Researchers at North Carolina State University (NCSU) and Sindre Metals have successfully used a three-dimensional (3D) printing technique called “direct metal laser sintering” to form a 4.5 centimeter-diameter cylinder made of a fully amorphous metal alloy also known as metallic glass. The sample’s dimensions are larger than previously reported, thus paving the way for novel applications that require materials with exceptional strength. The results of their work were published in a recent issue of Applied Materials Today.

Bulk metallic glasses (BMGs) are multicomponent, noncrystalline metallic alloys, which can be formed in samples with thickness typically in the millimeter scale.  Very high cooling rates are required to help the alloy transition from its liquid state to the solid glassy state with the highly disordered atomic structure without crystallizing. While glass is brittle, BMGs have twice the strength of steel, are hard and elastic, and show good corrosion resistance. The ability to form BMGs easily from common transition metals like iron has naturally aroused much interest and led to rapid development in the field.

“We are not the first who have successfully 3D printed bulk metallic glasses (BMGs), but what sets our work apart is that we are the first group to successfully produce this iron alloy with a thickness larger than its critical casting thickness,” says Zaynab Mahbooba, first author of the article and PhD student in the team of Ola Harrysson in the Center for Additive Manufacturing and Logistics at NCSU.

The critical casting thickness, the maximum thickness that a BMG can be cast, is different for each BMG alloy composition. To make the alloy amorphous, the (critical) cooling rate is also different. “If the BMG is cast into dimensions larger than [its] critical casting thickness, then the cooling rate in the center of the sample will be too slow and the material will not be fully amorphous,” Mahbooba says. 

The team chose to work with the well-documented FeCr-MoCB. This alloy has a critical casting thickness <2 mm when synthesized by conventional copper mold casting. The sample produced by Mahbooba and co-workers using direct metal laser sintering (DMLS) had a thickness of 3 cm, which is 15× larger than its critical casting thickness and also larger than any traditionally produced iron-based BMG recorded thus far.

For fabricating the FeCr-MoCB metallic glass, the “printer” spreads the alloy powder on stainless steel substrates that are then placed on a building platform inside a chamber flooded with argon gas. Driven by computer aided design (CAD) data, the laser melts the powder, creating a 20 μm solid layer. The platform is then lowered 20 μm, metal powder is spread again, and the process is repeated until, layer by layer, a solid, uniform sample of the desired shape and dimensions is produced.

“Our BMG alloy was extremely brittle and challenging to process,” Mahbooba says, adding that the team went through hundreds of iterations of different process parameters before finding the parameter space that could produce the dense, amorphous sample reported in the article. “The processing space we ended up with was, to our surprise, far outside of what we initially expected and of what we have used to process crystalline metals,” Mahbooba says.

Using the optimized process settings, two cylinders measuring 30 mm × 450 mm were printed.  X-ray diffraction measurements confirmed no crystalline peaks, which means that the material was >99% amorphous. However, the researchers also reported the presence of stress-induced micro-cracks within the bulk material, products of the rapid solidification during the laser processing. Furthermore, electron backscatter diffraction analysis, which has higher resolution than x-ray diffraction, revealed the presence of a low concentration of nano-grain clusters within the BMG microstructure. The researchers say that the localized and isotropic nature of the nano-grains suggest that nucleation was not a product of thermal annealing, but of mechanical stress during DMLS processing.

The team is currently working to eliminate the micro-cracking by targeting Fe-based BMG alloys with enhanced plasticity.

“I think the group has achieved their goal to a certain degree, since they have prepared bulk BMGs. However, further research will be required to eliminate the crack defects and nano-grain clusters that appear and make the material ready for real applications. High cooling rate is important for the formation of BMGs, but stress accumulates during the preparation of large bulk BMGs under this situation; this makes cracks very hard to be controlled in the brittle BMGs,” says Meng Wang, a professor at Northwestern Polytechnical University, China.

Shengfeng Guo of the Faculty of Materials and Energy, Southwest University, China, says “This work is an exciting contribution because it shows that Fe-based BMG with a large dimension can be obtained by using the DMLS. However, the fabrication of highly dense, crack-free, and fully Fe-based BMGs remains a great challenge using laser 3D printing techniques due to a trade-off between high cooling rate for glass formation and the slow solidification process [required] for a crack-free structure. There is a lot of potential in this area, but there is still a long way to go,” he says.

Read the abstract in Applied Materials Today.