With several different fusion welding processes, the melted weld pool profile which ultimately solidifies to form the fusion zone, diverging greatly by a wide variety of factors, e.g. base material, workpiece size, machine setups and extensive range of other process variables. For each distinctive welding setup, the weld pool geometry could vary considerably, and thought to be largely dependent on the hydrodynamics of the weld pools [1]. The Marangoni Effect or thermo-capillarity is seen to be the a dominant force influencing weld pool flow patterns under Gas Tungsten Arc Welding (GTAW), inducing liquid metal to flow to regions with higher surface tension (γ) caused by surface tension thermal gradients ∂γ⁄∂T, this in turn would greatly alter the weld pool thermal history, hence the fusion zone geometry [2]. As a general trend, for a negative ∂γ⁄∂T, outward flow from the pool centre to the edge tends to produce wide and shallow pools; whereas for a positive ∂γ⁄∂T, the liquid metal would flow inward to the pool centre, thus creating deep and narrow pool shapes [3]. Fig. 1 Schematic illustration of GTAW process with negative surface tension temperature gradient. This research group believes that the Marangoni Effect is the dominant force in weld pool shaping. To better understand the weld pool behaviours, a two-dimensional simulation model was constructed in CFD package Fluent®, based on stationary arc GTAW welding conditions. In addition, GTAW welding experiments were also performed on titanium alloy Ti-5Al-5Mo-5V-3Cr as reference data for the numerical results to evaluate against.
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