Apr 12, 2018 | By David
A team of researchers based in Singapore have recently carried out an advanced study on the SLM (Selective Laser Melting) 3D printing technique, with findings that could prove invaluable for manufacturing projects that use metal AM processes. The results of this study were outlined in a paper entitled ‘Simultaneously enhanced strength and ductility for 3D-printed stainless steel 316L by selective laser melting’, in the journal NPG Asia Materials. The authors were Zhongji Sun, Xipeng Tan, Shu Beng Tor and Chee Kai Chua.
The engineers explored a range of different types of lasers, as well as simulating and measuring specific geometrical features of the powder bed and the melting process, in order to work out a creative approach to improve the efficiency of the SLM technique. They found that both the ductility and toughness of metal AM parts could be enhanced by in-process engineering of their crystallographic texture.
The Selective Laser Melting technique involves a laser beam, pre-programmed by a digital 3D model, being used to melt specific areas of a bed of metal powder, in order to fuse the particles of metal together into a specific shape. This 3D printing technique is valued due to its potential for automatically producing complex geometries in one stage, without excessive labor or tooling costs.
SLM is preferable to similar metal AM processes due to its improved resolution and the strength of the resulting parts. Despite its strength advantages compared to other techniques, SLM 3D printing still needs improvement in terms of ductility and toughness. Extensive post-processing is often required to improve the performance of SLM-produced parts, which can be strong but excessively brittle due to internal pores, anisotropy, lack-of-fusion defects and other problems. Engineers have been attempting to solve this issue of the trade-off between strength and toughness in metal 3D printing for many years.
The research project attempted to address this issue by studying the micro-structure of parts produced using SLM 3D printing. They found that there were two different crystallographic textures that could be observed, referred to as <011> and the more common <001>. The former enables a much tougher and more ductile part to be produced.
When deformation forces are applied to a SLM 3D printed part, there are two different mechanisms by which the crystallographic texture can deform. One is known as dislocation slip, and the other is known as deformation twinning. Deformation twinning is better at dislocating energy in the structure, and is therefore the preferred mechanism in terms of ductility and toughness. Parts with the <011> structure were found to exhibit this mechanism much more frequently.
Using complex simulations of melt-pool geometry and behaviour, the researchers explored different ways to achieve the desired crystallographic texture. They found that a higher laser power was more likely to achieve the improved structure, as was a shorter, deeper melt pool.
(Melt pools with different laser sources. All images, source: NPG Asia Materials)
Making use of marine-grade stainless steel 316L, a common material used for industrial purposes and in SLM 3D printing, the researchers were able to tailor the crystallographic structure in-situ in order to achieve improved performance. The levels of ductility and toughness were up to 40 percent greater in parts that had been engineered to have the <011> crystallographic texture.
Their work points the way forward for SLM 3D printing techniques to be improved, with in-process engineering according to the study’s recommendations. This will enable improved material performance, and increase metal AM’s implementation for the manufacturing of heavy load-bearing parts.
Posted in 3D Printing Technology
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