As countries around the world attach great importance to energy conservation and emission reduction, the development of pure electric new energy vehicles has become a trend. In addition to battery performance, the quality of the body is also a crucial factor affecting the driving range of new energy vehicles. Promoting the development of lightweight automobile body structures and high-quality connections can improve the comprehensive driving range of electric vehicles by reducing the weight of the entire vehicle as much as possible while ensuring the strength and safety performance of the vehicle. In terms of lightweighting of automobiles, the steel-aluminum hybrid body takes into account both the strength and weight reduction of the body, becoming an important means to achieve lightweighting of the body.
The traditional connection method for connecting aluminum alloys has poor connection performance and low reliability. Self-piercing riveting, as a new connection technology, has been widely used in the automotive industry and aerospace manufacturing industry because of its absolute advantage in connecting light alloys and composite materials. In recent years, China domestic scholars have conducted relevant research on self-piercing riveting technology and studied the effects of different heat treatment methods on the performance of TA1 industrial pure titanium self-piercing riveted joints. It was found that annealing and quenching heat treatment methods improved the static strength of TA1 industrial pure titanium self-piercing riveted joints. The joint forming mechanism was observed and analyzed from the perspective of material flow, and the joint quality was evaluated based on this. Through metallographic tests, it was found that the large plastic deformation area was refined into a fiber structure with a certain tendency, which promoted the improvement of the yield stress and fatigue strength of the joint.
The above research mainly focuses on the mechanical properties of the joints after riveting of aluminum alloy plates. In the actual riveting production of car bodies, the cracks of the riveted joints of aluminum alloy extruded profiles, especially high-strength aluminum alloys with high alloying element content, such as 6082 aluminum alloy, are the key factors restricting the application of this process on the car body. At the same time, the shape and position tolerances of the extruded profiles used on the car body, such as bending and twisting, directly affect the assembly and use of the profiles, and also determine the dimensional accuracy of the subsequent car body. In order to control the bending and twisting of the profiles and ensure the dimensional accuracy of the profiles, in addition to the die structure, the outlet temperature of the profiles and the online quenching speed are the most important influencing factors. The higher the outlet temperature and the faster the quenching speed, the greater the bending and twisting degree of the profiles. For aluminum alloy profiles for car bodies, it is necessary to ensure the dimensional accuracy of the profiles and ensure that the alloy riveting does not crack. The simplest way to optimize the dimensional accuracy and riveting cracking performance of the alloy is to control cracking by optimizing the heating temperature and aging process of the extruded rods while keeping the material composition, die structure, extrusion speed, and quenching speed unchanged. For 6082 aluminum alloy, under the premise that other process conditions remain unchanged, the higher the extrusion temperature, the shallower the coarse-grained layer, but the greater the deformation of the profile after quenching.
This paper takes 6082 aluminum alloy with the same composition as the research object, uses different extrusion temperatures and different aging processes to prepare samples in different states, and evaluates the effects of extrusion temperature and aging state on the riveting test through riveting tests. Based on the preliminary results, the optimal aging process is further determined to provide guidance for the subsequent production of 6082 aluminum alloy body extrusion profiles.
1 Experimental materials and methods
As shown in Table 1, the 6082 aluminum alloy was melted and prepared into a round ingot by semi-continuous casting. Then, after homogenization heat treatment, the ingot was heated to different temperatures and extruded into a profile on a 2200 t extruder. The profile wall thickness was 2.5 mm, the extrusion barrel temperature was 440±10 ℃, the extrusion die temperature was 470±10 ℃, the extrusion speed was 2.3±0.2 mm/s, and the profile quenching method was strong wind cooling. According to the heating temperature, the samples were numbered 1 to 3, among which sample 1 had the lowest heating temperature, and the corresponding billet temperature was 470±5 ℃, the corresponding billet temperature of sample 2 was 485±5 ℃, and the temperature of sample 3 was the highest, and the corresponding billet temperature was 500±5 ℃.
Table 1 Measured chemical composition of the test alloy (mass fraction/%)
Under the condition that other process parameters such as material composition, die structure, extrusion speed, quenching speed remain unchanged, the above No. 1 to 3 samples obtained by adjusting the extrusion heating temperature are aged in a box-type resistance furnace, and the aging system is 180 ℃/6 h and 190 ℃/6 h. After the insulation, they are air-cooled, and then riveted to evaluate the influence of different extrusion temperatures and aging states on the riveting test. The riveting test uses 2.5 mm thick 6082 alloy with different extrusion temperatures and different aging systems as the bottom plate, and 1.4 mm thick 5754-O alloy as the upper plate for SPR riveting test. The riveting die is M260238, and the rivet is C5.3×6.0 H0. In addition, in order to further determine the optimal aging process, according to the influence of extrusion temperature and aging state on riveting cracking, the plate at the optimal extrusion temperature is selected, and then treated with different temperatures and different aging times to study the influence of aging system on riveting cracking, so as to finally confirm the optimal aging system. A high-power microscope was used to observe the microstructure of the material at different extrusion temperatures, an MTS-SANS CMT5000 series microcomputer-controlled electronic universal testing machine was used to test the mechanical properties, and a low-power microscope was used to observe the riveted joints after riveting in various states.
2Experimental results and discussion
2.1 Effect of extrusion temperature and aging state on riveting cracking
Sampling was taken along the cross section of the extruded profile. After rough grinding, fine grinding and polishing with sandpaper, the sample was corroded with 10% NaOH for 8 minutes, and the black corrosion product was wiped clean with nitric acid. The coarse grain layer of the sample was observed with a high-power microscope, which was located on the surface outside the rivet buckle at the intended riveting position, as shown in Figure 1. The average coarse grain layer depth of sample No. 1 was 352 μm , the average coarse grain layer depth of sample No. 2 was 135 μm , and the average coarse grain layer depth of sample No. 3 was 31 μm . The difference in the depth of the coarse grain layer is mainly due to the different extrusion temperatures. The higher the extrusion temperature, the lower the deformation resistance of the 6082 alloy, the smaller the deformation energy storage generated by the friction between the alloy and the extrusion die (especially the die working belt), and the smaller the recrystallization driving force. Therefore, the surface coarse grain layer is shallower; the lower the extrusion temperature, the greater the deformation resistance, the greater the deformation energy storage, the easier it is to recrystallize, and the deeper the coarse grain layer. For the 6082 alloy, the mechanism of coarse grain recrystallization is secondary recrystallization.
(a) Model 1
(b) Model 2
(c) Model 3
Figure 1 Thickness of coarse grain layer of extruded profiles by different processes
Samples 1 to 3 prepared at different extrusion temperatures were aged at 180 ℃/6 h and 190 ℃/6 h, respectively. The mechanical properties of sample 2 after the two aging processes are shown in Table 2. Under the two aging systems, the yield strength and tensile strength of the sample at 180 ℃/6 h are significantly higher than those at 190 ℃/6 h, while the elongation of the two is not much different, indicating that 190 ℃/6 h is an over-aging treatment. Since the mechanical properties of the 6 series aluminum alloy fluctuate greatly with the change of the aging process in the under-aging state, it is not conducive to the stability of the profile production process and the control of the riveting quality. Therefore, it is not suitable to use the under-aging state to produce body profiles.
Table 2 Mechanical properties of sample No. 2 under two aging systems
The appearance of the test piece after riveting is shown in Figure 2. When the No. 1 sample with a deeper coarse-grained layer was riveted in the peak aging state, the bottom surface of the rivet had obvious orange peel and cracks visible to the naked eye, as shown in Figure 2a. Due to the inconsistent orientation inside the grains, the deformation degree will be uneven during deformation, forming an uneven surface. When the grains are coarse, the unevenness of the surface becomes larger, forming an orange peel phenomenon visible to the naked eye. When the No. 3 sample with a shallower coarse-grained layer prepared by increasing the extrusion temperature was riveted in the peak aging state, the bottom surface of the rivet was relatively smooth, and the cracking was suppressed to a certain extent, which was only visible under microscope magnification, as shown in Figure 2b. When the No. 3 sample was in the over-aging state, no cracking was observed under microscope magnification, as shown in Figure 2c.
(a) Cracks visible to the naked eye
(b) Slight cracks visible under microscope
(c) No cracks
Figure 2 Different degrees of cracking after riveting
The surface after riveting is mainly in three states, namely, cracks visible to the naked eye (marked “×”), slight cracks visible under microscope magnification (marked “△”), and no cracks (marked “○”). The riveting morphology results of the above three state samples under two aging systems are shown in Table 3. It can be seen that when the aging process is constant, the riveting cracking performance of the specimen with higher extrusion temperature and thinner coarse grain layer is better than that of the specimen with deeper coarse grain layer; when the coarse grain layer is constant, the riveting cracking performance of the over-aging state is better than that of the peak aging state.
Table 3 Riveting appearance of samples 1 to 3 under two process systems
The effects of grain morphology and aging state on the axial compression cracking behavior of profiles were studied. The stress state of the material during axial compression was consistent with that of self-piercing riveting. The study found that the cracks originated from the grain boundaries, and the cracking mechanism of Al-Mg-Si alloy was explained by the formula.
σapp is the stress applied to the crystal. When cracking, σapp is equal to the true stress value corresponding to the tensile strength; σa0 is the resistance of the precipitates during intracrystalline sliding; Φ is the stress concentration coefficient, which is related to the grain size d and the slip width p.
Compared with recrystallization, fibrous grain structure is more conducive to cracking inhibition. The main reason is that the grain size d is significantly reduced due to grain refinement, which can effectively reduce the stress concentration factor Φ at the grain boundary, thereby inhibiting cracking. Compared with fibrous structure, the stress concentration factor Φ of recrystallized alloy with coarse grains is about 10 times that of the former.
Compared with peak aging, the over-aging state is more conducive to cracking inhibition, which is determined by the different precipitation phase states inside the alloy. During peak aging, 20-50 nm ‘β (Mg5Si6) phases are precipitated in the 6082 alloy, with a large number of precipitates and small sizes; when the alloy is in over-aging, the number of precipitates in the alloy decreases and the size becomes larger. The precipitates generated during the aging process can effectively inhibit the movement of dislocations inside the alloy. Its pinning force on dislocations is related to the size and volume fraction of the precipitate phase. The empirical formula is:
f is the volume fraction of the precipitate phase; r is the size of the phase; σa is the interface energy between the phase and the matrix. The formula shows that the larger the size of the precipitate phase and the smaller the volume fraction, the smaller its pinning force on dislocations, the easier it is for dislocations in the alloy to start, and the σa0 in the alloy will decrease from the peak aging to the over-aging state. Even if σa0 decreases, when the alloy goes from the peak aging to the over-aging state, the σapp value at the time of cracking of the alloy decreases more, resulting in a significant decrease in the effective stress at the grain boundary (σapp-σa0). The effective stress at the grain boundary of over-aging is about 1/5 of that at the peak aging, that is, it is less likely to crack at the grain boundary in the over-aging state, resulting in better riveting performance of the alloy.
2.2 Optimization of extrusion temperature and aging process system
According to the above results, increasing the extrusion temperature can reduce the depth of the coarse-grained layer, thereby inhibiting the cracking of the material during the riveting process. However, under the premise of certain alloy composition, extrusion die structure and extrusion process, if the extrusion temperature is too high, on the one hand, the bending and twisting degree of the profile will be aggravated during the subsequent quenching process, making the profile size tolerance not meet the requirements, and on the other hand, it will cause the alloy to be easily overburned during the extrusion process, increasing the risk of material scrapping. Considering the riveting state, profile size process, production process window and other factors, the more suitable extrusion temperature for this alloy is not less than 485 ℃, that is, sample No. 2. In order to confirm the optimal aging process system, the aging process was optimized based on sample No. 2.
The mechanical properties of specimen No. 2 at different aging times at 180 ℃, 185 ℃ and 190 ℃ are shown in Figure 3, which are yield strength, tensile strength and elongation. As shown in Figure 3a, under 180 ℃, the aging time increases from 6 h to 12 h, and the yield strength of the material does not decrease significantly. Under 185 ℃, as the aging time increases from 4 h to 12 h, the yield strength first increases and then decreases, and the aging time corresponding to the highest strength value is 5-6 h. Under 190 ℃, as the aging time increases, the yield strength gradually decreases. Overall, at the three aging temperatures, the lower the aging temperature, the higher the peak strength of the material. The characteristics of the tensile strength in Figure 3b are consistent with the yield strength in Figure 3a. The elongation at different aging temperatures shown in Figure 3c is between 14% and 17%, with no obvious change pattern. This experiment tests the peak aging to over-aging stage, and due to the small experimental differences, the test error causes the change pattern to be unclear.
Fig.3 Mechanical properties of materials at different aging temperatures and aging times
After the above aging treatment, the cracking of the riveted joints is summarized in Table 4. It can be seen from Table 4 that with the increase of time, the cracking of the riveted joints is suppressed to a certain extent. Under the condition of 180 ℃, when the aging time exceeds 10 h, the appearance of the riveted joint is in an acceptable state, but unstable. Under the condition of 185 ℃, after aging for 7 h, the appearance of the riveted joint has no cracks and the state is relatively stable. Under the condition of 190 ℃, the appearance of the riveted joint has no cracks and the state is stable. From the riveting test results, it can be seen that the riveting performance is better and more stable when the alloy is in an over-aged state. Combined with the use of the body profile, riveting at 180 ℃/10~12 h is not conducive to the quality stability of the production process controlled by the OEM. In order to ensure the stability of the riveted joint, the aging time needs to be further extended, but the verification of the aging time will lead to reduced profile production efficiency and increased costs. Under the condition of 190 ℃, all the samples can meet the requirements of riveting cracking, but the strength of the material is significantly reduced. According to the requirements of vehicle design, the yield strength of 6082 alloy must be guaranteed to be greater than 270 MPa. Therefore, the aging temperature of 190 ℃ does not meet the material strength requirements. At the same time, if the material strength is too low, the residual thickness of the bottom plate of the riveted joint will be too small. After aging at 190 ℃/8 h, the riveted cross-sectional characteristics show that the residual thickness is 0.26 mm, which does not meet the index requirement of ≥0.3 mm, as shown in Figure 4a. Considering comprehensively, the optimal aging temperature is 185 ℃. After aging for 7 h, the material can stably meet the riveting requirements, and the strength meets the performance requirements. Considering the production stability of the riveting process in the welding workshop, the optimal aging time is proposed to be determined as 8 h. The cross-sectional characteristics under this process system are shown in Figure 4b, which meets the interlocking index requirements. The left and right interlocks are 0.90 mm and 0.75 mm, which meet the index requirements of ≥0.4 mm, and the bottom residual thickness is 0.38 mm.
Table 4 Cracking of sample No. 2 at different temperatures and different aging times
Fig.4 Cross-sectional characteristics of riveted joints of 6082 bottom plates at different aging states
3 Conclusion
The higher the extrusion temperature of 6082 aluminum alloy profiles, the shallower the surface coarse-grained layer after extrusion. The shallower coarse-grained layer thickness can effectively reduce the stress concentration factor at the grain boundary, thereby inhibiting riveting cracking. Experimental research has determined that the optimal extrusion temperature is not less than 485 ℃.
When the thickness of the coarse-grained layer of 6082 aluminum alloy profile is the same, the effective stress of the grain boundary of the alloy in the over-aging state is less than that in the peak aging state, the risk of cracking during riveting is smaller, and the riveting performance of the alloy is better. Taking into account the three factors of riveting stability, riveted joint interlocking value, heat treatment production efficiency and economic benefits, the optimal aging system for the alloy is determined to be 185℃/8h.
Post time: Apr-05-2025
