Power batteries provide power for automobiles and largely determine the safety, reliability and durability of automobiles. The manufacturing process of power battery cells and modules are heavily used in laser welding, a special process, AIAG CQI-15 stipulates the special requirements of the laser welding process in the passenger car industry.
Taking the square shell cells commonly used in power batteries as an example, there are laser welding of the cover explosion-proof valves, laser welding of the poles, laser welding of the cover and the shell, etc. The laser welding of aluminum alloys is a very important part of the manufacturing process. Laser welding aluminum alloy process, it is easy to produce bursting point, porosity, weld cracks, melt depth exceeds the poor, melt width exceeds the poor and other unqualified phenomena, therefore, the need to define the impact of unqualified items, analyze the impact of the consequences of the impact (degree of harm), to understand the mechanism that produces the laser welding unqualified, and thus to take effective measures to improve the quality of the laser welding of the mass production stage and the consistency of the laser welding, to improve the output of the production line, reducing the number of Welding failure, reduce the cost of material waste.
Common Laser Welding Failures (Part)
01 Battery Explosion-proof Valve Seal Welding Common Failures
The explosion-proof valve is a round sheet of pure aluminum (1060 or 3003) with a thickness of 0.08 ~ 0.1 mm. When welding with an infrared fiber laser, due to the high reflectivity of solid aluminum to the infrared laser, and the material is very thin, such as welding process is not appropriate, the explosion-proof valve in the laser welding process is prone to overcooking perforation or blow holes, so that it loses the function of the pressure relief and explosion-proof.
Potential Failure 1: Overshooting/Melt-through
Reason: When using infrared laser welding, because the surface of solid aluminum alloy has a high reflectivity to infrared laser, so often use a higher laser power, and the explosion-proof valve 0.08 ~ 0.1 mm thickness is too small, it is easy to melt through.
Solution Suggestion: Choose suitable welding process parameters to achieve a steep rise and slow fall of laser power and control heat input. Adopt a waveform with a front spike and attenuate exponentially. The front spike can increase the absorption rate of the laser by the aluminum material, and the subsequent exponential attenuation wave can prevent perforation caused by too high a power density.
Potential Failure 2: Blown holes
Causes: Caused by the escape of gas from the molten pool during the laser welding process.
Sources of gas: 1) Power battery covers and explosion-proof valves are thin-thick stamping parts, which are prone to residual lubricants and cleaning fluids after processing. In the high power density of the laser effect, these liquids are very easy to vaporize and float to the surface of the molten pool, bursting at the same time produce a lot of spatter and leave a crater on the surface of the weld, the formation of blowholes. 2) explosion-proof valve width-to-thickness ratio is generally up to 30 or so, due to the heat of the weld and is very easy to produce thermal deformation warping, resulting in explosion-proof valves and the top cover of the assembly gap in the presence of a large amount of air. Welding of these residual air thermal expansion, spray out of the molten pool, the formation of blowholes.
Suggested solutions: 1) Thoroughly clean the cover and explosion-proof valve before welding; 2) Optimize the welding process with pre-spot welding + seam welding, and prevent warping and deformation by spot welding to reduce blowhole defects.
02 Common failure of battery pole welding
When the power battery is grouped, the connection between the positive and negative poles and the adapter block is made by laser welding.
Potential Failures: Blow holes
Cause: The pole weld is the mating surface between the aluminum adapter and the pole with a diameter of about 6 mm, so it is very prone to residual impurities such as lubricants and cleaning agents. The heat from the laser welding causes the impurities left at the pole to vaporize quickly, and the gas escapes from the molten pool, resulting in blowholes.
Suggested solutions: 1) Thoroughly clean before welding to remove gas impurities; 2) Optimize the welding process to reduce blowholes through the slow rise in power. Optimize the conventional one-pass welding process into two-pass welding, low-power first welding to preheat the material and exhaust gas, high-power second welding to achieve the required depth of fusion.
03 Battery shell and cover welding failure
Power battery shell is mainly used Al3003 aluminum alloy, its thickness is generally between 0.6 ~ 0.8 mm, commonly used small power fiber continuous laser welding. The main quality problem of the laser weld there is porosity, which will reduce the sealing of the battery.
Potential Failure: Porosity
Solution Recommendation: Porosity is primarily a metallurgical type of porosity caused by oxide film or water vapor decomposition. Reducing the laser power and increasing the welding speed reduces the porosity to a maximum of 1.1%. In order to further reduce the porosity, it is recommended to use vibrating mirror laser instead of pulsed laser spot welding. The stirring effect of the keyhole on the molten pool can accelerate the escape of bubbles, which can almost completely eliminate the porosity of the sealed weld.
Laser welding of power battery is a special process, all aspects of the loss of control may affect its own and the vehicle's safety, performance and life. Comprehensive understanding and mastery of the laser welding process of power battery, and systematic introduction of the best practices of laser welding process quality supervision are the basis for ensuring high qualification rate and consistency of laser welding in the mass production stage.
The second edition of CQI-15 welding process system evaluation standard released in 2020 has become the standard of laser welding process and quality monitoring for the head manufacturers in the power battery industry, and it is also an important reference standard for mainstream passenger car OEMs to recognize the quality system and laser welding process of power battery manufacturers.
