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

A review of dynamic recrystallization phenomena in metallic materials

A review on selective laser sintering/melting (SLS/SLM) of aluminium alloy powders: Processing, microstructure, and properties

http://nanomechanics.mit.edu/sites/default/files/documents/2007_Acta_nc_Overview.143.pdf

Toward a quantitative understanding of mechanical behavior of nanocrystalline metals

Microstructural investigation of friction stir welded 7050-T651 aluminium

Microstructural characteristics of friction stir processed (FSP) 7075 Al samples in different regions behind the pin tool were investigated. Furthermore, the microstructure evolutions during friction stir welding (FSW)/FSP has been revealed. Multi-mechanisms, including dDRX, grain growth, dislocation introduction, dynamic recovery (DRV) and cDRX, were found to be inherent in the process at different stages. Because inhomogeneous plastic deformation was introduced by the process, individual grains in the final microstructure have undergone different evolution mechanisms. These are either from growth of initially recrystallized grains or are the result of cDRX of subgrains formed during DRV. Grains within the processed region exhibit different densities of dislocations and are in various degrees of recovery.

Microstructure evolution during FSW/FSP of high strength aluminum alloys

The microstructure and mechanical properties of friction stir (FS) welded SAF 2507 super duplex stainless steel were examined. High-quality, full-penetration welds were successfully produced in the super duplex stainless steel by friction stir welding (FSW) using polycrystalline cubic boron nitride (PCBN) tool. The base material had a microstructure consisting of the ferrite matrix with austenite islands, but FSW refined grains of the ferrite and austenite phases in the stir zone through dynamic recrystallisation. Ferrite content was held between 50 and 60% throughout the weld. The smaller grain sizes of the ferrite and austenite phases caused increase in hardness and strength within the stir zone. Welded transverse tensile specimen failed near the border between the stir zone and TMAZ at the retreating side as the weld had roughly the same strengths as the base material.

Microstructure and mechanical properties of friction stir welded SAF 2507 super duplex stainless steel

Friction stir processing of large-area bulk UFG aluminum alloys

The accurate 3D finite element simulation of the Friction Stir Welding (FSW) process requires a proper knowledge of both material and interface behaviors, but friction, the key phenomenon of this process, is quite difficult to model and identify. According to the extreme encountered conditions and the highly coupled nature of the material flow, simple tribological tests are not representative enough, so the welding process itself has been utilized in most analyses of the literature, although its complexity has led to use simplified numerical models and approaches. The recent development of more accurate 3D simulation software, which allows modeling the entire complexity of the FSW process, makes it possible to follow a much more rigorous inverse analysis (or calibration) approach. FSW trials are conducted on an Al 6061 aluminum plate with an unthreaded concave tool. Forces and tool temperatures are accurately recorded at steady welding state, for different welding speeds. The numerical simulations are based on an Arbitrary Lagrangian Eulerian (ALE) formulation that has been implemented in the Forge3® F.E. software. The main feature of the numerical approach is to accurately compute the contact and frictional surface between the plate and the tool. A first study using Norton's friction model show the great sensitivity of welding forces and tool temperatures to friction coefficients, the need to take into account the changes brought to the contact surface by slight friction variations (thanks to the ALE formulation), the possibility to get very accurate calibrations on forces, and the impossibility to properly render the tool temperature profile. On the other hand, the use of Coulomb's friction model allows obtaining realistic temperature profiles and so calibrating a friction coefficient that offers an excellent agreement with experiments, on forces as much as on tool temperatures, for various welding speeds.

Tracy W. Nelson

Papers

Microstructural investigation of friction stir welded 7050-T651 aluminium

Microstructure evolution during FSW/FSP of high strength aluminum alloys

Microstructure and mechanical properties of friction stir welded SAF 2507 super duplex stainless steel

Friction stir processing of large-area bulk UFG aluminum alloys

Friction model for friction stir welding process simulation: Calibrations from welding experiments