parameter optimization of butt welding, metallographic structure, hardness, EBSD, should be stress strain curve, scanning electron microscope and infrared thermal imager on magnesium alloy friction stir welding joint of high cycle fatigue crack fracture behavior and infrared temperature characteristics, and finally puts forward some effective methods to improve the fatigue life of FSW joints of AZ31 magnesium alloy. The experimental results show that AZ31 magnesium alloy FSW welding joint fatigue cycle number reaches 107 and the fatigue limit is 55.11 MPa, the fatigue crack of welded joints of welded joints along the advancing side (AS). To obtain fatigue fracture of FSW welding using infrared thermography The temperature field distribution, found through metallographic analysis that AS friction heat is higher than RS., the heat affected zone (HAZ) the existence of a large number of coarse grains and a small part of the twin, TMAZ (TMAZ) compared to the grain size of HAZ is small, and there are some grains, indicating that the region in the FSW process in dynamic complete recrystallization in the weld nugget zone (NZ) of all grains are small equiaxed grains, shows that the welding process of NZ are completely dynamic recrystallization phenomenon. The microhardness of welded joints shows that friction stir welding, AS HAZ hardness is lowest, further proved that this region poor mechanical properties for welding joint softening zone. During cyclic deformation, welding joint of AS strain is higher than the retreating side friction stir welding (RS) strain. By using scanning electron microscopy (SEM) on the fatigue fracture morphology were studied, the conclusion is obtained AZ31 magnesium alloy FSW joint fatigue fracture mode is brittle cleavage fracture, while fracture can be observed in the tear ridge and two crack. Detection results of infrared thermography showed that evolution in the presence of five section specimen surface of AZ31 magnesium alloy under cyclic load temperature: the temperature rise speed, temperature drop, temperature level two, temperature rise and the temperature dropped.AS side of the HAZ FSW joint temperature is higher than that of other regions, and then verify the maximum strain in the same area under cyclic loading. The fatigue limit of double FSW joint single FSW improved 67%. double weld joint on both sides of the heat input is equal, symmetrical distribution of microstructure, avoid by welding at the site of AS coarse grain weakening of welded joints under cyclic loading. The same phenomenon, AS single FSW joint strain range is about 2.5%-5.25%, RS strain range is about 1.5%-3.75%. double face FSW The head of one strain range is about 1.25%-3.75%, the joint and the single FSW RS strain is very close, also shows that the fatigue strength of duplex FSW is high. By using scanning electron microscopy (SEM) on the fatigue fracture morphology were studied. The conclusion is: the surface of AZ31 magnesium alloy FSW joint double surface fatigue crack was adorable AS or near the surface, the micro morphology of "font" pattern, eventually in the RS area on the other side of the fault transient weld, its morphology is characterized by brittle cleavage morphology.

【学位授予单位】：太原理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG453.9

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