Page 83 - MSAM-1-3

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

depowdering and removal from the build box. Finally, all 16-bit tiff stack files and imported for processing into ImageJ
samples were sintered in a fully inert atmosphere filled software for image preparation and gray scale correction.
with argon gas. A tube furnace (Carbolite Gero Ltd.) was AVIZO was used for the visualization and analysis of XCT
used with the following temperature profile, as shown data. Segmentation and thresholding of the data were
in Figure 4: (i) 3°C/min ramp to 700°C, 60 min dwell; performed to extract the material (SS316L) of the printed
(ii) 3°C/min ramp to 1120°C, 140 min dwell; (iii) cool cylinder. The total volume of the solid cylinders (without
down at 3°C/min to 850°C, 90 min and to 300°C, 183 min; accounting for porosity) was calculated using AVIZO’s
and (iv) cool down at 3°C/min to room temperature. internal tool. Figure 5 shows the printed cylinders from the
A similar sinter cycle can be observed in previous SS316L XCT scans segmented and displayed in AVIZO. The higher
binder jetting work . part resolution achieved with 10 µm can be observed along
[24]
with segmented porosity, which is shown in black.
3. Part evaluation techniques
3.2. Mechanical testing
3.1. Sintered density
Sintered test coupons (n = 5/group size) were subjected
Sintered density was calculated for all cylinder samples to ultimate flexural strength (UFS) 3-point bending test
from each group (n = 4) using both physical and micro- as per ASTM B528-16 standard at 2.5 mm/min loading
X-ray computed tomography (XCT) methods. Specimen rate until complete rupture in an MTS Landmark 22 Kip
dimensions were recorded using a digital caliper (10 µm servo hydraulic test. UFS and flexural modulus (FM)
accuracy) for the cylinder dimensions and a digital scale described in Equation II were computed based on F, L,
(0.1 mg) for weight recordings. In Equation I, the formula w, t – force required to rupture, span distance, specimen
used for caliper density calculations is shown, where m cylinder width, thickness, and slope of the line tangent to the load
is the total mass of the cylinder and v cylinder is the volume using displacement curve, respectively.
the radius and height of the cylinder. This value was later
divided by the theoretical density of SS316L stainless steel σ = 3FL (II)
(8.00 g/cm ) for relative density % present in the printed part. max flex 2wt 2
3
m
cylinder
ρ sintered = v cylinder (I)

On collecting sintered data from all printed samples
(n = 4/group), a more precise calculation of density was
further explored using XCT on one sample per group size.
This technique was selected as it is a non-invasive tool that
enables the evaluation of internal features of printed parts.
A GE Vtomex L300 CT scanner with a microfocus tube was
used. Scans were collected using a 250kV 40 µA X-ray beam
with 10 µm voxel resolution. Raw XCT data were collected as

Figure 5. Pores reduced in bimodal powder size groups as observed from
Figure 4. Time-temperature profile used for sintering SS316L samples. X-ray computed tomography results.

Volume 1 Issue 3 (2022) 5 https://doi.org/10.18063/msam.v1i3.20