Mechanical performance and failure modes of self-piercing riveted joints between AA6061 and solution-treated TC4 alloy

TC4 titanium alloy and AA6061 aluminum alloy are widely used in the transportation industry because of their excellent mechanical properties and lightweight. In this work, the TC4 titanium alloy was solution heat treated between 800 °C and 990 °C for 1 h, and water cooled to room temperature. The riveting and tensile tests at room temperature were conducted to evaluate the joint performance. The tensile strength and failure morphology were used to discuss the mechanical performance of joints. Solution heat treatment significantly improves the elongation, mechanical performance, and hardness of TC4 titanium alloy. Compared with the as-received material, the elongation of the treated TC4 titanium alloy is increased by 13% at the solution temperature of 900 °C, the tensile strength was added by 175 MPa at 930 °C, and the hardness was significantly increased. The optimal performance of the TC4 titanium alloy can be obtained at 930 °C. The tensile strength of the joint with the TC4 alloy solution heat treated at 930 °C is the highest of all joints. When the TC4 alloy was solution treated between 800 °C and 850 °C, the rivets were pulled from the AA6061. While at 900 °C and 930 °C, the AA6061 sheet was broken at the rivet. At 960 °C and 990 °C, the TC4 sheet was broken near the rivet. The crack size of TC4 titanium alloy gradually decreases from the rivet outward, and the crack spreads around the rivet. Severe friction can be found, which causes the peeling of the lower plate AA6061 alloy. The breaks of TC4 alloys were the plastic broken. The failure morphology of the TC4 alloy sheet is different under different solution heat treatment temperatures.


Introduction
To promote green and low-carbon development, lightweight has achieved wide development prospects in many industries, and lightweight is a key development direction in the future [1][2][3]. TC4 (Ti-6Al-4V) titanium alloy has high tensile strength, low density, and stable mechanical properties at high temperatures, which is widely used in the structure and fasteners of aircraft bodies [4,5]. Generally, aluminum alloy are the ideal materials for versatile applications. For example, the AA7075 and AA2024 alloys are widely used in aviation industry [6,7]. However, due to the complicated forming and heating process, the cost of AA7075 and AA2024 alloys is relatively high, which is not the ideal section for the automobile industry. Due to the higher cost-performance, light weight and good plasticity, the AA6061 aluminum alloy is extensively used in multi-material design of body-in-white and aircraft [8,9]. Self-piercing riveting (SPR) is a new deformation connection technology. Compared with bolting, welding, and traditional riveting, SPR has lower pollution, lower energy consumption, and service reliability due to the reduced residual stress, stress concentration [10,11]. Therefore, it is important to achieve a reliable connection between TC4 and AA6061 alloys using the SPR process.
Self-piercing riveting has attracted research interests around the world. Ang [12] reviewed the SPR process from three aspects (joint failure, corrosion, and optimization technology). He et al [13] reported the content of SPR joints in mechanics, corrosion, and free vibration aspects. Huang et al [14,15] found that the composite aluminum sheets can change the failure mode, and increase the tensile strength of SPR joints. Abe et al [16] found that the joining ability of the joints with upper steel plate and lower aluminum plate is higher. Xie et al [17] reported a new way to calculate the shear strength of SPR joints. Jiang et al [18] studied the framework of selfpiercing riveted semi hollow rivets. Zhang [19] revealed that the different riveting sequences affect the deformation of steel and aluminum plates in SPR. Steel plates are usually concave, while aluminum plates are usually convex. Lin et al [20] used finite element simulation to study the cross tensile strength of SPR joints, and verify the simulation results with experimental results. Kim et al [21] calculated the formation of the SPR joint with the help of the machine learning method. Karathanasopoulos et al [22] predicted SPR joint formation through neural network modeling.
The riveting parameters have significant impacts on the performance of the joints. Zhang et al [23,24] reported the fatigue property, crack growth mechanism, and performance of TA1 self-piercing riveted joints. Lee et al [25] found that the fatigue life was related to the amplitude of stress, and the smaller the amplitude of stress, the longer the fatigue life. Presse et al [26] studied the fatigue life of different combination SPR joints according to the stress-life curve. Xing et al [27] found that increasing the overlapping region decreased the static strength and deformed elongation of the overlapping region. Hirsch et al [28] used ABAQUS software to simulate SPR of fiber reinforced polymers and metal plates. Zhang et al [29] optimized the undercut amount, bottom thickness, and remaining thickness of SPR joints on aluminum alloy. Zhou et al [30] discussed the impact of the rotational speed of the pin and the welding speed on the microstructure and mechanical performance of friction stir welds of 5052 aluminum alloy. Zhao et al [31] discovered that the fatigue failure of TA1 titanium plate SPR joint was caused by fretting wear. Iyer et al [32] investigated the fatigue of SPR joints with different numbers of rivets, and found that the number and position of rivets affected the tensile strength and fatigue strength of the joints. Liu et al [33] found that the amount of missing data for the remaining floor plate thickness increases rapidly with the shifting displacement and angle.
Heat treatment is one of the means to enhance the strength and elongation of the metals [34]. Li et al [35] found that proper solid solution treatment can enhance the wearing quality of steel. Hussein et al [36] found that the heat-treated Ti20Nb13Zr alloy is enhanced in solidity, wearing quality, and anti-corrosion. Shi et al [37] found that the microstructure evolution of titanium sheets was stable, and the essential components remain stable during the annealing. Pan et al [38] discovered that when the TC4 materials are heat treated under specific parameters, the tensile strength and extensibility of the TC4 are added by 23% and 55% compared with the sintered materials. Ma et al [39] found that improving the warming power can reduce the wear of the combined material.
However, there is little discussion on the connection between titanium and aluminum alloys using SPR technology. There are some technical difficulties in welding titanium with other lightweight alloys of different melting temperatures, and new methods are needed to realize the high-quality connection of these materials. In this work, the TC4 titanium alloy and AA6061 aluminum alloy is connected by SPR, and the mechanical performance and failure modes of the joints are analyzed. The TC4 plates were solution heattreated, and the SPR technology was used to achieve the connection between the solution-treated TC4 and AA6061 alloys. The fracture morphology of the joint was recorded under a scanning electron microscopy, and the principle of fracture was analyzed through microscopic fracture morphology. The impacts of solution treatment on the joint formation, hardness, static properties, and tensile failure of TC4/AA6061 SPR joints were revealed.  Then, the TC4 plate was water quenched, and the plates were cooled to room temperature with 10 seconds [40].
The tensile properties of the heat-treated TC4 plates were tested.

Self-piercing riveting experiments
The Tucker ERT80 SPR machine was applied for riveting. The AA6061 alloy and the heat-treated TC4 alloy were connected to form SPR joints. Several studies have revealed the influence of process parameters on the properties of SPR joints [41]. For instance, the plates with extreme hardness and low extensibility are used as the upper plates, while plates with low hardness and extreme extensibility are used as the lower plates. Due to the high hardness and low plasticity, the TC4 titanium alloy is used as an upper board. The AA6061 aluminum alloy is the ideal alloy for the lower plate to form the self-locking structure due to its low strength, low hardness, and good ductility [42]. The riveting position is in the rectangular centerline, and the rectangular size is 20 mm × 20 mm. The overlapping dimensions are exhibited in figure 1. In this work, the rivets made of boron steel with a hardness of H5 and a height of 6 mm were selected. Figure 2 shows the sampling locations in the tensile and hardness tests, as well as the tensile test of joints. To obtain the mechanical properties of the heat-treated TC4 alloy, tensile experiments and hardness tests were performed in. The WPL100 tensile machine was conducted for the tensile examinations, and the tensile examinations were carried at room temperature. Each tensile test was repeated 5 times. The stretching rate was 2 mm min −1 . The hardness of TC4 alloy was measured by HRS-150 digital Rockwell hardness tester. Adjacent  hardness test points were separated more than 2 mm apart. Before the hardness tests, the samples were polished using abrasive paper from 400 # to 2000#. To ensure the accuracy of the tests, the average of five hardness values was selected. After the SPR experiments, the joint was cut alongside the center line of the rivet to display the geometric shape. The joints were made smooth with abrasive paper, and the section geometry of the joints was recorded by a high-definition camera. The tensile properties of the joints were tested. The stretching rate was 2 mm min −1 . The Su8010 field emission scanning electron microscope (SEM) was applied to record the fracture morphology of the stretching joints. In the meantime, an x-ray energy spectrometer was used to determine the constituent elements of the joint.

Results and discussion
3.1. Influence of solution heat treatment on the performance of TC4 alloy Solution heat treatment has major impacts on the performance of TC4 titanium alloy [43]. Figure 3(a) shows the hardness and scalability of the TC4 titanium alloy after thermal treatment. When the alloy is heat treated at temperature ranges of 800°C to 990°C, the hardness goes up with the rise of temperature and the elongation cuts down with the rise of temperature [44] and the elongation cuts down with the rise of temperature. This is because after the solution treatment, the TC4 titanium alloy becomes hard and brittle, resulting in the increased hardness and decreased elongation. Figure 3(b) shows the tensile strength of the TC4 titanium alloy after thermal treatment. Among 800°C and 930°C, the tensile strength goes up with the rise of the temperature. While at 930°C and 990°C, the tensile strength cuts down with the rise of temperature. This is because that below the phase transition temperature of the TC4 alloy, with the increase of solid solution temperature, the αphase is transformed into β-phase, i.e., the small diameter α-phase is dissolved, and the fraction of larger diameter α-phase is decreased. During the cooling process, a portion of β-phase is transformed into α'-phase. Generally, α'-phase and β-phase are hard phases. The diameter of α'-phase is small, and its strength is high. As  the solid temperature increases, α'-phases are more numerous, leading to the increased hardness of the material. At 990°C, only β-phase exists, and without the limitation of α-phase, the diameter of β-phase increases, which decreases the strength of the alloy [44,45]. So, the performance of TC4 titanium alloy is optimal after solution heat treatment at 930°C.

Joint strength of SPR joints
The SPR process of TC4-AA6061 alloys is exhibited in figure 4. The SPR in figure 4 can be approximately divided into four periods: clamping, puncture, expansion, and forming. In the clamping stage, the positions of the rivets, and AA6061 are fixed. In the puncture stage, the rivet pierces the TC4 alloy. In the expansion stage, the rivet legs gradually are opened, and the AA6061 takes place deformation and fills into the concave mold little by little. In the forming stage, the rivet legs are fully opened and tightly connected to TC4 and AA6061 plates. Figure 5 shows the intersecting surface and schematic tensile failure diagram of the joints. Figure 5(a) shows the intersecting surface and shape parameters of the joint. The head height is 0.36 mm, the interlocking width is 0.73 mm, and the foot thickness is 1.03 mm, which displays good riveting level. TC4 alloy can not fill the cavity of the rivet due to its high deformation resistance. That's why there's a gap in figure 5(a). In figure 5(a), the rivet has slight horizontal instability in the forming process, i.e., the intersecting surface of the joint is not completely symmetrical. During the joint formation process, TC4 alloy is partially enclosed in the rivet cavity, causing horizontal instability and limiting the deformation of the rivet [46].
To get the mechanical performance of the joints, the mechanical performance was tested. The general failure patterns of joints are displayed in figures 5(b)-(d). The tensile load causes the bending of the TC4 alloy, and pulls the rivet from the AA6061 alloy, as shown in figure 5(b). This situation occurs when the elongation of TC4 alloy is well. When the elongation of TC4 alloy is reduced, the tensile load makes AA6061 break at the rivet, as shown in figure 5(c). When the elongation of TC4 alloy is very low, the TC4 alloys break near the rivet, figure 5(d).   is that the rivet is pulled away from the AA6061 alloy, and the AA6061 alloy is split by the rivet in the tensile direction. This is because that the elongation of 800°C and 850°C treated TC4 plate in figure 3(a) is close to that of AA6061 plate, table 2. The tensile strength of the TC4 alloy in figure 3(b) is higher than that of the AA6061 plate in table 2. The rivet is pulled away from the AA6061 alloy, and the AA6061 alloy in contact with the rivet angle is stripped. The failure form in figures 6(c)-(d) is that one side of the AA6061 plate is broken. This is because the elongations of 900°C and 930°C treated TC4 alloys are decreased, figure 3(a). The failure site occurs in AA6061 alloy. The joint is first subjected to tension, and the concentration of stress happens to the contact region between the rivet head among the AA6061 alloy, where one side of the AA6061 alloy is broken. The failure form of figure 6(e) is a broken TC4 alloy. Because both the elongation and tensile strength of 960°C treated TC4 alloy are declined, figure 3. The failure site occurs in TC4 alloy, and one side of TC4 alloy is broken. The failure  form in figure 6(f) is a completely broken TC4 alloy. Due to the decreased elongation and tensile strength of 990°C treated TC4 alloy, in figure 4, the 990°C treated TC4 alloy becomes brittle, and the 990°C treated TC4 alloy is subjected to the rivet pressure along the tensile direction and is prone to broken. When the TC4 alloy is treated at 960°C and 990°C, the number of dimples in TC4 alloy significantly decreased, and the depth became shallow in figure 10. This also suggests a severe decrease in elongation in TC4 plate.
Solid solution heat treatment has an important impact on the performance of SPR joints. Figure 7 displays the tensile properties of the solid-solution TC4-AA6061 joints. Figure 7(a) displays the maximum load and failure displacement of joints after solid solution heat treatment at different temperatures. When the TC4 alloy is treated at 800°C and 990°C, the failure displacement decreases with the increase of temperatures. At 800°C and 930°C, the maximum load increases with the temperatures. While at 930°C and 990°C, the maximum load decreases with the increased temperatures. In the repeated experiments, the error of the maximum load is small, indicating high accuracy of the data. Figure 8(b) reveals the load-displacement curves of the joints at different temperatures. Tensile tests are performed at room temperature. For instance, the 930°C joint is the riveted joint made of 930°C solution heat-treated TC4 alloy and the AA6061 alloy at room temperature. During the initial elasticity stage, i.e., A-B process, there is a linear relationship between load among displacement, and the load adds as increasing displacement. The slope represents the elastic modulus of the joint, there is no apparent yield point. It is that mechanical interlocking exists between rivets, upper and lower plates. The contact stress increases between plates and the rivet. In this process, the TC4 alloy is winding and the rivet is slightly deformed. In the B-C phase, the slope of the curve decreases and the rate of load increase slows down. It is that the winding TC4 alloy has less binding force on the rivet. AA6061 alloy bears a certain load, and the rate of load increase slows down. In the C-D phase, the rivet continues to stretch the TC4 plate. The tensile load is gradually decreased until the joint is completely failed. This is because the rivet begins to separate from the AA6061 alloy with increased tensile displacement. The contact area of the rivet and AA6061 alloy reduces, causing a decrease in frictional resistance. The elongation of the 990°C treated TC4 alloy is small, figure 4(a), and the TC4 alloy is completely broken after bearing the load at point E of figure 7(b). At point F of figure 7(b), the elongation of the upper plate is high, figure 4(a), and the upper plate is greatly deformed until the rivet is pulled away from the AA6061 alloy.  Figure 8 shows the morphologies of cracks in AA6061 alloy during tensile tests. The peeling of AA6061 alloy occurs in figure 8(a). Figures 8(b)-(c) displays the magnified observation of metal peeling in figure 8(a). In the SPR experiment, the punch squeezes the rivet into form. The legs of the rivet bend and the bottom of the rivet opens. In the tensile examination, the rivet is pulled away from the AA6061 alloy. The foot of the rivet is ground by AA6061 alloy, resulting in a flaking of the AA6061 alloy. In the tensile process, the AA6061 alloy is deformed and eventually broke. Figure 8(d) shows the fracture morphology of AA6061 alloy. The evenly distributed dimples and voids on the fracture morphology of AA6061 indicate that the elongation rate of AA6061 is very good. The AA6061 alloy has a sliding wear phenomenon during the tensile test process. Adjacent voids are merged by pulling forces to form a crack. Figure 9 is the micrographs of the 930°C joint. It can be found out from figure 9(a) that there are cracks along the outward direction of rivets in solution heat treatment TC4 alloy. This is because the stress is concentrated at the rivet during the tensile test process. The crack in TC4 alloy arises in the region where it intersects with the rivet. The crack gradually reduces from the rivet outward. The generation of the crack in 930°C treated TC4 alloy is a consequence of reduced elongation, figure 4(a). Figure 9(b) is the crack marked in figure 9(a). The compositions of the crack in figure 9(b) are analyzed using the x-ray spectrometer (EDS). The elemental composition and mass scores of crack regions are displayed in figures 9(c)-(d). The crack regions are named 1741 and 1742. There are lots of elements like O, Ti, and C in the crack region, and the O and Ti elements account for a great scale. This indicates that the cracked surface is primarily titanium oxide. The ratio of Ti and O atoms is 2:1, which also indicates the existence of TiO 2 .

Failure morphology
In the tensile test of TC4-AA6061 joints, the major breaks of TC4 alloy occur at the 900°C, 930°C, 960°C, and 990°C treated samples. The above four failure joints are scanned to study the failure mechanism. Figure 10 displays the crack morphologies in TC4 alloy during tensile tests at different solid solution temperatures. The solid TC4 alloy has a toughness fracture, and the characteristics of plastic toughness are different. Figure 10  distributed and the spacing of adjacent voids is small. This indicates that 900°C-TC4 alloy is uniformly deformed, significant deformation occurs during plastic deformation, and the TC4 alloy shows good elongation [47]. The dimple size of 930°C-TC4 alloy gradually became inconsistent and unevenly distributed in figure 10(b). The number of voids in 930°C-TC4 alloy exceeds that of the voids in 900°C-TC4 alloy, indicating a decrease in the elongation of 930°C-TC4 alloy. Many dimples also appear in figure 10(c). The dimples in figure 10(c) become uneven, when compared with those in figure 10(b). The number of voids in 960°C-TC4 alloy exceeds that of voids in 930°C-TC4 alloy. The tearing edges appear in 960°C-TC4 alloy. This indicates a decrease in elongation of 960°C-TC4 alloy when compared with the 930°C-TC4 alloy. The dimple size of 990°C-TC4 alloy is inconsistent in figure 10(d). The distribution of dimples in figure 9(d) is more chaotic when compared with that in figures 10(b)-(c). Adjacent voids are less spaced, and some voids merge to form larger cracks. The number of voids and torn edges in 990°C-TC4 alloy are more than those of 960°C-TC4 alloy. This indicates a decrease in elongation of 990°C-TC4 alloy compared to 960°C-TC4 alloy. The fracture morphology of figure 10 explains the failure form of figure 6 and the failure cause at point F of figure 7(b).

Conclusion
The impacts of solution heat treatment on TC4/AA6061 self-piercing riveting joints are reported in this essay. The major conclusions are as below: (1) The solution heat treatment can significantly raise the performance of titanium alloy TC4 alloy. The elongation is increased by 13% when the alloy is solution treated at 900°C. At 930°C, the strength is increased to 1072 MPa. The optimal comprehensive performance is obtained at 930°C.
(2) In the tensile test, The TC4-AA6061 joints have different failure forms. Treated at 800°C and 850°C, the rivets are pulled from the AA6061 alloy. At 900°C and 930°C, the AA6061 alloy is broken at the rivet. While at 960°C and 990°C, the TC4 alloy failures near the rivet.
(3) The failure of TC4-AA6061 joints was observed, and the crack size of TC4 titanium alloy gradually decreases from the rivet outward, and the crack is to spread around the rivet. There is severe friction between the rivet and AA6061 alloy, causing the metal peeling of the AA6061 alloy lower plate due to the high hardness and strength of rivets. All the breaks of TC4 alloys are plastic breaks. The morphology of TC4 alloy is different under different solution heat treatment temperatures.