Abnormal mechanochemical effect in ultraprecision machining of an additively manufactured precipitation-strengthened high-entropy alloy

Abstract

Recently, researchers have explored the use of precipitation strengthening and finer microstructures with high-density dislocations in additive manufacturing to produce high-entropy alloys (HEAs) with adjustable properties. However, the inherent surface roughness and lack of machinability research in AMed HEAs limit their engineering applications. In this study, we systematically investigated the microstructural characteristics, mechanical properties, and machinability of Fe29.3Co28.7Ni28.6Al6.8Ti6.6 (at.%) HEAs with three different structures: single FCC phase cellular (SPC), dual precipitation-strengthened (DPS), and single precipitation-strengthened (SPS). These structures were fabricated by selective laser melting and isothermally annealing at 780 and 940 °C. Compared to SPC HEA, DPS HEA exhibits a significant increase in yield strength and ultimate tensile strength but with a dramatic sacrifice in ductility. SPS HEA exhibits similar mechanical properties to SPC HEA due to the pronounced coarsening of L21 precipitates. The ultraprecision machining micro-cutting test showed that SPC HEA had a significant mechanochemical effect, as evidenced by a sharp drop in cutting force for inked workpieces, but not DPS HEA. An abnormal finding was that the negligible reflection of cutting force for SPS HEAs suggested a negative mechanochemical effect, even though SPS HEA had equally excellent plasticity like SPC HEA. It was found that nanocrystallization-induced strength enhancement and ductility reduction of SPS HEA lead to chips’ deformation dominated by shear avalanche rather than chip folding of SPC HEA, which involves the reduction of surface energy and friction of chips’ interfaces. Overall, these results and our research findings may guide the machining of AMed precipitation-strengthened HEAs and accelerate their engineering application.