EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Wire arc additive manufacturing (WAAM) is a group of technologies suitable for producing large and/or thick parts due to high material deposition and building rates. Among many materials processed by WAAM, austenitic stainless steels, e.g. 316L, are of the most industrially relevant. The microstructure of WAAM 316L thin parts has extensively been studied, however, multiwalled WAAM 316L remains largely unexplored.

Hence, in this study, the microstructure over many beads, i.e. the repeating AM structure, of multiwalled 316L WAAM parts, produced with different building strategies, is characterized in detail, using optical microscopy with a specially designed automated stitching algorithm, scanning electron microscopy, electron back-scatter diffraction, energy dispersive X-ray analysis, and micro-indentation. The macrostructure of the part consists of large and highly-oriented columnar grains dominated by epitaxial and competitive growth. Long grains, crossing several layers, aligned with the building direction, were identified at the overlapping regions between adjacent weld beads. The long grains exhibited a strong <100> texture aligned with the building direction, demonstrating a strong influence of the building strategy on the microstructure. The microstructure consisted of two main phases, austenite matrix and ferrite with vermicular and lathy morphologies, with dispersed micron-sized silicon-rich oxide inclusions.

All this full-field microstructural information is mapped to an average bead shape to obtain maps of statistically averaged microstructure maps, by employing a novel micrographs mapping methodology for obtaining the representative repetitive microstructure cell of additively manufactured parts [1]. The statistically-averaged spatial variations across an additively manufactured part will be discussed and compared to thin-wall WAAM 316L literature.

[1] L. Palmeira Belotti, J.P.M. Hoefnagels, J.A.W. van Dommelen, M.G.D. Geers, A method for extracting average microstructure cells from micrographs of additively manufactured parts, submitted for publication (2021).