Shot peening as a pre-treatment to anodic oxidation coating process of AW 6082 aluminum for fatigue life improvement

Abstract

Shot peening with glass beads was used as a pretreatment to anodic oxidation of AW 6082 aluminum alloy to eliminate the negative effect of the anodized layer on fatigue strength. Shot peening with intensity 6N and coverage of 100% was able to significantly improve the fatigue strength of the experimental material and when N = 3 × 109 cycles is considered the fatigue strength increased from σ a = 80 MPa to σ a = 110 MPa. Creation of the anodized surface layer with thickness of approximately 30 μm on the pre-treated surface decreased the fatigue strength, when compared to the shot peened surface, almost in the whole measured region; however, the fatigue strength was still higher than for not treated material. The positive effect of shot peening on fatigue strength was a result of creation of compressive residual stress field with maximum of −200 MPa and depth of 350 μm. Fracture surface analysis showed that this deformed surface layer had increased resistance to fatigue crack initiation and propagation in the early stages and cracks created on the anodized surface layer were unable to propagate directly to the material and new initiation of fatigue crack was required; however, it was still accelerated by the sharp notch which the crack created.