【作者】 马丽莉；

【导师】 王文先；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2007， 硕士

【摘要】 随着科学技术的发展，以及人们对能源和环境保护的重视，被称为21世纪“绿色”工程材料的镁合金受到越来越多的关注。对于大型复杂的结构而言，焊接技术是其最重要的加工技术，而疲劳破坏是焊接结构失效的一种主要形式。因此了解镁合金焊接结构疲劳性能的特点，采用合理的评定方法指导镁合金焊接结构的疲劳设计，对于镁合金的发展和广泛应用都具有极其重要的作用。本文通过疲劳试验对AZ31B镁合金板材TIG焊横向对接接头和横向非承载十字接头的疲劳性能和相应的S-N曲线进行了分析，将试验结果与同形式、同参数的铝合金、不锈钢焊接接头的疲劳数据进行了比较。用热点应力方法对镁合金横向接头进行了疲劳评定研究。利用有限元软件对焊缝几何尺寸与镁合金接头焊趾部位应力集中系数的关系做了分析。研究发现，AZ31B镁合金母材和两种横向接头试验数据直接拟合得到的S-N曲线斜率m分别为4.45、4.43和3.05，在2×10~6的循环次数下的名义应力疲劳强度分别为57.81MPa、24.60MPa、17.25MPa，两种接头试件的热点应力疲劳强度分别为29.5MPa和20.7MPa。镁合金横向接头名义应力和热点应力S-N曲线的斜率和疲劳强度均高于国际焊接学会(ⅡW)推荐的m=3.0及相应的疲劳强度，这说明对于镁合金，使用m=3.0也具有一定的安全裕度。通过试验比较，AZ31B镁合金两种横向接头的疲劳强度是同形式铝合金接头的40％左右，横向非承载十字接头的疲劳强度还不足同形式不锈钢接头疲劳强度的10％。将镁合金接头的试验数据用国际焊接学会(ⅡW)推荐的m=3.0处理得出FAT值，与规范给出的同形式铝合金和结构钢接头的疲劳级别进行对比，结果表明经过安全评定方法处理的镁合金接头的疲劳强度仅为规定的铝合金疲劳级别的一半，为结构钢的10％～16％。通过有限元模拟，外推得出接头的热点应力，并计算得出横向对接接头和横向非承载十字接头的热点应力集中系数分别为1.2和1.6，说明热点应力集中系数与接头形式有重要关系。AZ31B镁合金两种横向接头的热点应力疲劳强度可用一条S-N曲线表示，说明用热点应力法进行镁合金接头的疲劳评定能够减少焊接接头按疲劳强度划分的等级，比用名义应力法进行评定具有一定的优越性。通过有限元的模拟，发现对于AZ31B镁合金对接焊缝和角焊缝，焊趾角度θ和焊趾处过渡圆弧半径ρ都是影响应力集中系数的主要因素。焊趾角度θ的增大、焊趾处过渡圆弧半径ρ的减小都会使焊趾处应力集中系数增大，从而恶化镁合金接头的疲劳性能，其中焊趾角度对应力集中系数的影响最大，并且当焊缝截面形状变化较大(θ较大或ρ较小)时，应力集中系数对于其他影响因素的敏感性较强。更多还原

【Abstract】 With the development of science and technology, and the attention of energy and environmental protection, magnesium alloy which known as the 21st century ’green’ engineering material, is paid more and more attentions. As to large-scale and complicated structures, the welding technique is the most important process technology, and the fatigue destroy is a main kind of failure form of welded structures. To the development and extensive using of magnesium alloy, it’s very important that realizing the characteristic of the fatigue performance of magnesium alloy welded structures, and adopting the rational evaluation method to guide the fatigue design of magnesium alloy welded structures.Fatigue performances and corresponding S-N curve of transverse butt joints and transverse cross joints with TIG-welding of AZ31B magnesium alloy plates was analyzed. The experimental results were compared with the fatigue data of aluminum alloy and stainless steel welding joints with the same form. The fatigue evaluation with the hot stress method to transverse welding joints of magnesium alloy were researched. The relation between coefficient of stress concentration of magnesium alloy weld toe and geometrical measurement of welding seams were analyzed by finite element analysis software.The result shwoed that the S-N curve slope of AZ31B magnesium base metal and the transverse joints were 4.45, 4.43 and 3.05 respectively. Under 2×10~6 cycle, the nominal fatigue resistance are 57.81 MPa, 24.60MPa, 17.25MPa respectively, and the hot stress fatigue resistance of two kinds of joints were 29.5MPa and 20.7MPa respectively. The S-N curve slope and the fatigue resistance of the nominal stress and the hot stress were higher than that recommended by International Institute of Welding (IIW). That proved that to the magnesium alloy, using m=3.0 had certain safe allowance.Comparing experimental results, the fatigue resistance of transverse welding joints of AZ31B magnesium alloy were about 40% value of aluminum alloy joints with the same form, and 10% that of stainless steel joints. The FAT of magnesium alloy joints which dealing with experimental dates with m=3.0 were compared with that of aluminum alloy and structural steel joints with the same form that recommended by International Institute of Welding (IIW). The results indicated that the fatigue resistance of magnesium alloy joints which were evaluated by secure assessment was the half of that of aluminum alloy joint, and 10%-16% of the structural steel joint.Hot stress value of magnesium alloy joints were caculated. The coefficients of hot stress concentration of transverse butt joints and transverse cross joints were 1.2 and 1.6 respectively, which proved that the coefficients of hot stress concentration were related to the form of joints. The hot stress fatigue resistance of two kinds of transverse joints of AZ31B magnesium alloy could be expressed with one S-N curve. It showed that the fatigue grade of magnesium alloy joints could be reduced by evaluation with hot stress method. It was superior to evaluating with the nominal stress method.To butt-jointed seam and fillet weld of magnesium alloy, the angle of weld toeθand the radius of weld toeρwere the main factors which influenced the coefficient of stress concentration. With the increment ofθand the reduction ofρ, the coefficient of stress concentration of toe place should increment. The welded toe angleθis the most influential factor to the coefficient of stress concentration. When the change in the sectional appearance of welding seam was bigger, the coefficient was more sensitive to the other influence factors.更多还原