Study on the shock pressure and surface property enhancement during laser shock peening of metal alloys

Abstract

In this dissertation, theoretical and experimental study of laser shock peening (LSP) of duplex stainless steel and 3D printed titanium alloy is investigated. The research consisted of two parts: shock pressure measurement during LSP and finding Johnson-cook constitutive model of 2205 duplex stainless steel (DSS) by comparing theoretical and experimental results (Chapter 2) and the investigation of effect of LSP on the properties of Ti-6Al-4V manufactured by laser powder bed fusion (LPBF) (Chapter 3). Because the dynamic responses during LSP process are very complex and observing them instrumentally is difficult and LSP process have many experimental parameters. Finite element method has been employed to predict the residual stress fields and to gain understanding about the LSP process. To conduct simulation, pressure conditions and material properties are needed. Because the peak value and profile of shock pressure varies depending on the LSP conditions, shock pressure measurement at our experimental condition is necessary. And plastic properties of the 2205 DSS at high strain rate impact conditions like LSP process are unknown. Because at various strain rates the materials behave differently, simulation results have discrepancy with the experimental results. For more accurate simulation, properties at various strain rates such as Johnson-Cook model parameters are necessary. In this thesis, we predicted sample properties using pressure conditions and residual stress inversely by comparing experimental and calculated residual stresses. Ti-6Al-4V components fabricated by LPBF suffer from high brittleness and poor toughness issues, which are largely attributed to the α^'-martensite phase of LPBF products. Post-heat treatment is often carried out to improve the toughness of LPBF products, which, however, has been reported to result in a decrease in strength, abrasion resistance, and corrosion resistance. In this study, laser shock peening (LSP) is applied to post-heat-treated Ti-6Al-4V LPBF products as a solution to achieve enhanced surface properties as well as improved toughness. By post-heat treatment only the impact toughness of the LPBF-fabricated Ti-6Al-4V could be increased by 150%, that too at a significant degradation of surface hardness, wear, and corrosion resistance. When LSP is applied to the post-heat-treated Ti-6Al-4V, impact toughness remains nearly the same, whereas surface hardness and wear resistance are recovered to approximately 92% and almost full values prior to heat treatment, respectively. LSP also decreased the corrosion rate by 64% from that of the heat-treated sample. The results showed that LSP combined with post-heat treatment is a plausible approach to achieve both high impact toughness and enhanced surface properties in LPBF-fabricated Ti-6Al-4V products.