A Study on Metallographic and Machining Characteristics of Functionally Graded Material Produced by Directed Energy Deposition

Abstract

Directed energy deposition (DED) stands as a key process in metal additive manufacturing (AM) and offers the unique capability of creating functionally graded materials (FGMs). FGMs have garnered significant interest in high-value industries by advantages such as performance optimization, reducing material defects, and resolving joining issues. However, post-processing remains a crucial step, indicating a need for further research to understand the machinability of FGMs. This paper focuses on the characteristics analysis of fabricating and machining an FGM based on stainless steel 316L (SAE 316L) and Inconel 718. The FGM was fabricated by starting with SAE 316L at 100 wt.% and adjusting the composition ratio by incrementally increasing Inconel 718 by 20 wt.% while simultaneously decreasing SAE 316L. Following the FGM fabrication, microstructure and mechanical properties were comprehensively analyzed by hardness testing, optical microstructure measurements, energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). To investigate the post-processing aspects, end-milling experiments were conducted using two distinct milling methods (upward and downward milling) and machining paths (from SAE 316L towards Inconel 718, and vice versa). The mean cutting force peaked at 148.4 N in upward milling and dipped to 70.5 N in downward milling, and tool wear measurements further provided insights into the optimal milling direction when working with an FGM of SAE 316L and Inconel 718.