About The Explosion Point of Lithium Battery Laser Welding

• What is the explosion point of laser welding?

• What is the impact of the explosion point on lithium batteries?

• Analysis of the cause of the explosion point

• Measures to control the explosion point

In laser welding, welding quality and efficiency have always been the focus of major lithium battery manufacturers. At the same time, the stability of the equipment is also the key to battery production efficiency and quality.

At present, in the production of square aluminum shell batteries, the welding quality around the top cover is very critical to the battery assembly production. At present, the best production efficiency can reach 99.5% (square aluminum shell battery, shell thickness 0.6mm), but in most cases it can only be maintained at around 97.5~98.5%.

In actual production, the weld quality is most affected by the weld, and there are many defects such as burst points and pinholes, followed by defects such as false welding and polarization.

What is the explosion point of laser welding?

The defects caused by laser welding of lithium batteries are divided into: forming defects (pore splashing, explosion point, poor forming consistency), connection defects (cracks, undercuts), performance defects, etc. according to their forming and performance.

Explosion point is the common name for point defects in laser welding in the lithium battery industry. Its essence is the problem of splashing (also known as fire explosion). There are many factors that cause splashing, 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 appearance of bulges, pores on the shell surface, bubbles inside, etc. are generally caused by the small core diameter of the optical fiber or the laser energy setting is too high.

When the laser beam continues to heat, the solid metal turns into liquid and forms a molten pool. Subsequently, the liquid metal in the molten pool is heated and "boils". Finally, the material absorbs heat and vaporizes, and the boiling changes the internal pressure, bringing out the surrounding liquid metal package, and finally producing "splashing".

The laser continues to act on the material, causing the material to vaporize and expand violently, generating pressure and forming a molten state. Subsequently, when the metal vapor escapes, high pressure is generated, pushing the material to the top of the small hole. Eventually, the spatter is pushed out of the top of the hole and adheres to the surface to form a melt.

Impact of explosion point on lithium battery

The explosion point is not just an appearance problem. The explosion points of different processes have different effects on the battery.

Battery connector welding process:

Affects the connection strength and current carrying capacity between the connector 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 poles. If it remains inside the battery, it will cause abnormal self-discharge. During the top cover welding and sealing nail welding process, the explosion point affects the airtightness of the battery cell and the mechanical strength of the shell to resist internal expansion. There are hidden dangers of liquid leakage and water vapor ingress during use. The module package level mainly affects the mechanical strength and overcurrent.

Causes of explosion points 

According to the appearance of explosion points, they can generally be divided into black explosion points and white explosion points.

(1) Causes of black explosion points 

When the laser passes through the stain, a violent oxidation reaction will occur to form gas, which will directly affect the stability of the molten pool keyhole and form a black explosion point. Specifically, the molten pool keyhole is abnormally closed, resulting in insufficient melting depth, insufficient melting width, unmelted surface or direct penetration of the parent material, which requires metallographic cutting to assist in judgment. The sources of contaminants generally include the following aspects:

① Residual foreign matter in the feed

It is mainly caused by the outflow of parts during the processing, such as lubricating oil of punching and blanking equipment, and tape for adjusting and sticking fixtures.

② Pollution during packaging and transportation of parts 

This is mainly due to organic residues caused by sharp edges of the top cover and aluminum shell and scratches on the packaging materials.

③ Stains generated during welding adhere to the weld

 For example, the operator grabs the battery core with dirty gloves, contaminating the position to be welded.

(2) Causes of silver explosion points

Mainly due to the unstable dynamic behavior of the keyhole during laser deep melting welding. If it is at the upper and lower limits of the process window, it is easy to cause the keyhole to be unstable due to changes in the external environment, causing the keyhole to break, and the spatter will cause insufficient melting depth and unqualified appearance. This situation is generally accompanied by broken welding (partial lack of fusion, insufficient melting depth, insufficient melting, insufficient energy).

• Shielding gas turbulence, typically around corners, and often accompanied by a change in weld color (oxidation).

• The laser welding head or shaft shakes, causing the lock hole to be unstable.

• The pre-weld spot weld position is unstable and the melting amount changes suddenly.

• In annular spot welding or composite welding, the ratio of core power to outer ring power is too large, the penetration depth is too deep, and the keyhole is unstable.

• The process parameters are close to the upper and lower limits of the process window, resulting in unstable keyhole.

• Abnormal moisture content on the surface of incoming materials.

Lithium battery laser welding defect-explosion point solution

The key is to verify the problem through experimental design, find the cause, and then solve it in a targeted manner.

(1) Ensure the consistency of the gap on the weld surface, and the gap should be small (set standards according to the site and product). 

(2) No stains (you can artificially apply blue film, glue and other stains to verify what the stain is, which station, which action, which fixture, and whether the program action is reasonable).

(3) You can use a small force to hit the steel plate to check the swing.

(4) Unstable molten pool: Be careful not to select the defocus amount, shielding gas flow, trajectory, energy, and inner and outer ring power ratio on the upper and lower boundaries of the process window, so as to avoid the small hole being in an unstable state and affected by environmental fluctuations.