Managing and remaking metals
Cast magnesium research
The U.S. Automotive Materials Partnership (USAMP-AMD), part of USCAR, recently began a new Structural Cast Magnesium Development (SCMD) project that will focus on resolving critical issues that limit the large-scale application of structural cast magnesium castings for automotive components. Project Chairman Richard Osborne, of General Motors Corp., indicated the five-year, $10-million project activities "will focus on developing the technology necessary to cast and implement a front cast magnesium structural cradle."
 A team of engineers from General Motors, Dana, and Alcoa have developed a one-piece aluminum propshaft to improve NVH on the Chevrolet Silverado and GMC Sierra Heavy-Duty pickups. |
The selection of a chassis component for the study provides many stretch goals for the project team to overcome. Key manufacturing issues include the production of high-integrity castings (high-pressure die, semisolid, low-pressure, squeeze, etc.) and appropriate joining methods. Harsh-service environments create significant material issues including erosion/corrosion and stress/relaxation.
The objectives of the SCMD project are:
- Improve understanding of cast magnesium alloys
- Develop a cost model that compares cast magnesium chassis component cost to alternative materials and processing techniques
- Provide comprehensive database and design guidelines
- Improve cast component integrity
- Identify and/or develop methods to improve corrosion resistance
- Develop accurate failure mode and effects analysis for design and manufacturing of chassis-type components
- Improve dissimilar material joining technologies
- Transfer knowledge and lessons learned to industry.
This project, sponsored cooperatively through USAMP-AMD and the DOE through a Cooperative Research And Development Agreement (CRADA), involves a number of research entities including Lawrence Livermore (LLNL), Oak Ridge (ORNL), and Sandia (SNL) National Laboratories, as well as General Motors, Ford, DaimlerChrysler, and more than 35 light-metal suppliers.
 With engine emissions standards for diesle engines becoming stricter, the International Copper Association believes its CuproBraze charge air coolers will meet the increased performance, pressure, and temperature demands of the next-generation engines. |
Osborne indicated there is a multitude of benefits to be gained from this research. "We anticipate successful completion of this CRADA will lead to positive applications, both tangible and intangible," he said. For instance, vehicle mass savings for ground and air transportation will lead to a reduction in fuel consumption and emissions, increased recyclability, and less dependence on foreign oil. Automakers in the U.S. are under increasing pressure to reduce CO2 emissions and increase federal Corporate Average Fuel Economy (CAFE) standards.
The ultimate goal of the SCMD project is the development of a mechanical property database and design/development of alloys for body and chassis applications (i.e., using ambient-temperature alloys instead of the high-temperature alloys that are necessary for powertrain components). USAMP also has had under way since January the Magnesium Powertrain Cast Components Project, the goal of which is to determine the potential benefit of using magnesium alloys in powertrain components The North American automotive industry currently uses approximately 3.5 kg (7.7 lb) of magnesium per vehicle. Cast magnesium structures have the potential to reduce 100 kg (220 lb) of vehicle mass, which could reduce emissions by 5% and increase fuel economy by about 0.43 km/L (1.0 mpg). The structural magnesium project comes on the heels of a successful five-year, $10-million cast light metal program that focused primarily on cast aluminum research and was recently completed by the same team.
Copper coolers
To meet the increasingly strict U.S. and European emissions standards for new diesel engines, the engines will require better volumetric efficiency for more complete combustion. Currently, most diesels use turbochargers and charge air coolers (CAC) to reduce engine emissions and to increase engine power and fuel economy. To meet these new standards, it will be necessary for engine builders to increase the inlet pressure of the engine. The increase in inlet pressure increases inlet temperature, and it is the CAC's job to remove the additional heat.
 A cutaway of the stainless-steel crucible shows the neodymium-iron-boron (Nd-Fe-B) magnet material at the bottom, that has been immersed in liquid magnesium. (Photo courtesy of Ames Laboratory.) |
The CAC is an integral part of the engine design in Class 7 and 8 trucks. The average inlet temperature in current CACs is 190°C (374°F). To reach the reduced emissions standards, the industry expects the average inlet temperature to increase to 246°C (475°F). The already high failure rate of current aluminum CACs is expected to rise with the increase in pressure and temperature that the new standards will bring. According to the International Copper Association (ICA), the industry will have to develop a copper/brass CAC because aluminum's tensile strength declines rapidly at 150°C (302°F) and the repetitive thermal cycling between 150-200°C (302-392°F) substantially weakens the product.
Because of the need for higher pressure, the need for highly efficient CACs will greatly increase in the next few years, according to the ICA. All six major U.S. diesel engine manufacturers will be forced to comply with the stricter regulations beginning with the 2002 model year, with remaining American manufacturers being brought into compliance by the 2004 model year. Europe's new standards will go into effect beginning with the 2005 model year. Owners of trucks, buses, and other vehicles that use diesel engines in the U.S. and Europe will all have to upgrade their CACs to meet these tougher requirements.
The ICA-developed CuproBraze manufacturing process for copper/brass CACs copes easily with higher temperatures, and the greater strength of copper/brass can withstand the increased pressure. Failures of current aluminum CACs are primarily due to material failure, according to ICA. A CuproBraze CAC—which is lead-free, using a brazing alloy that is 75% copper, 15% tin, 5% nickel, and 5% phosphorus—can withstand temperatures of 290°C (554°F) with no metal fatigue.
 An electron micrograph of a piece of Nd-Fe-B magnet shows the zone near the surface where the neodymium has been leached out by the liquid magnesium. (Photo courtesy of Ames Laboratory.) |
"CuproBraze technology takes advantage of copper's superior conductivity, and our copper technology increases the capacity of the charge air cooler to dissipate heat," said Anthony Lea, Vice President of the ICA. "These features provide automotive manufacturers with the technology to address the current and future emissions standards in the U.S. and the EU."
ICA claims that because existing equipment can be used with minor adjustments, retooling costs to adopt the CuproBraze process are low. The process uses common vacuum and controlled atmospheric gas brazing furnaces and existing brazing equipment used for aluminum radiators. Components are brazed at approximately 620-650°C (1148-1202°F), or about 300°C (540°F) below the melting point of brass, resulting in less scrap than aluminum processes in which only a 40°C (72°F) margin of error exists between melting and brazing. Also, first-run rejects can be re-brazed, further reducing the overall scrap rate.
