Deformation based additive manufacturing of a metastable high entropy alloy via Additive friction stir deposition

Abstract

Additive friction stir deposition (AFSD) is a novel solid-state additive manufacturing process which offers unique capability of producing 3-D parts with no solidification related defects and having homogeneous, equiaxed microstructure. However, to fulfill the full potential of the process for structural applications, AFSD needs intense process optimization for a wide range of alloys with excellent mechanical properties. The present study explored AFSD of a novel metastable high entropy alloy (HEA), CS-HEA (Fe 40 Mn 20 Co 20 Cr 15 Si 5 (at.%)), enabled with transformation and twinning induced plasticity during deformation. Intense shear deformation at elevated temperature and strain rate during AFSD led to the operation of restoration mechanisms such as recovery, recrystallization, and grain growth which resulted in refined grains with excellent strength and work hardenability. The average grain size for as-deposited CS-HEA is 3.0 ± 0.5 µm, and the average tensile yield strength of as-deposited CS-HEA is 450 ± 20 MPa. The microstructural variation and mechanical response of the alloy as a function of process parameters were correlated to the AFSD process variables, phase transformation, and recrystallization kinetics. Further, the interaction between recrystallization kinetics and transformation kinetics on microstructural evolution of the material was explored. Additionally, the microstructure and stacking fault energy of the alloy were used to predict the mechanical response of the deposited material using a five parameter work hardening model. • A Metastable high entropy alloy, CS-HEA is additively manufactured using a deformation based additive manufacturing process – additive friction stir deposition. • Operation of discontinuous dynamic recrystallization during AFSD deposition of CS-HEA led to formation of refined equiaxed grain structure in as deposited state. • Variation in AFSD processing parameters affected the microstructure of the alloy, higher heat input resulted in larger grain size with higher fraction of high temperature γ phase. • γ→ε transformation and twinning in ε phase provided excellent strength-ductility combination in as deposited state.