It is one of the great misconceptions about automotive design and engineering: The notion that just by building cars out of aluminum, you’ll automatically save weight and make them stronger. We posted it ourselves when we talked about the new F-150 today. Aluminum works well in some applications, but it’s not the space-age wonder material some would have you think it is, and in some applications, it’s got serious disadvantages. Here’s what makes that true:
There’s Nothing New About Aluminum
As early as the 1908 Bugatti Type 10, aluminum has been put to use by automotive manufacturers. Featuring aluminum castings and widespread use of the alloy on the body, it was one of the first cars to feature the alloy in this type of application.
During World War II, aluminum was the material of choice for aircraft, and after the war, a lot of the skills and technology that made it viable moved over to automotive production. Many hand-crafted, coach-built cars coming out of Italy in the era immediately following the war were built with extensive aluminum bodies.
Strength vs. Stiffness
It’s accurate to say that some aluminum alloys are stronger than some steel alloys, but aluminum isn’t inherently stronger than steel. It is, in fact, stronger than some steel alloys. It is also, in fact, not as strong as other steel alloys. KVA M3 stainless steel, for example, offers twice the tensile strength of 7075-T6 aluminum, with only a 10-percent weight penalty.
Many factors other than weight and strength dictate whether aluminum is the right application. The biggest consideration is stiffness. Listen to any of the press conferences given by any of the OEMs the last few days, and one of the areas for improvement in any given car is its overall stiffness.
“Strength” is measured as the maximum load a material can be subjected to without yielding.
“Stiffness” is refers to how much a material will bend when a load is applied.
For the sake of simplicity, pick a common structural shape: 2-inch square tube. Relative stiffness is measured in something called “modulus of elasticity.”
Aluminum’s modulus is about 10 million psi. Steel is about 30 million psi. So, a piece of 2-inch square tube would have to be three times as thick to offer the same stiffness as steel, thereby eliminating any weight advantage it might offer.
Bicycle manufacturers understand this. No material offers the durability of steel at anywhere near the price.
Modern High Strength Steel
High strength steel used in automotive applications today is a far cry from the 18-gauge stuff a Model T was built out of. High tensile steel is six times stronger than the conventional steel, and it’s still significantly more cost-effective than aluminum or carbon fiber.
In 2013, Volkswagen replaced a whole lot of parts in the Golf that were once aluminum with high-strength and ultra-high strength steel alloys. Volkswagen saved weight and expense. It’s also easier to fabricate and to weld than aluminum alloys.
While everybody seems to be talking about the aluminum in the F-150, high-strength steel is just as big a story. The use of high-strength steel in the F-150′s frame went from 23 percent to 77 percent, resulting in a frame 60 pounds lighter, with greater stiffness than ever.
Ford says that up to 95-percent of the F-150′s body is going to be made of “high-strength, military-grade aluminum,” cutting up to 700 pounds from the body.
Since the F-150′s body sits on a steel frame, stiffness isn’t as much of an issue as with a monococque, so it seems like a natural application. But what isn’t clear is how much more these aluminum bodies cost to produce, and how much more they might cost to repair in a collision.
Whether your next car is made of steel or aluminum, though, has a lot less to do with whether its the best application, and a lot more to do with what kind of price concessions the OEM can grind out of its suppliers. The American Iron and Steel Institute and the Aluminum Association have been at each others throats for decades, and given the government’s mandate that auto manufacturers reach 54.5 mpg by 2025, that competition is going to get a lot more fierce in the future.