The use of lightweight metal materials such as aluminum alloys is an effective way to reduce the weight of the vehicle body and reduce fuel consumption and emissions of the entire vehicle.
This paper introduces a new type of steel plate and aluminum plate connection technology for automobiles. This technology realizes steel-aluminum connection by steel nail punching and staking aluminum plate and steel nail and steel plate spot welding process, which is called steel-aluminum resistance welding connection (IW). .
Compared with Self Pierce Rivet (SPR) and Flow Drill Screw (FDS), it can achieve low cost and high quality connection between steel and aluminum under the condition of ensuring joint strength. Development prospects.
1 Introduction
As China's automobile production and possession continue to rise, automobile energy consumption and harmful gas emissions are increasing year by year. In the increasingly shortage of energy conditions and deteriorating environmental conditions, the development of energy-saving, low-emission vehicle technology is the only way to reduce energy consumption and environmental pollution.
From the perspective of international development trends, lightweighting is the most effective measure for vehicle energy conservation and environmental protection. At the same time, since the body generally accounts for 25% to 30% of the total vehicle quality, the weight reduction of the vehicle body is crucial for the vehicle to lose weight.
Due to the inherent advantages of low density of lightweight metals such as aluminum alloys, the application of these materials has gradually become the development direction of lightweight bodies.
However, due to the difference in melting point and thermal expansion coefficient of iron, it is impossible to connect with the steel plate by spot welding. Although self-piercing riveting (SPR) and self-tapping threaded connections (FDS) ensure high-strength mechanical connections between steel and aluminum, due to the monopolization of technology patents and the large investment in production equipment, the current production costs are high.
SPR and FDS will have reverse bulging and puncture in practical applications, affecting the perceived quality of the surface of the part, so it is only suitable for single-sided implementation.
Introduced a new type of steel plate and aluminum plate joining technology (Intelligencte Welding, IW), introduced the specific implementation and scope of the technology, and compared the cost structure of IW and SPR and FDS and the mechanical properties of the joint, indicating IW A low-cost, high-quality connection between steel and aluminum can be achieved under the condition of ensuring the joint strength.
2, steel and aluminum IW connection implementation
The specific embodiment of the steel-aluminum IW connection is as follows. The special steel rivet is conveyed to the rivet by the conveying mechanism, and the positioned aluminum plate is punched at the pre-joining position by the steel rivet and the rivet die, and the punching waste is discharged. The structure of the steel rivet embedded in the aluminum plate is completed, as shown in Fig. 1.
Figure 1 Steel nail punching aluminum plate
Appropriate amount of structural adhesive on the side of the steel plate in the pre-joining position area to ensure anti-corrosion and improve the joint strength (the latter is baked by the electrophoresis oven to solidify the structural adhesive to achieve the effect of improving the joint strength); finally, by resistance spot welding The machine completes spot welding between the steel rivet and the steel plate to realize the connection between the aluminum plate and the steel plate, as shown in Fig. 2.
Figure 2 Gluing and steel nail and steel plate spot welding
After the rivet and welding, the steel rivet mainly has three connecting functional areas, namely the welding nugget area of ​​the steel rivet head and the lower steel plate, the riveting lock zone formed between the steel rivet bar and the aluminum plate, and the steel plate, the aluminum plate and The glued reinforcement zone formed between the structural adhesives is shown in Figure 3.
Figure 3 IW's three connection ribbons
3, steel aluminum IW connection performance
In order to ensure that the connection properties of steel-aluminum IW can meet the design requirements, the joint-level joint static shear and tensile tests are designed, and the maximum shear of SPR, FDS and IW under the same plate thickness and material conditions. Cut the failure load to measure.
The test method is shown in Fig. 4, in which the steel plate grade is GMW3032M-ST-S-CR340 and the thickness is 1.0 mm; the aluminum plate grade is GMW15192M-AL-S-5000-RSS-110-U, and the thickness is 1.4 mm.
Figure 4 joint static test
The sample before and after the shear test is shown in Figure 5, and the sample before and after the tensile test is shown in Figure 6.
Figure 5 Samples before and after the shear test
Figure 6 Samples before and after the tensile test
It can be seen from Figures 7, 8, and 9. In the shear load state, the displacement-force curve trajectory and inflection point of the IW test piece are basically consistent with SPR and FDS:
The displacement is in the range of 0~0.5mm. Since only the elastic deformation of the aluminum plate occurs, the curvature of the three curves is basically consistent;
At a displacement of 0.5 mm, the three curves appear similar to the "elastic strain to plastic strain" transition in the material tensile curve;
After the displacement of 0.5 mm, the three curves show inconsistencies in curvature and extremum due to differences in connection strength and stiffness due to differences in connection.
Figure 7 SPR maximum shear failure load
Figure 8 FDS maximum shear failure load
Figure 9 IW maximum shear failure load
Among them, the average shear failure load of IW increased by 5.19% and 2.08% compared with SPR and FDS respectively; the average displacement of IW in the maximum shear failure decreased by 7.14% and 10.46% compared with SPR and FDS, respectively. It can be seen that the shear resistance of IW is slightly higher than SPR and FDS, and the failure deformation is small.
It can be seen from Figs. 10, 11, and 12 that the displacement-force curve trajectory and the inflection point of the SPR, FDS, and IW test pieces are largely different under the tensile load state.
Figure 10 SPR maximum tensile failure load
Figure 11 Maximum tensile failure load of FDS
Figure 12 IW maximum tensile failure load
Due to the presence of threads in FDS, the maximum tensile failure load is much larger than SPR and IW, while the maximum tensile failure load of IW is 91.95% higher than SPR; the average displacement of SPR in the event of maximum shear failure is much smaller than FDS. And IW, and the average displacement of IW in the event of maximum tensile failure is 30.09% higher than that of FDS.
It can be seen that the tensile strength of IW is much lower than FDS but higher than SPR, and the failure deformation is large.
4. Applicable area and cost advantage of steel and aluminum IW
Figure 13 shows the distribution of all the aluminum parts of a steel-aluminum hybrid body. These aluminum parts are connected to the steel parts by SPR or FDS. Considering the existence of reverse convex and puncture phenomena in SPR and FDS, there are large constraints in design, and generally can only be implemented on one side.
At the same time, the SPR and FDS processes require investment in specialized production equipment, so the production costs are high. IW can get rid of this limitation, using a uniform punch-and-spot welding process instead of SPR and other steel-aluminum joints except the exterior part and the second surface part (no obvious fasteners are allowed to protrude). FDS, to achieve an effective connection between the steel plate and the aluminum plate, and can be implemented on both sides, the design flexibility.
Figure 13 Distribution of all aluminum parts of a steel-aluminum hybrid body
Referring to a steel-aluminum hybrid body, the steel and aluminum plate thicknesses connected by SPR and FDS are correlated. According to Figure 14 and Figure 15, the thickness of the aluminum plate using SPR and FDS is 1.0~2.85mm, using SPR and FDS. The thickness of the steel plate is 0.65~1.1mm.
Figure 14 Thickness distribution of aluminum parts using SPR and FDS in a hybrid body
Figure 15 Thickness distribution of steel parts using SPR and FDS in a hybrid body
In order to improve the design efficiency while reducing the process complexity and cost of IW, it is considered to serialize the development of steel rivets to avoid the problem of excessive number of steel rivets in the IW connection due to the difference in thickness of steel and aluminum.
As shown in Fig. 16, the steel rivet has the following main dimensions in the section: flange diameter A, flange height B, rivet length C, rivet length D, rivet diameter E, and rivet diameter F.
Figure 16 Main dimensions of steel rivet section
Considering that the dimension B is independent of the thickness of the steel aluminum plate, and the dimension D is the fixed key dimension of the punching and locking area; and because the ordinary spot welding process is between the steel plate and the steel rivet, the thickness variation of the steel plate does not affect the steel rivet and the steel plate. The quality of the spot welding between them is affected, so the steel rivets are serially developed on the basis of only changing the size C related to the thickness of the aluminum plate, as shown in FIG.
Figure 17 Series development of steel rivets
Combined with a large number of related tests, it is concluded that the height P of the steel rivet rivet exposed to the rivet is controlled within 0.3~0.5mm without affecting the subsequent welding quality, as shown in Figure 18.
Figure 18 Height of the aluminum plate exposed by the rivet after punching and riveting
According to the above analysis results, a series of design schemes for steel rivets suitable for steel-aluminum IW connection in a steel-aluminum hybrid body is obtained. As shown in Table 1, only five steel rivets of different sizes C are used to cover all steel and aluminum. IW connection.
Table 1 Series design of IW-connected steel rivets in a steel-aluminum hybrid body
In terms of production efficiency, since the FDS process requires a high-speed rotary tapping process of 2~3s, the production efficiency of a single piece is lower than that of SPR and IW. At the same time, SPR and FDS are single-piece due to technical patent monopoly and professional production equipment. The cost and equipment cost are too high, and IW's punching and riveting equipment does not have technical barriers and has a simple structure. At the same time, the subsequent welding equipment can use the current common spot welding equipment, so the cost is much lower than SPR and FDS.
5 Conclusion
a. Compared with SPR and FDS, IW has the advantages of high appearance quality, low cost and high degree of design flexibility.
b. The shear resistance of IW is slightly higher than that of SPR and FDS, and the failure deformation is small; the tensile strength of IW is lower than FDS but higher than SPR, and the failure deformation is larger.
Therefore, in the development design of the hybrid vehicle body, the IW connection method can be adopted for the steel-aluminum joint region where the shear load is stronger than the tensile load, and the FDS should be adopted for the steel-aluminum joint region where the tensile load is stronger than the shear load. Connection method.
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