Fatigue Life Enhancement of Titanium Alloy by the Development of Nano/Micron Surface Layer Using Laser Peening.

Abstract

Ti-15V-3Cr-3Al-3Sn (Ti-15-3) is a metastable beta alloy which is considered to be a potential alternative for Ti-6Al-4V alpha+beta alloy for aerospace applications, especially for sheet products. This paper describes the work carried out to enhance the fatigue life of Ti-15-3 in an economical way by means of laser peening without coating (LPwC) using Nd:YAG laser operating at a power density of 5 GW cm-2. In order to have a sufficient bulk hardness and high compressive stresses on the surface, as-received beta solution treated (ST) Ti-15-3 was subjected to aging (520 °C/10 h/Air-cooled) and then to LPwC. Laser peening induced a notable increase in Ra (arithmetic mean roughness), which was measured using MAHR GD-120 profilometer. The Electron Back Scatter Diffraction (EBSD) analysis of the aged sample (STA) revealed a significant increase in the alpha precipitation (20 vol%), and this led to a substantial increase in the hardness (~40%) and UTS (~50%). In addition to this, peening of aged (STA+LPwC) sample resulted in a considerable increase (~12%) in near-surface microhardness and compressive residual stress (maximum stress of -195 MPa at a depth of 150 μm). This increase in compressive stress and microhardness led to an enhancement in the fatigue life of the STA+LPwC sample by 210% when compared to STA sample. In spite of high surface roughness induced by the LPwC, fractography studies revealed that crack initiation was independent of surface roughness.