Experimental Optimization for Fatigue Life Maximization of Additively Manufactured Ti-6Al-4V Alloy Employing Ultrasonic Impact Treatment

Ultrasonic Impact Treatment (UIT) is an effective surface modification method used to increase fatigue life of welding application by inducing compressive residual stresses. In this paper, we extend the application of UIT to include additively manufactured metallic parts with particular focus on Ti-6Al-4V manufactured with Direct Metal Laser Sintering technology. Design optimization for the application parameters of the UIT was conducted with the objective of fatigue life maximization. Application parameters of the UIT included the static pre-stressing force of UIT impact needle against the treatment surface and its scanning speed during application while the scanning pattern is maintained constant. Results show that a static load of 30N and a scanning speed of 2000mm/min can provide an optimal fatigue life increase up to 250% compared to untreated parts. It is shown that the increase in fatigue performance correlates strongly with an increase in the micro-hardness and compressive residual stress and less strongly with surface roughness. Two Stress-Life (S-N) curves are fully characterized experimentally based on a set of samples treated with the determined optimal UIT parameters and another set of untreated versions. Endurance limit of the optimally treated specimens is increased by 25% from about 200-250MPa.