Center-Punching Mechanical Clinching Process for Aluminum Alloy and Ultra-High-Strength Steel Sheets

Abstract

In recent years, with the rapid advancement of automotive lightweight technology, the mechanical clinching process between aluminum alloy and ultra-high-strength steel sheets has received extensive attention. However, the low ductility of ultra-high-strength steel sheets often results in conventional mechanical clinching processes producing joints that either fail to establish effective interlocks or cause the steel sheets to fracture. To address this issue, a novel mechanical clinching process is presented, called center-punching mechanical clinching (CPMC). This innovative process employs a method of punching, flanging, and bulging gradation to achieve the mechanical clinching of aluminum alloy and ultra-high-strength steel sheets in a single step. In order to determine the effects of different parameters on the quality and strength of the joint, an experimental study was carried out for various die depths and diameters based on the condition of constant punch size. Based on tensile and shear tests, the static strength and failure modes of CPMC joints were analyzed. The results indicated that the CPMC process significantly enhances the connectivity of joints for AA5052 aluminum alloy and DP980 ultra-high-strength steel. Optimal tensile and shear strengths of 1264 and 2249 N, respectively, were achieved at a die depth of 2.2 mm and a diameter of 10.4 mm. The CPMC process provides new ideas for the mechanical clinching of aluminum alloy and ultra-high-strength steels.

1. Introduction

Mechanical clinching, also referred to as imprinted connection, involves the application of an imprinted connection mold. Under its action, the connected materials undergo cold extrusion, resulting in an interlocking connection point. This method is particularly suitable for joining soft and thin sheet materials. This process is characterized by minimal raw material consumption and does not require either pre-treatment or post-treatment. Compared with traditional riveted and spot-welding methods, mechanical clinching offers distinct advantages, classifying it as a green, low-carbon manufacturing technology [1,2]. In recent years, the drive for lightweight automotive design has sparked significant interest in high-strength, lightweight materials, thereby accelerating the development of reliable joining techniques for dissimilar materials [3,4].

To enhance the quality of mechanical clinching, researchers are constantly proposing novel clinching processes. Peng et al. [5,6] investigated a two-stroke flattening clinching (TFC) technique for flat surfaces, which utilized flat sheet surfaces to achieve high-strength connections, with joints having good mechanical properties. Chen et al. [7,8] proposed a mechanical clinching renovation process to repair deformed and damaged joints by embedding rivets in the pits of the joints. Babalo et al. [9,10] developed an electro-hydraulic clinching (EHC) joining technique for thin sheets, completing hole clinching for sheets with a thickness of 0.5 mm. The results indicated that this process enhances joint strength, achieving joint efficiency approximately twice that of other clinching methods. Zhang et al. [11] conducted a study on the stepped mechanical clinching of aluminum alloy sheets, and the results showed that at the same forming force, the stepped punch has more advantages over the round punch. Ma et al. [12] investigated the dynamic bonding behavior in a clinch–bonding hybrid process involving high-strength steel and aluminum alloy through numerical simulations. The authors elucidated the impact of adhesive pockets on sheet metal deformation and process-induced cracking.

Nowadays, ultra-high-strength steel and aluminum alloy sheets with high specific strength are widely used, and the development of mechanical clinching processes suitable for aluminum alloy and ultra-high-strength steel sheets is of considerable practical importance for advancing lightweight automotive technology. Abe et al. [13] showed that using dies to control metal flow is effective in joining high-strength steel and aluminum alloy sheets, but the interlock value is small for higher-strength SPFC980 steel sheets. Lee et al. [14] developed a novel mechanical clinching technique called “hole clinching”, enabling the joining of ductile materials to high-strength, low-ductility materials. This method requires the precise alignment of the centers of the punch, die, and hole in the lower sheet during joining to avoid fracture of the upper sheet. Han et al. [15] investigated a heat-assisted hole-clinching process using numerical methods and experiments to join magnesium alloy and 22MnB5 ultra-high-strength steel. Abe et al. [16,17] proposed a mechanical clinching process involving lower sheet preforming. This process not only investigated the influence of the preformed punch shape on the deformation behavior of the steel sheets, but also increased the interlock value and improved the quality of the clinched joints. Fu et al. [18] demonstrated that for preformed clinching joints, the upper aluminum sheet extruded in terms of neck thickness due to the high resistance of the bottom steel sheet. Consequently, a punch with a pressure-step structure was added to improve the joint quality. Chen et al. [19] developed a hot-stamping clinching tool with an integrated forming system, heating system, and cooling system, and the tensile strength of the clinched joints increased by 3–4 times due to the high cooling rate in the experiments.

In conclusion, to effectively join ultra-high-strength steels and aluminum alloys, previous researchers have introduced innovations in optimizing molds and processes, and some good results have been achieved. The traditional mechanical clinching process, while simple, fails to create an effective interlock. The majority of contemporary research employs multi-step or thermally assisted mechanical clinching techniques to join ultra-high-strength steel sheets. This makes the mechanical clinching of aluminum alloy and ultra-high-strength steel sheets somewhat limited in practical production. In comparison to existing methodologies, the straightforward and operationally efficient one-step clinching process streamlines production by reducing the number of steps, shortening the cycle time, and enhancing suitability for practical application. Therefore, to enhance clinching efficiency while ensuring joint strength, it is imperative to explore and investigate novel mechanical clinching processes.

In this study, a novel center-punching mechanical clinching (CPMC) process is proposed for joining aluminum alloy and ultra-high-strength steel sheets. The CPMC process uses a combination of punching, flanging, and bulging to achieve mechanical clinching in one step. To verify the feasibility of the CPMC process, clinching tests were conducted with various die diameters and depths. The influence of process parameters on the geometry of the clinched joints was thoroughly analyzed. Meanwhile, the static strength and failure mode of the joints were evaluated through tensile and shear tests. The results demonstrate that the CPMC process effectively joins AA5052 aluminum alloy and DP980 ultra-high-strength steel, enhancing clinching efficiency while ensuring high-quality joints.

2. Mechanism and Plastic-Forming Process of CPMC

2.1. Mechanics of Plastic Forming of CPMC

Figure 1 illustrates the conventional mechanical clinching process, where the deformation of the lower ultra-high-strength steel sheets is mainly achieved through stretching and extruding. This process results in only small interlocks due to the large forming force required to thin the lower sheet [17]. Additionally, the low ductility of the ultra-high-strength steel sheets may lead to tensile fractures when the die depth is significant. The red arrow indicates the direction of material flow. The center-punching mechanical clinching process modifies the deformation method of the traditional clinching process by employing round hole flanging and bulging, as shown in Figure 2. The thinning of the lower sheet is primarily attributed to the increased hole diameter resulting from round hole flanging. With a smaller forming force, ultra-high-strength steel can be bulged outward through aluminum alloy sheets, facilitating the formation of an “S” shape. The stress state in the stretching and flanging stages of both conventional mechanical clinching and CPMC processes is essentially the same. Axial stress is negligible compared to radial and tangential stress, and the radial strain during flanging is zero. Analyzing the stress–strain state of the lower sheet during the clinching stage of both processes reveals that the lower sheet in traditional mechanical clinching is subjected to triaxial compressive stress, with the steel sheet elongating in the radial and tangential directions and undergoing compressive deformation in the axial direction. Nonetheless, in the CPMC process, the lower sheet is subjected to two-way compressive stress and one-way tensile stress, with the steel sheet elongating and deforming in the tangential direction while compressing in the axial direction. Mechanical analysis indicates that the CPMC process facilitates easier deformation during the clinching molding stage compared to traditional mechanical clinching. Therefore, under the same conditions, the CPMC process is more likely to generate larger interlocks.

2.2. Principle of CPMC

A schematic diagram of the center-punch mechanical riveting process, designed according to the mechanical mechanism of plastic forming, is shown in Figure 3. The CPMC process incorporates a punching technique that includes round hole flanging and bulging. This technique is designed to reduce forming force; prevent tensile fractures in ultra-high-strength sheets; and enhance the strength of the clinched joint. The tools used in the CPMC process include the punch; holder; slider; die; and button die. The CPMC process flow consists of the following stages: (a) Positioning the upper and lower sheets on the upper surface of the die and adjusting the sliders so that the button die aligns flush with the upper surface of the die, forming a punching mold. (b) The punch moves downwards to punch the upper and lower sheets, and the waste is discharged from the circular hole in the middle of the button die. (c) Upon completion of the punching, the punch remains stationary while the slider is swiftly adjusted, aligning the button die flush with the lower surface of the die’s groove, thus forming a clinching die. (d) The punch continues its downward motion until the punch load reaches the predetermined value. This process generates a neck thickness at the connection point, achieving geometric interlocking and thereby completing the mechanical clinching. As shown in Figure 4, the neck thickness (tn) and interlock (ts) are crucial indicators for evaluating the connection quality of clinched joints [20,21].

3. Materials and Experimental Methods

3.1. Materials

Thin sheets of 1.5 mm thick AA5052 aluminum alloy and 1.2 mm thick DP980 ultra-high-strength steel were used in this study. The main mechanical properties of the sheets, shown in

Table 1. Main mechanical properties of the materials.

Material	Thickness (mm)	Elastic Modulus (GPa)	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)
AA5052	1.5	69	144	223	22
DP980	1.2	207	653	1064	12

, were assessed by conducting uniaxial tensile tests using an InspektTable100 testing machine at a speed of 1 mm/min [22,23]. The chemical compositions of DP980 and AA5052 are listed in

Table 2. Chemical composition of DP980.

Element	C	Si	Mn	P	S	Al	Fe
Mass fraction (%)	0.09	0.55	2.71	0.012	0.001	0.032	Remain

and

Table 3. Chemical composition of AA5052.

Element	Cu	Si	Mn	Mg	Zn	Cr	Fe	Al
Mass fraction (%)	0.10	0.25	0.10	2.40	0.10	0.25	0.40	Remain

.

3.2. Experimental Setup

The mechanical clinching tools along with the mold schematic diagram are shown in Figure 5. According to the working conditions in this article, the CPMC process employed a 135 kN servo-hydraulic clinching machine, with the test forming force set at 80 kN. The CPMC process was divided into two primary stages: punching and clinching. In the punching phase, the punch approached the sheet at a speed of 40 mm/s, decelerating to 2 mm/s approximately 5 mm above the sheet until the punching was complete, with a punching stroke of approximately 4 mm. In the clinching stage, the slider moved outward, the button die descended to the specified position, and the punch continued to travel downward at 2 mm/s until it reached the maximum pressure, completing the clinched joint. To ensure the quality of the clinched joints, the punch held the pressure for 3 s before returning to its initial position at a speed of 40 mm/s.

The geometric dimensions of the mechanical clinching mold are shown in Figure 6. An increase in punching diameter resulted in a corresponding rise in forming force. Based on the thicknesses of the upper and lower sheets in the clinching process, the punching diameter used in this study for the CPMC process was 2 mm, with a unilateral clearance of 0.1 mm. In order to investigate the capability of the CPMC process in joining aluminum alloy and ultra-high-strength steel, a series of clinching experiments were conducted using dies with varying depths (H = 2, 2.1, and 2.2 mm) and diameters (D = 10, 10.2, and 10.4 mm). The experimental plan is shown in

Table 4. List of experiments of the CPMC process.

Experiment	Depth of Die H (mm)	Diameter of Die D (mm)
1	2.0	10.0
2		10.2
3		10.4
4	2.1	10.0
5		10.2
6		10.4
7	2.2	10.0
8		10.2
9		10.4

.

3.3. Tests of Static Strength

To evaluate the mechanical properties of the CPMC joints, tensile and shear tests were conducted using an InspektTable100 tensile testing machine. The sheets were clinched with an overlap of 25 and 50 mm between two sheets for tensile and shear tests, respectively, as shown in Figure 7. The testing speed was set to 2 mm/min [24]. To prevent the effect of additional loads during the shear test, additional sheets were placed on both sides of the specimen [25,26]. The clamping methods of the specimens for the static strength test are shown in Figure 8. To ensure the accuracy of static strength testing for clinched joints, three specimens were tested under each joint geometry condition [27]. The average strength of the three specimens was taken as the joint strength for that specific size condition.

4. Results and Discussion

4.1. Material Flow of Sheet Metal

The efficacy of the proposed CPMC process was evaluated by mechanical clinching experiments. Figure 9 shows the forming curve (punch load versus stroke) during the CPMC process. In stage I (punching stage), once the punch fully contacts the upper sheet, the load gradually increases as the punch descends. This stage concludes with the completion of punching and the ejection of waste material through the central hole of the button die. In stage II, the lower steel sheet is mainly formed by flanging. Throughout this process, the thickness of the steel sheet remains nearly constant, and the punch load incrementally increases. By the end of stage II, the steel sheet makes contact with the die. In stage III, the metal flow between the upper and lower sheets is significantly enhanced, leading to a sharp increase in punch load. The diameter of the circular hole in the center of the lower sheet continuously enlarges, causing axial thinning of the metal at the bottom, which takes on a conical shape, with the thickness of the standing wall of the sheet flap remaining almost unchanged. In this process, the upper aluminum sheet enters into the central region of the steel sheet, becoming elongated and thinned at the neck. In stage IV, the extrusion of the aluminum sheet at the punch’s lower end drives the bottom metal of the steel sheet to slide into the outer cavity, resulting in the outward bulging of the vertical wall area of the steel sheet flap. During this process, the wall thickness of the interlocking area slightly decreases due to the bulging of the steel sheet, while the punch load continues to increase. After the punch load reaches a predetermined value, a geometric interlock is finally formed between the upper and lower sheets.

4.2. Geometric Features of Clinched Joints

The cross-sections of the CPMC joints under varying conditions of die depth (H) and diameter (D) are shown in Figure 10. The joint cross-sections exhibit reasonable neck thickness and interlock across all process conditions. The maximum neck thickness and interlock are 0.455 and 0.24 mm, respectively. Compared with the literature [17], CPMC exhibits greater neck thickness and interlock for the same material. This indicates that the CPMC process is effective in joining aluminum alloys and ultra-high-strength steel sheets. Figure 10 demonstrates that varying die shapes significantly impact the joint geometry, which subsequently affects the connection quality of the clinched joints. Due to the non-uniform deformation of metal sheets during the CPMC process, which results in asymmetry in neck thickness and interlock values on both sides of the joint, the average values from both sides are considered as the final result in the experiment. To accurately investigate the variations in joint geometry due to different die dimensions, the exact values of neck thickness and interlock, measured under various joint geometries, are presented in

Table 5. Specific figures for neck thickness and interlock values.

Experiment	Depth of Die H (mm)	Diameter of Die D (mm)	Interlock Value (mm)	Neck Thickness (mm)
1	2.0	10.0	0.340	0.110
2		10.2	0.400	0.115
3		10.4	0.455	0.125
4	2.1	10.0	0.315	0.150
5		10.2	0.370	0.160
6		10.4	0.450	0.170
7	2.2	10.0	0.270	0.240
8		10.2	0.360	0.235
9		10.4	0.400	0.240

.

The influence of varying die shapes on the interlock value is shown in Figure 11. As die depth increases, the metal at the punch and die corners has more room for plastic deformation, thereby increasing the interlock value. As die diameter increases, the change in the interlock amount is not significant. The interlock value peaks at 0.24 mm with a die depth of H = 2.2 mm and die diameters of D = 10.0 and 10.4 mm. Figure 12 demonstrates the influence of varying die shapes on neck thickness. An increase in die diameter enlarges the metal flow space between the punch and the die, resulting in a thicker neck thickness. Conversely, as die depth increases, neck thickness gradually decreases. This occurs because the side of the punch is externally conical; the deeper the press-in, the larger the diameter, and the greater the deformation resistance of the steel sheet, making it difficult to press thin. Consequently, only the neck of the aluminum sheet thins. When the die depth is 2.0 mm and the die diameter is 10.4 mm, the neck thickness is larger, measuring 0.46 mm.

4.3. Static Strength of Clinched Joints

The load–stroke curves obtained from the static strength test are illustrated in Figure 13. Before the tensile shear test, the clamping specimen was preloaded with 100 N. To ensure the accuracy of static strength testing for clinched joints, three specimens were tested under each joint geometry condition. The tensile strength of CPMC joints under various die shapes is shown in Figure 14. The data reveal that with increasing die diameter and depth, the tensile strength of the joint correspondingly increases. However, this trend does not correspond with changes in the joint neck thickness and interlock value, indicating that the joint strength is governed by a combination of these factors rather than a single one. Figure 15 depicts the shear strength of CPMC joints under various die shapes. The trend of shear strength is similar to that of tensile strength. Notably, the shear strength is approximately twice the tensile strength. In this experimental study, the tensile strength of most joints exceeded 1100 N, while the shear strength surpassed 2100 N. The maximum tensile and shear strengths of the joints were recorded at 1264 and 2312 N. Despite the geometric inaccuracies of the clinched joints due to uneven deformation, the deviation in the test results indicates that the repeatability of the joints’ static strength is satisfactory. In summary, die diameter plays a significant role in increasing tensile and shear strengths as well as interlock values, but may slightly reduce neck thickness. Die depth enhances neck thickness, which may help the joint withstand larger shear and tensile forces. The optimal combination of larger diameter (10.4 mm) and moderate depth (2.2 mm) seems to provide the best balance between high tensile/shear strengths and adequate interlock and neck thickness. This analysis indicates that both the die depth and diameter need to be optimized together, with a preference towards larger diameters for higher joint strength, despite minor trade-offs in neck thickness.

4.4. Failure Mode

The main failure modes of CPMC joints can be classified into three types: neck fracture, button separation, and a combination of both failure mechanisms [28]. And these three main failure modes are shown in Figure 16. Button separation typically occurs when a small interlock value is produced during the clinching process or when very thick metal necks are formed [29,30]. Additionally, neck fractures primarily occur when the interlock value of a joint is high. Figure 17 illustrates the failure modes of CPMC joints under tensile tests. It is apparent that neck fractures occurred in joints with die depths of 2.2 mm, where the interlock value exceeded 0.22 mm. The ‘group’ numbers below the pictures correspond to the experiment numbers in Table 4. Mixed failure is observed in groups 1 and 4, which have smaller neck thicknesses, while button separation occurs in the remaining groups. As shown in Figure 18, all CPMC joints with die depths of 2.1 and 2.2 mm exhibited neck fracture failures in the shear test, in contrast to the tensile test results. Among these joints, the smallest interlock value observed was 0.15 mm. Groups 1 and 2 exhibited mixed failure due to small interlock values. Notably, group 3 experienced bottom fracture because of the thin aluminum alloy bottom thickness.

5. Conclusions

This study proposes a novel mechanical clinching process called center-punching mechanical clinching (CPMC) for joining aluminum alloy and ultra-high-strength steel sheets. The principal findings of this study are as follows:

(1)

The CPMC technique proposed herein is particularly suitable for joining aluminum alloy and ultra-high-strength steel sheets, as good neck thickness and interlock can be observed in the cross-section of the joint.

(2)

The CPMC process, which involves punching, flanging, and bulging in a single mechanical clinching operation, eliminates the challenge of extruding ultra-high-strength steel sheets encountered in traditional mechanical clinching methods for thin sheet deformation, thereby significantly improving the interlocking strength of the joint.

(3)

Variations in die diameter and depth significantly influence the quality of CPMC joints. An increase in die depth enhances the interlock value, while an increase in die diameter augments the neck thickness.

(4)

In this study, the maximum tensile and shear loads of the CPMC joints achieved 1264 and 2312 N, respectively. Considering the geometry of the CPMC joints and the strength tests, it was concluded that joint quality was optimal when the die depth was 2.2 mm and the die diameter was 10.4 mm. At these parameters, the neck thickness and interlock value were 0.4 and 0.24 mm, respectively, with tensile and shear strengths of 1264 and 2249 N.