Simulation Of Ultrasonic-assisted Perforated Plasma Arc Welding Of Stainless Steel Thermal Field

316L stainless steel has the advantages of good pitting corrosion resistance,high temperature strength and good welding performance.It has a wide application prospect in shipbuilding,chemical pipeline and other fields.Therefore,the research on the welding of316 L stainless steel plate has important theoretical significance.The keyhole plasma arc welding(PAW)has the advantages of high energy density,good weld formation,and can realize one-sided welding and two-sided forming without groove.However,the stability of small holes and small adjustable margin still exist in PAW,which restricts the wide application and development of PAW.In recent years,as an auxiliary energy,ultrasonic is more and more used in welding.It is found that when combined with other welding methods,ultrasonic has the advantages of reducing welding defects and increasing arc pressure.Therefore,in this project,the thermal field of 316 L stainless steel was simulated by ultrasonic assisted plasma arc welding(U-PAW),and compared with that without ultrasonic,so as to reveal its internal thermophysical mechanism.The numerical simulation of temperature field and keyhole of PAW and U-PAW welding under different welding conditions shows that the shape of the pool and the hole increases and the punching hole time is shortened after ultrasonic is applied.This is mainly because the ultrasonic vibration makes the plasma arc column shrink further,increases the energy density and plasma arc force of the plasma arc,promotes the flow of liquid metal in the molten pool,so the melting depth increases after reaching quasi steady state and the melting width slightly increases.The punching hole ability of plasma arc is increased by ultrasonic,and the perforation time is shortened.At the same time,the flow fields of PAW and U-PAW under different welding conditions were simulated.The results show that under the same welding parameters,the maximum velocity of liquid metal in the molten pool after applying ultrasonic is higher than that without ultrasonic.The flow of fluid is mainly related to the effect of force.The ultrasonic vibration increases the force of plasma arc by restraining the plasma arc,promotes the flow of liquid metal in the molten pool,and intensifies the heat conduction process in the molten pool.More heat is transferred to the bottom and around the molten pool,which increases the depth of the molten pool and the depth of the keyhole,and slightly increases the width of the molten pool.The flow direction of fluid is mainly related to whether the keyhole is opened,when the keyhole is blind,the fluid flows from bottom to top under the action of force;After the keyhole passes through the workpiece,the liquid metal at the bottom of the molten pool flows from top to bottom along the wall of the keyhole,and the liquid metal at the middle and upper parts of the molten pool flows from bottom to top.In the upper part of the molten pool,when the liquid metal flows to the upper surface of the workpiece under the action of the force,the flow rate decreases and the backflow occurs,forming a circulation;When the keyhole is a blind hole,the circulating current of liquid metal at the bottom of the molten pool is strong,and the circulating current is weakened after open keyhole.The experiments of the keyhole plasma arc welding(PAW)and ultrasonic vibration assisted keyhole plasma arc welding(U-PAW)were carried out for 316 L stainless steel under different welding conditions.The experimental results show that the penetration depth and width of the weld can be increased slightly after ultrasonic application under the same welding parameters;The microstructure of weld area can be refined by ultrasonic.The microstructure of weld area changes from columnar to smaller equiaxial crystal.Finally,the numerical simulation results are compared with the fusion lines of the cross-section of the weld,and the change trend of the two is consistent.