Phase Transformation and Mechanical Behavior of Laser Powder Bed Fusion (LPBF) Manufactured Co-28Cr-6Mo

Abstract

This thesis analyzes how laser powder bed fusion (LPBF) additive manufacturing print settings affect the phase transformation properties, tensile mechanical properties, microstructure, and fracture behavior of Co-28Cr-6Mo. Four different LPBF print settings were analyzed and compared in this study, with varying laser powers, scan speeds, and hatch spacings. It was found that for the phase transformation properties, Co-28Cr-6Mo's phase transformation rate from a metastable γ-phase face-centered-cubic (FCC) structure to ϵ-phase hexagonal-close packed (HCP) structure varies in speed and sensitivity to phase transformation aging temperature depending on the print setting. Phase transition time reached a minimum for all the samples at around a Tp = 850 or 860◦C. The 250W-400mm/s-100μm sample transformed at the fastest rates with the most sensitivity of these rates to aging temperature, while the 350W-800mm/s-125μm sample was one of the slowest to transform at the peak transformation temperature, but also least sensitive to changes in the aging temperature. For tensile behaviors, the print settings affected the ultimate strengths and elongations at fracture, leading to the 250W-400mm/s-100μm outperforming the other sets when mostly FCC, and 350W-800mm/s-125μm outperforming the others after the phase transformation. As the phase composition transformed from the as-built γ-phase FCC structure to the ϵ-phase HCP structure, samples became more brittle, but also had much higher resulting ultimate strengths. Analysis of the microstructure of these samples revealed that the massive phase transformation occurring in LPBF Co-28Cr-6Mo had lamellar structures of HCP forming within the FCC matrix, and that grain size decreased while orientation preference slightly increased in the samples as HCP percent increased within the phase composition of Co-28Cr-6Mo. Common fracture features were also identified for the varying phase compositions and print settings, which found that cleavage fracture structures and a surface dimpling texture were the most common features identifiable within the fracture surfaces. Varying the print settings and temperature at which samples were tensile tested modified which structures were most present in the fracture, allowing for correlation to be made between the testing conditions used, tensile properties analyzed, and the resulting features present on the sample.