Mechanisms of damage initiation in a titanium alloy subjected to water droplet impact during ultra-high pressure plain waterjet erosion

Impingement of ultra-high pressure plain waterjets (PWJ) has been considered as a promising surface treatment technology (e.g. peening, milling, cleaning) for some difficult-to-machine engineering materials (such as titanium alloys). However, a comprehensive understanding of the mechanisms of damage initiation on such materials under PWJ impingement has not been established. This paper presents an in-depth investigation into the initial material damage mechanisms which are observed during impingement of a PWJ onto polished samples of an α−β titanium alloy—Ti–6Al–4V, and the influence of the microstructure of the alloy on the initial damage modes. A series of single-track erosion trials were carried out and scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to examine the resulting damage. It was found that the microstructural features of Ti–6Al–4V influence the development of damage, with grain boundaries exhibiting lower resistance to damage than grains themselves. Under waterjet impingement, the initial stage of damage is associated with deformation resulting in grain tilting; a combination of impact damage and lateral outflow jetting then results in preferential damage to the grain boundary regions. Hydraulic penetration into the damaged grain boundaries results in the formation and expansion of microvoids and pits, firstly along the grain boundaries and then across the grains. The subsequent damage is linked with the effects of both hydraulic penetration and lateral outflow jetting on the increasingly roughened surface.