Crystal growth during keyhole mode laser welding

Evolution of microstructure during keyhole mode welding involves several special features such as multiple inflections of weld pool boundary curvatures, strong spatially variable thermal cycles and negligible undercooling. These systems are difficult to characterize rigorously, because depending on the sections selected, significantly different grain structures and topological features are observed. Here we uncover the special features of crystal growth during keyhole mode laser welding considering the motion of the melt pool and the interdependence of the grain growth in both the fusion zone and the heat affected zone. The temperature distribution and the transient thermal history of welds were combined with the grain growth simulation using a Monte Carlo approach in a computationally efficient manner. The computed results were tested against independent experimental data for keyhole mode laser welding of copper where the grain structure can be easily resolved. The results showed that the curved columnar grains growing from the fusion zone boundary coexisted with axial columnar grains near the centerline of welds. The effects of welding speed on the dimension, distribution, orientation and morphology of the columnar and equiaxed grains were studied.