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3D-printed flexible stainless steel stronger than conventional steel

By Prachi Patel January 10, 2018
3D-printed flexible
A high-angle annular dark-field (HAADF) scanning transmission electron microscope image of solidification cellular structures after deformation of 316L stainless steel. Inset: Trapping of dislocations at the cell walls. Credit: Nature Materials.

Researchers have found a way to three-dimensionally (3D) print stainless steel that is 2­–3× stronger than conventional steels while still remaining flexible. This additive manufacturing method could lead to rapid, low-cost manufacturing of everything from medical implants to parts for airplanes, nuclear power plants, and oil rigs.

Lightweight, tough, and flexible steels are useful for many applications. To make stainless steel strong, manufacturers use metallurgical techniques like forging and cold rolling. These methods give the steel a microstructure with tightly packed grains and thin boundaries between the grains. But making steel stronger usually comes at the expense of reducing its ductility.

This strength-ductility tradeoff has been a long-standing challenge for materials researchers. To overcome it, researchers have recently tried to engineer the microstructure of steels, but this typically requires compositional engineering and heat treatment.

3D printing is a simpler way to manipulate the microstructure. To 3D print stainless steels, researchers use laser powder-bed-fusion, a technique involving a laser beam that moves back and forth across a flat layer of powdery metal alloy particles, melting and fusing the particles together. The process is repeated by adding the powder layer-by-layer. But steels made this way are porous and weak.

Making dense, porosity-free alloys requires an optimal set of laser parameters. Researchers at Lawrence Livermore National Laboratory (LLNL) have used computer modeling and experiments to understand the relationship between the microstructure and properties of 3D-printed metal alloys. Using a special rapid cooling technique, the team has in the past made metal alloys with densely packed cell-like grains.

They have now come up with the specific laser and design parameters to 3D print low-porosity, dense 316L stainless steel, a low-carbon marine stainless steel containing chromium, nickel and a small percent of molybdenum that has excellent resistance to corrosion and oxidation.

The 3D-printed 316L stainless steel contains a densely packed cellular structure with cells ranging in size from nanometer- to sub-millimeter-scale. Cross-sectional scanning transmission electron microscopy images also show that Cr and Mo are segregated along the thin walls of these cells. The cell walls contain a high density of dislocations, which can lead to fractures. “The elemental segregation could play a role in pinning down cellular walls, which act as blockage sites for dislocations,” says Morris Wang, LLNL materials scientist and lead author of the study published recently in Nature Materials.

Using two different conventional laser powder-bed-fusion machines, the researchers made square pillars and rectangular plates. Tests showed that the steel had a combination of strength and flexibility surpassing that of conventional 316L stainless steel.

As a demonstration, the researchers also printed a small rocket engine part. Depending on complexity, it typically takes weeks to print real parts, Wang says, but this is still substantially faster than conventional approaches, which can take months to years to develop a component.

Huajian Gao, a professor of engineering at Brown University, says that the new 3D-printed steel shows an “exceptional combination of strength and ductility as compared to conventional 316L steels. The knowledge of the deformation mechanisms observed in the 3D-printed steels gained from this work will guide future additive manufacturing to create high performance metals and alloys for structural applications.”

That is just what the researchers now plan to do. “We plan to apply our 3D printing strategies to other metals and alloys,” Wang says. “There are well over five thousand types of other alloys that cannot be 3D-printed at the moment. There is a lot of work that needs to be done.”

Read the abstract in Nature Materials.