304不锈钢激光焊接接头形貌与组织性能研究

发布时间：2018-10-13 07:31

【摘要】：AISI 304不锈钢是应用最为广泛的一种奥氏体不锈钢,很多情况下它的应用都需要利用到一种重要的加工方法——焊接。相对于传统的焊接方法,激光焊接因其具有能量集中、热影响区小、热变形小及焊接速度快等优点,故能提高304不锈钢焊接接头的质量。由于接头质量与宏观形貌和组织性能密切相关,所以很有必要对304不锈钢激光焊接接头形貌和组织性能进行研究。本文以304不锈钢激光焊接接头为对象,旨在研究焊接工艺参数对接头宏观形貌、微观组织和机械性能的影响规律,得到合适的焊接工艺参数。此外,建立适用于激光焊接的三维热源模型和有限元分析模型,实现焊接温度场的三维动态模拟,从宏观形貌和微观组织两方面将模拟结果与试验结果联系起来,指导工艺参数的设计和优化。首先,采用板厚为0.7 mm的304不锈钢板进行焊接工艺试验,分析了工艺参数对焊缝宏观形貌的影响规律。当工艺参数选择得当时,所获得的焊缝具有合适的焊缝尺寸且没有宏观焊接缺陷的产生。然后,将得到的焊接接头制成金相分析试样并进行电解腐蚀,利用光学显微镜对焊缝的微观组织进行观察,发现焊缝的微观组织不同于母材,且工艺参数对焊缝的微观组织有影响,主要体现在焊缝边缘柱状晶区域宽度和焊缝中心等轴晶尺寸的变化上。对接头不同区域进行显微硬度测试,发现焊缝的显微硬度值高于母材。此外,对不同工艺参数下的焊接接头进行拉伸试验,并将接头的拉伸强度与焊缝的宏观形貌和微观组织联系了起来。当焊缝拥有较好的宏观形貌和较小的晶粒尺寸时,能获得拉伸性能最好的焊接接头,且接头在拉伸试验后于母材处断裂。利用扫描电镜对该拉伸断口的形貌进行拍摄,发现断面上存在大量韧窝,断裂模式为韧性断裂。最后,根据激光焊接的焊缝成形特征,构造了双椭球体热源和高斯面热源叠加的组合热源模型,利用有限元分析软件ABAQUS和FORTRAN语言编写的用户子程序,实现了激光焊接温度场的三维动态模拟。在宏观形貌方面,模拟结果中的熔池边界线和试验结果中的熔合线基本吻合,验证了模型的可靠性;在微观组织方面,通过焊接温度场预测焊缝液相中的温度梯度和焊缝中心的冷却速度,以此定性地推断焊缝结晶形态的倾向和晶粒的相对大小,探究焊接温度场与焊缝微观组织的关系。
[Abstract]:AISI 304 stainless steel is one of the most widely used austenitic stainless steels. Compared with the traditional welding method, laser welding can improve the quality of 304 stainless steel joint because of its advantages of concentrated energy, small heat affected zone, small thermal deformation and fast welding speed. Because the joint quality is closely related to macroscopic morphology and microstructure and properties, it is necessary to study the microstructure and properties of 304 stainless steel laser welded joint. The purpose of this paper is to study the effect of welding process parameters on the macroscopic morphology, microstructure and mechanical properties of 304 stainless steel laser welded joints, and to obtain appropriate welding process parameters. In addition, a three-dimensional heat source model and a finite element analysis model for laser welding are established to realize the three-dimensional dynamic simulation of the welding temperature field. The simulation results are related to the experimental results in terms of macroscopic morphology and microstructure. Guide the design and optimization of process parameters. Firstly, the welding process of 304stainless steel plate with a thickness of 0.7 mm was carried out, and the influence of process parameters on the macroscopic morphology of the weld was analyzed. When the process parameters are selected at that time, the obtained weld has a suitable weld size and no macroscopic welding defects. Then, the welded joints were made into metallographic analysis samples and electrolytic corrosion. The microstructure of the weld was observed by optical microscope, and it was found that the microstructure of the weld was different from that of the base metal. The effect of process parameters on the microstructure of the weld is mainly reflected in the variation of the width of the columnar crystal zone and the equiaxed grain size of the weld center at the edge of the weld. The microhardness of weld is higher than that of base metal. In addition, tensile tests were carried out on the welded joints with different process parameters, and the tensile strength of the joints was related to the macroscopic morphology and microstructure of the welds. When the weld has better macroscopic appearance and smaller grain size, the welded joint with the best tensile properties can be obtained, and the joint breaks at the base metal after tensile test. Scanning electron microscopy (SEM) was used to photograph the morphology of the tensile fracture. It was found that there were a large number of dimples on the fracture section and the fracture mode was ductile fracture. Finally, according to the weld forming characteristics of laser welding, the combined heat source model of double ellipsoid heat source and Gao Si surface heat source superposition is constructed, and the user subprogram written by finite element analysis software ABAQUS and FORTRAN language is used. The three-dimensional dynamic simulation of laser welding temperature field is realized. In terms of macroscopic morphology, the boundary line of the molten pool in the simulation results is basically consistent with the fusion line in the experimental results, which verifies the reliability of the model. The temperature gradient in the liquid phase of the weld and the cooling rate of the weld center are predicted by the welding temperature field. The tendency of the crystal morphology of the weld and the relative size of the grain are deduced qualitatively, and the relationship between the welding temperature field and the microstructure of the weld is explored.
【学位授予单位】：武汉理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG456.7

【参考文献】