Is Aluminum Really the Savior of Auto Weight Savings?
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.