BJAM Printing of Mo-Mn-C Alloy Powders

WA low-alloy steel powders (Rio Tinto Metal Powder, Sorel-Tracy, QC) were used in this study. The chemical composition which was determined by inductively coupled plasma optical emission spectroscopy was given in Table 1. The volume percentile powder size distribution (PSD) and sphericity were analyzed using a particle size and shape analyzer (Camsizer X2, RETSCH). The surface morphology was observed using scanning electron microscopy (SEM) (Vega3, Tescan).Table 1

Chemical composition of the powders studied.

Table 1

Items	Mn	Mo	C	O	S	P
Content/wt.%	0.20	0.80	0.60	0.074	0.01	≤0.01

The BJAM process was carried out in an BJAM system (M-Flex, ExOne) printer with the following processing parameters: 85 μm layer thickness, recoating speed of 300 mm/s, roller speed of 350 rpm, binder saturation of 35%, drying speed of 15 mm/s, and emitter power of 60%. Green parts were printed into a cuboidal shape of 10 × 10 × 10 mm³. For each printing batch, 16 replicates were printed. After printing, the samples were cured at 180 °C for 12 h in a curing oven (CT-333, JPW) under flowing argon, followed by the depowering step.
Thereafter, the specimens were subjected to the debinding and sintering treatments in a tube furnace (GSL-1600X, MTI Corp.) under a 5% H₂–95% N₂ atmosphere. The specimens were oriented during sintering in the furnace similarly to the as-printed orientation, with the x-y printing surface coinciding with the horizontal plane and with the z-printing direction aligned with the vertical orientation. Debinding was performed at 400 °C for 2 h (h), while sintering was conducted using two distinct thermal profiles, namely, direct-sintering and step-sintering. Schematic illustrations of the direct-sintering and step-sintering profiles are shown in Fig. 1(a) and (b), respectively. After ramping up to 1000 °C at 10 °C/min, direct-sintering treatment was achieved by gradually heating up to temperatures ranging from 1405 to 1452 °C (i.e., approximate SLPS region) with a ramp rate of 1, 3, or 5 °C/min, hold for up to 4 h, followed by furnace cooling. On the other hand, step-sintering treatment entails three consecutive stages: 1) The samples were firstly heated from 1000 to 1405 °C at 1, 3, or 5 °C/min; 2) after holding for 2 h, the samples were then heated to a temperature between 1432 and 1452 °C at 1, 3, or 5 °C/min; 3) after holding for up to 4 h, the samples were furnace cooled to room temperature.Fig. 1

Schematic illustrations of the two applied sintering schedules, with the debinding and sintering under 5% H₂–95% N2: (a) direct-sintering schedule, and (b) step-sintering schedule.

Fig. 1Characterization of the samples was carried out in both the green and sintered states. The oxygen contents in the green and sintered samples were determined through LECO oxygen analysis (O-836, LECO corporation) conducted at Rio Tinto Metal Powder according to the ASTM E1019. The bulk green density was assessed by measuring the sample volume and mass using a caliper (Digimatic Caliper, Mitutoyo) and precision balance (Secura 225D, Sartorius) with 0.0001 g accuracy, respectively. The sintered density was measured and calculated using the Archimedes method according to ISO 5017 and details were given in Ref. [27 (link)]. Phase of selected sintered samples were characterized by X-ray diffraction (XRD) (D8 Diffractometer, Bruker) with Co–K $\alpha$ radiation (λ = 1.79026 Å) operating at 40 kV and 45 mA. The as-sintered samples were polished and etched using 2% Nital for metallographic examination. Imaging of sample cross-sections was conducted with a digital microscope (VHX-700, Keyence). Quantitative measurements of grain size for selected sintered samples were undertaken according to ASTM E112-13. At least 200 grains were measured for each sample. The microstructure and chemical composition of powders in as-received, printed and cured, and sintered states were evaluated using a field-emission SEM (FE-SEM) (Leo 1530, Zeiss) equipped with an energy dispersive X-ray spectroscopy (EDX) detector (Oxford Inc).
DSC and TGA were performed (STA 449 Jupiter® instrument, NETZSCH) on the as-received powders and green parts. The experiments were performed under 95% N₂–5% H₂ gas flow to simulate the actual sintering atmosphere used in the tube furnace. The temperature program consisted of a heating stage up to 1500 °C with 5 °C/min and a controlled cooling rate of 5 °C/min down to room temperature. The melting point of the as-received powder (with 0.60 wt% C) was estimated using Thermo-calc software with TCFE 10 database.