【作者】 张红霞；

【导师】 卫英慧； 王文先；

【作者基本信息】 太原理工大学 ， 材料学， 2010， 博士

【摘要】 镁合金具有轻质高强,易于回收等一系列优点,被誉为“21世纪绿色金属结构材料”,其最具发展前途的应用领域是“陆、海、空、天”等交通运载装备,这些结构离开焊接技术的支持是无法完成的。资料表明,70%～90%的焊接结构失效是焊接接头在动载负荷作用下造成的,镁合金及其焊接接头中裂纹的启裂和扩展对焊接结构的疲劳性能及疲劳寿命产生较大的影响,而其失稳断裂临界裂纹尺寸又决定了材料或结构的疲劳寿命。因此,研究镁合金及其焊接接头的疲劳性能、裂纹扩展速率,对疲劳性能进行评定,具有重要的理论意义和应用价值。本研究针对AZ31B镁合金及其焊接接头疲劳性能、裂纹扩展速率及断裂韧度进行研究,利用OM和SEM等手段对材料的组织、裂纹扩展行为及断口进行分析；采用疲劳裂纹扩展理论(Parise公式)对疲劳寿命进行预测；利用临界距离法和热点应力法对不同形式的焊接接头进行疲劳评定；分析了织构对疲劳性能和裂纹扩展速率的影响。研究认为,在2×106循环次数下,AZ31B镁合金母材的疲劳强度为66.72 MPa,对接、横向十字、纵向非承载十字和侧面连接接头的疲劳强度分别为39.00 MPa,24.38 MPa,32.18MPa和24.40 MPa；焊接接头裂纹均起裂于焊趾部位,裂纹以穿晶方式扩展,塑性变形方式为滑移和孪生。镁合金及其焊接接头的疲劳断裂均为河流花样组成的脆性断口。采用TIG熔修和超声冲击(UIT)改善对接和横向十字接头疲劳性能,经TIG熔修处理后,AZ31B镁合金对接和横向十字接头的疲劳强度分别为41.68 MPa和34.13 MPa,与处理前相比较,分别提高了6.9%和39.9%。经过UIT后,十字接头的疲劳强度为24.7 MPa,比焊态试样提高了43.6%。疲劳裂纹扩展试验结果表明,T-L、L-T(第一、二个字母分别表示裂纹面的法线方向和表示预期的裂纹扩展方向；L表示长度或主变形方向,T表示宽度方向)方向试件裂纹扩展方向与切口方向平行,疲劳裂纹在AK达到5.5 MPa·m1/2、5.7 MPa·m1/2左右时,开始进入快速扩展阶段。45°方向试件裂纹扩展方向与切口方向成10°左右,疲劳裂纹在AK达到4.1 MPa·m1/2左右时,开始进入快迅速扩展阶段。疲劳断口为解理形貌的脆性断裂,断口中存在二次裂纹。L-T(W)和T-L(H)试样裂纹扩展过程中,裂纹沿焊缝中心和热影响区即切口方向扩展,L-T(W)试样当AK达到6.12 MPa·m1/2时,裂纹扩展速率减慢,T-L(H)试样裂纹扩展速率趋于一致。L-T(H)试样当AK达到8.2 MPa·m1/2时,裂纹扩展速度趋于缓慢；疲劳裂纹尖端以穿晶方式向前扩展,T-L(H)试样裂纹存在少量沿晶扩展。断口均为脆性特征的解理断口,断口中存在二次裂纹。平面应变断裂韧度试验结果表明,0,45和90试样的平均平面应变断裂韧度KIC平均值分别为9.36,9.40和9.89 MPa·m1/2。焊缝金属和热影响区的平均平面应变断裂韧度KIC值分别为18.28和15.62 MPa·m1/2,缺口在焊缝中心的断裂韧度大于缺口在热影响区的,因此焊缝中心部位的阻止裂纹扩展能力比焊接热影响区强。AZ31B镁合金母材及其焊缝的断口均为准解理断裂特征的脆性断裂,热影响区断口有沿晶断裂特征。采用临界距离法和热点应力法对焊接接头进行疲劳评定,在2×106循环次数下,临界距离法中采用点法和线法对接接头和横向十字接头的疲劳强度分别为79.55、49.10 MPa和79.01、44.38 MPa,两种方法差别不大,因此可以采用临界距离法预测疲劳断裂位置。用热点应力对焊接接头进行评定时,试验数据分散性明显比名义应力法减小。对接接头和纵向非承载十字接头疲劳强度可以用一条S-N曲线表示。板材中在挤压过程中产生的织构对疲劳性能和裂纹扩展速率产生影响,织构分析结果表明AZ31B镁合金心部存在{0002}<2110>和{0002}<1010>基面织构,基面织构与取向因子总体大于棱柱面织构的取向因子,外力在挤压方向时晶粒更容易发生滑移,横向试样不易滑移,疲劳累积损伤的程度就大,容易发生断裂,因此横向的疲劳寿命比挤压方向的小镁合金的织构ODF分析结果表明镁合金(0002)基面在挤压方向和横向的Schmid因子均为零,滑移所需临界剪切应力(CRSS)较大,基面织构在各个方向为硬取向,产生织构硬化,不易滑移。非基面滑移系在挤压方向和横向的取向因子不同,mED>mTD；挤压方向塑性变形较容易,造成镁合金横向疲劳裂纹扩展门槛值AKth大于挤压方向,挤压方向的裂纹扩展速率较高。更多还原

【Abstract】 Magnesium alloys have excellent mechanical properties such as low density and high specific strength, easiness of recycling, etc; they are prospective green engineering structure materials of the 21st century. This makes them ideal candidates in the traffic vehicle, automotive industry, aerospace industry etc. Welding technology is one of the most important methods for developing complicated and large-scale magnesium alloy structure. Fatigue fracture is a major failure pattern in metal structures, and 70% to 90% of failure accidents involving welding structures are caused by the fatigue fracture of welded joints. The crack initiation and micro-crack propagation have an improvement affect on fatigue properties and fatigue life of the magnesium alloy and its welded joints, the fatigue life are decided by the instability fracture critical crack size, investigations on the fatigue properties and fatigue crack propagation rate of the magnesium alloy welded joints have theoretical and practical significance in engineering applications.The fatigue properties, crack propagation rate and fracture toughness were researched for AZ31B magnesium alloy and its welded joints. The microstructure, crack propagation behavior and fracture were determined by OM and SEM; the fatigue life were forecasted by Parise crack propagation theory; the welded joints were assessed by critical distance method and the hot spot stress approach; the textures of AZ31B magnesium alloy and the effect on fatigue properties and crack propagation rate were analyzed.It is concluded that the fatigue strengths of AZ31B magnesium alloy and its butt joint (BJ), transverse cross joint (TJ), longitudinal cross joint (LJ), lateral connection joint (LCJ) (under 2×106 cycle times) are 66.72 MPa,39.00 MPa,24.38MPa,32.18 MPa and 24.4 MPa, respectively. The crack propagation of the AZ31B magnesium alloy base metal in a transgranular manner, the twin is the main format in the course of crack propagation. The crack propagates from the weld toe for the welded joints. Moreover, the fracture of the base metal and welded jointS are brittle and of cleavage-type, based on quasi-cleavage fracture. Secondary cracks between the base metal and welded joints fracture are also observed.The TIG dressing and UIT were used to improve the fatigue property of BJ and TJ. The fatigue property of BJ and TJ are 41.68 MPa and 34.13 MPa, respectively by TIG dressing, and 6.9% and 39.9% of the fatigue property are advanced. Under 2×106 cycles, the fatigue property of TJ improved to 24.7 MPa by UIT, and it have 43.6% advanced.Fatigue crack propagation rate on as-extruded AZ31B magnesium alloy is tested. C(T) specimens of the incision direction are parallel (T-L), vertical (L-T) and inclined 45°to extrusion direction. Results indicated that crack propagate along the extrusion direction for T-L and L-T specimens, Inclined 45°specimen has an angular deflection about 10°to extrusion direction. Fatigue crack begin to rapidly propagate when AK are about 5.5 MPa·m1/2,5.7 MPa·m1/2 and 4.1 MPa·m1/2,respectively for T-L, L-T and 45°specimen. Fatigue fracture consists of quasi-cleavage and it was brittle fracture. The fatigue crack is along the notch direction for L-T(W) and T-L(H) specimens, the crack fatigue propagation begin to slower propagation when AK is 6.12 MPa·m1/2 for L-T(W), the crack propagation rate of T-L(H) specimens is identical. Fatigue crack begin to slow propagate when AK is about 8.2 MPa·m1/2 for along welding direction. The crack tip propagates by both transgranular and intergranular fractures. The fracture is brittle fracture characterized by a cleavage plane, some secondary cracks and river pattern can be seen.Plane-strain fracture toughness KIC of a 10 mm-thick as-extruded AZ31B magnesium alloy and its welded joints are tested by three-point bending test. The result shows that fracture toughness KIC values of AZ31B magnesium alloy base metal in pallrel (0-sample),45 and vertical (90-sample) to the as-extruded direction is 9.36,9.40 and 9.89 MPa·m1/2, respectively, and the KIC of 90-sample is the highest. The KIC of notch in welding center and HAZ are 18.28 and 15.62 MPa·m1/2, respectively, and the fracture toughness of welded joints is well than base metal, that is, the ability resisting fracture of welding is well. The fracture appearance of AZ31B magnesium alloy and its welded joints consist of quasi-cleavage and are brittle type of fracture; the fracture appearance of notch in HAZ is an intergranular fracture.The critical distance method and the hot spot stress approach are used to assess the welded joints. The critical distance assess including PM and LM. The BJ and TJ fatigue strength are 79.55 MPa and 49.10 MPa for PM and 79.01 MPa and 44.38 MPa for LM. Hot spot stress approach evaluated result shows that the dispersity is reduced largely, the fatigue strength can be expressed by only one S-N curve for BJ and LJ, and fatigue level is FAT33. The tested fatigue level of two evaluation methods is higher than fatigue level obtained used 3.0 recommended byⅡW, which indicates there is greater safety factor in welded structures design if 3.0 applied.The texture is often formed in the course of molding and its existence effect on fatigue property and crack propagation rate. There are obvious basel texture{0002}<2110> and{0002}<1010> in the AZ31B magnesium alloy plate. The schmid factor average of non-basal texture in ED is greater than TD, so it is easy for non-basal texture slipping and crack source forming under the alternating loads, this causes the fatigue life difference between ED specimen and TD specimen.Polar diagram and ODF are analysized and the results indicate that the Schmid factors of non-basal slip systems are different in two directions: mED>mTD, and the threshold of fatigue crack propagation rateΔKth is different: the TD is greater than the ED. Plastic deformation will easier in ED specimen and the crack easy to propagate.更多还原