Study of the law between the weld pool shape variations with the welding parameters under two TIG processes

A double-shielded TIG method was proposed to improve weld penetration and has been compared with the traditional TIG welding method under different welding parameters (i.e., speed, arc length and current). The strength of the Marangoni convection was calculated to estimate the influence of the welding parameters on the variations in weld pool shapes. The results show that the changes in the welding parameters directly impact the oxygen concentration in the weld pool and the temperature distribution on the pool surface. The oxygen content and heat distribution on the weld pool surface are determinants of the pattern and strength of the Marangoni convection. For a negative temperature coefficient of surface tension (∂σ/∂T < 0), an outward Marangoni convection leads to a wide and shallow weld pool shape. The narrow and deep weld pool shape occurs when the Marangoni convection flows along an inward direction (∂σ/∂T > 0). The oxide layer that may appear with the relatively high oxygen content in the weld pool is harmful for the heat flow along the pool surface so as to reduce the welding efficiency especially in the double shielded TIG process.