The role of hardening and roughening during the incubation period in water droplet impingement erosion of Ti–6Al–4V

The present work investigates the role of surface hardening and roughening on the damage accumulation during the incubation period in water droplet erosion (WDE). Water droplet erosion tests were carried out on Ti–6Al–4V alloy at an impact velocity of 250 m/s. The evolution of hardness and surface roughness during the incubation period were evaluated using microhardness tester and Confocal Laser Scanning Microscope (CLSM). Scanning Electron Microscopy (SEM) was conducted on the impact area at several intervals during the incubation period. Finite Element (FE) simulations were performed to obtain the impact stresses on both smooth and rough surfaces. It was found that the solid surface plastically deforms by the action of the initial impingements, and as a result, hardness and surface roughness increase progressively until the end of the incubation period. The cyclic increase in hardness results in gradual improvement in the erosion strength, making the surface more resistant to the subsequent droplet impacts. However, the finite element results revealed that the dynamic surface roughening process results in higher impact stresses for the same impact pressure due to geometrical stress concentration. It is concluded that the increase in impact stresses resulting from surface roughening negates the improvement in the erosion resistance due to hardening. Hence, cyclic hardening and roughening are crucial parameters in the damage accumulation process during the incubation period.