Hybrid Multitasking—Machining and Friction Stir Welding

September 29, 2022

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The lithium-ion batteries in electric vehicles (EVs) require precise temperature control to maximize range and recharging capabilities, and to protect the battery itself. Optimum operating-temperature range is approximately 59 to 95 F. High temperatures reduce a battery’s working life, and extreme heat can damage battery chemistry or cause catastrophic thermal runaway.

Hybrid milling friction stir welding battery tray

As a result, automakers engineer complex cooling systems for battery protection. Most systems feature a battery container or tray with a network of tubes or channels that circulate coolant around the cells. The trays usually are located low in the vehicle and in some cases fill the entire underside of the car from the front to rear axle.

Fabricating the network of cooling tubes presents challenges that typically require more than subtractive machining alone. While shops may employ subtractive machining to mill shallow channels across the floor of the battery tray, attaching the cover plate over these channels usually requires some type of welding process.

Calling on Friction Stir Welding

Friction stir welding (FSW) represents an efficient and economical process for bonding the cover plates to the battery trays. FSW joins metal components using frictional heat, created by a rotating tool with a non-consumable central pin that moves along the joint between the parts, plasticizing the parts being joined. The process creates defect-free, strong joints that require little post-processing, use no filler material, resist corrosion and even can join dissimilar or low-melting point materials, all without the arcs, toxic fumes and molten spatter produced by traditional welding processes.

FSW also is a lower-temperature process compared to traditional welding processes that typically apply a significant amount of heat to melt and join the materials. And the resulting heat-affected zone (HAZ) can compromise joint strength. Because FSW joins materials without actually melting them, it creates much less heat and minimizes or avoids distortion—especially beneficial in battery-tray production.

In addition, welds made with the FSW process have the same strength as the base material. And, by using the FSW process, manufacturers can design battery trays with less material at the weld joints. Doing so enables them to reduce tray size and minimize weight, which in turn improves energy efficiency.

Hybrid Milling/Welding

An optimal solution for battery-tray manufacturing would be a hybrid process that combines the FSW process for assembling the cover to the milled battery tray. In such a case (the Mazak MegaStir process, for example), an intelligent tool holder mounts in the spindle of a vertical machining center that performs normal tool changes between a milling cutter (or other cutting tool) and an FSW tool holder. Fully closed-loop FSW software allows for real-time process monitoring, manual thrust-offset capabilities, process logging and part-program design, all at the machine control. An operator then can manage the three main FSW process steps—plunging, traversing and extracting.

The combination of FSW and subtractive machining creates a highly productive, lean, and hybrid multitasking platform, able to machine and assemble components in one setup with one operator. The FSW/subtractive machining hybrid operation facilitates prototyping and full production operations. Prototype shops can create a design, machine and weld the assembly, as well as finish-machine it all on the same piece of equipment with minimal setups.

The hybrid platform also enables commercialized high-rate production. If a product is designed to exploit the advantages of the hybrid manufacturing process, manufacturing speed increases by multiples. A part that theoretically might take 30 hr. to machine can be built in 1 hr. with hybrid multitask manufacturing.

The size of products that can be processed with hybrid multitasking is limited only by the size of the machine tool. “Most battery-tray applications use sheet metal in the range of 1⁄16 to 1⁄8 in. thick,” says Mazak general manager Dale Fleck. “The worktable of Mazak’s VTC 800 machine, for example, is long enough to handle an entire axle-to-axle battery cooling tray for an EV.”

Beyond Battery Trays

The advantages of the hybrid multitasking system in EV manufacturing extend beyond battery trays. Housings for electronic components such as control modules and transmissions can leverage the benefits of combining FSW with subtractive machining and provide a competitive advantage in the high-demand EV market.

Further, many other industry segments, including aerospace and medical products, and even household appliances, seek compact design and weight saving to increase efficiency and economy. As manufacturers become aware of the benefits of the hybrid process system and learn how to apply it, the range of applications will continue to grow. MF

Article provided by Mazak Corp.; www.mazakusa.com.