Why Pin tool design is critical in Friction Stir Welding

Because it avoids many of the common problems of standard fusion welding, Friction Stir Welding (FSW) is a widely used solid state joining process in a growing number of industries. Although the outcome of FSW is similar to fusion welding, the strength and precision of the weld, the process and machinery are very different.

Friction stir welding allows aluminum, copper and even tough steel alloys to be permanently joined without arc welding. This operation is done on equipment that often resembles standard milling machines.

These FSW machines use a cylindrical tool with a profiled threaded probe, called a pin tool, which is rotated at constant speed and then plunged into the joint between two pieces of metal. Friction heat created between the welding tool shoulder, pin and metals being joined causes the metals to soften to a point just below their melting point. Essentially, the metals soften to a plastic-like state but do not melt. In this plasticized state, the two metals are literally stirred together – creating a solid-state bond of the now mixed material. The entire process allows for minimal distortion of joined parts, and an extremely high weld strength.

Why is Pin tool design critical?

When we think about FSW, and the tools needed for this type of weld – the importance of the pin tool stands out. After all, it is the only consumable component in the FSW process.

The design that goes into a pin tool becomes critical to the success Friction Stir Welding. The FSW tool is subjected to severe stress and high temperatures. This is particularly true for the welding of hard alloys like steels and titanium alloys. (Currently the commercial application of FSW to these alloys is now limited by the high cost and short life of FSW tools.)  For every application, tool material selection, geometry and load bearing ability, mechanisms of tool degradation and process economics are crucial. (Read more about this here.)

 How are pin tools designed and tested for durability and quality?

The design and manufacturing of pin tools involves exacting tolerances and attention to detail. The features of pin tools needed to get desired flow characteristics are often quite complex and specific to the intended purpose. Designers must have a strong knowledge of manufacturing and inspection methods to ensure the design is achievable at a reasonable cost.

For example, a threaded tri-flute pin requires complex machining operations that are only possible on multi-axis machining centers. Since forces produced by FSW are quite high, pin tools must be designed to survive their intended use under the expected process conditions. As the only consumable component in the FSW process, eventually, every in tool will fail.

It is strongly preferred that the pin tools need replaced because of anticipated wear — rather than through catastrophic breakage. By designing pin tools to fail through wear, their lifetime can be predicted, and the tool changed out before it causes the process to go awry.

 How do you match the correct pin tool for the application?

For each application, there are many pin tool designs that can produce an acceptable weld, but certain designs will provide larger process windows than others. Some pin tools are best for lap weld joints, others work best for butt welds. For production applications, it is generally required to define the process window and establish acceptable limits for process parameters. Before performing a detailed Design of Experiment (DOE), Bond recommends a screening DOE to determine the optimum pin tool. Attempting to define the entire process capability of various pin tools can lead to excessive cost and delay. Once a preferred pin tool is selected, it is valuable to find the fastest reasonable feed rate possible for a given material thickness, alloy and geometry.

Quick Facts about Pin tools by Bond Technologies:

  1. Conventional pin tools are available in one- or two- piece configurations
  2. Retractable pin tools are available for machines equipped to accommodate these tools
  3. Fixed gap bobbin tools are also available
  4. Pin tools for welding steel, titanium and copper are available
  5. Pin tools that include thermocouples are available for those with the BondStir family of products
  6. Custom pin tools are available but require a small design fee. Prices come down quickly as quantities increase
  7. Bond pin tools are all serialized to provide full traceability

 How easy is it to reorder Pin tools when needed?

For customers with Bond friction stir welding machines, ordering pin tools is as simple as an email or phone call. We will ask some questions about the alloy being welded, thickness, joint geometry, etc. In addition to individual pin tools, Bond offers kits that include tools covering a range of sizes. Contact Bond Technologies to learn more about their FSW Machines and pin tool options.

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