Defects and solutions that are prone to occur when laser welding square shell battery explosion-proof valves for power batteries

For example, the commonly used square shell battery cells for power batteries include laser welding of cover explosion-proof valves, laser welding of pole columns, and laser welding of cover plates and shells. During the process of laser welding of aluminum alloy, it is easy to generate unqualified phenomena such as explosion points, pores, welding cracks, excessive depth and width of fusion. 

Therefore, it is necessary to define the impact of unqualified items, analyze the consequences (degree of harm) of the impact, understand the mechanism of laser welding unqualified, and take effective measures to improve the quality and consistency of laser welding in the mass production stage, improve the output of the production line, and reduce welding unqualified, Reduce the cost of waste materials.

Common failures in sealing welding of battery explosion-proof valves
Explosion proof valve is a circular thin sheet of pure aluminum (1060 or 3003) with a thickness between 0.08 and 0.1 mm. When using infrared fiber laser welding, due to the high reflectivity of solid aluminum material towards infrared laser and its thin material, if the welding process is not appropriate, the explosion-proof valve is prone to overheating, perforation or explosion during the laser welding process, causing it to lose its pressure relief and explosion-proof function.

Potential failure 1: Over burning/melting through
Reason: When using infrared laser welding, due to the high reflectivity of the solid aluminum alloy surface to infrared laser, higher laser power is often used. However, the thickness of the explosion-proof valve from 0.08 to 0.1 mm is too small, making it easy to melt through.
Solution suggestion: Select appropriate welding process parameters to achieve a steep increase and slow decrease in laser power and control heat input. Adopting a waveform with a pre peak and exponential attenuation can improve the absorption rate of aluminum material to laser, while the subsequent exponential attenuation wave can prevent perforation caused by high power density.

Potential Failure 2: Burst Hole
Cause: Gas escape from the molten pool during laser welding.
Source of gas:
1. The power battery cover plate and explosion-proof valve are thin stamping parts that are prone to residual lubricating oil and cleaning fluid after processing. Under the action of high-power density laser, these liquids are easily vaporized and float up to the surface of the molten pool, causing a large amount of splashing and leaving pits on the surface of the weld, forming explosive holes.

2. The width to thickness ratio of explosion-proof valves can generally reach around 30, and during welding, it is easy to cause thermal deformation and warping due to heating, resulting in a large amount of air in the assembly gap between the explosion-proof valve and the top cover. During welding, these residual air expands and sprays out the molten pool, forming explosive holes.

Suggested solution: 
1. Thoroughly clean the cover plate and explosion-proof valve before welding; 
2) Optimize the welding process by using pre spot welding and seam welding, and prevent warping and deformation through spot welding fixation to reduce blast hole defects.

In the laser welding of power square shell batteries, welding process technicians will select appropriate laser and welding process parameters based on the customer's battery material, shape, thickness, tensile requirements, etc., including welding speed, waveform, peak value, welding head tilt angle, etc. to set reasonable welding process parameters to ensure that the final welding effect meets the requirements of the power battery manufacturer.

Source: Shangtuo Laser