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Materials Science in Additive Manufacturing Additive manufacturing of 316L-Cu alloys

flowability due to agglomeration, while coarser particles 2.3. Compression and hardness testing
lead to increased porosity and surface roughness on Cylindrical compression test specimens were machined
the final part. Two alloy compositions were prepared: using a computer numerical control mill (770MX,
SS-3Cu and SS-5Cu. These powders were tumble-mixed Tormach, Madison, WI, USA) equipped with a lathe
in batches of 250 g and mixed at 160 rpm for 2 h with no attachment. The diameter of the specimens was milled
pause time to reach a uniform particle size distribution. to 7.5 mm, and the length was faced to 15 mm, resulting
The powder mixtures were contained in 500 mL jars with in a length-to-diameter (L/D) ratio of 2. Compression
an air atmosphere. All samples were fabricated using a testing was performed following ASTM E9 – 19. The
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FormAlloy laser DED system (Spring Valley, CA, USA). tests were conducted using a universal test machine
The processing parameters consisted of 350 W laser (600DX, Instron, Norwood, MA, USA) at a constant
power and 575 mm/min scan speed for contour and crosshead displacement rate of 0.1 mm/min. These
infill paths, with 0.3 mm layer thickness, 0.78 mm hatch tests were terminated once the specimens reached
spacing, and 20°C build plate temperature. Powder feed 4% engineering strain, with n = 3 tests performed for
settings included 0.5 rpm powder feeder disc rate, 9 L/min each composition. Hardness testing was conducted
carrier gas flow rate, and 13 L/min shield gas flow rate. according to ASTM E92 – 23, using a Micro Vickers
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It was observed that the Cu addition did not affect the Hardness Tester (Phase II Plus, Upper Saddle River,
processing parameters, as the same settings were used NJ, USA). Hardness samples were prepared in the same
for all three compositions. To minimize oxidation, the manner as the microstructure samples. Vickers hardness
printing environment was carefully maintained with an measurements were taken at 0.5 mm intervals, starting
oxygen concentration below 20 ppm, and the chamber from the base of the sample and extending up to a height
was filled with argon gas. The samples were designed as of 5 mm. A load of 1.961 N (HV 0.2) was applied, with
vertical cylinders with a diameter of 9 mm and a height a dwell time of 15 s.
of 18 mm and were printed onto a 316L substrate.
2.4. In vitro bacterial studies
2.2. Microstructure and phase analysis
The antibacterial efficacy of Cu addition into 316L was
Samples for microstructural analysis were produced evaluated in vitro against S. aureus and P. aeruginosa
by sectioning the cylindrical specimens longitudinally, bacterial strains (Carolina Biological, Burlington, NC,
exposing a vertical cross-section. These sections were USA) at 24, 48, and 72 h. The AM-produced cylindrical
then mounted in phenolic resin and ground using silicon specimens were sliced into 1 mm thick sections using a
carbide (SiC) pads in sequential order of increasing grit low-speed diamond saw, then mounted and ground with
size from 320 to 1200. The samples were subsequently SiC pads up to 1200 grit. These samples were subsequently
polished with alumina suspensions of 1 μm, 0.3 μm, cleaned via ultrasonication and sterilized by autoclaving at
and 0.05 μm particle size, using deionized water as the 121°C for 1 h. S. aureus and P. aeruginosa were rehydrated
medium. Etching was performed following ASTM E407 – with rehydration media and incubated at 37°C for 24 h to
23. The etching solution comprised 10 mL HNO , 35 mL reactivate. Serial dilutions of the bacterial cultures were
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HCl, and 30 mL H 0, with a 60-s immersion, followed by performed to achieve a concentration corresponding
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a rinse in water and air drying. Microstructure imaging to the 0.5 McFarland standard, equivalent to 1.5 × 10
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was done using a digital microscope (VHX-970F, Keyence, colony-forming units (CFU)/mL, as confirmed by optical
Itasca, IL, USA). The grain size was calculated using the absorbance measurements within a range of 0.08 – 0.1
average grain intercept method, where the number of grain at a wavelength of 625 nm. For antibacterial testing, the
intercepts was divided by the line length. Phase analysis sterilized samples were placed into 24-well plates, with 10
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was performed using X-ray diffraction (XRD) with a CFU applied on the surface of each sample, followed by
Rigaku MiniFlex 600 diffractometer (Rigaku, Japan). The 1 mL of nutrient broth. The plates were then incubated at
samples were scanned using Cu-K α radiation (1.54 Å at 37°C until the specified time points. At each time point, the
40 kV and 20 mA) with a 2θ theta range of 25° – 100° and a nutrient broth was carefully removed and replaced with a
scan rate of 5°/min. Energy dispersive spectroscopy (EDS) fixative solution (2% paraformaldehyde/2% glutaraldehyde
was conducted using a field emission scanning electron in 0.1 M phosphate-buffered saline [PBS]) overnight. After
microscope (FE-SEM; Sirion, USA) for 5 min. For EDS removal of the fixative solution, the samples underwent
analysis, samples were cut in the XY plane, which refers secondary fixation with 1% osmium tetroxide (OsO )
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to the horizontal plane parallel to the build platform and overnight, followed by a rinse in deionized water. The
perpendicular to the build direction (Z-axis), to map samples were then subjected to a serial dehydration
elemental distribution across the print plane. process using increasing concentrations of ethanol (30%,

Volume 4 Issue 1 (2025) 3 doi: 10.36922/msam.7357