The research team, working across French institutions, built a coupled thermo-metallurgical-mechanical finite element model of tungsten inert gas (TIG) welding on S355J2N steel plates. What sets the work apart is its systematic decomposition of transformation-induced plasticity, or TRIP, the extra plastic strain that arises when steel changes its crystal structure while under stress. Previous simulations often ignored phase transformations entirely or included them only partially, treating the associated volumetric strains while neglecting TRIP altogether. The new study shows that this omission can inflate predicted residual stresses dramatically, potentially leading designers to underestimate the fatigue performance of real welded assemblies.

The experimental foundation of the study was meticulous. Six identical S355J2N specimens, each consisting of two plates measuring 100 by 50 by 15 millimeters joined in a butt configuration, were welded under identical conditions: 200 amperes of current, 14 volts of arc voltage, a travel speed of 2 millimeters per second, pure argon shielding, and no filler metal. Type-K thermocouples recorded the thermal cycles at 50 hertz, while optical microscopy and scanning electron microscopy mapped the microstructures of the fusion zone and the heat-affected zone. Three specimens had to be excluded from the thermal analysis because their thermocouples shifted during welding, a candid limitation the authors acknowledge, but the three retained welds showed cooling-time repeatability within one second.

On the modeling side, the arc’s heat input was represented by the Goldak double-ellipsoid model, whose parameters were calibrated from the measured fusion zone geometry: a half-width of 6 millimeters and a half-depth of 3 millimeters. Diffusive transformations among austenite, ferrite, pearlite, and bainite were described with the V. I. Machnienko model, while the diffusionless martensitic transformation followed the Koistinen-Marburger relation. The mechanical response used a temperature-dependent elastic-plastic von Mises formulation with isotropic hardening, implemented in Abaqus through custom subroutines handling the moving heat source, phase evolution, and stress integration. The mesh contained 67,650 elements, refined down to half a millimeter near the weld.

The thermal validation was convincing. Predicted peak temperatures matched the six thermocouple measurements with errors below 7 percent, and the cooling stage was reproduced with good accuracy, although the cooling time between 800 and 500 degrees Celsius was overestimated by roughly 12 to 14 percent at the two hottest measurement points. The metallurgical predictions fared well too: the model identified bainite as the dominant phase in both the fusion zone, at about 44 percent, and the heat-affected zone, at about 32 percent, exactly as the electron microscope images confirmed. Martensite remained a minor constituent, and the predicted hardness of roughly 240 Vickers in the heat-affected zone sits comfortably below the 380 HV10 limit recommended by the relevant ISO standard.

The heart of the paper lies in its comparison of seven mechanical modeling configurations. Approach A2, which neglects transformation strains entirely, predicted peak longitudinal residual stresses of 632 megapascals on the surface and 760 megapascals through the thickness, values approaching the steel’s yield strength. Adding only the volumetric strain of phase transformations, approach A1, trimmed those peaks by 21 to 35 percent. But the complete model, A4, which adds TRIP, cut the through-thickness peak to 473 megapascals and the region-averaged longitudinal stress from 641 to 280 megapascals, a reduction of more than half. Volumetric strain alone, in other words, tells only a fraction of the story.

Even more striking was the decomposition of TRIP itself into three distinct contributions: classical plasticity driven by stress variations, plasticity driven by temperature variations, and plasticity driven by the rate of change of phase fractions. When each term was isolated, the phase-fraction term emerged as the overwhelming driver of stress relaxation. On the surface path, it reduced the mean longitudinal stress by 51 percent, nearly matching the 53 percent achieved by the complete model, while the stress-only term managed just 9 percent. This finding is qualitatively consistent with the theoretical estimate of Leblond and colleagues, who calculated that plasticity from phase fraction variations could be roughly 25 times greater than that from temperature variations during martensitic transformation.

The study also quantified a subtler effect: the evolution of the effective yield stress during cooling, computed as a phase-weighted average of the individual phase yield stresses. Keeping the yield stress identical during heating and cooling, as many earlier simulations did, raised the predicted peak stress by about 8 to 24 percent compared with accounting for its evolution. Sensitivity analyses reinforced the robustness of the conclusions: plausible variations in high-temperature thermal properties, mesh refinement, and high-temperature mechanical assumptions shifted the mean longitudinal stress by at most a few percent, whereas the TRIP activation threshold proved to be the single most influential parameter, changing predicted stresses by up to 32 percent.

Transverse residual stresses, by contrast, remained much lower than the longitudinal ones, peaking at 178 megapascals on the surface and 384 megapascals through the thickness for the complete model. The authors attribute this asymmetry to the greater plastic deformation the plate undergoes in the transverse direction and to the narrow width of the weld bead relative to its length. The longitudinal dominance aligns with trends reported in earlier studies of arc-welded joints, including work showing that tensile residual stresses in tubular K-joints can drive fatigue cracks to half the chord thickness even under nominally compressive loading.

The practical implications are significant for anyone designing or fabricating welded steel structures. Models that ignore TRIP may substantially overestimate residual stresses and thereby underestimate fatigue performance, potentially triggering unnecessary post-weld treatments or overly conservative designs. Conversely, the finding that the phase-fraction term governs relaxation gives modelers a clear target: capturing transformation kinetics accurately matters more than refining thermal parameters. The authors caution that direct experimental validation of the residual stresses is still pending, with X-ray diffraction measurements on reserved specimens planned for a forthcoming study, along with Vickers microhardness validation and a dedicated investigation of the contact conductance between plate and welding table. Until those results arrive, this work stands as a compelling demonstration that the invisible drama of phase transformations inside cooling steel is not a modeling nicety but the decisive factor in predicting the stresses that determine whether welded structures endure.

Subject of Research: Thermo-mechanical modeling of residual stresses and transformation-induced plasticity in TIG-welded S355 structural steel joints

Article Title: Thermo-mechanical modeling of residual stresses in TIG-welded S355 steel joints

Article References: Niangaly, I., Abdennadher, M., Zouari, S., Leprêtre, E., & Dieng, L. (2026). Thermo-mechanical modeling of residual stresses in TIG-welded S355 steel joints. Results in Engineering, 32, Article 113233. https://doi.org/10.1016/j.rineng.2026.113233

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113233

Keywords: residual stress, TIG welding, S355 steel, phase transformations, transformation-induced plasticity, TRIP, finite element modeling, bainite, martensite, fatigue, heat-affected zone, structural steel

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