Friction stir welding (FSW) is a relatively new solid-state joining process. This joining technique is energy efficient, environment friendly, and versatile. In particular, it can be used to join high-strength aerospace aluminum alloys and other metallic alloys that are hard to weld by conventional fusion welding. FSW is considered to be the most significant development in metal joining in a decade. Recently, friction stir processing (FSP) was developed for microstructural modification of metallic materials. In this review article, the current state of understanding and development of the FSW and FSP are addressed. Particular emphasis has been given to: (a) mechanisms responsible for the formation of welds and microstructural refinement, and (b) effects of FSW/FSP parameters on resultant microstructure and final mechanical properties. While the bulk of the information is related to aluminum alloys, important results are now available for other metals and alloys. At this stage, the technology diffusion has significantly outpaced the fundamental understanding of microstructural evolution and microstructure–property relationships.

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Friction Stir Welding and Processing

Recent advances in friction-stir welding : Process, weldment structure and properties

Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered essentially unweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while introducing extensive localized deformation. Studies were performed on these solid-state welds to determine mechanical properties both in the longitudinal direction, i.e., within the weld nugget, and, more conventionally, transverse to the weld direction. Because of the unique weld procedure, a fully recrystallized fine grain weld nugget was developed. In addition, proximate to the nugget, both a thermomechanically affected zone (TMAZ) and heat affected zone (HAZ) were created. During welding, temperatures remained below the melting point and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a banded microstructure that influences room-temperature fracture behavior was created. In the as-welded condition, weld nugget strength decreased, while ductility remained high. A low-temperature aging treatment failed to fully restore T651 strength and significantly reduced tensile ductility. Samples tested transverse to the weld direction failed in the HAZ, where coarsened precipitates caused localized softening. Subsequent low-temperature aging further reduced average strain to failure without affecting strength. Although reductions in strength and ductility were observed, in comparison to other weld processes, FSW offers considerable potential for welding 7075 T651 aluminum.

Properties of friction-stir-welded 7075 T651 aluminum

Effects of friction stir welding on microstructure of 7075 aluminum

The microstructural distribution associated with a hardness profile in a friction-stir-welded, age-hardenable 6063 aluminum alloy has been characterized by transmission electron microscopy (TEM) and orientation imaging microscopy (OIM). The friction-stir process produces a softened region in the 6063 Al weld. Frictional heating and plastic flow during friction-stir welding create fine recrystallized grains in the weld zone and recovered grains in the thermomechanically affected zone. The hardness profile depends greatly on the precipitate distribution and only slightly on the grain size. The softened region is characterized by dissolution and growth of the precipitates during the welding. Simulated weld thermal cycles with different peak temperatures have shown that the precipitates are dissolved at temperatures higher than 675 K and that the density of the strengthening precipitate was reduced by thermal cycles lower than 675 K. A comparison between the thermal cycles and isothermal aging has suggested precipitation sequences in the softened region during friction-stir welding.

W. M Thomas

Papers

Friction stir process welds aluminium alloys : The process produces low-distortion, high-quality, low-cost welds on aluminium