Ultra-fine grained steel (UFGS) with an average grain size of less than 1μm has been
developed and is expected to demonstrate superior properties. However, its welded heat-affected zone,
HAZ, substantially affecting the strength of a welded joint, will be easily softened after welding.
Therefore, minimization of UFGS’s HAZ size during laser welding was carried out using the cooling
conductor liquid nitrogen. It was found that a shielding gas with adequate flow rate for the liquid
nitrogen depth was used to displace nitrogen on the area of laser beam irradiation to stabilize the weld
bead. Also, because YAG laser system was mainly used because it has a lower laser induced plasma or
plume temperature, which results in a decreased occurrence of pit and blowhole. HAZ size
minimization strongly depends on the initial plate temperature. Reduced initial plate temperature
shrinks the specific heated temperature range in which softening occurs. However, due possibly to
decreasing thermal conductivity under room temperature, which prevents heat removal, the benefit of
reducing the initial plate temperature is limited. The optimal initial temperature to minimize the HAZ
size, in the present work, was found to be 123K.
Remote Laser Welding of Steel and Aluminum Alloys
