Fatigue behaviour of PBF additive manufactured TI6AL4V alloy after shot and laser peening

Abstract

Additive manufacturing (AM) of metallic parts is a relatively new manufacturing procedure. Many industry sectors, such as the aerospace or automotive sectors, have started to apply this technology to produce some elements, thus reducing costs and weight. Several metallic alloys have been employed for AM. Due to the high strength-to-density ratio, Ti6Al4V alloy is probably the alloy most used for AM in the aerospace industry. This alloy usually shows good static strength properties. However, the presence of internal defects and the surface roughness result in a fatigue strength that is clearly lower than that of materials produced by traditional processes. Moreover, the scatter of the fatigue results is generally higher than in the case of wrought pieces.

Different treatments have been proposed to improve the fatigue behavior by reducing internal defects and roughness or generating a favorable residual stress field. In this work, selected surface treatments were considered to improve the fatigue strength of AM parts, including shot and laser peening as well as a combination of shot peening plus chemical assisted surface enhancement (CASE®). Three groups of specimens, each with one of the surface treatments, were fatigue tested to compare the results produced by these treatments. The residual stresses, roughness and hardness produced by the treatments were analyzed. After testing, the fracture surfaces were also analyzed to better understand the fatigue process of the different groups of specimens. The results indicate that laser peening produced the best results, followed by shot peening plus CASE and shot peening. In all three cases, the fatigue strength was much higher than that of the reference group without surface treatment. It was also observed that all failures initiated from an interior defect in the shot peening plus CASE group, four out of six failures in the laser peened group, but only one failure in the case of shot peened group and none in the reference group. Failures of specimens with initiation from internal defects started from defects located deeper than the compressive residual stress layer produced by the treatments.