Investigation And Numerical Simulation On Laser Beam Welding Of Ultra-thin Austenitic Stainless Steel Sheet

Austenitic stainless steels have good plasticity,corrosion resistance and easy processing and formability.They have been widely applied in many fields,such as engineering structure,medical devices,kitchen utensils,textile industry,petroleum industry,atomic energy industry,and so on.In practice,austenitic stainless steel is often used as welded structure.During the welding of ultra-thin stainless steel sheet whose thickness is lower than 0.5mm,the welding defects such as welding deformation and burning through are easily generated.Aiming at the main problems during the welding of ultra-thin stainless steel sheet,the pulsed laser beam welding(LBW)is used to weld two kinds of 321 austenitic stainless steel sheets with thickness of 0.15 mm and 0.1mm respectively.It is because that LBW has the characteristics of low heat input,narrow heat-affected zone(HAZ),low welding deformation,and well purification effect on weldment.Firstly,the MSC.Marc software is used for the numerical simulation analysis of LBW process,according to the results of numerical simulation,the laser butt welding parameters is optimized for stainless steel sheet with thickness of 0.15 mm.Moreover,the method of orthogonal experiment is used to optimize the laser lap welding parameters for stainless steel sheet with thickness of 0.1mm.The microstructure and mechanical properties of welded joints under different welding conditions are systematically investigated.The numerical simulation results of LBW shows that,the distribution of isotherm is elliptical,and the constant temperature line are dense in front of the moving heat source,indicating that this area has a high temperature gradient.The constant temperature line is sparse in rear of the heat source,indicating that this area has a low temperature gradient.The peak temperature is different at different nodes away from weld center.In weld center,peak temperature is the highest,and the farther node away from weld center is,the lower peak temperature is.With the increase of pulse energy,the maximum temperature and weld width increase,and the transverse residual tensile stress which is vertical to welding direction gradually increases.With the increase of welding speed,the maximum temperature and weld width gradually decreases,the residual stress which is vertical to welding direction gradually increases.Microstructure analyses of butt joint show that,different heat input will influence grain morphology,which results in different microstructure of welded joints.When the heat input is high,and the microstructure of weld center and fusion zone are columnar crystal.While when the heat input is low,weld center is equiaxed crystal,and fusion zone is columnar crystal.The relationship between weld thermal cycle curve and joint microstructure is investigated,and result shows that weld thermal cycle has great effect on microstructure of welded joint.With the increase of distance from weld center,cooling rate gradually decreases,and the content of delta ferrite gradually decreases.The XRD analysis of phase constituent indicates that the microstructure of weldment is composed of austenite and ferrite.The results of tensile tests for butt joint show that when the LBW parameters are E=1.4J and v=6mm/s,the tensile strength of welded joint is 541 MPa,which reaches 86% of that of base metal.The SEM analysis of tensile fracture indicates that welded joint presents the characteristic of ductile fracture.The results of microhardness test shows that the hardness value is the highest near fusion line,the hardness value near fusion line is about 222 HV.The content of delta ferrite in microstructure also has a certain influence on the hardness of weldment.From weld center to HAZ,the cooling rate gradually decreases during welding,and the content of delta ferrite gradually decreases.Consequently,the hardness of weldment decreases at a certain extent compared with the hardness near fusion line.Through orthogonal experiments,the optimized laser lap welding parameters are as follows: welding speed 0.5m/min,laser power 150 W,defocusing amount +2 mm.The well appearance of weldment can be obtained.The microstructure analysis of joint show that there is a clear boundary between weld metal and base metal,and the fusion zone is columnar crystal,while the weld center is equiaxed crystal.HAZ is very narrow,and no welding defect such as cracks and gas pores is found in welded joint.The XRD analysis shows that weldment is mainly composed of austenite and a small amount of ferrite.The results of tensile shear tests for lap joint show that,the shear load of welded joint with optimized welding parameters is maximum,which reaches 94% of the tensile force of base metal.The results of microhardness test show that hardness value is the highest near fusion line,that of weld center is the second and that of base metal is the lowest.The SEM observation shows that the tensile shear fracture of joint obviously presents the characteristic of ductile fracture.