Investigation, modelling and validation of material separation mechanism during fiber laser machining of medical grade titanium alloy Ti6Al4V and stainless steel SS316L

Abstract In the laser machining of titanium alloy (Ti6Al4V) and stainless steel (SS316L), it was observed that Ti6Al4V was machinable at higher cutting speeds and lower laser power than SS316L, despite higher melting temperature of Ti6Al4V. Three millimeters thick Ti6Al4V and SS316L substrates were machined using 400 W fiber laser machining system with cutting velocities ranging from 75 to 800 mm/min in the presence of Argon (at 10 bar pressure) as the shield and assist gas. At 300 W laser power and velocities of 150 mm/min or less, material separation was observed in Ti6Al4V but not in SS316L. An investigation into the laser machined surfaces confirmed that the primary mode of material separation in both the materials was melt & blow cutting. However, the secondary modes of machining investigated via XPS, XRD and EDX confirmed substantial transformation of Ti6Al4V constituents into oxides regardless of the fact that argon gas was used for shielding. Analytical modelling of the laser machining process also suggested substantial dilatational and transformational strains in Ti6Al4V while thermo-elastic, plastic and transformational strains were relatively equivalent in both materials. It was hypothesized that laser machining of Ti6Al4V was assisted by larger amount of chemical transformation but not in SS316L. The hypothesis was validated by laser machining experiments in the presence of varying pressures of oxygen and argon.