# Topology Optimisation in Structural Steel Design for Additive Manufacturing
## 7 Designing for Additive Manufacturing with Topology Optimisation
Thus, emerging technologies such as Electrochemical Additive Manufacturing (ECAM) [129; 538] was not included. On the opposite field, some AM techniques have long been proposed before the concept of AM was defined. That is the case of WAAM, which dates back to 1925 [424] and is now highly productive. Hence, some researchers regard WAAM as a leading option for steels manufacture [421], provided that shortcomings with high residual stresses, material and geometrical properties are successfully addressed (as can be seen in Figure 12). On the other hand, PBF-L (also in Figure 12), PBF-EB, and DED-L have been mentioned as the most relevant technologies for producing metallic alloys in the current context of AM [423]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**AM Categories to** & & & \\
**ASTM’s Committee F42,** & & & \\
**ASTM F2792-12a [531]** & **AM Techniques** & **Remarks** & **References** \\
**(Withdraw) and** & & & \\
**ISO/ASTM 25900-2015** & & & \\
The role of TO as an enabler for AM justifies the unparalleled growth of both [130]. Currently, the Design for Additive Manufacturing (DfAM), which succeeded the Design for Manufacturing (DfM) traditional methods [541, 542], employs more or less advanced optimisation tools, spanning practical lattice-based ones, as depicted by [PERSON] et al. [543] and illustrated in Figure 13, to complex approaches [544, 545], and is already paving its way into structural steel connections [546, 547], albeit mostly for tension-only nodes. As highlighted in recent Review Articles [130, 424, 425, 530] as well as Buchanan and Gardner 2019 [421], TO is yet to overcome important challenges to meet AM requirements. A stronger link between the DfAM and production is needed to overcome the current  
Figure 12: Examples of additively manufactured metallic parts. Left to right and top to down: (**a**) Nozzle manufactured with PBF-L [423]; (**b**) MX3D bridge manufactured with WAAM [129, 420]; (**c**) rib manufactured with WAAM [420, 539] (reproduced under the Creative Commons Attribution 4.0 International License, [[http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)]([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)) accessed on 25 February 2021); (**d**) turbine part manufactured with PBF-L [530]; (**e**) aerospace structure component manufactured with PBF-L [540]. Figure 13: Lattice-based design for additive manufacturing (DfAM). From left to right: (**a**) CAD model; FEM; lattice model; (**b**) boundaries and loading definition; and (**c**) final optimized structure [543] (reproduced under the Creative Commons Attribution 4.0 International License, [[http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)]([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)) accessed on 25 February 2021). bottleneck, including better accounting for AM metallurgical aspects, such as residual stresses, internal defects, interlayer bonding conditions, and anisotropy [536; 548]. In fact, composite materials TO for AM is still afar from the industrial practice. Other challenges for the current DfAM can be found in the difficulty to avoid overhangs and general support structures [549], which promote waste, require labour, time, and cost to post-process and hinder surfaces' quality. Furthermore, for the AM process convenience, the present DfAM makes extensive employment of open-walled infills which, amidst important contributions for material reduction, have a frequently detrimental effect on the product stiffness [550], and especially on the torsional stiffness. Another important and often overlooked problem lies in the difficulty in testing additively manufactured parts, especially by Non-Destructive Testing (NDT) techniques [551], which result in a poor understanding of material properties yielded by different AM techniques [552]. On the other hand, recent research brought innovative design techniques to assist AM.