Selective laser melting of Ti6Al4V alloy: Effect of post-processing on fatigue life, residual stress, microstructure, microhardness and surface roughness

In this study; turning, milling, heat treatment and shot peening (SP) post-process operations alone or in different combinations were performed on Ti6Al4V specimens which were produced by selective laser melting (SLM); then the effects of these operations on the fatigue life, residual stress, microstructure, microhardness and surface roughness were investigated. SP processes were carried out in two different types, traditional and multiple, and it was observed that the multiple SP process, which was carried out with the same almen intensity as traditional, with a smaller ball diameter, was effective in reducing the surface roughness. As a result of the heat treatment, alpha + beta transformation was observed in the microstructure. alpha phase was seen in both lamellar and globular forms. Traditional SP and multiple SP processes provided a significant increase in microhardness values in the 0-250 mu m depth range. A significant decrease in the microhardness values of the heat-treated specimens was observed due to the alpha + beta transformation in the microstructure. Traditional SP and multiple SP processes enabled the residual stress type to become compressive type in both the x-axis and the y-axis in the 0-150 mu m depth which is a very critical region for fatigue cracks despite the x-axis direction surface unmachined. Fatigue test were performed on machined, multiple shot-peened and machined after heat-treated specimens. The best results in terms of fatigue strength were obtained in the specimens that were heat-treated. It was observed that the most important cause of fatigue crack initiation in multiple shot-peened specimens was an internal defect located deeper than 250 mu m. Fisheye formation was observed on fracture surfaces of some multiple shot-peened fatigue specimens.

Automated summary

In this study, various post-process operations such as turning, milling, heat treatment, and shot peening were performed on Ti6Al4V specimens produced by selective laser melting. The effects of these operations on fatigue life, residual stress, microstructure, microhardness, and surface roughness were investigated. The results showed that the multiple shot peening process with a smaller ball diameter effectively reduced surface roughness. Heat treatment resulted in the alpha + beta transformation in the microstructure. Traditional and multiple shot peening processes significantly increased microhardness values in the 0-250 μm depth range. The best fatigue strength was observed in the specimens that underwent heat treatment.