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Heat treatment of products post additive manufacture are considered hugely important since the metallurgical condition post process is suboptimal. In the case of nickel-based superalloys, grain size, precipitate distribution and precipitate size are distinct from wrought equivalents. Appropriate heat treatment is required to ensure material performance. In this study, LPBF alloy 718, post-processed using a standard heat treatment, is explored under thermal and thermomechanical exposure conditions (with and without applied stress) to illustrate textural and microstructural evolution. The results show the instability of the LPBF microstructure in terms of grain size, precipitate density, and crystallographic orientation, illustrating the need for an appropriate heat treatment in relation to future service conditions. During thermal exposure only, the instability of the LPBF alloy microstructure was evident as the texture increased with time before decreasing and almost disappearing at the time of fracture. This contrasts with wrought alloy whose texture increases throughout creep testing and reaches a maximum at the time of fracture. An ideal microstructure for improved creep performance was identified and includes large equiaxed grains, elimination of texture, dissolution of Laves and δ phase and the precipitation of small carbides and γ’’ precipitates. Recommendations on how to heat treat LPBF alloy 718 to reach this microstructure are given. Overall, this work showed that LPBF components may become more performant than wrought and conventional equivalents. more...

Lattice structures are an important aspect of design for additive manufacturing (DfAM). They enable significant component light-weighting and the tailoring of a wide range of physical responses; mechanical, thermal, acoustic, etc. In turn, lattice design relies on fundamental research to uncover useful structure-property relationships, such as the influence of cell geometry and volume fraction. A number of commercial computer-aided-design (CAD) programs exist that offer lattice generation, but these tend to prioritise product design. This paper describes the FLatt Pack program (or Functional Lattice Package), which was created to address the paucity of research-focussed lattice design software. It possesses a number of features with this in mind, including; (i) it is free to use for research, (ii) it is standalone software with minimal, and also free, dependencies, and (iii) it undergoes frequent and rapid development based on state of the art lattice information and modelling methods. FLatt Pack includes twenty-three lattice cell types covering a broad range of pore connectivity, structural anisotropy, and surface area; a clear GUI presenting the lattice design stages in a sequential manner; and the option to export designs in appropriate formats for AM and finite element (FE) simulation. The program also features conformal lattice generation in arbitrary shapes, arbitrary volume fraction grading, and resource-efficient computation through an adaptive spatial resolution based on the user's design choices. The most recent version of FLatt Pack is freely available at: www.github.com/ian27ax/FLatt_Pack_dist. more...

Laser Powder Bed Fusion (LPBF) enables complex structures to be manufactured, which is attractive to industries where augmented service performance can be achieved. However, the build time of LPBF can be slower than traditional manufacturing processes, especially for higher volumes of parts. Multi-laser machines have the potential to significantly reduce process time, but there is little understanding of how interlaced scan strategies impact upon mechanical properties. The results of this work show that multi-laser scan strategies have no adverse effects on the creep properties of LPBF alloy 718 at different build orientations, demonstrating the potential of using multi-laser strategies for faster build rates without compromising the mechanical properties. Indeed, it is shown that for samples built vertically (i.e. where the build direction is parallel to the loading direction), multi-laser samples outperformed their single-laser counterparts and had a similar creep life and secondary creep rate to wrought alloy (1% difference). The single and multi-laser samples built horizontally (i.e. with the build direction perpendicular to the loading direction) and at 45° from the loading direction performed similarly and all failed soon after the onset of tertiary creep. It was also found that the failure of components could not be attributed to the interlacing scan strategies used with multi-laser processing, but rather to the build orientation and its effect on the grain orientation with respect to the loading direction and the stress state of the sample. However, the scan strategy, number of lasers, fraction area of solidified layer, powder underneath the layer and the interlayer rotation meant that the layers themselves as well as the samples were heterogenous. Despite this, the creep life for any given sample was within 73 h (i.e.17%) of its repeats, giving confidence in the results. It is therefore possible, as a result of this work, to understand the effects of multi-laser scan strategies, particularly the laser-to-laser interfaces on LPBF microstructure. Multi-laser scan strategies are shown to have no detrimental effects on the creep performance, while cutting the build time by more than half. more...

This work investigates the use of hierarchical mesh decomposition strategies for topology optimisation using bi‐directional evolutionary structural optimisation algorithm. The proposed method uses a dual mesh system that decouples the design variables from the finite element analysis mesh. The investigation focuses on previously unexplored areas of these techniques to investigate the effect of five meshing parameters on the analysis solving time (i.e. computational effort) and the analysis quality (i.e. solution optimality). The foreground mesh parameters, including adjacency ratio and minimum and maximum element size, were varied independently across solid and void domain regions. Within the topology optimisation, strategies for controlling the mesh parameters were investigated. The differing effects of these parameters on the efficiency and efficacy of the analysis and optimisation stages are discussed, and recommendations are made for parameter combinations. Some of the key findings were that increasing the adjacency ratio increased the efficiency only modestly – the largest effect was for the minimum and maximum element size parameters – and that the most dramatic reduction in solve time can be achieved by not setting the minimum element size too low, assuming mapping onto a background mesh with a minimum element size of 1. more...