In this study, synergistic double-sided friction stir welding (DS-FSW) is proposed to solve the problems of large deformation and the time-consuming of traditional DS-FSW. The microstructure, mechanical properties, and fracture paths of novel and conventional joints are studied under different welding parameters. Results show that defects in novel joints have been improved. However, the grain size in the stir zone of novel joints is larger than that of conventional joints. The microhardness map of the novel joint shows a more uniform distribution compared to that of the conventional joint. At a rotational speed of 1800 rpm and transverse speed of 1000 mm min−1, the distortion of the novel joint and conventional joint is 0.1 and 1 mm, respectively. The tensile force of novel joints is higher than that of conventional joints for the same welding parameters. The maximum tensile force of novel and conventional joints is 36.8 and 34.9 kN, respectively.

A new Al–Cu–Ni alloy was repaired by arc additive remanufacturing and T6 heat treatment. The results show that the grain size in the upper part of the weld is slightly larger than that in the lower part, and the whole distribution is uniform without delamination. The orientation of grains in the weld is disordered, and most of them are stable crystal structures with a large angle. The average tensile strength at 300°C is 148.3 MPa, the yield strength is 106.6 MPa and the elongation is 10.5%. δ-Al3CuNi and γ-Al7Cu4Ni increase the strength of grain boundary, and the strength of θ″ and θ′ are dispersed from the α-Al matrix during aging, which are the main reasons for obtaining high-temperature properties.

This study investigated the effect of beam oscillation on the solidification behaviour during laser welding of Al-5754 to Al-6061 alloy. In this study, a finite element model has been developed to simulate temperature and fluid flow fields by implementing different combinations of volumetric heat source models. Solidification parameters such as temperature gradient (G), solidification rate (R), cooling rate (G × R) and G/R are evaluated to understand the mechanism of microstructure formation. It was found that beam oscillation improves the tensile strength by 21.4% for full penetration welding due to an increase in the percentage of formation of equiaxed grains. Modelling results revealed that the cooling rate increases with an increase in oscillation frequency. However, tensile strength followed a parabolic distribution with a peak at the oscillation frequency of 300 Hz.

Aluminium and titanium are important structural metals; however, their joining is difficult because of different microstructural responses and intermetallic compounds (IMCs). The present work underlines the mechanisms associated with friction stir welding of pure Ti and Al alloy. The results show no visible defect, while multiple particles are consolidated in the stirred zone (SZ). Chemical composition across Al-Ti interfaces is different and dependent on location and particle size. This affirms intensive mechanical mixing and thermal diffusion of Al-Ti substrates and IMCs formation. The UTS and ductility of the Al-Ti joint are ∼73% and 37% higher than Al base metal, respectively. Deformation of Ti in SZ is responsible for phase evolution and joint formation with better joint characteristics.

The dissimilar AZ31B/AZ61 Mg alloy joints were fabricated by pulse current-assisted friction stir welding (PCaFSW). The maximum tensile strength and elongation were 314.28 MPa and 13.66%, respectively. A micro interlocking structure was formed and the grain refinement effect was enhanced by the PCaFSW process, which was improved relative to that in the conventional FSW. The enhanced strength and plasticity with increasing pulse current were attributable to the grain refinement, weakening of texture strength, formation of nano-scale precipitates and increase of short dislocation lines. The fracture mode was transformed from a brittle fracture mode to a combined fracture mode of brittle and ductile fractures.

ABSTRACTLaser welding of NiTinol is a promising fabrication method. An understanding of the effects of laser heat input on different surface properties and corrosion resistance behaviour is needed for laser-welded sea water-flooded components of NiTinol. Here the effect of laser heat input on various physical, chemical and mechanical surface properties along with the corrosion resistance behaviour of the laser welded samples was investigated. The corrosion resistance performance of the phase-separated samples was found to be inferior to the other laser-welded samples. The top surface showed a superior corrosion resistance performance because of the recirculation of the melt pool and Ti enrichment on top surface.

As an energy-efficient solid-state joining technique, friction stir welding (FSW) shows enormous advantages compared to conventional fusion welding methods. This work comprehensively reviewed the recent studies on FSW of commonly-used metals including Al, Mg, Fe, Ti and their alloys, with a particular focus on thermal cycles experienced during FSW. The role played by thermal cycles was comparatively summarised for different metals, trying not only to understand the physical effect of welding conditions on joints’ microstructural evolution and mechanical properties, but also provide a guide for engineers to take proper FSW strategies when facing different metals, which has not been discussed in detail in other reviews. Finally, several ideas were proposed for a possible advancement in FSW of the investigated metals.

The tailor joining of thin metal sheets frequently suffers from the assembly error of air gap. Herein, an improved laser welding scheme, integrating the pulsed laser power and the modulated beam spot size is developed and applied on 1.5 mm-thick Ti6Al4V sheets with a predetermined air gap range. Weld beads of high continuity and homogeneity are obtained while the tensile property has a nonlinear decrease when increasing the air gap. During the welding, the over-gap metal bridge is obtained due to melt accumulation and periodical keyhole penetration, and the gap bridging capacity, mainly featuring bilateral metal bridges can be weakened as the air gap increases. This evaluation would provide a basis for processing optimization regarding seam appearance and property.

Three sleeve structures (conventional sleeve, threaded sleeve, grooved sleeve) were used for refill friction stir spot welding (RFSSW) of 2A12-T42 aluminium alloy to investigate the effect of sleeve geometry on hook formation and mechanical properties of the joints. The high-pressure area generated during the plunge stage was responsible for the hook formation. Threads and grooves sleeves not only enhanced the mixing effect but inhibited the hook formation by breaking up the alclad layer or promoting the downward material flow, respectively. The welds using a grooved sleeve had the lowest hook height and highest tensile-shear failure load. As the hook height decreased, the upward tendency of crack propagation was suppressed, which confined the propagation paths within the regions of higher hardness.

Resistance spot welding is employed to join the 9Cr oxide dispersion strengthened steel and the effects of different welding parameters are studied. Within the ranges of parameters used in this experiment, the failure load and energy increased from 4.3 kN, 1.0 J to 6.8 kN, 7.5 J as the welding current increased, while as the electrode force increased, the failure load and energy first decreased from 6.8 kN, 3.8 J to nearly 4 kN, 1.2 J, then remained stable. As the welding current exceeded 12.8 kA, the failure mode changed from pull out failure along the fusion line to the pull out failure along the fusion line partially and base metal partially. The welding current and electrode force strongly affect the mechanical properties and fracture mode of the joint.

The selection of parameters affects the surface roughness in the additive manufacturing process. This study aims to determine the optimal combination of input parameters for predicting and minimising the surface roughness of samples produced by Fused Deposition Modelling on a 3D printer using a cascade-forward neural network (CFNN) and genetic algorithm. Box–Behnken Design with four independent printing parameters at three levels is used, and 25 parts are fabricated with a 3D printer. Roughness tests are performed on the fabricated parts. Models generated by the hybrid algorithm achieve the best results for predicting and optimising surface roughness in 3D-printed parts. The surface roughness prediction accuracy of the trained CFNN with optimised parameters is more accurate compared to previous random test results.

Printed metallic parts often suffer from thermomechanical defects such as delamination, buckling and warping, and this effect is exacerbated in multi-arc additive manufacturing (AM) due to the extensive heat input and large molten pool. These defects originate primarily because of high residual stresses accumulated during layer-by-layer deposition. Here we develop, validate and employ a three-dimensional finite element model with two independent heat sources to analyse the thermomechanical responses in dual-arc parallel AM of Ti6Al4V thin-walled parts. The results are compared with those of the conventional single-arc AM. Although the deformation in dual-arc AM is slightly larger than that in single-arc AM, the stresses at the substrate-deposit interface for dual-arc AM are reduced by 53% due to the lower cooling rate.

To understand the underlying microstructure evolution and joining mechanism of ultrasonic spot welded Cu/Cu joint, the microstructure and crystallography analysis were investigated. The grain size, grain boundary, misorientation angle, Kernel Average Misorientation, Schmid factor and crystal orientation along the weld thickness were heterogeneous and unsymmetrical. The changes of corresponding microstructural and crystallographic features were more obvious for the top sheet comparing with the bottom sheet. The {111} texture, nano-sized equiaxed induced by discontinuous dynamic recrystallization and refined elongated grains because of continuous dynamic recrystallization were formed at the welding interface. The nucleated fine grains between the elongated grains attributing to continuous dynamic recrystallization were beneficial to the grain boundary migration and consequent the achievement of the interfacial metallurgical bonding.

Cross-sectional asymmetry of melt pools along adjacent tracks results in an uneven build surface in gas metal arc-directed energy deposition. We present here a novel attempt on real-time monitoring of melt pool cross-sectional asymmetry in terms of its lateral and angular deviations from the track centreline using a two-colour quotient thermal camera. The melt pool lateral and angular deviations are found to be 2–3 mm, and 8–12°, respectively, for the range of process conditions. A unique analytical methodology is proposed that shows a hatch spacing of around 72–76% of the width of a deposited track can reduce melt pool asymmetry and layer height variations along adjacent tracks, which is also observed experimentally.

Residual stresses created from welding have a direct influence on fracture mechanisms of cracks and flaws in and around welded regions. Excessive residual stress can lead to unexpected failures and an important property in fitness-for-service calculations. In this work, an experimental database of the residual stress of low heat input austenitic girth welds is analysed for statistical correlation. The combined measurement profiles indicate normality throughout the weld thickness. Two-sample tests of grouped datasets were additionally carried out to examine potential relationships within the data. Presented measurements were characterised by location from weld centre, and similarly, by pipe aspect ratio. Results indicate a moderate influence of dimension ratio on residual stress in piping components suggesting potential for additional consideration in fitness-for-service procedures.

A new low transformation temperature (LTT) welding material 16Cr8Ni was developed to satisfy the optimal characteristics in diluted welds due to all repair welding positions. Its good weldability for all welding positions was tested. The measured Ms temperature of weld metals with different dilutions was between 150°C and 250°C which is the optimal range for compressive residual stress generation and fatigue life extension. The in-situ observation of phase evolution by the synchrotron-based X-ray diffraction technique validated the solidification mode of LTT welds for the prevention of solidification cracks. Moreover, the mechanical properties in the diluted LTT welds were measured by miniature uniaxial tensile tests. Large compressive residual stresses at the welded joints due to all welding positions were confirmed numerical analyses.

An astigmatic electron beam with an ellipticity ratio of approximately 2:1 was examined using the Enhanced Modified Faraday Cup (EMFC) diagnostic to measure its properties and show how the power density distribution varies near the beam waist. Results show that the beam has two power density peaks, one on either side of the beam crossover point, resulting from different focal distances of the major and minor axes of the elliptical beam shape. Quantification of the beam properties was used to establish a depth-of-field where the beam diameter and peak power densities are relatively constant. For the 1 kW beam studied here, the depth-of-field measured approximately ±5 mm from the beam crossover point, illustrating how electron beam diagnostics can be used to identify process control limits for repeatable and reliable welds under non-ideal electron beam power density distribution conditions.

Creep-resistant nickel, cobalt based superalloys, selected for a high-speed flight application, deposited using Wire + Arc Additive Manufacturing (WAAM), was reported. Three different alloys, Haynes 188, Inconel 718, and Rene 41, were deposited, and tested for their high-temperature tensile properties, and the results compared with wrought data. The alloys were tested from ambient temperature to 1000°C in their as-deposited condition and after undergoing industry standard age-hardening and solutionising heat-treatments, to down select the best performing alloy under two different processing conditions. The mechanical strength of the alloys fell short of the maximum achievable in wrought condition. Precipitation-strengthened alloys, Inconel 718 and Rene 41 were found to have underperformed the most significantly, whereas solid-solution-strengthened Haynes 188 suffered the least due to WAAM.

The metallurgical joining between cast A359 aluminium alloy and AISI321 stainless steel using pure Zn as the interlayer was performed slightly above Al–Zn eutectic temperature. The liquid phase was formed by a eutectic reaction between Al and Zn and subsequently isothermally solidified by diffusion of Zn into the A359 base metal (i.e. reactive transient liquid phase bonding). The formation of a Zn-rich diffusion affected zone in A359, incorporation of Si particles into the bond zone, formation of a thin Al–Fe–Si intermetallic at the steel interface, and isothermal solidification of the liquid phase and subsequent Al–Zn eutectoid transformation govern the microstructure of the joint. The resultant bond microstructure does not deleteriously affect the heat sink function of Al base metal.

This paper presents the experimental and numerical results of the influence of dwell time (30, 60, 90 s), down force (2450, 4900, 7350 N), and rotational speed (1000, 1500, 2000 RPM) on microstructure, microhardness, and material flow during solid-state welding. Probeless friction stir spot welding (P-FSSW) with a flat tool shoulder was assisted to form a metal compound of AA2024, AA6082, and AA5754 aluminium plates with different thicknesses in lap configuration. The analysis of the experimental data and numerical results showed that down force had a significant influence on the material flow and the quality of the welds. High friction energy, and subsequent intensive material flow, promoted the vortexes formation, which improve metals mixing and grain refinements.