3D printing for stronger stainless steel Friday, 10 November 2017

A research team at Lawrence Livermore National Laboratory in California have developed a way to 3D print tough and flexible stainless steel, allowing them to strengthen the steel without compromising on its ductility.

According to the researchers, who are led by Yinmin Morris Wang, a materials scientist at the Lawrence Livermore National Laboratory, traditional approaches for strengthening steels typically came at the expense of useful ductility, but the new approach allows austenitic 316L stainless steels to be additively manufactured via a laser-powder-bed-fusion technique, with the result having a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels.

This is a breakthrough, not just for the strengthening of steel, but also in the application of 3D printing technology. Current 3D printing works best with plastic and porous steel, which are too weak for tougher applications. The ability to use 3D printing to manufacture objects stronger than 316L steels could lead to faster and cheaper ways to make complex rocket engine, nuclear reactor and oil rig parts.

Stainless steel is made by melting conventional steel, which is a combination of iron and carbon, then adding in chromium and molybdenum. Following a process of cooling, reheating and rolling, the material gains a microscopic structure with tightly packed alloy grains and thin boundaries between the grains, creating a cell-like structure.

The strength of these boundaries withstands the fractures that can occur when the metal ils bent or stressed, making stainless steel tough and flexible enough to be formed into desired shapes.

3D printing attempt to reproduce such a structure have previously been unsuccessful. In a typical 3D printing setup, a layer of powdered metal alloy particles are laid on a flat surface. A computer-controlled laser beam then scans across the surface.

Particles hit by the laser melt and fuse together. The surface then drops down a step, another layer of powder is added, and the laser melting process begins again, binding the new material to the layer below. This eventually builds up into a 3D printed structure.

However, stainless steel objects built in such a way have highly porous microscopic structures, and so they are weak and prone to fracture.

Wang and his colleagues came up with an approach where they used lasers and rapid cooling in order to fuse metal alloy particles together in a dense, tightly-packed structure. They then extended that work by designing a computer-controlled process to increase their control over the structure of the material from the nanoscale to micron scale.

Under these controlled conditions, the printer is able to build tiny cell wall-like structures into the material on each scale, preventing fractures. In their tests, the researchers found that under certain conditions the final 3D printed stainless steels were up to three times stronger than steels made by conventional techniques and yet still ductile.

Because the scientists used a commercially available 3D printer and laser to achieve their results, it’s likely that other groups of researchers and industry players will be able to replicate their approach and create a range of high-strength stainless steel parts.

[Image: LLNL materials scientist Joe McKeown looks on as postdoc researcher Thomas Voisin examines a sample of 3D printed stainless steel. Photos by Kate Hunts/LLNL.]