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Special Report: Automotive

Automotive heats up

By Tom Klemens

It’s fabricating and forming’s turn to contribute to fuel economy

April 2013 - There’s something uniquely American about the new car mystique—it’s a perfect fit for a still young nation, bristling with new technology, expansion, mobility and opportunity. Now in its second century, America’s love affair with the automobile has shaped the nation’s character from the earliest days of the 20th century, giving rise to a can-do attitude and a network of more than four million miles of road.

Today automakers continue to develop new materials, designs and manufacturing techniques in an effort to balance consumer appeal, fuel economy and vehicle safety while remaining profitable. Although consumers obviously decide what features sell, government regulations play a major role in spurring change and innovation.

Way back when

In early horseless carriages, metal was relegated to boilers and internal combustion engines. But metal offered automobile builders formability, economy and strength, and soon became the predominant material as more autos hit the road.

Henry Ford made automobiles affordable in 1908 when the Model T entered mass production, taking advantage of metal’s superior strength-to-weight ratio compared to wood’s, as well as having better formability and durability. Ford also increased the number of potential buyers by paying a good wage and over the next two decades sold 15 million Model Ts.

In the mid-1920s, General Motors president Alfred P. Sloan introduced the idea of the automotive model year and created a tiered family of models all produced by the same company.

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Over the years aesthetics and performance have driven new development in this highly competitive industry, but always with a watchful eye on economy. However, in the 1970s, environmental awareness and personal injury lawsuits moved the industry to significantly boost fuel economy and increase passenger protection. Although automakers knew it was in their competitive best interests to improve these areas, it was the federal government that stepped in with specific fuel efficiency and safety standards.

On the safety side, the decade of the 1970s saw the birth of the crash test dummy. Members of the Hybrid III family—a man, a woman and three children (ages 10, 6 and 3)—were introduced in 1976 and quickly became familiar figures in popular culture. Data from their test participation led to structural changes in auto frames, as well as the addition of shoulder belts and the development of airbags.

Concurrently, efforts were undertaken to improve fuel efficiency, partly to address environmental concerns—the first Earth Day occurred in 1970, the same year the EPA was established—but also in response to the increasing price of fossil fuel. In 1975, soon after the Arab oil embargo of 1973, Congress created the first Corporate Average Fuel Efficiency standard. The goal was to double existing manufacturer fleet averages and reach 22.2 mpg by the 2007 model year. These requirements remained in effect, essentially untouched, until Congress passed the Energy Independence and Security Act of 2007. That legislation set a new overall goal of 35.5 mpg by 2020.

In 2009 the federal government, state regulators and the auto industry established a national program to coordinate and facilitate automotive fuel efficiency and pollution control goals and requirements. The new standards, which were announced in July 2011 and finalized in August 2012, require the industry-wide fuel economy to average 54.5 mpg for cars and light-duty trucks by the 2025 model year.

Reaching agreement on that future requirement dispelled much of the uncertainty regarding future regulations and is just one reason industry R&D is running in high gear today. Consumers are pushing for improved fuel economy, as well.

Much already has been accomplished in the realm of engine performance, although improvements to auto air-conditioning systems are expected to support better fuel economy in coming years. Tire design and aerodynamics have become more sophisticated and may already have made their largest contributions to better fuel economy. Most future improvement hinges on reducing vehicle weight.

How better fuel economy happens

Many of the more massive auto parts originally made of steel or cast iron, such as drivetrains and engine blocks, already have been reengineered to use lighter materials. 

“The drivetrain parts have made great progress in terms of reducing weight,” says Taylan Altan, professor at Ohio State University and director of its Center for Precision Forming. “But the other part of the car where you can reduce weight is the body—shell pieces and the outer body.”

ffj-0413-automotive-image1Altan says there are basically four ways to reduce the weight of the sheet metal in a vehicle. One is to design a sheet metal part with ribs to increase its stiffness. This can be effective, but it is limited. A second alternative is to make certain parts out of lighter weight materials, such as magnesium or aluminum. This already is done in some vehicles, but because these materials are more costly, it typically is only in high-end models.

Magnesium is widely used in castings, but as a sheet product it is difficult to form and requires warm forming. Although aluminum is easy to form, its earlier use typically was limited to inner parts where dent resistance is not a prime requirement. But that’s changing, says Kevin Lowery, director of corporate communications for Pittsburgh-based aluminum producer Alcoa Inc. “Based on feedback from OEMs in the latest Ducker survey, they’re going to double the amount of aluminum they’re using by 2025,” he says. “And that is coming off a base of aluminum already being the number two material used to make a car.” Even now, according to Lowery, as many as 40 percent of North American hoods are being made from aluminum.

“The next frontier driving this will be the use of aluminum in doors and body-in-white applications, essentially the skeleton of the car,” Lowery says. “There are projections of a 750 percent increase in global auto body sheet consumption by 2025.”

The changeover to aluminum, with its easy formability, may not be terribly difficult, but it can be tricky. One Chicago-based electronics manufacturer’s representative, who asked to remain anonymous, described a material handling glitch that arose not long ago when one facility started producing aluminum doors and hoods. When the company switched from steel to aluminum on its hood line, he quickly got a call because every hood suddenly had four dents in it.

“The problem was that the proximity switches are metal detectors,” he says. “They’re set on the robots’ arms that come in with the suction cups to move the material.” Before changing materials, the proximity sensor would see the hood and know it was close enough to turn on the vacuum. When the vacuum sensors confirmed good suction, the robot would pull the part out of the die and transfer it to the next die.

“There could be six or seven transfer dies, all in a row, where the robot takes the part that just got stamped in stage one and puts it in stage two,” he says. “Meanwhile, another robot’s putting a blank in stage one. After seven stampings it’s a finished part, in this case a hood.” With the change in materials, the company’s $70 million stamping press suddenly was making a large piece of scrap every 10 seconds. 

“Many standard sensors, including those in use on the hood line, are rated to detect metal at 10 mm, but that assumes it is ferrous and flat,” he says. “With aluminum you lose half of your sensing range.” That meant the sensors had to get 1⁄2 in. closer to sense the hood’s presence, which pushed the suction cups 1⁄2 in. into the aluminum hood. Upgrading to sensors with no material derating factor quickly put the line back into production.

A third way to reduce sheet metal weight is by using high-strength steel (HSS), also known as high-strength low-alloy steel (HSLA). For example, some automakers use 270 MPa steel sheet, which is on the lower end of the HSS range, for exterior parts. However, HSS is more widely used for internal parts, such as floor and door panels, where 600 to 1,200 MPa material is specified. For even greater weight savings, advanced high-strength steel (AHSS), or advanced high-strength low-alloy steel (AHSLA), can be used. The tradeoff in using HSS or AHSS is it is more
difficult to form and requires a high-capacity press with high-strength tooling. The wear and tear on the equipment is greater, as well.

Strong, light and springy

“Springback is a big issue,” says Shrinivas Patil, product manager with press maker Aida-America Corp., Dayton, Ohio. “And because the strength is so high, your tools have to be modified accordingly to be able to handle this kind of steel.” Another issue with high-strength materials is snapthrough, or reverse tonnage, “which can kill the tool, and can kill your presses,” he says. “And as you go with the higher strength steel, your reverse tonnage is going to go higher, as well. Using servos and being able to slow the blanking process, we are able to reduce the reverse tonnage by more than 90 percent.”

Another drawback to using HSS or AHSS stems from the fact that material inconsistencies are more likely. Because these products are proprietary and developed by the individual steel producers, material specifications are less comprehensive than for more traditional grades of steel. In particular, only a minimum tensile stress is specified, meaning two coils of HSS that have the same nominal grade may, in fact, have different strength characteristics. The coil with the higher yield stress will be stiffer and generally more difficult to form.

Patil says running computer simulations to determine formability becomes more difficult when using HSS. “When you go to this new high-strength steel, there is very limited information available,” he says. “So you need to do some basic research to establish materials properties before you can run your simulation.”
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FFJournal senior contributing editor J. Neiland Pennington wrote about Hyundai’s solution to this problem of material inconsistency in the March 2013 issue of Modern Metals.  Because the automaker owns its own steel mills, it can be more sensitive to carefully controlling HSS production and also manage quality control through its internal supply chain.

The fourth way

Hot stamping, also known as press hardening, is a fourth method automakers are using to reduce vehicle weight. A fledgling technology with regard to U.S. adoption, this involves the use of manganese-boron steel, which is relatively strong but cannot be formed at room temperature.

Hot stamping technology was developed in Sweden in the late 1970s and originally used for making saw and lawnmower blades, as well as other farm implements. 

To form the material it is first heated to about 1,200 degrees C. Along the way, at about 910 degrees C, it becomes austenitic and easily formable. Once in the press and formed, it is held for several seconds as it is quenched at a very specific cooling rate. The quenching returns much of the steel’s crystalline structure to martensite, giving the material its high strength. The austenite that is allowed to remain provides some degree of flexibility. Depending on the quenching process, hot-stamped manganese-boron steel can exhibit strength in the 1,500 to 1,650 MPa range.

European automakers have been using manganese-boron steel for a decade. In some vehicles, hot-stamped manganese-boron steel now makes up 10 percent to 15 percent of body parts, by mass. U.S. automakers are increasingly turning to the technology, as well, with at least one of the Big Three shooting for 25 percent of its auto body components to be manganeseboron steel. 

The heating and quenching requirement reduces the production rate, cutting it by half or more. It also requires high-performance tooling with piping to enable the circulation of cooling liquid as the part is quenched. And because the machinery has to stop while each part is held in the press, it eliminates the possibility of using a mechanical press.

The technology allows stampers to do a lot in a single forming step, says Rich Marando, president of machinery supplier Graebener-Reika Inc., Reading, Pa. “There are some complexities if multiple forming steps are required,” he says. “It’s not as conducive to multi-step forming as traditional stamping with traditional material is. For a very complex deep draw part, you can use an indirect process, where you’re preforming the blank, then heating it, and then forming and quenching and finish-forming it in the second step.” Although some European automakers do their own hot stamping, others rely on Tier 1 suppliers to provide hot-stamped parts, which is also the course U.S. automakers are taking. That has been a concern for Tier 1 suppliers because gearing up to do hot stamping means investing millions of dollars in new equipment, while new materials continue to be developed. Transformation-induced plasticity steel (TRIP) and twinning-induced plasticity steel (TWIP), two such innovative steel types, could cut into future demand for hot-stamped parts. Both use sophisticated metallurgy to keep the steel austenitic at room temperature, and thus formable without heating. They also can be work hardened, so that as a part was deformed in a crash it would suddenly increase in strength. However, both TRIP and TWIP are still in development. It is likely to be a while before either is used in mainstream production, so for now, hot stamping in the U.S. is on a growth trajectory.

Is hot stamping limited to automotive? “Right now it’s primarily for automotive, but I don’t think it’s going to stay that way,” Marando says. “Automotive has a tendency to take new forming technologies and pump money into them by creating demand that drives investment and R&D. When they’re done refining the process and cutting down the cost, it usually finds its way into other industries, and I don’t think it will be anything different in this case.” FFJ

 

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