Laser powder bed fusion of Cu alloys: from process parameter optimization to oxidation analysis

Abstract

This dissertation investigates two Cu-based alloy systems, Cu–Cr–Zr and Cu–Ag, with distinct research focuses for each alloy. The first part focuses on Cu–Cr–Zr, aiming to optimize laser powder bed fusion (LPBF) processing conditions and evaluate mechanical performances of the alloy under high strain rate loading. The second part focuses on Cu–Ag, with the goal of understanding its oxidation behavior at high temperatures, particularly when processed by LPBF. Through a systematic approach, this work establishes direct links between processing parameters, microstructural evolution, and functional properties, providing critical insights into tailoring these alloys for advanced structural and functional applications. In the first part, a comprehensive multi-stage statistical optimization is applied to identify the optimal LPBF process parameters to maximize relative density and minimize surface roughness of Cu–Cr–Zr. A Plackett-Burman design (PBD) is first employed to screen 23 process parameters and identify the most influential factors. The key parameters, including laser power, scanning speed, hatch spacing, and layer thickness, are then fine-tuned using a central composite design (CCD) to develop predictive models for both relative density and surface roughness. The optimized process achieves near-full densification, with relative density exceeding 99.5％ and surface roughness below 15 μm. To evaluate dynamic mechanical behavior, the optimized Cu–Cr–Zr samples are subjected to high strain rate loading using a split Hopkinson pressure bar (SHPB), with strain rates ranging from 4400 1/s to 11300 1/s. The alloy demonstrates significant strain hardening, followed by thermal softening and adiabatic shear band (ASB) formation. At the highest strain rate of 11300 1/s, the flow stress exceeds 450 MPa, while the alloy maintains considerable strain accommodation due to the dynamic activation of slip systems and localized deformation within ASBs. In the second part, the feasibility of in-situ alloying of Cu–Ag using LPBF is systematically investigated. Single-track experiments show that increasing scanning speed reduces melt pool size, limiting Ag dissolution, while higher laser power promotes homogeneity at the cost of increased keyhole porosity. Through process optimization, the study achieves a high relative density exceeding 99.2％, along with a uniform distribution of Ag throughout the matrix. The high-temperature oxidation behavior of optimized thin-walled and triply periodic minimal surface (TPMS) Cu–Ag components is then examined across temperatures ranging from 300 ℃ to 800 ℃. Thermogravimetric analysis (TGA) reveals a clear transition from sub-parabolic to parabolic oxidation behavior as temperature increases. At lower temperatures (300 ℃ to 600 ℃), Cu–2Ag initially oxidizes faster than pure Cu; however, its oxidation rate decreases significantly over time, ultimately resulting in lower total mass gain than pure Cu. At higher temperatures (700 ℃ to 800 ℃), Cu–2Ag exhibits superior oxidation resistance from the outset, with a slower and more stable oxidation rate throughout the exposure period. The presence of Ag shifts the oxidation mechanism toward a more protective parabolic regime, indicating the formation of a stable, adherent, and refined oxide scale. Together, these findings confirm the viability of in-situ alloying via LPBF and highlight the potential of Cu–Ag alloys for high-performance applications, particularly in environments where thermal stability and oxidation resistance are essential.