Spot welding quality control in body manufacturing

Spot welding is the main joining method for modern automobile bodies and other components, and plays an irreplaceable role in the automobile manufacturing industry. The quality of the joint welding of the body determines the overall structural rigidity and integrity of the car. Therefore, it is important to check the connection quality of spot welding. Stamping, welding, painting and final assembly of the vehicle body in the production process, the steel press plate pressed into a variety of body parts, and welding the body parts assembled by welding stamped into the vehicle body. Spot welding is a major component of the welding process. According to statistics, the number of soldering points on a light passenger car or car body is about 4,000.

Basic principle of spot welding

Resistance spot welding is to press the workpiece to be welded between the two electrodes, and to pass the current, and use the resistance heat generated by the current flowing through the contact surface of the workpiece and the adjacent region to heat it to a molten or plastic state to form a metal bond. Spot welding is a high-speed, economical connection method. It is suitable for the manufacture of sheet metal parts that can be punched and rolled without requiring airtightness and thickness less than 3mm. It is widely used in the welding of low carbon steel products such as automobile bodies.

The advantages of spot welding include: spot welding heating time is short, welding speed is fast; cost is low; operation is simple, easy to operate, easy to mechanize and automate; spot welding produces less smoke and harmful gases, and welders have good working conditions.

The most basic welding cycle for spot welding is:

(1) Pressurization procedure In this process, only pressure is applied, no electricity is applied, and the purpose of pressurization is to make the workpiece contact tightly to obtain a good contact resistance.

(2) Welding procedure Power is applied in the program, heat is generated by the resistor, and the metal of the workpiece is melted to form a molten pool, and the process is completed under pressure.

(3) Maintenance procedure This process only applies pressure and no electricity, and the solder joint metal is cooled under pressure.

(4) Resting procedure This process is a decompression process until the next welding cycle.

In order to improve the joint performance, it is sometimes necessary to add some auxiliary procedures outside the basic cycle, including: increasing the pre-pressure to eliminate the gap between the workpieces and making them fit tightly; using preheating pulses to improve the plasticity of the metal to make the workpiece fit tightly, preventing Splash; increase forging pressure to compact the nugget to prevent cracks and shrinkage; use tempering or slow cooling pulse to eliminate the quenching structure of alloy steel, improve the mechanical properties of the joint, or prevent it under the condition of no forging pressure Cracks and shrinkage holes.

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