FEATURE ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2025 67 powder size distribution and the cumulative frequency of powder particles in Fig. 1d shows that the powders have a d90 of 70 µm, d50 of 54 µm, d10 of 37 µm respectively. SINTERING WINDOW ESTIMATION To determine the sintering temperatures, the approach proposed by Wright et al. coupled with CALPHAD[6,7] was used. Figure 2a shows a schematic of the correlation of heat capacity to critical sintering temperatures, helping to estimate the SLPS window. Based on the heat capacity curve of an alloy, critical temperatures such as the onset of sintering (TOS, point 1 on the heat capacity curve), maximum sintered density (Tm, point 2 on the heat capacity curve), and the distortion window (Td, points 3 and 4 on the heat capacity curve) can be estimated. Based on this methodology, the heat capacity (solid line) for nanostructured bainite and corresponding volume fraction of liquid evolution (dotted line) was calculated via the CALPHAD approach as shown in Fig. 2b. The computed heat capacity curve shows similar trends to the theoretical curve in Fig. 2a. Furthermore, heat capacity predicts distortion (points 3 and 4) when the liquid volume fraction is between 35-40 vol% as reported by German et al.[8] Based on the CALPHAD data in Fig. 2b, sintering was conducted at series of temperature from 1275° to 1350°C. Sintering time was estimated from SLPS sintering times for other high carbon tool steels like H13[9]. Table 2 shows the sintering conditions studied in this work. Fig. 1 — Powder characterization of pre-alloyed nanostructured bainitic steel, where (a) and (b) represent low magnification and high magnification secondary electron SEM images, (c) represents EDS elemental maps representing surface chemistry of powders, and (d) represents the powder size distribution. (a) (b) (c) (d) Fig. 2 — (a) Theoretical sintering window estimation using heat capacity curve proposed by Wright et al. (b) CALPHAD-computed heat capacity for nanostructured bainitic steel alloy (light gray zone indicates temperatures to achieve maximum density and dark gray zone indicates temperatures where distortion happens). (a) (b) TABLE 2 — SINTERING TEMPERATURE AND TIME USED IN THE EXPERIMENTAL RUNS Sintering temperature, °C Sintering time, hours 1275 5 1300 2 1300 5 1325 2 1350 2 SINTERING AND EVALUATION Pre-alloyed powders were filled in an alumina crucible and were sintered in vacuum. Following sintering, the samples were cooled in a furnace. The density of sintered parts were evaluated using ZEISS Metrotom x-ray computed tomography (XCT). Figure 3 shows the macro- scopic images of the sintered samples (Fig. 3a) along with XCT cross-sections to indicate the porosity (Figs. 3b to 3f). It can be seen from the macroscopic images that when the sintering temperature is 1275°C, the sintered sample holds the shape without much distortion along the Z axis. When the sintering temperature is 1300°C, shorter hold time results in good shape retention (two-hour hold at sintering temperature), whereas longer hold time results in excess liquid phase formation and distortion along the Z axis, indicating that kinetics of liquid formation is crucial in determining the optimum sintering time. This can be correlated to the heat capacity curve in Fig. 2b where it can be seen that 1300°C is at the border between temperature for maximum densification and distortion. When the sintering temperature is above 1300°C excess liquid phase formation results in distortion along Z axis. The XCT cross-sections taken at the middle of the samples indicate porosity distribution. Overall, all the sintering temperatures result in minimal porosity. For sintering 13
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