【作者】 李扬；

【导师】 胡绳荪；

【作者基本信息】 天津大学 ， 材料加工工程， 2015， 博士

【摘要】 黄铜与不锈钢异种金属焊接结构在制冷装置、机械制造及汽车工业等领域的应用越来越广泛。这种复合结构不仅具有黄铜优良的导热性能,而且还具有不锈钢良好的力学性能。但是由于黄铜与不锈钢的化学成分、物理性能存在很大的差异,其焊接性研究得到关注。激光焊具有能量密度高且集中、加热与冷却速度快等特点,有利于异种金属材料的焊接。为此,本文开展了黄铜与不锈钢异种金属薄板激光焊接的研究,该研究对于探索异种金属激光焊接成形机理以及工程应用具有重要的理论意义和实际应用价值。本文首先对黄铜板在上、不锈钢板在下和不锈钢板在上、黄铜板在下的两种搭接方式激光焊接的焊缝成形、焊缝化学成分与分布、接头微观组织特征以及力学性能等进行了实验研究与测试分析。研究表明,黄铜板在上、不锈钢板在下搭接激光焊时,焊缝呈“杯”状,熔深比较大;不锈钢板在上、黄铜板在下搭接激光焊时,焊缝呈“盘”状,熔深比较小;黄铜-不锈钢激光焊接焊缝中没有金属间化合物生成,焊缝中心位置的黄铜与不锈钢以机械混合物形式存在,且分布不均匀,在固液界面处存在很窄的过渡区,不同的搭接形式过渡区形态不同,过渡区中两种材料发生了一定程度的混合;通过分析研究提出了黄铜板在上、不锈钢板在下的搭接方式适合于薄板激光焊接。本文建立了黄铜-不锈钢薄板搭接激光焊接的数值分析模型,开展了温度场、流场的研究,结合温度场、流场的数值分析结果,分析研究了黄铜-不锈钢薄板激光焊焊缝成形以及焊缝化学成分及其分布的机理,说明了不同搭接方式激光焊缝成形的特点、焊缝及过渡区黄铜与不锈钢混合的特点。本文开展了黄铜-不锈钢薄板搭接连续与脉冲激光焊接的研究,分析了连续激光焊中激光功率、焊接速度,脉冲激光焊中脉冲激光峰值功率、平均功率以及脉冲时间等焊接参数对焊缝成形、焊缝化学成分及其分布、焊缝微裂纹以及力学性能的影响,总结了规律;提出了连续激光焊时,保持热输入不变时,调节焊接速度是控制焊接接头质量的关键;脉冲激光焊时,保持激光脉冲能量不变时,激光的脉冲激光峰值功率和加热时脉冲持续时间对接头的成形起着相反的作用。同时,本文还开展了黄铜-不锈钢薄板搭接激光焊接微裂纹产生机理以及影响因素的分析测试研究,研究表明,黄铜在上不锈钢在下时形成的微裂纹宽度小于0.2μm,而不锈钢在上黄铜在下的微裂纹宽度可以达到4.08μm;随着焊缝中不锈钢含量的增大,焊缝中产生微裂纹的机率增大;连续激光焊时,微裂纹的数量随着热输入的增加而增加;脉冲激光焊时,当脉冲激光能量恒定不变时,如果不锈钢熔化量差异不大,微裂纹的数量随着激光功率密度的增大而减少,保持激光功率密度不变时,随着平均功率的增加,微裂纹的数量及尺寸也随着焊接热输入的增加呈增加趋势。分析了微裂纹产生的机理,那就是在焊缝结晶过程中,由于不锈钢与黄铜的熔点不同,奥氏体不锈钢首先冷凝结晶,液态黄铜沿不锈钢奥氏体晶界扩展,在结晶拉应力作用下,产生微裂纹。最后,本文开展了黄铜-不锈钢薄板搭接激光焊接接头拉剪性能测试,结果表明,连续激光焊接接头的拉伸-剪切力比脉冲激光焊接接头的拉伸-剪切力平均大200N左右;焊接接头的断裂均发生在黄铜一侧的过渡区,焊缝中过渡区的微裂纹数量及焊缝熔合区粗大的组织是影响焊接接头拉伸-剪切力的主要因素。更多还原

【Abstract】 The welding structure of brass and stainless steel has been used more and more widely in the field of refrigeration equipment, mechanical manufacturing and automobile industry. This kind of composite structure not only has excellent thermal conductivity characteristics of brass, but also has superior mechanical properties of the stainless steel. However, there are many differences in the thermal physical properties between the brass and stainless steel, so weldability between brass and stainless steel has attracted increasing attention. Laser welding with the features of high energy density, high heating and cooling speed is conducive to dissimilar metals welding. So, in this paper, laser welding on brass and stainless steel was researched. This research has important theoretical significance and practical application value on welding mechanism for dissimilar welding of high energy density of heat.In this paper firstly researched the macroscopic feature, microstructure, interface characteristics, elements diffusion and mechanical property in the welded joint on the two different lap joint types(brass on top and stainless steel on top). This result showed a fusion zone with “cup” shape during laser lap welding with brass on top, and that a fusion zone with “dish” shape during laser lap welding with stainless steel on top. There was no intermetallic compound formation in the welded joint of brass and stainless steel. Brass and stainless steel existed in the form of mechanical mixture, and non-equilibrium distribution in the welded joint. There was a narrow transition region in the solid-liquid interface. The different lap joint types had the different transition form. In the transition zone was mainly the compound of brass and stainless steel. This research demonstrated that the brass on top was more suitable for the welding of brass sheet to stainless steel sheet.In this paper, the lap joint numerical analysis model was presented on brass to stainless steel laser welding. The temperature field and flow field was researched. The mechanism of chemical composition and distribution of welded joint was analyzed. The mechanism of the weld forming characteristics, and the chemical composition and distribution was researched. The forming characteristics of the weld joint, mixing characteristics of the welded joint and the interface, and the influence factors of non-equilibrium distribution of brass to stainless steel were explained.This paper carried out the continuous and pulsed laser welding of brass sheet to stainless steel sheet in lap joint. The influence of parameters of laser power, welding speed in continuous laser welding and parameters of pulse laser peak power, average power and pulse time in pulsed laser welding on formation, composition and distribution, weld micro-crack and the mechanical properties of the welded joint were researched, in addition, the law was summarized. For continuous laser welding, when the heat input was constant, the welding speed was the main factor to control the welding quality. For pulsed laser welding, peak power and pulse duration have opposite effect on the penetration-to-width ratio of welded joint when the pulsed laser energy was constant.At the same time, this paper also carried out the micro-crack mechanism and influence factors of the brass sheet to stainless steel in laser welding of lap joint. The researches showed that the crack width was less than 0.2μm when brass on top, and was 4.08μm when stainless steel on top the crack width. The appearance probability of the micro-crack was increased with the increase of stainless steel content in the weld. For continuous laser welding, the micro-crack number was increased with the increase of welding heat input. For pulsed laser welding, and the pulsed energy was constant, the micro-crack number was decreased with the increase of laser power density when to the molten pool of the fusion quality of stainless steel was similar to each other. For pulsed laser welding, the micro-crack number and size were increased with the increase of average power when the laser power density was constant. The mechanism of crack appearance was that the molten pool and the transition region had mechanical mixture of the brass and stainless steel. In the weld crystallization process, austenitic stainless steel firstly appears condensing crystallization because of the difference crystallization temperature between the stainless steel and brass. The liquid brass diffused along the austenite grain boundary, and with the crystallization of tensile stress, micro-cracks appeared in the interface.At last, this paper also carried out the tensile-shear force of the lap joint. The researches showed that the welded joint’s tensile-shear force value of continuous laser welding was 200 N larger than pulsed laser welding. Welded joint fracture occurred in the transition zone on the brass side. Cracks number in the transitional zone and in the coarse microstructure in fusion zone were the main factors to influence the tensile-shear force.更多还原