Characterisation of Inhomogeneity and Anisotropy in Cold Metal Transfer Wire Arc Directed Energy Deposition of Austenitic Stainless Steel

Abstract

Wire Arc Directed Energy Deposition (WA-DED), also known as Wire Arc Additive Manufacture (WAAM), is a niche additive manufacturing technique for metals that is increasingly offering a competitive advantage to traditional forging and casting methods. Characteristics of WA-DED are high deposition rates and feedstock that is inexpensive compared to powder processes, making it highly efficient for manufacture of large components. The aim of the research is to investigate and characterise the homogeneity and anisotropy in material strength of AISI 316LSi austenitic stainless steel produced using wire-arc directed energy deposition (WA-DED).

The study employed a comprehensive experimental approach to assess the effects of heat input, arc mode, and thermal management strategies on the homogeneity and anisotropy in mechanical performance of the deposited material. A WA-DED system was specified, procured, calibrated, and commissioned to have capabilities as close to an industrial system as possible. An active cooling system was implemented to mitigate heat accumulation. Samples were subjected to detailed microstructural analysis using Optical Microscopy, Electron Backscatter Diffraction (EBSD), and Energy Dispersive Spectroscopy (EDS), followed by mechanical testing to evaluate properties such as Young’s modulus, yield strength, ultimate tensile strength, ductility, and microhardness.

A distinct heterogeneity in mechanical performance was observed. This was linked to a coarsening of microstructure, leading to less texture. Furthermore, an increase in δ-ferrite and the intermetallic σ-phase with distance from the substrate. This led to the general trend with distance from the substrate of a decrease in the Young’s modulus in the horizontal direction, an increase in the yield strength, an increase in the ultimate tensile strength and a significant reduction in ductility. This led to substantial variation in isotropy as well as a more general heterogeneity in mechanical performance. Conversely, it was shown that although there was variation in microhardness along the height of the wall, this could not be correlated to any microstructural findings. However, the use of an active cooling system significantly increased the microhardness.

Date of Award	7 May 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Alborz Shokrani Chaharsooghi (Supervisor), Stephen Newman (Supervisor) & Vimal Dhokia (Supervisor)