Solidification-joinability correlation of hypoeutectic aluminium casting alloys for self-piercing riveting (SPR)

Conventional mechanical joining processes are typically rigid in their tool systems and can only react to changing process and disturbance variables to a limited extent. At the same time, various industries are increasingly trending towards multi-material systems consisting of parts with varying geometric and mechanical properties. Due to the varying properties, rigid mechanical joining processes require sampling procedures and periodic changes of tool components or auxiliary joining parts. Consequently, research is focusing on versatile mechanical joining processes that allow increased control by modifying the process parameters. Two processes based on self-piercing riveting can achieve a significant increase in process influence possibilities through a multi-linear actuator as versatile self-piercing riveting (V-SPR) and a tumbling superimposed actuator as tumbling self-piercing riveting (T-SPR). Initial research into V-SPR has shown that this process can be used to achieve a higher variation in overall package thickness by adapting the rivet geometry and using multiple linear actuators. The T-SPR process also enables increased material flow control by means of targeted compression of the rivet using the tumbling actuator, thereby extending the range of joints that can be manufactured. Based on these two processes, a combination of the two mechanisms of action is to be developed.

Mechanical joining processes, in particular joining by forming like self-pierce riveting or clinching, require homogenous material characteristics for reliable joint properties. Joining processes in production are called safe, when the estimated properties for each manufactured joint, e.g. strength, remain in predefined narrow boundaries. Thus, an automated process monitoring is possible. A key reliability factor in automated production are varying joining materials properties. In case of steel, the knowledge of mutually dependent manufacturing steps at the coil build-up are crucial: First, milling conditions, such as dressing grade, predetermine the mechanical strength and fracture strain of the later joint. Second, the structure of corrosion protection layer by electrolytic or hot-dip galvanizing combined with forming and corrosion protection lubricants define the gliding abilities at later joining processes. It was found that a high dependency exists between solidification behaviour as well as surface friction level of the joining material and the stability of the joining process and the load capacity of the joint. To achieve safe mechanical joining results, these mentioned material properties must be guaranteed precisely by production monitoring techniques at the coil manufacturing. These include among others, methods for the non-destructive testing of mechanical properties at different coil areas, as well as systems for monitoring surface conditions and layers. As the material properties are build up over several production steps, production data are digitally collected today, analysed under quality aspects and if necessary corrective measurements are set online in the production.

Due to the increasing importance of reducing CO2 emissions, lightweight construction strategies are highly emphasized in the automotive industry. One of these strategies is lightweight moulding, which is used, for example, in the space frame design. Here, lightweight materials are joined with castings to form a structure that is well adapted to the application of force. Especially for multi-material joints, self-piercing riveting is frequently used as joining technology due to the wide range of joining possibilities and the high load-bearing capacities. However, the susceptibility of cast aluminium alloys to cracking due to the brittle microstructure causes great challenges for mechanical joining by self-piercing riveting. Therefore, this study investigates the suitability of the cast aluminium alloy AlSi9Mn for self-piercing riveting. Various improvement strategies are utilized to increase suitability considering of joining as well as material parameters. An evaluation of the joint and its damage characteristics is provided by metallographic analysis and industrial X-ray computed tomography (XCT). Metallographic analysis is used to determine the quality relevant characteristics of the joint, while XCT provides volumetric and non-destructive testing of the joint for internal cracks.

Self-piercing riveting (SPR) is a cold mechanical joining process used to join two or more sheets of materials by driving a rivet piercing through the top sheet or the top and middle sheets and subsequently lock into the bottom sheet under the guidance of a suitable die. SPR is currently the main joining method for aluminium and mixed-material lightweight automotive structures. SPR was originated half century ago, but it only had significant progress in the last 25 years due to the requirement of joining lightweight materials, such as aluminium alloy structures, aluminium-steel structures and other mixed-material structures, from the automotive industry. Compared with other conventional joining methods, SPR has many advantages including no pre-drilled holes required, no fume, no spark and low noise, no surface treatment required, ability to join multi-layer materials and mixed materials and ability to produce joints with high static and fatigue strengths. In this paper, research investigations that have been conducted on self-piercing riveting will be extensively reviewed. The current state and development of SPR process is reviewed and the influence of the key process parameters on joint quality is discussed. The mechanical properties of SPR joints, the corrosion behaviour of SPR joints, the distortion of SPR joints and the simulation of SPR process and joint performance are reviewed. Developing reliable simulation methods for SPR process and joint performance to reduce the need of physical testing has been identified as one of the main challenges.

Cast aluminum alloys are promising materials that can simplify the manufacturing process of automobile body structures. However, the low ductility of cast aluminum poses significant challenges to existing riveting technologies. In the present work, dissimilar AA6061-T6 aluminum alloy and Al–Si7Mg cast aluminum were joined by self-piercing riveting (SPR) and friction self-piercing riveting (F-SPR) processes to reveal the effect of friction heat on rivetability of low-ductility cast aluminum alloys. The joint macro-morphology, microstructure, peak tooling force, microhardness distribution, tensile-shear, and cross-tension performance of the two processes were comparatively studied. Results indicated that the in-situ softening effect of friction heat in the F-SPR process could effectively improve the ductility of cast aluminum, avoid cracking, and reduce the tooling force by 53%, compared to the SPR process. The severe plastic deformation and friction heat induced by rivet rotation results in refined equiaxed grains of aluminum near the rivets and solid-state bonding between aluminum sheets in the rivet cavity. The F-SPR joints are superior to SPR joints in both tensile-shear and cross-tension performance due to the avoidance of cracking, increase of mechanical interlocking, and solid-state bonding of interfaces. Significantly, when Al–Si7Mg is placed on the lower layer, the peak tensile-shear and cross-tension loads of the F-SPR joints are 7.2% and 45.5% higher than the corresponding SPR joints, respectively.

Self-pierce Riveting and Hybrid Joining of Boron Steels in Multi-material and Multi-sheet Joints

Mechanical joining of complex car body components is an essential part of lightweight construction concepts in the field of car body manufacturing. Besides the mechanical behavior of the joints, the influence on the dimensional accuracy is of particular interest, as joining techniques like clinching or self-piercing riveting cause distortion comparable to spot welding. In recent years, a lot of simplified models using the FE-Method to predict the distortion of assemblies caused by welding (weld seams, spot welds) were presented and commercialized. In contrast to thermal joining technologies, there are no such simplified models with practical relevance existing in the mechanical joining technology sector. In this paper, a new method to predict distortion, caused by different mechanical joining technologies, including effects from previous forming processes, and clamping conditions, is presented. The validation of the simplified model takes place due to an extensive design of expe-riments. It can be proved that the distortion of simple as well as of complex specimens can be relia-bly predicted.

Self pierce riveting (SPR) as mechanical joining is widely used for aluminum alloy, high strength steel or dissimilar materials such as polymer and metal in vehicle structures. The purpose of this study is to evaluate the tensile-shear strength of AM50 magnesium sheets jointed by a newly developed in-situ heating SPR joining system. It is difficult to joint magnesium sheets by riveting due to less ductility from a limitation of slip at room temperature, although there are few studies. In-situ heating SPR joining process has been developed to improve reliability of the joining sheet of magnesium, and to prevent occurrence of cracks which are initiated at beginning of riveting process. AM50 magnesium alloy sheet has 0.8 mm in thickness, and commercial aluminum rivet with 3.3 mm in diameter of the leg and 4 mm in height was used in this process. Cracks occurred in a lower sheet in both as-rolled and annealed AM50 sheet when conventional SPR joining process was applied at room temperature. No cracks are observed during in-situ heating SPR joining process at 250°C. The tensile-shear test was carried out at room temperature in order to estimate an effect of in-situ heating on reliability of the joint. The tensile-shear strength of AM50 joint heated at 250°C is higher than that of the joint from 5052 aluminum alloy with the conventional equipment.

Abstract. Mechanical joining processes have gained extensive importance in recent years. In contrast to temperature-based processes, mechanical joining approaches allow the joining of dissimilar materials such as steel and aluminum. What most of the processes have in common is that they are often designed for a specific combination of materials and sheet thicknesses. Consequently, the versatility of these processes is limited. This article will therefore provide an overview of four innovative manufacturing processes, which allow a higher adaptability than conventional mechanical joining processes. In this context, the basic strategies, their applications as well as their advantages and limitations are presented. The contribution closes with a summarizing evaluation and an outlook.

Organic sheets made of fiber-reinforced thermoplastics can make a crucial contribution to increase the lightweight potential of a technical design. They show high specific strength- and stiffness properties as well as good damping characteristics, while being able to show a higher energy absorption capacity than comparable metal constructions. In addition, organic sheets provide good recycling capabilities. Nowadays, multi-material designs are an established way in the automotive industry to combine the benefits of metal and fiber-reinforced plastics (FRP). Currently used technologies for the joining of organic sheets and metals in large-scale production are mechanical joining and adhesive technologies. Both require large overlapping areas to achieve the desired joint strength and stiffness of the technical design. Additionally, mechanical joining is usually combined with “fiber-destroying” pre-drilling and punching processes. This will disturb the force flux at the joint zone by causing unwanted fiber- and inter-fiber failure and inducing critical notch stresses. Therefore, the multi-material design with fiber-reinforced thermoplastics and metals needs optimized joining techniques that don’t interrupt the force flux, so that higher loads can be induced and the full benefit of the FRP material can be used. This article focuses on the characterization of a new joining technology, based on the Cold Metal Transfer (CMT) welding process, that allows to join organic sheets and metals in a load path optimized design. This is achieved by realigning the fibers around the joint zone by the integration of a thin metal pin. The alignment of the fibers will be similar to load paths of fibers inside structures found in nature. A tree with a knothole is always going to align its fibers in principle stress direction. As a result of the bionic fiber design, high joining strengths can be achieved. The increase of the joint strength compared to blind riveting was performed and proven with stainless steel and orthotropic reinforced composites in tensile shear-tests, based on the DIN EN ISO 14273.

Vehicle weight reduction has been discussed by automotive industry for decades. The current solution for weight reduction purpose is the use of aluminum stamping and casting parts to replace the steel parts. However, joining aluminum with aluminum or aluminum with steel parts would be difficult to select a proper joining technology to ensure a qualified joint. Self-pierce riveting (SPR) as a mechanical joining technique has been used by automotive industry owing to its simplicity, versatility, and environment friendly nature. Although SPR can avoid the joining quality problems caused by welding to join aluminum with steel parts, the other joining problems from SPR still hinder the industry to widely use this joining technology. This article analyzed the cause of cracks on joint button from the understanding of joined material. The modified heat treatments were used to investigate the relationship between eutectic silicon morphology and cracking condition on joint button. In addition, the lap-shear strength according to the samples with various heat treatments was also tested and analyzed. Eventually, the results indicated that silicon content and its morphology take main responsibility on crack initiation and propagation on SPR joint button. But the influence of cracks on joining strength still needs to be investigated and studied in the future work.

Due to economic and ecological requirements and the associated trend towards lightweight construction, mechanical joining technologies like self-piercing riveting are gaining in importance. In addition, the increase in lightweight multi-material joints has led to the development of many different mechanical joining technologies which can only be applied to join a small number of material combinations. This leads to low process efficiency, and in the case of self-piercing riveting, to a large number of required tool changes. Another approach focuses on reacting to changing boundary conditions as well as the creation of customised joints by using adaptive tools, versatile auxiliary joining parts or modified process kinematics. Therefore, this study investigates the influence of increased die-sided kinematics on joint formation in self-piercing riveting process. The aim is to achieve an improvement of the joint properties by superimposing the punch feed. Furthermore, it is intended to reduce required tool changes due to the improved joint design. The investigations were carried out by means of a 2D-axisymmetric numerical simulation model using the LS-Dyna simulation software. After the validation of the process model, the die was extended to include driven die elements. Using the model, different kinematics as well as their effects on the joint formation and the internal stress concentration could be analysed. In principle, the increased actuator technology enabled an increase of the interlock formation for both pure aluminium and multi-material joints consisting of steel and aluminium. However, the resulting process forces were higher during the process phases of punching and spreading.KeywordsSelf-piercing rivetingVersatilityDie-side kinematicFEM

Self-piercing riveting (SPR) is a mechanical joining technique widely used in the automotive industry to assemble similar or dissimilar sheet metals in two, three, or four layers without pre drilling. In this process, a rivet penetrates the upper sheet and flares into the lower sheet, creating a robust interlock. This study examines the effect of riveting speed on the mechanical performance of 1 mm thick Al 5182 sheets joined in double and triple sheet configurations. Specimens were produced at six riveting speeds (170, 190, 210, 230, 250, and 270 mm s −1 ) and tested in lap shear and T peel modes. Results show that increasing riveting speed markedly enhances joint strength: in lap shear tests, the maximum load for both double and triple sheet joints occurred at 270 mm s −1 , while in T peel tests of double sheet joints the peak strength was observed at 210 mm s −1 . Macro structural examination of the riveted zones confirms that higher speeds promote greater rivet flaring and sheet interlock, correlating with improved load capacity and more favorable failure modes. These findings underscore the importance of optimizing SPR parameters especially riveting speed to achieve reliable, high strength aluminum assemblies for lightweight automotive applications.

Innovative joining technology for the production of hybrid components from FRP and metals

In the context of electromobility, ensuring the leak tightness of assemblies is of paramount importance, particularly in battery housings. Current battery housings, often featuring base assemblies crafted from extruded aluminum profiles, address the challenge of leak tightness at joints through methods like friction stir welding, a process known for its time and cost intensiveness. The aim of this study is to develop and implement a new type of extruded profile concept to produce tight base assemblies for battery housings by a longitudinal mechanical single stroke joining process. The geometry, the process and the properties of the aluminum profiles are investigated to get a joint that meets the tightness requirements and achieve high load-bearing capacities in agreement with the high homologation requirements set to vehicles with high-voltage systems. The joint is formed by means of a single stage press stroke, which eliminates the need for complex tool designs that are necessary for continuous joining (roll joining). Flat steel contact surfaces are used as joining tools. To evaluate the joint quality, force curves from the joining process are analyzed and the resulting joint geometries are assessed using micrographs. The resulting leak tightness of the linear joints is measured by a helium sniffer leak detector and the load-bearing capacities are investigated by shear lap and bending tests and fatigue strength test. The study also explores whether a difference in strength between the two joining partners has a positive effect on the joint properties.