Introduction
In today's rapidly evolving new energy vehicle industry, power batteries serve as the core component, and their manufacturing processes directly determine vehicle safety and driving range. Laser welding technology—thanks to its high efficiency, precision, and low heat input—has become the "golden process" in power battery production. The rational selection of welding methods and processes directly affects battery cost, quality, safety, and consistency. This article provides a comprehensive overview of laser welding for power batteries.
1. Principles of Laser Welding
Laser welding leverages the excellent directionality and high power density of a laser beam. Through an optical system, the beam is focused into a very small area, rapidly creating a highly concentrated heat source zone that melts the workpiece and forms strong weld points and seams.
2. Main Types of Laser Welding
1. Conduction Welding vs. Deep Penetration Welding
Conduction welding: The laser beam melts the workpiece surface along the joint; molten material flows together and solidifies to form the weld. Used mainly for relatively thin materials, where maximum weld depth is limited by thermal conductivity, and weld width always exceeds weld depth.
Deep penetration welding (keyhole welding): When high-power laser energy is focused on the metal surface, heat cannot dissipate quickly and weld depth increases sharply. This technique offers extremely fast processing, a very small heat-affected zone, and minimal distortion—suitable for deep welds or welding multiple layers simultaneously.
The key difference between the two is the power density applied per unit time on the metal surface, with different critical thresholds for different metals.
2. Through Welding vs. Seam Welding
Through welding: The connector tab requires no punching—simpler processing—but needs a higher-power laser welder. Weld depth is lower than seam welding, with relatively lower reliability.
Seam welding: Requires only a lower-power laser welder. Weld depth is higher than through welding, with better reliability—but the connector tab needs punching, making processing more difficult.
3. Pulsed vs. Continuous Wave Welding
Pulsed mode welding: Proper welding waveforms should be selected—common options include square wave, peak wave, and double-peak wave. Aluminum alloy surfaces have very high reflectivity, so when a high-intensity laser beam strikes the material surface, 60%-98% of laser energy is lost to reflection, and reflectivity varies with surface temperature. For aluminum welding, peak and double-peak waveforms are optimal—their longer-pulse-width trailing ramp-down portion effectively reduces porosity and cracking.
Because of aluminum's high reflectivity, the welding head is typically tilted at an angle to prevent vertical reflection from damaging the focusing lens. Weld spot diameter and effective bonding surface diameter increase with tilt angle, reaching maximum at 40°. Weld penetration and effective penetration decrease as tilt angle increases, dropping to zero above 60°. Tilting the welding head appropriately can increase both weld depth and width. Additionally, the laser spot should be positioned with 65% on the cover plate and 35% on the shell to reduce flash caused by cover-fit issues.
Continuous mode welding: Continuous laser welding doesn't experience the rapid heating/cooling of pulsed machines, so cracking tendency is less pronounced. Continuous lasers produce smooth, uniform welds with no spatter and no internal cracks. In aluminum welding, continuous lasers offer clear advantages: higher productivity than traditional methods with no filler wire required; they solve post-weld defects seen in pulsed welding (cracks, porosity, spatter), ensuring good mechanical properties; no post-weld depression, reduced polishing and grinding, and lower production costs. However, because continuous laser spot sizes are smaller, workpiece assembly precision requirements are higher.
In power battery welding, technicians select appropriate lasers and welding parameters—welding speed, waveform, peak value, head tilt angle—based on the customer's battery material, shape, thickness, and tensile requirements.
3. Advantages of Laser Welding
Concentrated energy, high efficiency, high precision, high depth-to-width ratio.
Flexibility: Laser beams are easy to focus, align, and guide via optical instruments—can be positioned at a suitable distance from the workpiece and guided between fixtures or obstacles, whereas other welding methods face spatial constraints.
Low heat input, small heat-affected zone, minimal residual stress and deformation.
Precisely controllable energy, stable results, good weld appearance.
Non-contact welding with fiber transmission—good accessibility and high automation.
No re-melting issues when welding thin materials or fine wires, unlike arc welding.
Power battery cells, following the "lightweight" principle, typically use lighter aluminum materials and must be even thinner—shell, cover, and bottom generally require thickness below 1.0mm, with mainstream manufacturers at around 0.8mm. Laser welding provides high-strength welds for various material combinations, particularly effective for copper-to-copper and aluminum-to-aluminum welding—it is also the only technology that can weld nickel-plated material to copper.
4. Why Laser Welding Is the "Must-Have" for Battery Manufacturing
1. Precision requirements
Battery shells are typically 0.6-0.8mm thick (mainly aluminum alloy). Traditional welding can cause deformation or burn-through, while laser spot diameters can be controlled at micron level for precision welding of ultra-thin materials.
2. Material compatibility
Can weld highly reflective materials (copper, aluminum), solving the challenge of negative electrode copper adapter welding (reflectivity over 90%), and enables aluminum-nickel dissimilar metal connections.
3. Zero contact and automation
Non-contact welding combined with robotic systems adapts to complex 3D battery module structures, improving yield by over 30%.
5. Process Challenges in Laser Welding
Over 90% of power battery shells use aluminum alloy. The welding challenges include aluminum's extremely high laser reflectivity and high porosity sensitivity. Key defects are porosity, hot cracking, and flash (spatter).
1. Porosity
Two main types occur: hydrogen porosity and bubble-collapse porosity. Because laser welding cooling rates are extremely fast, hydrogen porosity is more severe, and a third type—keyhole collapse voids—also appears.
2. Hot Cracking
Aluminum alloy is a typical eutectic alloy, prone to hot cracking including weld solidification cracking and HAZ liquation cracking. Composition segregation in the weld zone causes eutectic segregation and grain boundary melting, forming liquation cracks under stress that reduce joint performance.
3. Flash (Spatter)
Many factors contribute: material cleanliness, purity, and inherent characteristics—but the decisive factor is laser stability. Shell surface protrusions, porosity, and internal bubbles are causes. The root cause is typically an overly small fiber core diameter or excessively high laser energy settings.
It's important to note: it's not—as some equipment vendors claim—that "better beam quality means better welding." Good beam quality suits deep overlay welding. Finding the right process parameters is the key to solving this problem.
4. Other Challenges
Pouch cell tab welding: High fixturing requirements—tabs must be firmly pressed to ensure proper weld gap. High-speed S-shape, spiral, and complex trajectory welding increases weld bonding area and strength.
Cylindrical cell welding: Mainly for the positive terminal. Since the negative electrode shell is thin, burn-through is easy. Some manufacturers now use weld-free negative electrode processes with laser welding for the positive.
Prismatic cell assembly welding: Contaminated posts or connectors decompose during welding, forming weld flash points and holes. Thin posts with plastic or ceramic components below are prone to burn-through. Smaller posts can also cause weld offset to plastic, creating burst points. Avoid multi-layer connectors—gaps between layers prevent solid welding.
The most important prismatic cell process is shell-cover sealing, divided by position into top cover and bottom cover welding. Some manufacturers use "deep drawing" to produce shells, requiring only top cover welding.
Prismatic battery welding divides into side welding and top welding:
Side welding: The main benefit is minimal impact on the cell interior—spatter doesn't easily enter the shell interior. Since welding may cause protrusion, slightly affecting subsequent assembly, side welding demands extremely high laser stability and material cleanliness.
Top welding: Welding on one plane lowers equipment integration requirements and simplifies mass production, but has two drawbacks: possible spatter entering the cell, and higher cost due to demanding shell front-end processing.
6. Six Core Welding Applications
1. Explosion-proof valve welding: The "gatekeeper" of safety
The explosion-proof valve is a critical pressure-relief structure. Laser must create a sealed weld on an 8mm aluminum disc withstanding 0.4-0.7MPa burst pressure. Since continuous lasers replaced pulsed welding, sealing improved 50%, eliminating electrolyte leakage risks.
2. Shell and cover sealing: The battery's "vacuum lock"
Uses side welding or top welding. Side welding keeps spatter out but demands extreme material cleanliness; top welding offers higher throughput but requires spatter control. New blue-light hybrid lasers effectively reduce aluminum porosity, pushing yield above 95%.
3. Terminal post welding: The "bridge" of current paths
Positive (aluminum) and negative (copper) posts must withstand ≥500MPa tensile strength. Pain point: residual contaminants cause "burst holes" (the 6mm mating surface easily retains oil). Solved via pre-weld plasma cleaning + power ramp control.
4. Adapter welding: The "neural network" of module connections
Copper-aluminum dissimilar welding easily generates brittle compounds. Laser-ultrasonic hybrid processing now prevents intermetallic compound formation and conductivity loss.
5. 4680 large cylindrical full-tab welding: New challenges, new solutions
Full-tab structures increase weld area 5-fold, but tab folding can misalign and short-circuit. Beam shaping (ring spot) enables simultaneous multi-tab welding, reducing heat input by 40%.
6. PACK module welding: The "final link" of system integration
When copper/aluminum connectors reach 2mm thickness, 6kW+ high-power fiber lasers are needed for penetration welding, with equipment investment of roughly 10-30 million RMB per GWh.
7. Factors Affecting Welding Quality
Laser welding raises workpiece temperature sharply, melting and reconnecting material into a permanent joint. Laser welds offer good shear and tear strength. Battery weld quality is evaluated by conductivity, strength, airtightness, metal fatigue, and corrosion resistance.
Main factors fall into three categories: welding equipment, workpiece condition, and process parameters.
1. Welding Equipment
The most critical requirements for the laser are beam mode and output power stability. Beam mode is the primary beam quality indicator—lower order means better focusing, smaller spot, higher power density at the same laser power, and greater weld depth-to-width ratio. TEM00 fundamental mode or low-order mode is generally required.
In the optical system, the focusing lens matters most—focal lengths typically range from 127mm (5in) to 200mm (7.9in). Shorter focal lengths help reduce the focused beam waist diameter but risk contamination and spatter damage.
Shorter wavelengths yield higher absorption. Materials with good conductivity generally have high reflectivity—for YAG lasers: silver 96%, aluminum 92%, copper 90%, iron 60%. Higher temperature means higher absorption (linear relationship). Surface coatings like phosphate, carbon black, or graphite improve absorption.
2. Workpiece Condition
Laser welding requires machined workpiece edges, high assembly precision, strictly centered spot and weld joint, with assembly accuracy and centering maintained against thermal deformation. Because the laser spot is small, the weld narrow, and no filler metal is generally added, excessive gaps let the beam pass through without melting the base material, or cause undercut and depression; slight centering deviation can cause lack of fusion or penetration.
Generally, butt joint assembly gaps and spot joint deviation should not exceed 0.1mm; misalignment should not exceed 0.2mm. In practice, failure to meet these requirements sometimes makes laser welding unfeasible. For good results, butt and lap gaps should be within 10% of thin plate thickness. Successful laser welding requires tight contact between base materials, requiring careful part fastening—hard to achieve on thin tab substrates, which bend easily, especially when tabs are embedded in large battery modules.
3. Welding Parameters
(1) Power density and welding mode
The most critical parameter is laser spot power density. As power density increases, welding transitions from stable conduction welding → unstable mode welding → stable deep penetration welding.
Power density is determined mainly by laser power and beam focus position (given fixed beam mode and focal length). Power density is proportional to laser power. Focus position has an optimal value: when the beam focus is 1-2mm below the workpiece surface (depending on plate thickness and parameters), the ideal weld is achieved. Deviation increases surface spot size, reduces power density, and eventually changes the welding mode.
Welding speed affects process form and stability less than laser power and focus position—only when speed is too high does insufficient heat input prevent stable deep penetration welding. In practice, select stable deep penetration or stable conduction welding based on penetration requirements, absolutely avoiding unstable mode welding.
(2) Effect of parameters on penetration in deep penetration welding
Within the stable deep penetration range, higher laser power means greater penetration (approximately 0.7 power relationship); higher welding speed means shallower penetration. At a given power and speed, the optimal focus position gives maximum penetration—deviation reduces penetration or shifts to unstable mode or stable conduction welding.
(3) Shielding gas effects
Shielding gas protects the workpiece from oxidation; protects the focusing lens from metal vapor contamination and molten droplet spatter; disperses plasma generated by high-power laser welding; and cools the workpiece, reducing the heat-affected zone.
Argon or helium is typically used; nitrogen is acceptable where appearance requirements are lower. They differ significantly in plasma generation tendency: helium, with higher ionization potential and faster heat conduction, generates less plasma than argon under the same conditions, enabling greater penetration. Within a range, increasing shielding gas flow increases plasma suppression and penetration, stabilizing at a certain point.
(4) Parameter controllability analysis
Among the four welding parameters, welding speed and shielding gas flow are easy to monitor and keep stable; laser power and focus position are prone to fluctuation and hard to monitor. Although laser output power stability is high and easy to monitor, transmission and focusing system losses change the power reaching the workpiece—losses related to optical component quality, service time, and surface contamination—making monitoring difficult and introducing uncertainty.
Beam focus position is the parameter with the greatest impact on welding quality and the hardest to monitor and control. Production currently relies on manual adjustment and repeated process trials to determine the appropriate focus position. However, workpiece deformation, thermal lens effects, or multi-dimensional spatial curve welding can shift focus position beyond allowed ranges.
For both cases: first, use high-quality, high-stability optical components with frequent maintenance to prevent contamination; second, develop real-time laser welding monitoring and control methods—optimizing parameters by tracking power and focus position changes reaching the workpiece, achieving closed-loop control, and improving welding quality reliability and stability.
Note that laser welding is a melting process—both substrates melt during welding. The process is fast, so total heat input is low. But because it's a melting process, dissimilar material welding may form brittle, high-resistance intermetallic compounds. Aluminum-copper combinations are particularly prone to this. These compounds have demonstrated negative impacts on short-term electrical performance and long-term mechanical performance of microelectronic lap joints. Their impact on long-term lithium battery performance remains uncertain.
8. Process Challenges and Breakthrough Solutions
1. Porosity and cracking
Causes: dramatic hydrogen solubility changes in aluminum melt pools, and rapid cooling.
Solutions: double-peak pulsed waveforms (longer ramp-down pulse width reduces porosity); welding head tilted 40° to increase weld width and reduce cracking sensitivity.
2. Spatter (flash)
Root causes: material contamination or excessive energy density.
Innovative processes: spot offset (65% cover + 35% shell); hybrid welding (laser + arc) to lower peak temperature.
3. High-reflectivity material control
For copper welding, infrared-green hybrid lasers raise absorption from 30% to 80%, reducing burst points.
9. Future Trends: Intelligence and New Material Adaptation
Real-time quality monitoring: Plasma spectroscopy analysis + AI defect recognition for online detection of porosity and lack-of-fusion defects, intercepting non-conforming products.
Solid-state battery welding challenges: Sulfide electrolytes are heat-sensitive, making ultrashort picosecond lasers (heat-affected zone <10μm) the new direction.
Accelerating equipment localization: Companies like Han's Laser and United Winners Laser have broken through 4kW fiber laser technology, cutting costs by 30%.
Conclusion
Laser welding has become the "golden process" in power battery manufacturing—covering every critical joint from explosion-proof valves, shell covers, terminal posts, and adapters to 4680 full-tab and PACK module welding. Yet challenges like porosity, hot cracking, and spatter still require precise parameter control and material adaptation. In the future, with real-time quality monitoring, picosecond lasers, and domestically produced high-power lasers, power battery laser welding will continue evolving toward greater intelligence, precision, and lower cost.
