Study On Laser Weld Defect Repair Of SUS301L Austenitic Stainless Steel

SUS301L austenitic stainless steel has good comprehensive properties,which has been widely used in rail car industry.At present,the side wall of the rail car body is connected by laser lap welding.It is found that the assembly gap is inevitable in the welding process of the car body,especially in the manufacturing process of the large-size components such as the side walls of the vehicle body.The surface collapse occurs,which is due to the lack of filler weld metal caused by too large gap size.It is urgent to repair the defective welds in actual production.Therefore,it is crucial to study the repairing process of laser weld defects of austenitic stainless steel and analyze the repairing mechanism.In this paper,the laser lap weld of austenitic stainless steel with plate thickness combination of 0.6 mm+2 mm and 1 mm+0.6 mm were selected as the research object.The effects of different assembly gaps on the surface collapse amount,microstructure and tensile shear force of the laser weld of semi-permeable and full permeable plates were studied,and we developed a new process to repair the surface defects of stainless steel by using laser remelting technology.Furthermore,we obtained the optimized laser remelting repair process parameters for two kinds of plate thickness combined welds.It reveals the mechanism of the laser remelting technology to improve the microstructure and mechanical properties of the weld,which provides reliable data explanations for the feasibility of the laser melting repair process,and provides a theoretical basis for the application of this technology on stainless steel car body.Firstly,to investigate the influence of assembly gap on weld quality,the microstructures and mechanical properties of welds with two sets of thinkness combination(0.6 mm+2 mm and 1 mm+0.6 mm)were analyzed contrastively.With the analysis of the results,it can be concluded that the microstructure of the welds mainly includes?-austenite and?-ferrite with the orientation relationship of K-S.The solidification mode of weld is AF mode.When there is a gap between the two plates,a triangular region composed of similar equiaxed crystals can be found,and the size of the cellular crystals in this region decreases significantly.With the increases of assembly gap size,the weld penetration state changes from full penetration to semi-penetration,the maximum tensile-shear load of the weld decreased,and the fracture mode of the weld changed from heat-affected zone fracture to joint surface fracture.In order to ensure that the surface morphology and properties of the weld can meet the quality standard and requirement,the assembly gap should be less than 0.1 mm.Therefore,the defective welds with an assembly gap of 0.2 mm were selected as the research object of the combined laser repair process of two plate thicknesses combinations.Subsequently,laser melting technology was used to remelt and repair the surface collapse defect of laser welds,and the effects of laser power,welding speed and focusing distance on the surface quality and mechanical properties of repaired welds were studied.Furthermore,the orthogonal optimization test method was used to optimize the laser repair process parameters.As for the welds of semi-penetration,the optimized parameters are as follows:laser power is of 1.6 k W,welding speed is of 3.81m/min and focusing amount is of 0 mm.As for the welds of full penetration,the optimized parameters are as follows:laser power is of 1.45 k W,welding speed is of2.74 m/min and focusing amount is of 0 mm.Under the condition of optimal repair process parameters,the weld surface collapse can be effectively repaired,and the tensile shear force of the repaired weld reaches the level of the non-defective weld.Moreover,it revealed that the bulging of the weld surface is mainly caused by the volume expansion of the weld metal in the absence of external welding materials.The densities of base metal and weld metal show that the average density of weld metal is about7.8387 g/mm~3,and that of base metal is about 7.8566 g/mm~3,which further verifies that the laser repair mechanism is based on the volume expansion of weld metal.Finally,the mechanism of laser remelting technology on the microstructure and mechanical properties of repaired weld was revealed by studying the microstructure and properties of weld before and after laser repair with the plate thickness combined of 1 mm+0.6 mm.After laser remelting repair,the volume of weld pool decreases,the cooling rate of metal per unit volume of weld pool increases,and the solidification rate of weld pool increases,leading to grain refinement in the laser remelting zone.Meanwhile,the recrystallization of weld pool in the non-equilibrium solidification process also promotes grain refinement.Compared with gapless welds,the proportion of small-angle grain boundaries in the repaired welds is greater than that in the primary welds,indicating that the repaired welds have strong anti-crack growth ability.With the repair of laser remelting,the average grain size in the center of the weld decreases from48?m to 29.8?m,the residual tensile stress of the repaired weld decreases and the fatigue life increases.The substantial improvement of the mechanical properties is related to the influence of grain refinement on dislocation of the weld.