Ultrasonic pulsed waterjet peening of commercially-pure titanium

Abstract

Commercially-pure titanium (CP-Ti) has proven indispensable in critical industries such as aerospace and automotive. To meet the ever-increasing demands of titanium applications, ultrasonic pulsed waterjet peening (UPWJ) has been evaluated for the first time on CP-Ti, as a novel method to optimise compressive surface residual stress. UPWJ offers the potential for an environmentally benign, non-contaminating peening process, paving the way for sustainable surface modification methods. UPWJ treatment involved directing a waterjet at a pressure of 69 MPa onto CP-Ti coupons, maintaining a stand-off distance of 25.4 mm and adjusting traverse speeds from 200 to 1000 mm/s. The effects of stand-off distance were also evaluated (varied from 25.4 to 44.4 mm), at a constant traverse speed of 1000 mm/s. Conventional shot peening was also applied on the same CP-Ti alloy as a baseline. The experimental results revealed relationships between the applied traverse speed and stand-off distance, and the measured surface roughness and residual stress. Furthermore, it was shown that UPWJ improves the surface mechanical behaviour, in terms of both the Rockwell hardness (HRB) and the scratch hardness. A higher stand-off distance and faster traverse speed contributed to increasing the induced compressive residual stress (to a maximum of −391 MPa), while keeping the surface roughness at moderate levels (i.e., Ra <3 μm). Comparing UPWJ with shot peening, this study highlights the potential of UPWJ for CP-Ti alloy surface modification, while removing the potential for surface contamination, which can be detrimental in many applications. Comparison of UWPJ used for peening Ti-6Al-4V (Grade 5) is also briefly discussed.