At present, the lightweight of automobiles has become a realistic demand for car companies and the whole industry to improve their core capabilities. The body weight accounts for about 40% of the total weight of the vehicle, and the space for weight reduction is the largest. In the future, the development direction of the body design will use light materials with different properties according to the requirements of the parts to achieve the best match between the functions of the materials and components. The application of lightweight materials such as aluminum alloys, magnesium alloys, composite materials, carbon fibers, high-strength steels, and ultra-high-strength steels will play a major role in the lightweighting process of automobiles.
Lightweight materials also reduce the weight of the car body, but also put forward new requirements for the body connection technology. Aluminum alloy has high resistance, and the use of resistance spot welding greatly increases energy consumption, and the electrode is easily contaminated, which is not conducive to continuous production; high-strength steel and ultra-high-strength steel are sensitive to heat, and MIG/MAG welding is prone to thermal deformation or weld crack; Laser welding is also difficult to achieve in connection with high reflectivity materials (aluminum, copper, etc.). In addition, the traditional body connection process is not suitable for dissimilar metal and non-metal materials.
In recent years, the self-piercing riveting process has overcome the shortcomings of the traditional riveting, and achieved the completion of the punching and riveting, opening up a new way for the connection of the car body; and the self-punching riveting technology can effectively solve the steel-aluminum overlap, aluminum-aluminum Lap joints, magnesium-aluminum lap joints and some composite material lap joint defects, to achieve an effective connection of lightweight materials, thereby promoting the use of lightweight materials on the body.
This paper mainly describes the process principle of self-piercing riveting technology, compared with the advantages and disadvantages of traditional resistance spot welding process, and its application in the automotive industry.
Self-punching work principle
The self-punching riveting technology is powered by a hydraulic cylinder or a servo motor to directly press the rivet into the plate to be riveted. The riveted plate is plastically deformed under the pressure of the rivet, and is filled into the riveting die after forming, thereby forming a stable A brand new plate joining technology for connection. The riveting nose first presses the riveted plate, the rivet is pressed and embedded by the riveting rod, penetrates the upper plate and expands into the next layer of plate, and then the rivet expands together with the plate, filling the riveting die, and the rivet legs are turned open to form " Buttons" to complete the firm connection of the upper and lower plates.
Self-punching riveting is a cold joining process. Its connection principle has its unique technical advantages compared with the traditional hot spot welding of resistance spot welding. It mainly includes: materials that can achieve multiple layers, different properties, and poorly welded properties. Reliable connection; no need to pre-open holes, can protect the coating coating, strong corrosion resistance, high riveting point strength, long life; environmentally friendly cleaning connection process, smokeless, steam, spark generation; compared with traditional spot welding energy consumption Less, no cooling water and compressed air consumption; riveting quality is stable, visual inspection, consistency is good; riveting process is easy to achieve automation and online monitoring, to meet the needs of large-scale production.
Self-piercing riveting mechanical properties
Force-displacement curve
The force-displacement curve of the self-punching riveting and the traditional spot welding joint position was obtained by the tensile force test. At the same time, the energy absorption data during the experiment was obtained, and the steel and aluminum alloy materials were tested respectively. The comparison was made at the riveting point performance. In the process of failure, the kinetic energy can be effectively absorbed, and the displacement deformation is large, which can prevent the joint portion from being completely disconnected in a short time.
2. Fatigue performance curve
Fatigue performance curves of self-piercing riveting and traditional spot welding joints were obtained through fatigue. Experiments were carried out on steel and aluminum alloy materials respectively, indicating that the riveting joints can withstand high-frequency fatigue of large loads; resistance electric welding on steel plates There is a large increase, the fatigue strength is basically flat under low load conditions, and it is improved under high load conditions; it shows a big advantage in the test of aluminum plate, and the resistance spot welding connection is improved under the same load condition. Close to 100 times.
Process requirements for self-piercing riveting on the body
1. Body panel lap joint requirements
When the two layers are overlapped, the thickness of the bottom plate accounts for more than 1/2 of the total plate thickness; when the three-layer plate is overlapped, the thickness of the bottom plate accounts for more than 1/3 of the total plate thickness; avoiding the overlapping combination of the same thickness plates; When riveting, the thin plate is on the top (the nose end is riveted) and the thick plate is on the bottom (the end of the riveting die).
2. Body panel strength requirements
The maximum strength of the top plate shall not exceed 1600 MPa, and the maximum strength of the bottom plate shall not exceed 600 MPa; the plate with high strength shall be placed in the upper layer, and the plate with low strength shall be placed in the lower layer; the elongation of the bottom plate shall be greater than 8%.
Flange size requirements
When using 3 mm rivets, the minimum distance from the edge of the flange to the centerline of the rivet is 6.0 mm and the minimum at the edge of the rim is 18 mm. When using a 5 mm rivet, the minimum distance from the edge of the flange to the centerline of the rivet is 8.0 mm, and the minimum at the edge of the rim is 20 mm. The minimum distance between the two rivets is 30 mm.
Component structure requirements
SPR belongs to the double-sided contact connection process, and the aluminum profile is not suitable for the SPR process; the rivet gun needs to be moved downward from the button to complete the demolding action, and the stripping distance requirement is ≥5 mm.
Rivet selection requirements
Diameter selection: total plate thickness <4 mm, diameter 3 mm rivet, total plate thickness ≥4 mm, diameter 5 mm rivet; choice of nail length: bottom plate thickness <2 mm, nail length to total plate thickness +2.5 ~3 mm; the bottom plate thickness is >2 mm, and the nail length is the total plate thickness +3~3.5 mm.
Riveting selection requirements
Riveting different materials and thicknesses, the choice of riveting die is also different. Table 1 shows the applicable occasions of common riveting die.
Self-piercing riveting equipment commonly used on body-in-white
The hand-held self-piercing riveting device adopts a micro hydraulic cylinder design. The motor drives a micro hydraulic pump to drive the hydraulic cylinder to provide power for riveting. The hydraulic cylinder adopts a bidirectional design, which not only provides power for riveting, but also provides homing after riveting. The required pressure to complete the entire riveting process. Hand-held self-pierce riveting equipment is mainly used in some automotive maintenance, industrial field installation, on-site maintenance and other occasions, not suitable for mass production.
The hydraulic pre-tightening self-piercing riveting equipment belongs to the mainstream models in the international automobile production field, and has many advantages such as good riveting quality, high work efficiency, simple and reliable structural design, strong continuous working ability and long equipment life. The hydraulic control unit adopts a monolithic structural design to provide power to the hydraulic actuator, control the riveting process, and adjust the riveting parameters. The unit consists of a fuel tank, a hydraulic pump, a solenoid valve hydraulic control unit, a PLC riveting control unit and an electrical cabinet. It is automated by automated production lines or robots.
The electric servo type self-piercing riveting equipment is similar in structure to the pre-tightening self-piercing riveting equipment, except that in the power supply, the hydraulic pressure is changed to the electric servo, and the servo motor is used to provide power, and the riveting process is realized by the mechanical mechanism. This process is controlled by PLC, which can easily realize the automatic production process and is the mainstream equipment in the automotive industry.
in conclusion
The self-piercing riveting technology is suitable for the interconnection between lightweight materials and dissimilar materials, and has a great improvement over conventional resistance spot welding technology in both static data and force-displacement experimental data. And to improve the safety factor of driving and driving will play a huge role. In the future, with the deepening of self-piercing riveting research, it will be more widely used in the automotive industry.
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