【作者】 司恩；

【导师】 王文先；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2010， 硕士

【摘要】 与其他结构材料相比,镁合金具有比刚度、比强度高、易回收等一系列的优点,在汽车、航空、航天和国防军事工业等众多领域都具有极其重要的应用价值。镁以其资源丰富而受到更加的重视。但是由于镁合金本身的焊接冶金性能差,其连接技术依然是制约镁合金进一步应用的技术瓶颈。因此,利用高能量密度的激光对镁合金连接技术的研究具有重要的实际意义。本文利用500W脉冲Nd:YAG固体激光器对薄板AZ31B变形镁合金进行焊接性研究。研究镁合金激光自熔焊工艺性能,探索了Nd?YAG脉冲激光焊接镁合金的一般原则;研究了YAG激光焊对接接头的物相、显微组织、力学性能、断口形貌以及电化学腐蚀性能;分析了激光焊接存在的主要缺陷及产生机理,并提出了相关的预防措施和改善方法。对2mm厚AZ31B变形镁合金进行自熔焊工艺性研究,发现在相同的平均功率下采用不同的脉冲宽度和脉冲频率对焊缝熔深熔宽的影响有很大差别,峰值功率和脉冲宽度是影响焊缝熔深熔宽的主要参数。为了提高脉冲YAG激光与镁合金的耦合效率,脉冲YAG激光焊接镁合金时应遵循较大脉宽(>3ms)、较低频率(<60Hz)、适当峰值功率的原则。基于以上脉冲YAG激光焊接AZ31B镁合金的原则,对1mm AZ31B镁合金进行对接焊接试验,确定最佳工艺参数为:脉冲宽度(tp)=4.5ms,脉冲频率(f)=30Hz,平均功率(P)=253W,峰值功率(pp)=1.87kW,焊接速度(v)=5mm/s,占空比(CT)=16%,保护气体为Ar,侧面保护气体流量为15mL/min,离焦量(△)=+3mm。YAG激光焊缝的显微组织主要是由晶粒细小的α-Mg固溶体和呈颗粒状弥散分布在α-Mg固溶体内的β-Mg17Al12相组成。由于激光焊接时产生的较大过冷度使焊缝区晶粒得到了明显的细化,晶粒尺寸从母材的35μm细化到了10μm。焊缝区除了在熔合线附近存在少量的柱状晶外,其它区域均为细小的等轴晶,晶粒尺寸从熔合线到焊缝中心有明显的变化,从柱状晶区到晶粒较大的等轴晶区到晶粒较细小的等轴晶区再到焊缝中心的晶粒较大的等轴晶区,过渡界面清晰可见。焊缝深度方向的晶粒大小变化不大。AZ31B镁合金母材的抗拉强度为234.75MPa,激光焊缝的抗拉强度为180.63MPa,达到母材的76.78%。母材的断口形式主要为解理断裂,焊接接头的断口形式为准解理断裂和韧性断裂的混合断裂,母材的脆性断裂倾向比焊接接头的大。母材的硬度大约为55HV0.05左右,焊缝的硬度比母材的提高了12.7%～30.9%,最大硬度值可达72HV0.05,这主要是受细晶强化和沉淀强化共同作用的结果。硬度的最大位置出现在焊缝的次中心区,非中心部位。母材的平均自腐蚀电位为-1471mV,焊缝的平均自腐蚀电位为-1384mV,比母材的提高了87mV,母材的平均自腐蚀电流为1184mA,焊缝的自腐蚀电流为1128mA,焊缝的自腐蚀电流比母材的减小了56mA。抗腐蚀性能得到提高。进一步对AZ31B镁合金YAG激光自熔焊和对接焊的缺陷进行了分析研究,发现主要缺陷是焊接裂纹和合金元素的烧损。裂纹主要是结晶裂纹,镁合金有较大的结晶裂纹倾向。少数裂纹在扩展过程中出现多次分叉的现象。将焊后没有出现裂纹的试样在一定拘束应力下静置48小时后,进行表面检查和解剖观察,没有发现任何形式的延迟裂纹。更多还原

【Abstract】 Compared with other structural materials, magnesium alloys have a large of advantages, and they have extremely important value in the automotive, electronics, transportation, aviation, aerospace and defense, military industry and many other field. Magnesium alloys have been paid more attention to because of its rich resources with the traditional metal mineral resources depleted. However, the connection technology is still technology bottleneck which restricts the further application of magnesium alloy due to its magnesium alloy poor welding metallurgy performance. Therefore, study on the connection technology of magnesium alloy has important practical significance using laser which has many advantages.In this paper, laser welding properties of AZ31B magnesium alloy thin sheet are researched using 500W pulsed Nd:YAG solid state laser, hoping to discover the general principles which can guide pulsed YAG laser welding of magnesium alloy; the phase, microstructure , mechanical properties, fracture morphology and the electrochemical corrosion of the YAG laser butt joints are studied; the mechanism and the major defects of the laser welding process are analysed , and relevant prevention measures and solutions are put forward.Firstly, laser self-fusion welding processing properties of 2mm thick AZ31B magnesium alloy are studied, finding different pulse width and pulse frequency have a large influence on weld width and deep penetration under the same average power , the peak power and pulse width are the main parameters which affected weld width and deep penetration. To improve the coupling efficiency between pulsed YAG laser and magnesium alloys, pulsed YAG laser welding of magnesium alloys should be followed by the principle of a larger width(>3ms), low frequency(<60Hz), proper peak power.Based on the general principles of pulsed YAG laser welding of magnesium alloys AZ31B, 1mm AZ31B magnesium alloy is welded in the butt-welded form, the optimum process parameters: pulse width(tp)=4.5ms, pulse frequency(f)=30Hz, average power(P)=253W, peak power(pp)=1.87kW, welding speed(v)=5mm/s, duty cycle (CT)=16%, protective gas is Ar, side protection gas flow 15mL/min, defocusing (△)=+3mm.The microstructures of YAG laser butt-welded joints mainly consist of fineα-Mg grain andβ-Mg17Al12 phases which dispersed in theα-Mg solid solution. grains in weld area is significantly refined by large undercooling rate during laser welding, grain size is refined from the original 35μm to 10μm. In addition to few columnar crystals near the fusion line, other regions consist of fine equiaxed grains. The grain size has significant change from fusion line to weld center, changing from columnar crystals area to the fine equiaxed grains area to more fine equiaxed grains area, and then to fine equiaxed grains area in weld center, and the transition interface is clearly visible. the grain size along weld depth direction has little change. Tensile strength of butt-welded joint is 180.63 MPa, which is 76.78% of that of the base material. The form of base metal fracture is mainly cleavage fracture, fracture of welded joints form is the mixed fracture of quasi-cleavage fracture and ductile fracture, brittle fracture tendency of the base metal of welded joints than big. compared with the base metal, micro-hardness of butt-welded joint increased by 12.7%~30.9% in the interaction of grain refinement and precipitation strengthening. the maximum value of macrohardness is 72HV0.05, but the location in which microhardness is maximum is not weld center. the average corrosion potential of YAG laser butt-welded joint is -1384mV, compared with substrate materail, increasing by 87mV. The average corrosion current of base metal is 1184mA, and corrosion current of the welded joint is 1128mA, compared with substrate materail, reducing 56mA. improving corrosion resistance.Finally, massive experimental results show that major defects in YAG laser butt welding and self-fusible welding of AZ31B magnesium alloy and butt welding are welding cracks and burning of alloying elements. Welding cracks are mainly solidification cracking, and magnesium alloy have greater tendency to solidification cracking. few crack appears the phenomenon of many-times bifurcation, there is no cracks is observed in the sample for 48 hours under certain restraint stress After surface examination and anatomical, finding none form of delayed crack.更多还原