Electron Beam Melting (EBM), a powder bed additive layer manufacturing process, was used to produce Ti-6Al-4V specimens, whose microstructure, texture, and tensile properties were fully characterized. The microstructure, analyzed by optical microscopy, SEM/EBSD and X-ray diffraction, consists in fine α lamellae. Numerical reconstruction of the parent β phase highlighted the columnar morphology of the prior β grains, growing along the build direction upon solidification of the melt pool. The presence of grain boundary α GB along the boundaries of these prior β grains is indicative of the diffusive nature of the β-α phase transformation. Texture analysis of the reconstructed high temperature β phase revealed a strong o0014 pole in the build direction. For mechanical characterization, tensile specimens were produced using two different build themes and along several build orientations, revealing that vertically built specimens exhibit a lower yield strength than those built horizontally. The effect of post processing, either mechanical or thermal, was extensively investigated. The influence of surface finish on tensile properties was clearly highlighted. Indeed, mechanical polishing induced an increase in ductilitydue to the removal of critical surface defectsas well as a significant increase of the apparent yield strengthcaused by the removal of a $ 150 mm rough surface layer that can be considered as mechanically inefficient and not supporting any tensile load. Thermal post-treatments were performed on electron beam melted specimens, revealing that subtransus treatments induce very moderate microstructural changes, whereas supertransus treatments generate a considerably different type of microstructure, due to the fast β grain growth occurring above the transus. The heat treatments investigated in this work had a relatively moderate impact on the mechanical properties of the parts.

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This paper investigates the influence of post treatments on the fatigue properties of 316L stainless steel produced by laser powder bed fusion. Miniaturised fatigue samples are built in vertical orientation with optimised process conditions to result in very low porosities and minimal scatter in results. Fatigue performance is evaluated for two different material conditions: as-built and stress-relieved, at a nominal load ratio of -1. Furthermore, the samples are tested with and without surface machining. A thorough microstructural and fractographic analysis is performed to evaluate the impact of the main fatigue influencing factors. The results show that the fatigue behaviour of machined samples with and without stress relief heat treatment exceeds that of conventionally manufactured 316L.

International audienceA major drawback of metal additive manufacturing is the surface roughness of the manufactured components. This is even more critical when complex lattice structures are considered. An octet-truss lattice structure was fabricated by Electron Beam Melting. A chemical post-treatment was applied in order to improve the surface quality. The morphology of individual struts was characterized experimentally by high resolution X-ray tomography after different chemical etching times. The chemical etching treatment was found to be beneficial as it decreases significantly the occurrence of surface defects. The evolution of the elastic mechanical properties with the etching time was determined by FFT computations directly applied to the 3D volume of the struts. A cellular automaton based model was also developed in order to predict the morphological evolution of the as-built strut during etching. The model enables to predict the kinetics of dissolution as well as the evolution of the surface defects and the elastic mechanical properties of the samples. It also enables to determine the required etching time to firstly remove powder particles stuck to the surface and secondly to reduce the plate-pile like defect occurrence. (C) 2016 Elsevier Ltd. All rights reserved

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In order to improve the tensile properties of additively manufactured Ti 6Al 4V parts, specific heat treatments have been developed. Previous work demonstrated that a sub-transus thermal treatment at 920°C followed by water quenching generates a dual-phase α+α′ microstructure with a high work-hardening capacity inducing a desirable increase in both strength and ductility. The present study investigates the micromechanical behavior of this α+α′ material as well as the thermal stability of the metastable α' martensite. To that end, annealing of the α+α′ microstructure is performed and the resulting microstructural evolution is analyzed, along with its impact on the tensile properties. A deeper understanding of the micromechanics of the multiphase microstructure both before and after annealing is achieved by performing in-situ tensile testing within a SEM, together with digital image correlation for full-field local strain measurements. This approach allows the strain partitioning to be quantified at a microscale and highlights a significant mechanical contrast between the two phases. In the α+α′ microstructure, the α′ phase is softer than the α phase, which is confirmed by nanoindentation measurements. Partial decomposition of the martensite during annealing induces a substantial hardening of the α′ phase, which is attributed to fine-scale precipitation and solution strengthening. A scale transition model based on the iso-work assumption and describing the macroscopic tensile behavior of the material depending on the individual mechanical behavior of each phase is also proposed. This model enables to provide insights into the underlying deformation and work-hardening mechanisms.

Wire feeding can be combined with different heat sources, for example, arc, laser, and electron beam, to enable additive manufacturing and repair of metallic materials. In the case of titanium alloys, the vacuum operational environment of electron beam systems prevents atmospheric contamination during high-temperature processing and ensures high performance and reliability of additively manufactured or repaired components. In the present work, the feasibility of developing a repair process that emulates refurbishing an “extensively eroded” fan blade leading edge using wire-feed electron beam additive manufacturing technology was examined. The integrity of the Ti6Al4V wall structure deposited on a 3 mm thick Ti6Al4V substrate was verified using X-ray microcomputed tomography with a three-dimensional reconstruction. To understand the geometrical distortion in the substrate, three-dimensional displacement mapping with digital image correlation was undertaken after refurbishment and postdeposition stress relief heat treatment. Other characteristics of the repair were examined by assessing the macro- and microstructure, residual stresses, microhardness, tensile and fatigue properties, and static and dynamic failure mechanisms.

Over the last years, additive manufacturing (AM) techniques such as laser powder bed fusion (L-PBF) have been frequently adopted for efficiently producing biomedical implants. L-PBF offers the advantage of low material waste and high accuracy enabling the production of complex and highly personalized geometries. However, when manufacturing time is considered, the L-PBF production rate is relatively low compared to conventional production techniques. The aim of this paper is to present the impact of layer thickness on static and fatigue properties of CoCr scaffolds produced by means of L-PBF. An increased layer thickness (from 30µm to 60µm) leads to an improvement in terms of production rate of 40 to 50% without affecting the final geometry of the structure. A fatigue test campaign was conducted on both 30µm and 60µm layer thickness samples in "as-built" condition. The analysis of the test results with a local stress method highlighted no significant differences in terms of fatigue performances. In addition, the effect of post-process treatments, such as hot isostatic pressing (HIP) and chemical etching on static and fatigue properties were investigated. It is shown that HIP does not affect the fatigue properties of the scaffolds whilst chemical etching is capable of improving fatigue resistance when the local stress approach is considered.