Self-piercing Riveted Joints Research Articles

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.

Effect of Top Sheet Materials on Joint Performance of Self-Piercing Riveting

Three types of self-piercing riveting (SPR) joints, i.e., steel/aluminum, carbon fiber reinforced polymer (CFRP)/ aluminum, and aluminum/aluminum, were constructed using three different top sheet materials with the same aluminum alloy as the bottom sheet. The effects of the top sheet material on the joint quality and mechanical behavior were evaluated. The top sheet materials’ characteristics dominate the rivet piercing process and the consequent interlock distance. The high-strength steel top sheet requires a comparatively higher rivet setting force and induces early flaring of the rivet tail, resulting in a larger interlock distance. Though the CFRP needs the highest rivet setting force to penetrate the rivet through the CFRP fibers, the CFRP/aluminum joint exhibits the smallest interlock distance because of the SPR process-induced damages to the CFRP and subsequently less flaring of the rivet tail. In strength tests, the damaged CFRP sheet resulted in rivet head pullout of the CFRP/aluminum joints, which exhibited the lowest lap-shear and cross-tension strengths. In contrast, the steel/aluminum joints demonstrated the highest strengths because of their comparatively larger interlock distances. In addition to the experimental analysis, simulations revealed the rivet penetration and flaring mechanisms with various top sheet materials, and their respective joint quality and strengths.

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%.

Simplified modelling of self-piercing riveted joints and application in crashworthiness analysis for steel-aluminium hybrid beams

In this paper, an equivalent simplified model of self-piercing riveting (SPR-ESM) validated by tests is constructed to characterize the macroscopic mechanical responses of SPR joints under quasi-static and dynamic loading conditions. The application of SPR-ESM in the crashworthiness analysis of steel-aluminium hybrid beam is investigated. The results reveal that the failure of the riveting point in the test specimen is accurately simulated by the numerical model connected by SPR-ESM, and relative errors of mechanical responses between simulation and test are controlled within 7 %. Parametric study is performed to explore the effect of the riveting point failure on the mechanical performance of steel-aluminium hybrid beam under different loading angles. The results indicate that: (1) a severe instability deformation mode of steel-aluminium hybrid beam is easily to happen when the loading angles increases; (2) the effect of riveting point failure on the mechanical performance of steel-aluminium hybrid beam is limited when the beam has a stable folding deformation; and (3) a significant decrease in the ability of resisting deformation and energy absorption is easily to happen when the steel-aluminium hybrid beam has a severe instability deformation mode.

Insight of Salt Spray Corrosion on Mechanical Properties of TA1-Al5052 Self-Piercing Riveted Joint

Self-piercing riveted (SPR) joints in automobiles inevitably suffer from corrosion damage and performance reduction. In this work, the influence of salt spray corrosion on the mechanical properties of TA1-Al5052 alloy SPR joints was studied. The TA1-5052 SPR joints were prepared and salt spray tests were carried out for different durations. The static and fatigue strengths of the joints after salt spray corrosion were tested to analyze the effect of salt spray duration on the performance of the joints. The results show that the joints' static strength and fatigue strength decrease with prolonged salt spray time. The salt spray duration affects the joint's tensile failure mode. The tensile failure without corrosion and with a short salt spray time is the fracture failure of the lower aluminum sheet, and the tensile failure of the joints after a long time of salt spray corrosion is the failure of the rivets. The fatigue failure form of the SPR joint is the formation of fatigue cracks in the lower aluminum sheet, and salt spray time has little effect on the fatigue failure form. Salt spray corrosion can promote the initiation and propagation of fatigue cracks. The fatigue crack initiation area is located at the boundary between the lower aluminum sheet and the rivet leg. The initiation of cracks originates from the wear zones among the sheet metal, rivets, and salt spray particles.

Study on the mechanical properties and fatigue failure mechanism of 7075-T6 aluminum alloy joints under different joining processes

To obtain the theoretical basis and reference basis for the selection of low ductility aluminum alloy joining methods, three kinds of joints were obtained using 7075-T6 aluminum alloy sheets for clinching, self-piercing riveting (SPR), and resistance spot welding (RSW) joints, and the single-lap shear tests were performed on the three kinds of joints, and fatigue tests were conducted on the clinched joints and SPR joints. The fatigue fracture of the SPR joint was analyzed using scanning electron microscopy (SEM) and X-ray energy-dispersive spectroscopy (EDS). The results showed that the SPR joint demonstrated the best static performance, followed by the clinched joint and the RSW joint. In the case of the fatigue test, the SPR joint showed better fatigue performance than that of the clinched joint. The fatigue failure of the SPR joint and clinched joint is associated with fretting wear. Fretting wear of the SPR joint is more severe than that of the clinched joint. The debris produced by fretting led to microcracking phenomena in the plate. Under the action of alternating load, the microcracks continued to expand and generate macrocracks, and the joint was broken and finally failed. This fretting wear is the root cause of the failure of the SPR joint. Thus, the SPR joints exhibited the best mechanical properties.

A study on non-destructive testing of geometrical parameters of self-piercing riveting joints using an acoustic microscope with a scanning focusing converter

ABSTRACT A non-destructive technique was proposed for geometrical parameters’ measurement of self-piercing riveting (SPR) joints based on ultrasonic C-scan image. Firstly, in order to enhance the detection precision, the characteristics of the ultrasonic echo signal at the edge of internal structure were analysed, and a theoretical formula on its sound intensity distribution was established. Then, an image segmentation process for the detection of SPR expansion diameter was suggested. Results showed that taking the greyscale value at the ‘inflection point’ from the C-scan image as the threshold of the segmentation, the non-destructive test results can get good correlation with the metallographic cross section, which provides a new method on the quality evaluation of SPR by using non-destructive test.

Equivalent characterization of pre-strained material properties and mechanical behavior prediction of steel/aluminum self-piercing riveted joints

Self-piercing riveting formation results in additional straining and property variations of the sheet material and rivet of the joint. As the riveted forming strain was concentrated within a limited area, it was difficult to characterize the properties of the pre-strained materials through conventional tensile tests. The present study dealt with the effects of considering riveted forming strain on the mechanical behavior prediction accuracy of the self-piercing riveted (SPR) joints of DP590 high-strength steel and AC43500-T7 cast aluminum alloy sheets. Main efforts were put on the evaluation of the failure parameters and behaviors of the pre-strained sheet materials. Equivalent specimens of the two sheet materials with different pre-strain levels were prepared via a flat rolling process for the characterization of the sheet deformation during the riveting process. Based on the material parameters obtained from the tensile tests of the rolled sheets and the compression tests of the rivet, a three-dimensional simulation model of the SPR joints was established. The mechanical behaviors of the joints were investigated under three loading conditions of 0° pull-out, 90° lap shear, and 45° tension–shear. The prediction accuracies of pull-off displacement under the three loading conditions reached 94.07%, 91.61% and 94.02%, and were 5.57%, 26.43% and 6.18% higher than those predicted by the model without considering the riveted forming history, respectively. The prediction accuracies of the peak force under the three loading conditions were 98.43%, 89.70% and 95.35%, respectively. The error difference of predicted peak force of the two models was less than 3%.

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.

Fatigue reliability analysis of 5052 aluminium alloy self-piercing riveted joints with given confidence

Fatigue reliability analysis of 5052 aluminium alloy self-piercing riveted joints with given confidence

Automatic identification framework of the geometric parameters on self-piercing riveting cross-section using deep learning

Self-piercing riveting (SPR) is one of the most important joining technologies used in light-weight vehicle body manufacturing. Traditionally, SPR joints are developed through trial-and-error tests on various rivet and die combinations. Given the hundreds of rivets and dies available in SPR joint design, huge combinations exist in the full solution space. How to optimize the joining process at a low cost with high reliability is critical both for new material, new vehicle and new production line development. Instead of relying on experience-based physical testing, data-driven approach has been believed to be a promising way for future automotive manufacturing and design. However, the lack of effective data acquisition and accurate characterization methods for joints becomes a barrier, which limits the volume and availability of joining data. In present research, an automatic identification framework of the geometric parameters on SPR cross-sections using deep learning is proposed, which integrates an innovated and complete flow from the image pre-process to postprocess. Firstly, cross-section images are transformed into material segmentation maps using deep learning. Then critical control point of a cross-section is identified accurately, and totally 9 key geometrical parameters are measured based on the locations and combinations of control points. The present research shows that SOLOv2 and Unet are the best models for SPR cross-section image segmentation. The proposed framework could provide high accurate measurement results (average error < 0.02 mm) with a very short process time (within secs), laying a solid foundation for data-driven design and optimization of joining processes within the whole design space at vehicle level.

Automatic and robust design for multiple self-piercing riveted joints using deep neural network

Automatic and robust design for multiple self-piercing riveted joints using deep neural network

Mechanical properties and fatigue failure mechanisms of purely self-piercing riveted (SPR) and hybrid (SPR-bonded) joints under salt spray environment

Automotive light-weighting has become an important direction for the development of the automotive industry, and the light-weighting materials are inevitable to be joined. Self-piercing riveting (SPR) is a kind of new process to joint various materials. The SPR and SPR-bonded (SPR-A) joints of AA6061 and DP590 sheets were prepared. The mechanical properties of SPR and SPR-A joints after exposure to neutral salt spray environments for different aging times were tested. The results showed that the shear and fatigue properties of the joints gradually reduced with the increase of salt spray time. In addition, the SPR-A joints not only have higher maximum failure loads but also have larger energy absorption values in shear tests when compared with that of the SPR joints under the same salt spray time. The fatigue test results showed that fretting wear between the rivet-foot and the lower sheet is the main failure mechanism of joints. Moreover, the salt spray time has no obvious effect on the fatigue failure modes of the two kinds of joints.

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.

Combined strengthening mechanism of solid-state bonding and mechanical interlocking in friction self-piercing riveted AA7075-T6 aluminum alloy joints

A recently developed friction self-piercing riveting (F-SPR) technique based on the combination of friction stir processing and riveting has been reported to possess both solid-state bonding and mechanical fastening characteristics. However, there is still a lack of quantitative understanding of the hybrid enhancement mechanism, hindering its engineering application. To fill in this gap, the current research investigated the microstructure evolution, microhardness distribution, and miniature-tensile performance of the aluminum alloy AA7075-T6 F-SPR joints by experiments. An accurate numerical simulation model was established to quantitatively evaluate the individual contributions of microstructure, local bonding strength, and macro interlocking to the performance of the joint, which could well explain the experimental results. It was found that due to the friction stirring of the rivet, solid-state bonding driven by dynamic recrystallization is realized between the trapped aluminum in the rivet cavity and the bottom aluminum sheet. The solid-state bonding zone has 75% yield strength, 81% ultimate tensile strength, and 106% elongation compared to the base material. This solid-state bonding enables the internal interlocking between the trapped aluminum and the rivet to withstand the additional load, which forms a novel dual-interlock fastening mechanism and increases the peak cross-tension force by 14.3% compared to the single-interlock joint.

Strength and Failure of Self-Piercing Riveted Aluminum and Steel Sheet Joints: Multi-axial Experiments and Modeling

The mechanical failure of self-piercing rivet (SPR) joints connecting seven series aluminum and high strength steel sheets is investigated, both numerically and experimentally. The joint strength and failure mechanisms are characterized for a total of four distinct loading modes, including the lap-shear, cross-tension, inclined cross tension and coach peel. For the analysis of the underlying influential parameters in each loading case, joint designs with equal total sheet thickness and equal rivet head diameters are considered. The highest strength values are obtained in lap-shear loading for all joint types, while high rivet interlock joints are observed to pair with increased cross-tension strength values. Moreover, the loading mode in which the highest energy is absorbed directly relates to the joint type. Depending on the material combination to be joined, either the cross-tension or the coach-peel cases yielded the highest energy absorption. The experimental results indicate that high rivet hardness and bottom sheet strength values have a favorable impact on the coach-peel strength and the associated deformation response. For all failure modes, the lowest rivet hardness employed (H4) was proven sufficient to prevent rivet failure in the joint types employed, despite the substantial equivalent plastic strains developed in it. Furthermore, high interlocks were noted to primarily affect the lap shear failure mode, inducing significant bottom sheet damage upon fracture, with the failure response observed in all other loading cases to remain practically insensitive to the interlock magnitude.

Mechanical testing of adhesive, self-piercing rivet, and hybrid jointed aluminum under tension loading

Joining technologies play a crucial role in facilitating vehicle weight reduction, while maintaining structural performance and vehicle crashworthiness. Adhesive and self-piercing rivet (SPR) joining have individually been assessed in joining multi-material structures including aluminum alloys; however, limitations include fixturing requirements for adhesive joints and non-uniform load distribution within SPR joints. Hybrid (adhesive-SPR) joining has been proposed to potentially address these limitations. Yet, there is limited data regarding the strength, stiffness and energy absorption of hybrid joints under tension loading, and a lack of quantitative comparisons between adhesive, SPR and hybrid joints. In the present study, adhesive, SPR and hybrid joints were created in aluminum H-shaped tension specimens and the joint morphologies were quantified. Joint strength, stiffness and energy absorption were compared for two aluminum sheet metal alloys and three sheet thicknesses. Adhesive joints exhibited higher strength (up to 20.5%) and stiffness (up to 422%) than SPR joints, while SPR joints demonstrated higher energy absorption (up to 352%). Hybrid joints in 3 mm sheet exhibited reduced strength and stiffness relative to adhesive joints. Importantly, hybrid joints for 1 and 2 mm sheet thicknesses demonstrated strength and stiffness comparable to adhesive joints (<8% difference), and improved energy absorption compared to adhesive (up to 336%) and SPR joints (up to 53.5%), enhancing the performance of the individual methods. The study results could facilitate identifying suitable joining method, sheet thickness and alloy type according to structural applications (e.g. intrusion prevention versus energy-absorbing structures), and support future adoption and improvement of hybrid joints.

A framework for calibration of self-piercing riveting process simulation model

This paper presents a framework that integrates machine learning and global sensitivity analysis to calibrate process simulation model of self-piercing rivet (SPR). In the present framework, surrogate models are trained to represent the intrinsic numerical relationship between selected model parameters (e.g., material properties, interface frictions, and clamping force) and cross-section dimensions of SPR joint obtained from simulation. Under the assistance of surrogate models, a global sensitivity analysis is performed to identify critical parameters that have significant effects on SPR joining process. Then, new surrogate models are trained to present the sensitive parameters and the cross- section dimensions obtained from process simulation. The parameter calibration of the simulation model is described as a two-objective optimization problem to minimize errors of key cross-section dimensions between simulation and experiment. Combined the newly trained surrogate models with genetic algorithm, the model parameters are calibrated efficiently. A case study of simultaneous calibration of four SPR process simulation models is used to illustrate the proposed framework and examine the effectiveness of the proposed framework. A good consistency is achieved between calibrated models and experiments.

Determining the properties of multi-range semi-tubular self-piercing riveted joints

By applying a variety of strategies, including both the development of new drive concepts as well as the use of lightweight constructions, it is intended to meet the climate targets set by the Paris Agreement. Multi-material design is often used when applying lightweight constructions, especially in the mobility sector. Herby, various materials with different properties are combined to well adapt the structure to the application of force in order to reduce weight. For example, this can result in the combined use of (high-strength) steel and aluminium. However, the increasing number of materials used and the number of resulting joints has led to the development of a number of different joining processes. These can only be used to a limited extent for joining multi-material joints and are usually inflexible facing changing boundary conditions. Examples of further difficulties affecting the established processes are metallurgical incompatibilities, which particularly pose difficulties for the use of thermal joining processes. A frequently used mechanical joining process is self-piercing riveting. Due to its high load-bearing capacities, a wide range of application and high process robustness it is often used when joining of dissimilar material is required. However, in case of self-piercing riveting used, the increasing number of multi-material joints and material-thickness combinations leads to the need of a large number of rivet-die-combinations to adapt the joining process to the respective joining task. Since the joining system cannot react to these changes, a tool change or an adjustment of the system is necessary, which leads to a reduction in efficiency and extended process times. To increase flexibility and process efficiency, new, versatile joining technologies are needed that can be adapted to changing boundary conditions. One possibility for this is the use of multi-range capable semi-tubular self-piercing rivets, which are inserted into the joint by using a new joining system with extended punch-sided actuator technology. The increased actuator technology enables the rivet to be set by an inner punch and subsequently to form a rivet head by embossing with an outer punch. All punch movements can be controlled independently of each other, enabling adaptive adjustment of the process parameters. Depending on the rivet geometry used, rivet head formation by the outer punch can be performed both with and without head deformation. The rivet without head deformation consists of a tubular shape with ring grooves in the rivet head area. Using the outer punch, punch-sided material is formed into the ring grooves creating an interlock in the head area of the rivet. The rivet with head deformation is designed differently. It is modified to enable subsequent forming to the respective thickness of the joint by forming the protrusion of the rivet head onto the punch-sided joining part. In the study presented here, the joining process of the versatile self-piercing riveting is presented, analysed and the property profile of the joints is determined on the basis of various material-thickness combinations. Here, both the characteristic parameters of the joints and the joint load-bearing capacities are determined. Finally the property profiles are compared with conventionally manufactured joints in order to identify potential for improvement.