The Influence of Porosity on Fatigue Crack Initiation in Additively Manufactured Titanium Components

Without post-manufacture HIPing the fatigue life of electron beam melting (EBM) additively manufactured parts is currently dominated by the presence of porosity, exhibiting large amounts of scatter. Here we have shown that the size and location of these defects is crucial in determining the fatigue life of EBM Ti-6Al-4V samples. X-ray computed tomography has been used to characterise all the pores in fatigue samples prior to testing and to follow the initiation and growth of fatigue cracks. This shows that the initiation stage comprises a large fraction of life (>70 %). In these samples the initiating defect was often some way from being the largest (merely within the top 35 % of large defects). Using various ranking strategies including a range of parameters, we found that when the proximity to the surface and the pore aspect ratio were included the actual initiating defect was within the top 3 % of defects ranked most harmful. This lays the basis for considering how the deposition parameters can be optimised to ensure that the distribution of pores is tailored to the distribution of applied stresses in additively manufactured parts to maximise the fatigue life for a given loading cycle

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

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

Methods for Rapid Pore Classification in Metal Additive Manufacturing

The additive manufacturing of metals requires optimisation to find the melting conditions that give the desired material properties. A key aspect of the optimisation is minimising the porosity that forms during the melting process. A corresponding analysis of pores of different types (e.g. lack of fusion or keyholes) is therefore desirable. Knowing that pores form under different thermal conditions allows greater insight into the optimisation process. In this work, two pore classification methods were trialled: unsupervised machine learning and defined limits. These methods were applied to 3D pore data from X-ray computed tomography and 2D pore data from micrographs. Data were collected from multiple alloys (Ti-6Al-4V, Inconel 718, Ti-5553 and Haynes 282). Machine learning was found to be the most useful for 3D pore data and defined limits for the 2D pore data; the latter worked by optimising the limits using energy densities

Material interactions in laser polishing powder bed additive manufactured Ti6Al4V components

Laser polishing (LP) is an emerging technique with the potential to be used for post-build, or in-situ, precision smoothing of rough, fatigue-initiation prone, surfaces of additive manufactured (AM) components. LP uses a laser to re-melt a thin surface layer and smooths the surface by exploiting surface tension effects in the melt pool. However, rapid re-solidification of the melted surface layer and the associated substrate thermal exposure can significantly modify the subsurface material. This study has used an electron beam melted (EBM) Ti6Al4V component, representing the worst case scenario in terms of roughness for a powder bed process, as an example to investigate these issues and evaluate the capability of the LP technique for improving the surface quality of AM parts. Experiments have shown that the surface roughness can be reduced to below Sa = 0.51 μm, which is comparable to a CNC machined surface, and high stress concentrating defects inherited from the AM process were removed by LP. However, the re-melted layer underwent a change in texture, grain structure, and a martensitic transformation, which was subsequently tempered in-situ by repeated beam rastering and resulted in a small increase in sub-surface hardness. In addition, a high level of near-surface tensile residual stresses was generated by the process, although they could be relaxed to near zero by a standard stress relief heat treatment

The effect of density and feature size on mechanical properties of isostructural metaffic foams produced by additive manufacturing

Simple models describing the relationship between basic mechanical properties and the relative density of various types of porous metals (such as foams, sponges and lattice structures) are well established. Carefully evaluating these relationships experimentally is challenging, however, because of the stochastic structure of foams and the fact that it is difficult to systematically isolate density changes from variations in other factors, such as pore size and pore distribution. Here a new method for producing systematic sets of stochastic foams is employed based on electron beam melting (EBM) additive manufacturing (AM). To create idealised structures, structural blueprints were reverse-engineered by inverting X-ray computed tomographs of a randomly packed bed of glass beads. This three-dimensional structure was then modified by computer to create five foams of different relative density ρr, but otherwise consistent structure. Yield strength and Young’s modulus have been evaluated in compression tests and compared to existing models for foams. A power of 3 rather than a squared dependence of stiffness on relative density is found, which agrees with a recent model derived for replicated foams. A similar power of 3 relation was found for yield strength. Further analysis of the strength of nominally fully dense rods of different diameters built by EBM AM suggest that surface defects mean that the minimum size of features that can be created by EBM with similar strengths to machined samples is ∼1 mm