During diffusion bonding of Al-Li alloys, the dense oxide layer limits joint quality. Using a nano-Cu interlayer can inhibit interfacial oxidation, but its mechanism is unclear. This study combines molecular dynamics simulation and experiment to investigate the diffusion-bonding behavior of 2A97 Al-Li alloy using a nano-Cu interlayer. Results show that the nano-Cu layer promotes element diffusion, with higher temperatures enhancing the effect. At 480–500 °C, oxides and continuous Cu intermetallics in the bonded interface hinder diffusion. At 520 °C, oxides and intermetallics decrease and disperse, allowing interfacial atoms to achieve sufficient diffusion. At 540 °C, coarsening of Cu-containing intermetallics reduces mechanical properties. Simulation matches experiments, providing a theoretical basis for nano-Cu interlayers in Al-Li alloy diffusion bonding.
Optimization for large industrial parts manufactured with directed energy deposition using a thermomechanical simulation is time-intensive, due to the complex thermomechanical interactions and high deposition rates necessitating small time steps and element sizes. This work develops a layer-dependent inherent strain method using the minimum thermal strain extracted from a thermomechanical simulation for distortion prediction, employing the thermal shrinkage as the predominant driver of distortion in welding processes. The method has been tested on a wall with 40 mm height and a cylinder, achieving 94.1% accuracy, improving predictions by 15% compared to simulations using a constant inherent strain tensor. The approach enables fast distortion prediction, while the thermal strain correlation enables future parameter estimations without prior thermomechanical simulation.
This study proposed material models for the S700MC/DP1180 dissimilar-metal butt-welded joint, which accounts for solid-state phase transformation and tempering softening. Based on the models, a numerical approach was employed to calculate the welding-induced stress and deformation. After experimental verification, the calculated results demonstrate that solid-state phase transformation and softening effect significantly affect the magnitude and distribution of residual stresses: the peak longitudinal residual stress shifts from the DP1180 side to the S700MC side, decreasing from 1092 to 798 MPa; the location of the peak transverse residual stress is also relocated, and its magnitude declines from 162 to 111 MPa. Although the solid-state phase transformation and softening effect induce a slight reduction in welding deformation, their influence is marginal.
Tandem gas metal arc welding (T-GMAW) utilizes simultaneous deposition from two wires to enhance the productivity for joining thick sections. Current knowledge on the actual energy consumption vis-à-vis filler wire deposition rate in T-GMAW is limited. We present here a detailed investigation on multi-pass single V-groove T-GMAW of a 30 mm thick structural steel plate with real-time monitoring of current, voltage and metal transfer modes for both filler wires. A novel electrical deposition efficiency (EDE) metric is realized using measured current and voltage transients to correlate electrical energy usage with the deposition rate. For a constant wire feed rate, the short-circuiting metal transfer mode resulted in much lesser energy input and 50% higher EDE in comparison to the pulsed mode of metal transfer.
In situ diagnostics for defect detection and compositional assessment remain critical for addressing persistent challenges in Additive Manufacturing (AM). Among the available diagnostic techniques, Optical Emission Spectroscopy (OES) stands out for its strong capability to provide in situ and non-intrusive monitoring of the AM process. As part of our ongoing efforts to develop a low-cost, real-time AM monitoring tool, this study represents an initial investigation focused on the concentration of key species in the Tungsten Inert Gas (TIG) wire-arc plasma generated during welding of an aluminum (Al-6061-T6) sheet using OES. Emission spectra from the arc plasma were analysed to identify and quantify key spectral lines associated with aluminum and potential alloying elements. The plasma temperature and electron number density were estimated to support quantitative analysis through Boltzmann and Saha methods to enable accurate concentration determination by calibration-free approach. The results demonstrated the potential of OES as an effective tool for monitoring and optimizing the aluminum welding operations through in situ non-intrusive plasma composition analysis.
In the ultrasonic welding (UW) process, achieving reliable bonding under low ultrasonic energy input is crucial for minimising surface damage and improving long-term service stability of joints. In this research, three Cu interlayers with horizontal (H-TBs), inclined (I-TBs) and vertical twin boundaries (V-TBs) were designed to accelerate weld formation during UW. At a welding time of 0.3 s, lap shear tests indicated that the I-TBs joint exhibited 38.6% and 54.9% higher strength than the H-TBs and V-TBs joints, respectively. The moderate microhardness, elevated coefficient of friction and activation of soft-mode dislocation motion in the I-TBs joint generated localised plastic instability, accelerated the flattening process and improved metallurgical bonding under the low energy inputs.
Joining NiTi shape memory alloy and 304 stainless steel (SS) presents significant technical challenges, and studies on micro-resistance butt welding (MRBW) of such dissimilar materials are extremely scarce. This work uncovers the nanoscale micro-structure evolution and inter-metallic compounds (IMCs) formation mechanism of NiTi to 304 SS micro-wire joints with a diameter of 0.45 mm during MRBW. Atomic diffusion of Fe and Ni promotes the formation of Fe2Ti and Ni3Ti phases, which determines the thickness, morphology, and distribution of the interfacial IMC layer. Benefiting from the high-performance Ni3Ti phase, the joint fabricated at a welding current of 20 A exhibits the optimal comprehensive performance, with a tensile strength of up to 446 MPa and an elongation of 1.5%.
This study aims to increase the penetration depth of an aluminium (A5052) weld pool in alternating current tungsten inert gas welding by controlling the cathode spot behaviour through the application of an external magnetic field. The external magnetic field direction was parallel to the base metal surface and perpendicular to the welding direction. The heat input from the distributed cathode spots was experimentally investigated using a high-speed video camera. The penetration depth almost doubled as a result of applying the optimal magnetic flux density of 5.5 mT. At this magnetic flux density, the largest number of cathode spots was concentrated in the weld pool, causing intensive heat input owing to the large cathode fall voltage and consequently leading to deeper penetration.
This study investigated the microstructure and mechanical properties of dissimilar vacuum electron beam welded joints between TU1 oxygen-free copper and 316L stainless steel. Sound, defect-free welds with a characteristic nail-head profile were achieved. The fusion zone exhibited a complex interpenetrating microstructure comprising Fe-rich and Cu-rich zones with dispersed particulates. EBSD analysis revealed a significant grain size gradient, with fine equiaxed grains at the fusion line and coarse columnar grains growing epitaxially into the fusion zone. Mechanical testing demonstrated that joint failure occurred in the weaker heat-affected zone rather than the weld joint, with a yield strength of similar to 123.5 MPa and elongation of similar to 6.8%. These results demonstrated that EBW was an effective technique for producing reliable copper/stainless steel dissimilar joints.
The non-equilibrium cooling of 304 SS by TIG welding, making the formation of dendritic delta-ferrite in fusion zone (FZ), whereas the lath delta-ferrite formed in the heat affected zone (HAZ) that accompanied by high proportion of high-angle grain boundaries and precipitation hardening. Consequently, welded joint has higher strength and hardness compares to the base metal (BM), and fracture exhibits ductile intergranular. The slower cooling rate in HAZ promotes precipitation strengthening and refinement of delta-ferrite, inducing microhardness gradient distributed as HAZ > FZ > BM and effectively improve tensile strength. However, electrochemical measurements reveal FZ and HAZ are more susceptible to corrosion than the BM. Cr segregation and depletion surrounded delta-ferrite phase, lowering protection film formation on welding bead of gamma-austenite.
The columnar/equiaxed grain (C/E) ratio governs the microstructural uniformity and mechanical performance of the weld metal (WM). As the ratio is intrinsically dictated by weld pool solidification kinetics, in situ observation of the solidification process becomes essential for elucidating grain morphology evolution. It has been demonstrated that increasing the SiO2 content in flux-cored wires reduces the secondary dendrite arm spacing from 42.75 to 31.46 mu m, promotes carbon enrichment ahead of the solidification front, enhances constitutional undercooling by 11 degrees C and suppresses the primary delta phase originating from the initial liquid/solid interface, where columnar grains predominantly form, thereby reducing the C/E ratio from 2.1 to 1.1. These findings may serve as guidance for microstructural design towards high-performance WMs.
Friction stir spot welding (FSSW) is a promising joining method for advanced high-strength steels. Understanding the factors influencing the failure behaviour of FSSW joints is crucial for using this method in the automotive industry. This study addresses the role of rotational speed on the microstructure and hardness across the weld, as well as on the geometrical features and failure behaviour of ultrafine carbide-free bainitic steel FSSW joints. Increasing the rotational speed from 600 to 2000 rpm is identified as effective in eliminating the hook feature. A small effective thickness (320 mu m), accompanied by a sharply upward hook, renders the weld susceptible to partial pullout failure at 600 rpm, beyond which interfacial failure emerges as the prevailing failure mode. A rotational speed of 1000 rpm is sufficient to develop an adequate bonding area with fine martensite, resulting in the maximum failure energy.
To address weldability challenges and achieve excellent tensile performance of low-density austenitic steel in friction stir welding (FSW), two low heat input parameters were used to join Fe-30Mn-9Al-0.85C steel. The results indicate that low heat input suppresses the dissolution of kappa-carbides in the stir zone, while promoting the growth of grains and carbides and increasing annealing twin boundaries (ATBs) in the heat-affected zone (HAZ). The welded joints exhibited a joint efficiency of 103%. This study uncovers a mechanism - during tensile deformation in the HAZ, ATBs transformed into high-angle grain boundaries, leading to grain refinement and giving rise to the dynamic Hall-Petch effect - that enables a strategy for achieving the excellent tensile performance of FSW joints in low-density austenitic steel.
Conventional two-dimensional optical microscopy is the most common tool for capturing and quantifying keyhole collapse porosity and other defects formed during the laser welding of aluminium alloys. This approach is of limited utility since only a single longitudinal or transverse metallographic cross-section, comprising only a small fraction of the overall weld volume, can be extracted or analysed at a time. X-ray computed tomography tools, on the other hand, enable three-dimensional visualisation and analysis of porosity and defects to be made across the full weld volume. When comparing these two measurement techniques, two-dimensional microscopy tools significantly underestimated the fraction of defects present within the weld at levels between 50% and 80%. Three-dimensional x-ray computed tomography tools also provide a means for accurately characterising the location, size and morphology of irregularly shaped defects. By categorising the defect shapes using a Zingg's shape analysis, in which the distribution of defect morphologies could be quantified as disk, rod and spherical shapes, a transition in defect morphology moving from stationary beam to oscillating beam welds was detected and quantified.
Oscillating/alternating magnetic fields have been proven to promote columnar-to-equiaxed grain transition (CET) in laser/arc welded joints, whereas the effectiveness of steady magnetic fields remains controversial. This study explored whether CET can be promoted by an external steady magnetic field in laser-MIG hybrid welding of aluminum alloy. Welds were produced at different magnetic flux densities; grain morphology and thermal-fluid flow were analyzed. The results showed that the steady transverse magnetic field interacted with the arc current to generate a backward Lorentz force in the rear molten pool, fracturing columnar dendrite tips and promoting CET. However, Joule heating induced by the magnetic field caused grain coarsening. This study will provide a more underlying understanding of weld grain growth dynamics.
This study focuses on the formation, quantification and morphology of retained austenite in the heat-affected zone (HAZ) of a thermomechanically controlled processed high-strength steel. Physical simulation using dilatometry combined with high-energy X-ray diffraction was employed for in situ phase quantification, while various electron microscopy methods were used for microstructural investigation. Compared to the initial state, which contained approximately 1% retained austenite, the content increased by 1.9% in the coarse-grained HAZ and by 3.9% in the intercritically reheated coarse-grained HAZ. Some of the formed retained austenite appears as stand-alone phase in both zones. However, the majority seems to be present within various types of martensite-austenite constituents.
A 316L stainless steel/Inconel 625 functionally graded material was fabricated by gas metal arc-based wire arc additive manufacturing assisted by a static magnetic field and external cold wire feeding. The effects of magnetic field assistance on surface morphology, microstructure, and mechanical properties were investigated. The observed improvements are consistent with enhanced molten pool convection induced by the magnetic field, resulting in refined grain morphology and reduced elemental segregation. Compared with the non-magnetic condition, the magnetic-assisted sample exhibited higher hardness, approximately 10% higher tensile strength, and 19% higher yield strength, while maintaining comparable ductility. These results indicate that the combined application of a static magnetic field and cold wire feeding improves structural homogeneity and mechanical performance of WAAM-fabricated functionally graded materials.
For demanding applications such as lightweight pipelines and structures, this study investigates the effects of MIG welding and post-weld heat treatment (PWHT; specifically solution treatment at 510 degrees C for 65 min followed by water quenching and ageing at 180 degrees C for 2 h) on the dendrite structure and properties of 2A12 aluminum alloy joints using ER2319 and ER5356 fillers. After PWHT, significant dendrite fragmentation and spheroidisation occur, reducing columnar dendrite fraction to 5% (ER2319) and 15% (ER5356). The uniform theta'-Al2Cu precipitates enhance tensile strength by 52% and 43%, respectively, with the ER2319-PWHT joint reaching 378 MPa, 11.5% higher than the base metal. While PWHT promotes <100> equiaxed grain recrystallisation and increases low-angle grain boundaries to 35%, corrosion resistance in 3.5% NaCl solution remains primarily governed by filler composition (ER2319 > ER5356). These findings provide critical insights for microstructure control in aluminum weldments.
