Static Strength Analysis and Experimental Research of Clinched Joints By Two-strokes Flattening Clinching Method

Clinching technology is widely used to join sheet materials in manufacturing fields, especially in automotive lightweight applications. However, the clinched joints have a weak static strength and high protuberance, which influence the application of the clinching technology. In order to improve the static strength and decrease the protuberance height of clinched joint, a new method to join aluminum alloy sheet materials with two - strokes flattening clinching (TFC) was investigated in this paper. The tension - shear strength, cross - tension strength, energy absorption and failure modes of clinched joint and TFC joint were investigated. Furthermore, the stiffness and the hardening exponent of the joints under different experimental tests were studied. The results indicated that the mechanical behaviors of the joints were optimal when the forming force was 35 kN. The maximum cross - tension and tension - shear strength of TFC joint were increased by 514 N and 1145 N on average compared with the initial clinched joint. The main failure modes of the joints were the neck fracture mode under the tension - shearing and cross - tension test. In addition, the stiffness and hardening exponent explained the variation of the mechanical properties of the joints under different forming forces.


Introduction
In recent years, aluminum alloys have been widely used in manufacturing fields due to their light weight and high hardness, especially in the automotive industry [1][2][3]. The joining method of these aluminum alloys has becomes a hot research topic. The methods of joining aluminum alloys in automobile manufacturing mainly include welding, resistance spot welding, riveting, adhesive bonding and clinching. Welding and resistance spot welding are both chemical joining methods. The joining process consumes a lot of energy and generates harmful gases. Furthermore, welding and resistance spot welding are not suitable for joining the different alloy sheet materials because of the different melting points. Different from welding, adhesive bonding and clinching can join the similar and dissimilar aluminum alloys using mechanical methods [4]. However, the quality of the adhesive bonded joints is severely affected by the temperature, and the process of the adhesive bonding is time consuming. The clinching process has the characteristics of short time-consuming and high economic efficiency.
The clinched joints are more stable than adhesive bonding joints and are less affected by the temperature [5,6]. Clinching is the most economical way to join sheet materials. However, the strength of clinched joints is lower than that of spot welded joints. Many researchers have studied the clinching process to optimize the quality of clinched joints.
Clinching of similar and dissimilar sheet materials of titanium and aluminum alloys were investigated by He et al. [7]. Different combinations of titanium alloys and aluminum alloys as well as placement positions affected the strength of joint. The results showed that the mechanical properties of joints were better when the titanium alloy was taken as the lower sheet. Lambiase et al. [8] researched the clinched joints with different combinations of aluminum alloys and Glass Fiber Reinforced Polymer (GFRP) using different punches. They study results showed that the mechanical behaviors of joints are largely affected by the thickness of the metal/GFRP sheets. The clinching processes of dissimilar materials of aluminum alloy and Carbon Fiber Reinforced Plastic (CFRP) sheets were investigated by Lee et al. [9,10]. The authors optimized the parameters of the hole clinching tools to improve the performance of hole-clinched joints. They found that an important alignment factor affecting the static strength of clinched joints was the center difference between dies and the hole in lower sheet. Abe et al. [11,12] investigated the mechanical behavior of clinched joint using ultra-high-strength steel sheets. The results of their study found that the clinched joints had better fatigue strength than the resistance spot welded joints. Furthermore, the parameters of conventional clinching tools were optimized by He et al. [13]. Eshtayeh et al. [14] optimized the clinched joint using the Grey-based Taguchi method. The found that an increase in the thickness of the bottom or the neck thickness leads to a decrease in the interlock lock. Han et al. [15] investigated the effect of the bottom die parameters on the static strength of the joints.
They found that the groove depth and die depth of the die have a significant impact on the quality of joints. Lambiase et al. [16] researched the effect of different dies on the static strength of clinched joints. The main results of the research found that static strength of the clinched joints created by the extensible dies are stiffer than that of the joints created by fixed dies. Mucha et al. [17] studied the influence of forming force and bottom thickness on static strength of the clinched joints. The authors found that the bottom thickness and static strength of clinched joints are negatively correlated within a certain range. It means that increasing the forming force within a certain range can improve the performance of the joint.
Moreover, the reshaping methods are also employed to enhance the quality of the clinched joints. Wen et al. [18] created a reshaping method using a matched pair of counter tools to increase the mechanical behaviors of the clinched joints. They found that the reshaping method can reduce the protuberance and enhance the strength of the joint. The average tension-shear strength of reshaped joints is enhanced by 17% versus to that of clinched joint. Chen et al. [19][20][21] studied height reducing methods to improve the quality of clinched joints. The protuberance of clinched joints can be reduced through the high reducing method with a pair of flat dies. The authors found that the reshaping force and the different combinations of material sheets affect the quality of the reshaped joints. However, as shown in Fig. 1, these methods cannot flatten the protuberances of the clinched joints.  [20] In current study, a new clinching process is studied, which is able to improve the quality of joints and make the joints almost free of protuberance. The AA5052 sheet materials with the thickness of 2.0 mm were adopted in clinching process. The tension-shearing tests and cross-tension tests were conducted to evaluate the static strength of clinched joints. The effects of forming force and stroke on the joint strength, materials flow, failure mode, the stiffness, the hardening exponent and energy absorption were evaluated experimentally. Furthermore, in order to study the sheet material flow at different strokes, profiles of joints with different process parameters were studied comparatively.
The results showed that the TFC process can greatly improve the mechanical properties of initial joints. The quasi-static test showed that the tension-shear strength and cross-tension strength are increased by 78.2% and 45.2%, respectively. Material flow analysis showed a 69.6% and 211.6% increase in neck thickness and interlock. Furthermore, the TFC joints exhibit a superior strength and absorb more energy during the cross-tension test.

Two-strokes flattening clinching process
Two-strokes flattening clinching (TFC) process was proposed to produce clinched joints with better mechanical behaviors, which is an optimization of conventional clinching process.
As demonstrated in Fig. 2 include the interlock ( ), the neck thickness ( ) and the bottom thickness ( ). In addition to these important parameters, the work hardening (n) and stiffness (K) of the joints are also significant parameters that affect the mechanical properties of the joints.

Materials
AA5052 sheet materials are used in automobile bodies, and it has good ductility and strength. The TFC process was conducted on the AA5052 sheets materials with the thickness of 2.0 mm. All sheets were obtained by cutting from the rolling direction. The size configuration of these sheets is thickness 2 mm × length 80 mm × width 25 mm. The mechanical properties of AA5052 sheet materials were determined by uniaxial tensile tests with Instron 5982 universal tester. The experimental results were summarized in Table 1. Table 2 lists the chemical composition of AA5052 sheet materials. Table 1 The principal mechanical properties of the AA5052 sheet materials  Table 2 The chemical composition of AA5052 sheet materials  Fig. 4(a). Another joint specimen employed for tension-shearing test is depicted in Fig. 4(b). As shown in Fig. 5, different types of joint specimens are held by different clamping tools. Fig. 5(a) illustrates the clamping tools for tension-shearing test specimens, and Fig. 5(b) illustrates the clamping tools for cross-tension test specimens. The tension-shearing and cross-tension tests were performed by CMT-5105GJ universal testing machine. In the experimental test, the ascent speed of the CMT-5105GJ machine was constant at 4 mm/min.
When the force and displacement curve on CMT-5105GJ tester suddenly drops, the joint fails completely, ending the test and recording the data. The static strength of each joint was obtained from the test results of three test specimens.
As shown in Fig. 6, button separation and neck fracture are the main failure modes during the static experimental test of the joints. The failure modes reflect the mechanism of action between the interlock and the neck thickness of the joint. Energy absorption is area enclosed by the force and displacement of joints before failure, which is an overall assessment of the mechanical properties of the clinched joints (see Fig. 7). Hardening exponent (n) describes the work hardening characteristics of the sheet materials during clinching process (see Fig. 8). The hardening exponent (n) can be obtained from fitting the displacement-load curves before the joint failure by the following formula: Where F and X are the load and the displacement of the joint, A and 0 are the coefficient and initial displacement, respectively. Coefficient A mainly affects the amplitude of the curves, and 0 is the offset of the initial position of the curves. The hardening exponent (n), which affects the main variation of the curves, was mainly studied in this experiment.
Furthermore, the stiffness of joint (K) is analyzed, which is the ability of joints to resist deformation during the elastic phase (see Fig. 7).

Cross-tension test
The cross-tension strength of initial clinched joints and TFC joint with various forming forces were obtained from three cross-tension tests. The neck thickness and the interlock determine the static strength of the joint [25]. As presented in Fig. 9, the cross-tension strengths of TFC joints are all greater than that of initial clinched joint under various forming forces.
Especial the increase extent is more significantly at forming force of 40 kN, which increasing extent reaches 39.3%. The cross-tension strengths of initial clinched joints and TFC joints are  The stiffness (K) and hardening exponent (n) of the joint with various forces of the punch are shown in Fig. 11. The stiffness of the TFC joints is lower than that of initial clinched joints at different forming forces, which indicated that the deformation-resistant capacity of the TFC joint is lower than that of the initial clinched joint. The stiffness of the initial clinched joint is largest when the punch force is 35 kN. Similarly, the hardening exponent of the initial clinched joint is greater than that of TFC joint at different forming forces. The hardening exponent of the initial clinched joint reaches a minimum value of 0.29874 when the forming force is 35 kN.   The displacements of the TFC joints under different forming forces before failure are less than that of the initial clinched joints in the tension-shearing test. The stiffness and hardening exponent of different joint under different punch forces are depicted in Fig. 15. As the punch force increases, the stiffness of the joint increases at first and then decreases. The stiffness of both joints reaches maximum value, which means that the joints displacements are short before failure in the tension-shearing test. Conversely, the hardening exponent of both joints decreases at first and then increases as the forming force increases. The smaller the hardening exponent of the joint, the higher the strength at the same forming force.
From the Fig. 9 and Fig. 13, it can be observed that the tension-shear strength of the joints is higher than its cross-tension strength in the experimental test. The average tension-shearing strength of TFC joints is 1.54 times greater than its cross-tension strength. The average tension-shear strength of initial joints is 1.23 times greater than its cross-tension strength.
Furthermore, the tension-shear strength of TFC joints is 1.55 times of its cross-tension strength when the forming force is 35 kN. This is mainly due to the combined effect of the cross-tension strength of the material being greater than its tension-shear strength and the transverse interference around the joint during the tension-shearing test.
The neck fracture mode is the primary fracture mode of joints in the tension-shearing test.
As illustrated in Fig. 16, the tension-shear strength of joints is mainly determined by the thickness of the joint neck. The neck thickness of the TFC joint is greater than that of the initial clinched joint, which results in the higher tension-shear strength than that of initial clinched joint. Furthermore, the greatest force that the joints neck can be withstood, which can be equivalent to a ring subjected to shearing force [27,28]. As shown in Fig. 17, the maximum force ( ) can be calculated as follows: Where is the fracture stress of joint neck. , the area of the ring. and are the radius of the punch and the thickness of the neck, respectively.

Energy absorption
Impact resistance is one of the important factors for assessing structural stability, which is especially important in automobiles that are subjected to frequent shocks. Energy absorption is an significant indicator to evaluate the impact resistance [29]. More energy absorbed before joint failure, the better its impact resistance.
The value of energy absorption is obtained by gauging the area between the load-displacement curve and the abscissa in the coordinate system. As displayed in Fig. 18 and

Conclusion
Clinching aluminum alloy sheet materials with two-strokes flattening clinching method was investigated in present work, which can significantly improve the mechanical behavior of the joints. The AA5052 sheet materials were adopted in the experimental test. The materials of protuberance flow to the joint neck, which improves the neck thickness and interlock as well as flattens the initial clinched joint. The TFC process is effective for increasing the neck thickness and the interlock of the joint. The main findings of this work are as follows: (1) The cross-tension strength and tension-shear strength of the joint can be enhanced by the TFC process. The cross-tension strength is significantly improved at the forming force of 40 kN, which increasing extent reaches 39.3%. The maximum increase in tension-shear strength of the TFC joint compared to the initial clinched joint is 38.8%.
(2) The stiffness of joint (K) is the ability to resist deformation during the elastic deformation stage. The greater the stiffness value, the smaller the deformation of the joint under the same load. A joint with a hardening exponent of about 0.3 has good mechanical behaviors.
(3) All the failure mode of joints was neck fracture in the tension-shearing tests. The failure mode of button separation occurred in the cross-tension test of initial clinched joints when the punch force is 30 kN.
(4) All the TFC joints have better energy absorption comparing to initial clinched joints.
The energy absorption of TFC joints is 142% at most than that of initial clinched joint joint at the forming force of 40 kN. Furthermore, the maximum increase in energy absorption of initial clinched joints and TFC joints are 24% and 38% at different forming forces, respectively.
(5) The TFC joints have better material flow than initial clinched joints. The protuberance material of initial clinched joint flows to the neck area through the TFC process and becomes almost flat.