Size effects on microstructure and mechanical properties of additively manufactured copper–chromium–niobium alloy

This study quantifies size effects on microstructure and mechanical properties of Laser Powder Bed Fusion (L-PBF) Copper–Chromium–Niobium Alloys, specifically GRCop-42 (Copper-4at% Chromium-2at% Niobium Alloy). Thin-wall structures made of copper-based alloys are desirable for applications requiring a combination of high thermal conductivity and strength. Additive manufacturing offers the ability to produce copper alloy thin-wall structures without the need for complex tooling. However, consideration must be given to the effect of wall thickness on solidification rate and process as it can affect material and part characteristics including the microstructure, porosity, and surface roughness. This investigation aims to determine which of these characteristics are most influenced by variations in wall thickness and how they affect the mechanical properties. Additionally, we investigate how these characteristics and their influence on mechanical properties are changed by heat treatment. The results show that surface roughness, grain size, and texture do not appear to be greatly influenced by the build thickness, while porosity appeared to be highly dependent on thickness. By increasing wall thickness from 0.7 mm to 2.0 mm, porosity decreases by 74.7% in the as-built condition and 99.7% in the hot isostatic press (HIP) condition. The variability in porosity has a notable effect on the mechanical properties of the thin-wall structures: yield strength and ultimate tensile strength decrease by more than 20% for both as-built and HIP samples as thickness decreases from 2.0 mm to 0.7 mm. This decrease in strength appears to be attributable to the increased porosity in thin-wall samples, as scanning electron microscopy (SEM) images show crack networks linking internal voids along the fracture surface.