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Accelerating AM design workflow
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Fig. 2 Discrete optimisation approach: (a) specify design domain, loading and support conditions; (b) discretisation using nodes; (c) interconnect nodes with line elements and use layout optimisation (LO) to identify the optimal arrangement of discrete members (shown in red/blue); (d) adjust joint positions to improve the solution (geometry optimisation, GO) optimisation (GO). This works particularly well when the engineer has a high degree of design freedom, as truss-like forms will generally be found to provide the most efficient design solutions. Smith et al. [1] recently demonstrated the approach, employing LO to obtain AM component designs. Also, GO can now potentially be applied in tandem with LO to obtain improved solutions, as described by He et al. [2]. The combined approach is shown in Fig. 2. Once a line-element representation has been obtained, this can readily be transformed into a continuum model, suitable for further verification or manufacture, as indicated in Fig. 3. There are four key benefits of this discrete, line-element, optimisation approach:
can specify in the optimisation that members should not be inclined at shallow angles to the horizontal • Engineers can easily check the absolute structural efficiency of the design, based on the subtended angles between members • Engineers can rapidly perform structural checks on the optimised form, such as global buckling stability checks.
checking global buckling stability. The discrete optimisation approach means that engineers can obtain a viable design very quickly after an initial optimisation. This is typically not the case when using topology optimisation.
Examples
• Engineers can take steps to ensure that the optimised design is easy to be manufactured using AM processes. For example, they
As ever, there is a trade-off between computational efficiency and optimisation capability. A pragmatic approach, adopted in the recently released LimitState:FORM software, is to use a relatively simple optimisation formulation - linear in the case of the LO phase. As a result, engineers can obtain solutions extremely quickly, typically in seconds or minutes. Engineers can then easily edit and analyse the resulting optimised line-element model, for example adjusting the design to remove thin elements, or
The simple beam design shown in Fig. 4 was obtained automatically using LO, and was shown, through load testing [1] to be capable of carrying the target design load. In this case, specimens were manufactured using titanium Ti-6Al-4V via the Electron Beam Melting (EBM) process. The discrete optimisation approach was also recently used to develop a new airbrake hinge design for the Bloodhound supersonic car project, as shown in Fig. 5. The resulting design is almost 70% lighter than the original. Also, the use of GO and the ability to interactively edit the solution using LimitState:FORM led to a simpler form compared to that proposed previously by [1].
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• Engineers can automatically generate a high-quality solid geometry CAD model, rather than a mesh geometry
Fig. 3 Conversion of a line-element structure into a continuum (after Smith et al., 2016): (a) loft elements (e.g. as cylinders); (b) add joints; (c) expand joints to avoid stress concentrations
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Metal Additive Manufacturing | Spring 2017
© 2017 Inovar Communications Ltd
Vol. 3 No. 1