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Article

High-Cycle Fatigue Characteristics of Aluminum/Steel Clinched and Resistance-Spot-Welded Joints Based on Failure Modes

by
Ákos Meilinger
*,
Péter Zoltán Kovács
and
János Lukács
Institute of Materials Science and Technology, Faculty of Mechanical Engineering and Informatics, University of Miskolc, 3515 Miskolc, Hungary
*
Author to whom correspondence should be addressed.
Metals 2024, 14(12), 1375; https://doi.org/10.3390/met14121375
Submission received: 22 October 2024 / Revised: 27 November 2024 / Accepted: 28 November 2024 / Published: 1 December 2024
(This article belongs to the Special Issue Manufacturing Processes of Metallic Materials)
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Abstract

:
Materials for lightweight vehicle structures play an increasingly important role in both economic and environmental terms; high-strength steels and aluminum alloys are suitable for this role. Resistance spot welding (RSW) and conventional clinching (CCL) methods can be used for joining vehicle bodies and can also be applied for aluminum/steel hybrid joints. Whereas vehicle structures are subjected to cyclic loading, damages can occur due to high-cycle fatigue (HCF) during long-term operation. Systematic HCF test results are rarely found in the literature, while HCF loading basically determines the lifetime of the hybrid joints. The base materials 5754-H22, 6082-T6, and DP600 were used for similar and hybrid RSW and CCL joints, and HCF tests were performed. The number of cycles-to-failure values and failure modes were studied and analyzed. Based on the experimental results, HCF design curves belonging to a 50% failure probability were calculated for all cases, and the curves were compared. Clear relationships were found between the failure modes and fatigue cycle numbers for both joining methods. Considering the steel/steel joints as a base, the load-bearing capacity of the hybrid joints is lower (48.7% and 73.0% for RSW, 35.0% and 38.7% for CCL) and it is even lower for the aluminum/aluminum joints (39.9% and 50.4% for RSW, 31.7% and 35.0% for CCL). With one exception, the load-bearing capacity of the CCL joints is higher than that of the RSW joints (156.1–108.3%).

1. Introduction

Different structures and equipment are made up of several different elements, the features (e.g., material quality, characteristic geometric dimensions, and surface area) of which may differ significantly. The construction of structures or equipment is inconceivable without the interconnection (joining) of the individual elements, and the correlation of joining technologies is achieved within the structure–material–manufacturing technology triangle. The diversity of structures, together with the economic and environmental demands and constraints, require continuous innovation in joining technologies [1,2,3], regardless of the size of the structures, but at different scales and/or levels [4].
Joining technologies play an exceptionally important role in all phases of a product’s life—these phases include the design, development, fabrication, utilization, maintenance, and repair of a product as well as its recycling and disposal [5]. As a direct consequence, joining technologies and joining processes can be classified in several ways. They can be classified according to the size scale (e.g., micro/macro [4,5]), the combined materials (e.g., similar/dissimilar [6]), the main effect that creates the joint (e.g., heat/force and associated (plastic) deformation [2,7]), the variations within a process (e.g., clinching sheet materials [8]), and the geometry of the joint (continuous/spot), and the list is continuous.
Among the various structures, particular attention should be paid to vehicular/automotive structures, where a distinction can be made between those made of thicker plates (e.g., the chassis) and those made of thinner plates (e.g., the car body). Although not exclusively, plate thickness influences the joining technologies that can be used. Focusing on welding, fusion welding technologies may be preferred for thicker plates (e.g., trucks, heavy machinery), while solid-state welding may be preferred for thinner plates (e.g., cars). However, there are certainly processes (e.g., laser welding) that can be used regardless of the plate or sheet thicknesses. For thinner plates, there is a wide range of combined processes, where the combined formulation can have multiple meanings. It can be interpreted as a combination of several technologies (e.g., forming and welding), as a combination of different processes from one technology (e.g., root welding by the tungsten inert gas (TIG) welding method, the welding of additional layers by the metal inert gas (MIG) welding method, or laser MIG (or TIG) hybrid joining), or there are many variations of joints made with auxiliary or additional elements (e.g., rivets, screws) [9,10].
Joining technologies involving significant plastic deformation are typical for thinner plates or sheets; one of the characteristic technologies for car bodies is clinching. It is possible to clinch steel elements [11], steel and non-steel elements, or metallic elements [12], and both metallic and non-metallic elements [10]. As can be seen from the list, clinching technology can be used to join materials with significantly different physical properties (e.g., metal and fiber-reinforced plastic, or steel and aluminum alloy). In addition to the more typical two-plate joints, three-plate joints are also used [13].
One of the most typical material combinations for dissimilar material connections is the joining of steel to aluminum. The welding technologies used are summarized in [9,14], friction-based technologies are reviewed in [15], and details of bump or projection welding and ultrasonic welding processes are summarized in [16,17].
In the case of spot-like welding technologies, essentially resistance spot welding (RSW) and ultrasonic welding (UW) can be applied for hybrid joints [18,19,20]. In recent years, special process variants have emerged to improve the joint properties, such as resistance element or resistance rivet welding (REW or RRW) [21,22], metallic bump-assisted RSW (MBaRSW) [23], (high power) ultrasonic welding ((HP)USW) [24], and a combination of ultrasonic and resistance spot welding [25]. Micro RSW (MRSW) [26] and vaporizing foil actuators welding (VFAW) [27] have also been investigated as prospective technologies for aluminum/steel joining. These advanced joining methods frequently require additional materials or/and process steps; or rather, the process cycle is longer in comparison with the basic RSW cycle. Different spot welding process variants used for aluminum/steel joining, including their main characteristics, are summarized in Table 1. An overview of clinching technologies suitable for making aluminum/steel hybrid joints can be found in Table 2, where conventional clinching (CLL) [28] was selected as a basic technology. The material grades in both tables have retained the designations used in the original sources and, within each technology, the applied order corresponds to the order of the aluminum groups. It is worth noting that the differences between the (clinching) technologies listed in Table 2 are more significant than in the case of the (spot welding) technologies found in Table 1.
After comparing the data found in Table 1 and Table 2, also with regard to the specific examples presented, two features should be highlighted. Firstly, for both technologies, the application of coatings on steel plates and auxiliary elements is common, which is clearly unrelated to the joining technologies, and secondly, the use of intermediate (third material) layers is less common in clinching technologies.
It is a well-known fact that car bodies are subjected to cyclic loads that fall within the range of high-cycle fatigue (HCF) or ultra-high-cycle fatigue (UHCF). Obviously, both the applied materials and the joints made with different technologies must withstand these loads.
On the basis of the above, the general objective of our research is the continuation of previous work [64], involving the comparison of aluminum/steel joining technologies and optimizing the technological parameters of joining with respect to different criteria (medium term). The direct aim of the research work and this paper provide newer information about the behavior of aluminum/steel RSW and CCL joints under HCF loading conditions (short term). The novelty content of the manuscript is twofold: On the one hand, a relationship was searched for and found between the magnitude of the high-cycle fatigue loading of joints and the failure modes. On the other hand, the load-bearing capacity of the two joining technologies under HCF loading was compared. The comparison provides a practical opportunity to select the joining technology based on the cyclic load range.

2. Materials and Methods

2.1. Investigated Materials

DP600 (SSAB EMEA AB, Borlange, Sweden) steel was chosen as the steel part, and 6082-T6 and 5754-H22 alloys (both from Comhan Aluminium GMBH, Hagen, (NRW), Germany) were chosen as the aluminum part; these materials are frequently used for automotive applications. The DP600 steel has relatively low strength compared to other dual phase (DP) steels and contains hard martensite islands embedded in a ferrite matrix with a dispersed distribution. The 6082-T6 aluminum alloy is heat-treatable and has a higher strength and lower formability. The material contains mainly Si and Mg alloying elements, causing ageing. Furthermore, it is more sensitive for welding, especially for softening in the heat affected zone (HAZ), and hot cracking can easily occur too. The 5754-H22 aluminum alloy has good formability and appropriate associated strength properties. Its main alloying element is Mg, and its strength can be increased by forming, and then softened to a quarter of the hardness. The weldability of the alloy is favorable compared to other aluminum alloys.
For the investigations, both similar (namely, steel/steel and aluminum alloy/aluminum alloy) and hybrid (such as aluminum alloy/steel) RSW and CCL joints were produced. For a better comparison, the base materials have a 1 mm thickness. Table 3 and Table 4 show the chemical composition of the steel and aluminum base materials, respectively, and Table 5 summarizes their essential mechanical properties (yield strength (Rp0.2), tensile strength (Rm), and elongation (A50)). The data in all three tables are taken from quality certificates from the base material manufacturers.

2.2. Resistance Spot Welding (RSW)

A TECNA 8007-type resistance spot welding machine (TECNA S. p. A., Bologna, Italy) was used with a TE550 control module to form the RSW joints. The machine operates with AC, and the frequency of the transformer was 50 Hz. A pneumatic system was used to ensure the welding force. The process was controlled for a constant current, meaning that the welding current remained the same throughout the process, while the voltage varied. The same welding electrode was applied for all material combinations; the electrode material was CuCrZr (AVIVA Metals, La Seyne sur Mer, France) (RWMA (Resistance Welding Manufacturing Alliance), class 2, where Cr = 0.5–1.5%, Zr = 0.02–0.2%, Cu = balance), and furthermore, Figure 1 shows the electrode geometry (adapted from [68]).
The electrode pin diameter was 5 mm (which is recommended for 1 mm thick base materials), but it was not flat, a radius of R = 50 mm was applied. There was no difference between the upper and lower electrodes. Different welding parameters were used for the different base material combinations; the parameter optimization was based on previous experiments [69,70,71] and possible practice (e.g., [72]). The optimization was aimed at achieving the maximum tensile-shear force by varying the welding current and the welding time. Table 6 shows the selected welding parameters for all material combinations.
The DP600/DP600 base material combination was welded with the lowest welding current and the longest welding time (compared with the others). The aluminum/aluminum RSW joints required the highest welding current and the shortest welding time. In the case of the aluminum/steel RSW joints, almost twice the welding current was applied compared with the steel/steel combination, but the welding time was shorter, because these parameters resulted in a thin intermetallic compound (IMC) layer with good tensile-shear strength. The reason for the large difference in parameters comes from the significant differences in the electrical resistance and thermal conductivity of the steel and the aluminum alloy. Based on our own investigations, Figure 2 shows the differences in the IMC layers in the case of the aluminum/steel RSW joints, illustrated by an optical microscope.
The shape and the geometrical dimensions of the RSW specimens can be seen in Figure 3 (adapted from [68]).

2.3. Conventional Clinching (CCL)

The clinching method is one of the most versatile mechanical joining methods, connecting two or more sheets through a one-step process of local plastic deformation. The main elements of the technology are the die (body and insert), the punch, and the blank holder, as shown in Figure 4a. During the conventional clinching process, the sheets are placed on the die and the blank holder moves down, pushing them. After that, the punch is moved downwards to stamp the sheets. The materials can flow in a downward direction, fill the gaps on the die, and form a mechanical interlock between the sheets. The geometrical parameters, such as the neck thickness (tN), the bottom thickness (tB), and the undercut (tC), are the most significant parameters affecting the strength of the clinched joints (see Figure 4b).
The clinched joints were made on an MTS electrohydraulic, computer-controlled, universal materials testing system (MTS Systems, Eden Prairie, MN, USA) with a 250 kN maximum compressive loading capacity. The equipment, together with a TOX tool (TOX® PRESSOTECHNIK GmbH & Co.KG, Weingarten, Germany) mounted on the load frame, is shown in Figure 5.
The shapes of the specimens and the tool, along with the geometrical dimensions, can be seen in Figure 6. The bottom thickness (see Figure 4b) of the investigated clinched joints was tB = 0.5 mm. The aim of the selection of the (optimal) bottom thickness was to achieve the maximum tensile-shear force, taking into account the favorable clinching force [73,74].

2.4. High-Cycle Fatigue (HCF) Tests

The shape and the geometry as well as the major dimensions of the test specimens of all the base material combinations for the RSW and CCL joints can be seen in Figure 3 and Figure 6, respectively. The two parts of the specimens were cut from 1000 mm × 2000 mm sheets into 100 mm × 30 mm strips with laser cutting. For the RSW specimens an overlap of 30 mm was used and for the CCL specimens an overlap of 35 mm. The investigated joints of both technologies can be found at the geometrical center of the overlapped area. Figure 7 and Figure 8 show the RSW and CCL joints, respectively, for all of the material combinations.
The HCF tests were executed based on the instructions of the relevant standard [75]. The tests were performed using MTS electrohydraulic, universal materials testing equipment with an MTS FlexTest 40 controller (MTS Systems, Eden Prairie, MN, USA). The 5 mm overlapping difference has no effect on the feasibility of the investigations and the comparability of the test results.
A sinusoidal loading wave form was applied, and during the entire test phase the load ratio (in our case Fmin/Fmax) was R = 0.1, with a frequency of f = 30 Hz. Several load levels were selected and used during testing of the RSW and CCL joints. Considering that the investigations were evaluated according to [76,77], the load range levels were chosen as described in these sources. All tests were performed at room temperature and in a laboratory environment (controlled air and humidity). Compliance with the testing conditions detailed above ensured that they did not significantly affect the reliability of the results.

3. Results and Discussion

3.1. Failure Modes

After performing the high-cycle fatigue tests, the mode of failure was determined by visual inspection (VT) of each specimen, for both technologies, one by one.
For the RSW technology, among the variations presented for static and cyclic failures ([46,78,79] and [79,80,81], respectively), the following were relevant in our case: pull-out failure; crack initiation in the HAZ and crack growth in the base material; crack initiation in the HAZ and interfacial failure; and interfacial failure. Figure 9 shows an example for each case, for all of the material combinations.
For the CCL technology, among the variations presented for static and cyclic failures [82,83,84,85] and [86], respectively), the following were relevant in our case: button neck fracture; base material fracture; base material fracture and button neck fracture; and crack initiation in the joint as well as crack growth in the aluminum alloy. Figure 10 shows an example for each case, for all of the material combinations.

3.2. Results of the High-Cycle Fatigue (HCF) Tests

The number-of-cycles-to-failure values were registered at the end of the high-cycle fatigue tests. The results were illustrated by the diagrams plotting load range (ΔL) against the number of cycles to failure (N), applying a logarithmic scale for the number of cycles in the ΔL-N curves.
Straight lines are fitted to the measured ΔL-N points. The straight lines were determined by applying the least squares method (in the lifetime stage) and calculating the mean values (in the endurance limit stage, ΔLel); therefore, these lines are associated with a 50% probability of failure.
Figure 11 shows the ΔL-N curves for the similar and hybrid RSW joints, including the failure modes. The figure clearly demonstrates that the most common failure modes across the entire range were “crack initiation in the HAZ and crack growth in the base material” and “crack initiation in the HAZ and interfacial failure” (Figure 9b,c); the mode “pull-out failure” (Figure 9a) occurred at higher loads and lower cycles, and the mode “interfacial failure” (Figure 9d) occurred at lower loads and higher cycles.
Figure 12 presents the HCF test results of all the RSW joint combinations, compared with data from the literature from two sources [36,87]. The HCF resistance of the DP600/5754-H22 hybrid joints is significantly better than that of the DP600/6082-T6 hybrid joints, and, furthermore, they are competitive with the results found in the literature. According to data found in the literature [36], the endurance limit is close to that of IF/6022-T4 hybrid and 6022-T4/6022-T4 similar joints, and demonstrates a slight difference compared to our result (DP600/6082-T6, 5754-H22/5754-H22). It should be noted that in source [36] the fatigue limit values were not determined; furthermore, wider and thicker specimens were used. For the DP590/DP590 material combination [36], there were few measuring points, no fatigue limit value was specified, and the tests were carried out on wider and thicker specimens. From this point of view, the difference between the data measured on the DP600 and DP590 materials can be evaluated as large.
Figure 13 shows the ΔL-N curves of similar and hybrid CCL joint combinations, including the failure modes. The figure clearly illustrates that different failure modes occurred for the similar DP600/DP600 and the dissimilar joints, with the typical modes “base material fracture” (Figure 10b) and “button neck fracture” (Figure 10a) occurring for higher loads and lower cycles, and the mode “base material fracture and button neck fracture” (Figure 10c) occurring at lower loads and higher cycles. For the dissimilar joints, all specimens failed in the mode “initiation in the joint and crack growth in the aluminum alloy” (Figure 10d).
Figure 14 and Figure 15 show the ΔL-N curves associated with a 50% probability of failure for the similar and dissimilar material combinations. The trend from the figures is that the load-carrying capacity of the conventionally clinched (CCL) joints exceeds that of the resistance-spot-welded (RSW) joints.
Table 7 summarizes the characteristics of the ΔL-N curves for 50% probability for both joint methods and all material combinations, using the Basquin-type [88] equation in the lifetime section, as follows:
ΔL = A × ln(N) + B
Among the RSW and the CCL similar joints, the DP600/DP600 combination had the best result, with endurance limit values of 1282.5 N and 2007 N, respectively. Furthermore, the 6082-T6/6082-T6 and the 6082-T4/6082-T4 combinations resulted in the worst values, of 513 N and 636.75 N, respectively. In all cases, results for the dissimilar joints were better than the corresponding results for the similar aluminum joints. The data found in the literature are in accordance with our test results, with the DP600/5754-H22 joints showing a better high-cycle fatigue limit than the DP500/5754-H22 joints [31], where the DP500/5754-H22 joints were performed using an REW process on thicker steel plates with the application of a steel rivet on the aluminum side.
Two ratios have been defined for the numerical comparison of the endurance limit values of joints, one within and one between the joining technologies, as follows:
ΔLel ratio(1) = (ΔLel,i/ΔLel,DP600/DP600) × 100
and
ΔLel ratio(2) = (ΔLel CCL,i/ΔLel RSW,i) × 100
where “i” represents the material combinations. The calculated values of the two ratios are given in Table 8.
Considering the steel/steel joints as a base, the load-bearing capacity of the hybrid joints is lower (48.7% and 73.0% for RSW, 35.0% and 38.7% for CCL) and even lower for aluminum/aluminum joints (39.9% and 50.4% for RSW, 31.7% and 35.0% for CCL). The results of our tests are in harmony with the literature data (DP500/5754-H22). With one exception (DP600/5754-H22), the load-bearing capacity of the CCL joints is higher than that of the RSW joints (156.1–108.3%).

3.3. Cost Comparison

Besides the HCF resistance, an overview of the aspects affecting costs can help find the applicable process for joining. Table 9 shows a comparison based on different aspects of the RSW and CCL technologies. The comparisons presented in the table are qualitative and relate only to the two technologies; the qualifying indicators need to be interpreted in relation to the two technologies. Quantitative comparisons require knowledge of global and local data (e.g., product characteristics, energy, wages, and overheads) that are beyond the scope and purpose of this article.
In addition to the aspects presented in Table 9, other criteria may also be important. The loading of joints and the quality requirements basically determine the applicable process. If the joint properties of clinched joints meet the quality requirements, CCL can be cheaper than RSW.

4. Conclusions

Based on the investigations performed and evaluated, and with further regard to their results, the following conclusions can be drawn:
  • The investigated joining technologies issued comparable high-cycle fatigue (HCF) test results for the similar and hybrid resistance-spot-welded (RSW) and conventionally clinched (CCL) joints made from DP600, 5754-H22, and 6082-T6 base materials, both within and between the two technologies.
  • For both the joining technologies used, the typical failure modes could be identified in the high-cycle fatigue investigations. In the case of RSW, the most common failure modes across the entire range were “crack initiation in the HAZ and crack growth in the base material” and “crack initiation in the HAZ and interfacial failure”; furthermore, the mode “pull-out failure” occurred at higher loads and lower cycles, and the mode “interfacial failure” occurred at lower loads and higher cycles. In the case of CCL, different failure modes occurred for the similar DP600/DP600 and the dissimilar joints, with the typical modes “base material fracture” and “button neck fracture” occurring at higher loads and lower cycles, and the mode “base material fracture and button neck fracture” occurring at lower loads and higher cycles. For the dissimilar joints, all specimens failed in the mode “initiation in the joint and crack growth in the aluminum alloy”.
  • Based on a Basquin-type equation, applying the least squares method in the lifetime stage and further calculating the mean values in the endurance limit stage, high-cycle fatigue limit curves (ΔL-N) can be determined corresponding to a 50% probability of failure. In the case of both the similar and the hybrid joints, the fatigue limit curves demonstrated that the load-bearing capacity of the CCL joints exceeds that of the RSW joints. Compared to the steel/steel joints, the load-bearing capacity of the hybrid joints is lower (48.7% and 73.0% for RSW, 35.0% and 38.7% for CCL), and it is even lower for the aluminum/aluminum joints (39.9% and 50.4% for RSW, 31.7% and 35.0% for CCL). The load-bearing capacity of the CCL joints is higher than that of the RSW joints (DP600/DP600: 156.1%, 6082-T4/6082-T4: 124.1%, 5754-H22/5754-H22: 108.3%, and DP600/6082-T4: 112.2%) except for in the case of one joint (DP600/5754-H22: 82.7%).
  • Since there is a difference between the two technologies in terms of the load-bearing capacity of the joints under high-cycle fatigue loading conditions, their application can be chosen depending on the real loading of a specified structural element.
  • The investigations that have been started should be systematically continued. In order to compare the behavior of hybrid joints, further investigations are needed: joints made with other technologies (e.g., friction stir welding) should be performed; a fatigue crack growth (FCG) test should be prepared, executed, and evaluated on similar and hybrid joints made using different joining technologies; and the two different technologies, as well as the HCF and FCG behaviors, should be compared to specify the optimal conditions and areas for application. Where it is possible, fracture mechanical tests and assessment methods should be prioritized.

Author Contributions

Conceptualization, Á.M. and J.L.; methodology, Á.M., J.L. and P.Z.K.; investigation, J.L.; writing—original draft preparation, J.L.; writing—review and editing, Á.M. and P.Z.K.; visualization, J.L., Á.M. and P.Z.K.; supervision, J.L. and Á.M.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research work was supported by the European Union and the Hungarian State and co-financed by European Structural and Investment Funds within the framework of the GINOP-2.3.4-15-2016-00004 project, with the aim of promoting cooperation between higher education and industry.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

Special thanks to our colleague László Szentpéteri at the Institute of Materials Science and Technology, within the Faculty of Mechanical Engineering and Informatics at the University of Miskolc, for the execution of the long-term high-cycle fatigue tests.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The geometry of the resistance spot welding (RSW) electrode.
Figure 1. The geometry of the resistance spot welding (RSW) electrode.
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Figure 2. The result of welding parameter optimization in the case of the aluminum/steel joints: (a) the thinnest IMC layer (1.1 µm average) with 16.5 kA, 220 ms, and 2.5 kN; (b) the thickest IMC layer (2.5 µm average) with 16 kA, 400 ms, and 2.5 kN.
Figure 2. The result of welding parameter optimization in the case of the aluminum/steel joints: (a) the thinnest IMC layer (1.1 µm average) with 16.5 kA, 220 ms, and 2.5 kN; (b) the thickest IMC layer (2.5 µm average) with 16 kA, 400 ms, and 2.5 kN.
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Figure 3. The shape and the geometry of the resistance-spot-welded (RSW) specimens.
Figure 3. The shape and the geometry of the resistance-spot-welded (RSW) specimens.
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Figure 4. Characteristics of the conventional clinching technology (adapted from [73]): (a) the main elements of conventional clinching technology, and (b) the structure of a clinched joint.
Figure 4. Characteristics of the conventional clinching technology (adapted from [73]): (a) the main elements of conventional clinching technology, and (b) the structure of a clinched joint.
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Figure 5. MTS electrohydraulic materials testing system with the clinching tool and the main elements of the tool.
Figure 5. MTS electrohydraulic materials testing system with the clinching tool and the main elements of the tool.
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Figure 6. The shape of the specimens and the clinching tool and their geometrical dimensions.
Figure 6. The shape of the specimens and the clinching tool and their geometrical dimensions.
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Figure 7. An example of the RSW joints/investigated high-cycle fatigue specimens for all material combinations.
Figure 7. An example of the RSW joints/investigated high-cycle fatigue specimens for all material combinations.
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Figure 8. An example of the CCL joints/investigated high-cycle fatigue specimens for all the material combinations.
Figure 8. An example of the CCL joints/investigated high-cycle fatigue specimens for all the material combinations.
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Figure 9. Failure modes during high-cycle fatigue testing of the RSW specimens: (a) pull-out failure, (b) crack initiation in the HAZ and crack growth in the base material, (c) crack initiation in the HAZ and interfacial failure, (d) interfacial failure.
Figure 9. Failure modes during high-cycle fatigue testing of the RSW specimens: (a) pull-out failure, (b) crack initiation in the HAZ and crack growth in the base material, (c) crack initiation in the HAZ and interfacial failure, (d) interfacial failure.
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Figure 10. Failure modes during high-cycle fatigue testing of the CCL specimens: (a) button neck fracture, (b) button neck fracture, (c) base material fracture and button neck fracture, (d) crack initiation in the joint and crack growth in the aluminum alloy.
Figure 10. Failure modes during high-cycle fatigue testing of the CCL specimens: (a) button neck fracture, (b) button neck fracture, (c) base material fracture and button neck fracture, (d) crack initiation in the joint and crack growth in the aluminum alloy.
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Figure 11. ΔL-N curves for the investigated similar and dissimilar RSW joints including the failure modes.
Figure 11. ΔL-N curves for the investigated similar and dissimilar RSW joints including the failure modes.
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Figure 12. Comparing our results on RSW joints with relevant results that can be found in the literature.
Figure 12. Comparing our results on RSW joints with relevant results that can be found in the literature.
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Figure 13. ΔL-N curves for the investigated similar and dissimilar CCL joints including the failure modes.
Figure 13. ΔL-N curves for the investigated similar and dissimilar CCL joints including the failure modes.
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Figure 14. Comparison of the RSW and CCL results on similar joints.
Figure 14. Comparison of the RSW and CCL results on similar joints.
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Figure 15. Comparison of the RSW and CCL results on dissimilar joints.
Figure 15. Comparison of the RSW and CCL results on dissimilar joints.
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Table 1. Spot welding-based joining technologies for aluminum/steel hybrid joints, including their main characteristics.
Table 1. Spot welding-based joining technologies for aluminum/steel hybrid joints, including their main characteristics.
Joining ProcessAluminum PartSteel PartAdditional InformationSource
MaterialThickness (mm)MaterialThickness (mm)
RSWA50521.0A366/A366M-971.0commercial steel cover plate (1.0 mm) on aluminum side[29]
A50521.0SUS3041.0commercial steel cover plate (1.0 mm) on aluminum side[29]
A50521.5DP6001.2pure Zn interlayer[30]
AW5754-H221.0DP5001.5DP500 uncoated[31]
5182-O2.0SAE 10081.41050 clad SAE 1006 transition material[32]
6008-T61.5H220YD1.0H220YD galvanised[33]
6008-T61.5H220YD1.0H220YD galvanised; 4047 AlSi12 interlayer[34]
AA6016-T41.0Interstitial-free steel (IFS)0.7IFS bare, galvanized (60 g/mm2), galvannealed
(40 g/mm2, 60 g/mm2)		[35]
AA6022-T41.2Rolled interstitial-free steel (IFS)2.0IFS hot-dipped galvanized[36,37]
AA6022-T41.2Low-carbon steel (LCS)2.0LCS hot-dipped galvanized[38]
AA6022-T41.2mild steel (MS)2.0MS hot-dipped galvanized[39]
X6260.8Low-carbon steel (LCS)0.9; 2.0LCS uncoated[40]
AA60221.2Low-carbon steel (LCS)0.9; 2.0LCS uncoated[40]
AA60221.2HSLA steel1.2; 2.0HSLA steel uncoated[40]
AA60621.2HSLA steel2.0N/A[41]
AA60221.2HSLA steel and CR780T *0.65 and 1.4N/A[41]
A60611.5AISI-SAE 10051.5pure Cu insert[42]
A60611.5AISI-SAE 10051.5pure Zn insert[42]
AA6061-T61.0DP5901.6DP590 bare[23]
6063-T61.516Mn1.016Mn uncoated[43]
Al6K321.0; 1.6SGARC4401.0; 1.4SGARC uncoated; PT3000 (CrNi) process tape on aluminum side and PT1407 (steel) process tape on steel side[44]
Al6K321.0; 1.6SGARC4401.0; 1.4SGARC Zn-coated; PT3000 (CrNi) process tape on aluminum side and PT1407 (steel) process tape on steel side[44]
MBaRSWAA6061-T61.0DP5901.6DP590 bare; ER4043 printed bump[23]
MRSWAA11000.4SS3010.2 circular low-carbon steel (LCS) interlayer (0.2 mm)[26]
AA11000.2SS3010.4N/A[45]
REW/RRWAl5052-H321.0; 2.0; 3.0DP7801.2DP780 Zn-coated; 20MnB4 solid steel element[46]
Al5182-O1.022MnB51.222MnB5 Al-Si-coated; 20MnB4 solid steel element[46]
AW5754-H221.0DP5001.5Q235 solid steel rivet on aluminum side[31]
EN AW-60161.222MnB51.520MnB4 solid steel rivet with numerically optimized geometry[47]
EN AW-60161.222MnB51.522MnB5 Al-Si-coated (AS150); 20MnB4 solid steel rivet with numerically optimized geometry and Zn-Ni-coated[47]
EN AW-6016-T41.222MnB5 and 22MnB5 *1.5 and 1.0both 22MnB5 Al-Si-coated (AS150); 20MnB4 solid steel rivet with numerically optimized geometry and Zn-Ni-coated[47]
EN AW-6016-T661.222MnB5 and 22MnB5 *1.5 and 1.0both 22MnB5 Al-Si-coated (AS150); 20MnB4 solid steel rivet with numerically optimized geometry and Zn-Ni-coated[47]
AA6061-T61.0HS1300T1.55HS1300T Al-Si-coated; SWRCH16A solid steel rivet on aluminum side[21]
AA6061-T61.0DP780 and press-hardened steel (PHS) *1.2 and 1.55DP780 Zn-coated and PHS Al-Si-coated; 35CrMo semi-tubular steel rivet on aluminum side[22]
(HP)USW6061-T61.5AISI 3041.5AISI 304 uncoated[48]
6061-T61.5ASTM A361.5ASTM A36 uncoated[48]
Al-60110.93DC040.97DC04 uncoated[24]
Al-60110.93DX53-ZF0.97DX53-ZF hard galvannealed Zn coating[24]
Al-60110.93DX56-Z0.75DX56-Z soft hot-dipped Zn coating[24]
AA7075-T62.0HSLA steel1.2AA7075-T6 clad with AA7072; HSLA steel hot-dip galvanized[49]
USW + RSWA6061-T61.0AISI 10080.9A6061-T6 insert (0.4 mm)[25]
VFAW5A061.8SS3214.03003 interlayer (1.02 mm)[27]
AA61112.5HSLA 3402.5HSLA steel bare[50]
* Three-sheet aluminum/steel dissimilar joint. N/A: not applicable.
Table 2. Clinching-based joining technologies for aluminum/steel hybrid joints, including their main characteristics.
Table 2. Clinching-based joining technologies for aluminum/steel hybrid joints, including their main characteristics.
Joining ProcessAluminum PartSteel PartAdditional InformationSource
MaterialThickness (mm)MaterialThickness (mm)
Conventional clinching (CCL)14201.5Q2151.5galvanized steel[51]
AA3004N/ASAE 1006N/AN/A[52]
AA3004N/AAISI 304N/AN/A[52]
AA50521.0ASTM A361.0N/A[53]
Al50522.0DP7801.6N/A[54]
AL50521.5ARC05 (EN 10130)1.5N/A[55]
AA5182-O0.85DX51D+Z1.2N/A[56]
AA6011-T41.0SAE10040.7pre-strained aluminum, galvanized steel[57]
EN AW 60141.0HCT-590X+Z1.5N/A[58]
EN AW 60141.0HCT-590X+Z1.5galvanized steel[59]
60612.03041.5N/A[60]
aluminum1.5mild steel1.5N/A[61]
Single-stage shear-clinchingAA6016-T42.022MnB51.5aluminum-silicon-coated steel[62]
Multi-stage clinching with pre-holeAA6016-T42.022MnB51.5aluminum-silicon-coated steel[62]
Conventional clinching with auxiliary layerAL50522.0HC340LA2.0AL1060 auxiliary layer (1.5 mm)[63]
AL60612.0HC340LA2.0AL1060 auxiliary layer (1.5 mm)[63]
Mechanical clinching and adhesive bondingA5052-H341.5JSC7801.2CEMEDINE EP138 adhesive[64]
Hybrid clinching–weldingAA57541.5DQSK0.8zinc-coated steel[65]
Friction stir hole clinching (FSHC)Al6061N/ADP980N/AN/A[66]
Electric-assisted mechanical clinching (EAMC)AA6061-T61.0DP5901.5galvanized steel[67]
N/A: not applicable.
Table 3. Chemical composition of the investigated steel, according to the quality certificate of the manufacturer, weight %.
Table 3. Chemical composition of the investigated steel, according to the quality certificate of the manufacturer, weight %.
Material GradeCSiMnPSNbVB
DP6000.0980.20.810.0150.0020.0140.010.0002
Table 4. Chemical composition of the investigated aluminum alloys, according to the quality certificates of the manufacturers, weight %.
Table 4. Chemical composition of the investigated aluminum alloys, according to the quality certificates of the manufacturers, weight %.
Material GradeCuFeMnCrMgTiSiZn
6082-T60.090.460.460.020.70.030.90.08
5754-H220.0550.2940.3580.0092.7960.0160.1930.034
Table 5. Base mechanical properties of the investigated materials, according to the quality certificates of the manufacturers.
Table 5. Base mechanical properties of the investigated materials, according to the quality certificates of the manufacturers.
Material GradeRp0.2 (MPa)Rm (MPa)Rp0.2/Rm (–)A50 (%)
DP6004486690.67018.7
6082-T63033480.87115.0
5754-H221372200.62322.0
Table 6. Welding parameters for the resistance spot welding (RSW) experiments.
Table 6. Welding parameters for the resistance spot welding (RSW) experiments.
Material CombinationWelding Current (kA)Welding Time (ms)Welding Force (kN)
DP600/DP6008.53204.0
6082-T6/6082-T623.01002.5
5754-H22/5754-H2224.01002.5
DP600/6082-T615.02202.5
DP600/5754-H2216.52202.5
Table 7. Characteristics of the ΔL-N curves for 50% probability based on the Basquin-type equation.
Table 7. Characteristics of the ΔL-N curves for 50% probability based on the Basquin-type equation.
Material CombinationABCorrelation CoefficientΔLel (N)Source
Resistance-spot-welded joints
DP600/DP600−552.986990.9851285.5Our previous study [68]
6082-T6/6082-T6−133.023130.963513Our previous study [68]
5754-H22/5754-H22−66.416680.668648Our previous study [68]
DP600/6082-T6−167.930830.929625.5Our previous study [68]
DP600/5754-H22−125.127950.770939Our previous study [68]
DP590/DP590−320.451600.984N/A[87]
6022-T4/6022-T4−274.244980.981N/A[63]
IF/6022-T4−449.279660.952N/A[63]
DP500/5754-H22−19.262167N/A882[59]
Conventionally clinched joints
DP600/DP600−196.750600.9542007This study
6082-T4/6082-T4−109.823920.928636.75This study
5754-H22/5754-H22−113.423990.974702This study
DP600/6082-T4−322.057460.945702This study
DP600/5754-H22−240.045160.975776.25This study
N/A: not available.
Table 8. Numerical comparison of the endurance limit values for the RSW and CCL joints.
Table 8. Numerical comparison of the endurance limit values for the RSW and CCL joints.
Material CombinationΔLel (N)ΔLel ratio(1) (%)ΔLel ratio(2) (%)
Resistance-spot-welded joints
DP600/DP6001285.5100N/A
6082-T6/6082-T651339.9N/A
5754-H22/5754-H2264850.4N/A
DP600/6082-T6625.548.7N/A
DP600/5754-H2293973.0N/A
DP500/5754-H22 [59]88268.6N/A
Conventionally clinched joints
DP600/DP6002007100156.1
6082-T4/6082-T4636.7531.7124.1
5754-H22/5754-H2270235.0108.3
DP600/6082-T470235.0112.2
DP600/5754-H22776.2538.782.7
N/A: not applicable.
Table 9. Comparison of the RSW and CCL technologies based on different aspects.
Table 9. Comparison of the RSW and CCL technologies based on different aspects.
AspectsRSWCCL
Workpiece preparationMore sensitivity, more expensiveLess sensitivity, cheaper
Electrical networkSerious, more expensiveRegular, cheaper
Manufacturing equipmentComplex machine, more expensiveSimple machine, cheaper
ManpowerWell-educated operator and technologist required, more expensiveBasic qualifications for operator and technologist acceptable, cheaper
Tool refurbishment possibilityPossible several times, characteristically cheaperNot possible, replacement necessary, characteristically more expensive
Tool lifetimeElectrode tip can degrade, especially in the case of aluminum welding [89,90], more expensiveHigher than an RSW electrode [91], especially in the case of aluminum, cheaper
Maintenance demand of the manufacturing processComplex machine, several parts can fail, frequent maintenance required, more expensiveSimple machine, less parts can fail, higher reliability of equipment, cheaper
Energy consumptionHigher, more expensiveLower, cheaper
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Meilinger, Á.; Kovács, P.Z.; Lukács, J. High-Cycle Fatigue Characteristics of Aluminum/Steel Clinched and Resistance-Spot-Welded Joints Based on Failure Modes. Metals 2024, 14, 1375. https://doi.org/10.3390/met14121375

AMA Style

Meilinger Á, Kovács PZ, Lukács J. High-Cycle Fatigue Characteristics of Aluminum/Steel Clinched and Resistance-Spot-Welded Joints Based on Failure Modes. Metals. 2024; 14(12):1375. https://doi.org/10.3390/met14121375

Chicago/Turabian Style

Meilinger, Ákos, Péter Zoltán Kovács, and János Lukács. 2024. "High-Cycle Fatigue Characteristics of Aluminum/Steel Clinched and Resistance-Spot-Welded Joints Based on Failure Modes" Metals 14, no. 12: 1375. https://doi.org/10.3390/met14121375

APA Style

Meilinger, Á., Kovács, P. Z., & Lukács, J. (2024). High-Cycle Fatigue Characteristics of Aluminum/Steel Clinched and Resistance-Spot-Welded Joints Based on Failure Modes. Metals, 14(12), 1375. https://doi.org/10.3390/met14121375

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