Microstructures and mechanical properties of β forging Ti17 alloy under combined laser shock processing and shot peening

Abstract

The aim of this paper was to utilize combined laser shock processing (LSP) and shot peening (SP) to study the grain refinement mechanism and mechanical properties modification of β forging Ti17 alloy. Firstly, LSP experiments were conducted on the surface of Ti17 alloy by a YAG laser system and square spots of 8% overlapping rate. LSP process parameters were pulse width of 15ns, pulse energy of 30J and spot size of 4mm×4mm. Secondly, SP experiments were carried on the surface of LSPed Ti17 alloy. ASH230 steel shots were used to SP with a diameter of 0.3mm and an intensity of 0.3mmA (A-type Almen stripe). Lastly, the microstructures and mechanical properties in the surface layer of Ti17 alloy were investigated by surface topography and roughness, micro-hardness, X-ray diffraction (XRD) analysis, residual stress, scanning electron microscopy (SEM) and transmission electron microscope (TEM). Results showed that surface topography and surface roughness amplitude were increased by combined LSP and SP. The amplitude and depth of the micro-hardness in the surface layer were also significantly improved. No new phase was formed after combined LSP and SP. High amplitude compressive residual stress with −613.5MPa was induced in the surface with combined LSP and SP. The smaller phase sizes in β phase and more obvious crystal defects (deformed twining and dislocations in α phase and dislocations in β phase) were generated by combined LSP and SP. Grain refinement mechanisms were attributed to high density dislocations in α and β phases and multidirectional twin intersections in α phase.