Thermo-mechanical process variables driven microstructure evolution during additive friction stir deposition of IN625

Abstract

Additive friction stir deposition is an innovative technique for solid-state additive manufacturing involving complex thermo-mechanical processes such as frictional heating, plastic deformation, and viscoplastic material flow, which repeats layer by layer during material deposition. The present study is an attempt to understand the relationship between the complex thermo-mechanical process attributes and the resultant microstructure in additive friction stir deposition of IN625. The estimation of thermo-mechanical process attributes, primarily temperature and strain rate during multi-layer deposition was based on the already established thermo-pseudo-mechanical description of friction stir based processes. The complex thermo-mechanical interaction associated with multi-layer material deformation, deposition, and flash formation can be attributed to variations in the average deposition temperature and the strain rate. Microstructural examination of the additive friction stir deposited sample was performed using electron backscatter diffraction. As a result of the additive friction stir deposition associated thermo-mechanical conditions, the microstructure exhibited fine equiaxed grains in the plane along the build direction along with bands of very fine grains within the interface regions between deposited layers. The observation of fine equiaxed grains and a lower degree of in-grain misorientation in the additively fabricated sample pointed to dynamic recrystallization as the governing restoration mechanism during the deposition process. The grain size refinement during additive friction stir deposition of IN625 resulted in substantial augmentation in the yield strength compared to IN625 feed material and as-cast IN625.