Numerical Simulation Of TIG Welding Process Of Stainless Steel Pipe Under Constraint Conditions

Ferritic stainless steel has good high temperature oxidation resistance,small thermal expansion coefficient,small density,light weight and low price.It is widely used in automobile manufacturing,pressure pipeline,industrial construction,medical equipment and many other fields.Aiming at the main welding process of ferritic stainless steel exhaust pipe of automobile engine.Tungsten inert gas(TIG)welding has the advantages of high welding quality,stable welding process and low cost.It is especially suitable for the welding of thin plate structure and widely used in automobile manufacturing.In the pratical production process,the stainless steel pipe is formed by bending and crimping the stainless steel plate through multiple rollers.Therefore,the stainless steel pipe is produced under constrained state.The constrained and pressure state during forming will change the stress release of the welding process,which affects the residual stress and deformation of the workpiece,and ultimately affects the stability and dimensional accuracy of the workpiece.The relevant data of the welding forming process of stainless steel pipes is not easy to be obtained through experiments.Therefore,the finite element method is used to analyze the stress-strain behavior during the welding process under the influence of the applied pressure and fixture constraints and reveal the distribution characteristics of the residual stress after welding.The stress distribution has important guiding significance for realizing low-deformation and high-quality welding.Based on the structural nonlinear comprehensive consideration of the physical process of TIG welding of 409L ferritic stainless steel plate under external pressure,a heat source model that is more consistent with the experimental results is adopted by using a more efficient thermal-mechanical sequence coupling calculation method and for different arc sizes and behaviors.A finite element numerical model for TIG welding of 409L ferritic stainless steel plate under external pressure was established.The temperature field and stress-strain field of the calculated results were verified by using the weld cross section morphology,HAZ thermal cycle curve,post-welding deflection and post-welding residual stress.The verification results show that the model is reliable and effective within the error range,and can be used to calculate the thermal evolution process of 409L ferritic stainless steel plate TIG welding.The effect of 10 MPa external pressure on the TIG welding temperature and stress-strain field of 1.2 mm 409L ferritic stainless steel plate is numerically analyzed.During the welding process,the external pressure will increase the compressive stress and area of the weldment,and slow down the appearance of the tensile stress of the weldment under the external pressure at the same time.The external pressure will reduce the transverse residual stress by 20 MPa and increase the transverse residual compressive stress by 10-15 MPa,after the welding and cooling is completed at 400 s.The external pressure will reduce the longitudinal residual stress by 3～13 MPa,which is slightly lower than the value of the external pressure.Generally,the external pressure can reduce the residual tensile stress of the weldment and increase the residual compressive stress,which can effectively improve the load-bearing capacity of the weldment surface,but it reduces the tensile strength of the weldment along the direction of force.Based on the finite element model for steel plate,the TIG welding finite element model was established for409L ferritic stainless steel pipe.The influence of the constraint of roller clamp on the evolution of stress field and the distribution of residual stress on the outer surface in range of 6.3°～173.7° and 186.3°～353.7° of pipe was analyzed.Constraint will transform the unconstrained area of the weld from the transverse residual stress of 10 MPa to the transverse residual stress of 170 MPa,and the unconstrained area of the weldment bottom will transform from the transverse residual stress of 15 MPa to 70～78 MPa.In terms of longitudinal residual stress,the constraint of the fixture will increase the residual tensile stress in the unconstrained area of the weld by 40 MPa,and at the same time increase the residual compressive stress at the bottom of the weldment,with an average value of 20 MPa.From the perspective of the overall residual stress distribution of the weldment,the constraint of the fixture will make the residual stress concentration exist in the weldment area and reduce the residual tensile stress in other areas,and increase the residual compressive stress At the same time,the fixture constraints will limit the post-weld deformation of the outer surface of the weldment within the constrained range and change the deformation direction of the unconstrained area of the weld at the welding and cooling process,so that the weld area with outward deformation of 5.2mm under free deformation is transformed into the deformation area with inward deformation of 2.3mm.