Controlling the Microstructure of Arc Sprayed Shells

Techniques for controlling the microstructure of sprayed steel structures are discussed in this paper. Steel is arc sprayed onto shaped substrates to form tooling. The quality of the tool is greatly influenced by the microstructure of the material and the interlamella regions of the deposit. This work is focused on characterizing the microstructure, improving the state of the inter-lamella regions, and discusses our success in forming pseudo-alloys and graded shells by mixing sprayed materials. Microstructure control has interesting implications for other research as well, such as the MASK & DEPOSITS approach of forming objects.Mechanical Engineerin

Sprayed Metal Shells for Tooling: Improving the Mechanical Properties

Tbis work describes methods to improve the quality of the metal resulting from thermal spray deposition, including both the mechanical and metallurgical behavior. The engineering context is the production of sprayed metal shells suitable for tooling applications. The sprayed metal shells are mechanically dominated by interparticle interfaces; the particles are largely mechanically interlocked with very little metallurgical bonding. Based on these observations, improvements are made to these interfaces, and the measure of the improvement is shown in mechanical tests.Mechanical Engineerin

Origin and Properties of the Gap in the Half-Ferromagnetic Heusler Alloys

We study the origin of the gap and the role of chemical composition in the half-ferromagnetic Heusler alloys using the full-potential screened KKR method. In the paramagnetic phase the C1_b compounds, like NiMnSb, present a gap. Systems with 18 valence electrons, Z_t, per unit cell, like CoTiSb, are semiconductors, but when Z_t > 18 antibonding states are also populated, thus the paramagnetic phase becomes unstable and the half-ferromagnetic one is stabilized. The minority occupied bands accommodate a total of nine electrons and the total magnetic moment per unit cell in mu_B is just the difference between Z_t and $2 \times 9$. While the substitution of the transition metal atoms may preserve the half-ferromagnetic character, substituting the $sp$ atom results in a practically rigid shift of the bands and the loss of half-metallicity. Finally we show that expanding or contracting the lattice parameter by 2% preserves the minority-spin gap.Comment: 11 pages, 7 figures New figures, revised tex

Processing, Thermal and Mechanical Issues in Shape Deposition Manufacturing

An overview of Shape Deposition Manufacturing (SDM) is presented, detailing manufacturing, thermal and mechanical issues of concern in making it a commercially viable method for creating arbitrarily shaped three-dimensional metal parts. SDM is a layered manufacturing process which combines the benefits of solid freeform fabrication and other processing operations, such as multi-axis CNC machining. This manufacturing process makes possible the fabrication of multi-material layers, structures of arbitrary geometric complexity, artifacts with controlled microstructures, and the embedding of electronic components and sensors in conformal shape structures. Important issues toward the production of high quality objects are the creation of inter-layer metallurgical bonding through substrate remelting, the control of cooling rates of both the substrate and the deposition material, and the minimization of residual thermal stress effects. Brief descriptions of thermal and mechanical modeling aspects of the process are given. Because SDM involves molten metal deposition, an understanding of thermal aspects of the process is crucial. Current thermal modeling of the process is centered on the issue of localized remelting of previously deposited material by newly deposited molten droplets. Residual stress build-up is inherent to any manufacturing process based on successive deposition of molten material. Current mechanics modeling is centered on the issues of residual stress build-up and residual stress-driven debonding between deposited layers.Mechanical Engineerin

Shape Deposition Manufacturing

One challenge for solid freeform fabrication has been to develop the capability to directly create functional metal shapes which are dense, metallurgically bonded, geometrically accurate and with good surface appearance. Shape Deposition is a manufacturing paradigm which attempts to address these issues. It incorporates the advantages of several processes including solid freeform fabrication (complex geometries, rapidly planned), 5-axis CNC machining (accuracy, smooth surfaces), shot-peening (for stress relief) and 'microcasting' (a high-performance, weldbased material deposition process). These processes are integrated within a CAD/CAM system using robotic automation. This paper will present the current research in this effort.Mechanical Engineerin

Laser Deposition of Metals for Shape Deposition Manufacturing

A laser/powder deposition process has been added to the Shape Deposition Manufacturing system at Stanford University. This process is more robust than previous SDM metal deposition processes, consistently producing fully dense, near-net shape deposits with excellent material properties Material is deposited by scanning the laser across a surface while injecting metallic powders into the melt-pool at the laser focus. A number of parts have been produced with the system, including an injection molding tool, multimaterial structures and simple mechanisms. Currently research is being perfonned to improve the finish quality of the parts. One of the main areas of research involves controlling thermal stresses which can lead to warpage and delamination. Selective deposition techniques and the use of low coefficient of thennal expansion materials such as INVARTM show promise for reducing defonnations caused by internal stresses.Mechanical Engineerin

Spontaneous generation of hydrogen peroxide from aqueous microdroplets

© 2019 National Academy of Sciences. All rights reserved.We show H2O2 is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 μm to 20 μm in diameter). Production of H2O2, as assayed by H2O2-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of H2O2. The generated H2O2 concentration was ∼30 μM (∼1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O2 or bubbling O2 decreased the yield of H2O2 in microdroplets, indicating that pure water microdroplets directly generate H2O2 without help from O2 either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for H2O2 formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH- at or near the surface of the water microdroplet. This catalystfree and voltage-free H2O2 production method provides innovative opportunities for green production of hydrogen peroxide11sciescopu