STUDY ON THE THERMAL LAG EFFECT OF KEYHOLE IN CONTROLLED PULSE KEY--HOLING PLASMA ARC WELDING

Abstract

The behaviors of keyhole formation determine to a great degree the deep penetration welding process and the weld depth-width ratio in welding of medium and large thickness plates. Obtaining a deep insight into the dynamic keyholing process in the weld pool in plasma arc welding is of great significance for widening the process parameter-widow and improving the process robustness as well as weld quality stability. Based on the Level-Set theory, a numerical model is developed to describe the keyhole behaviors in the weld pool in stationary plasma arc welding (PAW), and employed to track the evolution process of keyhole boundary. In this paper, the fluid flow fields in both the plasma and the keyhole regions are constructed according to the experimental and simulation data in the previous literatures. The combined volumetric heat source model is used to numerically analyze the transient temperature field and then to determine the weld pool geometry. The algorithm of Level-Set theory combined with the transient thermal conduction model is used to determine the evolution of both keyhole and weld pool geometry at different time steps. The dynamic information on the weld pool and keyhole geometry and sizes under a few process conditions is obtained by the numerical simulation of keyholing phenomena in welding of stainless steel plates. It is found that a complete keyhole is established at 2.7 and 2.5 s for the current levels of 170 and 180 A, respectively. During the stationary PAW process, the cross-section geometry of keyhole transforms from U-shape at initial stage to V- shape later and shows finally a hyperbola shape after a complete keyhole is formed. The numerical analysis of keyhole formation is verified by measuring the efflux plasma voltage signals at the moment of keyholing.