Orme, Chris A. | Search NASA

DOE OSTI2021

Transformations of Ti-5Al-5V-5Cr-3Mo powder due to reuse in laser powder bed fusion: A surface analytical approach

Reactive Ti-5Al-5 V-5Cr-3Mo (Ti5553) alloy feedstock powder is used in laser powder-bed fusion (LPBF). Due to the large quantity of powder necessary for LPBF, powder is reused numerous times under oxidizing environments. Transformations to the powder’s native surface oxide may impact the LPBF process and lead to deviations from expected behavior of printed parts. In this work, we present a multimodal characterization of new and reused powder by combining X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and transmission electron microscopy (TEM) of focused ion beam (FIB)-prepared cross sections of individual powder particles to understand how the surface oxide composition changes, grows, and becomes hydroxylated and hydrated due to reuse. We show that the native oxide film of Ti5553 powder is a hydroxylated mixed oxide composed of TiO 2 and Al 2 O 3 that grows from 5.6 +/- 0.7 nm to 8.3 +/- 1.1 nm due to reuse. Al oxide regions of the surface oxide become more hydroxylated during reuse in comparison to Ti oxide regions. This indicates that an outer oxide of pure TiO 2 may enhance recyclability of Ti5553 powder while the presence of Al oxides may decrease recyclability. Further characterization of surface transformations due to recyclability may eliminate unnecessary variation in LPBF.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Phase changes in fluoropolymer binders

Motivation: Understanding fluoropolymer binder structure and aging response continues to be an important goal for the prediction and assessment of polymer bonded explosive compositions within the stockpile. As part of ongoing efforts within NNM, we are developing new methods to quantify crystallization and chemical degradation within aged Fluoropolymer materials. Significance: Time, temperature and radiation are known to alter the mechanical properties of fluoropolymers as crystalline domains nucleate and grow within the material. However, current methods do not accurately quantify and assess the degree or rapidity of these physical aging processes. For this reason, we are developing methods that directly measure crystalline domain, composition, size and shape and cross-correlating them with traditional techniques. Progress: Most recently, we thermally aged fluoropolymer binder, FK800, for various durations up to 23 days at several temperatures including: 25C (red), 35C (green), and 60C (blue). We used wide angle X-ray scattering (WAXS) to observe when crystalline peaks emerged, signaling a phase transition from amorphous to partially crystalline. Some of these data are summarized in Figure 1. As expected, samples held at 25C (red), which is below the glass transition temperature at 28C, remain amorphous – the WAXS data (Figure 1b) lacks sharp peaks indicative of crystallinity and the ultra-small angle X-ray scattering (USAXS) also has a smooth background without peaks indicative of structure (Figure 1a). By contrast, samples held at 35C and 60C have new peaks in both the USAXS and WAXS data. The structure is clearer after subtracting off the amorphous background from the WAXS data (Figure 1c). These data show that samples aged at 60C are more crystalline as determined by the more defined peaks and also have a larger lattice constant as seen by the shift in peak position as compared to the 35C samples. The corresponding USAXS data (Figure 1a) indicate that the size of the crystals was larger at 60C than at 35C as seen by the shift in the knee position to lower q-values. Interestingly, the shape of the scatter curve also changed, which indicates a change in morphology. Atomic force microscopy (AFM) was used to image the fluoropolymer (Figure 1c-e). At 35C, ~50nm long stacked crystal plates are observed. High resolution imaging shows that the stacks consist of 2-6 lamellae that are spaced approximately 3nm apart. At 60C, the individual lamella appear both thicker and longer and are intertwined to fill the surface. Morphological data such as these images help to interpret & model volume averaging techniques like X-ray scattering and NMR. We also expect the shift from isolated islands to intertwined clusters to impact mechanical properties.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Progress Summary: Developing experimental methods to quantify the degree of crystallinity in fluoropolymer binders

Understanding fluoropolymer binder structure and aging response continues to be an important goal for the prediction and assessment of polymer bonded explosive compositions within the stockpile. As part of ongoing efforts within NNM, we are developing new methods to quantify crystallization and chemical degradation within aged Fluoropolymer materials.

36 MATERIALS SCIENCE↗