About the Laser Welding Explosion Point of Lithium Battery
After reading this article, you will know the answers of the following problems:
- What is the laser welding explosion point?
- What impact does the explosion point have on lithium batteries?
- Cause analysis of burst point
- Burst point control measures
In laser welding, welding quality and efficiency have always been the focus of major lithium battery manufacturers. At the same time, the stability of equipment is also the key to battery production efficiency and quality.
At present, in the production of square aluminum shell batteries, the welding quality of the welding around the top cover is very critical to the impact of battery assembly production. At present, the best production efficiency can reach 99.5% (for square aluminum shell batteries, the thickness of the shell is 0.6 mm). But most of the time, it can only be maintained at about 97.5~98.5%. In actual production, the welding seam has the greatest impact on the quality of welds. And there are more explosion points and pinholes, followed by such as virtual welding, polarized light etc.
What is the laser welding explosion point?
Defects generated by laser welding of lithium batteries are classified according to their forming and performance: forming defects (porosity splash, explosion point, poor forming consistency), connection defects (cracks, undercuts), performance defects, etc..
Explosion point is a common saying of laser welding point defects in lithium battery industry. Its essence is the problem of splash (also known as fire explosion). There are many factors that cause spatter, such as the cleanliness of the material, the purity of the material itself, the characteristics of the material itself, etc.. But the stability of the laser is the decisive factor. The reason for shell surface protrusions, air holes and internal air bubbles is that the fiber core diameter is too small or the laser energy setting is too high.
When the laser beam is continuously heated, the solid metal turns into a liquid state, forming a molten pool. Then, the liquid metal in the molten pool is heated and “boils”. Finally, the material absorbs heat to vaporize, and the boiling changes the internal pressure, bringing out the surrounding package of liquid metal, eventually producing a “splash”.
The laser continues to act on the material, causing the material to vaporize and expand violently to generate pressure and form a molten material (left picture). Then, when the metal vapor escapes, high pressure is generated to push the material to the top of the keyhole (middle picture). Finally, the spatter is pushed out of the top of the keyhole and adheres to the surface forming a melt.
The impact of explosion point on lithium battery
The explosion point is not just a matter of appearance, and the explosion point of different processes has different effects on the battery.
Welding process of the connecting piece of the battery:
it affects the connection strength and current carrying capacity of the connecting piece and the top cover. If the splash falls on the surface of the bare battery, it will burn the diaphragm and cause a short circuit between the pole pieces. If it is left inside the battery, it will cause self-discharge abnormal.
In the top cover welding and sealing nail welding process, the explosion point affects the airtightness of the cell and the mechanical strength of the shell against internal expansion. There are hidden dangers of liquid leakage and water vapor entering during use.
The module pack level mainly affects mechanical strength and overcurrent.
Cause of explosion point
According to the appearance of the explosion point, it can generally be divided into two types: black explosion point and white explosion point.
(1) The cause of the black explosion point
When the laser passes through the stain, a violent oxidation reaction will occur to form gas, which directly affects the stability of the keyhole in the molten pool and forms a black explosion point. The specific manifestation is that the keyhole of the melting pool is abnormally closed, resulting in insufficient penetration and melting width, the surface is not melted, or the base material is directly penetrated. And that requires metallographic cutting to assist judgment. The sources of pollutants generally include the following aspects:
① Residual foreign matter in incoming materials
It is mainly caused by the outflow of parts during processing, such as equipment lubricating oil for punching and blanking, adhesive tape for adjusting and pasting fixtures, etc..
② The pollution during the packaging and transportation of parts
That is mainly caused by the sharp edge of the top cover and the aluminum shell, and the residue of organic matter caused by scratching the packaging materials.
③ Stains caused by the process adhere to the welding seam
For example, the operator uses dirty gloves to grab the battery core and pollutes the position to be welded.
(2) The cause of the silver explosion point
Mainly due to laser deep penetration welding, the dynamic behavior of the keyhole is unstable. If it is at the upper and lower limits of the process window, it is very easy to cause the keyhole to become unstable due to changes in the external environment. The keyhole will be broken, and the splash will cause insufficient penetration and unqualified appearance. This situation is generally accompanied by broken welding (partially unfused, insufficient penetration, insufficient melting, insufficient energy).
- Turbulent flow of shielding gas, which generally appears near corners, and is usually accompanied by changes in weld color (oxidation).
- Laser welding head or shaft shaking, resulting in keyhole instability.
- Pre-welding spot welding position is unstable, and the melting amount changes suddenly.
- Annular spot or composite welding, the ratio of the core power to the outer ring power is too large, and the keyhole is unstable due to too deep penetration.
- The selection of process parameters is close to the upper and lower limits of the process window, resulting in unstable keyhole.
- The surface moisture content of the incoming material is abnormal.
Solutions for lithium battery laser welding defect - explosion point
The key lies in the experimental design verification problem, locating the cause, and then solving it in a targeted manner.
(1) Ensure the consistency of the gap on the surface of the weld, and the gap should be small (set the standard according to the spot and product).
(2) No stains (you can artificially smear blue film, glue and other stains to verify what kind of stain it is, from which station, that action, that fixture, and whether the program action is reasonable).
(3) The swing can be checked by hitting the steel plate with low power.
(4) The molten pool is unstable: Be careful not to select the defocus amount, protective gas flow, trajectory, energy, and inner and outer ring power ratios on the upper and lower boundaries of the process window, so as to avoid the keyhole being in an unstable state and being affected by environmental fluctuations.