Mechanical Joining Processes Research Articles

Cross-process chain influence of process variations in self-piercing riveting with regard to semi-finished product dimensions

Semitubular self-piercing riveting is an important and well-established mechanical joining process. A new approach, which involves the use of high strain hardening rivet materials, has the potential advantage of increasing resource efficiency in rivet production by eliminating the normally necessary post-treatment of the rivets by means of heat treatment and coating. However, as the high strain hardening of the rivet materials leads to extraordinarily high tool loads during rivet forming, process fluctuations can be particularly critical. For example, fluctuations in the dimensions of the semi-finished products used can influence the forming process itself, but also the quality of the formed rivets, which in turn can affect the subsequent joining process and the joint formation. An increased reject rate or premature tool failure caused by this is detrimental to the resource efficiency of the entire process chain. Therefore, a fundamental knowledge of the cross-process chain cause-and-effect relationships in conjunction with fluctuating billet dimensions is needed and is acquired within this research work. The investigations show that the forming and the joining process as well as the manufactured components themselves are influenced by the billet dimensions. The forming forces and tool stresses rise with increasing billet volume. Tool failure due to excessive or uneven stress can be the result. As the billet volume increases, the head diameter and the head thickness of the rivets increase. Consequently, the joining process and the joint are also indirectly influenced by the billet volume. With increasing volume, the joining force rises and, due to the increase in the head diameter, so does the joint strength. Knowledge of the detailed relationships is therefore highly relevant for the successful use of high strain hardening rivet materials.

Influence of the process time on a self-piercing riveting process with tumbling kinematic

The increasing significance of ecological responsibility, stricter political regulations and economic objectives are driving innovation in research fields such as lightweight construction. One of the most important popular methods is the use of multi-material systems. Due to the different geometric and mechanical properties of the various materials used, resource efficient applications and utilizations are possible. Great challenges arise for the joining processes to realize these multi-material systems, since conventional joining processes reach their limits. In the field of mechanical joining processes, there are continuously new approaches, such as superimposing the punch in a self-piercing riveting process with a tumbling kinematic, to increase the number of adaptable process parameters and enhance the process control. Through various preliminary tests, a good understanding of the process has been developed, which allows to directly control the geometric joint parameters by configuring the tumbling strategy. A major challenge, particularly with regard to future industrial applications, is the process time, which is comparatively high due to the tumbling kinematics. In the investigations, a reduction of approximately 90% of the process time is targeted by adapting the joining and tumbling strategy. Therefore, the correlation of the traverse velocity and the tumbling velocity are examined in a gradual series of experiments. To represent realistic applications, the experiments are carried out with a dual-phase steel and a precipitation-hardening aluminum alloy. For identifying the influence of the process parameters on the joining process, a constant rivet–die combination is applied. Further, the examination of force–displacement curves is conducted. Moreover, the determination of geometric joint parameters is reliant upon macrographs to assess the influence of the joining time on the geometric joint formation. The test results show that a significant increase in joining speed with a resulting reduction in process time is feasible. Although the joining properties are affected, reliable joining is possible. In particular, the shaft thickness of the rivet is influenced by the varying proportion of the tumbling process in the joining operation and increases with higher joining speeds.

Mechanically Joined Extrusion Profiles for Battery Trays

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.

On the transferability of nominal surrogate models to uncertainty consideration of clinch joint characteristics

The mechanical joining process clinching is becoming increasingly important, especially for multi-material car body elements enabling lightweight design. However, the design of these joining points is usually based on expert knowledge and time-consuming experiments. For this reason, simulations are used in combination with surrogate models to predict the characteristics of clinch points for specific process parameters. Thereby, the process parameters are assumed to be ideal, so that inherent uncertainties are currently neglected. In order to overcome this deficiency and ensure a realistic prediction of clinch point characteristics, a consideration of variations is inevitable. Therefore, this paper analyses the transferability or significance of the nominally predicted values of existing surrogate models for clinch joints, taking into account process parameters with unavoidable variations. This shows, that the nominal parameter configuration affects the scattering of joint characteristics when considering variations. Moreover, implications for the necessary additional sampling in order to increase the significance of existing (nominal) surrogate model predictions are given, while considering the additional computational effort involved.

Double-Sided Self-Pierce Riveting: Electro-Mechanical Analysis of Dissimilar Al-Cu Half-Lap Butt Joints

Double-sided self-pierce riveting (DSSPR) can be utilized as an alternative to self-pierce riveting (SPR) to produce butt joints through half-lap joints in dissimilar materials. The mechanical joining process makes use of tubular rivets with simple geometry that are here employed to join two sheets made from aluminium (Al) and copper (Cu). This research work analyses the influence of the stainless-steel rivet on both the electrical and mechanical performance of the joint. The electrical resistance variation of the joined assembly is measured at different temperatures and compared with conventional fastened joints made from the same material combination. The mechanical performance of the aluminium–copper connections is evaluated by means of shear tests and compared to the original fastened Al-Cu joint. An experimental approach is utilized to analyse the combined influence of different mechanical and electrical parameters to assess the performance of DSSPR in electrical applications.

Self-Piercing rivet joining of multi-material including CFRP: Experiments and FE modeling

In this study, finite element (FE) analyses based on the arbitrary Lagrangian–Eulerian technique were used to investigate the complex deformation behavior and numerous process variables that arise during the self-piercing riveting (SPR) joining of multi-materials, including carbon-fiber-reinforced polymer, GA590DP, and Al5052-H32 sheets in two- and three-layer combinations. Tensile tests and inverse analysis were conducted to determine the mechanical responses of each joining material, including the rivet, and equivalent strain-based fracture modeling approaches were adopted for a simple but effective crack initiation prediction during the mechanical joining process. The damage distribution and joining characteristics during the SPR joining were then numerically investigated by considering different die profiles. The high predictability of the proposed FE modeling approach in terms of the cross-sectional profile with joint quality indicators, such as the interlock, residual distance between the rivet leg and die, minimum thickness of the bottom sheet, load history, and peak load during mechanical joining, was validated through comparisons between the experiments and FE simulations.

Shear testing of clinch joints at different temperatures: Explanation of the failure sequence

Clinching is a reliable, high-speed, cost-effective, and environmentally friendly mechanical joining process that enables sheet metals to be joined in large scale productions including whiteware, ventilation and air conditioning systems and automotive applications. Currently, the mechanical behavior and durability of clinch joints under different thermal conditions is poorly understood. This study evaluates the mechanical performance of clinched joints at room temperature, 100 °C and 250 °C experimentally using uniaxial shear tests. Results revealed that all clinched specimens had a similar shaped load displacement curve. Four stages were observed: an initial linear behavior with low sensitivity to temperature; a force plateau caused by necking (the force was only moderately reduced for higher temperatures); a phase where the neck began to crack, resulting in a letdown of the load which showed high thermal dependence, and a final phase of tearing completely through the neck, with joint separation. The ultimate tensile shear strength of the joints was relatively unchanged by increases in temperature. At higher temperatures the stiffness and energy absorbed by the joint decreases. This work shows the results of thermal shear testing for clinched joints, and describes the mechanisms whereby the joint progressively collapses under load.

Mechanical joining of high-strength multi-material systems − trends and innovations

In conjunction with mechanical joining processes. Mechanical joining processes play a key role for the realization of multi-material lightweight structures, which are essential with regard to environmental protection. However, joining of dissimilar high-strength materials is challenging due to the varying properties of the joining partners and due to their high flow stresses and often limited ductility. Thus, the evolution of established processes as well as the development of innovative and highly productive joining technologies are necessary. Requirements for a highly volatile production environment are versatility, flexibility, resilience and robustness. Within this contribution, current trends and innovations related to selected mechanical joining processes for enabling the material mix are outlined in order to point out opportunities to address these requirements in the future. In this context, joining using cold formed pin structures is presented as a promising approach for connecting dissimilar materials like metals to fibre-reinforced plastics. Furthermore, it is shown how the shear-clinching technology can be combined with a process-adapted application of locally limited heat treatment in order to promote the joinability and control the material flow during joining. A novel approach for reducing process forces and expanding process windows is the use of ultrasonic assistance for mechanical joining operations, which is demonstrated by the example of a nut staking process with superimposed high frequency oscillation. As concerns the widely used self-piercing riveting technique, current research activities relate not only to the further development of the joining process itself, for example by combining self-piercing riveting and tumbling, but also to the use of new rivet materials like high strain hardening stainless steels. In addition, the evolution towards mechanical joining 4.0 against the background of data-based process control in conjunction with of mechanical joining processes is also subject of the considerations.

Numerical Simulation of Mechanical Joining of Three DP600 and DC06 Steel Sheets

There are many reasons to utilize various grades of steel in car body production. Automotive producers tend to choose steels with great formability and the capacity to absorb impact energy. The dominant method used for joining car body sheets has for many years been resistance spot welding, but the use of various steel sheets leads to research into alternative joining methods. Mechanical joining - clinching, is the innovative method to join these materials. Numerical simulation tools are used to optimize the joining of materials. Simufact Forming software was used to analyse the clinching joining of three sheets of material DP600 and DC06. According to the axisymmetric character of the mechanical joining process, the simulation was stream-lined to a 2D representation. The results of the simulation of the mechanical joining process were compared with the real samples prepared for metallographic observation.

Study of Convolution Neural Network Based Deep Learning to Classify the Quality of Self-Piercing Riveting Joint

The SPR(Self-Piercing Riveting) process is a mechanical joining process that is mainly applied to assembling multimaterial parts to reduce the weight of the car body. Because the quality of SPR joints is mainly evaluated through cross sectional inspection, which is a type of destructive inspection, it is expensive and time-consuming. Machine learning technology is being proposed as a non-destructive testing because it can predict the quality based on the signals generated during the process. However, research result on the quality prediction modeling of SPR joints have not yet been reported. In this study, the prediction accuracy according to the signal combination was compared and evaluated by applying the CNN algorithm using the displacement and load signals generated during the SPR process and the acoustic signal obtained from the acoustic sensor. The materials used in the experiment were SGAFC 1180Y, CFRP, and SPFC 590 and the thickness were 1.4 mm, 1.8 mm, and 1.0 m respectively and a three-layer SPR process was performed. After evaluating joining was performed by selecting the abnormal process conditions, with 12 conditions that may occur during the process. Consequently, in the case of the first model in which the CNN algorithm was based on displacement and load signals, the quality prediction accuracy was estimated to be 90.0%. In the case of the second model in which the CNN algorithm added acoustic signals to the displacement and load signals, the quality prediction accuracy was estimated to be 77.5%.

Estimation of Clinch Joint Characteristics Based on Limited Input Data Using Pre-Trained Metamodels

Given strict emission targets and legal requirements, especially in the automotive industry, environmentally friendly and simultaneously versatile applicable production technologies are gaining importance. In this regard, the use of mechanical joining processes, such as clinching, enable assembly sheet metals to achieve strength properties similar to those of established thermal joining technologies. However, to guarantee a high reliability of the generated joint connection, the selection of a best-fitting joining technology as well as the meaningful description of individual joint properties is essential. In the context of clinching, few contributions have to date investigated the metamodel-based estimation and optimization of joint characteristics, such as neck or interlock thickness, by applying machine learning and genetic algorithms. Therefore, several regression models have been trained on varying databases and amounts of input parameters. However, if product engineers can only provide limited data for a new joining task, such as incomplete information on applied joining tool dimensions, previously trained metamodels often reach their limits. This often results in a significant loss of prediction quality and leads to increasing uncertainties and inaccuracies within the metamodel-based design of a clinch joint connection. Motivated by this, the presented contribution investigates different machine learning algorithms regarding their ability to achieve a satisfying estimation accuracy on limited input data applying a statistically based feature selection method. Through this, it is possible to identify which regression models are suitable to predict clinch joint characteristics considering only a minimum set of required input features. Thus, in addition to the opportunity to decrease the training effort as well as the model complexity, the subsequent formulation of design equations can pave the way to a more versatile application and reuse of pretrained metamodels on varying tool configurations for a given clinch joining task.

Robust estimation of clinch joint characteristics based on data-driven methods

Given a steadily increasing demand on multi-material lightweight designs, fast and cost-efficient production technologies, such as the mechanical joining process clinching, are becoming more and more relevant for series production. Since the application of such joining techniques often base on the ability to reach similar or even better joint loading capacities compared to established joining processes (e.g., spot welding), few contributions investigated the systematic improvement of clinch joint characteristics. In this regard, the use of data-driven methods in combination with optimization algorithms showed already high potentials for the analysis of individual joints and the definition of optimal tool configurations. However, the often missing consideration of uncertainties, such as varying material properties, and the related calculation of their impact on clinch joint properties can lead to poor estimation results and thus to a decreased reliability of the entire joint connection. This can cause major challenges, especially for the design and dimensioning of safety-relevant components, such as in car bodies. Motivated by this, the presented contribution introduces a novel method for the robust estimation of clinch joint characteristics including uncertainties of varying and versatile process chains in mechanical joining. Therefore, the utilization of Gaussian process regression models is demonstrated and evaluated regarding the ability to achieve sufficient prediction qualities.

Geometric and mechanical joint characterization of conventionally and tumbled self-piercing riveting joints

In view of economic and ecological trends, the concepts for lightweight construction in transport systems are becoming increasingly important. These are frequently applied in the form of multi-material systems, which are characterized by the selective use of materials and geometries. One major challenge in the manufacturing of multi-material systems is the joining of the individual components to form a complete system. Mechanical joining processes such as semi-tubular self-piercing riveting are frequently used for this application but reach their limits concerning the number of combinations of geometry and material. In order to react to the requirements and to increase the versatility of semi-tubular self-pierce riveting, a process combination consisting of a tumbling process and a self-pierce riveting process has been presented previously. This process combination is used in this work to investigate the versatility and to identify the influencing parameters on it. For this purpose, experiments are conducted to identify process-side influence possibilities. The tests are performed with a dual-phase steel aluminum alloy to represent the varying mechanical characteristics of multi-material systems. Furthermore, the initial sheet thicknesses of the joining partners are varied in several steps. In addition to the geometric joint formation used to describe the undercut, the rivet head end position and the residual sheet thickness, the joining process, is also analyzed during the investigations. Further, the innovative joining process is evaluated by comparing it with a conventional self-piercing riveting process. The knowledge obtained represents a basis for the identification and evaluation of the versatility of the process combination.

Joining of Metal–Plastic Composite Layered Tubes by Hydroformed Threaded Coupling

Abstract Metal–plastic composite tubular structures that combine the high strength and stiffness of metallic tubes with the lightweight and flexible properties of plastic tubes, exhibit considerable potential to provide lightweight structures with improved performance compared to conventional monolithic tubular parts. However, the lack of well-confirmed technologies for connecting metal–plastic composite tubes that can ensure good structural stability and strength has proven problematic in actual applications. In this study, an innovative mechanical joining process via a hydroformed threaded coupling is proposed to achieve the successful joining of a metal–plastic composite layered tube (CLT). For the threaded connection, the CLT with an internal thread to act as a sleeve and the coupling tube with an external thread to act as a screw were joined together. A three-layer CLT composed of AISI 304 stainless steel/polyvinyl chloride (PVC)/AISI 304 stainless steel was investigated by free bulging and thread hydroforming. For successful forming into a threaded CLT, the optimal loading path was designed using analytical forming pressure and numerical contracting stroke. The mechanical tests based on compression and lateral three-point bending tests provided several key indicators for the quantitative performance of the thread-coupled parts. The reliability and applicability of the threaded coupling confirmed that the proposed joining process has strong potential for successful utilization in connecting metal–plastic composite tubes for structural applications.

Further Development of an Adaptive Joining Technique Based on Friction Spinning to Produce Pre-Hole-Free Joints

Mechanical joining processes are an essential part of modern lightweight construction. They permit materials of different types to be joined in a way that is suitable for the loads involved. These processes reach their limits, however, as soon as the boundary conditions change. In most cases, these elements are specially adapted to the joining point and cannot be used universally. Changes require cost-intensive adaptation of both the element and the process control, thus making production more complex. This results in high costs due to the increased number of auxiliary joining element variants required and reduces the economic efficiency of mechanical joining. One approach to overcoming this issue is the use of adaptive auxiliary joining elements formed by friction spinning. This article presents the current state of research on pre-hole-free joining with adaptive joining elements. The overall process chain is illustrated, explained and analyzed. Special attention is paid to demonstrating the feasibility of pre-hole-free joining with adaptive joining elements. The chosen mechanical parameters are subsequently listed. Finally, a comprehensive outlook of the future development potential is derived.

Data-Based and Analytical Models for Strength Prediction of Mechanical Joints

Currently the design of mechanical joining processes like self-pierce riveting with semi-tubular rivet (SPR-ST) and clinching for production is subject to complex and experimental test series in which process parameters such as the rivet or die geometry are varied iteratively and based on experience until a suitable joint contour and strength is achieved. To simplify the use of mechanical joining technologies these development cycles and thereby the effort for implementation into production must be reduced. In this paper, the numerical data-acquisition and the development of algorithm-based and analytical models for strength prediction for SPR-ST and Clinching is described. Therefore, an extensive experimental and numerical database regarding the SPR-ST process and strength of steel and aluminum joints with tensile strengths of the sheets between 200 - 1000 MPa was generated for the building of the models. This process data could then be used for the training and evaluation of different prediction models. The goal of the research presented here is to enable an immediate prediction of the quasi-static joint strength, based on the input parameters such as properties of the parts to be joined and process parameters.

Corrosion Phenomena and Fatigue Behavior of Clinched Joints: Numerical and Experimental Investigations

Lightweight construction has increasingly become the focus of scientific research in recent years, not least due to the constantly increasing fuel price, which is a key factor in the economic viability of many companies. In this respect, the use of hybrid structures, made of dissimilar materials offers many advantages. However, such hybrid structures often have undesirable side effects. For example, brittle intermetallic phases are formed when aluminum and steel are welded. Clinching as a mechanical joining process does not produce such intermetallic phases since the connection is realized through form and force closure. In this process, a punch passes through two or more sheets and forms them into a permanent joint in a die. In the present work, the corrosion phenomena of an aluminum-steel clinched joint have been investigated by both experiments and numerical simulations in order to explain the superior fatigue behavior of pre-corroded joints. Therefore, the clinched joints have been corroded by a three-week salt-spray test. In addition, the electric potential and the von Mises stress are calculated under the assumption of a static loading. The results of both experiments and numerical simulations can explain the improvement in the fatigue behavior of the corroded specimens. This phenomenon can be attributed to the accumulation of corrosion products in small gaps between the joined metal sheets.

Injection Lap Riveting of Aluminum Busbars—A Thermo-Electro-Mechanical Investigation

This paper presents a new mechanical joining process to assemble aluminum busbars in energy distribution systems. The process is based on the extension of injection lap riveting to the connection of busbars made from the same material as the rivets and requires redesigning the joints to ensure complete filling with good mechanical interlocking and appropriate contact pressures on the overlapping area. The experimental work was carried out in unit cells and involved the fabrication of the riveted joints and the evaluation of their electrical resistance at different service temperatures. Comparisons with the bolted joints that were fabricated and tested for reference purposes show that injection riveted joints provide lower values of electrical resistance and require much less space for assembly due to the absence of material protrusions above and below their surfaces. Numerical simulation with finite elements allows the relating of the reduction in electrical resistance with the changes in the electric current flow when the bolts are replaced by the new type of rivets. The experimental and numerical predictions revealed that the new type of rivets experience an increase in electrical resistance of up to 6 μΩ (30%) when the service temperature approaches 105 °C. Still, the resistance at this temperature (26.2 μΩ) is more than 3 times smaller than that of the bolted joints (80.5 μΩ).

Research advances in the mechanical joining process for fiber reinforced plastic composites

Fiber reinforced plastic (FRP) composites have received extensive attention in the auto industry and aviation field. For realizing the mechanical joining of fiber reinforced plastic composite, much research has been conducted. In this article, the latest development of the mechanical joining process for fiber reinforced plastic composites was reviewed. The application of numerical analysis methods in the mechanical joining process was also briefly described. Nine diverse mechanical joining processes were discussed, including bolted joining, hole-clinched joining, mechanical clinching, self-piercing riveting, electromagnetic riveting, friction riveting, injection clinching joining, Z-pin joining, and hybrid bond-bolted/riveted joining. The superiorities and drawbacks of diverse mechanical joining techniques were completely summarized. Furthermore, several future research trends for FRP composites were presented, such as establishing a comprehensive numerical analysis model of FRP composites, developing new hybrid joining processes, and developing non-destructive testing technique for defect visualization.

Numerical investigation of a friction test to determine the friction coefficients for the clinching process

Clinching as a mechanical joining process has become established in many areas of car body. In order to predict relevant properties of clinched joints and to ensure the reliability of the process, it is numerically simulated during the product development process. The prediction accuracy of the simulated process depends on the implemented friction model. Therefore, a new method for determining friction coefficients in sheet metal materials was developed and tested. The aim of this study is the numerical investigation of this experimental method by means of FE simulation. The experimental setup is modelled in a 3D numerical simulation taking into account the process parameters varying in the experiment, such as geometric properties, contact pressure and contact velocity. Furthermore, the contact description of the model is calibrated via the experimentally determined friction coefficients according to clinch-relevant parameter space. It is shown that the assumptions made in the determination of the experimental data in preliminary work are valid. In addition, it is investigated to what extent the standard Coulomb friction model in the FEM can reproduce the results of the experimental method.