A STUDY ON PARAMETER OPTIMIZATION FOR CIRCUMFERENTIAL GAS TUNGSTEN ARE (GTA) WELDING OF SMALL PIPES CONSIDERING BACKING GAS-PRESSURE .1. ANALYSIS OF WELD POOL SURFACE PROFILE

It is well known that the weld bead becomes wider and the weld pool ha ngs down as the circumferential welding of small-diameter pipes progre sses, if constant welding conditions are maintained over the entire jo int length and/or no appropriate backing gas is supplied into the pipe . In order to obtain a weld bead which is uniform in width and does no t hang down over the whole circumference of the pipe, the welding para meters such as welding current, welding velocity and backing gas press ure should be optimized as the welding progresses. In order to optimiz e the welding parameters, a mathematical model for determining the tem perature distribution in the pipe workpiece and the surface profile of the resultant weld pool is indispensable. An efficient finite differe nce model was adopted for calculating the three-dimensional transient temperature distribution in circumferential gas tungsten are (GTA) wel ding of pipes. Its solution was obtained by employing the alternating direction implicit (ADI)finite difference method, in which a periodic boundary condition and a periodic cubic spline function were used. For calculating the weld pool surface profiles in full penetration circum ferential welding of pipes, a governing equation was derived in the cy lindrical coordinate and solved using a simple finite difference model with the ADI scheme. In Part 2 of this paper, an efficient parameter optimization method is used to evaluate the optimal welding current fo r a required bead width when the welding velocity is given.