The welding industry is increasingly interested in applying the solid-state welding benefits of friction stir welding (FSW) to thick-section steel. Recent advancements in tungsten-based FSW tools have resulted in full-penetration welds in steel up to 19 mm (0.75 in.) thick. At increased thicknesses, friction stir welds generate higher forces and have a lower cooling rate due to higher heat input. Considerations must be made in terms of weld thermal management, tool material, tool design, and parameter selection. The ability to produce a fully consolidated weld in thick-section steel is a careful balance of heat input, tool design, and weld parameters.
Over the course of the last 20 years, friction stir welding (FSW) has been established as a fundamental joining method for a wide variety of soft metals, aluminum alloys in particular. Since its invention in 1991 by TWI (Thomas, Nicholas, Needham, Murch, Templesmith, Dawes), it has become a well-established welding process for all grades of aluminum. Due to the hardness differences between the friction stir tool and the material being joined, the development process has been wide and successful. These development efforts have demonstrated significant benefits to using FSW over arc welding in aluminum applications. With FSW, thicknesses in aluminum of up to 76 mm (3 in.) have been joined in a single pass (Colligan, 2007). In addition, the solid-state nature of this process allows for the joining of aluminum alloys which are challenging to join with arc welding, such as 2195, to be joined with ease. The low heat input of the FSW process results in low distortion and excellent weld mechanical properties (Konkol and Mruczek, 2007). With benefits like these, the FSW of aluminum alloys has been adopted by several industries including the defense, aerospace, railroad, and shipping industries.