Related: Automotive Materials
Chances are, more people are familiar with magnesium through health and diet issues (you can get your fill by loading up on leafy green vegetables) than they are through the use of the element (which happens to be the eighth most abundant on the planet) for automotive applications. In fact, Eric R. Showalter, director, Advanced Manufacturing Engineering, Intermet Corp. (Troy, MI), which is a supplier of various types of cast-metal automotive components—including those produced with magnesium—points out that compared to the metals with which it competes, including steel and aluminum, there is a dearth of information about magnesium. When asked whether the application of magnesium is being slowed by those who remember chemistry experiments (academic or otherwise) involving magnesium in its brightly burning form, Showalter suggests, “What’s impeding the progress of magnesium more is probably the lack of published properties and general knowledge about the material. There are books written on aluminum. Phase diagrams. Information about properties at different temperatures.” But for magnesium? Well, that’s another story. Although they are developing SAE and similar papers at Intermet, and while USCAR is working to promulgate information about the material, comparatively speaking, you’re more likely to find out how magnesium can contribute to a healthy body than about why it is a valuable material for certain automotive components.
(A word about that brightly burning magnesium. Yes, it is a highly reactive material. No, you’re not likely to have an automotive component turn into some sort of a Fourth of July novelty due to the conditions under which the burning takes place. Showalter explains that when the material is in its molten state for purposes of casting great care is taken, with the overall environment being particularly clean and a shielding gas used on top of the molten material.)
|Instrument panel beam for Cadillac is a two-piece assembly (riveting and adhesive bonding are used). A steel beam could consist of 20 or more pieces welded together.|
MORE THAN MEETS THE EYE.
Which is not to say that magnesium is a material that isn’t being used today in automotive applications. In fact it is being used in a number of them. For example, one of the big applications—both literally and figuratively—for the material is for the instrument panel (IP) beams on Cadillac models including the CTS, SRX, STS, and Seville. According to Showalter, a key reason why the move was made to magnesium for these beams is because they greatly simplify things. Whereas a similar stamped steel component would consist of something on the order of 20 to 25 individual pieces that would need to be welded together, the magnesium IP is one that has significant part consolidation. While it could be done (and is being done in other applications) as a single-piece IP, according to Gary L. Yager, product development manager at Intermet, Intermet personnel worked with people from General Motors to develop a two-piece IP beam. The benefits of this approach are realized both in terms of product and process. From the product point of view, the IP-beam for the CTS, for example, readily lends itself to either right-hand or left-hand steering applications. There is a common passenger side piece and then one each for the right- or left-hand configuration. This is a serious savings as related to tooling costs. As for the process benefit, when the work was being done starting in 1996 on developing the magnesium IP component, apparently there weren’t a whole lot of molding machines with 3,000+ tons of clamping force around. By making two smaller pieces rather than a larger one, the job can be done in 1,400- or 1,600-ton machines.
Magnesium is also used under the hood. Cam covers are being made for the material on vehicles ranging from the Dodge Viper to the new Ford F-150. One of the characteristics of the magnesium that makes it good for such applications is that it provides good sound dampening. Other applications include steering column components, grille opening reinforcements, and audio structures. Speaking of the last application, Nanda N. Gopal, director, Process Research and Development at Intermet, points out that magnesium provides good electromagnetic interference shielding. No doubt that characteristic will become even more appealing to people as the amount of electronic equipment in vehicles increases.
The big feature of magnesium is, of course, the fact that it is a lightweight structural material. Roughly speaking, magnesium is about two-thirds the weight of aluminum on a volumetric basis. That said, magnesium has historically been light—but expensive. The cost now is in the vicinity of $1.10 to $1.13 per pound. At that price point, Yager says it’s “about a wash with aluminum.”
|Although magnesium cam covers are featured on high-end engines, such as the Viper’s, they’re also used on more mainstream products, such as the Ford Triton 4.6-liter V8 used in the F-150.|
One of the primary application areas for aluminum in vehicles is under the hood—as in blocks and heads. What about magnesium blocks? “There are magnesium blocks,” Showalter says. “On racing cars.” A problem with more conventional engine applications is that the coolant tends to react with the magnesium, causing corrosion. One of the all-time greatest uses of magnesium did occur under the hood—at least the hoods that were located where the trunk is ordinarily located. The VW Beetle and the VW Super Beetle both had magnesium engine blocks (AS21 and AS41, respectively). They were, it should be noted, air-cooled engines. (Audi, which is part of Volkswagen AG, continues to work with the light metal in powertrain-related applications. For example, the air intake module on its W12 engine is magnesium, as are various components, including the cylinder head covers, on its V8. The company’s multritronic CVT and five-speed manual transmission both have magnesium housings.)
Another of the aspects that has to be dealt with is related to the modulus of the material (which is about half that of aluminum). It seems that when magnesium is brought to temperatures like that encountered in engines and transmissions, there is a tendency for deformation under load to occur (creep). This could be particularly deleterious vis-à-vis bolt-hole retention, for example. Creep-resistant alloys are being developed. According to Showalter, these alloys (adding such things as strontium, calcium or rare earths for stable performance at elevated temperatures) may cause a sacrifice with regard to castability, but they do open up additional opportunities.
Working toward reducing the cost of magnesium is a NIST/ATP-funded program designated “Cormag,” for “Cost Reduced Magnesium Casting Using Heated Runners.” This program is in the third of its four years. It is being led by Ford. What they’re working toward is a means by which casting can be done with less scrap (at least 10%) and reduced costs (e.g., by being able to make larger parts in lower tonnage machines and by being able to produce parts more quickly). Gopal likens it to the plastic injection molding process—but points out that while molten plastic is comparatively benign, molten magnesium is far more reactive. Consequently, creating the hot runner system is a demanding task. Still, Gopal thinks that the program will prove to be successful.