Abstract
To explore the failure principle of single-point butt clinched joints, the AA5052 and AA6061 sheets were employed to produce the single-point butt clinched joints. The microscopic morphology of the fracture surface, geometrical parameters, static strength, energy absorption and failure details of the joints were analysed. The experimental results showed that fracture forms of sheet materials are the same as ductile fracture. The sheet combination has a significant influence on the mechanical properties of the single-point butt clinched joint, which is ultimately due to the different mechanical properties of the basal sheets. The single-point butt clinching process proposed in this work contributes to simplifying the clinching process while increasing the joining efficiency.
Introduction
Recently, with the improvement in living standards, cars have been popularised rapidly [1]. Automobile manufacturers are facing unprecedented challenges while experiencing the opportunity of the explosion in the number of vehicles [2]. The contradiction between fuel economy and automobile weight is one of the main obstacles restricting the development of the automobile industry, and the joining technology of lightweight materials is the key to solving the lightweight problem of vehicle body [3-5]. Currently, the common joining techniques of lightweight materials are welding [6], adhesive joining [7], riveting [8] and mechanical clinching [9]. Compared with other conventional joining methods, energy-saving, low cost, environmental protection and easy to automation are the main advantages of the mechanical clinching process [10,11]. Since no additional rivets are required in the joining process, and the static strength of the clinched joint is stable, the mechanical clinching technology has become a research focus in the field of joining lightweight materials [12].
At present, researches on mechanical clinching technology are mainly focused on process parameters and die configuration [13,14]. Kumma et al. [15] investigated the influence of the blank holder force and edge radius of the clinching tools on the joints strength in the flat-clinching process. The experimental results show that the blank holder force and the die edge radius have a significant influence on the joint strength, and proper parameter selection can realise the mechanical clinching of the same and dissimilar materials. Wang et al. [16] established an accurate intelligent system to improve the clinching tools by the genetic algorithm approach and integrated finite element model. The accuracy of the intelligent system is verified by using Al6061-T4 alloy sheet with a thickness of 1.4 mm, which provides guidance for industrial application. Chu et al. [17] employed the mechanical clinching process to join the different combinations of titanium alloy, aluminium alloy and galvanised steel. And the effect of material combination sequence on the mechanical behaviours of clinched joint also has been systematically studied experimentally. The conclusions of this study indicate that materials with better strength and stiffness performance are more suitable as an upper sheet in the mechanical clinching process. To a certain extent, reasonable die parameters and sheet material combination sequences can effectively improve the joint strength and enhance the mechanical performance of the clinched joints.
The mechanical clinching method is also conducted in the field of joining high-strength metals and dissimilar materials [17]. For the connection of some thin-walled components with poor ductility, the method used to adapted is to add an auxiliary energy field to improve the joinability of the clinched joint, such as partial heating assisted clinching [18,19], friction-assisted clinching [20], laser shock micro clinching [21,22], ultrasonic assisted clinching [23], etc. The addition of the energy field promotes the material flow of joined sheet in the forming process of the mechanical interlock, avoids the crack development and defects generated in the mechanical interlock structure effectively, increases the mechanical properties and improves the joining quality of clinched joints. This method is specially adapted to the connection of metal materials with poor forming properties. As for the joining dissimilar materials, the mechanical clinching technology also performs well [24,25]. Currently, the most common clinching process for dissimilar materials are mainly focused on the connection of high-strength steel/aluminium alloy [17], aluminium alloy/carbon fibre reinforced polymer [26] and aluminium alloy/titanium alloy [27]. Some studies even attempted to join aluminium and wood-based materials by a flat-clinched joint [28]. The influence law of the wood moisture content on the joint strength was also proven by the static strength test, and the understanding of the flat-clinching process for this material combination was improved. Zheng et al. [29] successfully joined two different materials with significant differences in strength and plasticity by a novel mechanical clinching process, namely laser shock hole-clinching. By exploring the effects of laser parameters and initial grain size of metal foil on the forming quality of the joint, the defect mechanism of the laser shock hole-clinching process was further clarified.
Despite the advantages of the mechanical clinching technology, the application area of the clinched joint is limited by the high exterior protrusion. Given the above limitation, some scholars attempted to reduce or eliminate the exterior protrusion by improving the mould structure or optimising the joining process. The optimised processes such as dieless clinching [30], flat-clinching [31], reshaping process [32] and dieless clinching process free of blank holder [33] were wildly employed to enhance the joint static strength, improve the mechanical performance and reduce the exterior protrusion height of conventional clinched joint. Moreover, for the irreplaceable feature of the failed clinched joints, that is, when the clinched joints served a long time or subjected to fatigue loads in a period time, the clinched joint cracks or breaks, and the most common solution is to replace it with a new clinched joint. Chen et al. proposed a novel repairing process for failed clinched joint to extend the joint service life [34]. The failed joint is repaired by an additional rivet, which prolongs the service life of the joint and gains time for the replacement of new components. All repaired joints have higher static strength and energy absorption under the load impact, which confirms the repair potential of failed joints.
In industrial applications, the mechanical clinching process has been applied to more and more automation equipments. Therefore, the process complexity during the mechanical clinching process directly affects the clinching efficiency. On the premise that the joined strength of the joint meets the usage requirement, simplifying the clinching process would produce higher economic benefits. In this paper, a single-point butt clinching process is proposed, which simplifies the clinching process while reducing the number of clinched joints and ensuring the joint strength in the butt form of the sheet connection. The single-point butt clinching process is beneficial to widen the application range of the mechanical clinching process, especially for industrial scenarios where the connected sheet do not have enough overlapping length and are not subject to tensile load. Moreover, the single-lap shear test was executed to assess the mechanical properties and failure behaviours of various single-point butt clinched joints. Both the optical microscope and scanning electron microscopy (SEM) were employed to observe the fracture morphology of the joints after the failure tests. Further, the effect of sheet category on microstructure and mechanical behaviours of the single-point butt clinched joint was systematically illustrated by the experimental method. The single-point clinching process proposed in this work contributes to ensuring the service safety of structures with clinched joints in engineering applications.
Principle of the single-point butt clinching process
According to the clinching position and configuration of the joined sheets, there are two major clinched forms of the clinched joints produced by the mechanical clinching method, i.e. butt joint and the single-lap joint [35]. Figure 1 distinguishes between single-point butt clinched joint and single-lap joint in sheet configuration and the location of the clinched joint. As can be seen in single-lap joint (Figure 1(b)), part of one joined sheet overlaps with the other side of the other joined sheet. The upper joined sheet is inserted into the material of the lower joined sheet, and the mechanical interlock structure is formed by inlaying two joined sheets. Nevertheless, in the case of single-point butt clinched joint (Figure 1(a)), the joined sheets have no overlapping area, and the joined sheets are in the same plane. The formation of the interlock structure mainly depends on the plastic deformation of the two joined sheets, which are embedded in the assisted sheet for the purpose of joining the joined sheets.
The configurations of (a) single-point butt clinched joint and (b) single-lap joint.
As depicted in Figure 2, the clinching system is mainly composed of a punch and an extensible die. The extensible die consists of three sliding sectors, a fixed anvil and a rubber ring. The single-point clinching process involves four stages, that is, extruding stage (Figure 2(a)), deforming stage (Figure 2(b)), interlock stage (Figure 2(c)) and forming stage (Figure 2(d)). First, the two joined sheets are butted together, which is depicted in Figure 1(a). The joined sheets are fixed by an underneath assisted sheet, and placed in the direction of the punch on the bottom die. The punch driven by the press machine moves downward at a preset speed, and joined sheets and the assisted sheet are plastically deformed by the punching process. As the deformation increases, the extruded material flows rapidly into the groove of the extensible die. As the assisted sheet material touches the surface of the fixed anvil, the volume of deformed material increases steadily, at this point, the sliding sectors slide along the radial direction of the fixed anvil. Because the resistance of material flowing to the annular groove is relatively small, the upper sheet material extrudes the material of the assisted sheet to the annular groove simultaneously. At the same time, a mechanical interlock structure is also formed and the forming stage ends. In order to prevent the deformed material from rebounding after the forming force is reduced, this experiment continues to hold pressure for 10 s after the forming stage to ensure that the upper and lower sheets are fully filled and ultimately shaped in the concave die. After the pressure-holding stage, the punch goes up and the sliding sectors also return to the initial position, which means that the single-point butt clinching process is finished.
Single-point butt clinching process with the extensible die.
Figure 3 presents the mechanical interlock structure of the single-point butt clinched joint in this work. As can be obtained from Figure 3, neck thickness (tn
), exterior protrusion height (H), interlock value (ts
) and bottom thickness (X) are the main parameters of the mechanical interlock structure on single-point butt clinched joint. It is noted that the mechanical interlock structure of the single-point butt clinched joint is quite different from that of the conventional clinched joint, that is, a semicircular mechanical interlock is formed separately by the left and right joined sheets of the single-point butt clinched joint, while the mechanical interlock structure of the conventional clinched joint is a complete structure. To investigate the effect of different mechanical interlock structure on the mechanical properties of the single-point butt clinched joints, dissimilar joined sheets were employed to investigate the mechanical behaviours of the joints produced by the single-point butt clinching process in this study.
Geometric parameters of the mechanical interlock on single-point butt clinched joint.
Experimental procedure
Materials
Mechanical properties of the AA5052 and AA6061 aluminium alloy sheets.
Joining process
In the joining process, a punch with the diameter of 5.8 mm was employed as the upper die, and the wildly used extensible die was selected as the bottom die to implement the single-point butt clinching process. The extensible die with a depth of 1.1 mm variable volume cavity for material flow during the joining process. The mechanical clinching process was carried out on the Sust CMT-5105GJ testing machine and the experimental clinching equipment used in this work is displayed in Figure 4. Testing velocity, loading size and displacement can be precisely controlled and recorded by the computer. The punch moves downward with a speed of 2 mm min–1. For the purpose of ensuring the consistency of the mechanical clinching process, the force control mode was adopted to switch the start and stop the movement of the testing machine, which drives the punch in the joining process. Considering the higher hardness of AA6061 aluminium alloy and the excellent quality of AA5052 single-point butt clinched joint with 45 kN forming force, the forming force of 45 kN was selected in this section based on the same die parameters and the process parameters as in the previous sections. At the end of the joining process, the punch driven by the testing machine moves back to the primal position with a presupposed speed of 10 mm min–1.
The clinching equipment employed in this work.
Failure test
Failure test is the main method to estimate the mechanical behaviours of the single-point butt clinched joint. In this work, the static shearing tests were also implemented on the Sust CMT-5105GJ testing machine. In order to decrease the effects of the bending moment on the single-point butt clinched joint in failure test, two aluminium alloys shims with 2 mm thickness were padded at the two ends of the specimens far from the single-point butt clinched joints. The test speed is set to 2 mm min–1, and when the clinched joint was damaged or the joined sheets were completely separated, the shearing test terminated. To ensure the reliability of the experimental results, each type of the single-point butt clinched joints was tested five times. The average of the five testing results was regarded as the effective static shearing strength of each type of the single-point butt clinched joint. Figure 5 depicts the specimen and clamping tools used in the static shearing test in this work.
Specimen clamping tools used for the shearing test in this work.
So as to explore the joinability of the single-point butt clinched joints more comprehensively, two types of aluminium alloy sheets, AA5052 and AA6061 sheets, were selected as the assisted sheet and the joined sheet for investigating the effect of sheet category on the mechanical clinching process and mechanical behaviours of single-point butt clinched joints. Four different stack forms of different sheet categories were introduced to deeply understand the effect of sheet categories.
Terminology specifications of dissimilar single-point butt clinched joints used in this paper.
Results and discussion
Geometrical parameters
As for single-point butt clinched joint produced by AA5052 and AA6061 materials, the static strength depends to a large extent on the mechanical interlock on the relatively weaker side. Since the left and right sheets are different materials, there is a difference in material flow during the forming process. Therefore, in this section, the single-point butt clinched joint is also dissected in the cutting direction in Figure 6 to measure the main parameters of the mechanical interlock structure accurately and analyse the material flow of the clinched joints. The cross-section profiles of the single-point clinched joints produced by different sheet categories are depicted in Figure 7.
Cutting direction of the single-lap butt clinched joint. Cross-section profiles of the single-point clinched joints produced by different sheet categories.
As observed in Figure 7, in the cross-section profile of the AA56-5 and AA56-6 single-point butt clinched joints, there is a significant difference in the mechanical interlock profile between the left side and the right side, especially in interlock values, due to the joined sheets are of different materials. The interlock value on the AA5052 sheet side of the mechanical interlock is 1.65 times higher than that on the AA6061 sheet side when the assisted sheet is the AA5052 sheet. If the assisted sheet is an AA6061 sheet, the interlock value of the mechanical interlock on the AA5052 sheet side is 2.14 times of that on the AA6061 sheet side. Simultaneously, by observing the demarcation interface between two joined sheets in the AA56-5 and AA56-6 sections, the material flow of the AA6061 sheet along the axial direction of the fixed anvil is significantly greater than that of the AA5052 sheet in these two types of single-point butt clinched joints produced by dissimilar materials. This is the main reason that the interlock value on the AA5052 sheet side is significantly larger than that on the AA6061 sheet side.
Under the fact that the neck thickness is similar, the AA6061 sheet material flows to the AA5052 sheet side in a large amount along the radial direction of the fixed anvil, so that less AA6061 material flows to the interlock position. While the material on the side of AA5052 sheet is obstructed when it flows along the radial direction of the fixed anvil to the side of AA6061 sheet, and a large amount of AA5052 material flows to the interlock position in the opposite direction, consequently raising the interlock value. In addition, the bottom thickness and exterior protrusion height of dissimilar joint with different sheets categories were also analysed. Figure 8 presents the exact value of the bottom thickness and exterior protrusion height produced by different sheet materials.
The bottom thickness and exterior protrusion height of dissimilar joint with different sheet categories
As shown in Figures 7 and 8, the AA55-5 joint has the highest exterior protrusion height among the four types of single-point butt clinched joints. The AA56-5 joint and the AA56-6 joint have similar exterior protrusion height, whereas the bottom thickness of these two types of joints is quite different, which is caused by different categories of assisted sheets. Since the exterior protrusion height of the clinched joint is mainly affected by the depth of the cavity of the extensible die, the height of exterior protrusion on various single-point butt clinched joints does not differ significantly. Furthermore, it is worth pointing out that when single-point butt clinched joints are performed to join the AA5052 sheet and AA6061 sheet, the bottom thickness of the mechanical interlock is significantly higher when the AA6061 sheet is used as the assisted sheet than the AA5052 sheet. This is mainly due to that AA6061 aluminium alloy has better deformation resistance than AA5052 aluminium alloy in the same forming conditions.
Failure mechanism of the single-point butt clinched joints
Considering that the single-point butt clinched joints are made of different materials, the mechanical interlock parameters have a limited influence on the mechanical behaviours of the joints. For the purpose of exploring the failure mechanism of single-point butt clinched joints during the failure process, the static shearing tests were conducted to perform mechanical performance on different types of joints.
The main failure modes in the static strength test are neck fracture, button separation and hybrid failure mode [36]. If the strength of the neck position on single-point butt clinched joints is the lower than that of the interlock position, a ring fracture is generated on the thinnest neck position of the joint after failure, and this type of failure mode is termed as neck fracture. On the contrary, if the neck position of the joint has a higher strength than the interlock position, the embedded sheet is completely pulled out from the mechanical interlock structure after the static strength test, and this failure mode is called button separation. If the clinched joint is partially fractured at the neck position, and the upper sheet is separated from the interlock position of the lower sheet during the failure process, this failure mode is regarded as hybrid failure mode. Figure 9 depicted the macrograph of the dissimilar failed joints after the shearing test in this work.
Macrograph of the failed single-point butt joint produced by different materials after the shearing test.
As depicted in Figure 9, all the single-point butt clinched joints produced by the AA5052 and AA6061 aluminium alloy sheets are fractured in only half of the joint in the failure test. The major failure mode of the dissimilar single-point butt clinched joints produced in this work is neck fracture. As for the single-point butt clinched joints produced by dissimilar materials, the fracture position of the joint in the failure process occurs on the side of the joining material with lower tensile strength, i.e. the AA5052 sheet side. This is because the AA5052 sheet with the same thickness has a great ductility as well as a lower static tensile strength when subjected to shear load compared to AA6061 sheet. That is, with little difference in neck thickness of the single-point butt clinched joints, the fracture or failure of the material mainly depends on its static tensile strength. This is the main reason that the joints joining AA5052 and AA6061 sheets using the single-point butt clinching process proposed in this paper are mostly AA5052 aluminium alloy on the failed side in destructive tests. To investigate the difference between the failure forms of the single-point butt clinched joints with the AA5052 and AA6061 sheets, the microscopic morphologies of AA55-5 and AA66-6 failed joints were investigated which are shown in Figures 10 and 11, given the fact that the fracture position of the AA55-5 joint is on the AA5052 sheet side, and the fracture position of the AA66-6 joint is on the AA6061 sheet side in this work.
Micro morphologies of fracture position on failed AA55-5 joint. Micro morphologies of fracture position on failed AA66-6 joint.
As shown in Figure 10(a), when the single-point butt clinched joint is subjected to an overload force, one of the upper joined sheets with lower static strength is fractured at the neck position of the clinched joints. The fracture surface presents a relatively flat and crack-free appearance, while the other joined sheet has no macroscopic defects at the joint position. In Figure 10(b), the surface of area A is covered with a lot of shear dimples, which are small and uniform in size. There are also many shear dimples in region B of the fracture surface in Figure 10(c). The size of the dimples varies greatly. The overall size of the dimples is more significant than that of the dimples in area A, and the number of dimples is less. This indicates that area A has a higher nucleation density, smaller spacing, and a higher number of inclusions or second-phase particles compared to region B. Actually, a high density of large and deep dimples, indicating that the material underwent severe plastic deformation, can absorb more energy during the static shearing test, and the higher energy absorption capacity can more safely protect passengers or drivers from impact loads during a car crash. The microstructure of area C is radially and fibrously distributed, and the dimples are elongated with the action of shear loading. This is because the AA5052 material first undergoes plastic deformation in the micro area under the action of shear loading, and then micro-pits are presented. The micro-pits grow in the way of nucleation and growth, agglomeration, and eventual interconnection, resulting in micro-cracks and the continuous expansion of micro-cracks finally leads to ductile fracture of the clinched joint.
It can be seen in Figure 11 that the fracture surface micromorphology of AA6061 material is significantly different from that of AA5052 material after the static shearing test. The number of shear dimples in the fracture surface of AA6061 material is less than that of AA5052 material. A large number of shallow dimples are distributed throughout the SEM image of the fracture position. These shallow dimples have a different shape from the usual dimples, stripes of smaller sized planes (mini-facets) can be observed in the centre of the shallow dimples, and these mini-facets are surrounded by tearing ridges. There are many micro-pores distributed on the tearing edges around these strip-shaped shallow dimples. As depicted in Figure 11(b and c), a large amount of curved tearing ridges as well as a few elongated dimples appear on the fracture surface of areas D and E. In particular, large numbers of high-density short and curved tearing ridges are presented around the dimples in area E. These tearing ridges are the product of the struggle of the materials on joined sheets with the applied load during the joint failure. In Figure 11(d), area F consists of flat “cleavage-like” steps, micro-pores and tearing ridges which are the characteristic of quasi-cleavage failures. More energy can be absorbed by the tearing ridges and cleavage steps before the joint is fractured. Thus, quasi-cleavage fracture is the major fracture morphology in area F of the AA6061 aluminium alloy material during the shearing test.
Static shearing strength and energy absorption during the failure process
In the failure process, the static shearing strength and deformation of the single-point butt clinched joint affect the failure evolution process to a certain extent. So the energy absorption and the static shearing strength of the single-point butt clinched joint in the shearing test were also analysed in this work. In the same way, each group of the shearing test was repeated five times and the average value of the testing results was regarded as the effective strength of this type of single-point butt clinched joints. Figure 12 further indicates the static shearing strength of single-point butt clinched joint produced by dissimilar materials in the static shearing test.
The static shearing strength of dissimilar joints in the shearing test.
As observed from Figure 12, the static shearing strength of the AA66-6 joint is significantly higher than that of the other three joints, that is, the static shearing strength of the AA66-6 joint is 71.1%, 43.77% and 35.36% higher than those of AA55-5, AA56-5 and AA56-6 joints, respectively. The explanation for this phenomenon is that the AA66-6 joint failed during the shear test on the AA6061 side of the mechanical interlock structure, while the other three joints failed on the AA5052 side of the mechanical interlock structure. In the final analysis, the yield strength of AA6061 material is significantly higher than that of AA5052 material. The AA6061 side static strength of mechanical interlock is higher than that of the AA5052 side in the case of similar neck thickness.
For further quantitatively analysing the effect of the sheet category on mechanical behaviour of the single-point butt clinched joint, this section quantifies the energy absorption performance of various single-point butt clinched joints. The typical force–displacement curves of dissimilar joints produced by different sheet categories in the static shearing test are exhibited in Figure 13. And the average energy absorption of the dissimilar joints in the static shearing test is shown in Figure 14.
The typical force–displacement curves of dissimilar joints in shearing test. The average energy absorption of dissimilar joints in shearing test.
From Figures 13 and 14, the difference in deformation displacement of the AA55-5, AA56-5, AA56-6 and AA66-6 joints before failure is insignificant. Nevertheless, the energy absorptions of these four types of clinched joints are significantly different in the shearing test, which is mainly on account of the static strength of different joints produced by dissimilar materials varies considerably. Under the premise that the deformations of all single-point butt clinched joints are almost the same, the static strength becomes the main factor affecting the energy absorption capacity. Therefore, the energy absorptions of the single-point butt clinched joints produced by dissimilar materials are similar to the variation pattern of the static shearing strength of those joints, that is, the energy absorption of AA66-6 joint is severally 45.92%, 35.56% and 15.81% higher than those of AA55-5, AA56-5 and AA56-6 joints. A comprehensive analysis of the energy absorption of the single-point butt clinched joints produced by dissimilar sheet combinations leads to the conclusion that the difference in energy absorption of the dissimilar single-point butt clinched joints is mainly influenced by the static shearing strength of the joint, because the deformation displacement of the joint before failure is almost the same.
Conclusions
A novel single-point butt clinching process is proposed in the present study. Further, AA5052 and AA6061 sheets were selected as experimental materials to explore the effect of sheet category on failure mechanism and mechanical behaviours of the single-point butt clinched joint. The microscopic morphology of the fracture surface, energy absorption, geometrical parameters, mechanical property and failure details of the single-point butt clinched joints were systematically analysed. The main conclusions are reported as follows:
In the case of using AA5052 and AA6061 materials as the basal sheet and assisted sheet respectively, the main failure modes of all the single-point butt clinched joints produced by the same and dissimilar aluminium alloy sheets are neck fracture in the static shearing test. The microscopic morphology of all neck fracture surfaces shows that both AA5052 and AA6061 materials are the same as ductile fractures during the failure process when they are employed as the basal sheets. The difference in energy absorption of the single-point butt clinched joints produced by dissimilar aluminium alloy sheets is mainly influenced by the static shearing strength of the joint, because the deformation displacement of the joint before failure is almost the same. The sheet combination has a significant effect on the mechanical properties of the single-point butt clinched joint, which is ultimately due to the different mechanical properties of the basal sheets. The static shearing strength of the AA66-6 joint is 71.1%, 43.77% and 35.36% higher than those of AA55-5, AA56-5 and AA56-6 joints, respectively, which is 1735.3 N. The energy absorption of AA66-6 joint is severally 45.92%, 35.56% and 15.81% higher than those of AA55-5, AA56-5 and AA56-6 joints, which is 1.204 J. The single-point clinching process proposed in this work is beneficial for industrial scenarios where the connected sheet does not have enough overlapping lengths and are not subject to tensile loads. And it contributes to effectively simplifying the clinching process while increasing the joining efficiency.
Footnotes
Disclosure statement
No potential conflict of interest was reported by the author(s).