Novel wet etching technique boosts absorptivity of metal powders for Additive Manufacturing
October 16, 2024
A team from the United State’s Lawrence Livermore National Laboratory (LLNL), Stanford University and the University of Pennsylvania, has developed a novel wet chemical etching process that modifies the surface of conventional metal powders used in Additive Manufacturing. By creating nanoscale grooves and textures, the researchers have increased the absorptivity of the powders by up to 70%, allowing for more effective energy transfer during the Laser Beam Powder Bed Fusion (PBF-LB) process, particularly for challenging materials such as copper and tungsten.
One of the persistent challenges in PBF-LB metal Additive Manufacturing is the high reflectivity of certain metals, which can lead to inefficient energy absorption during the manufacturing process and can even damage some AM machines. This inefficiency often results in inadequate build quality and increased energy consumption, the researchers explain.
“Currently, with standard commercial laser-based machines, high-quality pure copper metal AM is generally considered infeasible,” said co-lead author and LLNL materials scientist Philip DePond. “Our method combines the effects of traditional surface treatments [that increase absorptivity] but doesn’t compromise the purity or material properties of copper that make it desirable — namely its high thermal and electrical conductivity. More fundamentally, we showed that laser-powder interactions extend to regions beyond the melt pool. This has been shown in simulations, especially those of high-fidelity done at LLNL, but not really detailed experimentally. We demonstrated that those interactions exist and can be beneficial to the process.”
The wet-etching technique is said to be relatively simple, but highly effective. The team immersed metal powders, such as copper and tungsten, in specially formulated solutions that selectively removes material from the surface. This process results in the formation of intricate nanoscale features that enhance the powder’s ability to absorb laser light. To characterise the surface features of the etched powders, the researchers employed advanced imaging techniques including synchrotron x-ray nanotomography, which provided detailed 3D representations of the powder particles, allowing the team to analyse and accurately model the of the electromagnetic influence of the nanoscale modifications.
The team conducted extensive experiments to demonstrate and attribute the mechanism of increased absorptivity to the modified powders. Process optimisation studies and eventually bulk and complex sample Additive Manufacturing was performed using custom built Laser Beam Powder Bed Fusion (PBF-LB) systems housed at LLNL’s Advanced Manufacturing Laboratory and MIRILIS laser-material interaction laboratory.
Researchers said that the enhanced absorptivity of metal powders is a promising step forward for reducing energy consumption in manufacturing, particularly as the demand for more sustainable and efficient manufacturing processes continues to grow. One of the team’s key findings was that they could manufacture high-purity copper and tungsten structures using lower energy input, less than 100 J/mm3 for copper, which is around the range for high-density titanium and stainless steel alloys, and ~700 J/mm3 for tungsten, around 1/3 less energy than is typically employed.
“In a broad sense, we are enabling the printing of copper without the risk of damaging the AM system itself,” DePond explained. “The process parameter window becomes wider as well, which allows a wider variety of scanning conditions to be explored, which often are needed when printing complex geometries. Finally, a handful of machine manufacturers have even gone the great lengths of creating entirely new machines to process copper and other highly reflective materials. These turn out to be nearly double the cost of a traditional machine, so the barrier of entry to printing these materials is prohibitively high.”
The potential applications of the findings could have an immediate impact on production. Researchers said the ability to manufacture with less energy not only reduces operational costs but also minimises the environmental impact of gthe manufacturing process and opens copper Additive Manufacturing up to a whole new contingent of producers.
“This method enables even commercial machines of fairly low laser power output to print copper, thus democratizing the process and providing access to a wider community,” Energy Security Program leader Dan Flowers said, adding that he hopes the work will allow industry to better utilize copper in advanced manufacturing. “From heat exchange to catalysis, more efficient printing of copper supports development of many clean energy and decarbonization technologies,” Flowers said. “The LLNL community and our low-carbon energy mission stand to benefit from this capability.”
The enhanced absorptivity and improved powder dynamics also could enable the production of high-quality AM parts with greater relative densities. In their experiments, the researchers achieved relative densities of up to 92% with half the energy input compared to other additively manufactured copper components, and over 99% with higher energies, indicating the potential for producing stronger and more durable metal parts.
The team is next looking to examine the effects of nanotexturing on elemental mixing of powders, such as materials that typically need drastically different energies to melt. Funding for the research came from the National Science Foundation, the US Department of Energy’s Office of Science, and LLNL.
Co-authors on the paper include LLNL’s Manyalibo “Ibo” Matthews; co-lead author Ottman Tertuliano and Luc Capaldi of the University of Pennsylvania; and Andrew Lee, Jiho Hong, David Doan, Mark Brongersma, X. Wendy Gu, Wei Cai and Adrian Lew of Stanford.
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