Microstructure and Mechanical Properties of Novel As-Cast and Additively Manufactured CoCrFeMo Alloy for Dental and Maxillofacial Implants

Cobalt-chromium alloys are mainly used in dental and reconstructive surgery thanks to their mechanical properties and corrosion resistance. These prostheses and superstructures are traditionally made by casting and milling processes. The conventional cobalt-chromium alloy contains around 60% cobalt. There has been a recent focus on reducing cobalt consumption. This is primarily due to its high cost and potential toxicity when released into the body. In this work, we focus on replacing a high amount of cobalt with iron which is a more economical candidate. Additionally, production methods can also reduce cobalt consumption. Rather than casting and milling implants and prostheses, Additive Manufacturing (AM) can significantly reduce material waste. Laser Powder Bed Fusion (LPBF) is one of the additive manufacturing techniques that provides several benefits over the conventional casting method.<br/><br/>The objective of this paper is to investigate and compare the phases, microstructure and mechanical properties of the newly designed CoCrFeMo alloy fabricated by Laser Powder Bed Fusion (LPBF) with casting using techniques such as X-ray diffraction, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Vickers micro-hardness (HV), tensile and bending fatigue properties.<br/><br/>The findings indicate that the as-cast specimen consists of an austenitic matrix (fcc) and precipitates of sigma phase. Moreover, TEM examination of the fcc matrix revealed the presence of numerous stacking faults. In contrast, the sigma phase formation in the as-printed specimen was fully suppressed due to the high cooling rate of the LPBF process leading to the formation of a bcc phase as the matrix along with a heavily faulted phase.<br/><br/>Furthermore, the effect of the fabrication process, phase and microstructure evolution on the mechanical properties of the specimens was investigated. The cast sample exhibited higher hardness due to the formation of sigma phase. The as-printed specimen, despite the presence of high residual stress, showed better tensile properties with an ultimate tensile strength of 1200 MPa and 17% elongation, as well as significantly better fatigue strength and fatigue life in comparison to the cast samples. The fatigue strength of the as-printed specimen was 33% higher than that of the cast one. Additionally, fracture surface analysis revealed a transition from an almost brittle in the as-cast sample to a more ductile fracture in the failure zone of the as-printed specimens, which correlates with the observed mechanical properties.<br/><br/>The results highlight the potential of using the LPBF process to produce newly developed CoCrFeMo alloys with tailored microstructures and promising mechanical properties based on standard criteria and the loads applied to implants in the body.<br/>Key words: CoCrFeMo alloy, Additive manufacturing, Laser powder bed fusion, Microstructure, Tensile test, Fatigue properties <br/><br/><i>The authors are grateful for the financial support provided by the Norwegian Research Council (NFR) from the bilateral cooperation project between India-Norway (OF-10716). We acknowledge Murugaiyan Amirthalingam and Jag Parvesh Dahiya from Indian Institute of Technology (IIT) Madras, for optimizing the printing parameters for the given alloy and printing the samples. Moreover, we would also like to thank Mona Wetrhus Minde and Wakshum Mekonnen Tucho for valuable discussions. </i>