The trouble with fusion welding aluminium begins with the metal’s own physics. Aluminium conducts heat rapidly, melts at a relatively low temperature, and reacts eagerly with oxygen, all of which conspire against conventional arc and laser welding. When a molten weld pool solidifies, it can crack under solidification shrinkage, trap hydrogen-driven porosity, fail to fuse completely with the base metal, undercut at the weld toe, burn through thin sections, or entangle slag and oxide fragments. These macroscopic defects reduce the effective cross-section of a joint and act as stress concentrators that invite crack initiation under cyclic loading. Beneath the visible damage, the microstructure suffers too: strengthening precipitates such as the beta-double-prime and beta-prime phases in 6xxx-series alloys, or Al2Cu in 2xxx alloys, dissolve or coarsen during the thermal cycle, alloying elements segregate to grain boundaries, and the heat-affected zone widens as grains coarsen, sapping strength according to the well-known Hall–Petch relationship.

The review documents how these problems vary by alloy chemistry. AA6061 proves highly susceptible to cracking under constrained strain conditions, while AA2219 resists cracking better thanks to a finer eutectic network that feeds liquid to grain boundaries during solidification. In Al–Mg–Si alloys, magnesium and silicon segregate to grain boundaries, and in some welds zinc segregates even more prominently, rendering joints vulnerable to intergranular stress-corrosion cracking and solidification cracking. Coarse Al–Fe–Si intermetallic particles in AA6061 fusion welds help nucleate cracks by decohering from the matrix. Micro-porosity forms during the liquid-to-solid transformation as local pressure drops from solidification contraction outpace the supply of liquid metal. Engineers have long fought back by optimising welding parameters, but the review’s central message is that optimisation alone cannot erase defects that are baked into the physics of melting.

Enter friction stir processing, a technique derived from friction stir welding, which The Welding Institute invented in the early 1990s. Where fusion welding melts metal, FSP never lets the workpiece breach its solidus. A hardened, non-consumable rotating tool with a shoulder and pin plunges into the material and traverses along a planned path. Friction at the shoulder-workpiece interface, which the review notes supplies roughly seventy percent of the total heat, and plastic dissipation around the pin soften the metal without melting it. The process unfolds in four stages: plunging, dwelling, travelling and retracting. The intense, simultaneous heat and severe plastic deformation drive dynamic recrystallization, replacing coarse dendritic fusion-zone structures with fine, equiaxed grains that can measure only a few micrometres or even sub-micrometres when parameters are balanced correctly.

Because the material never melts, FSP sidesteps the entire class of solidification defects. The review shows that it eliminates or substantially reduces porosity, hot tears, solidification cracks, shrinkage cavities and micro-segregation, redistributes strengthening precipitates, reduces residual tensile stresses and narrows the effective heat-affected zone. The processed region divides into characteristic zones: a stir zone of dynamically recrystallised fine grains, a thermomechanically affected zone with partial recrystallisation, a heat-affected zone subject only to thermal exposure, and the untouched base metal. Achieving this outcome depends on a delicate three-way balance between rotational speed, traverse speed and tool geometry. Moderate rotational speeds paired with intermediate traverse speeds consistently yield refined grains and uniform particle distribution, while excessive speeds overheat the material and trigger post-weld annealing that coarsens grains and erodes strength.

The evidence compiled in the review is striking. FSP applied to TIG-welded AA2024 eliminated solidification cracks and porosity while producing ultrafine, equiaxed microstructures with substantially improved tensile strength and hardness. On TIG weldments of AA7020, hardness and strength rose 118.5 percent over the untreated weld and 103 percent over the base alloy itself. TIG-welded AA6061-T6 gained 47 percent in tensile strength and 10 percent in microhardness after FSP. In AA7075 TIG joints, tensile strength climbed from 228 to 320 megapascals, accompanied by a dramatic rise in coincidence site lattice boundaries and high-angle grain boundaries that confirmed dynamic recrystallization at work. FSP on AA5052 TIG welds produced three-micrometre grains with 52 percent high-angle boundaries, delivering gains of 50 percent in hardness, 42 percent in yield strength and 25 percent in ultimate tensile strength.

Multipass processing amplifies these benefits. In dissimilar AA8011-H14 and AA5083-H321 TIG joints, the dendritic fusion-zone morphology vanished after a single pass, and by three passes the stir zone held ultrafine grains averaging 3.42 micrometres, with ultimate tensile strength rising from 79.82 megapascals in the unprocessed weld to 126.92 megapascals. Researchers have even used FSP to embed reinforcing particles directly into weld zones: adding alumina particles to TIG-welded AA5083 boosted yield strength by 29 percent, ultimate tensile strength by 18 percent and impact energy by a remarkable 56 percent, effectively converting a repaired weld into an in-situ metal-matrix composite. Scandium-modified filler combined with FSP on marine-grade AA5083 reduced grain size to two micrometres and pushed strength to 331 megapascals with 20.5 percent elongation, surpassing the base metal.

The technique performs equally well on MIG and cold metal transfer welds. FSP of AA6082-T651 T-fillet MIG welds cut grain size by 96 percent, improved average hardness by 37 percent and yield strength by 9 percent, and extended corrosion-fatigue life to roughly eight hundred thousand cycles thanks to the elimination of porosity. On dissimilar AA5083-H321/AA5356 arc welds, FSP erased coarse constituent phases and raised ultimate tensile strength from 259 to 306 megapascals. Compared directly against post-weld heat treatment on AA6082-T6 MIG joints, FSP achieved 99 percent joint efficiency versus 85 percent for heat treatment, with a 305 megapascal tensile strength and 23 to 24 percent hardness gains, though the thermomechanical route sacrificed about 7 percent elongation, a trade-off the authors flag as a matter of tailoring process choice to service requirements. Corrosion resistance improves as well; FSP of AA2219-T6 TIG welds cut corrosion rates from 25 to 4 mils per year by eliminating copper-depleted regions and refining AlCu2 eutectic particles.

The review concludes that friction stir processing has matured into a highly efficient, sustainable post-weld treatment, generating no harmful gases, radiation or excessive noise and consuming far less energy than conventional routes. Optimal parameters for TIG-weld repairs cluster between 600 and 1300 RPM rotational speed and 30 to 100 millimetres per minute traverse speed, with tool shoulder diameter, pin profile, the shoulder-to-pin diameter ratio and axial forging force all playing decisive roles. The authors chart future opportunities that read like a roadmap for the next decade: coupling FSP with ultrasonic vibration or electromagnetic assistance, designing novel scroll and triflute tool geometries, deploying real-time monitoring with force, torque and acoustic emission sensors, applying machine-learning-based optimisation across long weld seams, and embedding ceramic or nano-reinforcements such as silicon carbide, alumina, TiB2 and graphene to create composite weld regions. For an industry racing to build lighter, stronger, greener structures, the message is clear: the humble rotating tool may be the weld doctor aluminium has been waiting for.

Subject of Research: Friction stir processing as a post-weld treatment for improving fusion-welded aluminium alloy joints

Article Title: Advancement in fusion welded aluminium alloy joints by friction stir processing technique: a critical review

Article References: Reyaz, M. S. B., Imam, M., Anshari, M. A. A., & Shah, M. S. (2026). Advancement in fusion welded aluminium alloy joints by friction stir processing technique: a critical review. Discover Industrial Chemistry and Materials, 1(1), Article 5. https://doi.org/10.1007/s44508-026-00005-y

Image Credits: AI Generated

DOI: 10.1007/s44508-026-00005-y

Keywords: friction stir processing, aluminium alloys, fusion welding, TIG welding, MIG welding, dynamic recrystallization, post-weld treatment, grain refinement, solidification cracking, porosity, heat-affected zone, mechanical properties