Numerical modelling and stability analysis of convective flows in weld pools

In arc welding electromagnetic forces were traditionally thought to be the major cause of fluid motion in the weld pool. Experiments have shown, however, that under some circumstances the surface tension forces can produce a radically different flow pattern from that produced by electromagnetic forces only. Additionally, it is now known that the magnitude of the surface tension gradient with respect to temperature can vary greatly, even changing sign, with the addition into the metal of very small amounts of impurities. As a consequence of these observations the axisymmetic motion of a semi-infinite region of fluid is investigated in this thesis, under the action of the electromagnetic and surface tension forces arising from coincident point sources of heat and current. A similarity solution can be obtained in this case and the resulting ordinary differential equations are solved numerically. Results indicate that when the surface tension gradient is large, but still within the experimentally obtained range, it dominates the flow for most values of the applied current. However, flow breakdown caused by the velocity down the axis of symmetry becoming infinite, which has been a problem with a similar model containing the electromagnetic force only, still occurs. It is shown that the value of the current at which this happens can be altered by several orders of magnitude through changing the magnitude and sign of the surface tension gradient.Under certain experimental conditions the spontaneous rotation of the molten metal in the weld pool when the current is increased has been observed. In addition some 'hysteresis' effects have been noticed when making measurements of the weld pool size.