Newer steels, aluminum, magnesium, and other materials lead to more lightweight, economical vehicles
Advanced materials for automotive manufacturing are helping automakers build lighter, more fuel-efficient vehicles. With the lighter and stronger steel, aluminum, and magnesium components, the current crop of cars and trucks can meet stringent crash safety standards, while also improving fuel economy to help achieve the more strict government Corporate Average Fuel Economy (CAFE) mileage requirements looming on the horizon.
Proponents for newer high-strength metals all make the case for using their respective materials in automobiles of the future, and each has its strengths and weaknesses. Steel still accounts for roughly 60% of the metal used in an average vehicle in North America. The percentage of aluminum has continued to grow, reaching an all-time high of 8.6% of curb weight in 2009 North American vehicles, according to a recent study by Ducker Worldwide (Troy, MI) commissioned by The Aluminum Association Inc. (Washington, DC). While accounting for a much smaller relative percentage of vehicle content than either, magnesium weighs much less than steel and aluminum, but it still presents some processing issues and is mostly limited to die-cast components.
Advanced high-strength steels (AHSS) are becoming much more widely deployed by automakers for structural parts where thinner, stronger metals not only help save weight, but also offer substantially improved crash protection. Compared to conventional steels, AHSS are said to enable lowering a vehicle’s body-in-white structural mass by up to 25%, according to the American Iron and Steel Institute (AISI). AHSS is a series of high-strength steels containing microstructural phases other than ferrite and pearlite. Most AHSS have a multiphase microstructure, and these phases include martensite, bainite, retained austenite, and/or austenite in quantities sufficient to produce unique mechanical properties, according to the Auto/Steel Partnership (A/SP), a consortium of the Detroit Three automakers and five North American steel producers. The A/SP recently issued its revised Advanced High-Strength Steel Applications Design and Stamping Process Guidelines. Available at its Web site, www.a-sp.org, the case study manual on AHSS helps designers and engineers specify and apply specific grades of AHSS.
Automotive manufacturers began using high-strength, low-alloy (HSLA) steels in a significant way in the late 1980s, recalls Dave Anderson, director of the AISI’s Automotive Applications Council. “By the end of the ’90s, you could almost look at a car and say, ‘Here’s where the HSLA steel parts are on the car.’ They had become pretty well standardized. The physics of the vehicle applications are pretty much the same, because they were used in crash-worthy parts, and everyone was working on the same crash standards—that was more or less the state of the industry in 2000. We have been moving from the base grade of those advanced high strength steels, which I call the 600 grade, and we’ve been marching up in strength and in number of pounds used ever since.”
Development of AHSS, and more recently dual-phase and ultra-high-strength steels (UHSS), has resulted in steels with very high tensile strengths, rated between 600 and 1100 MPa, due to higher percentages of martensite. With higher strengths in steels, there’s a trade-off in the formability and machinability of the much harder metals. “The evolution of steel in automotive is really dependent on what our customers are designing, and what they need,” notes Roger Heimbuch, executive director of ASP.
“In the 2000s, there were worries about cost, fuel efficiency, crash, rollover, side intrusion, and front integrity, so you don’t get damage into the passenger compartment,” Heimbuch says. “That’s when we started introducing the multiphase steels—the dual-phase, the TRIP [Transformation-Induced Plasticity steel], the complex phase—and those were produced by varying the chemistry, as well as by the thermomechanical process at the mill, and then through the subsequent forming operation and bakehard operations at the auto companies. So we’ve got very strong material that can help reduce weight, but meet all crash and energy-management performance requirements.”
Aluminum automotive components can be found nearly everywhere in cars and trucks today. Lighter than steel, aluminum is very ductile, highly recyclable, and can easily be formed into parts, including those used for powertrain applications such as engine blocks and heads. The material makes dramatic reductions in motor-vehicle weight possible.
Honda and BMW are now the aluminum-content leaders, according to the Ducker study, replacing General Motors and Nissan, with both companies averaging more than 340 lb (154 kg) of aluminum per vehicle. GM, Honda, Toyota, BMW, Hyundai, and Volkswagen all increased the aluminum content of their North American vehicles from 2006 to 2009. On a component basis, the study cites engine blocks and steering knuckles as exhibiting the largest increase in growth over the last three years, with penetration of aluminum blocks reaching nearly 70%, the largest driver of aluminum growth in this decade. More than 22% of vehicles currently made in the US have aluminum hoods, an all-time record, according to the study.
“The use of aluminum in the automotive industry has grown every year for the past 30 years,” observes Kevin Lowery, of Alcoa Inc. (Pittsburgh) and chairman of the Aluminum Association’s communications committee, Auto and Light Truck Group.
Aluminum space frame technology appears in many upscale cars, including the Corvette, the Audi TT, Ferrari, and the Acura TL, notes Lowery. “The space frame is like the skeleton of a car,” Lowery states. “It’s essentially the structural aspect of the car. More and more OEMs are understanding that you can get high strength from aluminum. You can work with aluminum and design it such that you can eliminate multiple pieces or multiple parts, if you’re involved early enough in the design phase. And in addition to the high strength, you’ll get the weight savings.”
Magnesium use in automotive shows promise, with much research on applicability to auto applications. The metal is predominantly used in die-cast parts including four-wheel-drive transfer cases, transmission cases, engine cradles, steering-wheel components, seats, and instrument panels. While magnesium is abundant and is about a quarter of the weight of steel and two-thirds the weight of aluminum, it can present processing problems. “Die-cast is probably the primary production methodology at this juncture,” notes Greg Patzer, executive vice president, International Magnesium Association (IMA, Wauconda, IL). “Work is being done in sheet and forming, but by far, die cast is the way most things are produced.”
Research into automotive use of magnesium continues with work by the Pacific Northwest National Laboratory (Richland, WA) under cooperative efforts sponsored by the US Department of Energy and the United States Automotive Materials Partnership (USAMP), part of the United States Council for Automotive Research LLC (USCAR, Washington, DC). USCAR is a joint research collaboration of GM, Ford, and Chrysler. In addition, a collaborative project between the Commonwealth Scientific and Industrial Research Organization (CSIRO, Australia) and USAMP is researching use of the magnesium AMSC1 alloy in a new sand-cast magnesium engine research project. That research shows the advanced magnesium alloy offers not just a higher strength-to-weight ratio, but also offers greater noise and vibration dampening than either aluminum or steel.
Use of magnesium, aluminum, and even titanium alloys, as well as the ultra-high-strength steels, will be covered at the upcoming Materials Science & Technology conference to be held in Pittsburgh October 25–29, sponsored by ASM International (Novelty, OH), and three other materials societies.
Composites in automotive also are a key research area within ASM, as the Ground Transportation Committee is studying alternative approaches, including glass, ceramics, and nanotechnology, as well as electronic, magnetic, environmental, and energy issues. Composite materials are used widely in more cost-conscious platforms, like family sedans. These materials include the polymer-based composites used in body panels for some Saturn models, and fiberglass composites.
Process issues can often arise with materials that present problems in formability and machinability. “My understanding was that people were starting to move away from magnesium, for a couple of reasons,” says Don Graham, manager of turning products and educational services, Seco Tools (Warren, MI). “It was all the rage about 3–5 years ago, and a lot of people were working on magnesium components. Then, for various reasons, they began moving back to aluminum.
“Magnesium typically is in cast components that require no machining, and if they do require machining, typically it’s drilling,” Graham says. “When it comes to magnesium, we find that the best cutting-tool material to be a polished, positive-rake, uncoated carbide insert, typically a micrograin carbide. Using a hard, uncoated carbide insert with a polished surface, very positive rake, is good. Typically you use highpressure coolant, which helps to keep temperature down.”
For machining harder materials, Seco developed its Duratomic grade of inserts. These tools have a special coating that, combined with high cutting speeds, can improve machining processes for higher-tensile-strength alloys. The coating allows chips to slide more easily, with a lower coefficient of friction, lower heat generation, less built-up edge, better surface finish, and less wear.
Machining dissimilar metal components for an electric motor on an electric vehicle program required developing a new machining strategy, according to Brian Hoefler, product development manager, Valenite LLC (Madison Heights, MI). The company has been working on electric motor prototypes for a Detroit Three automaker that sourced the first 50 parts to Valenite, which is producing the components in its lab.
“In a traditional electric motor, you have copper windings,” Hoefler notes. “They’ve come up with a way to use cast aluminum surrounding the metal plates, in order to create the same electric motor without having to wind it with copper. There’s a process where they stamp out the metal, stack it, then cast the aluminum around it, and we machine it so it’s basically a generator shaft for an electric motor.”
To machine the dissimilar metals, Hoefler says he employed a fairly traditional tool. “We used a traditional diamond material on the sections that are mostly 6–8% silicon-aluminum, and then in the composite area, we actually used our new microform technology, and had great luck with it. We invented microform for something else; it just so happened it ran very well in the composite area. So we made a proposal to have a strategy for machining the parts, and we met with the OEM here at our facility. We showed them how it worked, and they’re going to give us some prototype orders, just to relieve the burden on them to produce them.”
Much of the company’s automotive work is producing valve seats for engines. “Our tools for producing valve seats on engines are among the Detroit Three’s global best practices so every engine that they produce in the world uses our valve seating tools,” Hoefler notes. “We have some boring technology that we’ve developed in the last couple of years that enables boring cylinders within 3–4 µm.”