Effect of Shot Peening on Fatigue Strength of Additively Manufactured Aluminum Alloy with Rough Surfaces

The objective of this study is to clarify the effects of the changes in surface roughness and residual stress by shot peening (SP) on the fatigue strength of aluminum alloys (AlSi12) with surface roughness after additive manufacturing. The following specimens were prepared: a specimen with surface roughness after AM (As built), an As-built specimen subject to SP (As-built+SP1), a specimen with residual stress removed by annealing As-built+SP1 (As built+SP2), and a smooth specimen (smooth). Arithmetic mean roughness R_a, residual stress, and hardness of these specimens were measured. The results showed that SP application reduced the arithmetic mean roughness R_a by 39%. In addition, compressive residual stress of about 66 MPa was introduced at the surface of the As-built+SP1 material, and the hardness was increased in the interior of the material. Subsequently, plane bending fatigue tests were conducted on four types of specimens, and the fracture surfaces of the fatigue-fractured specimens were observed. Fatigue strength increased in the order of As-built material ⟨ As-built+SP2 material ⟨ As-built+SP1 material = Smooth material. Comparing the results of the fatigue test and the property of these specimens, it was clarified that the reduction of surface roughness and the introduction of compressive residual stress by SP construction were effective to improve the fatigue strength.