• Ni-1 intermediate buffer layer effectively restricted carbon migration, , which led to resistance against the formation of CDZ and CEZ • Ni-1 buffer with PWHT enhanced P91 HAZ impact toughness by 14%. • GTAW with Ni-1/IN625 provided enhanced P91–304 L weld performance. • The Ni-1 buffer resulted in homogeneous delta ferrite distribution. • Highest hardness was in CGHAZ and lowest in ICHAZ across all joints. The pursuit of efficiency in nuclear power plant components operating at high steam temperatures and pressures has driven the use of advanced alloys like P91 steel and 304 L stainless steel, yet the structural integrity of their dissimilar welds remains challenged by carbon migration and premature failure; prompting this study to investigate the effectiveness of a Ni-1 (∼92% Ni) intermediate buffer layer combined with ERNiCrMo-3 (IN625) filler metal via multi-pass Gas Tungsten Arc Welding to enhance their metallurgical and mechanical performance. The study compared three IN625 buttering applications: as-welded, post-weld heat-treated, and post-weld heat-treated with an intermediate Ni-1 buffer layer. Microstructural analysis, employing optical microscopy and scanning electron microscopy, revealed peninsula features and an unmixed zone at the interface. The fusion zone features included Mo, Nb, and Ti-rich precipitates. The P91 heat-affected zone demonstrated a soft delta ferrite zone. Mechanical testing showed tensile strengths of 674 MPa (as-welded), 653 MPa (post-weld heat-treated), and 633 MPa (post-weld heat-treated with buffering). Notably, the Ni-1 buffer with post-weld heat treatment resulted in a 14% improvement in P91 heat-affected zone impact toughness compared to the as-welded joint. Microhardness profiling revealed the highest hardness in the coarse-grain heat-affected zone and the lowest in the intercritical heat-affected zone. The study concludes that the Ni-1 buffer layer successfully suppresses carbon migration, preventing the formation of detrimental carbon-depleted and enriched zones. By mitigating metallurgical degradation and improving toughness, the Ni-1/IN625 dual-filler approach provides a robust solution for the structural reliability of P91-304 L SS joints in demanding nuclear applications.

Experimental investigation of short fatigue crack propagation behavior within and across heat-affected zone in WAAM Ti-6Al-4V

Effect of t8/5 cooling time on microstructure and mechanical properties of simulated coarse-grained heat-affected zone in marine cryogenic high-manganese steel

This study investigates the performance of Flux-Bounded Tungsten Inert Gas (FB-TIG) welding using TiO2 flux for butt welding of SS304L stainless steel plates. The primary objective was to evaluate weld penetration, bead geometry, microstructural evolution, and tensile properties of the welded joints. Experimental trials established optimised parameters at 170 A welding current, producing full penetration without undercut formation. Macrostructural analysis revealed deep, narrow fusion zones with high depth-to-width ratios, while microstructural examination confirmed refined dendritic growth, stable δ-ferrite distribution, and a narrow heat-affected zone. The fusion line exhibited clean epitaxial solidification and defect-free bonding. Mechanical testing demonstrated that FB-TIG welded joints retained over 93% of the base metal’s ultimate tensile strength and more than 50% ductility, validating the robustness of the process. These findings highlight FB-TIG welding with TiO2 flux as a reliable, industrially viable technique for producing high-quality stainless-steel joints in critical applications such as pressure vessels, chemical processing, and energy systems.

Microstructural and thermal characterization of laser-welded Grade 91 steel joints of dissimilar thicknesses

This study characterizes CuCrZr/304 stainless steel dissimilar welds using HR-TEM and EDS. A 1.5–2.0 μm interfacial diffusion layer was identified, driven primarily by Cu migration. Analysis confirmed the formation of brittle Fe-Cu intermetallic compounds (FeCu 3 , Fe 2 Cu 5 ) and significant sulfur segregation (FeS, Cu 2 S), leading to grain boundary embrittlement and hot cracking. Furthermore, high-density dislocation networks and twin boundaries indicated strain-induced martensitic transformation in the heat-affected zone. Based on these nanoscale insights, employing diffusion barriers and post-weld heat treatment is proposed to mitigate defects and enhance joint stability.

This study presents a systematic methodology and multi-criteria optimisation of dissimilar welding between Al7075 aluminium alloy and polypropylene (PP) using a full-factorial design. It addresses the challenge of achieving reliable metal–polymer joints in lightweight structural applications, where mismatched thermal and mechanical properties limit conventional techniques. Four process parameters: pulse energy, repetition rate, welding speed, and defocus distance, were varied to determine their effects on shear strength, weld width, and heat-affected zone (HAZ). The approach integrates statistical analysis of main and interaction effects, response surface modelling, and Derringer–Suich desirability-based optimisation. The optimal parameter window (20–23 μJ, 500–700 kHz, 10–12 mm/s, 0 to +0.1 mm) yielded predicted values of 24.3 MPa shear strength, 149 μm HAZ, and 51 μm weld width. Model predictions were validated through replicated experiments, TOST equivalence testing (±10%), lack-of-fit assessment, and high process capability (Cpk > 1). SEM and EDS analyses confirmed a defect-free, continuous bonding zone. The novelty lies in combining full-factorial experimentation with statistically robust multi-objective optimisation, thereby demonstrating reproducible, industrially viable femtosecond laser welding. Future work will extend this framework to complex geometries, in-line monitoring, and broader hybrid material systems.

Repurposing the existing natural gas pipeline infrastructure for hydrogen applications poses safety concerns regarding hydrogen embrittlement. Pipeline weldments are particularly considered as vulnerable sites for hydrogen-induced degradation effects. To this end, this study evaluates the micromechanical interaction of hydrogen with the base metal (BM) and heat affected zone (HAZ) of API 5 L X65 pipeline steel. In-situ microcantilever bending complemented with high-resolution post-mortem characterisation was employed to reveal the hydrogen-assisted degradation of the BM and HAZ microstructure. As a second novel variable, the role of the surface condition in hydrogen environment was highlighted by comparing a polished reference surface with one containing a non-protective oxide layer. This unique approach reveals that hydrogen facilitates localised deformation by enhancing dislocation nucleation and reducing dislocation mobility. The polished HAZ exhibits a marginally better resistance to hydrogen embrittlement than the BM, attributed to the microstructural features. However, the oxide layer interacts synergistically with hydrogen, significantly deteriorating the mechanical integrity. Thus, the complete state of the pipeline determines the degree of hydrogen-assisted degradation, including both the microstructure and the surface condition. • Hydrogen embrittlement evaluation via in-situ cantilever bending of pipeline welds • Hydrogen-dislocation interactions drive localised deformation and degradation • The microstructure determines the hydrogen degradation of the BM and HAZ • Synergistic deterioration effects between hydrogen and a corrosion surface layer

Fracture behavior of spiral welded API X70 pipeline steels under monotonic and cyclic loading

Mechanical field control in powder bed fusion is critical in mitigating distortion and cracking by reducing residual stresses, and enhancing mechanical properties by regulating dislocation structures. However, achieving these while preserving optimal melt region dimensions for desired build quality and microstructures remains challenging due to the inherent positive correlations among input energy, melt region and heat-affected zone (HAZ) dimensions, and residual stresses and strains. This study establishes a framework for effectively manipulating residual stresses and strains in melt region and HAZ, on the premise of preserving melt depth with error margins <8%. Through thermomechanical analyses and experimental validations, we investigate the effects of melting strategies and heat accumulation on residual stress–strain constitutive behaviors. Although conventional strategies such as shortening beam path length or spot melting are commonly employed to reduce residual stresses, we reveal their limited effectiveness under typical conditions where heat accumulates. In contrast, we propose the concept of equivalent infinite cooling time interval, based on which the real-spot (RS) melting strategy can significantly reduce macroscopic residual stresses. The underlying mechanisms are elucidated by revealing the dual effect of heat accumulation on residual stresses and strains, which induces a trade-off between their peak levels and total affected areas. Moreover, we demonstrate that the RS melting strategy alleviates continuous compressive plastic deformation in melt region and HAZ by reshaping principal plastic strain orientations and alternating compressive and tensile plastic strain components. This enables reducing residual stresses while increasing cumulative plastic strains, thereby overcoming the limitations in mechanical field control imposed by the positive correlation between residual stresses and strains.

Microstructure evolution of dissimilar AISI 304L and AISI 410S stainless steel joints by the friction stir welding

Influence of plunge depth on microstructural evolution and mechanical performance of Al/steel friction stir riveted joints

Strengthening mechanisms and mechanical properties of dual-stabilized ferritic stainless-steel joints processed by high-speed laser welding

Achieving defect-free repair in ZM6 magnesium alloy through cold metal transfer welding combined with preheating

Microstructural gradient architecture and heterogeneous strengthening mechanism in ultra-narrow gap MAG welded 27SiMn steel joints

ABSTRACT Linear friction weld (LFW) between EN8 medium carbon steel and AISI304 austenitic stainless steels was successfully made. EN8 was clamped on the stationary side, and AISI304 was clamped on oscillating side. Welds were fabricated at a frequency, friction pressure, forging pressure and weld time of 22 Hz, 225 MPa, 485 MPa and 20 s, respectively. Microstructural observation revealed the presence of characteristic microstructural zones, including the weld centre zone (WCZ), thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ). The presence of EN8 grains forming a thin layer on AISI304 side of the WCZ indicated adequate intermixing. Microhardness was traced in the middle and at the ends of welded joint, and it was found to be greater in the middle than at the ends. Maximum hardness was observed on AISI304 side of joint interface in the middle region. The presence of stress-induced martensite was observed in the AISI304 TMAZ and HAZ. Tensile strength of 604.5 MPa with a 78% weld joint efficiency was obtained. The AISI304 and EN8 dissimilar welding has enormous potential in automobile, rapid transport systems and can bring about significant cost and weight savings, and design flexibility because of composites structures made from stainless steel medium carbon steel dissimilar joint.

This study investigates the crack propagation paths and interaction mechanisms of dual cracks in dissimilar metal welded joints (DMWJs) under material heterogeneity. A phase-field modeling framework was first developed in Abaqus to systematically analyze crack evolution and damage progression across the continuously graded interfacial region. To overcome the limitation of conventional multilayer discontinuous models in capturing the realistic continuous gradient of welded joints, a phase-field scheme coupling user-defined material subroutines and user-defined elements was established. The spatially continuous distribution of material properties was achieved by inversely identifying the yield strength profiles of the base metal, heat-affected zone, and fusion zone from microhardness experiments. By varying the crack spacing and crack length, the competition in crack initiation, path deflection, and coalescence behavior of dual cracks in the SA508-52Mb and 316L-52Mw regions was quantitatively analyzed. The results reveal that mechanical heterogeneity drives cracks to propagate preferentially toward the lower yield strength side and significantly governs the mutual attraction between cracks. Dual cracks in the 316L heat-affected zone tend to coalesce as they approach each other; in SA508-52Mb, short cracks exhibit pronounced deflection whereas longer cracks are inhibited. In 316L-52Mw, coalescence is observed only when the cracks have comparable lengths. This work elucidates the propagation mechanisms of dual cracks in DMWJs and provides theoretical insight for structural integrity assessment of nuclear reactor safe end components.

The model of thermo-fluid-solid coupling for Mg-3Al-Zn alloy was established based on the Coupled Eulerian–Lagrangian (CEL) method, which enabled the acquisition of velocity vector-based material flow patterns. This model revealed distinct material flow characteristics during the fabrication of single-sided/double-sided friction stir welding (SS/DS-FSW) joints. DS-FSW joints were experimentally prepared, with their cross-sectional elemental distribution analyzed using Energy Dispersive Spectroscopy (EDS). The friction coefficient, wear depth and wear rate of weld surface were studied in detail through friction and wear test. Results demonstrate that DS-FSW exhibits superior uniformity in material flow and temperature distribution. The material not only flows circumferentially around the stirring pin, but also flows alternately between layers with different thicknesses. The temperature distribution across the weld cross-section presents a ‘hourglass-shaped’ profile. At a welding speed of 180 mm/min, DS-FSW joints fabricated at 600 rpm and 700 rpm exhibited distinct Al–Mn precipitates. Notably, Si element aggregation was observed in the 600 rpm joint; the joint strength is reduced by 6.92% compared with 700 rpm. Conversely, the presence of Al–Mn phases enhanced wear resistance, with the wear rates in both nugget zone and heat-affected zone of the 600 rpm joint reduced to 1.15 × 10−3mm³/(N·m) and 1.13 × 10−3mm³/(N·m) respectively, compared with the 700 rpm counterpart.

This paper investigated the development of a novel graphene nanoplatelets reinforced IN182 composite electrode designed to address the microstructural and mechanical challenges associated with welding 9Cr-1Mo-V (P91) martensitic steel. The sensitivity of P91 steel to thermal cycles during multi-pass shielded metal arc welding often led to grain coarsening and reduced creep-rupture strength. To address this, graphene nanoplatelets were combined with ferrosilicon via high-energy ball milling to ensure uniform distribution of the nanoplatelets and then integrated into the electrode basic flux. This experimental electrode was then compared to standard dissimilar IN182 and matching E9015-B9 electrodes. Microstructural analysis using a scanning electron microscope and energy-dispersive spectroscopy confirmed that the graphene nanoplatelets effectively served as heterogeneous nucleation sites, leading to significant grain refinement in both the weld metal and the coarse-grained heat-affected zone. The inclusion of graphene nanoplatelets promoted a transition from coarse columnar dendrites to a more uniform equiaxed grain structure. Mechanical analysis demonstrated a significant strengthening effect where the yield strength increased from 412 ± 3 MPa to 428 ± 5 MPa, while the ultimate tensile strength rose from 636 ± 6 MPa to 645 ± 4 MPa. Furthermore, impact toughness increased by approximately 72%, rising from 95 ± 2 J to 164 ± 5 J, due to the breakup of brittle interdendritic phases. The novel IN182/GNPs composite electrode significantly enhances the microstructural stability and mechanical integrity of P91 weldments. Consequently, this material provides a high-performance alternative for ensuring the long-term structural reliability of components within nuclear heat exchangers and ultra-supercritical boilers.

Welded joints are widely recognized as the most critical point in structures made of armour steels due to pronounced thermal effects, microstructural heterogeneity, and the degradation of mechanical and fatigue properties. This study investigates the mechanical properties and fatigue crack growth resistance of a welded joint produced on SA 500 armour steel, with the aim of preserving the properties of the base material as much as possible. To achieve this, a welding procedure incorporating a high-strength filler wire and optimized welding parameters was applied. Hardness and tensile testing was conducted to evaluate the extent of property degradation caused by welding. The results demonstrate that the applied welding process effectively limited the reduction in hardness and tensile strength, achieving values reasonably close to those of the base material. In addition, fatigue crack growth behaviour was investigated in accordance with ASTM E647, using both the Paris law and the McEvily law. The obtained fatigue crack growth curves and threshold stress intensity factor (ΔKth) values indicate the nearly identical fatigue behaviour of the base material and the heat-affected zone, confirming the successful preservation of base material fatigue behaviour in the thermally affected zone. Moreover, the weld metal exhibited superior resistance to fatigue crack initiation and growth. Overall, the results confirm that the proposed welding approach provides favourable mechanical and fatigue performance for welded joints in armour steel applications.