Abstract
Laser Powder Bed Fusion (LPBF) of copper-based alloys provides a promising route for fabricating geometrically complex, high-performance components for energy, electronics, and thermal management applications. However, achieving a balance between mechanical strength and electrical conductivity remains challenging due to rapid solidification, non-equilibrium solute redistribution, and microsegregation inherent to LPBF processing. In Cu–Cr systems, residual Cr solubility within the Cu matrix and limited segregation control can compromise both properties.
This dissertation develops a microalloying-enabled solute redistribution framework to intentionally manipulate solidification pathways in LPBF Cu-based alloys. Binary Cu–1.2 wt% Cr and ternary Cu–1.2 wt% Cr–0.01 wt% Zr alloys were investigated. The central hypothesis is that trace Zr modifies thermodynamic phase equilibria and solidification response, thereby enhancing Cr rejection during rapid solidification and promoting purification of the Cu dendritic matrix.
CALPHAD simulations and focused pseudo-ternary phase diagram analysis demonstrate that minor Zr additions significantly reduce solid-state Cr solubility by steepening the solidus boundary with minimal impact on the liquidus. This shift increases the effective partitioning driving force during rapid directional solidification, amplifying Cr microsegregation into interdendritic regions. Based on these findings, a solidification response model is proposed to describe Zr-induced enhancement of Cr redistribution under LPBF conditions.
Microstructural and chemical characterization—including SEM, ToF-SIMS, and KPFM—validates the predicted segregation behavior. The ternary alloy exhibits wider interdendritic regions (0.6–1.2 μm vs. 0.4–1.0 μm in the binary), higher Cr-rich particle density, and greater nanoscale work-function contrast, confirming intensified Cr segregation and cleaner Cu dendrites.
Nanoindentation further corroborates the mechanism. The ternary alloy shows a markedly greater hardness contrast between dendritic and interdendritic regions compared to the binary alloy, providing quantitative evidence that Zr-induced segregation strengthens Cr-rich interdendritic regions while purifying the Cu matrix. This establishes a direct microstructure–property linkage.
To assess the generality of the proposed framework, Hafnium (Hf) was introduced as a secondary microalloying element. The segregation amplification and phase redistribution trends observed in Cu–Cr–Hf alloys mirror those of the Zr-modified system, providing mechanistic validation of the thermodynamics-driven segregation enhancement model.
In summary, this work establishes a thermodynamics-guided microalloying strategy for controlling solute redistribution in LPBF Cu alloys. By integrating computational modeling, solidification theory, multiscale characterization, and property validation, this dissertation presents a predictive framework for designing high-performance copper alloys through controlled microsegregation engineering under rapid solidification conditions. The insights extend beyond Cu-based systems and contribute broadly to alloy design principles in additive manufacturing.
