Role of molten pool thermo cycle in laser surface alloying of AISI 1020 steel with in-situ synthesized TiN

Significance of the molten pool thermal history in mitigation of cracks and improving the surface properties during laser surface alloying of AISI 1020 steel with in-situ synthesized TiN has been investigated. The molten pool thermo-cycle was monitored using an IR pyrometer and from this heating and cooling rates and molten pool lifetime were estimated for different process parameters. Good correlation was found between the molten pool lifetime and the evolved microstructure and mechanical properties. Molten pool lifetime was found to have an optimum range for crack-free alloyed layer formation with maximum volume fraction of TiN in the matrix that could provide improved hardness and wear resistance. Molten pool lifetime below this range resulted in Fe-Ti intermetallics formation and micro-cracks while the above resulted in heavy dilution resulting in a relatively low volume fraction of Ti in Fe and overall TiN content, both of which found to deteriorate the hardness and wear characteristics of the coating. Based on the results, a process map with molten pool lifetime as a function of a combined process parameter, Pd/V-2, is developed, where P, d and V respectively are laser power, beam diameter and scan speed.