To make thin sheets of metal the traditional way, ingots of metal are heated in a furnace and then pressed and rolled over and over again, sometimes at both hot and cold temperatures, to get the right properties. It is a time-consuming, energy-consuming, expensive process.
No genie ever comes out of ORNL’s plasma arc lamp, but this powerful lamp shows promise for magically transforming metallic powders into thin sheets of metal that are even less likely to deform when exposed to high temperatures. The powders are heated within minutes, and by adjusting the lamp’s setting, the sheet produced has the desired grain size, ductility, strength, toughness, and other mechanical properties. The process, if perfected, should produce thin metallic sheets with desired properties better, faster, and cheaper than traditional methods.
ORNL’s novel method has already been demonstrated for fabricating nickel sheet. The work was performed by Craig Blue, a materials researcher in the Materials Processing Group in ORNL’s Metals and Ceramics Division (M&C); M&C’s Vinod Sikka (group leader), Evan Ohrinher, and David Harper; and graduate students John Rivard and N. Jayaraman, both from the University of Cincinnati. Blue is heading the effort in infrared processing at the Infrared Processing Center, a Department of Energy user facility at ORNL, where the plasma arc lamp is located.
The researchers made the nickel sheets from powder, using the Infrared Processing Center’s 300,000-watt plasma arc lamp, which delivers 3500 watts/cm2 of an infrared beam that can irradiate areas ranging in width from 10 to 35 cm. The lamp was built by Canada’s Vortek Industries. It generates high-density infrared radiation when an arc of direct current strikes atoms of argon (an inert gas) at high energy as the current passes between a tungsten anode and cathode encased in a water-cooled quartz tube. A robotic arm precisely moves the infrared source to process the samples.
The ORNL group has received funding from the Defense Advanced Research Projects Agency and the National Aeronautics and Space Administration to use the lamp to demonstrate that a thin titanium aluminide sheet can be made from powder. Titanium aluminide “skins” cover the outside of military aircraft and spacecraft to protect them from the searing temperatures of 700 to 800°C these vehicles are exposed to when flying through the earth’s atmosphere at high speeds.
“A titanium aluminide sheet has a room-temperature ductility of only 1.5% and maintains its high-temperature mechanical properties at 700 to 800°C,” Blue says. “It costs $10,000 per square foot to make a skin that is 40 mills thick. Our process of pressing titanium aluminide powder at room temperature into a sheet and scanning the green powder sheet with the plasma lamp to liquid-phase sinter it into a final sheet without rolling it drops the cost 100 times, from $10,000 to $300 per square foot.” In liquid-phase sintering, a metallic powder is rapidly heated so that it forms a coherent mass without completely melting.
The ORNL technique shows promise for producing thin sheets of rhenium from powder. This process is of interest to the Missile Defense Agency of the U.S. Department of Defense because rhenium is an important material needed for components of “kill” vehicles, used to knock down incoming missiles armed with nuclear warheads before they hit populated areas. “Our goal is to produce rhenium in sheet form faster and cheaper while improving its properties by forming 10 times as many grains. As a result, when the sheet is formed into tubes, we believe the tubes will be less susceptible to cracking and failing.”
ORNL metallurgical engineers are experimenting with using the lamp to fabricate composite sheets from metallic powders mixed with fibers of ceramics, such as silicon carbide. “There is demand for sheet products that can be produced in a cost effective manner,” Blue says. “We have an incentive to find faster, better, and cheaper ways to make metallic sheets since metal-matrix composites with continuous fibers have limited application because of their high processing costs.”
The plasma arc lamp is also being used in projects related to moving vehicles. M&C’s Dave Stinton, Ron Ott, and Craig Blue are working with Ford Motor Company under DOE’s Advanced Automotive Materials Initiative to provide extruded aluminum underbodies for cars with softened areas that function as preferential crumple zones during a crashareas called crash triggers.
“By using 2-second pulses from the plasma arc lamp, we can reduce the hardness of the extruded aluminum frame by 50%,” Stinton says. “Our goal is to make the aluminum frame soft in predetermined areas so that it will absorb the maximum amount of energy in a head-on crash. We are doing computer modeling to predict how much energy will be absorbed by a bumper that has a certain level of softness. By making an aluminum frame that absorbs the most energy in the crash, we can improve the ability of the car to hold up in a crash, helping to protect the passengers from serious injuries.”
When bulldozers, backhoes, and other heavy earth-moving equipment push and lift boulders and soil mixed with rocks, their steel buckets would be rapidly abraded were it not for their abrasion-resistant coatings. Caterpillar, Inc., applies thermal spray coatings to its excavation equipment to protect it from abrasion. The problem is that Caterpillar’s thermal spray coatings eventually peel off like wall-paper strip-ped from wallsa phenomenon called delamination.
Using the infrared arc lamp at ORNL as part of a pro-ject led by M&C’s Gail Ludtka, Caterpillar researchers are finding they can spray the coating on first and then use the lamp to fuse it to the steel. The infrared lamp has a large-area beam and gives users the ability to precisely control the beam position and energy to fuse coatings to steel substrates.
“The rapid heating by the lamp enables the coating to bond metallurgically to the steel substrate without changing its properties,” Blue says. “The fused-on coating has higher wear properties and does not peel off. The coated steel should be able to resist damage from high-impact wear much longer than before.”
Blue and his colleagues have also been demonstrating the use of the infrared beam to fuse corrosion- and wear-resistant carbide coatings to H-13 steel pins and housing for aluminum dies used to cast automotive parts. The problem is that when liquid aluminum is injected into the dies, it reacts with the H-13 steel, degrading it and gradually making the die unusable. The fix is to coat the H-13 steel components with a carbide coating that protects them from attack by liquid aluminum. According to Blue, “We combine carbide particles with metallic bindings such as nickel and phosphorus to make a powder that is sprayed on the substrate and then fused to it using the Vortek plasma arc lamp.”
MAKING CHEAPER POWER SOURCES
Smart cards, radiofrequency identification tags, implantable medical devices (e.g., defibrillators and hearing aids), and integrated flexible circuits will be less expensive and more practical to use if they are powered by smaller, lighter, longer-lasting batteries. Nancy Dudney of ORNL’s Solid State Division (SSD) is using the ORNL lamp to determine whether thin films can be recrystallized on a substrate to make a lithium-ion battery. The goal is to determine whether infrared beam pulses, or energy bursts, lasting under 20 milliseconds can crystallize lithium-ion thin films deposited onto polymer substrates. Normally this process requires furnace anneals at 400° to 700°C, well above the maximum working temperature for polymers. Because a polymer substrate weighs and costs less than most substrates, a thin-film lithium battery made on a polymer would be both lighter and cheaper, increasing its market share.
Solid-oxide fuel cells that would power clusters of buildings are being developed in ORNL’s Fuel Cell and Functional Materials Program, managed by M&C’s Tim Armstrong. In a project led by M&C’s Ted Huxford, the plasma arc lamp is being used to sinter electrolyte films on substrates, to reduce the production cycle time and the costs of making fuel-cell stacks. It now takes 24 hours to do this sintering conventionally; using the ORNL lamp, the processing time has been reduced to 1 hour. “But the goal,” Blue says, “is to get the processing time under 10 minutes to reduce the cost of making solid-oxide fuel cells.”
Another ORNL user of the plasma arc lamp is SSD’s David Geohegan. He is interested in comparing laser ablation with infrared processing as a means of fabricating carbon nanotubestiny cylinders of carbon atoms in pentagonal and hexagonal arrangements resembling rolled-up chicken wire. Geohegan is looking at ways to use carbon nanotubes as reinforcing fibers in a metal or polymer matrix, to make a very strong structural material.
DOE’s Infrared Processing Center is being used for $2 million worth of research projects. Blue says that some 20 ORNL researchers are using the plasma lamp and that about 12 metallurgical and materials engineers are working with both ORNL and outside users to ensure that the lamp is fully utilized.
The infrared work includes research by three graduate students. John Rivard is pursuing a Ph.D. degree from the University of Cincinnati; he is studying the use of the plasma lamp for direct sheet fabrication. Greg Engleman from the University of Tennessee at Knoxville (UTK) is looking at coating fusing. Engleman is working with Nartendra Dahotre, director of the Center for Laser Applications in UTK’s Materials Department. The two are doing research that compares plasma-lamp-based and laser-based fusing of coatings. “Lasers work best for fusing needed in small areas, and the plasma lamp works best for fusing coatings over large areas,” Blue says. “There are pros and cons for each method.” The third graduate student, Hui Lu, who is from Northeastern University, is looking at rapid heating of aluminum-based materials and grain refinement.
“We have a formal memorandum of understanding with Vortek Industries,” Blue says. ORNL continues to work with Vortek Industries, the Canadian manufacturer of the lamp, to design and build an even more powerful, one-million-watt lamp for the Infrared Processing Center. Thanks to the hard work of many researchers and the teamwork of talented men and women from a broad range of backgrounds, the future for the user center looks even brighter.
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