Role of Superficial Defects and Machining Depth in Tensile Properties of Electron Beam Melting (EBM) Made Inconel 718

Abstract

Since there is no report on the influence of machining depth on electron beam melting (EBM) parts, this paper investigated the role of superficial defects and machining depth in the performance of EBM made Inconel 718 (IN718) samples. Therefore, as-built EBM samples were analyzed against the shallow-machined (i.e., only removal of outer surfaces) and deep-machined (i.e., deep surface removal into the material) parts. It was shown that both as-built and shallow-machined samples had a drastically lower yield strength (970 ± 50 MPa), ultimate tensile stress (1200 ± 40 MPa), and ductility (28 ± 2%) compared to the deep-machined samples. This was since premature failure occurred due to various superficial defects. The superficial defects appeared in two levels, as (1) notches and pores on the surface and (2) irregular pores and cracks within the subsurface. Since the latter occurred down to 2 mm underneath the surface, shallow machining only exposed the subsurface defects to outer surfaces. Thus, the shallow-machined parts achieved only 68% and 8% of UTS and elongation of the deep-machined parts, respectively. This low performance occurred to be comparable to the as-built parts, which failed prematurely due to the high fraction surface voids and notches as well as the subsurface defects.

Post-processing treatments–microstructure–performance interrelationship of metal additive manufactured aerospace alloys: A review

“…Insufficient melting of powder particles on and beneath the surfaces of the as-built parts often leads to the formation of pores/voids. Pores are also formed as a result of an interruption in the melt pool or when thermal shrinkage from the solidification cannot be efficiently fed by the molten melt/liquid [57]. The rough border defect is common with the electron beam melting-based AM processing and this defect is considered to have been due to high processing temperature and excessive heating effect [57].…”

“…Defects in MAM-produced parts usually consist of semi-molten powder particles-induced roughness, surface voids and subsurface defect [57]. These superficial defects in as-built parts can be classified as levels I and II.…”

“…These superficial defects in as-built parts can be classified as levels I and II. The Level I defects are surface pores and notches on the surface of the parts while the subsurface cracks and pores (within 2 mm underneath the surface) are the level II defects as revealed in Figure 10 [57]. Insufficient melting of powder particles on and beneath the surfaces of the as-built parts often leads to the formation of pores/voids.…”

“…Pores are also formed as a result of an interruption in the melt pool or when thermal shrinkage from the solidification cannot be efficiently fed by the molten melt/liquid [57]. The rough border defect is common with the electron beam melting-based AM processing and this defect is considered to have been due to high processing temperature and excessive heating effect [57].

Figure 10 Defects in EB-PBF produced IN718 parts: (a) outer surface defects (cracks and notches) and (b) subsurface defects (pores) [57].

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Post-processing of additively manufactured metallic alloys – A review

“…4 for PBF-EB/Alloy 718, where a large density of superficial defects, including surface voids and notches and subsurface defects, after a shallow machining led to premature failure during the tensile test. The tensile properties significantly improved after changing the printing strategy [53] and removing superficial defects using a larger depth of cut during machining [54]. The density of defects can also be reduced by application of HIP post-treatment, improving the ductility of AM materials [55][56][57].…”