High energy laser shock peening of Ti6Al4V alloy without any protective coating

Industrial laser shock peening (LSP) is gradually moving away from using protective overlay as it is tedious and time-consuming making the whole process inefficient for a production run. However, the peening of titanium alloys is still performed with the protective coating to protect the surface from thermal exposure to the laser. In this work, we study the effect of uncoated LSP (without protective coating) of Ti6Al4V alloy on its surface microstructure and mechanical performance. Our results show that uncoated LSP produces a thin oxide layer consisting of TiO2 and Al2O3 due to oxygen diffusion facilitated by the thermal effect of the laser. A brittle alpha layer around 5-8 & mu;m thick is also generated on the surface. We find that similar compressive residual stress conditions as that in coated LSP can be generated beneath the surface by increasing peak power density in uncoated LSP. Despite this, the fatigue performance of the peened alloy does not improve, which is attributed to the presence of several microcracks in the alpha layer. On the other hand, the removal of the alpha layer by abrasive grinding is found to increase the fatigue life by almost 9 times compared to that from uncoated LSP. The findings indicate that the uncoated LSP of Ti6Al4V alloy must be combined with a post-cleaning step to reap the significant improvement in fatigue life.