Friction stir welding of aluminum alloys – Part 2

Gary Kardys | October 13, 2019

Pros

Friction stir welding has many advantages compared to fusion welding. Friction stir welds have smaller heat-affected zones (HAZs) with fewer undesirable metallurgical changes. Since the FSW process is a solid-state process, there is no melting-related cracking or porosity. The lower heat input and absence of melting results in reduced distortion, less shrinkage and lower residual stress generation.

Friction stir welding can join aluminum alloys that are unweldable with fusion processes, and can join many 2xxx and 7xxx series aluminum alloys, which are unweldable by conventional fusion methods due to hot cracking. Non-weldable alloys tend have higher strength than weldable alloys due to higher alloying addition levels, which segregate on cooling and result in liquation and solidification cracking. Friction stir welding can join dissimilar alloys including different grade aluminum alloys and even steel to aluminum joints.

Figure 1. NASA is using FSW to produce high-quality, high-strength welds in critical aluminum alloys components on the Orion spacecraft and Space Launch System rocket. Source: NASA

A fusion weld bead can vary in shape along the length of a weld from undersized to oversized dimensions, especially during manual welding in difficult positions or orientations. Friction stir welding produces a clean, uniform weld joint regardless of the workpiece orientation or position. Another advantage of FSW is the ability to weld in any position or orientation, since the weld zone metal never melts. Some aluminum alloys with high fluidity might be difficult to fusion weld in certain orientations.

Friction stir welding has lower operational costs compared to fusion welding. End users quote cost savings of 10% to 60%. A major cost saving benefit of welding aluminum with FSW is the lower energy or power consumption requirements. In addition, no consumables are required for aluminum FSW.

Friction stir welds on precipitation hardened alloys typically have 20% higher tensile strength levels compared to fusion methods. The metal in FSW welds can reach tensile strengths 90% of the base metal strength. Friction stir welds have improved fatigue properties due to the higher cleanliness, fine grained structure in weld nugget and elimination of crack initiation sites (e.g., pores, cracks, defects). The lower processing temperatures (800° F) reduces or eliminates HAZ damage.

Friction stir welding is highly amenable to automation because there are few variables and components to control. Friction stir welding is much quieter than fusion welding processes and has no harmful fumes or arc-generated UV and infrared radiation. The absence of loud noises, molten metal splatter, slag, radiation, metal and slag fumes improves the overall quality of the welding workplace.

Cons

One of the major barriers to utilizing FSW is the high cost of equipment. Milling machines are sometimes adapted to perform FSW for experiments and prototyping, but milling machines are not designed to handle the loads and stress developed during FSW.

Figure 2. Example of "kissing bond" in lack of penetration defect in a friction stir weld. Source: NASA

Usually, a hole is left at the end of the weld, which is another FSW drawback. While FSW is superior to arc fusion in many respects, defects can still occur in FSW joints. Difficult to detect “kissing bonds” can occur if the pin position relative to the anvil is not tightly controlled. Less common defects found in aluminum friction stir welds are wormholes, galling and hook (i.e., surface oxide uplift) defects. These defects can be easily resolved by adjusting FSW parameters.

A welder can compensate for variation in mismatch and section thickness during arc welding. Most FSW is automated, so a welder cannot make adjustment on the fly. The joint between workpieces must have an extremely good fit or low tolerance. FSW processes for lap welding have also been developed, but hook defects can be problematic. A gap of more than 10% of the sheet or plate thickness can result in poor weld quality. FSW is typically limited to linear butt welds with sheets or plates of uniform thickness. Complex 3D welds are typically not viable.

New FSW systems are overcoming traditional disadvantages and expanding the types of welds FSW can make. For instance, friction stir spot welding (FSSW) can provide aluminum welds superior to resistance welding. NASA patented an FSW tool with a retractable pin, which permits welding of workpieces with varying or tapered cross sections, because the pin can be lengthened or shortened. Self-reacting FSW eliminates the need for an anvil, but the process still leaves a hole at the end of the weld. In self-reacting FSW, the workpieces are pinched by the FSW tool.

Friction stir processing (FSP) is when the friction tool is scanned over the surface of a casting or part to refine the microstructure through dynamic recrystallization. Additive part fabrication is another variant of the FSW process where metal is fed through a hollow friction stir tool. New FSW technology from Meld Manufacturing adds a filler alloy through the center of the FSW tool, which can be useful in additive, resurfacing and repair applications.

Conclusion

New FSW tool designs made of wear-resistant materials will increase aluminum FSW applications and productivity. Welding researchers are developing new FSW processes, which will expand their use beyond aluminum. As the technology continues to evolve and equipment costs decrease, FSW will continue to make a stir in new industries and enable more innovations in markets it has already impacted.

For more information about friction stir welding, check out AWS’s D17.3/D17.3:2016, Specification for Friction Stir Welding of Aluminum Alloys for Aerospace Applications, and AWS’s Welding Handbook, Chapter 7, “Friction Stir Welding,” Ninth Edition, Volume 3, “Welding Processes Part 2.”