In thermal management and electrical conductivity applications, it is critical to have the ability to manufacture copper (Cu) parts that are fully dense, highly thermally/conductively conductive and have excellent mechanical properties. Additive manufacturing (AM), or 3D printing, provides an unprecedented opportunity to produce Cu parts with complex geometries. However, pure Cu is highly reflective of infrared lasers, so pure Cu parts printed with commonly used laser additive manufacturing equipment tend to have high porosity, which reduces their mechanical and thermal/electrical conductivity. Although high densities of pure copper parts can be produced by additive manufacturing equipment equipped with short-wavelength green lasers or electron beams, the inherent low strength of pure copper and its inability to resist thermal softening have prevented the application of laser additively manufactured copper parts at high mechanical loads and high temperatures.
In order to solve the above problems, the team of Prof. Xingxing Zhang from the University of Queensland, Australia, in collaboration with Prof. Christopher Hutchinson from Monash University, Prof. Julie Cairney from the University of Sydney, Prof. Miao-Quan Li from Northwestern Polytechnical University, Prof. Xiaoxu Huang from Chongqing University, Prof. Jesper Henri Hattel from the University of Science and Technology of Denmark, and Prof. Mark Easton from RMIT University, have been working together to produce copper parts with high densities. Prof. Mark Easton, RMIT University, and other teams have collaborated to propose a design strategy for 3D printing of high-strength and high-conductivity copper. The key to the design strategy is to select an additive particle that is homogeneously mixed with the pure copper powder to ensure that it enhances the laser absorption of the pure copper when the laser interacts with the powder. In addition, the additive particles are dispersed in the copper matrix by dissolving into the melt pool when the powder is melted and re-precipitating during solidification, thus strengthening the copper without significantly reducing its thermal/electrical conductivity. The screening criteria for the additive particles are as follows: (1) the solid solubility of the constituent elements of the particles in copper should be minimal to minimize their adverse effect on thermal/electrical conductivity and to maximize the potential for reprecipitation of the nanoparticles upon solidification; (2) the particles should have a low melting point to facilitate their melting into the molten pool and to weaken the potential for coarsening of the reprecipitated nanoparticles during solidification; (3) the particles should have a low wetting point in the liquid copper should have a low wetting angle to prevent agglomeration of the reprecipitated nanoparticles in liquid copper. Based on this design idea, we found that lanthanum hexaboride (LaB6) meets the above criteria. By adding trace amounts of LaB6 nanoparticles, high density and high performance copper and its geometrically complex parts were realized by laser additive manufacturing.
The related work was published in the top international journal Nature Communications under the title of "Manufacturing of high strength and high conductivity copper with laser powder bed fusion". Communications. Dr. Yinggang Liu (now a professor at the School of Aeronautics, Northwestern Polytechnical University) and Dr. Jingqi Zhang of the University of Queensland are the co-first authors, while Prof. Mingxing Zhang of the University of Queensland, Dr. Ranming Niu of the University of Sydney, and Prof. Christopher Hutchinson of Monash University are the co-corresponding authors.
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Additive manufacturing (AM), or 3D printing, enables the rapid fabrication of geometrically complex copper parts and has a wide range of applications in thermal management and electrical conductivity. However, pure copper is soft, while its high reflectivity to infrared lasers typically results in 3D printed parts with high porosity, which reduces their performance. Although additive manufacturing using green lasers or electron beams can print pure copper parts with high densities, the inherent low strength of pure copper at room temperature and its inability to resist thermal softening limit the application of additively manufactured copper parts subjected to high mechanical loads and high temperatures. Adding elements such as Cr, Co, Fe, and Zr to pure copper by alloying it can increase laser absorption and strengthen the substrate, but this method significantly reduces the thermal/electrical conductivity of copper due to their high solid solubility in copper. Another approach is to add external particles (Al2O3, TiB2, etc.) that are immiscible with pure copper to strengthen the copper while maintaining high thermal/electrical conductivity. However, in practice, due to nanoparticle agglomeration, it proves to be extremely difficult to obtain significant strengthening without compromising the ductility and damage tolerance. As a result, alloying or adding incompatible external particles can increase strength and improve laser absorption properties, but usually results in a significant decrease in thermal/electrical conductivity and ductility.3D printing of high-strength, high-conductivity copper parts remains a pressing challenge.
Here, we demonstrate a laser additive manufacturing method for preparing high-density, high-performance copper parts by adding a small amount of lanthanum hexaboride (LaB6) nanoparticles to pure copper powder by laser powder bed fusion (L-PBF). The key to this method is the introduction of appropriate particles into the pure copper that enhance the laser absorption of the pure copper, followed by dissolution in the melt pool and reprecipitation during solidification. LaB6 was chosen based on its high laser absorption, good electrical conductivity, low melting point, and low wetting angle with liquid copper.LaB6 has a dual role. First, it improves the laser absorption of pure copper, thus favoring better fusion of the powder. Second, its ability to melt during powder fusion and subsequently reprecipitate as diffusely distributed nanoparticles during solidification not only enhances the strength of the material, but also maintains greater ductility and high thermal/electrical conductivity.1wt% LaB6-doped copper exhibits a yield strength of 346.8 MPa, 3.7 times higher than that of pure copper, as well as a fracture ductility of 22.8%, 98.4%, and a high thermal/electrical conductivity of 1.4%.1wt% LaB6-doped copper is also a good candidate for the IACS (International Annealed Pure Copper Scheme). IACS (International Annealed Copper Standard) electrical conductivity, a thermal conductivity of 387 W/m-K, and excellent resistance to softening at 1050°C, which is close to the melting point of pure copper. In addition, the applicability of the method to geometrically complex parts is also demonstrated in this study. The newly developed LaB6-doped copper fills an important gap in 3D printing of alloys and is suitable for high mechanical loads and high temperature environments. Since uniformly dispersed nanoparticles are commonly used to strengthen metallic materials, this design strategy of reprecipitation upon melting and solidification can be extended to other alloy systems for the development of print-ready, high-performance materials.
Figure 1 Microstructure and laser reflectivity test results of pure and LaB6-doped copper prepared by laser powder bed melting
Figure 2 Nanoparticle analysis of LaB6-doped copper prepared by laser powder bed melting
Figure 3 APT elemental characterization of LaB6 doped copper prepared by laser powder bed melting
Figure 4 Mechanical properties and electrical conductivity test results of LaB6-doped copper prepared by laser powder bed melting.
Figure 5 Compression test results of LaB6 doped copper dots prepared by laser powder bed melting
