The present work compares the dynamic effect of a self-piercing riveted (SPR) joint with that of a mechanical clinched joint having the dissimilar materials combination. The substrates used in this investigation are aluminum alloy AA5182-O and deep drawing steel DX51D+Z. The static and dynamic behaviors and the failure modes of the SPR and clinching joints are characterized by lap-shear, cross-tension, and coach-peel tests. The influence of the strain-rate-dependent mechanical behavior of the substrates on the joints is examined; this can help improve prediction of the energy absorption of the joints under impact loading. Considering the realistic baking process in a painting shop, the deforming and hardening effects on the SPR and the clinched joints induced by baking are also studied. The specimens are heated to 180°C for 30 min in an oven and then cooled down in air. The SPR and the clinched joints before and after the baking process are compared in terms of the mechanical behavior.
Currently, manufacturers of lightweight vehicles not only pursue more excellent structural performance of materials but also put effort into selecting or developing reliable technologies for joining dissimilar materials to achieve sufficient structural stiffness and crashworthiness. Among the combinations of dissimilar materials, joints with steels and aluminum alloys are the most prevalent ones applied to achieve viable and sustainable products. Mechanical, chemical, thermal, or hybrid joining processes can be selected to connect steel and aluminum alloy. The process could become complicated considering factors such as manufacturing conditions and cost. Self-piercing riveting (SPR) and mechanical clinching have many advantages and are quite suitable for manufacturing steel-aluminum joints. SPR demonstrates good mechanical and fatigue strength, while clinching has a lower manufacturing cost.
Suitable and cost-efficient mechanical joining technologies must be developed to utilize the weight reduction potential of steels combined with aluminum alloys and thus enable further affordable weight reduction in mass vehicle production. Mechanical joining technologies like SPR and mechanical clinching have been established in many automotive productions for joining multimaterial or light-metal car bodies, as these cold joining processes can be applied to joining dissimilar metals [
SPR is used to join two or more sheets of materials by driving a rivet piercing through the top sheet or the top and middle sheets and partially piercing and locking into the bottom sheet to form a mechanical joint. During the SPR process, the spreading of the rivet skirt is guided by a suitable die, and the punched slug from the top sheet and the middle sheet is embedded into the rivet shank [
The mechanical clinching process is a method of joining sheet metal by localized cold forming of materials, which is also capable of connecting three layers of sheets [
Comparing the production costs of these joining processes, clinching shows a lower running cost in the automotive industry. However, the mechanical characteristics of the joints are very different [
In this study, an aluminum alloy AA5182-O and mild steel DX51D+Z material combination is used. Exemplary results from lap-shear, cross-tension, and coach-peeling tensile tests are shown. The static and dynamic behaviors and the failure modes of both joints are characterized. In addition, as an increase in temperature can lead to a reduction in the strength of aluminum-magnesium alloys such as AA5182 [
Aluminum alloy AA5182-O and mild steel DX51D+Z are conventionally used for automobile body panels. The mechanical properties of the aluminum alloy and steel sheets are given in Table
Mechanical properties of AA5182-O and DX51.
AA5182-O | DX51D+Z | |
---|---|---|
|
6800 | 207000 |
|
0.34 | 0.3 |
|
90.7 | 165.4 |
|
506.9 | 473.2 |
|
0.004 | 0.0055 |
|
0.302 | 0.229 |
The cross-sectional shapes of the joints are shown in Figure
Tool geometries and cross-sectional shapes of (a) self-piercing joint and (b) mechanical clinched joint.
Lap-shear, cross-tension, and coach-peel test specimens used to determine static strength are illustrated in Figure
Specimens of joints for testing: (a) lap-shear, (b) cross-tension, and (c) coach-peel.
AA5182-O material sheets for automotive BIW applications show softening behavior with respect to the prestrained sheets after the paint bake cycle. The specimens were placed in an oven at 180°C for 30 min and tested under static conditions to determine the influence of the baking process on the mechanical behavior of the joints.
A universal testing machine (Zwick Z020) and hydraulic testing machine (Zwick H5020) were used to conduct the static and dynamic tests, respectively. The global loading speeds were 0.083 × 10−3 m/s, 0.1 m/s, and 2 m/s. The digital image correlation method (VIC-2D) was adopted for deformation measurement. The gauge length of lap-shear and coach-peel specimens was 50 mm, and the displacements measured in cross-tension tests were the separative displacement of the fixtures, which are regarded as rigid.
The substrate materials have different responses to the strain rate effect. Previous studies have reported that a negative strain rate dependence of the material strength has been observed for Al-Mg alloys at a quasistatic strain rate, but there is a positive strain rate dependence for a high strain rate [
Tests for lap-shear and coach-peel specimens under loading speeds of 0.1 m/s and 2 m/s were also performed to understand the mechanical behavior and failure modes of joints using a hydraulic high-speed testing machine. All tests were repeated three times to guarantee reproducibility, and the median result is shown.
The force-displacement curves and failures for static lap-shear tests of joints obtained by both joining processes are illustrated in Figure
Static lap shear tests: (a) force-displacement curves, (b) material fracture failure mode of the SPR joint, and (c) pulling-out failure mode of the clinched joint at 0.083 × 10−3 m/s.
In these tests, as the strength of steel sheet is higher than that of the aluminum one, steel sheets on the top are bent up and large local deformation occurs on the button of the aluminum sheets, which results in joint failure. In the SPR joint, fracture can be found on the aluminum sheet from the button to the edge, bottom sheet tearing mode, which is not a common failure type in similar materials’ joints. On the contrary, no large deformation occurs out of the button area in the clinched joint. The steel sheet is pulled out from the interlock on the aluminum sheet, and there is no material fracture on either sheet.
The force-displacement curves and failures for static cross-tension tests of the joints are illustrated in Figure
Static cross-tension tests: (a) force-displacement curves, (b) pulling-out failure modes of SPR joint, and (c) clinched joint at 0.083 × 10−3 m/s.
In terms of failure, pull-out is the dominant mode in both joints. Severe local doming occurs on the steel and aluminum sheets, and the strength of the steel sheets and rivets is higher, which leads to the expansion of clinched region on the bottom sheet. The interlocks in both joints are unbuttoned, so the steel sheets/rivets are plugged out, and no obvious material damage is found in either joint.
Similar mechanical behaviors are also observed in the static coach-peel tests, as the joints in these two forms primarily undergo a normal direction force, as shown in Figure
Static coach-peel tests: (a) force-displacement curves, (b) deformation process of SPR joint, and (c) clinched joint at 0.083 × 10−3 m/s.
From these tests, it can be seen that in the AA5182-O and DX51 joints under static conditions, the stiffness of the two joints is similar, but the toughness of the SPR joint is remarkably higher than that of the clinched joint. The normal strengths of these joints are much lower than their shear strengths. It is worth mentioning that plateaus appear when the loading reaches the peak of SPR joints under lap-shear and coach-peel conditions. This indicates that when the button begins to separate, the rivet can still help to bear the loading, but the clinched joint fails rapidly. Tail pull-out is the main failure mode in the cross-tension and coach-peel tests, which implies that it is better to prevent the joints from being exposed to a normal direction load.
Dynamic tests were conducted under global loading speeds of 0.1 and 2 m/s. Figure
Dynamic lap-shear tests of SPR joints: (a) force-displacement curves and (b) failure modes at three speeds.
Although the bottom sheet tearing is observed as the dominate failure mode, the modification of the curves could be the result of different dynamic behaviors with respect to the substrate materials. Generally, steels are more sensitive to strain rate effects than aluminum alloys, which contributes to the loss of load-bearing capability of the rivet tail. This leads to a transition of the deformation patterns on steel and aluminum sheets. At 0.083 × 10−3 m/s, the aluminum sheet remains flat during the tensioning process, and the steel sheet is bent. Conversely, as the yielding stress increases because of the strain rate effect, the steel sheet remains flat, while the aluminum sheet is bent, after which material failure occurs. The bent bottom sheets under dynamic conditions lead to a larger loading angle like peeling and a rapid failure.
Considering the coach-peel tests, Figure
Dynamic coach-peel tests of SPR joints: (a) force-displacement curves and (b) failure modes at three speeds.
The dynamic response of a clinched joint to shear loading is different from that of an SPR joint. As shown in Figure
Dynamic lap-shear tests of clinched joints: (a) force-displacement curves and (b) failure modes at three speeds.
Correspondingly, the clinched joint under coach-peel condition is enhanced by the loading speed. Both the maximum strength and ductile are enlarged. The total energy absorption capability increases from 1.8 (5 mm/min) to 1.9 J under 0.1 m/s and 2.2 J under 2 m/s, as shown in Figure
Dynamic coach-peel tests of clinched joints: (a) force-displacement curves and (b) failure modes at three speeds.
To conclude, both joints of DX51 and AA5182-O combination under shear and normal load conditions have dynamic effects. However, the dynamic effects perform quite conflicting. The SPR joint tends to be weakened and the clinched joint tends to be strengthened by the effect due to the different strain rate effects of substrate materials and dynamic interface contacts.
AA5182-O attains its strength through work hardening and exhibits softening during the paint bake cycle, which is very likely to occur for BIW, as shown in Figure
Quasistatic stress-strain curves of baked AA5182-O at 0.05 plastic deformation.
Figure
Comparison of lap-shear tests of SPR and clinched joints before and after the baking process at 0.083 × 10−3 m/s.
The cross-tension tests results demonstrate very different baking effect on the joints, as shown in Figure
Comparison of cross-tension tests of SPR and clinched joints before and after the baking process at 0.083 × 10−3 m/s.
In terms of coach-peel tests, completely converse responses to the baking process appear, compared with the lap-shear tests, as shown in Figure
Comparison of coach-peel tests of SPR and clinched joints before and after the baking process at 0.083 × 10−3 m/s.
The test results indicate that both SPR and clinched joints of AA5182-O and DX51 suffer from the baking process. The baking effect, however, is loading condition-dependent and affects the specimens differently. Table
Test result summary (units: speed m/s, force (N), and energy (J)).
Test speed | SPR | SPR baked | Clinch | Clinch baked | ||||
---|---|---|---|---|---|---|---|---|
Peak force | Total energy absorption | Peak force | Total energy absorption | Peak force | Total energy absorption | Peak force | Total energy absorption | |
|
||||||||
0.083 × 10−3 | 2280 | 21.0 | 2251 | 7.7 | 2180 | 2.1 | 2123 | 1.5 |
|
2320 | 10.1 | — | — | 2366 | 3.5 | — | — |
|
2477 | 11.9 | — | — | 2461 | 5.1 | — | — |
|
||||||||
|
||||||||
0.083 × 10−3 | 614 | 5.5 | 574 | 5.4 | 302 | 1.8 | 282 | 1.6 |
|
536 | 5.4 | — | — | 322 | 1.9 | — | — |
|
524 | 5.4 | — | — | 350 | 2.2 | — | — |
|
||||||||
|
||||||||
0.083 × 10−3 m | 1180 | 5.9 | 1069 | 6.1 | 675 | 3.1 | 652 | 2.9 |
Experiments on SPR and mechanical clinched joints of AA5182-O and DX51 with various loading directions, speeds, and heat treatments were realized. The energy absorption of SPR joints is much higher under all conditions. The dynamic and baking effects on the mechanical behaviors of these two joining methods were compared. The failure modes of SPR joints of dissimilar materials are direction-sensitive. Substrate material failure on aluminum is the primary mode under lap-shear conditions, and tail pulling-out dominates the cross-tension and coach-peel conditions. Clinched joints commonly fail by pulling-out, where the steel sheet separates from the bottom sheet. Dynamic effects on joints of dissimilar materials are loading condition-dependent. The strength of the two joints under lap-shear conditions increases with increasing loading rate. However, the dissipated energy of the SPR joint decreases because of the loss of load-bearing capability of the rivet tail, and that of the clinched joint increases. Under the coach-peel condition, the strength of the SPR joint decreases with increasing loading rate and the dissipated energy stays the same. However, both strength and the total energy absorption capability of the clinched joint increase under dynamic conditions. The baking process significantly affects the mechanical behavior of SPR and clinched joints and is loading direction-dependent. Both joints are weakened under lap-shear and coach-peel conditions, but the baking process has a negligible influence under cross-tension conditions.
The experimental data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This was sponsored by the Key D&R Program of China under contract no. 2016YFB0101606. The author would like to acknowledge the staff at Suzhou Automotive Research Institute of Tsinghua University (TSARI) for providing technical support in conducting experiments.