Design for SLM: topology optimisation of metallic structural nodes in architecture applications

Traditional node (left) and redesigned version after topology optimisation and SLM-specific re-design [4].

Selective Laser Melting offers design flexibility: complex products, usually built in numerous successive steps, can now be built in a single step and still have thin walls, deep cavities or hidden channels. An optimised design for SLM will limit the need support structures. The technique is mostly used in industries that benefit from weight reduction and where products are complex and produced in low quantities. Let's have a look at the topology optimisation process of a one-of structural node found in tensional integrity structures.

Topology optimisation

Tensegrity

Tensegrity – tensional integrity – is a structure based on isolated cables used in tension and linked together using compressive nodes to define a spatial system [1,2,3]. Due to the irregular shape of the structure, the structural nodes, connecting the cables to the struts within the tensegrity, all have slightly different shapes. Given the large potential combination of cable attachment positions and pulling directions, the conventional manufacturing of each node is very labour intensive and time consuming. It usually involves machining, welding and precision positioning of up to 7 unique sub-parts.

That’s what makes these structural nodes the perfect candidate for redesign and production with SLM [4]. Why use 7 steps when you can manufacture it in 1? Topology optimisation for SLM process uses 2 successive steps: 1) Topology optimisation [5]; 2) Design optimisation for SLM.

Buckminster Fuller coined the term tensegrity and popularised these structures. The sculpture above is by Kenneth Snelsons and shows an exhibit from the Mies Van der Rohe Berlin Modern Art Gallery.

Compressive node to be optimised.

In the design of load bearing structures, topology optimization is used to adjust material distribution and layout according to performance targets. Numerous commercial products based on mathematical methods are used to solve complex design problems with numerous design parameters such as original 3D geometry, loads direction and value, materials characteristics, Van Mises stress, allowing space for connection and handling, fixed holes diameters, weight and material reduction, etc…

Von Mises stresses [MPa] in the original design (left) and the design after topology optimization (right). Highest stresses can be observed in red, lowest in blue [4].

re-design for SLM

Areas and overhangs exceeding 45 angle (in red) require supports to be built using SLM [4]

The functional topology optimization method generates an optimal form for the required function, getting rid of unnecessary materials in the process. This however doesn’t account for the production method. That’s why the initial condition is modified to make the node self-supporting to facilitate and take full advantage of SLM.

This requires taking into account such things as: size of the node in relation to the building chamber, number of parts to fit on the building platform, production time and costs, printing direction, amount of support structure or form changes to avoid support structure, wall thickness and gaps, heat dissipation, heat treatment, surface treatment, etc

Design after topology optimisation (left) and after additional SLM-specific redesign (right) [4].

As a result, many improvements have been added to the 1st optimised component. Most notably, this is a shorter self-supporting structure with active supports that get rid of the top plate and reduce weight further. Overhangs are designed to avoid the need for support structures and post processing. For similar reasons, fixed-diameters holes have been discarded in favour of gothic shapes arches. 

To sum-up

This topology optimization results in a functional, beautiful organic form using less material while retaining its original functions as cable connectors. SLM-specific design improves production efficiency while reducing materials consumption and weight further.

All that is left to do is to combine FEA analysis with SLM-specific design optimisation in a single environment and make it available to AM engineers!

Do you know of any software that can handle FEA topology and production optimisation in a single environment? If yes, let me know in the comments, thanks!

References
[1] Galjaard, S.: Tensegrity structures with integrated street lighting. IABSE Future of Design, IABSE UK Newsletter, 35th edn. (2013)
[2] Snelson, K.: The art of tensegrity. Int. J. Space Struct. 27(2 & 3), 71–80 (2012)
[3] Strauss, H.: AM Envelope – The Potential of Additive Manufacturing for Façade Construction. Delft University of Technology, Faculty of Architecture, Architectural Engineering Technology department, Delft (2013)
[4] S. Galjaard, S. Hofman, S. Ren, Arup, Amsterdam, The Netherlands © Springer International Publishing Switzerland 2015 P. Block et al. (eds.), Advances in Architectural Geometry 2014, pp 79-93
[5] Bends®e,M.P., Sigmund, O.: Topology Optimization; Theory, Methods and Applications, 2nd edn. Springer, Berlin (2004)