Surface and sub-surface integrity of Ti-6Al-4V components produced by selective electron beam melting with post-build finish machining

The emergence of metal additive manufacturing (AM) processes offer manufacturers a promising alternative to traditional forging and casting techniques for the production of near net shape titanium alloy components. However, limitations in both the surface finish quality and the geometric accuracy of parts produced by AM means that post-build finish machining of the part remains to be a requirement to produce high precision components. Furthermore, the fatigue performance of material produced directly by these processes is often limited by both the poor surface finish and porosity related defects which occur within the material. This study investigates the implications of machining stock allowance on the surface integrity of Ti-6Al-4V specimens produced by selective electron beam melting (SEBM) followed by post-build finish machining. The study revealed that the exposure of porosity related defects on the newly machined surface varied depending on the depth of material removed from the as-built specimen surface during machining. Four point bend fatigue testing of the specimens was carried out to determine the effect of the exposed surface defects on the fatigue performance of the material. This study highlights that the non-uniform distribution of pores within SEBM Ti-6Al-4V means that careful considerations must be given regarding machining stock allowance in the design of these components due to the implications of material removal depth on surface integrity

Abrasive wear resistance of Ti-6AL-4V obtained by the conventional manufacturing process and by electron beam melting (EBM)

i–6Al–4V is one of the most used commercial titanium alloys, in part due to properties that are desirable in high value sectors such as the aerospace industry, such as good strength-to-weight ratio and corrosion resistance. These are of importance in gas turbine engines and structural components, where titanium alloys complement heavier steel or nickel alloys. It is also a good candidate to be used in the manufacturing of prostheses, since it has good biocompatibility, but applications in other areas are limited due to its poor bulk wear resistance. Conventionally processed Ti-6AL-4V is typically used, however additive manufacturing (AM) techniques such as electron beam melting (EBM) can also be applied to this alloy. These techniques are increasingly attractive across many industries because of geometrical freedom and control of mechanical properties, both key, for example, to the successful production of highly personalised prostheses and complex thermofluids channels in aerospace and automotive components. Parts produced by EBM are typically denser than those obtained by other AM processes, but still experience increased wear over their traditionally obtained equivalents, particularly in sliding, thus surface treatment is common. This work compares the wear observed when specimens manufactured by either conventionally or EBM were subjected to abrasion via means of a dry sand-rubber wheel tribometer capable of testing to the ASTM G65 test method. The specimens and resulting wear scars were characterised (hardness, grain size, roughness) and details of the wear mechanism(s) identified. The EBM specimens exhibited much greater wear, over twice that of the conventionally obtained specimens, and feature significantly more scratches. Although there are several studies variously considering two-body sliding wear and the efficacy of surface treatments of this type of alloy, this work addresses the paucity of information about the comparative abrasive wear performance of the alloy obtained via the two process routes

A data-driven approach for predicting printability in metal additive manufacturing processes

Metal powder-bed fusion additive manufacturing technologies offer numerous benefits to the manufacturing industry. However, the current approach to printability analysis, determining which components are likely to build unsuccessfully, prior to manufacture, is based on ad-hoc rules and engineering experience. Consequently, to allow full exploitation of the benefits of additive manufacturing, there is a demand for a fully systematic approach to the problem. In this paper we focus on the impact of geometry in printability analysis. For the first time, we detail a machine learning framework for determining the geometric limits of printability in additive manufacturing processes. This framework consists of three main components. First, we detail how to construct strenuous test artefacts capable of pushing an additive manufacturing process to its limits. Secondly, we explain how to measure the printability of an additively manufactured test artefact. Finally, we construct a predictive model capable of estimating the printability of a given artefact before it is additively manufactured. We test all steps of our framework, and show that our predictive model approaches an estimate of the maximum performance obtainable due to inherent stochasticity in the underlying additive manufacturing process. © 2020, The Author(s)

On the additive manufacturing, post-tensioning and testing of bi-material tensegrity structures

An investigation on the additive manufacturing and the experimental testing of 3D models of tensegrity prisms and columns is presented. An Electron Beam Melting facility (Arcam EBM S12) is employed to 3D print structures composed of tensegrity prisms endowed with rigid bases and temporary supports, which are made out of the titanium alloy Ti6Al4V. The temporary supports are removed after the additive manufacturing phase, when Spectra cross-strings are added to the 3D printed models, and a suitable state of internal prestress is applied to the structure. The experimental part of the study shows that the examined structures feature stiffening-type elastic response under large or moderately large axial strains induced by compressive loading. Such a geometrically nonlinear behavior confirms previous theoretical results available in the literature, and paves the way to the use of tensegrity prisms and columns as innovative mechanical metamaterials and smart devices

X-ray Tomography Characterisation of Lattice Structures Processed by Selective Electron Beam Melting

Metallic lattice structures intentionally contain open porosity; however, they can also contain unwanted closed porosity within the structural members. The entrained porosity and defects within three different geometries of Ti-6Al-4V lattices, fabricated by Selective Electron Beam Melting (SEBM), is assessed from X-ray computed tomography (CT) scans. The results suggest that horizontal struts that are built upon loose powder show particularly high (~20 × 10−3 vol %) levels of pores, as do nodes at which many (in our case 24) struts meet. On the other hand, for struts more closely aligned (0° to 54°) to the build direction, the fraction of porosity appears to be much lower (~0.17 × 10−3%) arising mainly from pores contained within the original atomised powder particles

Dimensional accuracy of Electron Beam Melting (EBM) additive manufacture with regard to weight optimized truss structures

The Electron Beam (EBM) additive manufacturing process is well suited to fabricating complex structural designs in Ti–6Al–4V because of the design freedoms it offers combined with strong and consistent material properties. However it has been observed that complications may arise when manufacturing truss-like structures (such as those produced via structural topology optimization) in the form of undersized features on the finished part. The issue appears to affect truss members that are not aligned with the vertical build direction, with an apparent lack of material on the negative surfaces. This effect appears to worsen with a greater angle between the truss member and the build direction, even with the use of support structures. This investigation has characterized and measured the dimensional errors that result from this issue through 3D scanning techniques. Process modifications have then been made which result in significant improvements in dimensional accuracy. This investigation highlights the importance of heat management at features with negative surfaces to yield parts that are dimensionally accurate without introducing excessive internal melt defects in the form of voids and porosity

Pitfalls in genetic testing: the story of missed SCN1A mutations

BACKGROUND: Sanger sequencing, still the standard technique for genetic testing in most diagnostic laboratories and until recently widely used in research, is gradually being complemented by next-generation sequencing (NGS). No single mutation detection technique is however perfect in identifying all mutations. Therefore, we wondered to what extent inconsistencies between Sanger sequencing and NGS affect the molecular diagnosis of patients. Since mutations in SCN1A, the major gene implicated in epilepsy, are found in the majority of Dravet syndrome (DS) patients, we focused on missed SCN1A mutations. METHODS: We sent out a survey to 16 genetic centers performing SCN1A testing. RESULTS: We collected data on 28 mutations initially missed using Sanger sequencing. All patients were falsely reported as SCN1A mutation-negative, both due to technical limitations and human errors. CONCLUSION: We illustrate the pitfalls of Sanger sequencing and most importantly provide evidence that SCN1A mutations are an even more frequent cause of DS than already anticipated

VAMOS: a Pathfinder for the HAWC Gamma-Ray Observatory

VAMOS was a prototype detector built in 2011 at an altitude of 4100m a.s.l. in the state of Puebla, Mexico. The aim of VAMOS was to finalize the design, construction techniques and data acquisition system of the HAWC observatory. HAWC is an air-shower array currently under construction at the same site of VAMOS with the purpose to study the TeV sky. The VAMOS setup included six water Cherenkov detectors and two different data acquisition systems. It was in operation between October 2011 and May 2012 with an average live time of 30%. Besides the scientific verification purposes, the eight months of data were used to obtain the results presented in this paper: the detector response to the Forbush decrease of March 2012, and the analysis of possible emission, at energies above 30 GeV, for long gamma-ray bursts GRB111016B and GRB120328B.Comment: Accepted for pubblication in Astroparticle Physics Journal (20 pages, 10 figures). Corresponding authors: A.Marinelli and D.Zaboro