Exploring of the effects of thermal ablation and plasma expansion on structure formation during ultrafast laser processing of Ti6Al4V alloy

Ultrafast laser processing has opened possibilities for surface modification of Ti6Al4V alloy, endowing them with enhanced and controllable properties. The interaction between ultrafast laser and metals is a complex process with energy transfer, material state evolution and mechanical dynamic at different spatial and temporal scales. However, the knowledge about material response of metals is still necessary to explore due to the complex ablation processes. Here, we study metal structure response induced by ultrafast laser thermo-mechanical ablation in a multi-scale perspective of micro-to-nanometer and femto-to-nanosecond region. Recast surface structure, dense crack, and nano oxidation layer are observed due to the thermal ablation after ultrafast laser processing. Hydrodynamic disturbance indued by plasma expansion contributes to the outwards ejection and recasting of molten materials to form extrusion craters and self-organized grooves. The mechanical effect induced by plasma expansion reconstructs internal structures with grain refinement and dislocation multiplication. Surface hardness is increased by multi-scale structure modification due to the thermo-mechanical ablation. Our work provides insights into the thermo-mechanical effects incurred by ultrafast laser pulse, which benefits the optimal laser conditions for precision processing, micro/nano fabrication, and surface engineering applications.