According to Eren Billur, an international engineering consultant to automotive manufacturers and tiers specializing in sheet metal forming, hot stamping has roots in traditional Japanese sword-making techniques of the 13th and 14th centuries. The 20th century saw the first patent for “press hardening” issued in 1914 in Switzerland, producing garden implements like shovel and spade blades in the 1930s.
Hot stamping today began seeing use in the automotive industry in 1984 Saab 9000 door beams. Speaking at a pressworking event in September at Urgent Design & Manufacturing (3dimensional.com) in Lapeer, Michigan, Billur identified lightweighting as an automotive issue particularly well-suited for investigating a hot-stamping response, as OEMs look for higher-strength, lighter steels, which are more difficult to form with conventional methods.
This is because hot stamping works well with new-generation ultra-high-strength steels (UHSS). Light but strong for improved crashworthiness and crumple-zone, performance, such UHSS grades are notoriously hard to form. Furthermore, steel suppliers say the main advantage with cold forming compared to hot forming is the difference in processing costs, including tooling costs, productivity, energy consumption and the need for more expensive laser trimming when hot forming is used.
Hot stamping works like this: Steel blanks are fed into tunnel or stacked furnaces and heated for a time and temperature (approximately 1500° F) that makes the blanks malleable. Blanks then move into a press capable of controlling stroke rate and dwell time for forming. Immediately after, in-die water channels provide quenching for 3 to 10 seconds. Heating shifts the steel to a fully austenitic phase for forming while quenching leads to a full martensitic phase, creating the hardened from while not overly stressing the tooling. Depending on the high-performance material, a number of high-strength, lightweight performance factors are achieved depending on the stamped component and its requirements.
“When getting into UHSS, some geometries can't be cold stamped due to work hardening of the material, strain limitations and increased springback,” says Chad Peterson, 3-Dimensional Services general. “Trying to achieve the geometries would require three to four forming operations, and material would work-harden to the point that it’s not practical or possible. But hot stamping, with material in a red-hot state, enables drawing of those geometries, with the material’s end strength far superior to what cold stamping can provide.”
Tools Will Be Taxed
Hot stamping is not without its challenges, many focusing on the tooling. In general, hot-formed parts are too hard to cut precisely with stamping dies, says Mike Austin of Diversified Tooling Group (diversifiedtoolinggroup.com). Since the process calls for 5-axis laser-cutting machines for trimming and piercing, in-die hot-piercing can help alleviate laser-cutting costs. “Developed trim is also very popular to reduce laser costs,” he says.
And while hot-stamping lines can be as much as 40 percent more expensive than a cold-forming transfer line (adding furnaces, increased automation and energy costs, and yes, specialized tooling all add up), the tooling can reduce process inefficiencies.
“With cold-stamping AHSS, you have a line of presses, meaning you’re building three to five dies to handle springback variations for the component,” Austin says. “With that, the stamper may still need to re-machine the dies several times over their life cycle. With hot forming, you build one die, admittedly a more-expensive die, but one that doesn’t have to be re-machined due to near-zero springback in an ultra-hard material.”
Take for example a transfer press cold-forming medium-sized parts. Every set of tools could cost as much as $1-million, Austin says, and you might require 40 sets of tools over the life of the press. “If by hot forming, you can cut tooling cost by nearly half for every set of tools, then guess what – you’ve just bought a couple presses,” he says.
Conformal Cooling for Hot Stamping
In-die quenching also offers its own challenges for die builders. Water channels are drilled and blocks are machined and then heat-treated before final hard cuts. Block dimensions must be very precise to obtain uniform quenching and cooling. Naturally channels must not leak at joints or through any die cracks, which can be difficult as cooling channels should be as close to the surface as possible without causing any cracks in the blocks in the first place.
Through metal 3D printing, die inserts with cooling channels built-in can be produced. Rather than requiring gun-drilled cooling channels in straight lines, cooling channels instead can closely follow intricate part contours, a process known as “conformal cooling.” As a result, quenching and cooling time can be shorter and parts cool more evenly, cutting process time.
With costs falling and process and tooling efficiencies rising, hot-stamped parts are growing in production vehicles. From his European perspective, Eren Billur reports the portion of press-hardened steel (PHS) parts in the European-spec Ford Focus grew from 9 percent in 2011 to 32 percent last year. Overall, hot-stamped automotive components totaled 85 million parts per year in 2007 and accelerated to more than 600 million parts per year in 2018. His figures report the total body-in-white portion allocated to hot stamping was 8 percent in 2015 and will more than double to 17 percent in 2025.
As a result, the use of lighter, ultra-high-strength steels will grow throughout the automotive chassis and expand from premium-priced luxury models to more affordable mass-market brands. Stiffness and NVH results will improve, energy-absorbing performance will be enhanced, overall crashworthiness will go up due to improved crash-dominant parts with complex shapes, all at lighter weight than cold-formed steels that will improve fuel efficiencies. The future for hot stamping is not only hot, it’s bright.
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