Spray Tips: Particle Impact States That Create Structural Features

April 10, 2017
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The most common source of coating porosity is trapped, unmelted, or resolidified particles. Depending on the particle temperatures, the arriving droplets may cover the full range of liquid to solid states. The liquid-state particles flow easily and fill most voids, as illustrated in the figure.

 Solid particles, some of which are reflected from the solid surface, may adhere locally to, or become trapped in, the rough finish of the coatings. These "overspray" particles are not well bonded, nor are they in intimate contact with the underlying splat, which creates voids that are not open to direct line-of-sight. As stated before, thermal spray is a line-of-sight process; hence, the next arriving particles cannot fill voids adjacent to trapped solid particles.

 In other cases, even in the same instant, some solid + liquid (i.e., partially molten) particles may deform and be completely densified, provided sufficient liquid is available to fill any voids that form around the small amount of existing solid core. It is also possible that partially melted particles act similarly to solid particles, leaving trapped voids around their solid portions.

Volume 5A, Thermal Spray Technology â‡’ Thermal Spray Processes and Coatings ⇒ Coating Structures, Properties, and Materials ⇒ Coating Structures 

Titanium powders produced by reducing TiO2 with magnesium in a hydrogen atmosphere

April 7, 2017
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The University of Utah, Salt Lake City,  announces that it recently received $3 million in funding to develop two process technologies for production of low-cost titanium powders, from the Advanced Research Projects Agency- Energy (ARPA-E) of the U.S. Department of Energy and its industrial partners. The first is Ti-6Al-4V spherical powder for 3D printing/additive manufacturing, and the second is commercially pure (CP) titanium powder.

This is a continuation of a project funded under ARPA-E's METALS program since 2013. The goal of the continuation project is to produce powders at a pilot scale and validate the technology for industrial applications. The patented processes use low-cost commercial TiO₂ as the raw material, and magnesium to reduce TiO₂ in a hydrogen atmosphere. The processes are relatively simple and promise drastically reduced cost of production for both CP-titanium and spherical Ti-6Al-4V powders.

The process is based on a patented de-oxygenation technology that relies on hydrogen to thermodynamically destabilize Ti-O solid solutions. It can produce titanium powder with extra-low oxygen content. Producing low-oxygen titanium alloy powder for additive manufacturing has been one of the major factors contributing to its high cost. The technology of this project could potentially disrupt the existing marketplace.

The team is led by Professor Z. Zak Fang. Boeing and Arconic Titanium Engineered Products are industrial partners.

[email protected]

3-D printed aluminum electronics enclosure flies on Lockheed Martin military satellite

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Lockheed Martin, Sunnyvale, Calif., announces that an electronics enclosure fabricated via additive manufacturing will be the first 3D printed part certified for service on a Lockheed Martin military satellite. By going from multiple machined parts to one 3-D printed part, the team was able to save time in the design and production of the Remote Interface Unit, as well as increase the quality and consistency of the units.

The lead time for manufacturing the part went from six months to only 1.5 months, with assembly time also reduced, from 12 hours to just three hours.

"3-D printing provides the ability to rapidly implement innovation by controlling production from design through implementation with one digital model," says Iris Bombelyn, vice president of Protected Communications at Lockheed Martin Space Systems. "By providing affordable, innovative solutions with a reduced timeline, we are able to adjust to the rapidly changing environment of military space."

AEHF is a global military satellite communications system that provides protected, assured communication for strategic commanders and tactical warfighters. Lockheed Martin will deliver the fourth AEHF vehicle in 2017. AEHF-5 and AEHF-6 are in production and are on track to launch in 2018 and 2019, respectively.

The qualified part onboard AEHF-6 was built by Laser Powder Bed Fusion additive manufacturing, in which a laser melts and fuses aluminum metal powder layer-by-layer to build a part based on a digital design. The electronic enclosure will serve as a model for other programs that are designed based on the A2100 satellite bus.

Across Lockheed Martin, 3-D printing is applied for design prototyping, tooling, and the production of affordable components. The company produced the first 3-D printed parts to fly on an interplanetary spacecraft, Juno, which is currently orbiting Jupiter, and a 3-D printed part flew on Orion's first flight. Additionally, technicians are qualifying large 3-D printed fuel tanks for the modernized A2100 satellite bus.

 www.lockheedmartin.com/aehf

Supercritical carbon dioxide carries thin-film coating to protect silicon

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The Agency for Science, Technology and Research (A-STAR) Institute of Materials Research and Engineering, Singapore, reports that its researchers have now developed a new way to coat silicon with a layer of protective molecules using supercritical carbon dioxide (scCO₂). Carbon dioxide is converted to scCO₂ under high pressure, when it becomes a free-flowing liquid that is chemically inert, inexpensive, and more environmentally friendly than traditional solvents.

The researchers used scCO₂ to carry molecules called alkylthiols, which contain long carbon chains with a sulfur atom at one end. Sulfur forms a stable bond with silicon, while the water-repelling carbon chains make a tightly-packed skin on silicon's surface.

To apply the coating they used alkylthiols containing between seven and 18 carbon atoms to coat silicon, germanium, and silicon nanowires. Each procedure took a few hours, and produced monolayers between 1.6 nanometers and 2.3 nanometers thick that resisted wear and repelled water. The greatest effect was seen for the longest alkylthiol chains.

The monolayers also protected the surface from oxygen for more than 50 days; versus those prepared of conventional solvents that were typically stable for less than seven days. "The increase in stability was expected, but such long-term stability was a surprise," says researcher Dr. Sreenivasa Reddy Puniredd.

Silicon nanowires are being tested for a range of biological applications, including biosensors and antibacterial surfaces. Although fragile and easily damaged by other monolayer formation methods, the silicon nanowires were undamaged by the scCO₂ process, allowing the researchers to test how they interacted with human liver cells. Those protected by the 18-carbon alkylthiol significantly reduced cell growth on the nanowires, compared with unprotected nanowires or a flat silicon surface. This is probably because the cells' proteins could not latch on to the monolayer's long carbon chains.

"This scCO₂ technology can be adopted for many kinds of inorganic surface modification," says Dr. Puniredd. "The technology is not only scalable, but also enhances the quality and stability of the film. It can potentially replace billions of pounds of organic solvents used every year in thin-film fabrication and cleaning applications."

www.a-star.edu.sg

Cold spray of metal particles entrained in supersonic gas jet rebuilds aircraft parts

April 4, 2017
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The University of Akron and Airborne Maintenance and Engineering Services, Wilmington, Ohio, announce that they have developed supersonic particle deposition, an aircraft repair method for cold spraying metal particles to rebuild the surfaces of corroded and worn parts.

Supersonic particle deposition (SPD) is a high-pressure spraying process in which metal particles contained in a supersonic jet of an expanding gas impact a solid surface with sufficient energy to cause bonding with the surface. This additive manufacturing process builds up and repairs the surface of the metal part without creating a heat-affected zone that would result from welding or high-temperature thermal spray.

Development and testing has been underway for two years, funded by the Ohio Legislature in the last Operating Budget, and supported by the University of Akron's National Center for Education and Research on Corrosion and Materials Performance (NCERCAMP). The collaboration includes Airborne, SAFEngineering, and U.S. Technology Corporation. The data gathered has provided the foundation with the information required for FAA approval of SPD dimensional restoration of aircraft structures.

By extending the useful life of aircraft parts, this public-private partnership initiative is anticipated to lead to the first FAA certification for full-scale commercial aircraft repair operations.

UA's Corrosion Engineering program is the first of its kind in the United States. Housed at the University of Akron, NCERCAMP provides a multidisciplinary approach to help government and industry develop solutions for corrosion and materials performance challenges.

In response to requests from private industry and the Department of Defense in 2006, the University of Akron launched an effort to help address the cost of corrosion on the nation's economy, estimated at more than $400 billion annually. In 2010, NCERCAMP was established by Congress and the U.S. Department of Defense.

www.uakron.edu