Analytical Description of the Criterion for the Columnar-To-Equiaxed Transition During Laser Beam Welding of Aluminum Alloys

An equiaxed grain structure in a laser welded seam is beneficial. In current literature the material and process perspective are addressed separately. This paper combined the material and process perspective in one analytical expression in order to access a deeper understanding of the key parameters for an equiaxed solidification during laser beam welding. For this, the steady state model of Hunt for growth of equiaxed dendritic grains was combined with heat conduction calculations and nucleation theory. The derived analytical expression identified the minimum values of the welding parameters required for the growth of equiaxed dendritic grains and describe their interaction with the metallurgically given nucleation density. The theoretically derived analytical description agrees well with experimental results obtained with two different batches of EN AW-6016 over a wide-range of laser welding parameters. The analytical description summarizes the effects of nucleation processes, local solidification conditions and the influence of process parameters on the growth of equiaxed dendritic grains. Furthermore, the equation identifies the key alloy properties, which influence the required process parameter for an equiaxed solidification. For the first time, the full range of phenomena, including metallurgical and thermal effects, was merged in one analytical expression.