Refill Friction Stir Spot Welding is a promising technique for joining lightweight aluminium alloys such as AA7075-T6. However, hook defects—microstructural discontinuities at the weld interface—can significantly compromise joint integrity. Additionally, variations in heat input during welding influence microstructure evolution and mechanical properties. This study investigates the influence of process parameters and the hook defect on the quasi-static performance of AA7075-T6 RFSSW joints. Joints with various parameter combinations were analysed using optical microscopy, electron backscatter diffraction, tensile lap shear testing, and microhardness mapping, alongside thermocouple measurements of process temperatures. A semi-quantitative heat input model was developed to provide new insights into thermal characteristics and their effects on microstructural evolution. The obtained results show a decrease in ultimate lap shear force for high upward or downward-directed hooks. A welding time of 4s and rotational speed of 2200rpm resulted in optimal joint performance, achieving an ultimate lap shear force of 12.2 kN and sufficient heat input to prevent refill defects. Extended welding durations led to overheating and additional defects in the stir zone, while low welding time or rotational speed significantly influenced welding temperatures, heat input, and microstructural characteristics at the shoulder plunge path periphery. The findings emphasise the critical role of dynamic recrystallisation in the resulting microstructures and their impact on mechanical performance. Fatigue strength of the optimised joints exceeds RFSSW joints reported in the literature, underscoring the effectiveness of the selected process parameters in enhancing joint durability. Furthermore, the established semi-quantitative heat input model links energy partitioning to defect formation and microhardness of the joints. This provides new insights into the RFSSW process and enables knowledge transfer to other RFSSW applications.

Current onboard hydrogen storage systems are volumetrically inefficient and represent a major constraint on the driving range of heavy-duty fuel cell vehicles. This work presents a conceptual model of an internally reinforced Type I rectangular-shaped pressure vessel as a solution to enhance the volumetric efficiency of hydrogen storage in heavy-duty vehicles. The pressure vessel’s geometry incorporates an internal reinforcing structure to ensure both the structural integrity of the vessel and compliance with the standards for onboard hydrogen storage. Initially, an analytical approach was employed to determine the base parameters of the wall and the internal structure of the reinforced pressure vessel. Finite element analysis was then conducted to validate the analytical solutions and assess the structural integrity of the pressure vessel under design pressure conditions. This was followed by a parametric optimisation study in which the design parameters were systematically varied to identify an optimal pressure vessel design. The 35 MPa reinforced titanium pressure vessel offers 29% more volumetric capacity than the conventional Type IV storage system. The gravimetric capacity of the titanium pressure vessel is low, 2.9 wt%; despite this, the mass of the vessel is applicable in HDVs. This design increases hydrogen storage capacity, offering a range increase of approximately 29% for the same design space.

The growing adoption of wire arc additive manufacturing (WAAM) requires an understanding of how WAAM-fabricated aluminium alloys respond to environmental factors that may degrade mechanical performance. This study investigates the effects of cathodic charging on the mechanical properties and fracture behaviour of WAAM AA2319 aluminium alloy. Cathodic charging was conducted in an electrolyte containing 3.5 wt.% NaCl and 3 g/L ammonium thiocyanate using different applied current densities. The resulting changes in mechanical performance were assessed through uniaxial tensile and Charpy impact toughness tests. The results demonstrate that cathodic charging led to a progressive reduction in ductility with increasing current density. Elongation decreased by up to approximately 45% relative to the uncharged condition, while ultimate tensile strength and yield strength were marginally affected. Charpy impact testing revealed a corresponding reduction in impact toughness of approximately 40% following hydrogen charging. Fractographic analysis showed a transition from ductile fracture dominated by microvoid coalescence in the uncharged material, to a mixed ductile–brittle fracture in hydrogen-charged specimens, characterised by shallow dimples and quasi-cleavage features. The observed changes in mechanical behaviour and fracture morphology suggest that cathodic charging promoted hydrogen-assisted mechanical degradation, with features consistent with hydrogen-enhanced localised plasticity (HELP) and hydrogen-enhanced decohesion (HEDE).

The very high cycle fatigue behavior of S275J2+N flux-core arc welded joints was investigated using the ultrasonic fatigue testing method at a loading frequency of 20 kHz. A bespoke specimen design featuring the weld toe was employed to more suitably represent in-service welded joints than typical ultrasonic fatigue testing specimens. This revealed that fracture occurs above 10 million cycles, beyond the classically accepted fatigue limit. Additionally, comparative fatigue tests were performed at 10 Hz to investigate the frequency effect on the fatigue behavior. A 35% increase in fatigue strength was measured at 20 kHz when compared to 10 Hz, indicating that a correction must be considered in the use of ultrasonic fatigue testing data for design purposes. Fracture surfaces for both test frequencies showed similar morphologies and typical characteristics for fatigue failures of ductile metals.

Rotary friction welding is a solid-state welding process that can manufacture high-integrity joints between similar and dissimilar materials with short weld times. However, access to expensive and complex industrial-grade friction welding machines is not always possible. This study explores the design process and functionality of a laboratory-scale friction welding setup following the fundamentals of large-scale machinery. The proposed setup is designed to be easily manufactured, employing the use of a calibrated drill press and load cell, thus ensuring welding parameters such as rotational speed and applied axial load are monitored. The decision to investigate rotary friction welding of aluminium bronze Ca104 to austenitic stainless steel AISI316 was taken to explore the limitations of this bespoke friction welding machine for prospective applications in the nuclear energy sector. The workpieces were friction welded at four sets of rotational speeds with constant friction and forging pressures. The microstructural evolution and mechanical properties of the dissimilar material welds were investigated via optical and scanning electron microscopy with energy dispersive spectroscopy, 4-point bend testing and microhardness measurements. Results show a change in the hardness along the weld interface and evidence of metallic diffusion between the dissimilar materials, demonstrating the successful application of the small-scale experimental setup.

There are limited data on the very high-cycle fatigue for structural steel welds for over 10 million cycles. The purpose of this research is a fatigue performance comparison of the welds made of steels S355JR+AR and S275JR+AR. The goal of reaching the gigacycle fatigue domain is achieved using the ultrasonic fatigue testing at 20 kHz. Fatigue samples are prepared to investigate the influence of two surface conditions-polished and precorroded. Fatigue failures are driven primarily by welding porosity with the fatigue life duration dependent on the size and location of pores. Visual comparison of fatigue data for steels poses a challenge, because of the vast scatter of experimental data points. Therefore, a statistical approach is used with a fatigue performance parameter applied to the fatigue data for welded samples to quantify the quality of the structural materials.

In this study, a fully coupled thermomechanical model of the refill friction stir spot welding process was developed using the coupled Eulerian–Lagrangian technique. The model was used to simulate joining of AA2024-T3 sheets with different welding times of 3 s and 1.5 s, and a fixed rotational speed of 2000 rpm. Model outputs of welding temperature, equivalent plastic strain, void content, and material flow were validated against experimental welds produced using the same welding parameters. Welding temperatures were accurately predicted for both welding times throughout the plunging stage yet overestimated during the refill stage compared with thermocouple measurements. Simulated regions of high plastic strain were found to correlate well with regions of higher grain refinement in experimental welds. Internal tunnel-like defects were predicted by the model in the shoulder plunge region; these predictions were validated by equivalent defects found in the microstructures of experimental welds. Stop-action analysis of experimental welds was used to validate the model’s ability to accurately capture material flow during the refill stage. This provides a valuable insight into refill flow behaviour and the formation mechanism of the internal tunnel defects, indicating that volume ratio is an important parameter for future study.

Precipitation hardening (PH) stainless steels, such as 15-5PH, have a high strength combined with excellent corrosion resistance. These properties make them valuable in critical industries such as defence, construction, aerospace, energy and maritime. Recent advancements in additive manufacturing (AM) technology enable the rapid and cost-effective production of components. In the case of 15-5PH components manufactured using wire arc additive manufacturing (WAAM), the as-deposited mechanical properties are not suitable at present for industrial applications. This paper explores the mechanical properties of this process and alloy combination without post weld heat treatment with the aim of eventual adoption in this condition by industry. The impact of weld heat input on the microstructure and mechanical properties of stainless steel 15-5PH produced using WAAM was investigated. The microstructure was examined using hardness testing in addition to optical and electron microscopy. Furthermore, mechanical properties were measured with tensile and impact testing. Investigations were conducted on material produced using weld heat inputs of 0.223 kJ/mm and 0.565 kJ/mm. These results indicate that reducing the weld heat input leads to a minor decrease in strength but an 80

Refill friction stir spot welding is a solid-state spot-welding technique suited to lap joining of thin aluminium sheets, including difficult-to-weld 2xxx series alloys that are prone to hot cracking during fusion welding processes. Long welding time is an ongoing challenge that hinders industrial adoption of the process. To address this, the present study explores much shorter welding times than those previously reported in the literature and assesses the impact on joint quality. Joints of 1.8 mm thick AA2024-T3 sheet were produced with welding times from 3 s, down to 0.75 s and rotational speeds of 1000 rpm to 2500 rpm. Defect formations within the welds were studied with the aid of optical microscopy. The mechanical properties were evaluated using tensile lap shear testing and microhardness mapping, and failure modes were characterised using scanning electron microscopy. Various weld defects were found at all welding times and rotational speeds, and the defects enlarged with decreasing welding time and increasing RS. The highest lap shear strength of 9.21 kN was achieved with a welding time of 3 s and rotational speed of 2000 rpm; lap shear strengths of 7.02 kN and 6.37 kN were achieved for 1.5 s and 0.75 s welds, respectively.

This work investigates refill friction stir spot welded joints of AA2024-T3 aluminium alloy, produced with short welding times between 3 s and 0.75 s. A novel tool geometry that incorporates a chamfer on the inner edge of the shoulder tip is investigated as a means of improving joint quality at short welding times by easing material flow during the refill stage. The influence of shoulder design on weld microstructure, defect formation, material flow, and mechanical properties was assessed. When compared with a standard shoulder geometry, it was found that the introduction of a chamfer on the inner tip edge improved material flow during the refill stage and led to improved material mixing at the weld periphery. The formation of voids in the region of the weld periphery was eliminated and tensile lap-shear strength of the welded joints was increased by 19% to 7.2 kN, and 27% to 8.16 kN, for 0.75 s and 1.5 s duration welds, respectively.

Inconel 625 is a nickel-based superalloy widely used in industries such as energy, space, and defence, due to its strength and corrosion resistance. It is traditionally time- and resource-intensive to machine, leading to increased environmental impact and material waste. Using additive manufacturing (AM) technology enables a reduction in resource consumption during the manufacture of high value components, as material is only deposited where it is required. This study compares the environmental impact of manufacturing an Inconel 625 impeller through machining and wire arc additive manufacturing (WAAM) by employing established life cycle assessment methods. WAAM shows significant advantages, cutting energy consumption threefold and reducing material waste from 85% to 35%. The current work also evaluates the mechanical properties of WAAM-produced components through tensile and axial fatigue testing, in addition to the use of optical and electron microscopy for metallurgical analysis and fractography. This demonstrates yield and ultimate tensile strengths exceeding industrial standards, with comparable or superior fatigue life to other AM methods. The improved fatigue performance extends the service life of components, bolstering sustainability by reducing the need for frequent replacements, thereby lessening associated environmental impacts. These findings underscore the promise of WAAM in enhancing both environmental sustainability and mechanical performance in manufacturing Inconel 625 components.

Structures and components across a range of industries exhibit service lives above 107 cycles, but there is limited fatigue data in this domain. Conducting fatigue tests in the gigacycle regime is only feasible by using ultrasonic fatigue testing machines in which specimens are typically excited at 20 kHz. This research focuses on the design of a novel specimen which captures the geometric stress concentration at the weld toe, as an alternative to typically used cylindrical ‘hourglass’ shaped specimens. The gas-shielded flux-core arc welding method was used to manufacture butt joints of 080A15 structural steel. Specimens were tested in fully reversed axial loading at room temperature using a Shimadzu USF-2000A ultrasonic fatigue testing machine. Specimens failed in the range of 104-107 stress cycles with all fatigue cracks initiating at the weld toe. One specimen was classed as a run-out after being subjected to 3 × 109 stress cycles without failure. The fatigue data obtained was assessed using the effective notch stress method and showed similar fatigue performance to the IIW guidelines.

Additive manufacturing (AM) offers advantages in many aspects over conventional manufacturing techniques. These include reduced lead times and material waste. A recent topic of interest is the environmental impact of manufacturing techniques. However, there is a lack of literature detailing the impact of titanium manufacturing processes. In addition, there is also a gap in knowledge comparing AM to conventional manufacturing methods with the aim of investigating the reduced environmental impact as a further advantage of AM. In this research, an environmental impact analysis is used to compare wire + arc additive manufacturing (WAAM) against conventional forging in the production of a Ti6Al4V component. This is achieved by comparing the material waste, energy consumption and carbon emissions of each production method. WAAM shows a 50% reduction in carbon emission and 40% reduction in energy consumption over forging. The largest difference is in the material waste, with a 55% reduction in discarded material. The comparison metrics of specific energy consumption (SEC) and specific carbon emissions are presented for evaluating the sustainability of processes, with the WAAM produced component in this study presenting a SEC of 574.9 MJ/kg compared to 958 MJ/kg for conventional forging.

The design life of welded structures and components extends into the very high cycle fatigue (VHCF) regime across various applications. However, the availability of data on the fatigue behaviour of welded joints in the VHCF regime is limited, particularly when compared to the low and high cycle fatigue regimes. The development of ultrasonic fatigue testing equipment has accelerated fatigue testing and allowed for the VHCF properties of welds to be investigated in a feasible timeframe. In the present review, the emerging research concerning the VHCF behaviour of welds of various steels and non-ferrous alloys are individually explored. Overall, it is observed that welded joints have significantly lower fatigue strength than the base metal in the VHCF regime and that welding defects have a considerable influence on fatigue strength. Through the discussion of the relevant literature, important findings concerning the effects of specimen geometry and fatigue improvement methods are underlined. Furthermore, the guidance provided within design standards is compared, and some examples of VHCF failures of in-service components are highlighted. Finally, perspectives on future directions of investigation are put forward with the aim of encouraging further research in the field of VHCF of welds.

Ultrasonic Joining (U-Joining) produces through-the-thickness reinforced (TTR) hybrid joints between thermoplastics and surface-structured metals. The joining parameters were previously optimized to join additively manufactured (AM) 316L stainless steel (316L SS) and 20% short-carbon-fiber-reinforced poly-ether-ether-ketone (PEEK-20CF) to maximize the joints' performance under quasi-static lap shear testing. However, further in-vestigations on the joint's fracture mechanisms and cyclic loading performance are still lacking. Therefore, this study describes the stress distributions, assesses the fracture mechanisms and evaluates the fatigue life of AM 316L SS/PEEK-20CF hybrid joints. A finite element model was developed to clarify the joints' mechanical behavior, and their fatigue performance was assessed under cyclic tensile condition. The fatigue tests were performed at different percentages of the reached ultimate lap shear force (ULSF) and analyzed via two-parameter Weibull distribution and load-life curves for different reliability levels. The results showed that a fatigue life of 1 x 106 cycles could be reached when a load of 1.52 kN, or 42% of the ULSF, is applied, demonstrating the joints' high mechanical performance and potential for engineering applications. Joints reaching the one million cycles threshold were stopped at this mark and tested under quasi-static lap shear to assess their residual force. The results significantly decreased from 3.6 +/- 0.3 kN to 2.4 +/- 0.5 kN for ULSF and residual force, respectively. Fractography analyses identified polymer delamination, partial TTR pull-out, and interfacial/net-tension failure as the main fracture mechanisms. Poly-mer detachment in fatigue specimens indicated the influence of secondary bending at low load levels, explaining the reduced residual force.(c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

The present study investigates the sliding wear properties of G350 grey cast iron following various spheroidizing annealing durations. The annealed samples were used as pins in sliding wear tests against AISI 4330 discs. Most testing combinations resulted in mild wear with little matrix damage to the cast iron pin or AISI 4330 disc. A transition to severe wear was observed for the highest load and longest annealing times where the overly spheroidized and ferritized cast iron matrix deformed significantly, prohibited beneficial oxidative wear from occurring, and thus damaged its disc counterpart. The depth of subsurface deformation increased with annealing time but not load. Instead, the depth peaked at 4 kg applied load for each condition due to the spallation of the affected surface layer at the higher load.

A novel approach for predicting the intermetallic compound (IMC) formation during friction stir welding (FSW) of AA6061 to commercially pure copper has been developed, in addition to their effect on mechanical properties. The temperature distribution of the aluminium to copper weld nugget determined by a finite element model, the use of an Al-Cu phase diagram and the elemental concentration of copper and aluminium in the weld nugget have been combined to predict and validate several IMCs present in the different zones of the weldment. The results of performing butt-welding of these dissimilar metals using the FSW process demonstrated that the highest ultimate tensile strength of 194.5 MPa was achieved at 1500 rev min(-1) tool rotational speed, 100 mm min(-1) traverse speed and a zero-tool offset.

The present study investigates the tribological properties of G350 grey cast iron in various microstructural conditions. Quench and temper heat treatments were conducted at four tempering temperatures, from 400 degrees C to 700 degrees C, to produce a range of tempered martensitic disc samples. Pins were slow furnace-cool annealed to produce a coarse pearlitic microstructure with some areas of ferrite. These samples were then used in pin-on-disc sliding wear tests. Hardness decreased with tempering temperature, from 400HV at 400 degrees C to 200HV at 700 degrees C. The two spheroidized carbide discs produced higher volume losses than the two acicular martensitic discs due to instability of the oxide layer on the softer substrate. Subsurface deformation was visible in the annealed pins and spheroidized discs which consisted of deformed cementite lamellae in the direction sliding and strained ferrite grains.

In this study, refill friction stir spot welding (RFSSW) was used to join AlSi10Mg, produced by laser powder bed fusion (LPBF), to high-strength wrought AA7075-T6 alloy.The investigation showed the best mechanical properties and integrity of the joint are achieved with medium heat input, where the optimal balance between hook height and integrity was observed.The welded additive manufactured alloy shows accentuated softening in the hook region of the thermo-mechanically affected zone.The feasibility of RFSSW for joining LPBF AlSi10Mg to high-strength wrought alloys was confirmed, showing high potential for further investigations and industrial applications.