Microstructural analysis of selective laser melted Ti6Al4V modified by laser peening and shot peening for enhanced fatigue characteristics

Highlights: • New insights into deformation characteristics for complex Ti6Al4V microstructures. • Optimization of laser peening process based on X-Ray stress analysis. • High-resolution EBSD and AFM resolve changes in deformation state after laser peening. The increased availability of additively manufactured materials provides a new extended design freedom for innovative products and applications, especially with view to high-tech demands as in the aerospace industry. Current microstructural challenges relevant for fatigue life like internal processing defects and tensile macro stresses still require innovative post processing methods, where laser peening shows some potential, but with need of more understanding of materials modification and fatigue outcome. This study aims at giving for the first time a thorough microstructural insight of selective laser melted Ti6Al4V, modified by both, laser peening and conventional shot peening. The results for the laser peened state highlight an outstanding depth of residual macro and micro stresses (~2.3 mm zero level-crossing), whereas deformation characteristics are accompanied by a low level however characteristic dislocation structure preferably close to the lath grain boundaries, as well as an orientation dependent grain deformation at the surface. Differences compared to shot peening become clearly obvious, whereas initial roughness is shown to play a substantial role along with both post process effects. Fatigue results prove the usefulness of both processes with significant benefits for laser peening by a minimum of a factor of 10 number of cycles.