Using heat to form complex metal parts
Hot dies, hot workpieces maintain ductility for forming complex parts
Just as hydroforming was “discovered” in the late 1980s or early 1990s, as it transitioned from just a few specialized industries into more mainstream manufacturing segments, another, related process is forging a similar path: Hot metal gas forming. Similar to hydroforming, but performed at elevated temperatures, it does many of the things hydroforming once did – forms difficult shapes and complex shapes, reduces part weight, exhibits less wall thinning than some competing processes – but takes them further because it works with more pliable material.
If you work in manufacturing, you already know that component designs tend to grow more complex as time goes on—nothing ever becomes simpler. For many consumer products, it's a matter of adding more features or improving the styling. In the automotive and aircraft industries, it's constant pressure to add or improve features while reducing the overall weight to help conserve fuel. Whether engineering an entire system or designing just one part or a subassembly, the goal is a modern-day application of an old adage: Trying to cram 10 pounds of features into a 5 lb. bag.
What is a manufacturer to do? Making a complex metal part can be challenging, but suppliers do have many processes to choose from, so it's a matter of matching the part to the process, whether it’s casting, forging, machining, or forming. If the part is really complex, it's a matter of making several parts and join them by welding.
That said, making a complex metal part isn't the issue; doing so economically is the key. And, just as parts become more complex, so too do machines and related equipment for manufacturing. As time goes on, forming and fabricating capabilities become more sophisticated.
Modern Updates to Traditional Processes
Two well-known forming processes, stamping and hydroforming, have been used in some form or another for many decades. Pressing a sheet of steel between two dies is old hat, and parts made by stamping can be found nearly everywhere—homes, offices, vehicles, and on and on. Hydroformed parts aren't that ubiquitous, but anyone who has opened a tap to get some water from a fancy gooseneck faucet probably has used a hydroformed plumbing fixture. Anyone who has listened to an orchestra or jazz band has heard music played on instruments that would be almost impossible to produce in any way other than hydroforming. Place a tube into a die half, clamp down the other die half, and fill the tube with a forming fluid. Voila! The tube is now a tuba, a saxophone, a French horn, or any of a handful of other brass instruments.
More Forming, Less Ductility. Two main factors limit a metal's ability to take on a complex shape. First is the metal's strength, which is its ability to resist deformation. High strength is favorable in a finished product, making it resistant to damage, but it's a drawback in a raw material because a high-strength material needs more forming force than a low-strength material. Strength also is associated with brittleness, so a high-strength material is less likely to stretch and more likely to fail than a low-strength material.
The second factor—actually, a series of three related factors—relates to changing the metal's shape at room temperature. First, the forming process usually causes thinning, especially at corners and along short-radius bends. Too much thinning leads to part failure during the forming process. Second, the metal's strength increases as it is worked at room temperature. Its ductility, or ability to stretch, is limited and inversely related to its strength; working a metal uses up its ductility, imparting more strength as the work progresses. Third, as the alloy's strength and brittleness increase, more force is required to form the part.
Managing Ductility. Fabricators have three ways to deal with these properties. All add cost and time to make the part, so it's a matter of finding the best one for the application.
First is an intermediate anneal. Rather than forming the part in one continuous forming process, fabricators can do some forming, then heat the part to restore some or all of the material's original ductility, then do some additional forming. This means investing in an annealing furnace, setting aside some floor space for the annealing furnace, and using more of two resources—energy and time—to make the part.
Second, if the part is stamped, heating the part and the dies before stamping improves the metal's ability to flow, allowing it to take on intricate die features. An immediate quench restores the metal's strength.
Third, if the part is hydroformed, a related process, called hot metal gas forming (HMGF), uses heat to make the material more pliable. HMGF relies on some combination of a heated part and heated dies to form metal at an elevated temperature (see Figure 1). Heating the gas is another factor, although it's rarely used.
Working the metal at an elevated temperature changes essentially everything about the forming process. Because increasing the process temperature causes metal to flow more freely than at room temperature, the metal doesn't thin nearly as much as it stretches to fill the die cavity. It also doesn't become increasingly brittle as the forming process progresses. The material's strength likewise is lower than that of a cold material, so the process requires less forming force.
In essence, working at an elevated temperature is like annealing while forming, maintaining the material's ductility during the forming process. Because the process maintains a consistently high ductility from start to finish, bend radii can be tighter, corners can be sharper, and parts can be increasingly complex.
For these reasons, parts that were considered marginal or impossible to form at room temperature are candidates for HMGF, as are component shapes that nobody would touch with a conventional, room-temperature process.
European Concept, Made in America
HMGF, a technology developed by German company HEATform™, was introduced to the Americas by a partnership between a new company, HEATform Americas, and an established company, Interlaken Technology Co. LLC. The system this partnership developed, in the form of the HTF-500, is similar to many modern hydroforming systems in that it uses a two-piece die to form the part, cylinders to grip the part, and boost cylinders to feed the tube into the tool during the forming process. It differs from hydroforming in that it relies on induction heating to raise the temperature of the workpiece and a proprietary system to heat the dies.
Automation such as a transfer system or a robot can be added between stations, as can stations for bending, lubrication, cleaning, and drying.
Applications. HMGF is a niche process suitable for difficult applications. A simple tubular part that has a slight change in diameter—a tapered chair leg, for example—can be formed by swaging or by drawing it into a tapered die. If the part has a more severe or abrupt change in diameter, or alternating large and small diameters, swaging might still be a suitable process, but as the features become more extreme, hydroforming becomes a candidate. If the features are really extreme, at or beyond the limits of hydroforming, HMGF becomes a candidate.
“Some three-dimensional parts traditionally were cast or assembled, but hydroforming has been a good alternative for many such components," said Tad Machrowicz, president of HEATform Americas.
"Two examples are a manifold with eight distribution nodes and a T fitting with a big expansion. In the past, the manifold would have been cast and the T fitting would have been assembled from two or three pieces.” HMGF can help to accomplish more complex manifolds and T fittings with more extreme expansions.”
Alloys. The process works on ferrous metals, red metals, aluminum alloys, and nearly any other metal used in manufacturing. Steels that require a strict quenching regimen, such as boron and vanadium steels, don't pose a problem, as long as a quenching tank can be located nearby. The process also is a possibility for alloys that have little ductility or machinability, including precious metals and alloys that never were considered good candidates for forming in the past.
HMGF generally requires less than 10 percent of the forming force of conventional hydroforming. This means that the dies and the press don't have to resist vast forming forces, so they are more economical to produce. It also means that process designers don't need to put as much attention on controlling those forces; they can focus on managing material flow, controlling the wall thickness, optimizing raw material use, and keeping the final product weight under control.
Sample Part. A conventional process, precision casting, is typical for making a highly styled aluminum part, such as a railing baluster. These days, HMGF is another viable process (see Figure 2).
Hydroforming is not a candidate because, when formed at room temperature, aluminum alloys don't provide sufficient elongation, nor do they form sharp corners and other severe design features.
“Calculated expansions of this part are in excess of 85 percent, but the practical expansion ratios at the ball corners are even higher,” Machrowicz said.
Stamping this part in a two-piece clamshell would lead to unacceptable wall thinning in these areas, according to Machrowicz. Preventing this requires starting with a higher-gauge material, which drives up the cost. While highly formable aluminum sheet is emerging from auto industry development, increased final weight, subsequent seam welding, and metal finishing operations needed for this part add more time and labor, increasing the cost of the clamshell approach, he said.
Acceptance. Because HMGF has broad potential, a thorough analysis is needed to determine if a particular part is a good candidate for the process.
“One part might be economical at thousands per year, but another part might be economical at just hundreds per year, ”Machrowicz said.
If it sounds reminiscent of the path that hydroforming took, it is. Once a niche process that catered to a small number of industries for decades, the process took off in the 1990s and now is used in countless industries. Perhaps HMGF will move down a similar path into the mainstream.
HEATform Americas, 8175 Century Blvd., Chaska, MN 55318, 952-856-4210, www.heatform.com
Interlaken Technology Co. LLC, 8175 Century Blvd., Chaska, MN 55318, 952-856-4210, www.interlaken.com
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