April 10, 2007
The demand for lightweight, high-strength components is increasing at the expense of traditional steel parts. Emerging technologies, such as steel thixoforming, may help reverse this trend. Thixoformed steel parts are significantly lighter than equally strong parts formed by conventional means.
|In the thixoforming process, steel must be transported as quickly and as precisely as possible from the heating furnace into the press tool. As a result, highly specialized material handling and press control systems have been designed.|
The global automotive industry is the largest customer of steel producers and stampers. Yet for decades the percentage of steel used in vehicles has steadily declined. To improve vehicle efficiency, automakers have been replacing steel with plastics, aluminum alloys, and other lighter-weight materials.
Parts made from materials lighter than steel, however, are more complex and command some of the highest margins. Even so, the demand for lightweight, high-strength components is increasing at the expense of traditional steel parts.
Emerging technologies, such as steel thixoforming, may help reverse this trend. Thixoformed steel parts are significantly lighter than equally strong parts formed by conventional means. At the same time, thixoformed parts require less raw material, machining or finishing, and energy than traditional steel parts. As a result, this technology may make steel parts competitive with alloy and plastic parts in weight, strength, and cost.
Steel thixoforming is still in its developmental stages; however, a recent European Union-sponsored steel thixoforming research project at the University of Hannover, Germany, produced some encouraging breakthroughs. Because of these advances, production-scale thixoforming now is closer to reality than a few years ago.
In the steel thixoforming processes, a metal such as aluminum or magnesium is heated to just below its melting point in an electric furnace and transferred to a press where it's formed. Because of its near-molten state during the forming process, material flow into the forming tool can be controlled better, creating a uniform structure stronger than is possible with conventional forging or die casting. A thixoformed product of equal or greater strength than a forged or die-cast part can be produced with less material (and weight).
|A thixoformed product of equal or greater strength than a forged or die-cast part can be produced with less material (and weight).|
The process also allows more precise forming of complex shapes with less waste. In the steel forging process, half of the material used may end up as scrap, while a steel thixoformed part can be formed close to its net shape in a single step, with almost no scrap. Postforming processes, including finishing stages, part transport between stages, and scrap removal and recycling, are minimized or eliminated, reducing time, labor, technology, equipment, and energy.
Tests performed for the EU thixoforming research project compared a thixoformed engine part for a Maserati with a conventionally forged part. The thixoformed part's raw material weight requirements were reduced by 45 percent. The forged part required four forming steps that eventually removed a total of nearly 68 percent of the initial billet's weight. The thixoformed part required one forming step, and the final part weighed about 11 percent less than the forged part. It also matched or exceeded the forged part's structural strength parameters.
Results like these are possible only if the heated steel is kept within a precise temperature range, called the thixoforming window, while the steel travels from furnace to press to forming tool. If the steel arrives too hot or cold, its forming capabilities and strength are severely degraded.
If near-molten steel is exposed to the open atmosphere, it will oxidize and scale, producing equally disastrous results during the forming process. However, the most complicating factor in steel thixoforming is the high temperatures involved. Aluminum thixoforming occurs between 500 degrees C and 650 degrees C, while steel thixoforming requires a temperature of about 1,400 degrees C. The thixoform window is much narrower for steel—it's only about 30 degrees C, compared to the 150-degrees-C window in aluminum thixoforming.
With its high temperature and narrow thixoform window requirements, steel must be transported as quickly and as precisely as possible from the heating furnace into the press tool. An inconsistency of a fraction of a second is enough to render it too hot or cold for proper thixoforming. At the same time, the heated steel must be protected from atmospheric contact to prevent oxidation and scaling. As a result, highly specialized material handling and press control systems have been designed.
A thixoforming material handling system transports heated billets in ceramic containers. Inside these protected environments, the billets are surrounded by nitrogen gas, which shields the steel from atmospheric contact. Propelled at speeds of 1.5 meters per second via the feed system, the billets complete an entire transport cycle from the furnace to the press feed tube in 2.2 seconds.
The press process must be equally fast. In the EU trials, the press cylinder had to move at a speed of up to 3 meters per second, with positional precision measured in a few hundredths of a millimeter. This is the only way to ensure the process consistently occurs well within the narrow thixoforming window.
Even with the advances in steel transport and forming, production-scale steel thixoforming is still several years away. For instance, an entirely new approach to tooling will be required. Traditional metal tooling will not survive the process's extreme temperatures. New tooling must be created using ceramics.
Ceramic tool design is still in its infancy, and a good deal of development is necessary before it can go from laboratory-scale testing to production-scale use. For example, ceramic material contains unfamiliar properties, such as a comparatively low tensile strength, which will cause a ceramic tool to shatter under contact pressures a metal tool could easily absorb.
Another critical factor is forming the material. Traditional steel grades are too limiting with the narrow thixoforming window. Therefore, new steel grades are in development with compositions and materials that will encourage better flow and forming characteristics. In addition, thixoforming steel must be more uniform than steels used in forging or die casting to ensure predictable and repeatable heating and flow behavior that is essential for a process with very narrow parameters. Further development of steel grades also may widen the thixoforming window, making the process much less temperamental than it is now.
As the basic processes, equipment, and materials undergo further development, those most familiar with the technology believe that steel thixoforming is an essential step the steel and metal forming industries must take. Thixoforming may create a renaissance for steel formers by helping them retrieve lost market share and create new opportunities in different industries.
Todd Helms is account manager—sales at AP&T North America Inc..
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