Riveting Process Research Articles

Elucidation of interface joining mechanism of aluminum alloy/dual-phase steel friction stir riveting (FSR) joint

The mixed use of aluminum (Al) alloy and high strength steel (HSS) is an effective way to reduce the weight of vehicles, which meanwhile brings great challenges to the traditional spot joining technologies. Three-stage friction stir riveting (FSR) process using a precisely designed semi-hollow rivet was developed for Al-steel joining. By coordination control the rotation and feed movement of the rivet, a “tube-sheet” annular solid-phase welding zone is formed between the semi-hollow rivet and the lower steel sheet, realizing the reliable joining of Al-steel dissimilar sheets combination. The rivet structure was designed to accommodate the trapped Al alloy in the rivet cavity during the friction welding stage, obtaining an inclusion-free friction welding interface which was not only mechanically mixed but also fully interatomic binding. The optimal FSR joint is composed of ferrite softening zone (FSZ), carbide precipitation zone (CPZ), ultrafine martensite zone (UMZ), and fine dual-phase zone (FDPZ). Moreover, due to the circumferential flow of materials during friction welding stage, the dynamic recrystallization (DRX) microstructures of the rivet and steel sheet both have shear texture consistent with {112}<111> and {110}<001> cube texture, which brings the uniformity of microstructure and performance. With the combined contribution of inclusion-free joining interface and the microstructure with uniform cube texture, the optimal FSR joints have the maximum average tensile-sheer strength of 281.1 MPa (90.7%·σ6061-T6) and peak load of 5.29 kN, presenting pull-out fracture mode with good fracture energy absorption.

A Study on Electromagnetic Riveting Behavior of High-Strength Rivets Depending on Hardness

Recently, the application of aluminum based on ultra-high strength steel is expanding around high-end models of electric vehicles, so it is necessary to develop strategic welding/joining technologies for these materials. In the case of overseas advanced automobile companies, such development is being carried out at the level of introducing mass production facilities through the development of joining technology, but in Korea, the localization rate of original core technology and related equipment in the field of welding and joining is still very insufficient. Therefore, in this study, in order to develop a high-strength rivet suitable for the rivet process during the multi-material bonding process, cold forging steel was used to attain an appropriate level of hardness through changes in the microstructure through heat treatment. Rivets with different hardness were fabricated through heat treatment, and flaring and tensile test were conducted. As a result of flaring test of rivets, buckling occurred with hardness values of Hv 447 and 486, while cracks occurred without buckling in rivets with hardness values of Hv 506. But as a result of tensile shear test using GA980 and Al5052 materials, there was no significant difference in tensile shear load values according to hardness values of the rivets.

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.

Research on the joining of three-layer sheets by flat bottom riveting process

Research on the joining of three-layer sheets by flat bottom riveting process

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.

Experimental investigation on the pre-holed screw-drilling riveting process for joining dissimilar lightweight sheets

The use of lightweight sheets such as carbon fiber reinforced composites (CFRP), magnesium alloys, and aluminum alloys is of great significance to realize the lightweight of automobiles. However, the differences between the sheets make the joining processes of dissimilar sheets a great challenge. A pre-holed screw-drilling riveting (PH-SDR) process is proposed based on a study of existing joining processes, the rivet with threads is riveted into the sheets by rotating, and the effective joining between the sheets is achieved by embedding the sheets into the rivet threads. The joint strength of the PH-SDR and self-piercing riveting (SPR) was compared through the lap-shear test. It was found that in the case of 1 mm Al6061-T6 + 2 mm AZ31B, the strength of the PH-SDR joint was 3.09 kN, which is 30.9% higher than the SPR of 2.36 kN. In the case of 2 mm CFRP + 2 mm Al6061-T6, when the spindle speed was 600 rpm, the strength of 5.43 kN and the energy absorption of 20.3 J were obtained for the PH-SDR joint, which is 51.2% and 89.7% higher than the optimal SPR joint. The results show that the proposed PH-SDR process provides a feasible solution for the joining of dissimilar lightweight sheets.

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.

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.

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.

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.

Single-sided friction riveting process of aluminum sheet to profile structure without prefabricated hole

Aluminum profile structures are extensively used in the transportation industry and pose a considerable challenge to single-sided spot joining technologies. The widely used Huck BOM® riveting process requires prefabricated holes and thus significantly increases cost and reduces efficiency. In this study, a novel single-sided friction riveting (SSFR) process with an internal-threaded semi-tubular rivet is proposed to realize a screwed-solid state hybrid joining of aluminum sheet to profile structure without prefabricated holes. The results show that the upper trapped metal is welded with the lower profile structure under the thermal-mechanical coupled action generated by rivet stirring. The internal thread could facilitate material filling into the rivet cavity and form a mechanical interlock, so as to realize the single-sided joining between the sheet and the profile without prefabricated holes. The one-stage SSFR process could form shear texture and promote continuous dynamic recrystallization, but lead to excessive material softening and internal cracking. A two-stage process including friction softening and screw tightening could better coordinate the effects of heat and force, thereby avoiding cracking and refining grains in the stirring zone. Finally, the tensile-shear and cross-tension peak loads were respectively improved by 9.0 % and 29.3 %, compared to the joints with one-stage process and non-structural rivet.

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.

Friction stud riveting (FSR) of thick high-strength aluminum alloy structure

7-series aluminum (Al) alloy is a promising material that can meet the ever-increasing lightweight demand for transportation tools. However, the assembly between Al alloy and steel components brings challenges to the existing stud pre-embedded technology. This study proposes a friction stud riveting (FSR) process to achieve high-performance and high-efficiency embedding of steel studs in Al alloy parts by shank undercut, grooves interlock, and intermetallic compounds bonding. The stud geometry and FSR process were first optimized by evaluating the macro morphology of the joints. Then, the microscopic characterizations were performed to understand the material flow behavior, plastic deformation, and microstructure evolution of the joint induced by the feeding and rotation of the stud. The results indicate that the transient thermo-mechanical coupled FSR process leads to a continuous transition of sliding, mixed sliding-sticking, and sticking contact conditions between the stud and the Al alloy, resulting in a thermo-mechanical affected zone (TMAZ) outside the stud and a trapped Al alloy (TAA) inside the stud with diverse dynamic recrystallization (DRX) states and microstructures. The rotation state evolution and temperature gradients of the material within the TAA create two DRX bands that dominate the fracture behavior of TAA under tensile and shear loads. The strengths of the optimized steel stud-thick Al FSR joints reach 101.3% and 118.5% of the 7A52 Al alloy base material in tensile and shear tests, respectively, which solves the problem of low joining strength between steel stud and Al alloy plate due to the brittle and thick intermetallic compounds layers in conventional stud welding processes.

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.

FEM Simulation of the Riveting Process and Structural Analysis of Low-Carbon Steel Tubular Rivets Fracture.

Riveted joints are a common way to connect elements and subassemblies in the automotive industry. In the assembly process, tubular rivets are loaded axially with ca. 3 kN forces, and these loads can cause cracks and delamination in the rivet material. Such effects at the quality control stage disqualify the product in further assembly process. The article presents an analysis of the fracture mechanism of E215 low-carbon steel tubular rivets used to join modules of driver and passenger safety systems (airbags) in vehicles. Finite element method (FEM) simulation and material testing were used to verify the stresses and analysis of the rivet fracture. Numerical tests determined the state of stress during rivet forming using the FEM-EA method based on the explicit integration of central differences. Light microscopy (LM), scanning electron microscopy (SEM) and chemical composition analysis (SEM-EDS) were performed to investigate the microstructure of the rivet material and to analyze the cracks. Results showed that the cause of rivet cracking is the accumulation and exceeding of critical tensile stresses in the rivet flange during the tube processing and the final riveting (forming) process. Moreover, it was discovered that rivet fracture is largely caused by structural defects (tertiary cementite Fe,Mn3CIII along the boundaries of prior austenite grains) in the material resulting from the incorrectly selected parameters of the final heat treatment of the prefabricate (tube) from which the rivet was produced. The FEM simulation of the riveting and structural characterization results correlated well, so the rivet forming process and fracture mechanism could be fully investigated.

Effect of adhesive application on friction self-piercing riveting (F-SPR) process of AA7075-T6 aluminum alloy

The combination of adhesive and other joining processes has been an increasing interest in the transportation industry. In this paper, a hybrid joining process combining friction self-piercing riveting (F-SPR) and adhesive bonding was developed to join AA7075-T6 aluminum alloy sheets. The formation process, macro morphology, microstructure, microhardness, and mechanical performance of F-SPR bonded joints were investigated comparing with the F-SPR joints. It was concluded that the adhesive played a role of lubrication for reducing the contact stiffness between the aluminum sheets, which further reduced the interlocking amount but did not affect the solid-state welding between the aluminum sheets. After the F-SPR bonding process, the initial 0.3-mm-thick adhesive layer was squeezed to less than 0.06 mm under a maximum riveting force of about 20 kN. The subsequent baking treatment for the adhesive curing ([email protected]℃) re-precipitated the η strengthening phase in the aluminum heat-affected zone, which further improved the aluminum hardness as well as the mechanical performance of the joint. The F-SPR bonded joint exhibited a combination of adhesive failure and rivet pull-out failure under quasi-static loading, which improved the tensile-shear strength by 128.7 % than the baked F-SPR joint but had no apparent effect on cross-tension performance. The F-SPR bonded joint also superposed the fatigue failure modes of the F-SPR joint and adhesive bonding joint, exhibiting the longest fatigue lives under the same cyclic load amplitude. The process provides a new method for aluminum alloy sheet joining in body-in-white production.

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

The increasing demands for the reduction of carbon dioxide emission require intensified efforts to increase resource efficiency. Especially in the mobility sector with large moving masses, resource savings can contribute enormously to the reduction of emissions. One possibility is to reduce the weight of the vehicles by using lightweight technologies. A frequently used method is the implementation of multi-material systems. These consist of dissimilar materials such as steel, aluminium or plastics. In the production of these systems, the joining of the different materials and geometries is a central challenge. Due to the increasing demands on the joints, the challenges for the joining processes itself are also increasing. Since conventional joining processes are rather rigid and can only react to a limited extent to disturbance variables or changing process variables, new methods and technologies are required. A widely used conventional joining method with these properties is self-piercing riveting. Because of the rigid tool combination and the fact that the rivet geometry that can be used is related to the tools, the joining of multi-material systems requires tool and rivet changes during the process. In order to extend the process window of joining with self-piercing rivet elements, the process is enhanced with a tumbling kinematic of the punch. The integration of tumbling results in a significant increase in the adjustable process parameters. This enables a higher material flow control in the joining process through a specific tumbling strategy. The materials investigated are a steel and an aluminium alloy, which differ significantly in their mechanical properties and have many applications in automotive engineering, especially for structural car body components. The steel material is a galvanized HCT590X+Z dual-phase steel, which is characterised by a low yield strength, combined with high tensile strength and a good hardening behaviour. The aluminium alloy is an EN AW-6014. The precipitation-hardening alloy consists of aluminium, magnesium and silicon with a high strength and energy absorption capability. The objective of this work is to obtain a fundamental knowledge of the new tumbling self-piercing riveting process. With different mechanical properties and different sheet thicknesses of the joining partners, the influences of these parameters on the tumbling strategy of the riveting process are analysed. Such a tumbling strategy is based on the tumbling angle, the tumbling onset and the tumbling kinematics. These parameters are investigated in the context of the work for selected combinations of multi-material systems consisting of HCT590X+Z and EN AW-6014. With the variation of the parameters, the versatility of the process can be investigated and influences of the tumbling on the self-piercing riveting process can be identified. To illustrate the results, force–displacement curves from the joining process of the individual joints are compared and the geometry of the rivet undercut and rivet heads are geometrically measured. Furthermore, micrographs allow the analysis of the characteristic joint parameters interlock, residual sheet thickness and end position of the rivet head.

Electromagnetic Self-Piercing Riveting (E-SPR) Process for Dissimilar Materials Joints

Automobile manufacturers have recently focused their research on the concurrent improvement of fuel efficiency and stability. To implement lightweight body structures, the joining of dissimilar materials, such as advanced high-strength steel (AHSS) and carbon fiber-reinforced polymer (CFRP), is the most important, whereas fusion welding, which is the conventional welding process, is inapplicable owing to the difference in mechanical properties between materials. Thus, the development of an appropriate dissimilar material joining process is of utmost importance, considering productivity and safety. Self-piercing riveting (SPR), a representative lightweight material joining process, has no limitation in terms of applicable materials and has the advantage of improved joint strength. However, as the requirements on the mechanical properties of materials in the industry are continuously increasing, there is a need to further develop the process. Therefore, this study introduces the electromagnetic self-piercing riveting(E-SPR), a joining process that uses high electrical energy to discharge the high electrical energy charged in the capacitor to the working coil located in the head of the C-frame. In particular, this process enables high-quality joining parts to be secured by improving the formability of a material with poor formability under a high strain rate. Consequently, a simple piece of E-SPR equipment was manufactured, and an Al/Steel and Al/CFRP joint was further fabricated. The results of a preliminary test reported a quality equal to or higher than that of the previously reported SPR joint. Based on these research results, further studies on processes and equipment for commercialization will be conducted in the future.

Effects of Residual Stresses on the Fatigue Lifetimes of Self-Piercing Riveted Joints of AZ31 Mg Alloy and Al5052 Al Alloy Sheets

During the self-piercing riveting (SPR) process, residual stress develops due to the high plastic deformation of the sheet materials. In this study, the effect of the residual stress on the fatigue lifetime of SPR joints with dissimilar magnesium AZ31 alloy and aluminum Al5052 alloy sheets was evaluated. The residual stress distribution was derived through a simulation of the SPR process by the FEA (finite element analysis). The measured values by the X-ray diffraction technique confirmed that the validity of the simulation has a maximum error of 17.2% with the experimental results. The fatigue strength of the SPR joint was evaluated at various loading angles using tensile-shear and cross-shaped specimens. It was found that the compressive residual stresses of the joint reduce the stress amplitude by 13% at 106 cycles lifetime, resulting in extension of its lifetime to approximately 3.4 million cycles from 106 cycles lifetime. Finally, it was confirmed that the fatigue life of SPR joints was appropriately predicted within a factor of three using the relationship between the fatigue life and the equivalent stress intensity factor. The fatigue resistance of the magnesium AZ31 alloy on the upper sheet was found to govern fatigue lifetimes of SPR joints of dissimilar magnesium AZ31 alloy sheets.