Investigations of the Microstructure and Mechanical Properties of 17-4 PH ss Printed Using a MarkForged Metal X

Abstract

The Markforged Metal X (MfMX) printing machine (Markforged Inc., Massachusetts, USA) is one of the latest introduced additive manufacturing (AM) devices. It is getting popular because of its safety, simplicity, and ability to utilize various types of powders/filaments for printing. Despite this, only a few papers have so far reported the various properties and performances of the components fabricated by the MfMX printer. In this study, the microstructure and mechanical properties of MfMX-fabricated 17-4 stainless steel (ss) in the as-printed and heat-treated conditions were investigated. XRD and microscopy analyses revealed a dominant martensitic microstructure with some retained austenite phase. The microstructure is generally characterized by patterned voids that were unfilled due to a lack of fusion between the adjacent filaments. Disregarding these defects (voids), the porosity of the dense region was less than 4%. Depending on the heat treatment conditions, the hardness and tensile strength were enhanced by 17–28% and 21–27%, respectively. However, the tensile strength analyzed in this work was low compared with some previous reports for L-PBF-fabricated 17-4 ss. In contrast, the hardness of the as-printed (331 ± 28 HV) and heat-treated samples under the H900 condition (417 ± 29 HV) were comparable with (and even better than) some reports in the literature, despite the low material density. The results generally indicated that the Markforged printer is a promising technology when the printing processes are fully developed and optimized.

Microstructure and Defect Analysis of 17-4PH Stainless Steel Fabricated by the Bound Metal Deposition Additive Manufacturing Technology

“…The presence of the triangular-like defects highlights how the deposition of tracks and the following squash of the melted composite material by the extrusion nozzle play a major role in defining the shape of the pores. These findings are in line with the literature concerning the material extrusion of metals where, despite a different form of the starting feedstock (composite wire instead of composite rods), large geometrically shaped voids are found in the final part, confirming the link between the defect and the 3D-printing process itself [10,26,39].…”

“…This effect is indeed independent of the infill deposition angle and is likely to be due to the combination of printing parameters such as the deposition strategy, overlap, and density profile chosen for the fabrication. While the literature shows continuous void lines [10,26,39], Figure 7a highlights discontinuous void lines preferentially located close to the outer wall of the 3D sample and suggests a better overall performance. During the sintering process, the evaporated residual binder increases the content of the pores since this thermal process takes place without the application of pressure and there is no further infiltration with additional material.…”

“…The highly bright inclusions shown in Figure 6c were characterized by energy dispersive spectroscopy and the results showed the high presence of Nb and C (~40 ± 10 wt.% Nb, 10 ± 2 wt.% C, calculated by averaging the EDS values acquired from every bright region), suggesting that these inclusions correspond to niobium carbides (NbC). The presence of these carbides can be expected since the temperatures reached during the sintering phase and the corresponding dwell times are similar to the conditions at which solution annealing is typically performed on 17-4PH stainless steel (also known as "condition A") [26,40]. Compared to the recently published results by Akessa et al [26], niobium carbides in the sample fabricated by BMD are located in different positions across the analyzed surface, while in the samples obtained by using the ADAM technology, carbides are found on the edges of the pores [26].…”

“…The presence of these carbides can be expected since the temperatures reached during the sintering phase and the corresponding dwell times are similar to the conditions at which solution annealing is typically performed on 17-4PH stainless steel (also known as "condition A") [26,40]. Compared to the recently published results by Akessa et al [26], niobium carbides in the sample fabricated by BMD are located in different positions across the analyzed surface, while in the samples obtained by using the ADAM technology, carbides are found on the edges of the pores [26].…”

“…Pellegrini et al [23] investigated the effect of two aging treatments applied on 17-4PH samples fabricated by three material extrusion technologies, such as BMD, fused filament fabrication (FFF), and atomic diffusion additive manufacturing (ADAM). Akessa [26], Bjørheim, et al [10] and Gonzalez-Gutierrez et al [24] fabricated specimens for tensile tests, the first two with the ADAM technology and the latter with a highly filled polymeric filament of 17-4PH; the authors attributed the high standard deviation (up to 38%) of the measured tensile data to the large number of defects and to the lack of cohesion between the layers and deposited tracks. Galati et al [25] mainly evaluated the density, accuracy, and surface roughness of cubic and complex reference samples, fabricated by the ADAM technology.…”