Self-piercing Riveting Process Research Articles

Investigation of the influence of the rivet geometry on joint formation for a versatile self-piercing riveting process

Investigation of the influence of the rivet geometry on joint formation for a versatile self-piercing riveting process

Study on the failure mechanism and mechanical properties of multi-layer hole-drilled self-piercing riveted three-layer steel/aluminum hybrid joints

To promote the application of the three-layer steel/aluminum hybrid sheet in the lightweight vehicle body, the multi-layer hole-drilled self-piercing riveting (MH-SPR) process is proposed. This paper aims to investigate the failure mechanism and mechanical properties of three-layer MH-SPR joints and explore the effects of overlap type and pre-drilled layer on forming quality, mechanical properties, failure modes, and fracture characteristics of the three-layer MH-SPR joint in detail. Three overlap types (e.g., type A, type B, and type C) and three pre-drilled layers (e.g., J0, J1, and J2) are designed for the experimental study and the shear experiments and full-field strain measurements are carried out. The results show that the pre-drilled layer can improve SPR joinability and reduce the peak force and energy consumption during the riveting process. Overlap types A and C can increase the peak force of the joint and type C can simultaneously enhance the peak force and energy absorption of the joint. The increase in the pre-drilled layer can increase the peak force and energy absorption of the joint. The overlap types and pre-drilled layers change the failure modes of the joints, which include five types of failure modes: middle sheet tearing, top sheet tearing, rivet fracture, interlock failure, and mixed failure. Full-field strain analysis shows that the overlap type improves the mechanical properties of the joints by altering the deformation mode of the steel sheet. This work provides a theoretical basis for expanding the industrial practice of the MH-SPR process and improving the joined quality of the three-layer joint.

Multi-objective optimization of rivet and die for self-piercing riveting using finite element analysis

This study numerically investigated the effects of self-piercing riveting (SPR) process parameters on the quality of joints between steel and aluminum alloy sheets. First, the mechanical properties of each metal were experimentally evaluated, and a single-lap shear test was performed to validate a constructed finite element analysis model. This model was subsequently applied to optimize the die depth, die radius, and rivet length parameters of the SPR process using a multi-objective optimization method considering the shear strength, interlocking distance, and minimum thickness of the bottom sheet as objective functions. The results indicated that the maximum interlock distance was achieved with a longer die depth and larger die radius, and the largest minimum thickness of the bottom sheet was achieved with a shorter rivet length and larger die depth and radius. The die radius and depth were determined to be the most critical variables for all objective functions. The findings of this study can be applied to optimize the SPR of steel and aluminum alloy sheets.

Finite element modeling and analysis of the self-piercing riveting forming process with the ball-shaped die

Self-Piercing Riveting (SPR) technique faces challenges as it expands to low ductile materials. In this paper, the ball-shaped die is suggested to prevent the formation of cracks in sheets during the SPR process. The SPR model with the ball-shaped die was developed using the finite element method, and simulations of the joint forming process and tensile test were conducted. The influence of die depth and diameter on SPR forming process and quality was analysed. The simulation results showed that as the die diameter increases, the undercut decreases, and the remaining thickness of bottom increases. The undercut and the remaining thickness of bottom decreases with the increase of the depth of the die. When employing a ball-shaped die with a 1 mm depth and 10 mm diameter, the SPR joint with tight interlock and excellent mechanical properties can be obtained. The results of the simulation and experimental tests are consistent.

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.

Low-frequency vibration assisted self-pierce riveting (LV-SPR) of carbon fiber reinforced composite and aluminum alloy

Self-pierce riveting (SPR) has been widely applied to join carbon fiber-reinforced polymer (CFRP) composites and high-strength metallic plates in the automotive and aerospace fields. However, the CFRP is often damaged by rivet piercing owing to its brittleness as well as the relatively small interlocking that forms between the rivet leg and high-strength plate owing to the hard deformation characteristics. Therefore, in this study, a novel low-frequency vibration-assisted self-pierce riveting (LV-SPR) technology is proposed, which utilizes the vibration effect in softening the metal and reducing interfacial friction at room temperature. In riveting experiments involving CFRP and 5052 aluminum alloy utilizing 37Cr4 semi-hollow rivets, LV-SPR exhibited a significant reduction of 68.2% in the riveting force compared to the traditional SPR process. The decreased riveting pressure in LV-SPR effectively mitigated the CFRP damage by 36.2%, which was caused by a reduction in the interfacial friction force between the rivet and CFRP laminate. Moreover, owing to the improved deformation capacity of the rivet and alloy plate by the vibration softening effect, the lateral expansion of the rivet leg in the aluminum alloy was enlarged by 36.4% compared to that using the traditional self-piercing riveting (T-SPR) process. Microscopic characterization revealed that vibrations notably promoted grain refinement and enlarged the subgrain structures of the rivets and alloy plates. Finally, by superposing oscillations, the shear strength of the connection joint increased by 13.5% compared to the T-SPR joint. The proposed LV-SPR was validated as an effective technique to increase the connection strength of high-performance plates, which can efficiently improve the structural safety and promote the widespread application of CFRP/alloys in automobiles and aircrafts.

Performance analysis of similar and dissimilar self-piercing riveted joints in aluminum alloys

In this work, similar (2A12) and dissimilar (6061) aluminum alloy sheets are validly joined using self-piercing rivet process. A quasi-static experiment is proposed to investigate the mechanical behaviors, failures mode, and mechanism of the different joints. Moreover, a method based on deep learning algorithm is anticipated to detect the appearance defects of the SPR welded joints. The results indicated that 2A12 joints of similar sheets contained the advantageous static strength and 6061 similar sheet joints had superior anti-vibration performance conducts. The joints with 6061-2A12 sheets introduced the most decent and comprehensive mechanical properties. The main failure mode of 2A12 similar sheet joints was substrate fracture. The performance of the substrate affects the failure mode of the joint and the plasticity of the substrate is better. When the time comes, the failure mode is mostly pull-off failure. Poor plasticity of the substrate can easily lead to substrate breakage. The reason for joint pull-off and button fall-off failure is that there is large plastic deformation in the lower plate of the joint and the mechanical internal locking structure is damaged. 2A12 substrate breakage belongs to a composite fracture that combines intergranular fracture and microvoid aggregation type fracture. The area of the 6061 substrate near the edge of the sample is shear fracture and the area near the center of the sample thickness is dominated by microvoid aggregation type normal fracture. The effectiveness of the method was verified by conducting a series of experiments and the detection accuracy of the method can reach about 90%. The detection speed was as high as 50 frames per second (FPS), which can effectively solve the problem that the rivet quality was difficult to monitor.

Fatigue performance of electromagnetic self-pierce riveting-bonding hybrid joint under alternating load and corrosion coupling conditions

Electromagnetic self-pierce riveting (E-SPR) process was suitable for joining carbon fiber reinforced plastics (CFRP) and aluminum sheets. The E-SPR joints of dissimilar material still suffered from corrosion, and bonding could be an auxiliary process to improve corrosion resistance of joints. This paper investigated the fatigue characteristics of electromagnetic self-pierce riveting-bonding hybrid joints under alternating load and corrosion coupling conditions. Neutral salt spray (NSS) test, microscope observation, and fatigue test were conducted. Results showed that the adhesives not only improved the joint’s corrosion resistance, but also its mechanical properties. Specifically, the shear curves contained two peak loads, namely peak A and peak B. Corrosion was the greatest at 840 h, where peak A was 42.86% less than at 0 h, but peak B had little effect. Moreover, the addition of adhesives significantly impacted the fatigue characteristics of the joints after corrosion. The E-SPR-bonded joints still appeared good anti-fatigue characteristics within 168 h of corrosion. The fatigue failure modes were fractures in the CFRP sheets and the rivets remaining in the aluminum sheets. The influence mechanism of corrosion on fatigue was revealed. The water molecule and chloride ions penetrated the adhesives to cause hydrolysis, increased the plasticity of the adhesives and decreased the bond strength, which reduced the fatigue characteristics. The results provided theoretical guidance for E-SPR-bonded joints applied in engineering applications.

Formation mechanism and mechanical strength evaluation of hybrid riveted/solid-state bonded aluminium alloy joint

In this research, an improved friction self-piercing riveting (F-SPR) process with internally threaded rivet and flat die was proposed to achieve hybrid riveted/solid-state bonded connection for low-ductility aluminium alloys. Instead of relying on rivet shank flaring to form mechanical interlock in the existing F-SPR process, a reliable threaded connection was established between the rivet shank and sheets by rapid filling of sheet material into the rivet thread grooves. A perfectly flat joint bottom was formed without inducing large-scale sheet distortion. A series of experimental tests were performed to clarify the joint quality characteristics, formation mechanism and mechanical performance. The results revealed that the quality of newly formed five aluminium threads demonstrated a decline trend from bottom to top. Two kinds of thread formation mechanisms were identified: The mechanism I is solely composed of Filling step and applied to upper four threads; The mechanism II consists of Filling-Fracturing-Welding steps and is applied to the lowest thread. Discontinuous solid-state bonding was formed at the sheet interface around the rivet shank periphery, whilst a bowknot-liked solid-state bonding zone with a weak welded zone and an unwelded zone was formed at the joint centre. Rivet pull-out failure from the bottom sheet was observed for the studied AA7075-T6 joints under both lap-shear and cross-tension loads. The peak loads reached 9.79±0.23 kN and 5.41±0.15 kN in the lap-shear and cross-tension tests, and the corresponding energy absorption was 13.82±0.59 J and 16.42±0.87 J.

Multi-fidelity data-driven optimization design framework for self-piercing riveting process parameters

Due to its excellent performance, the self-piercing riveting (SPR) process has been widely used in auto body manufacturing in recent years. The need for efficient and reliable design of SPR process parameters is growing in industry. Traditional trial-and-error design methods rely on historical experience and can be limited by physical experimental materials and equipment. Data-driven methods rely on data accuracy and data volume, and data from a single source is often inadequate to meet design needs. To address these issues, a multi-fidelity data-driven optimization design framework is proposed in this paper. The most significant feature of this framework is the fusion of physical experiment data and simulation data to build surrogate models. In this framework, a modified optimal Latin hypercube sampling method and a multi-fidelity surrogate model based on transfer learning and neural networks are proposed for SPR process. This multi-fidelity surrogate model can use a very small amount of experiment data to modify the surrogate model built on simulation data based on transfer learning, so that the model predictions can more closely match the physical experiment results at a lower modeling cost. Benefit from this method, the proposed framework can balance the contradiction between design accuracy and development cost compared to a single-fidelity data-driven framework. The application cases show that the prediction errors of the multi-fidelity models are <0.1 mm for the key geometric parameters of the SPR joint. As verified by physical experiments, the rivets and dies selected by the framework are the optimal solutions within the optional range. To the authors' knowledge, this is the first time that a multi-fidelity modeling method has been introduced to the field of SPR process, to solve the process parameter optimization design problem. Further, the method is not limited to the use of SPR process but can be applied as a paradigm in other engineering optimization design problems, especially in joining process parameter design problems.

Strength modeling of Al-alloy sheet self-piercing riveting considering different failure modes

Self-piercing riveting (SPR) is an efficient joining process for bonding sheets through the interlock of rivet flaring. The strength of SPR joints is commonly measured by destructive test. In this study, the metal deformation behaviors in the SPR of aluminum alloy sheets and the mechanical behaviors of the SPR joints in the destructive tests were investigated in order to develop the general strength models of SPR joints. As the important feature in the SPR process, the rivet flaring was quantified by an empirical estimator and utilized to establish the general strength models considering the geometric dimensions of components and tools, the material properties of rivet and sheets, and the load–displacement curve of punch. The predicted strengths of the SPR specimens were verified by the experimental results. All the calculation errors are lower than 8%. Finally, an industrial internet of things (IIoT) was developed to realize the data processing and the SPR joint strength predication.

Experimental and Numerical Investigations on the Impact of Surface Conditions on Self-Piercing Riveted Joint Quality

In this study, experimental and numerical investigations were carried out to achieve a comprehensive understanding of the impact of surface conditions on self-piercing riveting (SPR) joint quality. Oil lubrication and sandpaper grinding were employed in experimental tests to change surface conditions at rivet/top sheet, top/bottom sheets and bottom sheet/die interfaces. A finite element (FE) model for the SPR process was also adopted to numerically assess the impact of surface conditions. Variations in surface conditions were modelled by changing friction coefficients at contact interfaces. The results revealed that the friction coefficient between the rivet and top sheet (μ1) imposed significant influences on the interlock (I1) by affecting the deformation of the rivet shank and top sheet. The friction coefficient between the rivet and bottom sheet (μ2) showed a lower influence on the joint quality because of a smaller contact area and shorter interaction time. The friction coefficient between the top and bottom sheets (μ3) led to opposite changing trends of remaining bottom sheet thickness at the joint centre (tc) and under the rivet tip (ttip). The friction coefficient between the bottom sheet and die (μ4) demonstrated crucial influences on the remaining bottom sheet at the joint centre. The riveting force was significantly influenced throughout the whole riveting process by the μ1, but only affected at the end of the joining process by the other three friction coefficients.

Joining Strength of Self-Piercing Riveted Vibration-Damping Steel and Dissimilar Materials

A vibration-damping steel panel is used for lightweight vehicles to block any noise subjected to the passenger cabin replacing heavy fiber-based insulators. Conventional weld joining methods often encounter problems due to the presence of viscoelastic compounds reducing the joint quality and making the joining process unproductive. In this work, we present experimental results that show the self-piercing riveting (SPR) process can be used to produce high-quality joints between vibration-damping steel and (i) commonly used steel alloy (SPFC590DP), (ii) carbon-fiber-reinforced-plastic (CFRP) panels. Various die shapes are used to investigate the resulting interlock width and bottom thickness of the joints and tensile shear load tests were performed to evaluate the joining strength. The results show that high-quality joints between vibration-damping steel and the steel alloy are possible for all the dye types and panel configurations, used in this study, producing up to 6.2 kN of tensile shear load. High-quality joints were also possible with CFRP producing up to 4.0 kN, however, acceptable joints were formed only when the CFRP panels were on top during the riveting process due to severe cracking.

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%.

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.

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 mechanism and damage of self-piercing riveted joints in carbon fibre reinforced polymer composites and aluminium alloy

The joining mechanism and damage of self-piercing riveted joints in carbon fibre reinforced polymer (CFRP) composites and aluminium alloy were investigated. The damage constitutive model for composite materials, considering the shear effect, was proposed to predict the mechanical behaviour and damage evolution of the CFRP. Self-piercing riveting (SPR) tests were performed to determine the damage patterns of joints with different laminate structures. Furthermore, numerical simulations of the SPR process were performed to understand the joining mechanism and analyse the damage evolution process of the joints. The results show that matrix cracks and fibre cracks existed in the SPR joints in the CFRP and aluminium alloy, located around the rivet head. With the increase in CFRP thickness, the damage degree of the joint surface decreases. Therefore, the proposed constitutive model can predict the complex damage behaviours of a CFRP subjected to large deformations. The matrix cracks are mainly caused by the matrix tensile damage, with the ply angle influencing the damage evolution trend. In addition, delamination exists between each ply of the SPR joint; the propagation direction of delamination damage is consistent with the fibre direction of the lower layer, especially for the joints with [0/90/0]s CFRP or [45/-45/45]s CFRP.

A Comparative Study of Self-Piercing Riveting and Friction Self-Piercing Riveting of Cast Aluminum Alloy Al–Si7Mg

Abstract 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.

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.

Investigation of the influence of the tumbling angle on a tumbling self-piercing riveting process

To achieve the climate objectives, various measures are taken to increase the efficiency of raw materials and energies used. A sector with a large proportion of the global consumption of resources is the mobility sector. To increase the efficiency in this field, large efforts are made to reduce the weight of moving masses. One approach is the use of multi-material systems, which utilises different materials and their specific properties depending on the local requirements. Multi-material systems consist often of materials which differ in strength and density, for example, high-strength steels, aluminium alloys or polymers. Additionally, such a system can utilise different geometries of the components to be joined, characterised for example by varying sheet thicknesses. A central challenge of producing these systems is the joining of the individual components. This requires robust joining processes capable of covering the entire spectrum of possible variants and is feasible for different physical properties of the materials. Since conventional joining processes are rather rigid and have difficulty reacting to changing process and disturbance variables, new joining processes are necessary. With the objective of being able to react versatile to varying parameters, a process combination consisting of a semi-tubular self-piercing riveting process and orbital forming process with adjustable tumbling kinematic is introduced. Due to the process combination of tumbling and self-piercing riveting, mutual influences of the two process components are analysed in regard to material flow and process forces. Further, the investigations show the influence of a varying tumbling angle on the joining process itself and how the characteristic properties undercut, rivet head end position and residual sheet thickness of the joint are affected. The material used for the joining partners is an aluminium alloy EN AW-6014 typical for multi-material systems in the automotive industry and the rivets are from type Rivset C produced by the Böllhoff company.