Page 81 - MSAM-1-3

Materials Science in Additive Manufacturing Increasing density and strength in binder jetting

processing. Section 3 describes the characterization and for bimodal mixtures with a corresponding weight ratio
evaluation techniques employed in this study. Sections 4 of 27:73 which was evaluated to increase powder packing
and 5 detail the results obtained in this study and discuss density by mixing for 2 h in a heavy-duty shaker mixer
[30]
their implications, respectively. Finally, Section 6 highlights (Turbula-WAB Group, CH). Figure 1B displays powder
the major findings and directions for future research. morphology obtained at 0.8 µm per pixel, showing high
sphericity in the SS316L stainless steel powder used in this
2. Experimental methods work. Some surface defects and satellites were observed as
2.1. Material selection well, which could be beneficial for powder flowability.
Nitrogen gas atomized (SS316L) 316L stainless steel powder 2.3. Part fabrication and printing parameters
with similar chemical compositions were obtained from
two powder manufacturers, Sandvik (Stockholm, SE) and An ExOne (North Huntingdon, PA) Innovent + binder
Carpenter Additive (Philadelphia, USA). Batch sampling jetting machine with 30 µm print resolution was used
was conducted using ELEMENTRAC ONH-p 2 and for the fabrication of all parts. BA005 aqueous-based
ELEMENTRAC CS-I (ELTRA, USA) for total C, S, O, N, binder (supplied by ExOne) was deposited which uses
and H content. The O%, N%, and H% content were extracted polyvinylpyrrolidone (PVP) polymer as a bonding
through inert gas fusion technique, while combustion was agent. A total of six builds were printed corresponding
used for C% and S% concentration (Table 1). to each powder size group. To evaluate and compare the
mechanical strength of the unimodal and the bimodal
2.2. Powder characterization distributions used, 31.7 mm × 12.7 mm × 6.35 mm bars
To evaluate the impact of powder size distribution, six were fabricated as per the ASTM B528-99 standard.
powder size groups were prepared using a vibratory sieve To evaluate the sintered density in the fabricated parts
(Retsch AS 200 Control, Haan DE), as shown in Table 2 for (unimodal vs. bimodal), 7 mm (D) × 20 mm (L) cylinders
four unimodal and two bimodal distributions. were printed, as shown in Figure 2.
Using dynamic image analysis (DIA) technique To isolate the effects of build layout and orientation, a
through Microtrac MRB-CAMSIZER X2 equipment total of five bars and four cylinders were printed per build
(Newtown, PA), particle size distribution and morphology and placed in different locations. The bars were fabricated
were measured following the ISO 13322-2 standard . As perpendicular to the load direction within the build
[29]
shown in Figure 1A, median particle size (D50) values for platform, as shown in Figure 3.
unimodal groups are around 12 µm, 22 µm, 31 µm, and Several studies have noted that process-related parameters
36 µm for Groups 1, 2, 3, and 4, respectively. Groups 5 and affect part density and mechanical performance [24,31] . Table 3
6 consist of bimodal particle size distributions, which is a provides an overview of the process parameters that were
combination of coarse and fine particles. Based on literature utilized for the fabrication of all SS316L samples in this
work, a coarse fine particle size ratio of 1:3 – 1:4 is ideal study. Layer thickness varied slightly across groups, as
this is a variable directly related to the powder size. It is
Table 1. Chemical composition of as sourced SS316L recommended that the thickness of the layers should be
powders around 3 times the particle diameter for higher packing
density and a smooth surface finish . Another parameter
[32]
Powder manufacturer C% S% O% N% H (ppm) that slightly varied across groups was the recoat speed,
Carpenter Additive 0.014 0.005 0.040 0.090 5.490 which is the speed at which the hopper traverses the build
Sandvik Osprey 0.013 0.004 0.149 0.145 16.720 while dispensing powder . It can be observed that a lower
[33]
speed (mm/s) was utilized for finer particles because of
Table 2. Powder size groups (μm) the difficulty in powder dispensing due to clumping and
agglomeration. In addition, bed drying time, which is the
Groups D10 D50 D90 time the heat lamp takes to pass over the deposited binder
Unimodal distribution 10 µm 5.3 12.5 22.6 for drying, was adjusted between groups due to differences
20 µm 16.3 22.1 29.8 in particle sizes. Observations of part bleeding and layer
30 µm 22.1 31.3 41.8 delamination guided the selection of bed drying time.
40 µm 25.7 36.4 48.3 2.4. Post-processing
Bimodal distribution 30 (73%)+10 µm 10.6 26.8 39.0 Following green part fabrication, all samples were
40 (73%)+10 µm 10.7 32.7 47.4 cured in an oven at 200°C for 5 h, followed by manual

Volume 1 Issue 3 (2022) 3 https://doi.org/10.18063/msam.v1i3.20