How to enhance the ductility and toughness of SLM metal 3D printed parts

On April 13, 2018, Antarctic bears learned from foreign media that a research team in Singapore recently conducted an advanced study on SLM (Selective Laser Melting) 3D printing technology for manufacturing projects using metal AM processes. The findings may be invaluable. The results of this study are summarized in a paper entitled "Enhanced Selective Laser Melting 3D Printing Stainless Steel 316L Strength and Extensibility" published in the NPG aisa materials journal. The authors are Zho ngji Sun, Xipeng Tan, Shu Beng Tor and Chee Kai Chua.

Engineers explored a range of different types of lasers, as well as simulating and measuring specific geometric features and melting processes of powder beds to propose innovative ways to increase the efficiency of SLM technology. They found that the ductility and toughness of metal AM components can be enhanced by the optimization of their crystal structure.

The selective laser melting technique includes a laser beam pre-programmed by a digital 3D model for melting a specific area of ​​the metal powder bed to fuse the metal particles together to form a particular shape. Because 3D printing technology has the potential to automatically generate complex geometries in one phase, it is valued without the need for excessive labor or tooling costs.

Due to its higher resolution and the strength of the resulting components, SLM is superior to similar metal AM processes. Despite its strong advantages over other technologies, SLM 3D printing still needs improvement in terms of ductility and toughness. Extensive post-treatment is often required to improve the performance of SLM-manufactured components that may be strong but excessively brittle due to internal porosity, anisotropy, lack of fusion defects, and other problems. For years, engineers have been trying to solve the trade-off between metal 3D printing strength and toughness.

The research project attempts to solve this problem by studying the microstructure of parts produced using SLM 3D printing. They found that two different crystallographic structures were observed, called <011> and the more common <001>. The former is able to produce tougher and more malleable parts.

When a deformation force is applied to the SLM 3D printing unit, there are two different mechanisms that can deform the crystal structure. One is called a dislocation slip, and the other is called a deformation twin. Deformation twins are the preferred mechanism for ductility and toughness, and parts of the <011> structure exhibit this mechanism more frequently.

Using complex molten pool geometry and behavioral simulations, researchers explored different ways to achieve the desired crystallographic structure. They found that higher laser power is more likely to achieve an improved structure, just like a shorter, deeper molten pool.

â–³ Use a different pool of laser sources. All pictures, source: NPG Asia Materials

â–³ Formation of <001> and <011> crystallographic structures in SS316L samples constructed using SLM with 380W and 950W laser power

Using ocean grade stainless steel 316L, a common material for industrial applications and SLM 3D printing, the researchers were able to adjust the crystal structure in situ to achieve improved performance. In designing parts with a <011> crystal structure, the ductility and toughness levels are as high as 40%.

In addition, scanning strategies and the like have a certain impact on the results.