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Nanostructured aluminum alloys produced through cryomilling have generated interest due to their potential to create consolidated parts with high strength and low density. Degassing prior to consolidation minimizes adsorbed and absorbed volatiles, but is accompanied by microstructural changes such as grain growth, dislocation annihilation, and formation of dispersoids. These changes can influence the mechanical behavior of consolidated components. Cryomilled AA5083 was degassed at temperatures from 473 K to 773 K (200 °C to 500 °C) with a vacuum at or below 2.7 × 10−3 Pa. Grain size in the as-cryomilled powder (ranging from 21 to 34 nm) increased with higher degassing temperature and reached a maximum size of up to 70 to 80 nm. The dislocation density of 1.11 × 1015 m−2 in as-cryomilled powder decreased to 1.56 × 1014 m−2 for powder degassed at 773 K (500 °C). The Al6(MnFeCr) dispersoid formed when powders were degassed at or above 573 K (300 °C). Oxygen and nitrogen concentrations were unaffected by degassing; however, hydrogen concentration decreased with increasing degassing temperature to a minimum of 45 ± 3.16 ppm. Evolutions in composition and microstructure in cryomilled AA5083 were correlated to the strengthening mechanisms of grain size reduction (i.e., Hall–Petch), dislocation forest, and Orowan. However, strengthening by grain size reduction was the dominant strengthening mechanism.

The sintering of a metallic alloy powder into bulk form using only direct current (DC) electric energy input at ambient room temperature is presented for the first time. It was found that a flash sintering phenomena is achievable in aluminum alloy powder, with no addition of external thermal energy, at applied DC electric fields in the range of 175–330 V/cm in which the formation of interparticle necks occurs rapidly and is characterized by a near-instant change in the physical properties of the compact from electrically non-conductive to electrically conducting in a time period on the order of seconds. It was found that the kinetics of this effect have a logarithmic dependence on the magnitude of the applied electric field. Above approximately 330 V/cm, the critical field strength is reached at which an incubation time for flash sintering is not required and sintering occurs instantly with the application of the DC field. This technique has promise to greatly reduce processing time and costs associated with sintering of powder metallurgy products as well as consolidation of nanostructured metals by limiting exposure to high temperatures which result in excessive grain growth.

Additive manufacturing (AM) of metallic parts is generating significant interest due to the ability to produce complex parts in a short period of time with minimal finishing required. However, the effect of laser scan strategy on the properties of finished parts is not well understood. In this paper the effects of laser scan line strategy on the microstructure and mechanical properties of stainless steel produced using metal Powder Bed Fusion (PBF) AM were characterized. Microstructure and phase identification were measured using X-ray diffraction and quantitative optical microscopy which found that all samples had a dual phase austenite-ferrite composition. Shorter scan lines perpendicular to the load direction resulted in 25% retained austenite, while elongated scan lines parallel to the load direction more than doubled the amount of austenite retained. A change of direction within the scan line path resulted in increased delamination porosity along the melt pool boundary and changes in volume fraction of retained austenite. Fractography, revealed cracks that propagated along melt pool boundaries. Understanding the effect of strategy on the microstructure and mechanical properties allows the producer of AM parts to implement materials by design strategies. Keywords: Additive manufacturing, Powder Bed Fusion, Stainless steel, Processing-microstructure-property relations, Failure analysis

Aluminum 5083 powder, both as atomized and cryomilled, was consolidated via spark plasma sintering (SPS). This study quantified and compared the effects of heating an aluminum alloy powder directly through Joule heating vs indirectly through thermal conduction from the die during SPS processing. When consolidated under the same processing conditions, the cryomilled powders showed faster heating rates and densification than the as atomized powder. It was also possible to process the cryomilled powder in a non-conductive die but not the as atomized powder. This could be ascribed to an improvement in electrical conductivity of the powder due to the break up and redistribution of surface oxides after cryomilling. The changes in behavior as a result of cryomilling and/or changing die material led to samples with different fracture morphologies and increased hardness values.

Direct current (DC) electric currents were applied during sintering of aluminum alloy (AA5083) green powder compacts and it was found that the kinetics of sintering were greatly enhanced compared to samples processed without a field. In situ sintering kinetics during pressure-less sintering employing electric field strengths and amperages ranging from 0 to 56 V/cm and 0 to 3 A were quantified using digital image correlation. It was found that the application of a DC field during sintering results in a discontinuous change in volume at a critical temperature along with a transition in electrical properties of the compact from insulating to conductive. This effect is similar to the phenomena observed in the flash sintering process currently being actively researched for ceramic powder processing. The temperature at which the flash event occurs was found to be field strength dependent and doubling the field strength was found to decrease the flash temperature by 25 pct. Joule heating of the specimen was measured using thermal imaging and it was found to not contribute enough additional thermal energy to account for the substantially increased sintering rates observed in specimens processed using electric fields.

Spark plasma sintering has been a well-studied processing technique primarily for its very high cooling and heating rates. However, the underlying phenomenon driving the sintering behavior of powders under an electric field is still poorly understood. In this study, we look at the effect of changing current pathways through the powder bed by changing die materials, from conductive graphite to insulating boron nitride for sintering aluminum alloy 5083 powder. We found that the aluminum powder itself was insulating and that by changing the current paths, we had to find alternate processing methods to initiate sintering. Altering the current pathways led to faster temperature raises and faster melting (and potentially densification) of the aluminum powder. A flash sintering effect in metallic powders is observed in which the powder compact undergoes a rapid transition from electrically insulating to conducting at a temperature of 583 K (310 °C).

Significant improvements to spark plasma sintering (SPS) processability and bulk properties can be realized by modifying parameters that influence current flow such as material type, current type, or die design. Redesign of the tooling can divert the flow of current from the die to the powder bed, minimizing the presence of thermal gradients. Replacing the die material type from conducting to insulating can force all of the current through the powder bed, increasing the current density and leading to higher heating rates, in some cases promoting flash-like behavior. While the differences between pulsed and continuous DC current have been well-documented, switching from AC to DC current, which also has the potential to affect the sintering process, has not been widely addressed. Furthermore, the same material can behave differently depending on how it was produced and processed (e.g., nanostructuring). Even slight changes to parameters that alter current pathways, whether they are related to the materials, tooling, or current type, can have a significant influence over the materials that are ultimately produced.

Nanostructured pure tungsten (W) powders have been fabricated through cryogenic attrition (i.e., cryomilling) in a liquid nitrogen medium for the first time. The microstructure and chemistry of W powders before and after 4 and 12 h of cryomilling were thoroughly examined by gas fusion chemical analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy (TEM). Cryomilling in liquid nitrogen protected the tungsten from oxygen and hydrogen contamination while introducing nitrogen. Results showed that the W grain size decreased with cryomilling time, and reached approximately 5 nm after 12 h of cryomilling. High resolution TEM suggested that nitrogen reacted with W to form tungsten nitride (WN). Additionally, amorphous W was identified in the 12 h cryomilled W powder. Tungsten carbide (WC) contamination from the milling media and minor Fe–Cr–Ni-containing impurities from the stainless steel vessel were also documented. The WC had grain size ranging from 20 nm to 150 nm, and was homogeneously dispersed in W matrix.

Aluminum 5083 powder was milled through cryogenic attrition for 8, 16, and 24 h with graphite as a hydrogen-free process control agent. The powders were degassed to remove moisture and other impurities. The degassed powders have a relatively low hydrogen concentration. The morphology, grain sizes, and stability of the milled powders were compared, and it was found that while there was little difference in grain size between the 3 milling times, powders milled for 16 and 24 h showed more grain stability than powders milled for 8 h.

The ability to make powders from battlefield scrap and general waste materials in-situ on the battlefield forward operating base (FOB) is investigated. Three different techniques have been employed to grind mm-size small scrap pieces of water bottles, which are made of polyethylene terephthalate (PET). The particle size distribution of the milled powder obtained in each technique was measured. The results are presented in this report.

Immunoglobulin E antibodies to Psuedomonas aeruginosa were demonstrated in patients with cystic fibrosis colonized with the bacterium.