1 Introduction
With the release of the "Science and Technology Support Carbon Peak Carbon Neutralization Implementation Plan (2022-2030)" policy, automobile lightweight has become an inevitable trend. Body light aluminum alloy and advanced high-strength steel and other materials, through reasonable application and distribution, can achieve a safer body structure while balancing the production cost of the all-aluminum body and future maintenance costs. It is the most effective vehicle lightweight means.
Nailless riveting and self-piercing riveting (Self-Piercing Riveting, SPR) are effective ways to realize the connection of steel and aluminum dissimilar metals, especially nailless riveting, no need for additional rivets, no increase in the quality of the connection point, and the overall cost of the connection is lower than that of SPR. The leaner lightweight connection process is still in the process and experimental research stage in China, and has not been widely used in the body structure. In this study, the process parameters and static performance of the nailless riveting technology were compared by combining steel and aluminum sheets with different material thicknesses, so as to provide material selection and connection design reference for the application of nailless riveting technology in the body structure.
2 process
Nailless riveting is a stamping mechanical connection process, which uses the local plastic deformation of two or more layers of sheet metal to complete the process of deep drawing and extrusion composite processing, and forms an interlocking undercut circle at the extruded joint. Shaped or rectangular connection points, so that it has a certain tensile strength and shear strength. The connection process is shown in Figure 1. The process mainly includes pre-tightening, occlusal, punching, pressure holding and ejection. Nailless riveting can be used for the connection between the same or dissimilar sheets with gluing, coating, and adhesive sealing requirements.
There is work hardening in the forming process of nailless riveting, which improves the yield strength of the material and the bearing capacity of the riveted joint. The profile parameters of the cross-sectional view of the nailless riveted joint are shown in Figure 2. The main parameters are the thickness of the upper plate neck S1, the upper and lower plates Material interlocking depth C1, the sum of the bottom thickness of the upper and lower sheets at the connection point (bottom thickness) ST.
3 Process parameters and static properties
The research on the process parameters of the nailless riveted connection mainly adopts the Taguchi method and the orthogonal test to evaluate the shape parameters such as the neck thickness and interlocking depth of the joint section view, determine the riveting direction and the optimal combination of process parameters; the static performance research mainly uses different steel Static load failure test of aluminum sheet combination, comparing the mechanical properties of nailless riveted connection and SPR connection, and analyzing the influence of material grade, riveting direction, and material thickness on the mechanical properties of nailless riveted connection.
3.1
Test materials and methods
The test material is 5000 series aluminum alloy, and the material thickness is 1.0mm and 1.4mm, which are commonly used in body structure; the steel plate is CR3, CR340, and the thickness is 0.7mm, 0.8mm, 1mm and 1.3mm;
Nailless riveted joints are tested for joint shear and tensile strength by static load failure tests. Because the single lap joint is a common joint form in the body structure, the sample specifications are shown in Figure 3, the shear sample size is 85mm×35mm, and the lap joint is 30mm; the cross tensile sample size is 120mm×35mm, and the diameter of the positioning hole is 10mm . The riveted sample was subjected to a static load failure test on a universal testing machine CMT4304, and the speed of the entire test process was controlled at 10mm/min.
The sectional view of the nailless riveted joint is obtained by wire cutting of the sample joint, and it is inlaid, polished and corroded, and the corresponding shape parameter data of the sectional view is obtained by observing under an optical microscope.
3.2
Process parameter selection
3.2.1 Determination of riveting direction for nailless riveting
In order to determine the riveting direction, CR3 steel plate and 5000 series aluminum alloy were selected, and different material thicknesses and riveting directions were selected to evaluate the topography parameters of the sectional view of the nailless riveted joint. The interlocking depth value was used as an important basis for judging the riveting quality.
It can be seen from Table 2 above that for steel-aluminum nailless riveted connections, the same material thickness and different riveting directions can form better interlocking, and the interlocking state is not very sensitive to the material; different material thicknesses, riveting direction from thin to When thicker, the interlock depth drops significantly. Therefore, the material thickness is the main influencing factor for the interlocking of the nailless riveted connection, and the direction of the nailless riveted connection is preferably from thick plate to thin plate.
3.2.2 Determination of riveting process parameters for nailless riveting
The process parameters of the nailless riveting die affect the riveting interlock depth and riveting quality. In order to obtain the optimal process parameters, the Taguchi method is used to select the die. mm 5000 series aluminum plate.
The control factors are respectively selected punch diameter, die depth and base thickness, and each control factor has 3 levels, see Table 3.
Depth of interlock as a result of response, noise factor as lubricant, symptom as joint protrusion or cracks in the sheet. Use the orthogonal list tool to optimize, and establish the orthogonal experiment L9 of Wangda characteristic. Orthogonal test combinations and test results are shown in Table 4.
It can be seen from Table 4 that the interlocking depth of Test 5 is the largest, so it is determined that the optimal process parameters for nailless riveting are 5.5 mm in punch diameter, 1.2 mm in die depth, and 0.8 mm in bottom thickness.
3.3
3.3 Comparison of mechanical properties
Since there is no suitable standard for judging the mechanical properties of steel-aluminum joints in the industry, and since SPR has been widely used in steel-aluminum hybrid body structures, the mechanical properties of SPR joints are used as a benchmark to judge the mechanical properties of nailless riveted joints. Under the conditions of the same material thickness and material type, a sample-level joint shear and cross tensile static load failure test was designed to measure the shear and tensile failure loads of two connection methods, nailless riveting and SPR.
The grade of the test sample steel plate is CR3, and the material thickness is 0.8mm; the aluminum alloy grade is 5000 series, and the material thickness is 1.4mm. The optimal riveting directions were selected for the two connection methods, among which the nailless riveting was from thick to thin, and the SPR was from thin to thick, and from hard to soft. There are 5 samples in each group of tests, and the load-displacement curves and failure modes of tensile and shear load failures of each group of samples are shown in Figures 5 to 8.
3.3.1 Analysis of shear static load failure test
It can be seen from Figures 5 and 6 that under the shear load state, the failure mode of the nailless riveted connection is the neck fracture of the upper plate, the maximum failure load is 1620N, and the average failure displacement is 0.46mm; the failure mode of the SPR connection is the tearing of the upper plate, The maximum failure load is 2364N, and the average failure displacement is 4.95mm.
Further analysis shows that under the shear load state, both of them have a certain plastic buffer energy absorption, and the shear strength of the nailless riveted joint reaches 68.5% of SPR, but the average displacement of the nailless riveted joint is significantly lower when the maximum failure occurs In terms of SPR, it is only 9.3% of SPR.
Further analysis shows that under the tensile load state, the failure of the joints of the two connection methods is brittle fracture, there is no plastic deformation buffer zone, the tensile strength of nailless riveting is about 60.6% of SPR, and the average displacement of nailless riveting failure is also lower than SPR , reaching 65% of SPR. In conclusion, compared with the SPR connection, although the mechanical properties of the nailless riveted joint are reduced, it can be applied in the non-main load-carrying body structure area.
3.4
Analysis of Factors Affecting Static Properties
In order to further analyze the static performance of the nailless riveted joints, apply the nailless riveted joints to form design guidelines for the body structure, from the three aspects of material grade, riveting direction, and material thickness, combined with joint cross-sectional view morphology parameters and static load failure tests The data were used to analyze its influence on the static performance of the steel-aluminum nailless connection.
The sample size and test method are as above. In the test, the grade and thickness of common materials in the low-load area of the body structure are selected. mm, 1.3mm, test combinations and test results are shown in Table 5.
3.4.1 Effect of Material Grade
The first four combinations with a material thickness of 1.0mm were selected to analyze the influence of material grade on the static performance of the nailless riveted connection. The test results such as maximum shear force, maximum tensile force, interlock depth value and failure mode are shown in Table 6.
From the analysis in Figure 9, it can be seen that the shear failure mode mainly depends on the strength of the upper layer. When the strength of the upper layer is higher than that of the lower layer, the shear failure mode is generally the fracture of the connection point of the upper layer material; With the increase of the strength of the lower layer, the shear failure mode changes from the pull-off of the connection point to the fracture of the connection point; similarly, the shear strength mainly depends on the strength of the upper layer material, and increases with the increase of the strength of the upper layer material.
Under the same material thickness, the failure mode of the cross tension is the pull-off of the connection point, which has nothing to do with the material grade; the tensile load decreases with the increase of the material strength.
The interlock depth decreases as the material load increases, because the stronger the material, the more difficult it is for the material to deform during the connection, making the interlock more difficult.
3.4.2 Effect of riveting direction
Similarly, based on the data of the first four combinations, the influence of the riveting direction on the static performance of the nailless riveted connection can be analyzed, as shown in Figure 10.
The connection direction of nailless riveting is from high load to low strength. Although there is little difference in interlocking depth, the shear load increases significantly. Combination 1 is 53.4% higher than combination 2, and combination 3 is 45.6% higher than combination 4; the connection direction is high From strength to low strength, although the difference in interlocking depth is not large, the tensile strength is significantly reduced. Combination 1 is 33.6% lower than combination 2, and combination 3 is 29.4% lower than combination 4.
3.4.3 Effect of Material Thickness
The selected combination and test result data are shown in Table 7, and the influence of material thickness on the nailless riveting process parameters and static load failure strength is compared and analyzed.
It can be seen from Table 7 and Figure 11 that, for the shear strength, the thicker the upper material, the greater the interlocking depth, the greater the neck thickness, the higher the shear strength; the thicker the lower material, the more difficult the deformation of the upper material, although The interlock depth increases, but the thinner the neck thickness, the lower the shear strength. Regarding the tensile strength, the thicker the upper and lower layers, the greater the interlocking depth and the higher the tensile strength.
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Therefore, to increase the shear strength, a thicker upper layer or a thinner lower layer is required; the increase in the thickness of the upper and lower layers can increase the tensile strength.
4 Conclusion
a. Although the static performance of the nailless riveted connection is lower than that of SPR, it can be applied to the non-main load-carrying body structure area;
b. The shear strength is positively correlated with the strength of the upper material; the tensile strength is negatively correlated with the strength of the connecting composite material;
c. The riveting direction is from high-strength plate to low-strength, and the shear strength is higher; the riveting direction is from low-strength plate to high-strength, and the tensile strength is higher;
d. The thicker upper material thickness and the thinner lower material thickness have higher shear strength; the increase of the upper and lower material thickness can increase the tensile strength.
