【作者】 封小松；

【导师】 林尚扬； 陈彦宾；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2007， 博士

【摘要】 镀锌钢板由于低熔点镀层金属的存在,给其焊接带来了困难。激光高温钎焊工艺的开发改进了在接头强度、气孔、成形、接头抗腐蚀性能及焊接效率等方面存在的问题。这在工业生产中给结构设计改进、成本降低提供了可能。由于激光的加热特点及焊丝实时填加的影响,激光填丝钎焊过程的影响因素众多、工艺复杂,过程稳定性和接头质量需要得到进一步的提高。全面研究及定量评价各参数对焊接过程的影响规律具有重要的理论与实际意义。本文针对汽车工业中常用的镀锌薄钢板卷对接接头的连接,研究了各参数对钎缝成形、界面特征的影响规律。在此基础上,计算、分析了不同加热条件下的钎料温度分布和流动铺展成形。为深入理解激光填丝钎焊连接机理、开发新的激光填丝钎焊工艺提供技术支持和理论基础。在激光加热条件下,采用高速摄像系统观察和记录了预置于母材表面的钎料熔化铺展过程,研究了激光功率、光斑直径、加热时间等参数对钎料铺展效果的影响。进一步的,观察和描述了实际填丝钎焊中钎料的熔化及铺展过程。发现钎料表面张力对其流动铺展成形有重要影响,钎料温度与母材温度共同决定了其铺展性能,良好的钎焊过程需要考虑激光功率、焊接速度、送丝速度、激光加热位置等参数的匹配。在对圆形单光束激光填丝钎焊进行研究的基础上,本文提出双焦点激光填丝钎焊新工艺。与单光束相比,光束特征参数增加了双束激光功率配比、焦点间距、离焦方式、光束排布等参数,使双焦点光束激光钎焊更具灵活性。结合激光加热参数与光斑特征参数,可以改变钎焊热源能量密度分布,实现不同要求的加热效果。试验发现,双焦点光束激光填丝钎焊过程对焊接参数的变化具有更大的适应性,能够提高激光能量利用率。基于延长钎料与母材作用时间、改进接头下部界面结合质量的出发点,进一步提出矩形光斑激光填丝钎焊工艺。采用积分镜来获得长度方向能量均匀分布的矩形光斑。试验结果发现,采用合适的钎焊参数,矩形光斑激光填丝钎焊接头具有更小的焊缝宽度、更宽的钎料与母材接触面积,能有效降低镀锌层的烧损,但焊接过程适应性较低,激光能量的利用率下降。在对钎缝成形性能研究的基础上,采用金相显微镜、电子探针扫描、SEM及EDS等手段,对接头组织及界面行为进行了观察和分析。发现以CuSi3为填充材料的钎缝组织为以胞状树枝晶为主的α固溶体;界面附近的母材受焊接热作用而形成粗晶区。对不同加热条件下的界面形态进行了SEM分析,揭示了钎缝界面金属间化合物形成机制,界面金属间化合物层厚度和形态与焊接热循环及界面温度梯度有很大关系。建立激光加热下钎料流动铺展的热流耦合数学模型,考虑钎料自由表面的演变,采用有限元方法,模拟了激光定点加热、圆形单光束加热、双焦点光束加热及矩形光斑加热下的钎料铺展过程温度场、流场分布和形状变化。计算结果表明,激光定点加热时,光斑直径对钎料铺展形状影响显著,自由表面上的温度梯度和钎料液滴形状是钎料内部流动的主导因素。采用圆形单光束激光加热时,钎料升温迅速,上下部分具有较大的温度梯度;双焦点激光和矩形光斑加热下,钎料温度升高缓慢,内部温度分布更为均匀。单光束加热初期,钎料以向下填充钎缝接头间隙的流动为主,钎缝形状趋于稳定后,钎料的流动过渡到以Marangoni流动为主;双焦点激光加热的整个过程中均以向下填充接头间隙的流动为主,钎缝的填充过程时间更长;矩形光斑加热下具有更小的流动速度,焊缝中心线附近的钎料以向下填充接头间隙的流动为主,而钎缝上表面边沿区域钎料以Marangoni流动为主,向钎缝边沿流动。对钎料铺展的自由表面形状的模拟结果与试验结果相比较,两者吻合较好。更多还原

【Abstract】 The joining of galvanized steel sheets is difficult due to the existence of Zn coat. The evaporation and oxidation of zinc can lead to an unstable arc, low corrosion resistance and other welding defects such as porosity, inadequate fusion and cracks and so on. These problems can be avoided with the development of laser brazing process, which provides the possibility to improve the structure design and reduce the cost in production. With the laser heating feature and real-time wire feeding, the process is complex and the influencing factors are various, and process stability and joint quality are required to improve further. So, comprehensive investigation and quantitative evaluation of the influence of process parameters are significant in theory and practice. Aiming at the laser brazing of flange butt joints of galvanized steel sheets, the influence regularity of process parameters on brazing seam forming and interface behaviors was studied in this paper. Based on the analysis, the filler metal flow and temperature distribution in brazing process with different heating condition were calculated. These results provide technique support and fundamental theory in comprehending mechanics of laser brazing process and developing new process. The melting and spreading process of filler metal with static laser heating were observed and analyzed using high speed photographing system in this paper.The influence of process parameters such as laser power, spot diameter and heating time on spreading ability was evaluated. In further investigation, the melting and spreading process of feeding wire in real brazing process was analyzed. The experimental results indicate that the surface tension is the most important influencing factor in the spreading flow and forming of filler metal, the temperature of filler metal and base metal determine the spreading ability. Furthermore, the matching of parameters such as laser power, brazing speed, wire feeding speed and laser heating position should be considered for a good appearance brazing joint.Based on the investigation of individual laser beam brazing, a new brazing process, dual-spot laser beam brazing was presented. Compared with individual beam brazing, new beam parameters include beam power ratio, focus distance, defocusing mode and spot arrange method, which make the brazing process more flexible. Combining the heating parameters and beam parameters, energy intensity distribution of brazing heat source can be changed to realize the different thermal distribution. Experimental results show that the better process adaptability and higher laser power availability ratio can be achieved in dual-spot beam brazing than in individual beam brazing.In order to expand the interaction time between filler metal and base metal and increase the interface combination quality at the bottom joint area, rectangular spot laser beam was used as brazing heat source. Integrator mirror was used to obtain rectangular laser spot with uniform laser power distribution in spot length direction. The rectangular spot brazing results indicate that narrower brazing seam surface and more joint interface area with suitable brazing parameters can be achieved, and the process can reduce the burning loss of Zn coat. Whereas, the adaptability and laser power availability ratio are lower.Further investigation was focus on the laser brazing joint structure and interface behaviors, in which metallographic microscope, electron microprobe scanning, SEM and EDS analysis were used. In experiments, cellular dendritic crystal ofαsosoloid was found in brazing seam with filler material of CuSi3 and the adjacent interface area of base metal is coarse grain zone. The joint SEM analysis with different heating condition was carried out to understand the mechanics of intermetallic compounds formed at the interface, and the results indicate that the thickness and shape of intermetallic compounds layer relate to brazing thermal cycle and interface temperature gradient.A FEM model of thermal-fluid coupling of laser brazing was developed. With the model, fixed beam heating, individual beam, dual-spot beam and rectangular spot brazing process were simulated, and the temperature distribution, flow field and filler metal shape were calculated. The simulated results show that beam spot size is the determining factor of brazing droplet shape, the temperature gradient on free surface and droplet shape determine the flow of filler metal for fixed beam heating. In individual beam brazing, the temperature of filler metal increase rapidly, and a great temperature gradient can be obtained. For dual-spot and rectangular spot brazing, the temperature filed is more uniform. The simulated results of flow field show that, the downward flow filling the joint gap is the main flow situation of the filler metal at the early stage of individual beam heating, and Marangoni flow is more important after brazing seam shape forming. For dual-spot laser brazing, the downflow is dominant in the whole process and the time of filling joint gap process is longer. The average velocity of filler metal flow is small in rectangular spot brazing and the filler metal adjacent seam center line flow downwards and Marangoni flow is dominant for the filler metal at the upper part of brazing seam. Simulated and experimental brazing shapes were compared, and a good agreement is obtained.更多还原