Early friction stir welding (FSW) machines, and some on the market today, represent one-trick ponies with high price tags that obligate shops to keep them exceptionally busy. Making the advantages of FSW available in a multi-tasking machine platform, on the other hand, offers a huge benefit to shops that otherwise must subcontract such joining operations. As most fabricators know full well, relying on a subcontractor raises its own set of challenges.
Taking the issues surrounding outsourcing and the traditional high cost of FSW into consideration, machine tool manufacturer Mazak designed and built its Hybrid Multi-Tasking machine that combines subtractive multi-axis machining with FSW joining capabilities. These FSW technologies represent developments pioneered by Mazak MegaStir, global leaders also developing and refining the technology as well as a recent Mazak acquisition.
Ample applications
For those not familiar with FSW, the technology produces lighter weight parts and pure, clean joints that are stronger, higher quality and more durable than MIG welding and many other joining processes. It also requires less electricity and eliminates the problem of weakened material properties that can occur with conventional welding.
These benefits are all included in the Hybrid Multi-Tasking system, which is particularly well-suited for several applications. Examples include semiconductor production as well as the manufacture of batteries and battery trays and vehicles, especially hybrid and electric models.
For electric car production, a manufacturing line uses FSW to attach servo-drive motors at each wheel, and in shipyards, FSW is the preferred joining method for producing decking for aircraft carriers. In the energy industry, deep-hole drilling restricts the use of welded components “down the hole” because welds constitute weak points on parts, but with FSW, those welds maintain the same strength as the overall material.
Stir it up
Primarily, FSW outdoes MIG welding because it uses a solid-state combination of frictional heat with forging pressure. That combination is able to create full-penetration defect-free joints like few other processes can. A non-consumable FSW tool called a pin softens and stirs part material in a process that involves no powder or wire additives.
The pin, a rotating round tool, plunges into workpiece material under vertical pressure, dwells momentarily to create a heat pool of plasticized material and then traverses along the workpiece to create the weld. The process moves material from one side to the other, breaking down grain structure and allowing the grains to regrow while the tool shoulder constrains the direction in which they form. That controlled, refined grain regrowth consolidates the welded joint, making the material stronger while retaining its original thermal and chemical properties.
During the FSW process, neither the tool nor the workpiece itself melts, which induces less heat into parts when compared to other welding techniques, reducing heat-affected zones. Typically, weld quality is a reflection of material type; tool angle; traversing and rotation speeds of the tool; pin length, diameter and geometry; and the diameter of the tool shoulder, which changes the diameter of the weld.
FSW joints also require little post-processing, use no filler material, resist corrosion and produce a wide range of joints, including hollow ones. The FSW welded area divides into four regions, including unaffected material, a heat-affected zone with no plastic deformation, a thermo-mechanically affected zone with plastic deformation and altered micro-structure, and a recrystallized area within the thermo-mechanically affected zone in which FSW changes the grain structure of the metal.
When applied to Mazak’s Hybrid Multi-Tasking machine, process flexibility is key to maximizing the production and efficiency. Additionally, the incorporation of FSW offers greater versatility than other alternatives.
Machining side
Currently, the Mazak VTC-300 FSW and VTC-800 FSW vertical traveling column machining centers add FSW to the machining spindle through a plug-and-play integrated system. These machines include standard 15,000-rpm, high-speed 40-taper spindles, full traveling column designs and fixed tables for machining extremely long and heavy workpieces.
In addition to optional table center partitions that allow for creating two work spaces from a single machine, VTC machines and their FSW heads make it possible to achieve done-in-one production of parts that require joining as well as milling. Compared to the cutting and removal processes of a mill, the spindle that incorporates the FSW tool spins in the opposite direction, grabbing and pushing material back into itself so it flows vertically and horizontally. The FSW capabilities use a direct-drive motor driven by the machine tool spindle.
To create plug-and-play convenience, Mazak’s Hybrid Multi-Tasking machines with FSW include human machine interface (HMI) software that makes the technology easy for shops to learn and use. This real-time monitoring also enables the operator to adjust thrust force and material temperature on the fly directly through the CNC controller. Additionally, the software records all data from the operation, giving the shop a charted representation of the actual process involved in producing a part.
The technical details of FSW expand multi-tasking beyond the number of processes a machine can perform to include the number of problems that machine can solve. FSW eliminates the porosity associated with large grain growth, converting an additive casting into a forged structure through grain refinement. The entire welding process takes place in a single pass, not through depositing multiple layers of added metal. The technique dramatically simplifies the processes of repairing welded parts and joining dissimilar metals, including insoluble materials such as nickel and iron.
Temperature range
Both low- and high-temperature forms of the FSW technique exist with the former operating below 932 degrees F and the latter between roughly 932 and 1,292 degrees F. Aluminum and aluminum alloys – important in the aerospace, marine and transportation industries – fall under the low-temperature heading while nickel, steel, stainless steel and refractory alloys belong in the high-temperature category.
Within the low-temperature segment, FSW handles difficult-to-weld materials from the 7000 series of aluminum alloys as well as brass and copper. Low-temperature FSW represents the overwhelming majority of customer applications, with only about five percent made up of high-temperature tools and processes.
Ideally, FSW suppliers cover both halves of the temperature spectrum with technologies that handle each appropriately. Mazak MegaStir developed diamond tools for its low-temperature applications and polycrystalline cubic boron nitride (PCBN) tools for high-temperature use.
PCBN, which falls just below diamond on the scale of hardest known materials, remains thermally stable at 1,652 degrees F, making it an ideal choice for high-temperature FSW. For high-temperature FSW, Mazak MegaStir also makes a modular liquid-cooled tool holder that includes a flood entry system as an enhancement.
From the innovative combination of multiple production processes on a single piece of equipment to the superior qualities of the output it creates, Mazak’s Hybrid Multi-Tasking represents the future of machining – and FSW brings that future into the present. Its superb joints and other advantages make it a worthwhile consideration for many shops.
