Microstructure Formation Mechanisms of Spinodal Fe–Cu Alloys Fabricated Using Electron-Beam Powder Bed Fusion

Abstract

We studied the microstructure formation mechanisms of spinodal Fe–10%Cu alloys fabricated using electron-beam powder bed fusion with various scanning speeds. Cross-correlation electron backscattered diffraction analysis was utilized to investigate the crack initiation and propagation mechanisms related to dislocation density and residual stress in the as-built alloys. The as-built alloys with low scanning speeds have equiaxed microstructures without lack-of-fusion (LOF) defects and micro-cracks. As the scanning speed increased, the grain size and Cu particle size decreased, and micro-cracks initiated at the edge of LOF defects and then grew along the grain boundary parallel to the built direction (BD). In addition, coarse Fe3O4 particles formed on the boundary caused a decrease in thermal conductivity and tensile strength. A strong compressive residual stress parallel to the BD acts as a driving force for micro-crack propagation. The rapid cooling rate enhances local dislocation density, and lattice rotation causes micro-crack growth, thereby deteriorating mechanical and thermal properties. Therefore, the scanning speeds should be controlled below 2000 mm/s to obtain good strength and superior conductivity of the spinodal Fe-Cu alloy.

Keywords: Fe‒Cu alloys, Electron-beam powder bed fusion, Cu particles, Lack-of-fusion defects, Micro-crack, residual stress