Study On Laser Welding And Melting Process Of Ferritic Stainless Steel For Automobile

The pipe fittings processed by expanding pipe or shrinking neck are prone to local cracks due to the lack of strength,toughness,plasticity,hardness and other mechanical properties.Aiming at the problem of reprocessing crack after welding of ferritic stainless steel in the housing of automobile catalytic converter,the laser weld test of 441 ferritic stainless steel sheet with 1.2mm thickness was carried out to study the microstructure and mechanical properties of the welded joint in this paper.Based on the optimized welding process,the surface of the welding joint was treated by laser melting,and the effects of the laser melting process on the joint morphology of the welding joint,microstructure and mechanical properties were analyzed.For the "girth waist" joint with good weld shape(laser power of 1.8 k W ~ 2.2k W,welding speed of 1.5m/s ~ 2.4m/s,defocus of-4mm ~ +2mm),the width of the weld seam increases firstly and then decreases with the increase of laser power,welding speed and defocus amount.The degree of the "girth waist" firstly increased and then decreased,on the meanwhile the growth trend of columnar crystal is curbed.The microstructure of weld is equiaxed grain at the center and columnar grain at the edge,and the heat-affected zone is slightly coarsened.The results of XRD phase analysis show that the weld is ferrite.The optimal laser welding process of 1.2mm ferritic stainless steel was obtained.At this moment,the laser power was 1.5 k W,the welding speed was 2.0 m/min,and the defocus was +2 mm.At this point,the size of equiaxed grains and the width of columnar grains are 55.3 μm and 49.1 μm,the tensile strength of the weld reaches the maximum 510 MPa,the elongation is 1.27 times that of the base metal,the central hardness is higher than 194.2 HV,and the dimples on the surface of the tensile fracture of the joint are dense.The tensile strength and elongation of the weld first increased and then decreased with the increase of laser power,welding speed and defocus.The joint hardness increases with the increase of laser power and welding speed.The joint hardness increases first and then decreases with the increase of laser power and welding speed.The joint hardness decreases first and then increases with the increase of defocus,and increases with the decrease of grain size.The microstructure and mechanical properties of the joint are better when the defocus is +7mm and the heat input is 0.33 J/mm ~ 0.50 J/mm.The tensile fracture morphology of the melt joint is characterized by local brittle fracture.With the increase of heat input,the joint forming gradually becomes worse,the area of the molten layer increases,the average grain width of the molten zone decreases first and then increases,the refining degree of remelted grain decreases continuously,and the brittle fracture characteristics become more obvious.The results of phase analysis show that the weld surface has no phase transition and the remelted grain is still ferrite.After laser remelting treatment,the pitting density of the weld decreases and the corrosion resistance increases.