Fatigue strength improvement in Ti-6Al-4V subjected to foreign object damage by combined treatment of laser shock peening and shot peening

The improving effects of combined laser shock peening (LSP) and shot peening (SP) treatment on the physical properties, microstructure characteristics and foreign object damage resistance of Ti-6Al-4V alloy aerofoil specimens were studied. The surface micro-hardness of specimens after combined LSP + SP treatment is 20% higher than that of the base material, with an affected layer depth of about 400 μm; while the maximum residual compressive stress of samples is −843.2 MPa with a layer depth of about 640 μm. Severe plastic deformation occurred on the subsurface of the specimen after combined LSP + SP treatment, and the depth of grain refinement layer was about 60 μm. The subsequent foreign object damage (FOD) test showed that the combined surface treatment improved the hardness of specimens to effectively resist the damage caused by low energy impact, but the effect is not so obvious for high energy impact. For LSP + SP treated specimens impacted by 1-mm-diameter steel balls, the high cycle fatigue (HCF) strength at 10⁷ cycles improved most, which is nearly 35% higher than that of the untreated specimens. The enhancement is mainly due to micro-hardness maximization and residual compressive stress field introduced by combined LSP + SP treatment.