Industries from across the United States send people to Oak Ridge National Laboratory’s High Temperature Materials Laboratory (HTML), and those people often leave with more than they dreamed of receiving.
One of those businesses is Walford Technologies, an Oak Ridge company headed by Graham Walford. “As an outsider to ORNL, I found it easy to work with the people and the analytic systems, but that’s just one aspect of this relationship,” Walford says. “What I’ve found to be of intense value is not only the basic science support but also the fact I’ve received knowledgeable input of how this technology could develop in the future.”
Walford Technologies has been perfecting the lost-foam-casting technology, which was patented in 1958 and is used to make engine blocks, cylinder heads, and other complex shapes for the automobile and other industries. In this process, an expanded polystyrene foam pattern is made as a mold for casting each part. The foam pattern is covered with a refractory coating and embedded in sand. As molten metal is poured into the foam pattern, the foam decomposes and the metal replaces it, precisely duplicating its shape.
The process offers qualities and properties not found in other casting methods; however, problems occur when foam residues and other contaminants become entrapped during the metal-fill process, causing casting defects. Walford, who has been working with Ralph Dinwiddie in HTML’s Thermography and Thermophysical Properties User Center (TTPUC), is especially interested in developing measurement technology and instrumentation for measurement and control of the lost-foam-casting process.
Walford and Dinwiddie, a researcher in ORNL’s Metals and Ceramics (M&C) Division, used infrared imaging, optical imaging and analysis, and gas flow studies and X-ray densitometry from Quintek Measurement Systems machines to better understand the lost-foam-casting process. They collected series of images from laboratory analyses and during the metals-casting process, which they digitized, analyzed, and correlated with measurements of other foam properties. Walford hopes all of this information will lead to further refinement of the method. Lost-foam casting is the most environmentally “green” of available casting processes because it allows reuse of all but 2% of the casting sands, its emissions are relatively low, and it uses less energy.
Other organizations providing technical support and input to Walford Technologies are the University of Tennessee’s Department of Materials Science and Engineering, HA International, General Motors Powertrain Division, and Saturn. “A foundation has been laid to provide standardized measurements for the lost-foam-casting process,” Walford says. “It’s these collaborations that lead not only to advances in the industry but also to more users coming to ORNL and taking advantage of these world-class resources.”
IMPROVING AUTOMOTIVE BRAKES
Ford Motor Company, like Walford Technologies, has a long-standing relationship with many of HTML’s researchers. The overall focus of this relationship has been better characterization of brake materials and their performance under various operating conditions so future brakes will perform better and last longer. Projects have ranged from using neutrons from an ORNL reactor to measure residual stresses in brake rotors to using infrared imaging and other techniques to measure heat conduction, friction, and wear in brake materials.
“Our collaborations with Ralph Dinwiddie and other staff at HTML have been enriching for Ford Motor Company on several levels,” says Rena Hecht Basch, a senior technical specialist in Ford’s Safety Research and Development (R&D) Department. “The collaboration is enhancing our scientists’ knowledge, enabling the development of new research tools and providing a resource for complex problem solving.”
Through the user center’s programs, Ford research staff members have developed better systems to test brakes. “Our most fruitful collaboration led to the development of an infrared-based, ultrahigh-speed system to map the temperature of a rotating disc brake on a dynamometer, a tool used to this day for advanced brake R&D,” Basch says. That work is leading to even better brakes in Ford Motor products.
In addition to TTPUC, HTML is home to five other Department of Energy user centers that assist industries, universities, and government agencies in developing advanced materials by providing a highly skilled staff and sophisticated instruments. The centers are Materials Analysis; Mechanical Characterization and Analysis; Residual Stress; Diffraction; and Machining, Inspection, and Tribology. Highlights of work being performed at some of these centers is described below.
MAKING AIR BAGS MORE RELIABLE
When it’s extremely hot or cold, air bags do not always deploy reliably. To remedy the problem, personnel from TRW Automotive Corporation and the University of Nevada at Reno came to Oak Ridge to work with researchers in HTML’s Diffraction User Center. The purpose of the project was to determine how to alter the crystal structure of the material that causes air bags to inflate so they work reliably even under temperature extremes.
“The problem,” says M&C researcher Claudia Rawn, “is that over the temperature range encountered by automobile air bag systems, pure ammonium nitrate, the material used for gas generation inside the air bags, exhibits phase changes, leading to volume changes.”
These phase changes (physical transformations, like turning graphite into diamonds) lead to irreversible swelling of the material that is supposed to generate gases during certain car crashes and cause the air bag to inflate. So to change the temperature of the phase transition out of the range that an automobile might experience during day-to-night temperature fluctuations, researchers added a second component to the ammonium nitrate.
“By changing the composition of the gas generators, there will be no degradation of the material that causes the air bag to inflate,” Rawn says. “We’ve effectively increased the ‘shelf life’ of air bags.”
To address the problem of determining phase stability of the different compositions that could be used in air bags, Rawn and her collaborators characterized these materials using HTML’s high-temperature X-ray diffraction and differential scanning calorimetry instruments. Armed with this information, they arrived at the optimum composition for air bag generators, enabling the air bags to inflate reliably under all temperature conditions.
HELPING THE PAPER INDUSTRY
For many years, the U.S. paper mill industry has had a problem with recovery boilers, which burn organic waste to generate steam and electric power for the mills and recover the inorganic chemicals used in the pulping process. Over time, some boilers develop cracks that create a potential for explosions, posing a hazard to workers and the mills themselves. Boiler repairs or replacements are costly, so the industry sought help from ORNL, including HTML.
M&C’s Pete Angelini, who manages the DOE’s Industries of the Future, Office of Industrial Technologies, research program at ORNL, assigned the problem to a group led by M&C’s Jim Keiser. The researchers conducted studies that showed that alloys 825 and 625 were far more resistant to cracking than 304L stainless steel, the industry standard.
Keiser’s group characterized the microstructures of different steels using transmission electron microscopes and other analytical instruments before and after subjecting the material to corrosion and fatigue tests in an environment simulating that of the recovery boiler floor. The researchers measured the samples’ residual stresses (which can lead to cracking) using X-ray diffraction at HTML’s Residual Stress Center and neutron diffraction at ORNL’s High Flux Isotope Reactor. They compared the microstructures and properties of the samples of the different steels to predict which alloys would be most resistant to cracking over time.
During the past few years, as a result of these studies at ORNL and paper institutes, new boilers have been built and older boilers retrofitted, using alloys 825 and 625 at paper mills across North America, from Washington to Saskatchewan to North Carolina. Each boiler saves about 370 billion British thermal units per year and, collectively, the boilers are helping make the process safer, cleaner, and more efficient.
LONG-LASTING CERAMIC KNEES
Until recently, people who needed knee replacement surgery often had two choices. They could wait until they were 60 or 65 to have the surgery or they could have the surgery, knowing they would likely need another surgery later because materials in the standard cobalt chrome implant usually last only about 10 to 15 years. All of that, however, is changing because of Smith & Nephew of Memphis, which has developed an artificial knee that is expected to last much longer.
Various metals, including titanium, have been used for implants because they provide strength but they do not solve the problem of wear. Knee implants made of ceramics may reduce wear but are brittle and can crack. Because oxidized zirconium components are made of a metallic zirconium alloy that is heated to convert the surface to a ceramic (zirconia), the best of both worlds can be achieved. Compared to cobalt chrome, oxidized zirconium, in knee-wear simulation testing, reduces the wear rate of the polyethylene by 85%.
“One of the primary reasons knee replacements last only 10 to 15 years is due to wear of the ultrahigh-molecular-weight polyethylenethe interface between the upper and lower part of the artificial knee,” says Randy Fesmire of Smith & Nephew. “This wear is primarily due to roughening of the cobalt chrome femoral component, which is caused by both scratching and oxidative wear of the surface.”
Smith & Nephew was helped by M&C’s Laura Riester and Peter Blau, who conducted a number of tests of the material. The information gathered from these tests was required before Smith & Nephew’s Gordon Hunter and Marc Long could submit the ceramic knee concept to the U.S. Food and Drug Administration for approval. Specifically, measurements of the nano-hardness of the zirconia-zirconium alloy were needed.
“The surfaces of materials are often subjected to a variety of processes that can change their appearance and function,” says Blau, an M&C researcher in HTML’s Machining, Inspection, and Tribology User Center. “In the case involving the replacement knee, it was important to understand how long materials rubbing together in artificial knee joints will resist abrasion.”
One of the tests performed was the single-point, diamond scratch test. Working with Long, Blau used a specially ground and polished diamond point to scratch the surface of the material under a range of pressures.
“By measuring the width of the scratch on a material or coating, we can calculate the dynamic hardness and compare its relative abrasion resistance with that of other materials or coatings,” Blau says. “By examining the material under an optical or electron microscope, we can see the unique ways in which different types of materials respond to abrasion.”
Riester performed tests to measure changes in hardness and elasticity of the zirconia-zirconium alloy. Her tests also demonstrated that the material, which is 15 times as hard as aluminum, would not chip off over time.
HTML: PAST AND FUTURE
HTML is supported by DOE’s Office of Transportation Technologies, which is within the Office of Energy Efficiency and Renewable Energy. It was built in the mid-1980s to enable industrial, university, and government materials research collaborations to help make the nation less dependent on foreign oil supplies. The goal was to develop “high-temperature materials,” such as advanced ceramics, which can withstand the harsh environments of transportation vehicle engines designed to run at unusually high temperatures so vehicles can go farther on less fuel.
“Since our beginning, we’ve added many capabilities and instruments, and we’ve upgraded many of our existing instruments,” says Arvid Pasto, director of HTML. “For instance, we have taken on the responsibility of operating a synchrotron beam line at the National Synchrotron Light Source at Brookhaven National Laboratory. And, we will add a new aberration-corrected electron microscope soon in a new facility next to HTML.”R.W.
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