Canton, Michigan – As automakers increasingly use aluminum in place of steel in vehicle bodies in an effort to cut weight, many structures are getting thicker because designers are using more metal to provide crash-protecting strength in critical structures. Thick aluminum parts are still lighter than thin steel ones, so weight savings continue to mount, but swelling part thicknesses can make jobs tougher for designers and can limit visual features such as large windscreens or windows.
Shifting to thinner, stronger aluminum alloys hasn’t been possible because those materials were too difficult to stamp into complex shapes in high-volume automotive operations, says Mohammed Gharbi, director of technology and process development at the Schuler Group. To broaden the range of suitable alloys for automotive production, Schuler engineers are working with two processes to improve formability of difficult materials.
“Within the last few years, there have been several high-strength aluminum alloys developed that cannot be formed in cold conditions,” Gharbi says. “There needed to be a new technology, a new way to form this material to allow customers to use it to create complicated geometries with more strength.”
Both processes involve heating aluminum blanks before sending them into stamping presses. A lower-temperature process (between 200°C and 300°C [392°F to 572°F]) makes the metal more pliable for stamping without changing the mechanical properties of the underlying aluminum. Called HFQ by developer Impression Technologies Ltd., the higher-temperature process (400°C to 500°C [752°F to 932°F]) further improves formability, but it does create material changes in the aluminum, forcing an aging, heat-treating process that can lengthen production times.
“Whatever you cannot form cold, you can form it in warmer conditions. What you have to ensure is that you understand the material characteristics. Increasing temperature just to have the desired geometry is only one goal, Gharbi says. “The other goal is to have the mechanical properties you need at the end of the process. This makes the process more challenging.”
With both temperature levels, companies would heat panels, stamp them into shape, let them cool on the die, then remove them for heat-treatment aging (for the hotter process) or delivery (for the cooler one). Using the Schuler system would add one or two new process steps, something companies are often loathe to do because extra steps mean extra time and floor space with higher costs.
With extra process steps as a given, Gharbi says Schuler’s challenge was to make hot forming attractive enough to justify the time and cost. A big attraction is the ability to use steel-like design features in aluminum – something that hasn’t been possible because of the metal’s poor formability.
Ford’s F-150 pickup, the biggest user of automotive aluminum worldwide, uses 5000-series and 6000-series alloys – grades with 239MPa to 276MPa tensile strengths respectively. Series 7000 aluminum alloys, materials that have been too tough for automotive parts, boast tensile strengths as high as 538MPa, according to ASM Aerospace Specification Metals Inc. – equivalent to conventional steel alloys.
“Hot forming could support the use of thinner, lighter aluminum parts with more complex shapes,” Gharbi says. “So a longer process with more equipment and steps could cut costs by reducing material use.”
It’s a calculus that automakers know well. Tougher alloys are more costly, but when used strategically, a thin, strong part can replace a thicker one. The net cost effect can balance out, but vehicles benefit from the lighter part. Changes could be more noticeable in aluminum because of the material’s higher cost.
Design freedom also rises. For the F-150, Ford designers tried to use a stamped aluminum part for the A-pillar, the component that supports the truck’s windscreen and roof. But to get the desired strength, the part was too thick and limited forward visibility. Instead, Ford used a hydroformed aluminum tube (Schuler’s not complaining, they make hydroforming equipment as well).
“With 5000 series and 6000 series, you need to have more thickness to get the same strength as 7000-series aluminum,” Gharbi says.
“The bottleneck of the process is heating up the blank. Depending on the technology you use, you will have different times,” Gharbi adds. “If you have a furnace, you need minutes. If you use other technologies such as induction heating, you could get up to temperature in a few seconds.”
Existing systems and appetites for large capital investments will drive many of those technology decisions. For example, if a plant already has a gas-fired furnace for other heat-treating processes, engineers could design a schedule that dedicates furnace time for the aluminum parts, eliminating a big spending item. Induction systems may be more attractive for higher-speed production or with plants that don’t have furnaces with spare capacity.
“Forming doesn’t take a lot of time. Aluminum, at elevated temperatures, needs very low forming forces,” Gharbi explains. “You could add a cooling medium to your die to cool down the part faster, or you could heat up a portion of your die to get more formability.” The actual stamping and cooling time gets measured in seconds, not minutes. And aluminum cools quickly, going from the 500°C forming temperature to handleable in two or three seconds.
Fuel economy regulations in North America and emissions rules in Europe are effectively forcing massive amounts of lightweighting in every vehicle design. So Gharbi says companies have been very receptive to Schuler’s initial reports on the heat-forming processes.
“We’re talking to automotive companies in Southern Germany, and they don’t want to talk about only having one process or the other. They want equipment that can support both the lower temperature and higher temperature options,” Gharbi says. “They want one press that they could use if the metal gets heat-treated or not, so we’re developing that technology.”
From the January/February 2016 issue of Today's Motor Vehicles.
Even at the 500°C high end for HFQ aluminum hot-forming process, the temperatures are low compared to other metal treatments. Press-hardened steel, a similar technology that heats ultra-high-strength steel until it is formable, runs at about 930°C, says Mohammed Gharbi, director of technology and process development at the Schuler Group.
“You try to have the same material characteristics after the process as when you started,” Gharbi says. “The melting point of aluminum is lower than that of steel. There are alloying limits within aluminum that could liquefy at higher temperatures. That’s why we chose the temperatures we’re using.”