Materials Science Forum Vol. 1185

Abstract: The interface between the body and recovery insulation in factory joints is susceptible to localized electromechanical weaknesses, potentially compromising the long-term reliability of high-voltage direct current (HVDC) submarine cable systems. This study introduces ultrasonic vibration into the insulation recovery process of these joints. The effects of ultrasonic treatment are evaluated through tensile tests, thermal elongation tests, DC breakdown tests, and space-charge characterization. Results demonstrate that ultrasonic treatment significantly enhances interfacial tensile properties, improves deformation stability under thermal loading, increases the DC breakdown strength of the insulation interface, and significantly suppresses negative space-charge accumulation. These improvements are attributed to the high-frequency acoustic pressure and localized temperature rise induced by ultrasonic vibration, which promote molecular-chain melting and rearrangement, thereby reducing microscopic defects. This study provides an innovative method for the insulation recovery forming process in factory joints to improve interfacial reliability.

Abstract: The direct reuse of End-of-Life (EoL) automotive sheet metal offers significant environmental benefits, particularly when panels are flattened rather than remelted. However, the geometric irregularity and variable formability of reclaimed sheets require joining solutions that operate with minimal tooling and are compatible with flat-die pressing. This work introduces a novel rivet-based joining-by-forming process designed to create a mechanical interlock using only flat tools, enabling integration into the same hydraulic press used for EoL panel flattening.Finite element simulations were performed to optimize rivet geometry and study flange formation, stress distribution, and failure mechanisms. The optimized rivet design achieves stable outward flaring under axial compression, producing a functional joint without shaped dies. Experimental tests on reclaimed automotive sheets validated the joining concept and confirmed the deformation behaviour predicted numerically.The proposed method provides a low-cost, low-energy joining strategy suitable for reclaimed steels of uncertain formability and supports the development of circular manufacturing routes based on the direct reuse of automotive sheet metal.

Abstract: Conventional self-piercing riveting (SPR) produces rotationally symmetric joints with largely uniform mechanical behavior. While this provides robust performance in many applications, increasing demands for material-efficient lightweight design, complex load paths and hybrid material systems require more versatile joining strategies. Recent experiments have demonstrated that an adapted tumbling SPR (T-SPR) process can intentionally induce non-rotationally symmetric joint geometries and thereby extend process limits beyond those of conventional SPR. Such asymmetric joints offer the potential to tailor load-bearing capacity and energy absorption to specific load directions, which could be particularly advantageous in crash-relevant or multi-material applications. Building on these findings, the present study shifts the focus from geometry control to the systematic evaluation of the mechanical performance of asymmetric T-SPR joints. Specimens were produced using T-SPR with tailored combinations of tumbling angle and velocity. The joints are manufactured with a versatile tumbling self-piercing riveting tool. To assess the resulting mechanical properties, cross tensile and tensile shear tests are conducted. From the resulting force-displacement curves, typical mechanical properties such as ultimate load, load-bearing capacity, displacement at failure and absorbed energy are derived. The mechanical performance of asymmetric joints is evaluated in comparison with symmetric reference joints produced by tumbling self-piercing riveting. This enables both the demonstration of direction-dependent mechanical behavior of asymmetric joints compared to symmetric references and a systematic evaluation of how geometric anisotropy affects load-bearing capacity, absorbed energy and failure characteristics.

Abstract: In Friction Stir Welding (FSW) tool geometry plays a critical role in governing heat generation, material flow, and microstructural evolution within the weld. In this study, the feasibility and performance of FSW tools manufactured by Laser Powder Bed Fusion (L-PBF) are experimentally and numerically investigated. A non-conventional FSW tool produced in AISI 316L by L-PBF was designed and compared with a conventional machined steel tool in the welding of AA6082-T6 sheets performed using already optimized process parameters. This was followed by tensile testing and macro-and micro-hardness measurements, and a punctual microstructural analysis. In addition, a 3D thermo-mechanical finite element model was employed to forecast and analyze the temperature distribution, the effective strain and the overall material flow. The results show that the tool manufactured using L-PBF enables FSW joints to achieve mechanical properties and welding efficiency similar to those of the standard tool. Finite Elements Models (FEM), in good agreement with experimental results, show that the geometry of the additive tool promotes greater plastic deformation and lower peak temperatures, confirming both the validity of the model and the suitability of L-PBF for the advanced design of FSW tools.

Abstract: The growing demand for advanced energy storage systems, power electronics, and electrical interconnections requires joining processes that ensure both electrical conductivity and mechanical stability. Applications such as battery cell-internal contacts, flexible busbar connections, and printed circuit board (PCB) conductor interfaces impose strict requirements on material combinations, layer thicknesses, and thermal management, which influence the suitability of the joining processes. Micro friction stir spot welding (µFSSW) offers advantages through its low thermal impact as well as its high process robustness and through the bonding of the joining partners in solid state. In the studies presented in this article, transferable simplified specimens were designed to systematically evaluate process parameters for different applications of µFSSW. The feasibility of a full contact using µFSSW was assessed through electrical resistance measurements, mechanical shear testing, and morphological examinations of the weld zone, as well as axial force measurements during the welding process. The results demonstrate the suitability of µFSSW for copper-based current collector foils in cell-internal contacts, busbar interconnections, and PCB junctions, and they highlight key relationships between the process conditions and the electrical, mechanical, and structural performance. The study also highlights challenges and opportunities for a future industrial implementation.

Abstract: Multi-material components that consist of copper and aluminum enable the combination of advantageous mechanical, thermal, and electrical properties at competitive cost. While roll bonding is an efficient-solid state joining technique, its implementation requires fully processed, cold-rolled strip material from two process routes. Continuous compound casting in contrast offers a more efficient approach by combining aluminum and copper during casting, followed by flat rolling in a single process route. However, the differences in flow stress between the metals can cause non-uniform elongation and therefore significant shear stresses at the interface during rolling. These stresses may lead to a delamination of the compound if process conditions are not well controlled. This study investigates whether a geometrically structured interface, introduced during compound casting, can contribute to withstanding interfacial shear stresses through mechanical interlocking. In finite element simulations varying process parameters including height reduction, initial temperature, rolling speed ratio, and pass schedule were examined. Results show that a structured interface can effectively resist shear stresses at the copper-aluminum boundary, thereby improving joint stability during deformation. Furthermore, the strain distribution as well as the fluctuation of the shear stresses can be controlled by the process parameters. The findings indicate that the mechanical interlocking by a geometric interface combined with optimized process parameters can enhance the rolling of compound-cast copper-aluminum composites.

Abstract: The production of Al–Cu bimetallic sheets is of increasing interest for applications requiring a combination of lightweight performance and high thermal or electrical conductivity. Conventional fusion welding techniques are unsuitable due to excessive intermetallic compound (IMC) formation and poor bonding quality. Solid-state processes such as Friction Stir Welding (FSW) provide an attractive alternative; however, most studies aim to minimize heat input in order to suppress IMCs. In this work, a different approach is proposed. A hybrid joining strategy is employed, intentionally using controlled heat input and tool penetration to generate an extended stirring zone and a pronounced hook geometry. This results in mechanical interlocking combined with metallurgical bonding at the Al–Cu interface. Large-area bimetallic sheets were fabricated by FSW lap welding using four parallel passes to enlarge the bonded region. Microstructural characterization revealed the formation of continuous hook structures along the interface, promoting effective mechanical interlocking between the aluminum and copper layers. The integrity of the bimetallic sheets was further evaluated by cold rolling, which demonstrated excellent resistance to delamination despite local cracking of brittle IMCs. The results confirm that exploiting hook formation, rather than suppressing it, can provide a robust and scalable strategy for manufacturing Al–Cu bimetallic sheets by FSW.

Abstract: Hybrid joints made of steel and aluminium alloy produced by rotary friction welding, enable load-adapted lightweight components. However, a major challenge is the inhomogeneous radial temperature distribution caused by different relative velocities between the specimen centre and edge during rotation. This effect leads to local insufficient bonding and reduces the overall joint strength, especially in the centre, where low relative rotation speeds occur. Previous studies mainly addressed preheating before the friction phase, whereas superimposed heating during the upsetting phase has not been investigated so far. To achieve temperature equalisation along the cross-section during rotary friction welding, a modified KUKA Genius plus machine equipped with joule heating was used to introduce an electric current during the upsetting phase. Experiments were conducted on EN AW-6082 (AA-6082) joined to 20MnCr5 (AISI 5120H). A three-step variation of current intensity (10, 24 and 36 A/mm2), alongside a reference without current, was investigated. Temperatures were monitored using type K thermocouples, confirming temperature equalisation. Mechanical performance was assessed by uniaxial tensile tests, while hardness measurements and metallographic analyses characterised the influence of superimposed heating on the interfacial microstructure. Joint strength improves up to 20% with increasing current, linked to a uniform temperature distribution and enhanced material flow, resulting in a defect-free specimen centre.

Abstract: Nowadays, the growing demand for sustainable solutions in manufacturing has shifted research attention toward innovative recycling strategies. Among these, the Solid-State Recycling (SSR) technique has emerged as a viable approach to transform metal swarf into new products. Within the SSR family, friction stir extrusion (FSE) has gained particular interest as a promising method for producing wires from metal scraps, but recently, it was also employed for tube manufacturing. In literature, tube production via chip recycling often involves multi-step approaches, first consolidating/homogenizing the recycled chips and then extruding. In other cases, the tubes are manufactured directly from a bulk material, losing the sustainable goal. For this reason, this study aims to propose a single-step process in which aluminium chips are directly turned into a consolidated tube without any intermediate step. In addition, specific attention was given to the study of tool geometry, aiming to investigate the effect of a tapered tool’s shape on the material flow and the overall process performance. Experimental tests were conducted to characterize the microstructure of extruded tubes and to calibrate numerical simulations employed for investigating process dynamics. Results revealed that the reduced contact diameter of the chamfered tool generated lower processing temperatures but higher strain levels, fundamentally shifting the bonding mechanism from thermal assistance to mechanical dominance in oxide film breakage. Microstructural analysis demonstrated that the flat tool, characterized by predominant frictional heating and lower deformation, produced larger grain diameters due to thermally induced coarsening. Conversely, the chamfered tool yielded significantly refined grain structures through severe plastic deformation and dynamic recrystallization under suppressed thermal conditions, indicating superior consolidation quality and enhanced particle bonding.