Mechanical Joining Technologies Research Articles

Influence of process variations on clinch joint characteristics considering the effect of the nominal tool design

Focusing on upcoming challenges in lightweight design, such as increasing emission targets or novel multimaterial connections, versatile applicable and environmentally friendly production technologies are crucial. In this context, mechanical joining technology clinching offers a fast and energy-efficient procedure for assembling sheet metals, being a proper alternative to established joining methods, such as spot welding. However, the design of clinch points is a challenge, which is partly supported by numerical or data-based approaches for optimal tool dimensions assuring proper joint characteristics. While this is usually done for an ideal environment, real joining processes are characterized by multiple inevitably varying parameters, e.g. of the material, which have a significant impact on the quality of clinch points. Therefore, this contribution addresses the current gap by analyzing the effect of parameter variations or uncertainties on the resulting joint characteristics and studying the impact of the nominal tool design. Thus, an efficient meta-model-based variation simulation procedure is proposed and used for analyzing the effect of different tool design configurations and variation scenarios. Based on the results, it was found that varying process parameters have a strong impact on the resulting joint characteristics, whereby the effect significantly depends on the nominal tool design. This reveals the potential for a robust tool design and implies that the nominal tool design and the tolerancing of parameters should be done simultaneously for a reliable virtual joining point design without extensive iterations and physical tests.

In-situ computed tomography and transient dynamic analysis – failure analysis of a single-lap tensile-shear test with clinch points

Clinching is a mechanical joining technology, in which a mainly form-fit joint is created by means of local cold forming. To characterize the load-bearing behavior of such joints, they are typically analyzed destructively, for example by tensile-shear tests in combination with metallographic sections. However, both the initiation and progress of failure can only be described to a limited extent by this method. Furthermore, these tests allow only limited conclusions about clinch points under in-service loading. More purposefully, clinch points can be analyzed nondestructively by combining in-situ computed tomography (CT) and transient dynamic analysis (TDA). The TDA continuously measures the dynamic behavior of the specimen and indicates failure events like crack initiation, which then can be evaluated thoroughly by stopping the test and performing a CT scan. To qualify the TDA for this task, it is necessary to link the observed damage behavior with specific dynamic characteristics. In this work, the complementation of in-situ CT and TDA is investigated by testing a clinched single-lap tensile-shear specimen made of aluminum. The testing procedure is stepwise: at certain displacement levels, the specimen is investigated by in-situ CT and TDA. While the in-situ CT provides the location, extent, and development of the failure phenomena, the TDA uses this information to evaluate the dynamic signal and detect relevant frequency ranges, which indicate damage events. The results demonstrate, that failure initiation and progression can be analyzed efficiently by combining both measuring systems. The TDA reliably detects relevant signal changes in the monitored frequency band. By means of in-situ computed tomography, the corresponding failure phenomena can be described in detail, enhancing the understanding of the load-bearing and deformation behavior of clinch points. The concatenation of characteristic signal changes and observed failure phenomena can henceforth be transferred to analyze complex structures during operation nondestructively by TDA.

Influence of local heat treatment of rivets on the joint formation of a versatile joining process

An efficient lightweight construction method is the combination of different materials in order to adapt the structure to the applied load. To join these multi-material structures mechanical joining technologies are applied. However, the rigid tooling systems cannot be adjusted to changing boundary conditions which is why new, versatile joining technologies are required. In the versatile self-piercing riveting (V-SPR) process presented in [1] different material combination are joined by using a multi-range capable rivet. The rivet head is formed onto the respective thickness of the joint by an outer punch. In order to punch thru the upper sheet a great rivet hardness is required whereas a lower hardness is required for the subsequent forming of the rivet head. To achieve a combination of these requirements, this study investigates a local heat treatment of the rivet. The aim is to determine the feasibility of such a heat treatment as well as to investigate the influence on the joint formation.

Investigations to improve the tool life during thermomechanical and incremental forming of steel auxiliary joining elements

Modern vehicles as well as structures consist more and more often of a multi material mix. In addition to the use of light materials (e. g. aluminium) to optimise lightweight construction potential, higher strength alloys are being used more and more frequently. With the different material combinations, the requirements for the joining technology also increase. Thus (due to restrictions in the area of thermal joining processes) the joining of different material combinations is often realised by mechanical joining technology. Even though different joining tasks (dissimilar material or sheet thicknesses) can be solved by mechanical joining technology in general, it is often required to switch between appropriate auxiliary joining elements and even joining technique. To adapt the auxiliary elements to the individual requirements of each joint an innovative manufacturing process of customisable auxiliary joining elements has been developed. The use of the incremental, thermomechanical manufacturing process called friction-spinning for the manufacturing of adaptive auxiliary joining elements enables to individualise each element for the subsequent joining process. However, this advantage comes at the cost of a high mechanical and thermal loads on the forming tools. These loads also depend on the process design (trajectory, feed, rotational speed) and have to be controlled in order to improve the tool life. For a more profound process knowledge and to better understand the corellation of the individual process variables, a series of tests with varying process parameters were carried out in a partial factorial design of experiments. The results show not only an possible improvement in the tool life, but also the required process design for a higher reproducibility of the auxiliary joining element geometry and the possibility of positively influencing the geometry to achieve a force-closure of the joint.

A punching process to join metal sheets and fibre reinforced polymer composites by mechanical interlocking

In the multi-material lightweight design of structural components for the automotive industry, the joint between different materials plays a significant role in reducing vehicle weight without compromising performance or safety. Conventional technologies to mechanically join metals and carbon fibre reinforced polymers result in either drilling a hole in the composite material or increasing the weight of the part because of the fasteners employed. This work presents a new, simple, cost-efficient and non-weight penalizing mechanical joining technology for metal sheets and fibre reinforced polymer prepregs. It consists of a single-step punching process where the metallic sheet is completely perforated, but the prepreg is not. The punch pushes the carbon fibres through the hole in the metal sheet with no or minimal fibre breakage, generating a mechanical interlock which, in turn, increases the shear strength and absorbed energy of the co-cured joint.

A mechanical interlocking joint between sheet metal and carbon fibre reinforced polymers through punching

The joint between different lightweight materials plays a significant role in multi-material design of structural components for the automotive industry, aiming to reduce the vehicle’s weight without compromising performance or safety. Yet, conventional mechanical joining technologies between metals and Carbon Fibre Reinforced Polymers (CFRP) result in either a hole being drilled in the composite material, leading to damages which reduce the load bearing capacity, or the weight of the part being increased due to the incorporation of fasteners. At the same time, alternative mechanical joining methodologies involve complex and costly processing, hindering their industrial application. This work presents a new, simple, cost-efficient and non-weight penalizing mechanical joining technology between a metal sheet and fibre reinforced polymer prepregs consisting of a single-step punching process. In this process, the metallic sheet is completely perforated, while the prepreg is not. The punch pushes the carbon fibres through the metallic hole, with no, or minimal fibre breakage, generating a mechanical interlock. The shear strength and the absorbed energy of the co-cured joint increase with the incorporation of the mechanical interlocking joint.

Development of a numerical simulation model for self-piercing riveting of additive manufactured AlSi10Mg

Laser additive manufacturing processes are used for the production of highly complex geometric structures due to their high geometric freedom. Additive manufacturing processes, in particular powder-based selective laser melting, are used to produce metallic additive manufactured components for the automotive and aerospace industries. Different materials are often joined together to realize sustainable lightweight construction. The production of such mixed construction joints is often realized using mechanical joining technology (e.g. self-piercing riveting). However, there is currently very little experience with the mechanical joining of metallic additive manufacturing components. Furthermore, there is insufficient knowledge about the effects that occur during the mechanical joining of additive manufacturing components. In this article, a method is presented to investigate the joinability of additively manufactured components with conventionally manufactured components using a numerical simulation of the self-piercing riveting process. For this purpose, the additive manufacturing materials are characterized experimentally, the simulation model is configured, and the joining process with additive manufacturing materials is represented in the numerical simulation. Furthermore, the influence of the building direction on the mechanical properties is shown using miniature tensile specimens. Besides the configuration of the simulation model, the influence of heat treatment on the self-piercing riveting process is presented.

Experimental and numerical investigation of the influence of multiaxial loading conditions on the failure behavior of clinched joints

Destructive testing methods are used to determine the joint characteristics of mechanical joining technologies by testing the joints under different types of loads. In real load cases, however, combined loading conditions arise in complex structures that cannot be investigated experimentally at present. The objective of this study is to adapt a so-called L-specimen geometry for mechanical joints to introduce combined peel and shear tensile loading, taking into account plastic pre-deformation of the joined parts. A new specimen geometry is presented to investigate three different clinched joints using different materials, namely HCT590X as well as EN AW-6014 T4. The load capacity behavior, as well as the failure behavior, is analyzed based on failure modes for pure shear, pure peel, and combined loading. Furthermore, the L-specimen is investigated by numerical simulation and compared with the experimental results.

Determining the influence of different process parameters on the versatile self-piercing riveting process using numerical methods

Owing to the increasing use of multi-material construction and the resulting material incompatibilities, mechanical joining technologies are gaining importance. The reasons for this are the variety of joining possibilities as well as the high joint load-bearing capacities. However, the currently rigid tool systems cannot react flexibly to changing process conditions, such as varying sheet thicknesses or strengths as they require a change in the tool configuration. Therefore, a novel, versatile self-piercing riveting process (V-SPR) is proposed to improve the applicability of joining several sheet material combinations by considering only one rivet-die combination. Although previous studies have demonstrated the potential of V-SPR for joining equal materials, the present understanding of the process lacks detailed investigations regarding optimal tool configurations and sheet-joining tasks, such as varying the total and individual blank thicknesses. In this regard, a systematic parameter study in combination with statistical analyses is conducted to determine the applicability and feasibility of V-SPR using EN AW-6014 as an example. The process recommendations and limitations discussed in this paper pave the way for meaningful use of novel rivet geometry in versatile process chains.

Mechanical Properties and Joinability of AlSi9 Alloy Manufactured by Twin‐Roll Casting

AlSi casting alloys combine excellent castability with high strength. Hence, this group of alloys is often used in the automotive sector. The challenge for this application is the brittle character of these alloys which leads to cracks during joint formation when mechanical joining technologies are used. A rise in ductility can be achieved by a considerable increase in the solidification rate which results in grain refinement. High solidification rates can be realized in twin–roll casting (TRC) by water‐cooled rolls. Therefore, a hypoeutectic EN AC–AlSi9 (for European Norm ‐ aluminum cast product) is manufactured by the TRC process and analyzed. Subsequently, joining investigations are performed on castings in as‐cast and heat‐treated condition using the self‐piercing riveting process considering the joint formation and the load‐bearing capacity. Due to the fine microstructure, the crack initiation can be avoided during joining, while maintaining the joining parameters, especially by specimens in heat treatment conditions. Furthermore, due to the extremely fine microstructure, the load‐bearing capacity of the joint can be significantly increased in terms of the maximum load‐bearing force and the energy absorbed.

Joining of multi-material structures using a versatile self-piercing riveting process

Due to the increasing use of multi-material constructions and the resulting material incompatibilities, mechanical joining technologies are gaining in importance. The reasons for this are the variety of joining possibilities as well as high load-bearing capacities. However, the currently rigid tooling systems cannot react to changing boundary conditions, such as changed sheet thicknesses or strength. For this reason, a large number of specialised joining processes have been developed to expand the range of applications. Using a versatile self-piercing riveting process, multi-material structures are joined in this paper. In this process, a modified tool actuator technology is combined with multi-range capable auxiliary joining parts. The multi-range capability of the rivets is achieved by forming the rivet head onto the respective thickness of the joining part combination without creating a tooling set-up effort. The joints are investigated both experimentally on the basis of joint formation and load-bearing capacity tests as well as by means of numerical simulation. It turned out that all the joints examined could be manufactured according to the defined standards. The load-bearing capacities of the joints are comparable to those of conventionally joined joints. In some cases the joint fails prematurely, which is why lower energy absorptions are obtained. However, the maximum forces achieved are higher than those of conventional joints. Especially in the case of high-strength materials arranged on the die side, the interlock formation is low. In addition, the use of die-sided sheets requires a large deformation of the rivet head protrusion, which leads to an increase in stress and, as a result, to damage if the rivet head. However, a negative influence on the joint load-bearing capacity could be excluded.

Realtime Prediction of Self-Pierce Riveting Joints - Prognosis and Visualization Based on Simulation and Machine Learning

Machine learning is used in many fields nowadays to predict events, be it a pure classification or the prediction of certain values. Thus, these methods are also increasingly used in mechanical joining technology, for example for the prediction of joint strengths, in the classification of defects and rivet head positions or in the prediction of discrete result values such as interlock. This paper further shows how the complete joint contour including the output of stresses, strains and damage can be predicted and visualized in real time for self-piercing riveting with semi-tubular rivet. First, classical sampling is carried out in experiments with steel and aluminum sheets of different types and thicknesses. These are used as a basis for the qualification of the numerical simulations. For this validation experiments and simulations are compared via joint contour and force curves. For the simulations validated in such way several tool variants are carried out in variation calculations for each material-thickness combination. The simulation meshes of the thus generated database are standardized with respect to comparability (same number of nodes) and a data reduction is performed. After testing different approximation approaches, the best possible results are predicted and can be visualized in the developed software demonstrator.

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.

Development of reduced activation ferritic/martensitic steels in China

Reduced-activation ferritic-martensitic (RAFM) steel is considered as the primary candidate structural material for blankets of ITER, CFETR and DEMO. Several RAFMs have been developed in China (CN-RAFMs) such as CLAM, CLF-1, modified RAFMs and oxide dispersion strengthened RAFMs (ODS-RAFMs). The mechanical properties, irradiation behaviors, additive manufacturing and joining technologies of structural materials have been comprehensively studied. Qualifications of CN-RAFMs are on-going for ITER-TBM fabrication, with emphasis on material property databases and standardization. A new neutron irradiation program is being carried out for a series of fusion materials e.g. RAFMs and ODS-RAFMs with doses up to 40 dpa. Joining technologies (e.g. electron beam welding, tungsten inert gas welding, laser beam welding and diffusion welding, etc.) and advanced additive manufacturing are performed to overcome the big challenges of the blanket fabrication before ITER, CFETR and DEMO. The databases and standards for CLAM and CLF-1 were established respectively, and the specification and qualification of them are underway for ITER-TBM.In this paper, the latest development and strategy on fusion energy in China are briefly introduced, and the progresses of R&D activities on CN-RAFMs aiming for the engineering applications are reviewed.

Development of a rivet geometry for solid self-piercing riveting of thermally loaded CFRP-metal joints in automotive construction

Due to excellent mechanical properties, carbon fibre-reinforced plastics (CFRP) are often used for lightweight structures in the mobility sector and combined to sheet metals in order to reduce the weight of the structure. Due to different melting temperatures, the use of conventional thermal joining technologies is restricted. Therefore, the demand of efficient mechanical and adhesive joining technologies is increasing. In this publication, the damage behaviour of self-piercing riveted CFRP-metal joints subjected to thermal loading is analysed via computed tomography. Particular attention is paid to the residual stresses introduced in the CFRP, resulting from an interference-fit of the rivet and the laminate. It is shown, that primarily radial compressive stresses are responsible for the observed failure of the CFRP. With regard to these effects, geometric adaptions of the self-piercing rivet are carried out, which prevent damages occurring during the thermal loading. Therefore, a calculation method based on complex valued potential functions to determine stress concentrations in the composite with interference-fit bolts is used. The calculation is validated via optical 3D image-correlation. The damage reduction using the developed rivet is validated by means of micro-sections and single-lap shear tests.

Mechanical joining in versatile process chains

The use of mechanical joining technologies offers the possibility of joining mixed material structures, which are used in particular in lightweight construction. An integrated securing of the joinability in versatile process chains is currently hardly possible as the number of combinable tool variants as well as variable force- and path-based process parameters is infinite. A versatile process chain, i.e. a sequence of all the processes and process steps required for product manufacturing, enables targeted changes to the semi-finished product, the joint, the component or the joining process that exceed the originally planned extend while still ensuring joinability. In detail, it leads to a unique joint with its own mechanical property profile, which, against the background of the resulting infinite number of combinations, makes it impossible to secure the joinability on the conventional experimentally based approach without extensive safety factors. The Transregional Colaborative Research Center 285 (TCRC285), which also initiated this special issue, is intended to enable mechanical joining technology to be versatile in the sense of high application flexibility. This is to be achieved with a numerical representation of the complete process chain from the incoming semi finished product via the joining part production and the joining process to the property profile of the joint in the operating phase. Thus a predictability of the joinability can be achieved and improvements in the individual life cycles of a joint can be realized by grasping the cause-and-effect relationships. On the basis of this knowledge, new possibilities for intervention in the joining process are to be created for the adaptation of the joining processes. With the aid of the methods developed for this purpose, tools will later be available to the end user to substitute the large number of mechanical joining processes or joining task-specific configurations with a smaller number of adaptable processes. This expands the flexibility in material choices, enabling challenges in environmental issues and sustainability to be overcome.

A novel ultrasonic-assisted staking process for mechanical fasteners

Increasing material variety in car body engineering causes new challenges regarding the joining processes and therefore, leads to an enhanced usage of mechanical joining technologies. Mechanical fasteners, as self-piercing nuts, can be staked cost-efficiently without a pre-punching operation to create detachable joints. Nevertheless, elevated workpiece strengths lead to high forming forces and a limited application range. One promising approach to reduce tool and workpiece loads in forming operations is superimposing a high-frequency oscillation to the tool movement. The so-called ultrasonic assistance enables an immediate and considerable reduction of the material's flow stress. This beneficial softening effect has been confirmed for several materials and processes. Since the process management and monitoring are highly affected by the frequency-dependent dynamic behavior of the whole test setup, the application of ultrasonic assistance is already challenging for basic forming operations. Hence, only a few studies have been investigating ultrasonic-assisted joining processes. In particular, joining processes with auxiliary elements have been hardly considered. Within the scope of this work, a novel ultrasonic-assisted staking process for mechanical fasteners is proposed. Therefore, the punch movement for the installation of a self-piercing nut into an aluminum sheet is superimposed with a 20 kHz oscillation of 10 µm, 15 µm, and 20 µm amplitude. Ultrasonic assistance in the initial staking phase, characterized by moderate loads, causes irregular tool–specimen contact. Since the effect is promoted by increasing amplitudes, a correlation with the elastic tool deflection is presumed. Additionally, the transferability of the ultrasonic-based force reduction to mechanical joining with auxiliary element is confirmed and a beneficial influence on the joint formation is identified. Thus, ultrasonic assistance is proven as a promising approach for the extension of existing process limits.

Study on the mathematical model related to the connection properties of cold pressing joining technology * *SRAC: steel reinforced aluminum conductor.

Cold pressing joining is an important mechanical joining technology in the application of wire rope. In this paper, a new simplified equivalent model is established to describe cold pressing joining process of multilayer structures and corresponding mathematical equations describing the equivalent model are derived based on the elastic-plastic theory. In addition, the cold pressing joining experiments are carried out using the compression strain clamp (NY-300/40) and the steel reinforced aluminum conductor (SRAC - JL/G3A-300/40) for verifying the rationality of the mathematical model. Results show that the trend of the experimental results is in good agreement with the theoretical results. Finally, the critical processing parameters and quality evaluation parameters of pressed strain clamp NY-300/40 have been calculated based on the mathematical model built in this work. The universal mathematical model can be extended to other types of wire rope.

Process-adapted temperature application within a two-stage rivet forming process for high nitrogen steel

Mechanical joining technologies like self-piercing riveting are gaining importance with regard to environmental protection, as they enable multi-material design and lightweight construction. A new approach is the use of high nitrogen steel as rivet material, which allows to omit the usually necessary heat treatment and coating and thus leads to a shortening of the process chain. Due to the high strain hardening, however, high tool loads must be expected. Thus, appropriate forming strategies are needed. Within this contribution, the influence of applying different temperatures for each forming stage in a two-stage rivet forming process using the high nitrogen steel 1.3815 is investigated. The findings provide a basic understanding of the influence of the temperature management when forming high nitrogen steel. For this purpose, the rivets are not formed at the same temperature in each stage, but an elevated temperature is applied selectively. Different process routes are investigated. First, cups are manufactured in stage 1 at room temperature, followed by stage 2 at 200°C. Second, cups are formed in stage 1 at 200°C and used for stage 2 at room temperature. By comparing the findings with results when applying the same temperature in both stages, it is shown that the temperature during the first forming operation has an effect on the forming behaviour during the second forming stage. The required forming forces and the resulting rivet hardness can be influenced by process-adapted temperature application. Furthermore, the causes for the temperature impact on the residual cup thickness in stage 1 are evaluated by a cause and effect analysis, which provides a deeper process understanding. The thermal expansion of the tool and the billet as well as the improved forming behaviour at 200°C are identified as the main influencing causes on the achieved residual cup thickness.

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.