Design: optimising supports for complex components

Fully dense components built using selective laser melting exhibit mechanical properties equivalent or even better than those of parts produced by conventional manufacturing [1] [2]. But building components using SLM sometimes requires external support structures. These support structures are necessary for a few reasons: they strengthen and fix the part to the building platform, they conduct excess heat away and they prevent warping or complete build failure.

They make non negligible impacts on production yields and costs. These additional structures increase build time, build costs as well as post-processing time and complexity.

In order to derive the build optimal orientation susceptible to minimise (eliminate?) the quantity of supports, it is necessary to analyse the component’s geometry and its limitations.

Sources of manufacturing defects and build failure [3]

There are three main defects frequently occurring during SLM: staircase effect, warping and low down-skin surface roughness.

Staircase effect

Staircase effect results from the digitalisation of the 3D CAD version of the object: it’s a stepped approximation of the nominal shape. The tessellation process determines the accuracy and contours of the whole component. Staircase effect is most obvious for sliced inclines and curvatures. It becomes more pronounced as the surface angle increases with respect to the z (vertical) axis.

Staircase effect [4]

Poor down-skin surface roughness

Supports also act as mini heat-sinks. They dissipate the excess of heat away from the melting area. This is particularly valuable for down-facing surfaces at the boundary between solid material and powder. The heat transfer difference between powder and solid material generally means that surface roughness tend to be large for boundary layers such as overhands. When laser irradiates powder-supported layer, the melt pool generated seeps through the powder particles as a result of wettablilty and capillary forces, producing a poor surface roughness and decreased dimension accuracy.

Warping

Warping is due to thermal stresses caused by rapid solidification during SLM process [4]. When the thermal stress exceeds the strength of the material, plastic deformation occurs. The component can get distorted and this leads to bad junctions between supports and components. For overhangs, the lack of supports to fix the layer in position generates a ‘peel-off’ that can be caught by the recoating blade and damage the component.

Self-supporting features, such as apertures smaller than 5min in diameter, are directly determined by build orientation. Wide internal channels and apertures as well as large overhangs typically require supports as they are not supported by resolidified layers. 

Optimum build orientation

Finding the most suitable build orientation and types of supports is a very time consuming effort still largely based on experience. Often overlooked, this step can make a non negligible difference in term of costs (material and post-processing time) and in term of production yield (inadequate support structures can fail spectacularly).

Overhangs [4]

To speed up and systemise this step, researchers and manufacturers alike have been developing algorithms to find suitable orientation automatically according to:
- the shortest build time [5];
- the minimal amount of supports [5, 6, 7];
- an easy access to and removal of supports [7];
- the best possible surface roughness (minimal staircase effect) [5, 7].

Based on the evaluation of several parameters, overhang, surface area, thickness wall, contour length, etc., surfaces requiring support structures and support types can be determined. 

Case study of a cardan U-joint [4]

The case study of a cardan U-joint illustrates the procedure adopted to determine the most suitable support structures for a realistic complex shape.  This joint is a nonassembly mechanism designed to be fabricated using selective laser melting. The clearances between mechanisms are very small and the trapped powder and/or the presence of inadequate supports within the joint can cause build failure. 

Various orientations and supports structures for cardam Ujoint [4]

First, build direction is chosen to ensure shortest build height and quick fabrication. However, the support generated by this orientation are very difficult to remove within the small clearance. Minimising the amount of supports generates the tallest, more time consuming build. In this case, the joint also need supports. Alternatives where the two joint axis are inclined so that the angle assembly of the mechanism can be adjusted along the unconstrained degree of freedom generates a configuration with no supports are required within the clearance. This orientation also has the smallest z-directional height. The use of optimized supports has avoided distortion of the joint and made it possible to easily remove the supports. The Cardan U-joint can be swung directly and smoothly after supports removal.

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References
[1]      G. Sun, R. Zhou, J. Lu, and J. Mazumder, “Evaluation of defect density, microstructure, residual stress, elastic modulus, hardness and strength of laser-deposited AISI 4340 steel,” Acta Mater., vol. 84, pp. 172–189, Feb. 2015.
[2]      Y. Zhu, D. Liu, X. Tian, H. Tang, and H. Wang, “Characterization of microstructure and mechanical properties of laser melting deposited Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy,” Mater. Des., vol. 56, pp. 445–453, Apr. 2014.
[3]      F. Calignano, “Design optimization of supports for overhanging structures in aluminum and titanium alloys by selective laser melting,” Mater. Des., vol. 64, pp. 203–213, Dec. 2014.
[4]      Wang D, Yang Y, Yi Z, Su X. Research on the fabricating quality optimization of the overhanging surface in SLM process. Int J Adv Manuf Technol 2013;65(9– 12):1471–84.
[5]      Frank D, Fadel G. Expert system-based selection of the preferred direction of build for rapid prototyping processes. J Intell Manuf 1995;6(5):339–45.
[6]      Allen S, Dutta D. On the computation of part orientation using support structures in layered manufacturing. In: Solid freeform fabrication symposium. Texas, USA: Austin; 1994.
[7]      Strano G, Hao L, Everson RM, Evans KE. A new approach to the design and optimisation of support structures in additive manufacturing. Int J Adv Manuf Technol 2013;66(9–12):1247–54