Laser beam shaping for microstructural control during laser surface melting

Laser surface melting of austenitic stainless steel at high speed carries the risk of centerline solidification cracking due to the formation of tear-drop shaped melt pool. Laser beam shaping can be effectively employed to engineer surface microstructure of laser melted region by controlling the shape of associated melt pool. Results of the present study, involving numerical thermal simulation and experiments, demonstrated that by using a square laser beam, in place of a circular one, the profile of the solidification front associated with the melt pool can be made flatter and more steeply oriented with respect to the direction of laser scanning. Development of this kind of melt pool during laser surface melting results in the growth of smoothly curved columnar grains with a broader region of axial grains at the centerline of the laser surface melted track. Surface microstructure generated by surface melting treatment with a square laser beam should enhance resistance against centerline solidification cracking (over that obtained with a circular laser beam) even at higher processing speed.