Study On The Microstructures And Mechanical Properties Of Laser Welded SUS301L Austenitic Stainless Steel

With a series of advantages such as lightweight, low energy consumption, corrosion resistance, good safety, long service life and low comprehensive cost, the stainless steel railway vehicle has become the first choice of railway vehicles. In this paper, the choosing material is SUS301L austenitic stainless steel, which is used in railway car body with a large amount. Recently, SUS301L austenitic stainless steel has generally been adopted in railway car body in the developed countries because of its excellent plasticity and corrosion resistance, and good high and low temperature mechanical properties. Aiming at problems of resistance spot welding (RSW) in production process of stainless steel car body, it is proposed that the laser welding replaces the RSW for the production of the car body. Characteristics of microstructures and mechanical properties of laser welded stainless steel joints have been investigated. Besides, the effects of welding parameters on microstructures, mechanical properties and characteristics of temperature field distribution have been also investigated. The conclusions are as follows:(1) Laser welded stainless steel joints with a fine appearance and compact microstructure contain the weld zone, heat-affected zone and base metal. The weld zone near to the fusion line is coarsening columnar crystals and its center region exists dendritic crystals. The width of HAZ is narrow, with a grain coarsening feature. Furthermore, the closer near to the fusion line is, the more obvious the grain coarsening is. The stainless steel base metal and weld metal consist of y-Fe phase and δ-Fe phase.(2) The experimental results show that microhardness distribution of joint is uneven. The microhardness of weld zone is higher than HAZ, while lower than the base metal. Moreover, the results of tensile test show that the facture mode of laser welded joints have two kinds:HAZ facture mode and interface facture mode. Analysis results of fracture morphology indicate that the fracture surface has a lot of dimples.(3) With the increase of laser power, the width of weld bead for front side increases, and the color change of front side and back side is more and more obvious. The increasing heat input leads to the coarsening of columnar grain and the increase of width of columnar crystal zone. Moreover, with the increase of amount of melting, weld width and depth augment. In addition, the effective cross-section area of tensile samples also increases, leading to enlarging the tensile shear load of joints.(4) With the increase of welding speed, the width of weld bead for front side decreases and color change of front side and back side become lighter. Besides, the decreasing heat input leads to the refinement of columnar grain and the decrease of width of columnar crystal zone. In addition, the effective cross-section area of tensile samples also decreases, leading to diminishing the tensile shear load of joints.(5) With the increase of defocusing distance, the width of weld bead for front side decreases and color change of front side and back side become lighter. Besides, the size of grain in weld center and the width of columnar crystal decrease. In addition, the effective cross-section area of tensile samples also decreases, leading to diminishing the tensile shear load of joints.(6) By means of the quadratic orthogonal optimization design experiment, the regression equations of joint tensile shear load and weld depth have been established. Besides, the effects of welding parameters on tensile shear load and weld depth were investigated. By the Matlab software, the optimum welding parameters have been obtained, which are the laser power of2.0kW, the welding speed of22mm/s and the defocusing distance of0.5mm.(7) The temperature stimulation results show that the constant temperature lines in front of heat source are dense, indicating that this area has a big temperature gradient. On the contrary, the constant temperature lines in rear of heat source are loose, showing that this area has a small temperature gradient. The thermal cycle graph of nodes shows that heating speed is very quick and the retention time of high temperature is very short. Moreover, after the heat source moving, the temperature falls rapidly. Temperature distribution curves along the path indicate that the farther nodes arc away from the surface center, the lower the peak temperatures arc. The numerical stimulation results show that the shape of simulated melting pool is basically same as that of the actual pool.