Since the invention of the automobile, body materials ranging from steel, iron and other metals to plastics, composites and even wood have jockeyed for position as the main structural material of the motor vehicle. Recently, aluminum has been popping up in automotive industry news more and more, touted as the material of choice for advanced, next-gen vehicles from electrics to pickup trucks. I spoke with Dr. Jody Hall, vice president of the automotive market division of the SMDI about steel’s place in the automotive materials market.
“There has always been a lot of competition between materials,” she said. “Throughout the history of the automobile, this has been going on in the background. Recently, the way this competition has been out in the media, is a new phenomenon. I’ve been a metallurgist working in this industry for 30 years, and I’ve never seen a time where automakers are unveiling new vehicles and talking about the materials the body structure is made of. It’s unheard of.”
So, if material selection is becoming more and more of a consideration for car buyers and builders alike, which material is indeed better?
Light Weighting: Aluminum vs. High-Strength Steel
On the periodic table, aluminum is about half as dense as iron. But does being lighter in this literal sense translate to light weighting of vehicles?
The aluminum industry says yes, pointing to automakers like Tesla and Ford choosing aluminum bodies in order to translate the weight savings to the battery or payload capacity. However, Dr. Hall disagrees.
The body and chassis make up over half the weight of the average vehicle, so material can make a big difference in weight, stiffness and strength of the vehicle. According to Dr. Hall, the tensile strength of steel is up to 2,000MPa (290,000psi), which is about four times stronger than the strongest aluminum alloys available today.
This tensile strength difference means that aluminum parts need to increase thickness to meet safety standards, while advanced high-strength steel (AHSS) can be made thinner, thereby reducing weight. Granted, these thicker aluminum parts may still come out lighter than their steel counterparts.
“You may get lower mass with aluminum, but it’s a relatively small difference, and you pay for that mass reduction because of the higher cost of the material,” said Dr. Hall.
As a rule of thumb, automakers largely ballpark the cost of aluminum materials at two to three times higher per kilogram than steel.
Dr. Hall pointed to another disadvantage of the thicker aluminum parts. “If you look at the A-pillar of an aluminum vehicle, it’s a larger geometry,” she said. “It has to be wider in cross-section to have the same performance. So, it can perform the same and it may weigh slightly less, but you must have a wider A-pillar. This can affect the driving experience, and if all the parts are aluminum, it can reduce the space efficiency of the vehicle’s interior.”
Aluminum Vehicle Safety vs. High-Strength Steel
According to drivealuminum.org, aluminum-bodied vehicles are safer than their steel counterparts thanks to better energy absorption, larger crush zones that fold more predictably and larger overall size.
Dr. Hall brings up an interesting counterpoint to these advantages: It’s not the material that matters so much as how you design with that material.
“You can design a safe vehicle with other materials [than steel], but it’s a combination of materials and design,” she said. “It’s absolutely, scientifically incorrect to say that aluminum, as a material, is ‘safer.’”
When I brought up the counterpoint that every aluminum-bodied vehicle on the road has a five-star safety rating, she pointed to the confounding variables that may explain why:
“All are newer, higher-end vehicles. Luxury vehicles can afford the extra engineering to get that rating, and in the case of the Ford F-150, I think it’s a very aggressive and risky move to convert a pickup to aluminum. I think they did absolutely everything they could to make sure it would work.”
So, aluminum can be as safe as steel—but at higher cost.
Is Aluminum More Sustainable than Steel?
While aluminum can be easily recycled through advanced processes, steel has the advantage of being the most recycled material in the world. The recycling process of steel is simpler, too: Its ferrous property allows for easy scrapyard sorting, and all alloys of steel can be melted together and remixed to produce any alloy of steel. Aluminum, on the other hand, is costlier to recycle, requiring the different grades to be separated before melting to preserve the grades.
Workability of Steel vs. Aluminum
While some grades of AHSS can be challenging to work with, steel is widely known for higher yield and ductility than aluminum alloys. Dr. Hall points to one design difference that could make the choice for certain manufacturers between aluminum and steel:
“The Cadillac body design, for example, would not be possible with aluminum, due to the lower ductility of the material. Aluminum has lower elongation, which is an indication of formability.
New AHSS alloys have different material properties than those of 20 to 30 years ago.“The steel industry continues to innovate,” Dr. Hall said. “In the 1970s, we started with about seven grades of steel. Over the last several decades, that number has increased to over 200 grades.”
These new grades of steel were developed by the suppliers to deliver the mass reduction automakers need to compete under ever-tightening fuel economy guidelines. Automakers are squeezed on both sides in a way, with efficiency guidelines pushing for lighter vehicles and safety guidelines pushing for stronger, stiffer, bigger vehicles. This could explain the recent boost to aluminum, which can meet these requirements, but it’s also lit a fire under the steel industry to create alloys that can compete and outperform the other materials. But when it comes to the question of whether steel will ever be unseated as the king of the auto body materials, Dr. Hall isn’t worried.
“I keep a magazine in my office, an issue from the year 1953. It claims that steel would no longer be a material in automobiles by the year 1960. Clearly, that hasn’t happened,” she said. “In my 30-year career at GM, I saw body closure panels be designed for aluminum because they were lower mass, and then literally months later, they were replaced with steel because of cost. The mass was removed another way, either from the steel components or in other areas.”
The same prediction happened in 2014, when the Ducker Worldwide survey predicted that 7 out of 10 pickup trucks would follow in the footsteps of the Ford F-150. Since then, the Chevrolet Silverado has been the only example, and it only uses aluminum for the swing panels, hood, doors and tailgate.
In the battle to shave pounds off the vehicle or eke out performance, automakers turn to alternative materials and new technologies. But as soon as they achieve the needed mass reduction, the next step is to reduce cost.
“The Chevy Silverado is a logical use case for aluminum,” Dr. Hall conceded. “Swing panels are nonstructural, so they’re a good option for mass reduction—if you can afford the cost.”
With all her experience in the automotive industry, Dr. Hall has observed this battle for a long time. She summed it up this way:
“I think it says a lot that I was responsible for advanced technology in die and stamping at GM for 30 years, working with both steel and aluminum, and I came to work in the steel industry,” she laughed.
For aluminum’s side of the story, check out “Automotive Aluminum Demand Growing: What it Means for Tier Ones.”