Influence of plasma characteristics on nitrogen mixing into shielding gas in helium gas tungsten arc welding

The influence of welding condition on the mixing of atmospheric nitrogen into the arc plasma in helium gas tungsten arc welding was analysed by numerical simulations. In order to evaluate the effects of the convection flow and the diffusion on the nitrogen mixing phenomenon, the distributions of the Peclet number were used. Elongation of the electrode length has low impact on the decrease of shielding gas concentration because the convection flow becomes dominant in this area, which indicates higher Peclet numbers. Meanwhile, nitrogen diffusion increases in the plasma area with a temperature of about 10,000 K, so that elongation of the arc length leads to a remarkable decrease of shielding gas concentration. Additionally, the impact of convection flow increases in the arc centre area where high-velocity plasma jet exists, and the shielding gas concentration tends to rise owing to higher welding current in the condition of sufficient shielding gas flow rate. © 2014 Taylor & Francis.