Cryogenic treatment and laser shock peening enhance performance of L-PBF alloy.
Metal 3D printing is well known for being able to produce complex geometries that would be impossible to manufacture using more conventional processes. However, a major drawback of using additive processes—such as laser powder bed fusion (L-PBF)—is that the resulting parts tend to be weaker than their conventionally manufactured equivalents.
This is a known issue with the L-PBF process, since the rapid heating and cooling of each layer of metal powder via one or more lasers can cause residual stresses or heterogenous microstructures to form in the printed part.
Post-processing parts made with metal additive manufacturing (AM) is the obvious solution to these issues, but that encompasses a wide variety of processes, from hot isostatic pressing (HIP) to shot peening.
Precisely which post-processing regime will yield the best results depends considerably on the materials and requisite tolerances involved, but a new approach from researchers at Jiangsu University’s school of mechanical engineering in China is showing promise in improving both the strength and ductility of a metastable high-entropy alloy consisting of iron (50%), manganese (30%), cobalt (10%), and chromium (10%).
Their technique consists of two main steps: first, a deep cryogenic treatment (DCT), immersing the 3D printed alloy in liquid nitrogen at -196C to relieve global thermal stresses and refine its microstructure. Next, laser shock peening (LSP) plastically compresses and strengthens the material.
“When we combined these treatments, the metal’s internal architecture reorganized,” explained Zhaopeng Tong, first author of the paper, in a press release. “We saw a gradient of tightly packed nanocrystals and a reversal of internal stresses — from harmful tensile to beneficial compressive.”
According to the researchers, the treated alloy showed a switch from tensile to compressive surface stress, peaking at −289 MPa, a surface hardness of 380.8 HV, as well as improved strength and ductility. These are the results of microstructural gradients, dislocation hardening, and atomic-level transformations that collectively make the alloy stronger yet more deformable.
“This integrated strategy lets us fine-tune both the surface and the interior of printed metals,” said professor Xudong Ren, who led the study, in the same release. “It helps overcome the long-standing trade-off between strength and ductility, which is a central goal in structural materials design.”
Ren and his team are now expanding the technique to other alloys and metal systems, aiming to generalize the approach into a new class of post-processing technologies for high-performance additive manufacturing.
Their research is published open-access in the International Journal of Extreme Manufacturing.
