Optimization of Clinching Joint Process with Preforming between Ultra-High-Strength Steel and Aluminum Alloy Sheets

Abstract

With the rapid development of lightweight automobiles, the clinching joint technology of ultra-high-strength steel with aluminum alloy sheets have been paid more and more attention. However, due to significant differences in plastic deformation capabilities between the two metals, particularly the difficulty of steel sheet deformation, conventional clinching processes often result in insufficient joint interlocking or fracture issues. Although the preliminary use of clinching processes with preforming methods has shown some effectiveness in connecting two types of sheets, the bond strength is not high. This study employs finite element simulation and orthogonal optimization methods to investigate the impact of relevant process parameters on joint morphology in clinching processes with preforming. Under the condition of optimizing process parameters, a clinching punch with an added pressure-step structure was proposed to compact the joint and further enhance joint quality. Experimental verification demonstrates the feasibility of the improved clinching processes with preforming for bonding ultra-high-strength steel and aluminum alloy sheets.

1. Introduction

The clinching process is a method of achieving attachment using the plastic deformation of sheet materials themselves, without the need for additional connectors. It features broad applicability, low cost, and simple operation, among other advantages [1], compared to traditional riveted and spot-welded joints. As a result, it has been widely utilized in fields such as the automotive and electrical appliances.

Since the beginning of the 21st century, the rapid advancement of lightweight automobile technology has spurred extensive scholarly research into clinching processes for high-strength steel and lightweight alloy sheets. Varis et al. [2] conducted early experimental studies on the clinching processes of high-strength steel sheets, confirming that circular joints can withstand higher shear loads than square joints. Lee et al. [3] studied the process optimization of clinching aluminum alloys and high-strength steel sheets, identifying the concave die radius as an important essential parameter. If the concave die radius is constant, the interlock will increase as the depth of the convex die increases, but excessive depth may cause the bottom sheet to break. Meanwhile, if the concave die radius is variable, the neck thickness increases as the concave die radius increases, but an oversized radius can reduce the interlock. To improve joint strength, it is necessary to determine relevant process parameters based on the form of joint failure. Lee et al. [4] used Al6061-T4 alloy, DP780 steel, hot-pressed 22MnB5 steel and other sheets as research objects, proposing a new process of pre-punching the lower sheet with poor ductility and then overlaying it with an upper aluminum sheet for riveting, with experimental results confirming positive riveting effects. Huang et al. [5] and Wiesenmayer et al. [6] utilized aluminum–magnesium alloys and 22MnB5, HCT780X dual-phase steel sheets for clinching, examining the feasibility of the process under heated steel sheet conditions. Eshtayeh et al. [7] studied the influence of mold parameters on joint morphology based on a grey-based Taguchi method. They analyzed the finite element simulation results to comprehensively determine how concave die depth and diameter, convex die diameter, and punch fillet radius affect the neck thickness, interlock, and bottom thickness of the joint. Based on actual work, different optimization values can be selected. Kova’cs et al. [8] and Kumara et al. [9] selected heterogeneous metal sheets for clinching processes, validating that the pre-treatment of materials before riveting can prevent sheet fractures and improve material flow ability, thereby enhancing clinching joint strength. Babalo et al. [10] used aluminum alloy sheets for riveting and studied the High-Speed Metal forming of Connections (HSMC) using electric-hydraulic driven fluid impact on the convex die to achieve clinching. Experimental verification showed robust joint connectivity and resistance to detachment failure. Nourani et al. [11] observed the grain morphology of riveted joints using electron backscatter diffraction (EBSD) microscopy and discovered equiaxed grains in the base material, while the grains in the neck region after riveting were significantly elongated and fine; this indicated substantial metal deformation in the neck region and induced dynamic recrystallization. Ma et al. [12] selected JSC780 and A5052-H34 sheets as the research objects, investigated the clinching processes with the addition of adhesives between the sheets. Experimental verification revealed enhanced clinching joint strength. In summary, a research system has been established encompassing clinching mechanisms, material forming, process parameters, clinching joint strength, joint failure modes, and microstructural changes.

In recent years, there has been a growing emphasis on researching clinching processes for ultra-high-strength steel sheets with tensile strength exceeding 1 GPa, aimed at enhancing joint fatigue life. Hörhold et al. [13] recommended a pre-punched clinching process for 22MnB5 ultra-high-strength steel and Al6016 aluminum alloy sheets. They indirectly pre-punched the lower steel sheet using inner and outer convex die structures, followed by the continuous pressing of the thin sheets between the dies to achieve clinch-bonding. Abe et al. conducted numerous experimental studies on clinch-bonding ultra-high-strength steel, high-strength steel, and aluminum alloy sheets. For instance, Abe et al. [14] achieved the joining of SPFC780 high-strength steel sheets and Al5052 aluminum alloy sheets using conventional clinching processes but encountered issues such as the bottom cracking of SPFC980 ultra-high-strength steel sheets and insufficient interlock in joints when connecting SPFC980 and Al5052. Abe et al. [15] investigated optimized clinching processes for two types of ultra-high-strength steel with varying ductility, confirming that high-ductility ultra-high-strength steel sheets can be connected conventionally using optimized dies, whereas low-ductility steel sheets cannot. Abe et al. [16] developed a mechanical clinching using counter pressure of a rubber ring to join the galvanized ultra-high-strength steel sheets with low ductility. In the proposed process, the interlock was increased by the increment of metal flow with the counter pressure of rubber ring in the die cavity. Abe et al. [17] developed a mechanical clinching process by preforming a lower sheet to join two ultra-high-strength steel sheets with low ductility. In this process, the reduction in the thickness of the upper sheet around the punch corner became small due to the decrease in the pressure from the preformed lower sheet, and then the interlock was increased by the expansion in the radius direction in the final compression. Chen, C. et al. [18] proposed a process for connecting aluminum alloys and restoring deformed joints. Lee, C.J. et al. [19] investigated the influence of geometric interlocking parameters on the strength of failure mode-related joints, in order to design the geometry of hole-riveted joints. Moria, K. et al. [20] developed a combined process of hot-stamping and mechanical joining for producing ultra-high-strength steel patchwork components. The main blank and patch were uniformly heated side by side without overlapping in a furnace, and then the overlapped main blank and patch were hot-stamped. Abe et al. [21] conducted pre-punched clinch-bonding experiments using preforming dies similar to riveting dies on 1.2 mm ultra-high-strength steel sheets and 1 mm aluminum alloy sheets, resulting in ideal joint morphology. However, due to the relatively thin thickness of the aluminum alloy sheet used, the joint connection strength is relatively low.

Preforming process parameters are crucial for the clinching processes with preforming of ultra-high-strength steel and aluminum alloy sheets. Inadequate preforming will result in an insufficient squeezing force of the aluminum sheet on the ultra-high-strength steel sheet during clinching and may fail to deform the ultra-high-strength steel sheet appropriately, leading to an unreliable interlock. Similarly, an oversized diameter of the preforming punch will result in an excessively large neck thickness of the aluminum sheet during riveting, hindering its ability to prevent aluminum material from overflowing upward, causing ineffective interlocking. Therefore, conducting optimization analysis on performing process parameters and innovating the die’s structure are imperative for exploring effective preforming and clinching processes that enable robust bonding between ultra-high-strength steel and aluminum alloy sheets.

2. Mechanism of Clinching Processes with Preforming

Due to the significant difference in plastic deformation capacity between ultra-high-strength steel and aluminum alloy sheets, especially the difficulty in deforming steel sheets, leading conventional clinching processes often results in insufficient interlocking or fractures at the joint. In order to ensure the sufficient deformation of the steel sheet joint area during clinching, preforming steel sheet prior to clinching allows for optimal joint shaping, reducing deformation difficulties during riveting and ensuring clinch-bonding quality. Therefore, clinching with preforming is divided into two stages: the first step is to use a preforming punch to press and deform the steel sheet; the second step is to replace the preforming punch with the clinching punch, then stack the upper layer of the aluminum alloy sheet on top of the lower layer of the steel sheet and rivet them together to form the desired shape, as shown in Figure 1.

3. Numerical Simulation Analysis of Preformed Clinching Processes

3.1. Establishing Numerical Simulation Model

Since the strength of ultra-high-strength steel is much greater than that of aluminum alloy, this study took an aluminum alloy 5052(AA5052) sheet with a thickness of 1.5 mm and a DP980 ultra-high-strength steel sheet with a thickness of 1.0 mm as the research objects. Finite element simulation and orthogonal optimization methods were utilized to analyze the influence of key process parameters of clinching with preforming on joint morphology. Additionally, the improvement of the riveting die structure was explored further to enhance joint quality for optimizing reasonable process parameters. The mechanical performance parameters of the DP980 ultra-high-strength steel sheet and the AA5052 sheet are shown in

Table 1. Mechanical properties of DP980 and AA5052 sheets.

Material	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation at Break (%)	${\mathit{\sigma }}_{\mathit{s}}=\mathit{F}{\mathit{\epsilon }}_{\mathit{p}}^{\mathit{n}}$
				F (MPa)	n
DP980	730	1270	11.5	1817	0.168
AA5052	145	244	22	301	0.112

.

This study adopted the Deform finite element software V11.0 for simulation analysis. The mold consisted of the preforming punch, riveting punch, die, and blank holder, among which the punch, die, and flange ring were set as rigid bodies. Based on the die structure, an axisymmetric model was used for finite element modeling, as shown in Figure 2. Both metal sheets were set as elastoplastic and meshed by quadrilateral elements with a mesh density of 80 cells/mm². The displacement and velocity of the punch in the preformed stage were 1.4 mm and 1.5 mm/s, respectively, the velocity of the punch in the clinching stage were 1.5 mm. The friction coefficient between the die and steel sheet was set to 0.12, while its friction coefficient with the AA5052 sheet was set to 0.4 according to the literature [22].

Process Parameter Setting

A die diameter of 10 mm was selected as the baseline condition. Preforming experiments revealed a punch-down depth of approximately 1.5 mm for the steel sheet fracture. According to the literature [21], a larger punch-down depth results in increased interlock, and relatively less change in neck thickness. Therefore, the punch-down depth of the preforming punch was set to 1.4 mm. The remaining parameters, including the preforming punch diameter, clinching punch diameter, and die depth, were set as variables, as shown in

Table 2. Values of process parameters.

Process Parameter	Value (mm)
Diameter of preforming punch (Dp1)	7.2	7.4	7.6	7.8
Diameter of clinching punch (Dp2)	6.8	7.0	7.2	7.4
Depth of die (Hd)	1.2	1.4	1.5	1.6

. The sampling data was set according to the literature [21]. The simulation results for a joint bottom thickness of 1.2 mm were provisionally selected as the benchmark for comparison.

3.2. Optimization of Process Parameters for Preformed Clinching Processes

3.2.1. Orthogonal Optimization

In the preforming clinching processes, the three parameters of preforming punch diameter, clinching punch diameter, and die depth mutually influence joint morphology. The orthogonal optimization method can facilitate the study of the impact of these variables on joint neck thickness and interlock values. The orthogonal design table is shown in

Table 3. Orthogonal design table (unit: mm).

Number	Depth of Die (Hd)	Diameter of Preforming Punch (Dp1)	Diameter of Clinching Punch (Dp2)	Minimum Wall Thickness	Interlock
1	1.2	7.2	6.8	0.40	0
2	1.2	7.4	7.0	0.40	0
3	1.2	7.6	7.2	0.40	0.04
4	1.2	7.8	7.4	0.39	0.05
5	1.4	7.2	7.0	0.44	0.01
6	1.4	7.4	6.8	0.48	0.02
7	1.4	7.6	7.4	0.39	0.25
8	1.4	7.8	7.2	0.48	0.13
9	1.5	7.2	7.2	0.48	0.12
10	1.5	7.4	7.4	0.44	0.20
11	1.5	7.6	6.8	0.55	0.11
12	1.5	7.8	7.0	0.56	0.15
13	1.6	7.2	7.4	0.45	0.23
14	1.6	7.4	7.2	0.45	0.26
15	1.6	7.6	7.0	0.50	0.26
16	1.6	7.8	6.8	0.67	0.06

. The corresponding neck thickness and interlock values are derived from the finite element simulation of each combination of parameters. The results exhibit little to no joint interlock phenomena in Number 1 to 6 and Number 16. Figure 3 demonstrates the joint morphology simulated for Number 1 and 16, where Figure 3a illustrates the lack of interlock due to aluminum overflow from the neck thickness position. This was caused by the inadequate preforming of the steel sheet during the clinching stage, wherein the aluminum sheet failed to deform the steel sheet due to insufficient pressure. In Figure 3b, an overly large preforming punch diameter of the steel sheet, coupled with a small clinching punch diameter, led to increased neck thickness in the aluminum sheet, limiting its ability to prevent upward aluminum overflow and achieve substantial interlocking. Morphology comparison at a joint bottom thickness of 1.2 mm can be considered representative, considering the two aforementioned extreme joint failure cases.

Table 4. Orthogonal data analysis of neck thickness.

Number	Evaluation Index	A	B	C
1	Kj1	0.40	0.44	0.53
2	Kj2	0.45	0.44	0.48
3	Kj3	0.51	0.46	0.45
4	Kj4	0.52	0.53	0.42
5	Rj	0.12	0.09	0.11

and

Table 5. Orthogonal data analysis of interlock value.

Number	Evaluation Index	A	B
1	Kh1	0.02	0.09
2	Kh2	0.10	0.12
3	Kh3	0.15	0.17
4	Kh4	0.20	0.10
5	Rh	0.18	0.08

depict orthogonal data analysis for neck thickness and interlock values. In the tables, Factor A represents the depth of the concave die, Factor B signifies the preforming punch diameter, and Factor C indicates the clinching punch diameter. The evaluation index K denotes the average deviation of each level factor, while R represents the range of the average values of each level. By comparing R values, it can be observed that Factor A has a greater impact on neck thickness and interlock values, followed by Factors C and B.

To better illustrate the data trends in Table 4 and Table 5, evaluation index curves were created, as shown in Figure 4. Regarding the depth of the concave die, the difference between K_j3 and K_j4 is 0.1, and the difference between K_h3 and K_h4 is 0.5. The depth of the concave die appears to be more sensitive to changes in interlock value. To prevent excessive interlock that could lead to bottom sheet fracture, a concave die diameter of 1.5 mm is considered suitable. Regarding the preforming punch diameter, the interlock value is at a maximum at D_p1 = 7.6 mm, with a moderate neck thickness. Therefore, a preforming punch diameter of 7.6 mm is deemed appropriate. Regarding the riveting punch diameter, considering both neck thickness and interlock value, choosing a riveting punch diameter of 7 mm or 7.2 mm is more suitable. For the analysis of the clinching process, a clinching punch diameter of 7.2 mm is selected, which results in a larger interlock value. The optimal combination of process parameters is determined as A3B3C3—notably, H_d = 1.5 mm, D_p1 = 7.6 mm, and D_p2 = 7.2 mm.

3.2.2. Analysis of Joint Deformation Procedure

The model conditions with H_d = 1.5 mm, D_p1 = 7.6 mm, and D_p2 = 7.2 mm were selected for clinching formation simulation. Figure 5 shows the velocity field of joint deformation at the instant of contact between the upper aluminum sheet and the lower steel sheet, the point of interlocking formation, and the end of clinching. Observations demonstrated that the bottom of the joint underwent overall downward deformation due to the aluminum sheet, which forced the steel sheet to deform downward, as shown in Figure 5a. At the neck thickness position of the joint, the aluminum sheet had a radial expansion component. This stage represents the rapid filling of the concave die as the two sheets were joined. Due to strong resistance from the steel sheet at the bottom of the joint, the aluminum sheets were observed to cease downward deformation and exhibit overall radial expansion, as illustrated in Figure 5b. The steel sheet also appeared to decelerate its downward deformation, primarily filling the concave die cavity at the corners. At the neck thickness position, the steel sheet’s strong resistance forced the aluminum sheet to rise, causing it to arch and initiating separation between the two sheets. At this stage, the neck thickness had not yet formed, and the relatively low resistance to aluminum sheet blockage caused an intense upward flow of the aluminum sheet. At the bottom of the joint, the two sheets mainly endured radial deformation, steadily filling the concave die cavity until the neck thickness developed, as seen in Figure 5c. Due to the resistance encountered by the aluminum sheet within the narrow gap, below the minimum neck thickness, both sheets primarily experienced radial expansion, forming an interlock. Above the minimum neck thickness region, the aluminum sheet predominantly exhibited upward overflow deformation, resulting in a pronounced separation between the two sheets and affecting the overall joint quality. When the punch reached the bottom thickness of 1.2 mm, measurements indicated a neck thickness of 0.45 mm and an interlock value of 0.20 mm.

4. Improvement and Experiment of Clinching Punch Structure

4.1. Structural Improvement Design

In practical applications, it is necessary to avoid the separation of the two sheets [16], which mandates improvements to the mold. The addition of a pressure-step structure to the clinching punch, as illustrated in Figure 6, enables the pressing of the area of separation between the two sheets during the subsequent phase of clinching formation. According to simulation data, a gap value of 0.2 mm ~ 0.4 mm between the diameter of the pressure step (Ds) and the inner diameter of the blank holder is ideal. The height of the pressure step (Hs) can be initially estimated based on the joint volume and further refined through simulation and experimentation. The simulated joint morphology, with the initial setting of a pressure-step diameter (Ds) of 9.2 mm and a height (Hs) of 2.2 mm, is shown in Figure 7. Figure 7a exhibits the joint morphology without a pressure step, where separation between the two sheets is observed. Figure 7b demonstrates the continuous downward pressure exerted by the pressure step, resulting in the compression of the aluminum sheet overflow and steel sheet gap. Clinching with the pressure step generated a measured neck thickness of 0.51 mm and an interlock value of 0.20 mm. Compared to riveting without the pressure pad, the enhanced mold contributes to a higher neck thickness of the aluminum sheet, with minimal impact on the interlock value. This is because the narrow neck thickness hinders the downward movement of the aluminum sheet.

4.2. Preformed Clinching Process Experiment

As shown in Figure 8, a 15-ton servo press and clinching joint dies were used. The geometric parameters of the dies were selected as follows: the depth of the die was 1.5 mm, the diameter of the preforming punch was 7.6 mm, and the diameter of the clinching punch was 7.2 mm. In order to study the effect of pressure-step height of the clinching punch on the interlock and neck thickness of the joint, two clinching punches with pressure-step heights of 2.0 mm and 2.2 mm, as shown in Figure 9, were selected for experimental comparative analysis. Other geometric parameters of the dies were H_d = 1.5 mm, D_p1 = 7.6 mm, and D_p2 = 7.2 mm.

After sectioning, polishing, and photographing the samples under a microscope, the joint morphology under a base thickness of 1.2 mm was obtained, as shown in Figure 10. The measured neck thickness and interlock value of each joint are shown in

Table 6. The undercut and neck thickness of the clinching joints.

Number	Height of Pressure Step (mm)	Minimum Wall Thickness (mm)	Interlock (mm)
		Experiment/Simulation	Experiment/Simulation
1	2.0	0.56/0.52	0.19/0.20
2	2.2	0.56/0.51	0.18/0.20

. Compared to the literature [21], the neck thickness has significantly increased, while the interlock volume has slightly decreased.

From Table 6, we can observe that the experimental values closely match the simulation values, confirming the accuracy of the simulation. When comparing neck thickness values, it appears that neck thickness slightly increases as the height of the pressure step decreases. Similarly, when comparing interlock values, it seems that interlock values marginally increase as the punch height decreases. This indicates that even under strong compression at a lower height of pressure step, aluminum can enter the interlocking zone through the neck thickness.

4.3. Clinching Joint Strength Test

The clinching joint strength test essentially evaluates the shear resistance and peeling resistance of the conjoined sheets. The two mentioned properties pertain to the relative size of the joint neck thickness and interlocking value. When the shear strength of the joint is less than the peeling strength, the joint will break; conversely, the joint will peel. Larger joint neck thickness results in greater joint shear strength; a larger interlocking value corresponds to greater joint peel strength.

The preparation of experimental samples is shown in Figure 11. A 10 kN electronic universal testing machine was used to conduct shear tests and peeling tests, as shown in Figure 12. The tensile rate was 1 mm/min, and the maximum load at joint failure was recorded.

We selected the above two sets of dies for shear and peeling tests, respectively. The failure diagrams for the shear tests are exhibited in Figure 13, wherein Figure 13a,b is the failure morphology of the joint when the pressure-step height is 2.0 mm and 2.2 mm, respectively. The hybrid failure of shear and peeling occurs when the pressure-step height is 2.0 mm, indicating that the shear strength and peel strength are close to equal at this time, and the joint quality is good. However, only the peel failure occurs when the pressure-step height is 2.2 mm, indicating that the joint interlock value is small, and the peeling strength is less than the shear strength. The relationship curves between tensile load and displacement are shown in Figure 14. The load is larger when the pressure-step height is 2.0 mm than that when the pressure-step height is 2.2 mm. The joint connection load reaches the maximum value at the displacement of 3.0 mm, at 3.15 kN and 2.84 kN, respectively, proving that the joint quality of the 2.0 mm height of pressure step is better, as shown in

Table 7. Failure diagrams and most load of each joint.

Number	Height of Pressure Step (mm)	Shear Test		Peeling Test
		Failure Form	Most Load (kN)	Failure Form	Most Load (kN)
1	2.0	Shear and peeling	3.15	Tearing	1.08
2	2.2	Peeling	2.84	Tearing	1.12

.

In the peeling tests, when the tensile load is about 1.1 kN, the AA5052 sheets were torn under pressing steps 2.0 mm and 2.2 mm in height, respectively, and the failure morphology diagram is shown in Figure 15, indicating that the joint’s peel resistance strength exceeds the tensile strength of the AA5052 sheet itself, resulting in the tearing of the joint.

5. Conclusions

(1)

Based on finite element numerical simulations, reasonable parameters for the depth of the die, diameter of the preforming punch, and diameter of the clinching punch were obtained through orthogonal optimization. Experimental verification confirmed the feasibility of achieving clinching processes between ultra-high-strength steel and aluminum alloy sheets using preforming in the clinching process.

(2)

It is imperative to design a clinching punch incorporating a pressure-step structure. In the latter stages of clinching, the pressure step is utilized to compress the arched cracks between the aluminum sheet and the steel sheet, thereby enhancing the overall joint strength.

(3)

Through shear and peeling tests, it was determined that the primary failure mode of the joint was the peeling and tearing fracture of the upper aluminum sheet. Consequently, in clinching processes involving ultra-high-strength steel and aluminum alloy thin sheets, with aluminum positioned on top and steel below, augmenting the interlock of the aluminum sheet is paramount, which can be increased by the proposed mold featuring a pressure-step punch.