3D printing can quickly manufacture copper components with complex geometries for electrical and thermal management applications. However, pure copper or copper alloys produced by 3D printing often have low strength or low conductivity at room and high temperatures. 3D Printing Technology Reference noted that a joint team from the University of Queensland, the University of Sydney, the Technical University of Denmark, Northwestern Polytechnical University, Chongqing University, Royal Melbourne Institute of Technology, and Monash University (seven universities) published an article titled “Manufacturing of high strength and high conductivity copper with laser powder bed fusion” in Nature Communications in February 2024.
It should be noted that Yingang Liu, Jingqi Zhang, Yu Yin, and Xiaoxu Huang (Huang Xiaoxu) of the joint team just published a study on high-performance titanium alloy 3D printing in Science as co-authors in the same month. This Nature Communications article has also made significant research progress. The 3D printed reinforced pure copper developed is superior to the pure copper printed by infrared laser, green laser, and electron beam, and is stronger than GRCop-42 and GRCop-84 developed by NASA!
Pure Cu has a high reflectivity for infrared lasers, so pure Cu parts made by the most commonly used LPBF 3D printers often have high porosity, resulting in poor mechanical and electrical properties. Although additive manufacturing using green lasers or electron beams is able to produce relatively dense pure copper components, the inherent low strength of pure copper at room temperature and its inability to resist thermal softening hinder the application of additively manufactured copper parts in conditions requiring mechanical loads and high temperatures.
Alloying Cu with elements such as Cr and Zr increases laser absorptivity and improves strength, but due to their high solid solubility in Cu, this method reduces the conductivity of the material. Another approach is to add immiscible heterogeneous particles to strengthen while maintaining high conductivity. It has proven extremely difficult to achieve sufficient dispersion and uniform distribution of externally added nanoparticles to improve material strength without agglomeration of particles and reducing ductility and impairing conductivity. The challenge of achieving both high strength and high conductivity in 3D printed copper parts limits its applications that require a good balance of mechanical and physical properties.
Design strategy
In this Nature Communications article, researchers present a design strategy for high-strength, high-conductivity copper 3D printing by uniformly dispersing a small fraction of lanthanum hexaboride (LaB6) nanoparticles in pure copper via laser powder bed fusion (L-PBF). The study demonstrates that trace additions of LaB6 improve the machinability, strength, and thermal stability of the pure copper L-PBF process while maintaining high conductivity. The proposed strategy could extend the applicability of 3D-printed copper components to more demanding conditions where high strength, high conductivity, and thermal stability are required.
The key feature of the design strategy is the selection of additives whose constituent elements have negligible solubility in solid copper (thus having negligible adverse effects on conductivity), but dissolve in the molten pool during laser melting (thus having a less high melting point), and subsequently reprecipitate with very fine dispersion during solidification (thus providing excellent reinforcement). The criteria are as follows:
The solid solubility of the constituent elements of the particles in Cu should be negligible to minimize their detrimental effects on conductivity and maximize the reprecipitation of nanoparticles;
The particles should have a relatively low melting point to maximize the chance of dissolution in the molten pool and minimize the coarsening of reprecipitated nanoparticles during solidification;
The particles should have a low wetting angle with molten copper to minimize the aggregation of nanoparticles in liquid copper.
Research Findings
Following this design strategy, the researchers identified LaB6 as a suitable additive candidate. LaB6 meets the criteria because it has a low melting point, minimal solid solubility of the constituent elements in copper, and a small wetting angle with molten copper. The initial LaB6 particles added to the pure copper powder raw material showed irregular morphology with sizes up to 300nm. Laser reflectivity testing showed that pure copper powder showed extremely high reflectivity in the infrared laser range, and the reflectivity decreased significantly when 1.0wt% LaB6 nanoparticles were incorporated. This reduction can be attributed to two factors: the inherent low laser reflectivity of LaB6 and the introduction of LaB6 nanoparticles, which enhance the surface roughness of pure Cu particles and promote multiple reflections within the powder bed.
The experimental results have indeed achieved improvements: pure copper without LaB6 has problems such as discontinuous molten pool, unfused, porosity, and surface roughness even at high laser power; after adding LaB6 nanoparticles, the density of the material has been improved, and the grains are relatively large. The researchers used Micro-CT and EBSD to prove this.
SEM, EDS, XRD, TEM and other tests found that uniformly distributed and non-agglomerated LaB6 nanoparticles were found in the material. The particles showed a rectangular shape with an average size of less than 100nm and had an incoherent interface with the Cu matrix. In addition, SEM tests found that the dissolution of La and B in the Cu matrix was very limited, which minimized the adverse effects of the solute on conductivity. XRD analysis further showed that the lattice parameters of the Cu matrix were the same as those of pure Cu, indicating that the La or B dissolved in the solid Cu matrix was negligible.
Therefore, it is concluded that the LaB6 particles with irregular shapes and large sizes added externally are dissolved in the molten pool during the melting process, and the observed LaB6 nanoparticles are the products of reprecipitation during the solidification process. Further experiments also exclude the possibility of solid-state phase change caused by thermal cycling.
Excellent mechanical properties and conductivity
3D printing technology reference notes that the 1.0LaB6-reinforced pure copper developed by the joint team has excellent mechanical properties and other physical properties. Specifically:
Stronger than pure copper 3D printed by near-infrared laser and green laser
In order to evaluate the effect of LaB6 nanoparticles on mechanical and conductive properties, the researchers conducted tensile tests and conductivity measurements on pure copper and reinforced pure copper 3D printed by L-PBF process.
The yield strength of pure copper is 73±2MPa, the ultimate tensile strength is 121±1MPa, the elongation at break is 10.8±1.1%, the conductivity is 88.3%IACS, and the corresponding thermal conductivity is calculated to be 347W m−1 K−1. The low strength and low conductivity of pure copper are due to the presence of high-density pores. The yield strength of the reinforced pure copper was 347 ± 2 MPa, the ultimate tensile strength was 412 ± 7 MPa, the elongation at break was 22.8 ± 1.2%, and the electrical conductivity was 98.4% IACS. The thermal conductivity was calculated to be 387 W m−1 K−1.
Although high-density pure copper parts can be manufactured using green lasers, the strength of these parts is still significantly lower than that achieved in this study. The significant increase in strength is mainly attributed to the dispersion strengthening of LaB6 nanoparticles. The improvement in ductility is partly attributed to the improved strain hardening caused by the uniform dispersion of shear-resistant nanoparticles, as well as the reduction of porosity and defects. The improvement in electrical conductivity stems from the higher density of the manufactured parts and the minimal negative impact of the uniformly dispersed LaB6 nanoparticles.
Better than GRCop-42 and GRCop-84 copper alloys
Overall, the combination of high strength and high ductility with high electrical conductivity makes 1.0LaB6-reinforced pure copper superior to conventional and additively manufactured pure copper, copper alloys, and copper-based composites. Even the NASA-developed GRCop-42 alloy and GRCop-84, whose electrical conductivity of 85% IACS and 75% IACS are still the results of this study. In addition to higher electrical conductivity, 1.0LaB6-strengthened pure copper Cu and 3d printing beryllium copper also exhibits higher strength compared to GRCop-42 and GRCop-84 alloys, with the former alloys showing yield strengths in the range of 172-208MPa.
Performance group comparable to conventionally manufactured copper materials
In addition, while 1.0LaB6-strengthened pure Cu shows a unique combination of electrical conductivity and mechanical properties comparable to copper-based composites produced by conventional processes, coupled with the greater design freedom provided by additive manufacturing, it is very attractive for practical applications that require high strength, electrical/thermal conductivity, and geometrically complex copper components.
Excellent softening resistance
In addition to room temperature mechanical properties, 1.0LaB6-strengthened pure copper also exhibits improved thermal stability at high temperatures. Copper and its alloys usually suffer severe strength loss in high temperature environments, which eventually leads to service failures. The materials studied here show excellent resistance to softening, retaining 80% of the ultimate tensile strength and 28% of the elongation at break even after annealing at 1050 °C.
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In summary, the joint team demonstrated a route to reliably manufacture high-density copper parts with high strength and high conductivity by adding a small amount of LaB6 particles to 3D-printed pure copper feedstock. The key to this method is the introduction of appropriate particles into pure Cu, which enables them to dissolve in the molten pool and reprecipitate uniformly during solidification. The newly developed 1.0LaB6-reinforced 3D-printed pure copper fills a major gap in the field of metal alloy 3D printing and can be used in applications requiring demanding mechanical and electrical/thermal conductivity properties. Since uniformly dispersed nanoparticles are often used to strengthen metallic materials, this design strategy and the associated dissolution and reprecipitation during solidification can be extended to other alloy systems to develop other high-strength materials for additive manufacturing.