Analysis Of Weld Puddle Distortion And Its Effect On Penetration

Analysis of W e l d Puddle Distortion and Its Effect on Penetration Weld puddle distortion contributes significantly to weld penetration characteristics, and the control of puddle surface tension is essential to the control of weld puddle shape change and the enhancement of penetration By E. FRIEDMAN ABSTRACT. An analytical finite element heat transfer model for the gas tungsten-arc (GTA) welding process is developed to account for distortion of the weld puddle due to pressure from the welding arc and gravitational forces in the puddle. The model is employed to assess the combined effects of heat flow in the weldment and changes in the puddle shape on weld penetration. The relationship between puddle configuration and penetration has not previously been studied from an analytical point of view. Introduction Extensive experimental data exist on the effects of such welding parameters as arc current, arc gap, weld speed and shielding gas on weld bead depth and width for the CTA welding process. What has been lacking, however, is a clear understanding of how these and other variables interact to produce the observed weld bead shapes. To help develop this understanding, a systematic effort has been undertaken to establish an analytical model of heat transfer in the molten weld puddle and the surrounding solid material, ll is the flow of heat in the weldment that ultimately determines the extent of the puddle and, therefore, the configuration of the solidified weld bead. Hence, characterization and study of the heat transfer phenomenon provide a vital link in gaining an improved comprehension of how weld penetration and shape can be controlled. Heat flow in a weldment for the GTA process is governed by the mechanisms of heat conduction in solid and liquid material and fluid motion in the puddle. These in turn are dictated by conditions at the weldment b o u n d aries, which include heat input from the arc, surface energy losses to the environment and distortion of the weld puddle due to the force of the welding arc impinging on its surface, as well as by convective and magnetohydrodynamic interactions. The welding thermal cycle has been simulated numerically using finite difference approximations' as well as finite element methods of analysis for transient heat conduction.- ' ' The heat conduction mechanism for stationary GTA welds was studied extensively in an analysis and test program designed to assess the effects of both the magnitude and the distribution of heat input from the arc and of surface energy losses on penetration, w e l d bead width and temperatures for stationary GTA welds.1' The finite element procedures previously discussed 1 were employed for the analytical part of this investigation. Implicit in all these efforts is the assumption that the molten weld puddle maintains a fixed shape as heat from the welding arc is being applied. Distortion of the weld puddle or w e l d ment has not previously been considered in analyses of temperature transients and weld bead penetration and shape. The weld puddle is, however, Paper presented at the AWS 59th Annual Meeting held in New Orleans, Louisiana, during April 2-7, 1978. E. FRIEDMAN is a Fellow Engineer in Reactor Technology, Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, West Mifflin, Pennsylvania. locally distorted by gravitational forces and arc pressure" as well as by fluid flow induced by magnetic and convective forces in the puddle. The magnitude of this local distortion is most