Numerical Simulation Of Heat And Mass Transfer During Pulsed GTAW And CMT Welding Processes

Abstract:Arc welding is the most widely-used welding method. For pulsed GTAW, surface rippling is associated with structure stress of weldment, strength of the welded joint, fatigue life and other micro-structural features. The formation of surface rippling is a complex process, which contains multiple mechanisms. Due to the trend of automobile lightweight, the connection between aluminum and steel is becoming a hot focus in welding field. Since conventional GMAW has many defects, when it is used to join aluminum to steel, a new welding technology, CMT-GMAW, has come into being.The complex transport phenomena and their effect on weld pool dynamics and the formation of weld bead in moving gas tungsten arc welding (GTAW) under the pulsed current are studied by using a3D transient numerical model. The transient distributions of the temperature and melt flow velocity in the weld pool, weld pool dynamics, and solidified weld bead are simulated. The results show that the current pattern has significant effects on the weld pool dynamics and weld bead shape in GTAW. It is found that the surface ripples are formed under pulsed current by the interplay between the up-and-down weld pool dynamics, caused mainly by the periodically varied current and the weld pool solidification rate. Under continuous current, weld ripple is not observed due to the continuous weld pool dynamics and solidification. As the welding current increases, the weld pool size and the weld penetration depth increase due to the more intensive arc heating in the same duration. The larger pulsed current frequency tends to decrease the solidification rate and thus the pitch and height of the ripples are reduced.At the same time, a3D transient unified model is developed to simulate the transport phenomena during CMT welding-brazing process of1mm thick aluminum alloy6061and1mm thick zinc-coated Q235steel, in which a hole or groove is configured in the top sheet with the convenience of brazing. The entire events of the CMT process are simulated, including arc generation and evolution; up-and-down movement of electrode, droplet formation and dipping into the weld pool; weld pool dynamics; zinc evaporation and zinc vapor diffusion in the arc. The results show that the diffusion of the zinc vapor into the arc occurs near the workpiece surface due to the evaporation of zinc layer from the workpiece. The arc pressure keeps changing during the CMT process, which is related to the welding current, arc length and zinc evaporation. It is also found that the zinc evaporation leads to the higher arc pressure near the top surface of the steel, which will provide the drag force for the droplet impingement. As the weldment geometry is improved for the expansion of the arc plasma by increasing the hole size or using groove configuration, the arc pressure can be effectively reduced to reduce the spatter.This study provides theory basis for the selection of optimal parameters and improving weldment performance during pulsed GTAW and aluminum-galvanized steel CMT welding processes.