Taking the steel/Al body of certain company as the research reference, the relevant steel/Al joining tests for the different material combination were carried out. The chassis structure of the body was similar to the rear structure of the Tesla floor, as shown in Figure 1, which was designed by large-scale DCAA part. DCAA, TFSS and other steel materials determine the choice of joining method and the complexity of joining process.

The materials tested in this paper include common steel, TFSS and DCAA, whose composition and mechanical properties are shown in Tables 1-4. The data comes from the German Volkswagen Materials Standard.

For BIW products, the test methods of joints include shear strength, tear strength and cross tensile strength. The tensile type and test sample design are shown in Figure 2.

Considering the workload of the test and the characteristics of the tensile test, only the tensile and tearing strength were analyzed in this test.

The new energy vehicle must have the characteristics of lightweight and high strength. The application of steel/Al hybrid BIW of large DCAA and TFSS, such as the application of CTC or CTB technology, not only simplifies the body manufacturing process, but also strengthens the body strength.

The application of DCAA has led to the combination of dissimilar materials, such as steel/Al two-layer plate, steel/steel/Al, steel/Al/steel and Al/Al/steel and so on. The welding of steel/Al dissimilar materials, especially using methods such as resistance spot welding, will face a series of difficulties. New joining methods and innovative joining methods must be developed.

Combined with the material distribution and thickness of DCAA, the DCAA is C611, the TFSS is TL 4225, and the covering material is CR5. Their thickness is 2 mm, 1 mm and 0.75 mm, respectively. The combination above is a common plate thickness of DCAA and peripheral parts.

Medium frequency resistance spot welding (MFRSW) can solve almost all the joining of BIW steel plate, has mature process and quality standards. For joining of Al alloys, the joining can also be realized by using the MF or sub-high frequency power. At present, the application of MFRSW is applied mainly to the parts that bear non-dynamic loads, such as the joining of the battery housing. Due to the particularity of Al alloy spot welded joints, the fatigue strength is difficult to reach the level of steel materials, so the application is limited, and other joining methods must be used [13] [14] [15].

Due to the special material properties of DCAA, that is, it has a large affinity for water or hydrogen, it is easy to produce pores and hydrogen polymerization cracks in the process of welding. In addition, in view of the softening phenomenon of Al alloy material, namely, it is easy to produce a “plastic ring” in MFRSW process. The strength of the hot affected zone (HAZ) will be greatly reduced, resulting in breaking here. Therefore, it must be welded with the DC power, which has higher current density and shorter welding time. The welding time has an effect on the porosity and crack as well as the “plastic ring” of the HAZ. The relationship between the two phenomena must be balanced to choose the welding time reasonably. In addition, Al alloy spot welding also has the phenomenon of nugget core “shift”, the nugget will shift to the anode side. It is because, for the Al alloy spot welding, the current is larger, the Peltier effect is more obvious, and namely, the anode side of the joint will produce more heat. The reason for this phenomenon is that the resistivity of Al alloy is low and the current is high during welding. On the other hand, the adhesion of the electrode cap surface during welding will generate P and N type semiconductor materials, such as various inter-metal compounds on its surface, triggering the physical conditions generated by the Peltier effect.

To solve the problem caused by the Peltier effect, the most effective technical means is to use variable-polarity power to eliminate the physical conditions of Peltier effect. In addition, if Peltier effect cannot be eliminated, the design of plate thickness combination should be considered reasonably

Medium frequency resistance spot welding (MFRSW) can solve almost all the joining of BIW steel plate, has mature process and quality standards. For joining of Al alloys, the joining can also be realized by using the MF or sub-high frequency power. At present, the application of MFRSW is applied mainly to the parts that bear non-dynamic loads, such as the joining of the battery housing. Due to the particularity of Al alloy spot welded joints, the fatigue strength is difficult to reach the level of steel materials, so the application is limited, and other joining methods must be used [13] [14] [15]. RES (Friction element welding) is a new joining technology that can realize the welding of Al alloy and TFSS. It makes the “element” with high-speed rotation to penetrate the upper lower-hardness plate, and realizes the friction spot welding with the higher-hard plate such as TFSS. Through the principle of rotating friction melting, completes the welding of the “element” (steel) with the TFSS under the effect of pressure, and form a solid joining [16] [17] [18].

Its principle is shown in Figure 3. RES can solve the problem that the NEV DCAA cannot be joined with the TFSS, and can get the high quality joining of steel/Al dissimilar materials.

Because the yield strength of DCAA is not high, and has a high elongation, DCAA parts do not need prefabricated hole treatment. In principle, if the yield strength of the upper plate is higher than 780 MPa, prefabricated hole must be done, otherwise it is difficult to achieve RES connection.

RES can realize the connection of DACC and TFSS with higher joint strength. However, due to the influence of the yield strength of the light alloy side, it has certain limitations, the equipment price is high, and the process is relatively complicated.

FSPR is a special riveting method. It is to send automatically a special rivet into the riveting head through riveting equipment. In the joining, the connected base

metals are punched and discharged by the rivet, and then the lower die is squeezed around the rivet to form a reliable joining, the principle of FSPR shown as in Figure 4.

FSPR can rivet various materials, such as high-strength Al alloy, TFSS, etc. It can achieve multi-layer riveting and good forming effect. It can be used for joining whose joint appearance without convex hull after riveting. Punch riveting is completed at one time without pre-punch for the riveting hole, having fast riveting speed, high efficiency and stable riveting appearance. It has an irreplaceable role in solving the two-layer or three-layer plate joining between TFSS and DCAA.

The FSPR joining process must meet the following conditions:

Firstly, the thickness of the down plate is at least 1/3 of the total thickness of the plate combination. Secondly, the high-hardness material is on the punch side, and the low-hardness material is on the die side. Thirdly, the thin plate is on the punch side, and the thick plate is on the die side. In addition, the maximum strength of the upper plate can reach 1500 MPa. The harder the upper plate, the lower plate needs to have lower strength and better plasticity. There is a certain lower limit on the thickness. Moreover, the maximum strength of the lower plate cannot exceed 600 MPa.

The FSPR enables multi-layer riveting of DCAA and TFSS up to a total thickness of 9.1 mm. The joining appearance is good, and can be applied for covering parts. Punching and riveting is done in one go, without pre-punch the riveting hole. In addition, in terms of joint strength, the shear strength of the joint is

high, and the riveted appearance is stable.

The FSPR can reduce the requirement for material elongation and can be applied to materials with less than 10% elongation. It is also suitable for scenes where the thickness of the die fluctuates on the material. In addition, FSPR rivets will not slip due to uneven material thickness, which will affect the joining strength.

SPR belongs to the material stretch forming, and FSPR belongs to the material compression forming. The material does not crack after the riveting is completed. The essence of material cracking in the riveting process is that the crack extends to the entire cross section that leads to the fracture. The crack expansion requires a certain intensity of tensile stress during the period. If the material is subjected to compressive stress, then the crack tip is not easy to reach the stress intensity required for crack expansion, the crack cannot expand, and the material will not be damaged.

FDS is an effective method to join plate and blind structure, and has the characteristics of high welding efficiency. This method is suitable for the joining of steel/Al mixed structure with yield strength lower than 780 MPa. At present, the joining method is also in continuous innovation, and by some innovation, it is hoped to solve the joining of materials with yield strength of 980 MPa. The increase in material strength will inevitably affect the FDS welding head and drive motor, reduces life. For joining high-strength materials, there is no perfect process and quality standard at present and it needs to be further improved [19] [20] [21].

As a new joining method, FSPR is still in the stage of laboratory testing and local

application in BIW, some experiments were done in order to set up the process and quality standards. The test samples are shown in Figure 6.

The test carried out the joining of TFSS and DCAA with two-layer plate, as well as the three-layer plate of TFSS, DCAA and covering plate, which is the most common plate combination. The strength and surface state have different requirements, for other joining methods, they are difficult to meet the requirements of product design.

The FSPR joints of two- and three-layer plates are shown in Figure 7, from which the final forming results and joint morphology of the joint can be seen.

Through the action of the forming die, the metal of the rivet attachment at the end of the die has local plastic deformation and squeegees into the gap at the side depression of the rivet, thus forming the effect of riveting.

As can be seen from the figure, the TFSS has less deformation due to its high hardness and yield strength, and the gap filling in its corresponding region is supplemented to a certain extent by the deformation of low-strength materials such as DCAA below.

Through the strength test, the tensile and shear strength of the joint with two layers can be obtained: 7500 - 9000 N and 3500 - 4000 N respectively. The tensile strength and shear strength of the joint with three layers are 2000 - 2200 N and 400 - 500 N, respectively.

For the shear strength of the three-layer plate, it is characterized by the strength of the outermost steel plate. Because of its thin thickness, the strength of the base metal is also low, so the strength of the joint is not high, especially for the shear strength, the reduction is more obvious. However, for covering parts, there is no need for the outer plate and the assembly to have higher strength.

As can be seen from Figure 7, the filling of the arc gap around the rivet has an important impact on the strength of the joint, especially the tearing strength. It is also of great significance to study the relationship between the filling amount and the feeding depth of the forming die, as well as the influence of the elastoplastic deformation of the DCAA and the TFSS part on the filling, if a physical and mathematical model can be established to reflect the internal relationship. This simplifies the selection of process parameters.

Through joint strength and welding accessibility test, general laws such as joint spacing, flange edge, strength, ratio of elongation and thickness in the joining methods such as FSPR, SPR, FDS and RES, can be summarized, as shown in Table 5.

Data verification and analysis has been done for two-layer plate and three-layer plate.

This is very important, if these parameters or material selections are not reasonable, it will bring a huge impact on the choice of welding methods, the complexity of the welding process, the weldability of the joint and the quality of the joint, and even difficult to achieve the high quality of the product joints. Therefore, the verification and subsequent improvement of these parameters are of great significance for the future development of the body.