ABSTRACT
The molten metal in the weld pool is usually in vigorous fluid motion; there are several reasons why. One is that the molten metal has to flow past the keyhole or other surface depression rather than through it, as is implicitly assumed by the simpler models based on Equation (3.1). It is true that vaporization takes place on the keyhole wall, but only sufficient vaporization is needed to maintain the shape of the keyhole. Most of the fluid flows around it, although the flow can have a component parallel to the laser beam as well as perpendicular to it. Another driving mechanism is the variation of surface tension with temperature. Mathematically, the surface tension condition generates a boundary condition on the equations of fluid motion. The fact that there is a strong fluid motion means that the temperature distribution itself is affected. The fluid motion simply cannot be ignored in the way that it is in the simple models discussed in Chapter 6.
