ENSAYO DE VARIEDADES DE ISI SEMENTI

3.2.1 Manufacturing Data

Production of the AF9628 powder was contracted to PAC in Cincinnati, a com- pany with experience creating small batches of custom powders. The powder was created by combining individual elemental powders, using argon gas atomization, for a lot of 288 lbs, and would adhere to the specifications listed in Table 3.2. The powder was delivered at the end of November, approximately three months after the contract was awarded, at a cost of $66 per pound.

Table 3.2. Powder Contract Specifications and Reported Values, by % Weight Element Specified Amount (Min/Max) Reported Value

Carbon 0.26/0.29 0.28 Manganese 0.5/0.8 0.7 Silicon 0.9/1.1 0.9 Chromium 2.5/2.8 2.8 Nickel 0.9/1.2 1.1 Copper 0/0.2 <0.1 Vanadium 0.05/0.15 0.12 Molybdenum 0.85/1.05 1.00 Phosphorus 0/0.10 <0.010 Sulfur 0/0.005 <0.005 Aluminum 0/0.015 0.009 Titanium 0/0.006 0.004 Oxygen 0/300 ppm 280 ppm P+Sn+As+Sb 0/0.035 <0.035 Hydrogen 0/2 ppm 2 ppm

Hall Cup Time - 19.6 s/50 g

Apparent Density - 4.4 g/cc

The powder was delivered with a report of its chemistry, size distribution, apparent density, and Hall Flowmeter time. Reported values were determined by independent laboratories that are GE, SAFRAN, NADCAP, and ISO17025 approved, and tests were performed per ASTM’s B214, B822, B212, and B213. The testing indicated that

the powder lot met all specifications, with values listed in Table 3.2. The full report delivered with the powder is available in Appendix A.

3.2.2 Sampling

Static powder sampling was conducted in accordance with the procedures found in ASTM B215-15. A representative virgin powder sample was collected from the top of five of the 10 lb containers delivered with the powder lot, after the containers had been inverted twice to account for settling during transport. A representative recycled powder sample was collected from two separate sieving batches, after multiple builds of dense parts produced with virgin powder.

3.2.3 Appearance and Morphology

Initial powder testing involved characterizing the powder’s appearance, confirming particle sizes and distribution, and evaluating the powder’s circularity and aspect ratio. To determine appearance, pieces of conductive double-sided carbon tape were dipped into the representative powder samples and placed in a Tescan Maia3 Scanning Electron Microscope (SEM). SEM images are valuable for surface visualization, which can provide insight into the quality of the powder manufacturing process. To confirm the reported particle size and distribution, 3 mm3samples of powder were dispersed in

a Malvern Morphologi 4, an optical instrument that measures particle size. Access to this equipment was provided by the University of Dayton Research Institute (UDRI). The Morphologi 4 measurements are comparable to X-Ray Diffraction (XRD), with dimensions accurate to ±4 µm [95]. After the powder samples were dispersed in the Morphologi 4, images were taken to confirm an even dispersion of powder particles (Figure 3.1). The raw data was filtered to remove any lint or dust particles that would skew the data. The Morphologi 4 also calculated the average particle circularity and

aspect ratio, which provide a quantitative evaluation of particle quality.

(a) Virgin powder dispersion (b) Sieved powder dispersion

Figure 3.1. Dispersion pattern of powder samples in the Morphologi 4.

3.2.4 Porosity

Entrapped gas within the powder can contribute to porosity of the finished parts, and large pores in particular can contribute to low density. To evaluate internal porosity, powder must first be mounted in a binder so that it can be held for sectioning. Powder samples of 0.25 cm3 were placed in 1.25 mm cylindrical plastic molds, which

were then placed under vacuum. Approximately 20 mL of thinset two-part epoxy, mixed by weight, was siphoned into the molds. The molds were then removed from the vacuum and allowed to cure for 24 hours. After curing, the resin pucks were removed from the molds and wet ground by hand on a succession of silicon carbide grinding disks. Polishing with diamond slurry and a polishing wheel was attempted, but resulted in powder being ripped out of the resin matrix. The best finish was achieved through wet sanding by hand on a 1200 grit grinding disk, then buffing with a dry cotton pad, though this did leave numerous scratches in the surface. Sputter coating with 5 nm of iridium was attempted for imaging in an SEM, but this obscured most features. Final images of the sectioned powder were obtained on an optical Zeiss Observer equipped with an Axiocam 503 mono camera and extended depth of field

z-stack image stitching software.

3.2.5 Chemistry

The primary objective of the chemical analysis was to characterize differences between virgin and recycled powder, not to confirm the detailed elemental analysis delivered with the powder. An initial powder chemistry characterization was con- ducted via an EDAX Energy Dispersive X-Ray Spectroscopy (EDS) unit, which is useful for determining which elements are present in a material and their relative concentrations, though it cannot identify compounds formed by those elements. EDS uses the characteristic x-rays released by elements upon excitement by an electron gun to generate a spectrum of wavelength peaks. Several elements have similar char- acteristic x-ray wavelengths, and the user should be careful to avoid misidentification of the peaks. The virgin and soot samples used for appearance characterization in the SEM were compared for differences in composition and relative changes in concentra- tion. The chemistry data was compiled from six points within each viewing window, with an excitation voltage of 10 kV. Powder chemistry was again evaluated using X-Ray Fluorescence (XRF) courtesy of Dr. Flater, AFRL/RWMW, Eglin AFB. The theory behind XRF is similar to that of EDS; a sample is bombarded with x-rays, and it releases an electron. This causes electrons to move between shells, which by conser- vation of energy causes the release of a secondary x-ray at a characteristic wavelength. XRF data can identify chemical compounds, and can also be used to determine the relative quantities of elements present, excluding light and heavy elements that are difficult to detect. These two similar characterization techniques will be compared to confirm the repeatability of the results.