ORNL researchers Joseph Vought (left) and Vinod Sikka (right) and Philip Morris' Seetharama Deevi (center), who was on a 1-year sabbatical at ORNL, developed the award-winning Exo-Melt process. This system enables low-cost manufacturing of advanced materials such as nickel and iron aluminides. Photograph by Bill Norris.
Nurtured by ORNL researchers for almost 15 years, nickel aluminides may have found their niche. ORNL's modified nickel aluminides are receiving considerable attention by the heat-treating industry in the United States and may have arrived just in the nick of time to make some companies more competitive.
Nickel aluminides are intermetallic materials that have long been considered potentially useful because, thanks to their ordered crystal structure, they are very strong and hard and melt only at very high temperatures. But they had a serious weakness: they were too brittle to be shaped into reliable components. Then, in 1982, ORNL researchers led by Chain T. Liu in the Metals and Ceramics Division found the secret recipe for producing a ductile nickel aluminide alloy: add trace amounts of a few alloying elements in the right proportion. It was like turning peanut brittle into taffy. Their most important discovery was that the addition of a small amount of boron (200 parts per million) to a nickel aluminide alloy (Ni3Al) makes the alloy highly ductile at room temperature.
Joseph Vought inspects a coil cast from nickel aluminide at Harrison Alloys in Harrison, New Jersey.
ORNL's modified nickel aluminides have been attractive for industrial applications because they are lighter and five times stronger than stainless steel. They are also affordable: they contain no expensive, difficult-to-obtain materials of strategic value. Unlike standard alloys, which have a disordered structure that becomes even more random and weaker at increasing temperatures, nickel aluminides with their ordered structure become stronger as temperature rises to about 800°C. At high temperatures, they are resistant to wear, deformation, and fatigue, which is failure by cracking resulting from repeated stress or temperature.
Since the mid-1980s, ORNL's nickel aluminide alloy compositions have been licensed to about a dozen companies for various uses, ranging from processing glass to making dies for forming beverage containers and other shapes from metal. Automobile and tool companies have found ORNL's nickel aluminides especially appealing. The reason: because of its high aluminum content, this class of materials is virtually unaffected by gases containing oxygen or carbon at high temperatures. Because of their resistance to oxidation and carburization at temperatures up to 1100°C (2000°F), nickel aluminides are considered potentially useful materials for "furnace furniture"assemblies used to contain parts during treatment at high temperatures to harden their surfaces. They are viewed as excellent candidate materials for tools for the metal-forming industry and for trays, belts, radiant tubes, mufflers, and other furnace components used to manufacture parts by the automobile industry.
"These vendors had experience adding less than 1% aluminum to steel to remove trapped oxygen," Sikka says, "but they resisted melting material that had as much as 8 to 12% aluminum. They feared that the molten aluminum would leak through tiny cracks in the furnace wall and attack the heating coils, possibly causing an explosion."
To address the safety concerns of the alloy preparation industry, Sikka and Joseph Vought developed a new process in collaboration with Seetharama Deevi, who was on a 1-year sabbatical at ORNL from the Research Center at Philip Morris in Richmond, Virginia. The development, called the Exo-Melt process, received a 1995 R&D Award from R&D magazine.
"Our process is a special way of loading the furnace crucible to take advantage of the heat that is generated in the reaction between nickel and aluminum, as in the case of reaction synthesis of materials," Sikka says (see sidebar "Aluminides: From Powders to Products Using Reaction Synthesis"). "The reaction that produces NiAl liberates a large amount of heat and is, therefore, called an exothermic reaction. The heat of the reaction can be used to dissolve the alloying elements, such as boron, chromium, molybdenum, and zirconium. These dopants give the aluminide special properties like ductility and strength.
Schematic of the furnace-loading sequence employed for the Exo-Melt process to form modified nickel aluminide (Ni3Al) by melting nickel, aluminum, and small amounts of alloying elements. A similar arrangement can be used to form iron aluminides.
"What we want to produce is Ni3Al, so we start by putting nickel on the furnace floor to provide the extra two atoms of nickel for each molecule of NiAl. For the next layer, we add the alloying elements. Then we add nickel, which is sandwiched between two stacks of aluminum at the top."
As the furnace elements are heated by induction coils, the exact temperature reached by each element depends on its properties. Nickel, which has a melting point of 1440°C, heats up to about 800°C as aluminum reaches its melting point of 660°C. As the aluminum melts and comes in contact with the heated nickel, NiAl forms, releasing large amounts of heat. In fact, droplets of NiAl, which has a melting point of 1639°C, begin to form and drip down along with unreacted molten aluminum. The superheated NiAl liquid dissolves the alloying elements on its way down. The molten aluminum also continues to react with the heated nickel and forms additional NiAl, which reacts with the nickel at the bottom of the furnace to form Ni3Al.
"The key to the success of the Exo-Melt process is the furnace-loading sequence," Sikka says. "It makes possible the NiAl reaction that provides much of the energy for melting the rest of the elements in the furnace. The efficient use of heat from the NiAl reaction gives the Exo-Melt process several advantages. It saves energy, using only one-half to two-thirds of the power required by the conventional process. Because it melts the elements rapidly, it minimizes oxidation of the alloying elements and increases furnace life by minimizing time at high temperature. It allows ease of vacuum melting because all alloying elements can be loaded at the start. Best of all, it eliminates vendors' safety concerns in melting nickel aluminides."
The Exo-Melt process has been communicated by ORNL to alloy preparation vendors, and journal articles have been published on the process (e.g., "Exo-Melt: A Commercially Viable Process," Advanced Materials & Processes, by Sikka et al., Vol. 147, No. 6, June 1995). The vendors using the Exo-Melt process to melt large heats of nickel aluminides and cast components are Alloy Engineering and Casting Company in Champaign, Illinois; United Defense in Anniston, Alabama; The BiMac Corporation in Dayton, Ohio; and Sandusky International in Sandusky, Ohio. The heats range from 150 to 3000 pounds.
General MotorsSaginaw Division uses modified nickel aluminide furnace trays to hold automobile parts transferred to a furnace for heat treating.
Nickel aluminide parts made by the Exo-Melt process are already being used in the industrial world on an experimental basis. At the General MotorsSaginaw Division facility in Saginaw, Michigan, automobile parts, valves, ball bearings, and gears are heated in a carbon atmosphere in a furnace. "Carbon diffuses into metal," Sikka says, "and hardens the component surfaces to reduce wear."
Recently, an experiment was tried in a batch furnace at the General MotorsSaginaw facility. Gears were loaded onto two different trays that were moved into the furnace. One tray was made of the chromium-nickel alloyed steel (HU steel) conventionally used by GM. The other was made of nickel aluminide. Both trays and their gears were subjected to a carbon source for the same time and at the same temperature. Then the gears and tray were moved to a tank of oil for quenchingthey were cooled rapidly from between 800 and 900°C to 25°C, or room temperature. The gears or other parts being heat-treated to harden their surfaces are subjected to the heat-treatment cycle only once, but the trays holding the parts must endure hundreds of cycles in the furnace. The repeated thermal cyclesheating and rapid coolingand the hardening effects of the carbon atmosphere caused the HU steel tray to fall apart in 6 months. The nickel aluminide tray was barely affected.
Sikka explains that, because of its high aluminum content, nickel aluminide forms a thin film of aluminum oxide on its surface. "This aluminum oxide film is like a brick wall," he says. "It keeps carbon from diffusing into the alloy because carbon has very low affinity for aluminum."
As a result of the batch furnace test, Sikka says, "General Motors concluded that nickel aluminide has twice the life of the HU steel that they've been using in their furnace trays. It is expected that continued tests will show that nickel aluminide lasts three to five times longer than the conventional material. Because use of nickel aluminide extends the life of furnace furniture, the furnace can be operated much longer before furniture replacements are needed, saving General Motors millions of dollars."
General Motors is considering the evaluation of complete nickel aluminide furnace assemblies consisting of a base tray, upper and lower grids, and support posts. The evaluation is part of a cooperative research and development agreement (CRADA), using funds from both DOE-ORNL and General Motors.
For another example, consider Rapid Technologies, a company in Newnan, Georgia, that manufactures furnaces to be used for heating metal for manufacture of horseshoes, pliers, and wrenches. This company is using ORNL's nickel aluminide from United Defense to make rails for its walking-beam furnaces. The heat-treating furnace moves steel bars to a very high-temperature region, softening them so they can be formed into tools and horseshoes. The company has found that use of nickel aluminide rails in walking-beam furnaces has made possible the commercialization of its rapid-heating furnace technology. Because this technology provides rapid heating, less natural gas and process cooling water are needed, resulting in lower emissions and reduced energy use. Such technology should enable U.S. companies to be more competitive and to save jobs.
A major steel company is interested in using transfer rollers made of nickel aluminide in its steel-mill heat-treating furnace. There it heat treats steel plates at 800900°C to soften them so they can be shaped into components for bridges and other structures. The current rollers, which are turned by a gear system to move each steel plate along through the mill, are fabricated from steel.
"These rollers eventually have two problems," Sikka says. "In the intense heat, they lose their strength, sag, and begin to wobble, jostling the steel plates. Also, they develop oxide particles on the surface that cause scratches. As a result, a large fraction of the steel plates produced at the steel mill are scratched. That's a problem because their competitors in Japan are selling steel plates that are not scratched."
The approach to the problem has been to shut down the furnace every third week for an inspection of the rollers. Sagging rollers are replaced with new ones. The needlelike oxides are ground out of the other rollers.
In the past year, the company has tried using rollers made from nickel aluminide in the mill. "Company engineers noticed that the oxides produced on the surface of the nickel aluminide are smooth," Sikka says. "Also, they concluded that nickel aluminides did not sag because they are three times stronger than the steel being used in the rollers.
"They hope to test a large number of nickel aluminide rollers soon. If the material performs as well as tests show, they should have to turn off the furnace only about once a year, not every 6 weeks, making it much more competitive in the steel industry."
Nickel aluminides have come a long way since 1981 when Liu received $10,800 from ORNL's seed money program to find ways to make nickel aluminides ductile. Over $21 million has been spent on ORNL research that has successfully developed ductile nickel aluminides and an efficient process to produce them that is becoming acceptable to the alloy preparation industry.
"At the end of 1995, 50,000 pounds of Ni3Al had been melted by industry," Sikka says. "Commercial sales of nickel aluminide are approaching a half million dollars. These amounts should climb rapidly now that a network of both nickel aluminide suppliers and users exists."
These successes would not have been possible without support from DOE's Office of Energy Efficiency and Renewable Energy, Advanced Industrial Materials Program; DOE's Office of Energy Research, Basic Energy Sciences Materials Program and the Energy Research Laboratory Technology Applications Program; and DOE's Office of Fossil Energy, Advanced Research and Technology Development Materials Program. These programs, especially Fossil Energy, also sponsored ORNL's research on iron aluminides described in the sidebar "ORNL's Iron Aluminides: An Emerging Success Story"
Thanks to nudges from and networking by ORNL researchers, nickel aluminides appear to have found a niche. Clearly, they possess special properties that make them well suited for specific uses. Because they may help U.S. industry become more competitive and save jobs, ORNL's nickel aluminides are now breaking into the marketplace.
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