Non-Equilibrium Thermodynamics Underlying Materials Processing History in Friction Stir 

Hang Z. Yu

Department of Materials Science and Engineering, Virginia Tech, USA

hangyu@vt.edu

Abstract

Characterized by rapid and extensive shear flow at elevated temperatures, additive friction stir deposition has emerged as a competitive solid-state additive technology that enables large-scale additive manufacturing, structural repair, and recycling. Unlike fusion-based metal additive manufacturing, which produces solidification microstructures, this process gives rise to fine, equiaxed microstructures with forged- or wrought-like mechanical properties. Owing to its processing complexity, however, establishing a quantitative understanding for the process-microstructure-property linkage has remained elusive. This article examines the thermal and deformation histories across various stages of additive friction stir deposition, including heating, rapid shear flow at elevated temperatures, remaining at elevated temperature with minimal deformation, and cooling — with a focus on the underlying non-equilibrium thermodynamic aspects. The discussion emphasizes the challenges in predicting material evolution under external force-driven and far-from-equilibrium relaxation conditions, which are not adequately described by linear irreversible thermodynamics. Experimental approaches are proposed to uncover kinetic parameters in such scenarios, and the unique dynamic phenomena enabled by the driven conditions are discussed, underscoring the potential to leverage additive friction stir deposition as a scalable non-equilibrium processing and manufacturing tool.

Keywords:

Friction stir; metal additive manufacturing; non-equilibrium thermodynamics; rapid shear; driven conditions; kinetics