# Topology Optimisation in Structural Steel Design for Additive Manufacturing
## 7 Designing for Additive Manufacturing with Topology Optimisation  
Metallic materials conventional design has been mostly based on subtractive techniques for centuries. Cutting and drilling flat sheets of various thicknesses and laminated profiles have been the rule in steel construction, in which joining by bolting and welding became a ubiquitous counterpart. As a result, the design is limited to geometrical bounds, stockage is profuse, waste is significant, the material is not necessarily located where it contributes the most, and the whole process is very labour intensive. A notorious exception can be found in casted elements, which had its broadest use in the iron construction and now is mostly limited to steel nodes. However, the need for moulds makes this technique only viable when many similar products must be produced. Such lack of versatility usually limits this option to large space structures with equal nodes. Contrarily to the former, recent techniques allow producing metallic alloys by addition. Despite some critical technological differences, these techniques have been nested under the informal name of 3D Printing in the context of the current fourth industrial revolution. Other terms include Additive Manufacturing [130], Additive Layer Manufacturing (ALM) [131], to allude to the layer-by-layer nature of the process [423; 425] or Solid Freedom Fabrication (SFF) [126]. While AM has been commercially used for more than 20 years, typically for rapid prototyping without commercialisation purposes [354], only now the industrial capacity for widespread and reliable manufacturing of steel, titanium, and aluminium has been achieved [423], offering a tremendous opportunity for the construction industry [129]. Making AM a reality in construction plants and yards will depend on several economic and technical factors. Among the latter, one can highlight the ability to introduce robotics in construction [421] as with Large-Scale Prefabrication (LSP) [426], having consistent and usable-by-practitioners design tools [130; 388; 529], as well as achieving commercial maturity, including reliability and scale in AM techniques. Plentiful AM techniques, diverging solely from its name or commercial branding to its nature fundamentally, hinder non-experts the understanding of the available options for manufacturing. Even if an encompassing knowledge on the matter requires consulting comprehensive Review Articles, such as [129; 423; 425; 530]. Table 6, which adheres to the American Society for Testing and Materials (ASTM) classification of AM techniques, may assist in the task. It shall also be noted that distinctions among categories and techniques are usually made concerning its feedstocks and binding mechanisms [423]. However, not all those categories bear AM processes currently suitable for manufacturing of steel alloys. Some, as depicted, do not allow using metal powder or wire, being limited to polymers, ceramics, and other low-strength materials. Regarding the need for post-processing, DED-PA and DED-GMA metal production require machining, DED-EB usually needs surface grinding, DED-L may entail surface treatments, while PBF-L and PBF-EB are less likely to be post-processed [425]. Therefore, PBF and some DED processes are commonly designated direct-to-metal. Other aspects of paramount importance for assessing the suitability for steel manufacturing include the fabrication speed, properties reliability, and the alloy cooling rate. The latter will have a profound impact not only in residual stresses but also in the steel composition, where the control of martensite and retained austenite is crucial [423]. The former aspects are not always easy to assess, mostly since the equipment, context, and several other industrial manufacturing details play a significant role. Nevertheless, a choice has been made to limit the Table 6 scope to proven and industrially viable technologies, even if not all of those are applicable to the metal alloys manufacture.