October 9, 2024
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by Jeremy Thomas, Lawrence Livermore National Laboratory
In a noteworthy development in the field of metal additive manufacturing, a team of researchers from Lawrence Livermore National Laboratory (LLNL) and their academic collaborators have created a method that improves the optical absorptivity of metal powders utilized in 3D printing.
The strategy, which focuses on developing nanoscale surface patterns on metal powders, holds the potential to enhance the efficiency and quality of printed metal components, especially with challenging materials such as copper and tungsten, as stated by researchers.
Additive manufacturing (AM), widely recognized as 3D printing, has revolutionized product design and production, enabling the fabrication of intricate shapes and tailored elements that conventional manufacturing techniques find difficult to accomplish.
Nevertheless, a significant hurdle in laser powder-bed fusion (LPBF) metal 3D printing is the high reflectivity of some metals, which can hinder effective energy absorption during the printing process and may even inflict damage on certain printing machines. This lack of efficiency frequently leads to poor print quality and greater energy consumption, according to researchers.
Addressing this challenge directly in a study published as the September feature in the journal Science Advances, a team of scientists from LLNL, Stanford University, and the University of Pennsylvania unveiled an innovative wet chemical etching technique that alters the surface of standard metal powders. By establishing nanoscale grooves and textures, the researchers claimed they have enhanced the absorptivity of these powders by up to 70%, facilitating more efficient energy transfer during the laser melting process.
“At present, utilizing standard commercial laser-based machines for high-quality pure copper metal additive manufacturing is generally viewed as unfeasible,” stated Philip DePond, co-lead author and materials scientist at LLNL. “Our approach leverages the advantages of traditional surface treatments that enhance absorptivity, without compromising the purity or the beneficial properties of copper, such as its exceptional thermal and electrical conductivity.”
“On a more fundamental level, we demonstrated that the interactions between the laser and powder extend beyond the melt pool. This has been confirmed through simulations, particularly those with high fidelity conducted at LLNL, yet lacked thorough experimental validation. We proved that these interactions exist and can significantly enhance the manufacturing process.”
The wet-etching technique utilized is relatively straightforward, yet highly effective, according to the researchers. The team immersed metal powders such as copper and tungsten in specially designed solutions that selectively eliminate material from the surface.
This process creates complex nanoscale features that improve the powder’s capacity to absorb laser light. To analyze the surface features of the etched powders, the researchers used advanced imaging techniques such as synchrotron X-ray nanotomography, which produced detailed three-dimensional representations of the powder particles, enabling the team to investigate and accurately model the electromagnetic effects of the nanoscale alterations.
The research team engaged in thorough experiments to elucidate and credit the mechanism behind the increased absorptivity observed in the modified powders. They conducted process optimization studies and subsequently printed bulk and intricate samples utilizing custom-built LPBF systems located at LLNL’s Advanced Manufacturing Laboratory and the MIRILIS laser-material interaction laboratory.
According to the researchers, the improved absorptivity of metal powders represents a significant advancement in efforts to lower energy use in manufacturing. This is especially relevant given the growing need for more sustainable and efficient manufacturing methods.
A major discovery made by the team was their ability to print high-purity copper and tungsten structures with reduced energy requirements—less than 100 J/mm3 for copper, which is comparable to the energy levels used for high-density titanium and stainless-steel alloys, and approximately 700 J/mm3 for tungsten, which is about one-third less energy than what is typically required.
“Broadly speaking, we are facilitating the printing of copper without jeopardizing the AM system itself,” remarked DePond. “This expands the process parameter window, allowing for a greater variety of scanning conditions to be examined, which is often essential for printing complex geometries. Furthermore, several machine manufacturers have even invested in developing entirely new machines tailored for processing copper and other highly reflective materials, which end up costing nearly twice as much as traditional machines, thus creating a prohibitively high barrier for entering the market of these materials.”
The implications of this research could significantly influence production methods. According to the researchers, the ability to print with reduced energy consumption not only cuts down operational expenses but also lessens the environmental footprint of the manufacturing process. This innovation paves the way for copper 3D printing to be accessible to a broader range of manufacturers.
“This approach allows even commercial machines with relatively low laser power to print copper, effectively democratizing the technology and broadening access,” stated Dan Flowers, leader of the Energy Security Program. He expressed hope that this advancement will enhance the industry’s ability to leverage copper in sophisticated manufacturing applications.
“More efficient copper printing contributes to advancements in various clean energy and decarbonization solutions, ranging from heat exchangers to catalysis,” Flowers remarked. “The LLNL community, as well as our low-carbon energy objectives, will greatly benefit from these capabilities.”
The improved absorptivity and enhanced powder dynamics may also lead to the creation of high-quality printed components with increased relative densities. In their studies, the researchers attained relative densities reaching up to 92% with half the energy input compared to other printed copper components, and over 99% with greater energy inputs, suggesting the potential for developing stronger and more resilient metal parts.
The team is currently focused on investigating how nanotexturing influences the mixing of elemental powders, especially for materials that typically require significantly different melting energies.
For further details:
Ottman A. Tertuliano et al, High absorptivity nanotextured powders for additive manufacturing, Science Advances (2024). DOI: 10.1126/sciadv.adp0003
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