Steel, Aluminum, or Plastic in Electric Cars?
As important as better battery technology in future EVs is the development of stronger/lighter/cheaper materials. With expensive race-cars where cost is no object, carbon-fibers, kevlars, and aluminum lead the way. For family cars and pickups, plastics, aluminum, and high-strength steel alloys are emerging in ways to make cars lighter and thus extend range and efficiency.
I joined the Electric Auto Association in 1988, which met at the San Bruno Public Library. It was mostly an assemblage of retired engineers, including John Newell, who had designed the electric car the astronauts drove on the moon. I soon bought my first electric car (a 108-volt converted Honda Civic) and learned firsthand how important weight, aerodynamics, and rolling-resistance were to my “range” (then around 40 miles on a charge). This was already a tiny car, but additionally weighed down by around 800 lbs of lead-acid batteries. Going up hills it felt like a tiny scooter, and downhill it handled like a runaway train. It took around 8 hours to charge. Soon I bought a set of “hard” tires that extended my range 3–4 miles (most tires give a softer ride at a slight decrease in mileage, which no one notices in a gas car…but for me this was critical). I started reading about electric car “range” records of around 2,000 miles, mostly in souped-up bicycle looking things, with fiberglass or carbon-fiber bodies. It became clear that weight was as much a critical element as batteries, in the EVs future.
It should be pointed out to the many electric car critics, that EVs are responsible for only 15–20% energy losses, while gasoline cars waste 64–75%. EVs also get an equivalent “mileage” of around 110 mpg. However, electric car power trains/batteries currently weigh about 10-15% more than equivalent internal combustion engines/fuel tanks, making more lightweight batteries and cars the next EV milestone. The rest of this article will focus on the weight of non-battery elements. (fueleconomy.gov, n.d.),(Berjoza, Jurgena, 2017)
There is a long history of competition between aluminum and steel in the auto industry, of which steel has mostly won (due to cost, paint-ability, and weld-ability). In the aircraft industry, aluminum won a long time ago (steel was never really in the running due to weight), and now aluminum and reinforced plastics are competing. In the race-car or high-end sports car industry (think Ferraris and Corvettes) reinforced plastics (currently carbon fiber) and aluminum have a long history, as cost is less of an object. In terms of safety, let’s just say that the notion that steel is “safer” is relative, since plastics and aluminum can be made to be as strong or “safe” as you like, at a higher cost. In race car design, there is an evolution in safety, which involves using aluminums and plastics while inserting the lightest safety elements (kevlars, air-filled energy absorbers, air bags, light roll cages, harnesses, better braking/traction systems, and scatter shields). At this point, it would be helpful to define some of the various materials used:
1) Carbon Steel-Traditionally iron with some carbon added to become stronger and more ductile
2) High Strength Low-Alloy Steel (HSLA)-Iron with carbon, 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium. This steel alloy is stronger and more corrosion resistant, but less ductile compared to the older alloys. HSLA is used heavily in the auto industry to reduce weight, while maintaining practical considerations such as easy welding and painting. It is more expensive, but then about 30% less steel is needed for a given application. (worldautosteel.org, n.d.)
3) Thermoplastics-These include PVC, ABS, polyethylene, polypropylene, nylon, etc., which can be injection-molded and now 3D sprayed. These can be used for the less-structural elements of a car, and are often used for trim-pieces.
4) Thermosets-These include reinforced plastics such as carbon-fiber and fiberglass. These are typically used for high-end performance cars, and have favorable strength/weight ratios and structural properties. (Lipohar, 2018)
5) Polycarbonate-An plastic alternative to glass, safer but more expensive. (Trombetta, 2017)
6) Glass-A silicon derivative, used primarily for windows. It shatters and is harder to make safe, but it is cheap.
7) Aluminum-Many auto engine blocks use aluminum alloys containing silicon, copper, nickel, titanium, manganese, iron, magnesium, and zinc (A356). Similar alloys are used to make body parts (6000 Series). Aluminum-Magnesium alloys (lighter and less flammable) are heavily used in aircraft (principally alloy 7075). These aluminum alloys are less corrosive, but hard to weld and paint. Though more expensive than steel, aluminum is lighter and has good strength/weight ratios. (Hohn, 2018)
8) Kevlar-A heat-resistant synthetic fiber (polymer), with a very high strength/weight ratio. Currently used in place of steel for tires. Sometimes used in brakes and as reinforcing fiber in reinforced plastics. Expensive.
Aluminum is a highly recyclable material, and used in high-end cars (sports cars, race cars, the DeLorean, high-end Teslas) and is attractive as an auto-body as it is less corrosive and lighter. However, it’s cost and some aforementioned production issues have kept it out of the mass market. As an example, the price drop from a Tesla $80,000 Model S to a $40,000 Model 3 has much to do with the former having a largely aluminum body/chassis and the latter having an HSLA body/chassis (which also accounts for the variation in ranges, 402 to 353 miles). However, aluminum as said, is being used in many engine blocks (made popular by the Japanese vehicle revolution in the last century), which lowers weight and corrosion potential (it should be noted that the crankshaft, connecting rods, valves, and related parts are often steel). With the EV revolution on its way, many of the electric motor parts and power train will likely be made of aluminum. (Lin, 2021),(Hohn, 2018)
Plastic for the moment is not much of a threat to replace HSLA steel for auto bodies, except in non-structural parts. As said, plastic can be made structural, but at an increase in weight and cost that is currently not practical. Someday, this may change, but then steel alloys are also evolving to become stronger and lighter. Experts look for kevlars and carbon fibers to compete heavily when technical/production advances are made, and as the EV revolution places a premium on weight and range. Put simply, a much better battery might make range issues relatively moot, while if battery tech stays near its current level, lighter materials will be at a premium. Aluminum makes a strong case for being used in chassis, but then so does HSLA. For cost reasons, polycarbonate will not soon replace heavier glass in vehicles. Automakers have also evolved glass to be much safer in recent years. (dupont.com, n.d.),
There are interesting trade-offs at work here, between cost, range, serviceability, and safety. It reminds one of the endless debates over WW II aircraft and tanks. Early WW II fighter planes were cheaply built, basically cloth over a wood frame, light but not safe (upgraded to aluminum later for better performance and safety). German Panzer tanks were expensive, heavy, powerful/safe but broke down often. As the war progressed, the US aluminum aircraft improved range (mostly with aerodynamics and engine improvements) but always requested more armor. Similarly, the US Sherman tanks had to be shipped, were lighter with less armor and gunnery, but quite practical, economical, and maneuverable (armor upgrades were later made). Fortunately, these “VW bugs” were able to successfully gang-up on the German “BMWs”. The future of EVs may be generally summarized as aluminum power trains (with copper windings), HSLA (or better) steel chassis and roll cages, reinforced plastic structural body parts, and injection-mold thermoplastic non-structural parts, polycarbonate windows, with a whole lot of electronics. Safety in these vehicles will likely occur with a system of harnesses, air-filled energy dissipaters and instantly inflatable air-bags front, side, and rear. (aviation.stackexchange.com, 2019),(pitpass.com, 2018)
Finally, the last chapter on automotive steel vs aluminum vs plastic vs glass may well hinge on their “recyclability”. Aluminum is the leader here, with a 95% energy savings using recycled product. Steel has a 60 to 74% energy savings with recycled product, and glass 25–30%. Plastics recycling is the big wild card. When it is recycled properly, it reduces energy use 90%, but there are so many different varieties of plastic (and Asian markets have collapsed) so that currently plastics are recycled very inefficiently. This has to change if plastics are to be a part of the overall solution to the energy/climate change problem. In addition, it should be mentioned that lithium and cobalt from spent batteries also need to find better recycling solutions. (Hallingstad, 2013),(Jacoby, 2019)
Enjoy these other Transportation-related articles by Vern Scott!
