Laser welding is currently the main method of high-end battery welding.

Laser welding is a process in which a high-energy beam laser irradiates the workpiece, so that the working temperature rises sharply, the workpiece is melted and reconnected to form a permanent connection, and the shear strength and tear strength of laser welding are relatively good. The quality of battery welding, its electrical conductivity, strength, air tightness, metal fatigue and corrosion resistance are typical evaluation criteria for welding quality.

There are many factors that affect the quality of laser welding, some of which are extremely volatile and have considerable instability. How to correctly set and control these parameters so that they can be controlled within a suitable range in the high-speed continuous laser welding process to ensure the reliability and stability of welding quality and weld formation is related to the practicality of laser welding technology, important issue of industrialization.

The main factors affecting the quality of laser welding are divided into three aspects: welding equipment, workpiece condition and process parameters.

1. Welding equipment

The most important requirements for the quality of the laser are the beam mode and output power and its stability. The beam mode is the main indicator of the beam quality. The lower the beam mode order, the better the beam focusing performance, the smaller the spot, the higher the power density under the same laser power, and the greater the depth and width of the weld. Generally, the fundamental mode or low-order mode is required, otherwise it is difficult to meet the requirements of high-quality laser welding. At present, the laser beam quality and output power stability of HGLASER are quite high, and will not become a problem in laser welding.

The most important factor affecting the welding quality in the optical system is the focusing lens. The focal length used is generally between 127mm and 200mm. A small focal length is good for reducing the diameter of the focused beam waist spot, but if it is too small, it is easy to be contaminated and splashed during the welding process. .

The shorter the wavelength, the higher the absorption rate; generally, materials with good conductivity have high reflectivity. For YAG lasers, the reflectivity of silver is 96%, aluminum is 92%, copper is 90%, and iron is 60%. The higher the temperature, the higher the absorption rate, which is linear; generally, coating the surface with phosphate, carbon black, graphite, etc. can improve the absorption rate.

2. Workpiece condition

Laser welding requires the edge of the workpiece to be processed, the assembly has high precision, the spot and the weld are strictly aligned, and the original assembly accuracy of the workpiece and the alignment of the spot cannot be changed during the welding process due to welding thermal deformation. This is because the laser spot is small and the welding seam is narrow. Generally, no filler metal is added. If the gap is too large if the assembly is not strict, the beam can pass through the gap and cannot melt the base metal, or cause obvious undercut and depression, such as the deviation of the spot to the seam. If it is too large, it may cause incomplete fusion or incomplete penetration.

Therefore, in general, the butt assembly gap of the plates and the deviation of the spot alignment should not be greater than 0.1mm, and the wrong side should not be greater than 0.2mm. In actual production, sometimes laser welding technology cannot be used because these requirements cannot be met. To obtain a good welding effect, the allowable butt gap and overlap gap should be controlled within 10% of the thickness of the sheet.

Successful laser welding requires close contact between the substrates being welded, which requires careful tightening of the parts for best results. This is difficult to do well on thin tab substrates, which are prone to bending misalignment, especially if the tabs are embedded in large battery modules or assemblies.

3. Welding parameters

(1) Influence on the laser welding mode and the welding seam forming stabilizer The most important welding parameter is the power density of the laser spot, which has the following influence on the welding mode and the welding seam forming stability: as the laser spot power density increases from small to large The order is stable thermal conduction welding, mode unstable welding and stable deep penetration welding.

(2) In the deep penetration welding range, the influence of welding parameters on the penetration depth: in the stable deep penetration welding range, the higher the laser power, the greater the penetration depth, which is about 0.7 power; and the higher the welding speed becomes , the shallower the penetration. Under certain laser power and welding speed, when the focus is in the best position, the penetration depth is the largest, and when it deviates from this position, the penetration depth decreases, and even becomes mode unstable welding or stable thermal conduction welding.

(3) Influence of shielding gas, the main function of shielding gas is to protect the workpiece from oxidation during the welding process; to protect the focusing lens from metal vapor contamination and sputtering of liquid droplets; to dissipate the plasma generated by high-power laser welding; to cool workpiece, reducing the heat-affected zone.

(4) Monitorability analysis of each parameter: Among the four welding parameters, welding speed and shielding gas flow are parameters that are easy to monitor and maintain stable, while laser power and focus position are the ones that may fluctuate during the welding process and are difficult to monitor. parameter.

Although the laser power output from the laser is highly stable and easy to monitor, the laser power reaching the workpiece will vary due to losses in the light guide and focusing systems, which are related to the quality, age, and surface contamination of the optical workpiece. The position of the beam focus is one of the welding parameters that has a great influence on the welding quality and is the most difficult to monitor and control.