Issue 53

C. Navarro et alii, Frattura ed Integrità Strutturale, 53 (2020) 337-344; DOI: 10.3221/IGF-ESIS.53.26 338 K EYWORDS . Additive manufacturing; Fatigue strength of AM elements; Shoot peening; Laser shock peening; Superfinishing; Fatigue strength improvement. I NTRODUCTION dditive manufacturing (AM) of metal parts is a relatively new technology, which is developing very fast. Different metal alloys are used for AM with relatively good strength. Stainless steels, cobalt-chrome, aluminium or titanium alloys are some of those used for structural proposes. Among the titanium alloys, T16Al4V is probably the most used, especially for aerospace applications. Specimens manufactured by AM are used since some years ago in satellites and launchers. They allow reducing the weight, not only because the low density material used, but also because the possibility of taking advantages of topological optimization, presenting monotonic mechanical properties similar to specimens manufactured through traditional technologies like casting. However, in the case of fatigue, AM elements still have problems of strength and reliability [1-3] and that prevents their application in the aeronautical industry, where the reliability is one of the main concerns. The porosity, residual stresses and anisotropy of the additive manufactured specimens as well as the surface roughness and microstructure, which frequently is not well controlled, make the fatigue strength of the elements an issue where there is a lot of work to be done in order to improve this mechanical property and the reliability. Selective Laser Melting (SLM) is one of the most used AM techniques for Ti6Al4V alloy. The main features regarding fatigue of the work specimens made by this procedure are, as afore said, residual stresses, porosity, roughness, non- uniform microstructure and anisotropy [4-7], which reduce the fatigue strength and increase the scatter of the fatigue lives produced for the same cyclic load applied to different specimens taken from the same manufacturing batch. In addition to select the optimum manufacturing parameters to reduce defects, anisotropy, residual stresses, etc., several mechanical, thermo-mechanical and thermal treatments have been proposed to improve the strength and reduce the scatter [8-20]. These range from annealing or sand blasting to machining [9-13], Hot Isostatic Pressing (HIP) [13-16] or shot peening [17-19], or any combination of them [20]. Many experimental analysis have been published about the effect of these treatments on the fatigue strength, but mainly dedicated to the effect of thermal treatments, machining or HIP or combination of them [8-16]] and only a few about the effect of other surface treatment like shot or laser peening [17-20]. Actually, two of these works only consider the effect of shot peening on roughness, and on residual stresses and microhardness profiles close to the surface, but not directly on the fatigue strength, although they consider that the effect on those parameters will have also a beneficial effect on the fatigue strength. On the other side, Wycisk et al. [19] and Bagherifard et al. [20], analyse directly the effect of shot peening on the fatigue strength of the specimens, Wycisk did it on Ti6Al4V and Bagherifard on AlSi10Mg, with much better results in the case of AlSi10Mg The main objective of this work is to analyse the effect of some surface treatments on the fatigue strength of additively manufactured specimens. The analysis was made by comparing the fatigue lives obtained with the different treatments selected. The treatments considered are mainly shot peening and laser shock peening. Considering the deleterious effect of roughness, anisotropy and residual stresses existing in the as built elements, before applying any surface treatment, all specimens were sand blasted and annealed. In the case of shot peening, two different surface treatments were considered, one was just shot peening and the other was shot peening followed by a surface treatment called Chemical Assisted Surface Enhancement (CASE), produced by Curtiss-Wright ® . This treatment improves the surface finish produced by shot peening. In order to have a reference for the fatigue strength of the surface treated specimens two other treatments were considered. One was just sand blasting and annealing and the other was HIP after sand blasting and annealing. Real as built specimens instead of sandblasted plus annealing could have been used as reference. However, considering that it is well known that the real as buildt conditions produce very poor fatigue strength results and that most applications of SLM manufactured specimens use previous soft surface treatments, to at least clean the specimen surface, as well as heat treatment, it was decided using sandblasted and annealed conditions as reference. The HIP treatment is not actually a surface treatment, but it was included thinking in its use as reference for future analysis of the effect of the combination of HIP with other surface treatments. This document is organized as follows. First, the manufacturing procedure, treatments considered and test procedure are shown. Following the testing results are described. Those results are discussed trying to understand the effect of the treatments and, finally, some conclusions about the treatments are obtained. A