Investigation on microstructural and texture evolution in additive friction stir deposited Inconel 718 alloy

Abstract

Friction stir-based additive manufacturing has shown promising results and has the full potential to fabricate near-net-shaped 3D metallic structures. Most existing research has focused on discontinuous multilayer deposition of nickel-based superalloys using friction surfacing (FS). In this study, a semi-automatic friction stir welding machine operating in vertical position control mode was used to continuously build up multiple layers of Inconel 718 alloy. The process parameters were optimized based on the single-layer deposition of Inconel 718 alloy with different parameter combinations (rotational speed, axial feed, and traverse speed), which resulted in uniform microstructure and hardness. At high axial feed, changes in process path direction in the multilayer process resulted in variations in the axial force and temperature during deposition. Electron back-scattered diffraction (EBSD) analysis revealed the occurrence of discontinuous dynamic recrystallization (DDRX), which led to refined grains during multilayer deposition. Among the evaluated axial feeds (10, 13, and 16 mm/min), 16 mm/min yielded defect-free deposition with optimal microstructure and hardness. Microstructure analysis also revealed banded structures with variations in the grain size, resulting in hardness variation across the layers. The high axial feed showed less variation in grain size in all layers, while few coarser grains were observed at the maximum axial load. The microtexture analysis revealed that at 13 and 16 mm/min axial feed, C texture component {001}〈110〉 evolved in all layers, and the selective grain growth mechanism resulted in a sharp recrystallization texture.