Self-piercing Riveting Research Articles

Friction self-piercing riveting (F-SPR) of aluminum alloy to magnesium alloy using a flat die

Friction self-piercing riveting (F-SPR) process based on a pip die has been invented to solve the cracking problems in riveting high-strength and low-ductility light metals, such as magnesium alloys, cast aluminum, and 7 series aluminum alloys. In this paper, in order to solve quality issues caused by the misalignment between rivet and pip-die in F-SPR, a flat-die based F-SPR process was proposed and employed to join 1.27 mm-thick AA6061-T6 to 3 mm-thick AZ31B. The results indicate that a 1.0 mm die distance is effective to avoid rivet upset and insufficient flaring. As the feed rate increases, the heat input in the whole process decreases, resulting in a larger riveting force, which in turn increases both the bottom thickness and interlock amount. Besides, solid-state bonding, including Al-Mg intermetallic compounds (IMCs), Al-Mg mechanical mixture, and Al-Fe atom interdiffusion was observed at the joint interfaces. The upper Al layer was softened, but the lower Mg layer was hardened, and both sheets exhibited a narrowed affected region with the increase of feed rate, while the rivet hardness shows no obvious change. Three fracture modes appeared accompanying the variations in lap-shear strength and energy absorption as the feed rate increased from 2 mm/s to 8 mm/s. Finally, the F-SPR process using a flat die was compared to those using a pip die and a flat bottom die to show the advantage of flat die on coping with the misalignment problem.

Study on shear performance and calculation method for self-pierce riveted joints in galvanized steel sheet

Self-pierce riveting (SPR) with high connection efficiency and automatic level is suitable for joining galvanized steel sheets in cold-formed steel structures. 51 SPR joints in galvanized steel sheet were tested to study their shear performance. The effects of sheet thickness, sheet thickness ratio, and rivet length on the failure mechanism and shear performance were investigated. And an empirical equation about the relationship of sheet thickness and rivet length was developed. This paper also analyzed the applicability for the existing calculation method of shear strength of SPR joints and the design methods of shear strength of self-drilling screw joints in some specifications, and then proposed a new calculation method of shear strength. The results showed that sheet thickness and thickness ratios were two critical factors in the shear performance and failure mechanism of SPR joints. There was an optimal rivet length for each sheet thickness, which could be calculated directly by the developed empirical equation. Compared with the existing methods, the calculation method of shear strength for galvanized steel sheet SPR joints proposed in this paper considered overall failure modes, and had superior calculation accuracy and favorable stability.

Microstructural evolution in friction self-piercing riveted aluminum alloy AA7075-T6 joints

Friction self-piercing riveting (F-SPR) is an emerging technique for low ductility materials joining, which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet. The severe plastic deformation of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures. The cut-off action of the work materials by the rivet further complicates the material flow during joint formation. This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction (EBSD) techniques. The results suggested that as the base material approached the rivet, grains were deformed and recrystallized, forming two distinct fine grain zones (FGZs) surrounding the rivet and in the rivet cavity, respectively. Solid-state bonding of aluminum sheets occurred in the FGZs. The formation of FGZ outside the rivet is due to dynamic recrystallization (DRX) triggered by the sliding-to-sticking transition at the rivet/sheet interface. The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum, which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX. Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.

Modelling and interaction analysis of the self-pierce riveting process using regression analysis and FEA

Self-pierce riveting (SPR) is a major joining method used in the automotive industry. However, there still lacks a fast and easy-to-use joint quality prediction tool available for the automotive engineers. In this study, the simple but effective regression analysis method was applied to quickly predict the SPR joint quality. Two regression models were developed for the prediction of the interlock and the minimum remaining bottom sheet thickness (Tmin). The prediction accuracy of the developed regression models was validated by comparing with the experimental results. Under the studied joint configurations, the mean absolute errors (MAE) of the interlock and Tmin were 0.047 mm and 0.053 mm, respectively, and the corresponding mean absolute percentage errors (MAPE) were 10.4% and 12.3%. With the developed models, the interaction effects between rivet and die parameters on the joint interlock and Tmin were also systematically analysed. The results revealed that the rivet and die parameters demonstrated significant influences on the interlock but not on the Tmin. These interaction effects were further examined by analysing the deformations of the rivet and substrate materials. Moreover, the die-to-rivet volume ratio (R) was found to be critical for the formation of interlock, and a larger interlock is more likely achieved when the R is close to 1.0.

Mechanical clinching and self-pierce riveting for sheet combination of 780-MPa high-strength steel and aluminium alloy A5052 sheets and durability on salt spray test of joints

To increase the usage of high-strength steel and aluminium alloy sheets for lightweight automobile body panels, the joinability of sheet combinations including a 780-MPa high-strength steel and an aluminium alloy A5052 sheets by mechanical clinching and self-pierce riveting was investigated for different tool shapes in an experiment. All the sheet combinations except for the two steel sheets by self-pierce riveting, i.e., the two steel sheets, the two aluminium alloy sheets, and the steel-aluminium alloy sheets, were successfully joined by both the joining methods without the gaps among the rivet and the sheets. Then, to show the durability of the joined sheets, the corrosion behaviour and the joint strength of the aged sheets by a salt spray test were measured. The corrosion and the load reduction of the clinched and the riveted two aluminium alloy sheets were little. The corrosion of the clinched two steel sheets without the galvanized layer progressed, and then the load after 1176 h decreased by 85%. In the clinched two galvanized steel sheets, the corrosion progress slowed down by 24%. In the clinched steel and aluminium alloy sheets, the thickness reduction occurred near the minimum thickness of the upper sheet and in the upper surface on the edge of the lower aluminium alloy sheet, whereas the top surface of the upper sheet and the upper surface of the lower sheet were mainly corroded in the riveted joint. The load reduction was caused by the two thickness reductions, i.e., the reduction in the minimum thickness of the upper sheet and the reduction in the flange of the aluminium alloy sheet. Although the load of the clinched steel without the galvanized coating layer and aluminium alloy sheets decreased by about 20%, the use of the galvanized steel sheet brought the decrease by about 11%. It was found that the use of the galvanized steel sheets is effective for the decrease of strength reduction due to corrosion.

Deep-Learning Approach to the Self-Piercing Riveting of Various Combinations of Steel and Aluminum Sheets

Deep-learning architectures were employed to simulate the self-piercing riveting process of steel and aluminum sheets and predict the cross-sectional joint shape with a zero head height. Four steels (SPRC440, SPFC590DP, GI780DP, SGAFC980Y) and three aluminum alloys (Al5052, Al5754, Al5083) were considered as the materials for the top and bottom sheets, respectively. The key objective was to consider the material properties of these metal sheets (Young's modulus, Poisson's ratio, and ultimate tensile strength) in a deep-learning framework. Two deep-learning models were considered: In the first model, the properties of the top and bottom sheets were adopted as the scalar inputs, and in the second model, the three properties were graphically assigned to the three channels of the input image. Both the models generated a segmentation image of the cross-section. To assess the accuracy of the predictions, the generated images were compared with ground truth images, and three key geometrical factors (interlock, bottom thickness, and effective length) were measured. The first and second models achieved prediction accuracies of 91.95% and 92.22%, respectively.

Forming Quality and Fatigue Behavior of Self‐Piercing Riveted Joints of DP590 and AA6061 Plates

Two kinds of self‐piercing riveted (SPR) joints were prepared with DP590 and AA6061 plates. The forming qualities of the joints were studied using the finite element method. The relationships between the fatigue life and failure forms of the joints with different upper plates were discussed. Finally, the failure mechanisms of the joints were analyzed. The results show that the maximum static tension of DA32 joints (with an upper plate of DP590) is significantly greater than that of the other kind, and the fatigue life of DA32 joints is always longer than that of AA40 joints when with a 2.0 mm thick AA6061 aluminum alloy upper plate under the same fatigue load. The failure mode of SPR joints changes obviously due to different upper plates, and the fatigue life of the joints can be effectively improved by reducing the microvibration wear.

Process and performance characteristics of an improved friction-stir riveting process

Friction-stir riveting is a new mechanical fastening technique developed in recent years. It combines the spinning and extruding actions of friction-stir spot welding, and the interlocking created by embedding a rivet in the sheets in self-piercing riveting, producing a joint which strength stems from a compact stirred or mixed zone, an embedded solid rivet, and solid bonding. In this study a set of improved rivet design and tooling, based on the understanding of friction-stir riveting process and the joint formation gained in previous investigations, was used to join aluminum alloy sheets. The evolution of microstructure during riveting was investigated using the new rivet and tooling, and the joint structural characteristics were analyzed. A detailed study was also conducted on the effect of rivet depth of feed, the joint structures created, and the performance of these joints, compared with those using the previous rivet design and tooling. Afterwards, a statistical analysis was conducted to optimize the riveting process. Using the new rivet and tooling, and optimized riveting parameters, joints with peak load consistently exceeding 8 kN were obtained in joining 2-mm AA5052 sheets. The strength of the new friction-stir riveted joints is much higher than those obtained through other mechanical joining techniques, and even higher than resistance spot welding in joining aluminum sheets.

Self-Piercing Riveting of Medium- and High-Strength Al and Mg Alloy Sheets Enabled by In-Process Electric Resistance Heating

Abstract Enduring joints of high-strength materials are becoming increasingly important for the manufacturing of highly loaded body-in-white structures, particularly for automobiles, which require structural light-weighting without sacrificing passenger safety. While self-piercing riveting has been proven suitable for various material classes, its application to high-strength, low-ductile materials is hindered by the occurrence of cracks and insufficient penetration. In this work, we demonstrated that these restrictions can be overcome by localized one-sided short-time electric resistance heating. The treatment can be integrated into the riveting process, allowing for short process times. The heating lowers the hardness of the materials to be joined and enables piercing and penetration of the sheets to yield crack-free joints. The local mechanical properties are hardly changed in case of peak-aged sheet material conditions but increase significantly in the close vicinity of the rivet in case of under-aged sheet material conditions. The outstanding static and dynamic mechanical properties of the resulting joints evidence their robustness.

Self‐Piercing Riveting of Hot Stamped Steel and Aluminum Alloy Sheets Base on Local Softening Zone

Self‐piercing riveting (SPR) has been used to joint steel to aluminum alloy sheets in multimaterial car body. It is a challenge to joint high strength steel to aluminum alloy by SPR process, especially for materials with high strength and large thickness. Herein, the SPR process for 1500 MPa hot stamped steel and aluminum alloy sheets is studied by experiment and simulation method. The laser heat treatment process is applied to the hot stamped steel sheet before joining. The relationship between heat treatment process and hardness distribution is studied and the softening zone with hardness less than 300 HV is obtained. Furthermore, the local softening zone is joined to an aluminum alloy sheet by SPR process. In the process, the rivets with high hardness are used to pierce the steel sheet. The failure mode and maximum tensile shear force of the joint are studied. At the same time, the joints without laser heat treatment are compared. The results show that the interlock and the maximum shear force for laser heat treatment increase by 91% and 26%, respectively.

Self-pierce riveting of tubes to sheets

This paper presents a new joining by forming process capable of attaching a sheet to the end of a tube without the need of accessories and extra materials. The process is named as ‘self-pierce riveting of tubes to sheets’ and is an extension to sheet-tube connections of the working principle of double-sided self-pierce riveting utilized for connecting two overlapped sheets. The process requires tubes with chamfered ends to be pushed against the sheets and its ends to flare (curl) in order to create an interlocking that holds the sheets and tubes tightly together. The resulting joints are invisible because the interlockings are hidden inside the sheets. The presentation draws from deformation mechanics and identification of the major parameters to characterization of the interlocking and evaluation of the pull-out destructive strength that the new joints are capable to withstand without failure. Aluminium sheets and carbon steel tubes are utilized to demonstrate the effectiveness of the new joining by forming process and to demonstrate its ability to connect sheets and tubes made from dissimilar materials at room temperature. Finite element modelling using an in-house computer program gives support to the presentation.

Study on Self-piercing Rivet and Mechanical Properties of Aluminum Alloy Joint

In this work, the self-piercing rivet (SPR) experiments were carried out and the mechanical properties tests under different SPR process were taken. The effects of SPR process on the geometric characteristics and strength of the joint were studied. It was found that the SPR joint under displacement-controlled process had larger interlocking value, higher mechanical properties, more stable geometric characteristics, and better mechanical properties. The failure mode of SPR joints was mainly pull-off failure. And the mechanical properties of SPR joints decreased with the increase of head height of SPR, while the residual thickness has no significant effect on the mechanical properties. The mechanical properties of SPR joints were significantly improved when structural adhesive was added into SPR joints, in which the failure mode was transformed into joint drawing failure. In addition, the fatigue behavior of SPR joint was studied. It was found that there was no significant difference between displacement-control process and energy-control process in fatigue performance of SPR joint. Moreover, cracks were initiated on the surface, which meet the surface initiation mechanism.

Fatigue life performance of multi-material connections hybrid joined by self-piercing rivets and adhesive

Abstract The multi-material design presented contains EN AW-6016 aluminum and high strength CR330Y590T-DP steel. This dissimilar combination is an example of an affordable lightweight design solution, but it requires an adapted joining technology. Hybrid joining technologies such as selfpiercing riveting (SPR) in combination with a structural adhesive enables an assembly of such dissimilar material combinations. In addition to higher manufacturing costs for mechanical joining the design process still requires a great amount of effort. This study provides a simple approach for assessing hybrid joined multi-material connections. Therefore, tests for several combinations of the most relevant parameters on fatigue life (material properties, sheet thickness, load cases) were performed under quasi-static and cyclic loads. Based on the data acquired, it is shown that the fatigue life of hybrid joined connections can be estimated by superposing the contributing fatigue life of purely SPR and purely adhesive joints.

Finite element analysis of self-piercing riveting of aluminum alloy and carbon fiber reinforced polymer composites

Self-piercing riveting (SPR) is a high-speed fastening process that can join similar and dissimilar sheet materials without the need for pre-processing such as drilling or punching. During SPR processes, two overlapping sheets are joined by a rivet. The upper sheet is punched first by the rivet and then the lower sheet is deformed between the rivet and the die, creating a mechanical interlock. In this study, self-piercing riveting of aluminum alloy and carbon fiber reinforced polymer composites (CFRP) sheets was analysed using finite element simulations. For the finite element simulation of SPR processes, the orthogonal elasticity, the fracture model, and the cohesive zone model were used for describing the behaviour of CFRP. For validation of the composite material model, the punching process of CFRP was performed and the results were compared with FE predictions. The SPR process of the aluminum alloy and CFRP was simulated numerically and the performance of the joint was evaluated.

A self-piercing riveting method for joining of continuous carbon fiber reinforced composite and aluminum alloy sheets

During the self-piercing riveting process of carbon fiber reinforced polymer (CFRP) and aluminum sheets, the CFRP is penetrated by the rivet, which weaken the mechanical properties of the joint of CFRP and aluminum alloy sheets to a certain extent. To improve quality of the joint, this study proposes a post-curing self-piercing riveting (PC-SPR) method to join CFRP with aluminum alloy sheets. The mechanical properties and failure modes of the joints were studied through tensile shear tests and the effects of joint structure and CFRP thickness on joint quality were examined. The failure mode of the joint was analyzed by observing the microscopic appearance, and the damage mechanism of the joint was systematically discussed. Results showed that compared with regular self-piercing riveting (R-SPR) joints, PC-SPR joints had better strength.

Stress-based approach for fatigue life calculation of multi-material connections hybrid joined by self-piercing rivets and adhesive

Dissimilar connections with aluminum EN AW-6016 and high strength steel CR330Y590T-DP are affordable and promising regarding lightweight potential. Self-piercing riveting (SPR) and structural adhesive enable an assembly of such dissimilar combinations. This study provides an approach to assess hybrid joints by superposing the contributions of each joining technique on the resulting fatigue life. The fatigue life estimation of SPR is based on a structural stress approach and material SN curves. For the adhesively bonded connections, a stress-based approach and a master SN curve are used. Both of the approaches for fatigue life estimation show a reasonable agreement with test data.

Evaluation of Mechanical Characteristic for CFRTP/SPFC980Y Dissimilar joint

In this study, the mechanical characteristics of CFRTP/SPFC980Y dissimilar joints were evaluated. Bolt/nut, SPR (Self Piercing Rivet), and hybrid (bolt/nut & adhesive) joining were selected to evaluate joint properties. In the bolt/nut joining case, the effect of washer application in the joint behavior was observed, and the tightening torque was optimized. In SPR joining, CFRTP and SPFC980Y lacked formability, so SPR joining was possible by applying the additional sheet. For hybrid (bolt/nut & adhesive) joining, cause the bolt’s preload, the tensile shear load was lower than the applied adhesive joint. Nevertheless, the highest toughness was obtained in the hybrid joint due to the stress distribution effect by adhesive. Key words: CFRTP, SPFC980Y, Bolt/Nut, Self-Piercing Rivet, Adhesive, Hybrid joint

Mechanical Clinching and Self-Pierce Riveting of Thin Three Sheets of 5000 Series Aluminium Alloy and 980 MPa Grade Cold Rolled Ultra-High Strength Steel.

One thin 5000 series aluminium alloy sheet and two thin 980 MPa grade cold rolled ultra-high strength steel sheets were joined by self-pierce riveting and mechanical clinching processes. The joinabilities for a combination of the aluminium and steel sheets in both processes were investigated for different die shapes in the experiment and finite element simulation. In self-pierce riveting, the three sheets were successfully joined for both combinations of the upper and lower aluminium alloy sheets by optimizing the shapes of a die and rivet. In mechanical clinching, the three sheets were successfully joined by an optimum die for the configuration of the upper aluminium alloy sheet. On the other hand, the three sheets for the configuration of the lower aluminium alloy sheet were not joined even by optimizing the die shape in the both finite element simulation and experiment, because the material flow of the steel sheets was insufficient to form the two interlocks. The tension-shear loads for the clinched and riveted sheets with the adhesive were almost the same, because the load for the adhesive was the highest. In the cross-tension test, however, the load by the adhesive was comparatively small.

Mechanical and Corrosion Assessment of Friction Self-Piercing Rivet Joint of Carbon Fiber-Reinforced Polymer and Magnesium Alloy AZ31B

Abstract In the present work, thermoset carbon fiber–reinforced polymer (CFRP) was spot joined to magnesium alloy AZ31B by a friction self-piercing riveting (F-SPR) process. Lap shear tensile and cross-tension testing were used to evaluate the mechanical joint performance. An average lap shear tensile load of 5.18 kN was achieved, while an average of 2.81 kN was found from cross-tension testing. All F-SPR samples showed a pullout of AZ31B after mechanical testing, indicating good mechanical interlocking between the steel rivet and AZ31B. Corrosion potential was measured for each material to establish the galvanic corrosion characteristics. As expected, AZ31B was found to be the most active, while thermoset CFRP was the most noble. The steel rivet fell between the AZ31B (active) and the thermoset CFRP (noble). Salt fog corrosion testing (ASTM B-117) was performed to evaluate the corrosion performance of the uncoated F-SPR joint. With up to 200 h of exposure, the post-corroded F-SPR joint integrity retained 81.2% of the pre-exposure F-SPR joint strength with AZ31B pullout failure mode. From cross-sectional analysis of the F-SPR joint, extensive corrosion of AZ31B was observed at the joint and other exposure areas. However, steel rivet was not significantly corroded due to sacrificial anode effect by which AZ31B corroded first in the galvanic couple.

Fatigue life performance of multi-material connections hybrid joined by self-piercing rivets and adhesive

Abstract The multi-material design presented contains EN AW-6016 aluminum and high strength CR330Y590T-DP steel. This dissimilar combination is an example of an affordable lightweight design solution, but it requires an adapted joining technology. Hybrid joining technologies such as selfpiercing riveting (SPR) in combination with a structural adhesive enables an assembly of such dissimilar material combinations. In addition to higher manufacturing costs for mechanical joining the design process still requires a great amount of effort. This study provides a simple approach for assessing hybrid joined multi-material connections. Therefore, tests for several combinations of the most relevant parameters on fatigue life (material properties, sheet thickness, load cases) were performed under quasi-static and cyclic loads. Based on the data acquired, it is shown that the fatigue life of hybrid joined connections can be estimated by superposing the contributing fatigue life of purely SPR and purely adhesive joints.