Related: Automotive Materials
Aluminum has had mixed success in infiltrating vehicle areas that have traditionally belonged to steel, but one component where it has become a clear winner is wheels. In 1980 steel was the material of choice for 90% of wheel production, but by 2003 aluminum had surged to 60% of production, leaving steel with less than half of the market it had once practically owned. What happened? The biggest factor was probably the rise in interest by consumers in upscale trim level packages that accompanied the economic bubble of the 1990s. Automakers found that adding styled aluminum wheels was an easy way to communicate that a vehicle was a cut above, so they speced them for entire trim levels, pushing volumes up. Also, the tight tolerances held by cast-and-machined aluminum wheels surpassed their stamped-and-welded steel counterparts, leading to greater uniformity and less vibration on the road. This became particularly important, given the rise in popularity of SUVs, since those tall, big-wheeled vehicles are more sensitive to transmitted road vibration. (Interestingly, the reason that aluminum is most often chosen for applications—it's light weight—was at best a tertiary consideration.) But the higher costs associated with aluminum, which were grudgingly borne during the high-margin bubble, have become onerous in the aftermath of its bursting, and lower-cost steel wheels are poised to regain a large chunk of their lost market share. More concretely, Tom Heck, director of product engineering at Hayes Lemmerz (Northville, MI), estimates that in the 2006 model year his company alone will be responsible for replacing 2 million low-end aluminum wheels with steel units on various new vehicle platforms—bringing steel back to parity in a single year.
Better Steel, Better Wheels. To be clear: we're not talking about those low-budget steel wheels that you only see unclad when someone is limping along on a space-saver spare, but new designs that offer large ventilation openings and mimic the thin-spoke appearance of aluminum. Manufacturers have long wanted to marry the production cost benefits of stamped and welded steel with sportier designs, but ran into material limitations. To get the fatigue strength of aluminum with conventional steel grades, like the high-strength low-alloy (HSLA) steels that currently dominate wheel production, they had to increase material thickness to the point where it is often both heavy and hard to form. Then they began experimenting with dual-phase steel and found that its properties are well-suited to their purposes: it allows for thinner walls and lower weight; it has a high elongation factor which gives better formability; and it offers a greater spread between yield strength and tensile strength, so that it will bend easily in deep draw dies without breaking. Another plus for dual-phase is that it is largely a known quantity. It has been the steel of choice for European wheel manufacturers since the late 1980s, and was used in trial production in the U.S. as far back as the '70s, though problems with "red scale" deforms due to its silicon-rich chemistry and lack of availability doomed its chances at adoption. Now, however, high-chromium formulations have reduced the red scale problems, and, according to Heck, greater availability is in the offing with one U.S. mill currently producing dual-phase (U.S. Steel), and three more slated to come on line within a year.
But even with the advantages of dual-phase, steel wheels probably wouldn't be ready to challenge aluminum without improvements in design and manufacturing. Zeferino Bacchineto, engineering manager, Wheels Division, ArvinMeritor (Troy, MI), says that simulation software has greatly improved over the last few years, allowing for far more precise designs. He adds that even in a standard operation like the gas arc welding procedure used to join the center disc with the rim, ArvinMeritor has been able to improve efficiency through a proprietary attachment method. Heck echoes those sentiments, saying that better CAD/CAM tooling data has allowed Hayes Lemmerz to improve wheel uniformity and dimensional precision. "We're 25 to 30% better at holding dimensions than 5 years ago," he offers.
Cladding and Cost. Still, it's almost impossible to get a stamped steel wheel to look as good as a machined aluminum one without a little superficial help. So a fundamental component of the new "high-vent" wheels is cladding, albeit a more sophisticated version than the plastic disks usually found on steel wheels. Using the spokes of the center disk as a backbone, wheel makers are experimenting with stainless steel and painted plastic attachments that can approximate the high-end look of machined or chromed aluminum. Of course, this adds cost to the overall wheel package, but Heck estimates that even with trim included, steel wheels cost about $15 less per wheel than aluminum. And given the high priority currently placed on cost-cutting, a $60 savings per car set is hard for OEM purchasing departments to pass up. But it should be noted that steel wheel manufacturers are limiting their ambitions to replacing high-volume, low-end aluminum units, not top-of-the-line models. Why? Two reasons: (1) expensive aluminum wheels will continue to hold a styling advantage that steel cannot match because of the versatility of the casting and machining process; and (2) at low, luxury nameplate volumes casting is actually cheaper than stamping. In fact, Heck says that in order to justify the $500,000 or so investment needed for a set of progressive stamping dies, steel wheels need to be produced at an annual rate of about 400,000 units to beat out aluminum. But that's not a problem since the volumes that steel wheels are expected to capture in the next few years are measured in millions.