Topographic Measurement of Individual Laser Tracks in Alloy 625 Bare Plates.

Additive manufacturing (AM) combines all of the complexities of materials processing and manufacturing into a single process. The digital revolution made this combination possible, but the commercial viability of these technologies for critical parts may depend on digital process simulations to guide process development, product design, and part qualification. For laser powder bed fusion (LPBF), one must be able to model the behavior of a melt pool produced by a laser moving at a constant velocity over a smooth bare metal surface before taking on the additional complexities of this process. To provide data on this behavior for model evaluations, samples of a single-phase nickel-based alloy were polished smooth and exposed to a laser beam at 3 different power and speed settings in the National Institute of Standards and Technology (NIST) Additive Manufacturing Metrology Testbed (AMMT) and a commercial AM machine. The solidified track remaining in the metal surface after the passing of the laser is a physical record of the position of the air-liquid-solid interface of the melt pool trailing behind the laser. The surface topography of these tracks was measured and quantified using confocal laser scanning microscopy (CLSM) for use as benchmarks in AM model development and validation. These measurements are part of the Additive Manufacturing Benchmark Test Series (AM-Bench).

Simulation of TTT Curves for Additively Manufactured Inconel 625.

The ability to use common computational thermodynamic and kinetic tools to study the microstructure evolution in Inconel 625 (IN625) manufactured using the additive manufacturing (AM) technique of laser powder-bed fusion is evaluated. Solidification simulations indicate that laser melting and re-melting during printing produce highly segregated interdendritic regions. Precipitation simulations for different degrees of segregation show that the larger the segregation, i.e., the richer the interdendritic regions are in Nb and Mo, the faster the δ-phase (Ni3Nb) precipitation. This is in accordance with the accelerated d precipitation observed experimentally during post-build heat treatments of AM IN625 compared to wrought IN625. The δ-phase may be undesirable since it can lead to detrimental effects on the mechanical properties. The results are presented in the form of a TTT diagram and agreement between the simulated diagram and the experimental TTT diagram demonstrate how these computational tools can be used to guide and optimize post-build treatments of AM materials.

The Influence of Annealing Temperature and Time on the Formation of δ-Phase in Additively-Manufactured Inconel 625.

This research evaluated the kinetics of δ-phase growth in laser powder bed additively-manufactured (AM) Inconel 625 during post-build stress-relief heat treatments. The temperatures ranged between 650°C and 1050°C, and the times from 0.25 to 168 hours. The presence of δ-phase was verified for each temperature/time combination through multiple techniques. A conventional time-temperature-transformation diagram was constructed from the time-temperature data. Comparison to the growth in wrought IN625 with a similar nominal composition revealed that δ-phase formation occurred at least two orders of magnitude faster in the AM IN625. The results of this study also revealed that the segregated microstructure in the as-built condition has a strong influence on the kinetics of δ-phase formation in AM IN625 as compared to a homogenized material. Since control of the δ-phase growth is essential for reliable prediction of the performance of IN625 components in service, avoiding heat treatments that promote the formation of δ-phase in AM components that are not homogenized is highly recommended. This will be particularly true at elevated temperatures where the microstructural stability and the consistency of mechanical properties are more likely to be affected by the presence of δ-phase.

The Influence of Post-Build Microstructure on the Electrochemical Behavior of Additively Manufactured 17-4 PH Stainless Steel.

The additive manufacturing (AM) build process produces a segregated microstructure with significant variations in composition and phases that are uncommon in traditional wrought materials. As such, the relationship between the post-build microstructure and the corrosion resistance is not well understood. Stainless steel alloy 17-4PH is an industrially-relevant alloy for applications requiring high-strength and good corrosion resistance. A series of potentiodynamic scans conducted in a deaerated 0.5 mol/L NaCl solution evaluated the influence of these microstructural differences on the pitting behavior of SS17-4. The pitting potentials were found to be higher in the samples of additively-processed material than in samples of the alloy in wrought form. This indicates that the additively-processed material is more resistant to localized corrosion and pitting in this environment than the wrought alloy. The results also suggest that after homogenization, the additively-produced SS17-4 could be more resistant to pitting than wrought SS17-4 in an actual service environment.

Designing a Uniaxial Tension/Compression Test for Springback Analysis in High-Strength Steel Sheets.

We describe an innovative design for an in-plane measurement technique that subjects thin sheet metal specimens to bidirectional loading. The goal of this measurement is to provide the critical performance data necessary to validate complex predictions of the work hardening behavior during reversed uniaxial deformation. In this approach, all of the principal forces applied to the specimen are continually measured in real-time throughout the test. This includes the lateral forces that are required to prevent out of plane displacements in the specimen that promote buckling. This additional information will, in turn, improve the accuracy of the compensation for the friction generated between the anti-bucking guides and the specimen during compression. The results from an initial series of experiments not only demonstrate that our approach is feasible, but that it generates data with the accuracy necessary to quantify the directionally-dependent changes in the yield behavior that occur when the strain path is reversed (i.e., the Bauschinger Effect).

Ab initio study of the mechanical and transport properties of pure and contaminated silver nanowires.

The mechanical properties and conductance of contaminated and pure silver nanowires were studied using density functional theory (DFT) calculations. Several nanowires containing O2 on their surfaces were elongated along two different directions. All of the NWs thinned down to single atom chains. In most simulations, the breaking force was not affected by the presence of the O2, and similar fracture strengths of ≈1 nN were computed for the pure and impure NWs. When the O2 became incorporated in the single atom chain, the fracture occurred at the Ag-O bond and a lower fracture strength was found. All of the simulations showed that the impurity interacted with the silver atoms to reduce the electron density in its nearby vicinity. A variety of conductance effects were observed depending on the location of the impurity. When the impurity migrated during the elongation to the thinnest part of the NW, it reduced the conductance significantly, and an ≈1 G0 conductance (usually associated with a single atom chain) was calculated for three- and two-dimensional structures. When the impurity was adjacent to the single atom chain, the conductance reduced almost to zero. However, when it stayed far from the thinnest part of the NW, the impurity had only a small influence on the conductance.

Electron transport in gold nanowires: stable 1-, 2- and 3-dimensional atomic structures and noninteger conduction states.

Experimental conductivity measurements made during highly stable tensile deformation of Au nanowires show a rich variety of behaviors, including noninteger quantum conductance plateaus, transitions, and slopes. Using tight binding conductance calculations on simulated nanowires previously deformed using density functional theory, we demonstrate that all of these phenomena arise from structural transitions between deeply metastable ordered atomic configurations that self-organize during tensile deformation.

Ultra-small-angle X-ray scattering from dislocation structures.

Ultra-small-angle X-ray scattering data were obtained from deformed single-crystal aluminium samples. These data are consistent with recent theoretical predictions of scattering from dislocation walls, allowing quantitative microstructural parameters to be extracted.

Critical behavior of a strain percolation model for metals.

Extensive simulations of a strain percolation model for a deforming metal have been performed to examine its strain behavior. We find that the total strain exhibits critical power-law behavior that is well explained by two-dimensional percolation theory. Near the critical point, most of the strained cells organize themselves around a state having the minimum or at least marginally stable strain regardless of the initial conditions. A strain much greater than the minimum stable strain generally decays to a lower value when transmitted to an unstrained cell. The universal behavior of the total strain in the system is a consequence of the self-organizing character of the strain in the critical cluster. Although the probability distributions for the total strain and cluster size appear to exhibit nonuniversal behavior, this may merely represent a transient response before crossover to a true asymptotic, universal behavior occurs. Other critical aspects of the model are also discussed.

Distinctive acral erythema occurring during therapy for severe myelogenous leukemia.

A distinctive acral erythema developed in four patients with myelogenous leukemia, subsequent to blood transfusions and intensive chemotherapy with cytarabine. The clinical and histopathologic features of the eruption were suggestive of a drug-induced toxic eruption. To our knowledge, only one previous similar case has been reported in the literature. For patients in whom this self-limited condition develops, reassurance should serve as the mainstay of therapy.

Reduced threshold to suction-induced blister formation in insulin-dependent diabetics.

Diabetics may have an increased susceptibility to cutaneous blister formation clinically manifest as the bullous eruption of diabetes. We evaluated the mechanical force necessary to induce suction blisters in fifteen insulin-dependent diabetics and twenty age-matched normal controls. The mean suction blister threshold for the diabetics was 31.9 minutes versus 68.0 minutes for the controls, a highly significant difference (p less than 0.01). Histologic appearance of suction blisters was similar in diabetic and normal skin, under both light and electron microscopic examination, with a noninflammatory subepidermal separation in the lamina lucida between the cell membrane and the basal lamina. These results demonstrate that insulin-dependent diabetics have a marked reduction in suction blister threshold as compared to age-matched controls.

Heparin-induced cutaneous necrosis unrelated to injection sites. A sign of potentially lethal complications.

Skin necrosis is a rare complication of heparin administration that is usually localized to injection sites. A 32-year-old insulin-dependent diabetic patient, receiving intravenous (IV) and low-dose heparin sodium therapy, had cutaneous necrosis in areas distant to the sites of injection. Prior to the onset of cutaneous lesions, thrombocytopenia develop]ed in the patient that may have been heparin induced. Heparin may induce the production of platelet aggregating immunoglobulins that predispose persons who are sensitive to the drug to thrombocytopenia, skin necrosis, and thrombotic events. Obesity, diabetes, and treatment with broad-spectrum antibiotics seem to increase the risk of such complications. Cutaneous necrosis secondary to heparin administration may serve as a warning of the potentially lethal complications of IV use. In patients in whom skin necrosis or thrombocytopenia develops, heparin therapy should be discontinued and anticoagulation with an oral agent should be considered.

Neuropeptides in the pathogenesis of obesity-associated benign acanthosis nigricans.

The association of acanthosis nigricans with pituitary tumors and insulin-resistant diabetes suggests that a pituitary peptide may promote papillomatosis and acanthosis characteristic of acanthosis nigricans. Although such a peptide has not been isolated, it may derive by sequential cleavage from the 31,000-dalton precursor peptide to ACTH and beta-lipotropin (beta-LPH). In order to evaluate the role of pituitary peptides in the pathogenesis of acanthosis nigricans, we compared plasma levels of beta-endorphin (beta-EP) and ACTH in plasma of 8 fasting patients with obesity-associated benign acanthosis nigricans and 7 fasting normal controls utilizing sensitive radioimmunoassay procedures. Mean plasma beta-EP levels for the acanthosis nigricans and control subjects were not significantly different (90 pg/ml vs. 140 pg/ml), nor was any significant difference observed between plasma ACTH levels of the 2 groups (42.3 and 31.2 pg/ml, respectively.) Our data indicate that plasma levels of the pituitary-derived peptides ACTH and beta-EP are not increased in obesity-associated benign acanthosis nigricans, and suggest that its proposed hormonal mediator might originate independently from the large peptide precursor of ACTH, beta-LPH and their fragments.