Microstructure and wear resistance of Ti6Al4V surfaces processed by pulsed laser

When pulsed laser beams deposit spatially and temporally localized power on the metallic surfaces, microstructure and mechanical properties could change significantly. During and after each pulse, the exposed surface experiences a cycle of thermal processes involving extremely high heating and cooling rates, which consequently cause significant microstructure evolution. Changes in mechanical strength follow such microstructural changes. In this work, the influence of pulsed laser processing (PLP) on microstructural evolution, resulting hardness and wear resistance of Ti6Al4V surfaces is investigated. Average pulse power is varied from surface heating to melting regimes in processing. A 2D axisymmetric finite element model of a single pulse is utilized to obtain the heating and cooling histories, and a phase transformation mapping procedure is used to obtain the microstructural evolution. Melting (MZ) and heat affected zones (HAZ) observed at the cross sections of the processed surfaces are found to correlate with the predicted temperature field. The resulting phases in those zones are predominantly martensitic α′ and α phase; higher laser power produces thicker martensitic α′ surface layers several microns into the processed surface. Nanoindentation, nano/microscale and mesoscale wear tests are then applied to the processed surfaces to identify the effects of microstructure on hardness and wear resistance. The surface processed by higher laser power shows higher hardness and wear resistance compared to the surface processed by low power laser. The hardness and wear resistance of the surface processed by low power laser shows no obvious change from substrate material. Mixture of hard martensitic surface layer and underlying ductile substrate resulting from PLP facilitate superior resistance to abrasive and partly low cycle fatigue wear.