Page 127 - MSAM-2-3

Materials Science in Additive Manufacturing Powder alteration caused by L-PBF process

composition is shown in Table 1 (EOS art-no.9011-0016) 2.2.1. Experiment 1: PSD variation caused by the
[37] . The parts were printed on an EOS M290 400W powder spread process
machine in a nitrogen atmosphere with <1.3% oxygen The first experiment examines the PSD of the powder
concentration, with a gas flow differential pressure of 0.7 spread over the build plate during a printing cycle at
mbar and a chamber temperature of 40°C. The MS1 040 different locations. We compared the PSD of powder loaded
performance M291 2.00 EOSPRINT template was used: in the dispenser (before printing), powder at 16 different
laser power = 285 W, beam offset = 20 µm, laser speed = sites (matrix) over the powder bed, powder accumulated
960 mm/s, layer thickness = 40 µm, hatch space = 110 µm, in the collector bin, and powder after sieving. To do so,
and laser pattern = stripes rotated at a 47° angle with a 30° we printed at x = 125 mm and y = 10 mm to produce a
restriction angle at each of the next layers. small vertical cylinder of Ø = 10 mm and h = 20 mm. The
After each print, the powder was sieved manually cylinder was located outside the sample matrix and close to
through an 80 µm mesh sieve and fed into the dispenser. the gas outlet, as shown in Figure 1.
The powder samples were analyzed with a Malvern
Panalytical Mastersizer 3000 particle-size analyzer 2.2.2. Experiment 2: PSD variation depending on the
equipped with a Hydro LV module. The stirrer speed was position of printed part
set to 3000 rpm, a speed sufficient to keep the particles 2.2.2.1. Experiment 2A: Single contamination
in suspension. Before the measurements, the sample was In Experiment 2A, we evaluated the PSD based on
subjected to processing using Hydro LV ultrasounds at horizontal and vertical distances from a centrally located
25% power for 60 s to help disintegrate aggregates. Three printed cylinder with a diameter of 50 mm and a height of
consecutive measurements of 30 s each (20 s with the red 20 mm. The printing process generates spatters and smoke
laser, and 10 s with the blue LED) were collected for each that contaminate the power bed. As shown in Figure 2, the
sample. The average statistics of the three measurements 12 samples collected are 40 mm apart.
were computed (the coefficient of variation in Dv10, Dv50,
and Dv90 of the three measurements was always <1%), 2.2.2.2. Experience 2B: Double contamination
indicating a good sample dispersion. The Mastersizer To study double contamination, we printed four concentric
general-purpose optical model for nonspherical particles cylinders and analyzed the PSD of the powder collected in
with a refractive index of 2.757 and absorption 1.0 for four different zones as shown in Figure 3.
stainless steel (values taken from the Malvern Panalytical
database included with Mastersizer 3000 software) was 2.2.3. Experiment 3: PSD changes when printing
employed. Instrument performance was confirmed using lattice structures of different cell sizes
Malvern Panalytical’s QAS4002 Quality Audit Standard The third experiment highlights the evolution of PSD and
and two silica powder secondary standards with median morphology of powder depending on the area and volume
diameters of 70 µm and 270 µm. In addition, the powder ratios of printed parts. For this experiment, lattice cylinders
morphology was analyzed with a NanoImage SNE 4500M were designed using nTopology software. First, we defined
scanning electron microscope. the area and volume ratios of a lattice cylinder as follows:

2.2. Experiments R = A /A (I)
A lattice filled
We conducted three experiments to investigate alterations R = V /V (II)
in powder PSD throughout the L-PBF printing process, v lattice filled
studying the effects on PSD of gas flow transporting spatters, where R is area ratio; A filled and A lattice are, respectively, the
A
powder distance from the printed part, and geometrical total area of filled or lattice cylinders of equal diameter
properties of lattice cylinders. The builds were not repeated and height; R is volume ratio; and V filled and V lattice are,
V
to obtain replicates. However, the repeatability of the respectively, the total volume of filled or lattice cylinders of
measurements was evaluated by analyzing three powder equal diameter and height.
samples for each zone of experiment 2B and four powder Eventually, we investigated the PSD of powder trapped
samples of each lattice structure of the experiment 3. inside 4 lattice structures. The cylinders had the same

Table 1. The chemical composition MS1 virgin powder (in weight %)
Element Fe Ni Co Mo Ti Al Cr Cu C Mn Si P S
Max (%) 19 9.5 5.2 0.8 0.15 0.5 0.5 0.03 0.1 0.1 0.01 0.01
Balance
Min (%) 17 8.5 4.5 0.6 0.05 - - - - - - -

Volume 2 Issue 3 (2023) 4 https://doi.org/10.36922/msam.1781