Researchers develop new energy-efficient technique to weld steel and aluminum

 

Cars could soon be lighter and stronger, thanks to vaporized foil actuator welding (Credit: Shutterstock) As manufacturers, particularly in the automobile industry, continue to work toward incorporating lighter metals like aluminum with heavier steel, the ongoing problem has been how to successfully weld them together. The problem is that the high heat created in the welding process actually weakens these lighter metals, creating a less than optimum weld. After 10 years of research, engineers at The Ohio State University have developed a new welding technique that may prove to solve this problem while also using 80 percent less energy and creating bonds that are 50 percent stronger. "With our method, materials are shaped and bonded together at the same time, and they actually get stronger," says Glenn Daehn, professor of materials science and engineering at Ohio State, who helped develop the new technique. The traditional technique, called spot resistant welding, works by passing an electrical current through pieces of metal using the natural electrical resistance within those metals. This generates heat that melts them together to form a weld. This process consumes a lot of energy, and the melted metals are left in a weaker state than they were before. What Daehn and his team have done is create a new welding method called vaporized foil actuator (VFA) using short high voltage electrical pulses (lasting millionths of a second) passed through an aluminum foil, and a burst of hot gas traveling at speeds approaching thousands of miles per hour, to bond the atoms of one metal to atoms on the other. Since the two different bonded metals don’t melt, neither metal is weakened and the resulting seam is stronger. The energy savings comes from the fact that the electrical pulse is so short and the required energy to vaporize the foil is less than is necessary to melt the parts together in a traditional process. Daehn and his team have already used the process to successfully bonded different combinations of copper, aluminum, magnesium, iron, nickel and titanium. They have created strong bonds between commercial steel and aluminum alloys – a feat which is impossible normally. VFA is evidently also able to shape metal parts during the welding process, which could save manufacturers an additional step. In 2012, Honda announced it had created a similar result, welding aluminum to steel using a variation of a process called friction stir welding (FSW). Rather than the high heat and gas applied to a third element like the aluminum foil used in the VFA process, FSW uses heat, friction and pressure with a third element to meld two metals together. Like VFA, it uses less energy and creates a bond that has been proven to be stronger than that created by a traditional welding process. Variations of FSW have since been used by Lincoln, Mazda and Audi. Source: The Ohio State University   http://www.gizmag.com/vaporized-foil-actuator-welding/40137/

Continuous fabrication system for highly aligned polymer films provides method for tuning mechanical and thermal properties in bulk polymers

 

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Sequence of scanning electron microscopy (SEM) images demonstrate drastic changes in polyethylene surface morphology resulting from the fabrication process. The initial polymer particulate material resembled tightly wound balls of string (a). Comparing this to the extruded sample (b), however, polymer disentanglement as a result of the high shear rate Couette-based extrusion process is evident. SEM images of 50¡Á (c) and 100¡Á (d) drawn films. As shown, film structure is uniform fiberous with minimal defects. Scale bar represents 100 microns (a), and 2 microns (b-d).

A team of researchers from the Massachusetts Institute of Technology (MIT) in Cambridge, MA have demonstrated a novel automated fabrication process consisting of a three-step sol-gel extrusion, structure freezing and drying, and mechanical drawing process which results in production of highly aligned polymer films. Alignment of molecular chains within polymers is a desirable trait for many applications as it results in superior mechanical and thermal properties in the polymeric materials. Although these highly aligned polymer films (HAPFs) are in demand, previous fabrication methods were limited to manual, lab-scale batch processes. This novel, scalable technology can enable deployment of low-cost and energy efficient polymer alternatives to traditionally used materials in heat transfer applications, such as electronic packaging and heat exchangers, with the additional advantages of energy savings, weight reduction, chemical resistance, and electrical insulation. The report appears in the September 2014 issue of the journal Technology.

"By taking advantage of the inherent high thermal conductivity of polymers' C-C bond, and a corresponding reduction in chain entanglements and defects, this process opens the door for transforming materials which are traditionally considered thermal insulators into something that is ideal for use in heat transfer applications," says Professor Gang Chen, Ph.D., of the Massachusetts Institute of Technology and Principal Investigator on the paper.

While lab-scale fabrication processes have been successfully demonstrated, however, the investigators at MIT were able to solve the significant challenges which exist in terms of scaling and automated handling of the numerous process variables. In this work, improvement, or 'tuning' of the material properties, is achieved through manipulating the polymers' molecular chains first by disentanglement followed by macroscopic plastic deformation-induced alignment. This high throughput platform has three advantages over previous methods: (1) utilization of Couette flow for enhanced chain disentanglement; (2) constant-force adaptive-thickness mechanical drawing system aiding in uniform film production; and (3) an automated scalable platform -- thus successfully demonstrating a desktop printer sized fabrication platform for HAPFs in a commercially attractive form factor.

Utilization of Couette flow produces a high degree of molecular chain disentanglement; liquid N2 cooling freezes the disentangled structure in the extruded polymer gel; and the constant-force mechanical drawing leads to highly crystalline and uniform aligned final films. The platform was demonstrated using ultra-high molecular weight polyethylene, producing HAPFs with crystallinity >99% and lengths exceeding 15 meters. Molecular chain disentanglement has two crucial effects: (1) allows for plastic deformation to high draw ratios without film rupture, and (2) helps in subsequent molecular chain alignment for improved material properties.

While commercial-grade production systems for highly aligned polymer fibers are already in use (and address an existing mature commercial market), new opportunities for highly aligned polymers in a film form factor must be addressed. "This is a great example of a market-ready technology, which can supplement existing fabrication processes used for producing aligned polymer fibers. While fibers are ideal for textiles, however, for practical applications, such as fins in heat exchangers, casings for electronic systems, and biomedical treatments for improved cooling, a film (vice fibrous) form of these materials is essential.

The difficulty lies in translating the remarkable material property enhancements seen in high performance fibers into a film form factor," says James Loomis, Ph.D., the lead author on this paper. "Furthermore, for widespread commercial implementation of these advanced materials, a scalable, continuous, and robust film manufacturing platform is needed."

The team from MIT is working now to further characterize structural changes in the polymers as a function of the draw ratio (amount of plastic deformation in the films), establishing relationships between varying molecular weights and realized material properties, and investigating orientation effects of highly aligned nanocarbon-polymer composites. Application of this technology towards composites which incorporate electrically conductive nanofillers presents an exciting new direction for commercial deployment of composites with tunable strength, and thermal conductivity, and electrical conductivity.

Additional co-authors of the TECHNOLOGY paper are Hadi Ghasemi, Ph.D., Xiaopeng Huang, Ph.D., Nagarajan Thoppey, Ph.D., Jianjian Wang, Jonathan K. Tong, Yanfei Xu, Ph.D., Xiaobo Li, Ph.D., and Cheng Te Lin, Ph.D., all from the NanoEngineering Group at the Massachusetts Institute of Technology.

This work was funded by the Department of Energy/Office of Energy Efficiency & Renewable Energy/Advanced Manufacturing Program (DOE/EEREAMO) under award number DE-EE0005756.

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Story Source:

The above story is based on materials provided by World Scientific. Note: Materials may be edited for content and length.

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Journal Reference:

1. Alvin Chen, Kevin Nikitczuk, Jason Nikitczuk, Tim Maguire, Martin Yarmush. Portable robot for autonomous venipuncture using 3D near infrared image guidance. TECHNOLOGY, 2013; 01 (01): 72 DOI: 10.1142/S2339547813500064

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