Heat transfer and fluid flow in laser microwelding

The evolution of temperature and velocity fields during linear and spot Nd-yttrium aluminum garnet laser microwelding of 304 stainless steel was simulated using a well-tested, three-dimensional, numerical heat transfer and fluid flow model. Dimensional analysis was used to understand both the importance of heat transfer by conduction and convection as well as the roles of various driving forces for convection in the weld pool. Compared with large welds, smaller weld pool size for laser microwelding restricts the liquid velocities, but convection still remains an important mechanism of heat transfer. On the other hand, the allowable range of laser power for laser microwelding is much narrower than that for macrowelding in order to avoid formation of a keyhole and significant contamination of the workpiece by metal vapors and particles. The computed weld dimensions agreed well with the corresponding independent experimental data. It was found that a particular weld attribute, such as the peak temperature or weld penetration, could be obtained via multiple paths involving different sets of welding variables. Linear and spot laser microwelds were compared, showing differences in the temperature and velocity fields, thermal cycles, temperature gradients, solidification rates, and cooling rates. It is shown that the temperature gradient in the liquid adjacent to the mushy zone and average cooling rate between 800 and 500°C for laser spot microwelding are much higher than those in linear laser microwelding. The results demonstrate that the application of numerical transport phenomena can significantly improve current understanding of both spot and linear laser microwelding.