Residual Stress Field Peculiarities During Laser Shock Peening of Cylindrical Titanium Grade 2 Specimens

Laser shock peening is one of the surface hardening techniques for parts and structures, alongside shot peening, laser thermal hardening, surfacing, etc. The advantages of laser shock peening include the controllability of the impact source parameters, low thermal load on the material, and applicability to parts with various geometries. The application of this technology to curved surfaces involves specific features related to the shape of the laser spot projection onto the treated surface, the direction of shock wave propagation within the material, its potential reflections, and, consequently, the resulting residual stress field distribution. Studying these features enables the determination of optimal and efficient laser shock peening regimes for complex-geometry parts. The primary objective of this work was to establish the influence of two laser shock peening regimes, differing in laser pulse energy density, beam spot application pattern, and laser spot shape, on the micro structure, formation of the residual stress field in cylindrical specimens made of commercially pure titanium VT1-0 (Grade 2), and their mechanical properties. Numerical results demonstrate that near the specimen surface (at a depth of -0.2 mm), one of the investigated regimes generates a field of compressive residual stresses with a maximum amplitude of 550 MPa, while the other produces 400 MPa. Compressive residual stresses are observed in the surface layer of the material, while compensating tensile residual stresses are detected in the bulk material. Laser shock impact, due to its ultra-short duration and high energy, forms high-amplitude compressive residual stresses with minor plastic deformations in the surface and subsurface layers. This is confirmed both by numerical modeling of the plastic strain intensity distribution and by the results of micro structural studies. This characteristic result was observed for both processing regimes. Mechanical testing revealed that, despite the presence of significant compressive residual stresses in the surface layer formed by different processing regimes, the tensile strength of specimens with the investigated geometry shows no statistically significant change and, within the margin of error, corresponds to the strength of untreated specimens.