What is the explosion point of laser welding?
What impact does the explosion point have on lithium-ion batteries?
Analysis of the Causes of Explosion Points
Measures to Control Explosion Points
In laser welding, welding quality and efficiency have always been key concerns for major lithium-ion battery manufacturers, while equipment stability is also critical to battery production efficiency and quality.
In the current production of square aluminum-cased batteries, the welding quality around the top cover has a significant impact on battery assembly production. The current best production efficiency can reach 99.5% (square aluminum casing batteries, casing thickness 0.6mm), but in most cases, it can only be maintained at around 97.5-98.5%. In actual production, the weld seam quality is most affected by defects such as weld seam, explosion points, and pinholes, followed by defects such as incomplete fusion and uneven weld seam.
What is the explosion point of laser welding?
Defects caused by laser welding in lithium-ion batteries are classified based on their formation and performance: formation defects (pores, spatter, explosion points, poor formation consistency), connection defects (cracks, undercut), and performance defects.
Explosion points are a colloquial term for point defects in laser welding in the lithium-ion battery industry. Essentially, they are spatter issues (also known as "hot spots"). There are many factors that cause spatter, such as material cleanliness, material purity, and material properties, but the stability of the laser is the decisive factor. The appearance of protrusions, pores on the surface of the casing, or bubbles inside is generally caused by an overly small fiber core diameter or excessively high laser energy settings.
When the laser beam continuously heats the material, solid metal transforms into liquid, forming a melt pool. Subsequently, the liquid metal in the melt pool is heated and "boils." Ultimately, the material absorbs heat and vaporizes, with boiling altering internal pressure and carrying away surrounding liquid metal, resulting in "spatter."
Impact of explosion points on lithium-ion batteries
Explosion points are not merely cosmetic issues; different manufacturing processes result in explosion points that have varying effects on the battery.
Battery connector welding process:
It affects the connection strength and current-carrying capacity between the connector and the top cover. If spatter lands on the bare battery surface, it can burn the separator, causing a short circuit between the electrodes. If it remains inside the battery, it can cause abnormal self-discharge.
During top cover welding and rivet welding, explosion points affect the cell's airtightness and the mechanical strength of the casing to resist internal expansion, posing risks of liquid leakage and moisture ingress during use.
Module package level primarily affects mechanical strength and overcurrent.
Based on 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 contaminants, it triggers a vigorous oxidation reaction that forms gas, directly affecting the stability of the melt pool keyhole, resulting in black explosion points. This manifests as abnormal closure of the melt pool keyhole, resulting in insufficient melt depth, insufficient melt width, unmelted surfaces, or direct penetration of the base material. Metallographic cutting is required for辅助 judgment. The sources of contaminants generally include the following aspects:
① Residual foreign objects in the feedstock
This is primarily caused by the flow of parts during the manufacturing process, such as lubricating oil from punching and blanking equipment, adjustment tools, or adhesive tape used to secure fixtures.
② Contamination during component packaging and transportation
This is primarily caused by organic residue from sharp edges of lids and aluminum shells, as well as scratches from packaging materials.
③ Contamination adhering to the weld seam during welding
For example, operators handling battery cores with dirty gloves contaminate the welded area.
(2) Causes of silver explosion points
This is primarily due to unstable dynamic behavior of the keyhole during laser deep penetration welding. If the process is at the upper or lower limits of the process window, external environmental changes can easily cause the small hole to become unstable, leading to hole rupture. The resulting spatter can cause insufficient penetration depth and外观不合格. This situation is typically accompanied by incomplete welding (partially unfused, insufficient penetration depth, insufficient melting, insufficient energy).
Turbulence in the protective gas, which typically occurs near corners, is often accompanied by changes in weld seam color (oxidation).
Laser welding head or axis vibration, causing keyhole instability.
Pre-weld spot welding position instability, sudden changes in melting volume.
Ring spot welding or composite welding, with an excessive ratio of core power to outer ring power, resulting in excessive penetration depth and keyhole instability.
Process parameter selection near the upper and lower limits of the process window, leading to keyhole instability.
Abnormal moisture content on the surface of incoming materials.
Lithium-ion battery laser welding defects - explosion point solutions
The key lies in experimental design verification to identify the cause and then address it specifically.
(1) Ensure consistency in the gap between weld surfaces, with the gap kept small (according to on-site and product-specific standards).
(2) No contaminants (artificially apply blue film, adhesive, etc., to verify the type of contaminant, its origin from a specific workstation, action, or fixture, and whether the process sequence is reasonable).
(3) Use a small force to strike the steel plate to check the swing condition.
(4) Unstable melt pool: Avoid selecting defocus amount, shielding gas flow rate, trajectory, energy, and inner/outer ring power ratio at the upper or lower boundaries of the process window to prevent the small hole from entering an unstable state and being affected by environmental fluctuations.
