Measurements of the force exerted by a forced pulsed water jet, and its effect on the mechanical properties of titanium alloys.

Abstract

Forced pulsed water jetting (FPWJ) is a type of high-speed water jet (HSWJ) with a modulated jet – made of water packets – where the primary use is a surface treatment for coatings removal and commercialized by VLN Advanced Technologies since 2002. Compared to standard HSWJ, the FPWJ is able to cleanly remove coatings from the substrate by a pump pressure (pp ) ten times lower. This thesis provides insights into the physical mechanism that lead to the increased efficiency of the FPWJ over HSWJ technologies.

We first explore using the FPWJ as an environmentally friendly alternative surface treatment for medical grade titanium (Ti-6Al-4V Grade 23) implants, which takes advantage of the material erosion when the FPWJ is outside of normal operation. The surface morphology below a traverse velocity (vT ) of 120 mm/s demonstrated erosion of the Ti α-phase, and Ti β-phase grain boundaries, which gave rise to nanoscale and micron-scale surface features and produced a residual strain in the Ti β-phase. The ideal vT setting of 75 mm/s produced a surface roughness Sa = 5 μm that is comparable to typical surface roughness for titanium implants. X-ray photoelectron spectroscopy shows a small increase in TiO2 as well as an increase of V at the surface.

We designed and constructed a high-frequency force sensor to search for evidence of water hammering and explore the role that cavitation may play in coating removal and peening. Commercially pure titanium Grade 2 and Ti-6Al-4V Grade 23 samples were mounted on the force sensor to compare the surface morphology, residual stress and applied force from a jet as a function of pp and standoff distance (S_D). The FPWJ showed the applied force was approximately double compared to a conventional HSWJ optimal for a given pp at the FPWJ S_D^optimal . The dominant force exerted by the FPWJ is continuous, despite stroboscopic images showing a modulation of the jet profile. There is an large impulse from the jet into the sample surface when the jet operates in a range of SD values about the optimal value. This indicates that there is a macroscopic origin to the increase peening and erosion efficacy of the FPWJ, in addition to any microscopic effects, such as cavitation and droplet water hammering.