I don't know many people like me. I can stare at the underside of a car for hours on end. For me, a car up on a lift is like Monday Night Football on a 60-inch HD flatscreen for most other guys. I recently toured the Audi assembly line in Ingolstadt, Germany, and as you can imagine, I was transfixed. The only thing better than a well-designed steel unibody are the aluminum components Audi hangs off of them. I'm still trying to figure out how I can turn one of the cast aluminum suspension arms from the front of an S3 into jewelry.
Sadly, cast, and even moreso forged, aluminum pieces are rare on cars simply because of cost. But luckily, the new focus on fuel efficiency has brought new interest in alloys for their weight savings. But the car market is so competitive that the cost benefit analysis sometimes still swings toward stamped steel. Ninety-nine percent of all end users will never know the difference between steel and alloy components, but guys like me know.
One of the reasons alloys are more rare is their cost of production. For the most part you have to either cast or forge them. Casting is essentially molding a metal part. You heat up the metal until it's molten liquid and then pour it into a casting tool, which can also be extremely expensive to build. Forging metal takes place at low temperatures and consists of forcing the metal into the desired shape with thousands of pounds of force.
Metals have a crystalline grain structure, meaning it is aligned on the atomic level. Forging allows the grain structure to be worked into the direction most advantageous to the component. Forging also allows for less voids and failure points within the component. Castings, on the other hand, often have a slightly more random grain structure that develops as the part cools inside the mold. But even a cast part will generally have a better strength-to-weight ratio than a part made of stamped steel components welded together.
The next step in upgrading individual components is, obviously, using titanium. While titanium is currently being used in a few high-end applications for things like connecting rods and even exhaust systems, it's still too expensive to be used in any large-volume mainstream applications. One of the reasons is the high cost of processing titanium. It is roughly 30 times as expensive to process a like volume of titanium as steel, and that's just to get the raw titanium. The expenses just keep compounding when you consider casting, forging, and machining it are all a good 10 times higher than doing so with steel or aluminum.
Things may not be so bleak forever. In the '90s, new alloys started appearing that might be just what the automotive community needs. "Glassy metals," as they are called, are amorphous metals that can be formed at relatively low levels of heat. That they're amorphous means they no longer have a crystalline structure. The alloy is comprised of many different metals that all have very different atomic sizes. These huge differences in size inhibit grain formation during cooling and make the alloy less prone to internal strain.
Titanium-based amorphous alloys also possess yield strength double that of stainless steel and nearly five times that of traditional aluminum alloys. They are also harder and less prone to wear and corrosion. In the event of deformation, they hold their shape better and so are less susceptible to plastic deformation and associated fatigue.
The real beauty of amorphous alloys is the ability to use low-temperature casting to shape them. Many of these alloys can be cast at temperatures similar to those used for plastics. Because of the lower temperatures and lower internal stresses, the parts can be used in an as-cast shape, meaning they don't require extra machining after the casting process. Obviously this saves money.