Advances in joining technologies for the innovation of 21st century lightweight aluminium-CFRP hybrid structures

In the twenty-first century, the application of carbon fiber reinforced polymer (CFRP) materials in the vehicle industry are growing rapidly due to lightweight, high specific strength, and elasticity. In the automobile and aerospace industries, CFRP needs to be joined with metals to build complete structures. The demand for hybrid structures has prompted research into the combination of CFRP and metals in manufacturing. Aluminium and CFRP structures combine the mechanical properties of aluminium with the superior physical and chemical properties of CFRP. However, joining dissimilar materials is often challenging to achieve. Various joining technologies are developed to produce hybrid joints of CFRP, and aluminium alloys include conventional adhesives, mechanical and thermal joining technologies. In this review article, an extensive review was carried out on the thermal joining technologies include laser welding, friction-based welding technologies, ultrasonic welding, and induction welding processes. The article primarily focused on the current knowledge and process development of these technologies in fabricating dissimilar aluminium and CFRP structures. Besides, according to Industry 4.0 requirements, additive manufacturing-based techniques to fabricate hybrid structures are presented. Finally, this article also addressed the various improvements for the future development of these joining technologies. Ultrasonic welding yields the maximum shear strength among the various hybrid joining technologies due to lower heat input. On the other hand, laser welding produces higher heat input, which deteriorates the mechanical performance of the hybrid joints. Surface pretreatments on material surfaces prior to joining showed a significant effect on joint shear strength. Surface modification using anodizing is considered an optimal method to improve wettability, increasing mechanical interlocking phenomena.

Development of a new joining technology for hybrid joints of sheet metal and continuous fiber-reinforced thermoplastics

Punctiform mechanical joining technologies, such as riveting, clinching, or screwing, which are widely used in sheet metal processing, are frequently applied because they have been established for many years. Depending on the process, they offer a variety of advantages such as one-sided accessibility, re-detachability, and no need for pre-punching operations or auxiliary joining elements. In addition, the processes often guarantee a high process reliability and extensive process monitoring. However, with thermoplastic composites, they lead to considerable stress concentrations at the joint due to the fibers. Undesirable fiber and inter-fiber breaks then result. With the development of the novel joining technology of joint stamp riveting, an improvement is achieved in this situation that has been described for hybrid joints on components made of thermoplastic composites and metal sheets. The joining principle is based on the formation of a form lock between the joining partners. The thermoplastic composite is thermomechanically formed by means of a joint stamp without using an auxiliary joining element. Within the scope of a research project, the joining process was characterized with regard to the structure of the joining spot, the geometry of the forming tools, and also the mechanical properties for purposes of analyzing and designing the joining process.

Assessment of dissimilar joining between metal and polymer hybrid structure with different joining processes

In this century, innovation and technology are required to fabricate the hybrid joint of metal and polymers. Due to their lightweight and anti-corrosion properties, the mixed components are increasingly used to produce lightweight hybrid structures such as aerospace and automobiles. It is essential to develop welding techniques for joining dissimilar materials and instead use them in engineering structures. The bonding mechanism of the weld joint has varied depending upon the welding process. In the present review, the bonding mechanism of various hybrid joints like Friction stir welding (FSW), Friction stir spot joining, Friction riveting, laser welding, ultrasonic welding and induction welding is discussed in detail. The defects observed in the different welding process is discussed in details. The mechanical properties and microstructure analysis of different hybrid joints are reviewed in detail for a different combination of hybrid joints.

Failure Mechanisms of Cu–Cu Bumps under Thermal Cycling

The failure mechanisms of Cu–Cu bumps under thermal cycling test (TCT) were investigated. The resistance change of Cu–Cu bumps in chip corners was less than 20% after 1000 thermal cycles. Many cracks were found at the center of the bonding interface, assumed to be a result of weak grain boundaries. Finite element analysis (FEA) was performed to simulate the stress distribution under thermal cycling. The results show that the maximum stress was located close to the Cu redistribution lines (RDLs). With the TiW adhesion layer between the Cu–Cu bumps and RDLs, the bonding strength was strong enough to sustain the thermal stress. Additionally, the middle of the Cu–Cu bumps was subjected to tension. Some triple junctions with zig-zag grain boundaries after TCT were observed. From the pre-existing tiny voids at the bonding interface, cracks might initiate and propagate along the weak bonding interface. In order to avoid such failures, a postannealing bonding process was adopted to completely eliminate the bonding interface of Cu–Cu bumps. This study delivers a deep understanding of the thermal cycling reliability of Cu–Cu hybrid joints.

Interface Formation and Bonding Mechanisms of Laser Welding of PMMA Plastic and 304 Austenitic Stainless Steel

Laser welding experiments involving amorphous thermoplastic polymer (PMMA) and 304 austenitic stainless steel plates were conducted to explore the influence of laser welding process parameters on plastic–metal joints. A high-speed camera was applied to record the dynamics of the molten pool and the formation of bubbles to reveal the bonding mechanisms of the hybrid joints. The influence of process parameters on the joints was analyzed using temperature measurements performed with thermocouples. The microstructure morphology of joints was observed using SEM. The mechanical characterization of the hybrid joints was carried out to understand the effect of the welding conditions on the weld morphology, flaws and shear stress. Different interface temperatures resulted in two types of bubbles and led to different weld morphology characteristics. A stable hybrid joint with the best shear stress was produced with a laser line energy of 20.16 J/mm2, a temperature of 305 °C and small bubbles. The shear stress of the effective joint under the maximum mechanical resistance was 4.17 MPa. The chemical bonds (M-O, M-C) and mechanical anchoring that formed on the steel’s surface contributed to the joint bonding. Range analysis provided guidance for identifying the impact of individual factors in the shear stress for the laser welding of plastic–metal.

Innovative Carpentry and Hybrid Joints in Contemporary Wooden Architecture

Timber frame structures have a long and rich tradition. In addition to their functional and structural value, they are important elements of the cultural landscape. At the turn of the 21st century, concern for nature, resulting from the threat of environmental degradation, contributed to a growing interest in wooden constructions. For these reasons, we have observed the erection of buildings with wooden frame structures in many countries around the world. This trend contributed to the rapid development of wooden structures, new technologies, and innovative architectural solutions. The conducted research mainly focused on the joints used in their construction, as well as their perception. From among many examples, some original and innovative solutions were selected and analyzed. Their creators are famous architects: Renzo Piano, Imre Makovecz, Jürgen Meyer, Kengo Kuma, and Shigeru Ban. The objects presented in this article are distinguished by the originality of their form and by the fact that they are clearly inspired by vernacular architecture. Crucial elements of these objects, such as wooden, steel, and hybrid connections, are analyzed in this article. Because they are intentionally exposed, they play an important aesthetic role in addition to a structural one.