(1) Overview of laser welding.
The connection between the battery cell and the battery cell generally adopts the method of connecting piece welding. In the module manufacturing process, the soft package cell and the square cell module are generally laser welded. The specific situation may be different for each company. The following is an introduction to the laser welding process used in the industry for relatively packaged cells and square cells.
The three elements of laser production: excitation source, medium, and resonant cavity. The medium is excited to a high-energy state, and the excited absorption transition light is reflected back and forth between the mirrors at both ends, which amplifies the light wave and obtains sufficient energy to start emitting laser light. The four distinctive characteristics of laser: monochromaticity, coherence, directionality, and high brightness, so a highly concentrated laser can provide functions such as welding, cutting and heat treatment.
Laser welding belongs to fusion welding, which uses laser beam as energy source to impinge on the weldment joints. The laser beam can be guided by a flat optical element (such as a mirror), and then a reflective focusing element or lens is used to project the beam on the weld. Laser welding is a non-contact welding. No pressure is required during the operation, but inert gas is required to prevent oxidation of the molten pool.
(2) Laser welding parameters.
In actual production and use, laser welding must have optimized process parameters to achieve stable and reliable welding results. The main parameters to be considered are:
①Laser power. Increasing the laser output power can increase the welding speed, increase the weld width and penetration.
②Welding speed. The welding speed has a great influence on the penetration depth. Increasing the speed will make the penetration shallower, but too slow speed will lead to excessive melting of the material and welding penetration of the workpiece.
③The focal length of the lens. The focal spot size is proportional to the focal length, the shorter the focal length, the smaller the spot.
④ Defocus amount. The amount of defocus is directly related to the power density of the laser acting on the workpiece. The focal plane above the workpiece is positive defocus, and vice versa.
⑤Beam focal spot. The size of the spot determines the energy density to a certain extent.
⑥ Protective gas. The laser welding process often uses inert gas to protect the molten pool. Its functions are: protect the molten pool from oxidation: protect the focusing lens from metal vapor pollution and liquid droplet sputtering; it is very effective to dissipate the plasma shielding produced by high-power laser welding .
⑦ Material absorption value. The absorption rate of the material is related to its own characteristics, the absorption rate is low, and the welding effect is poor.
(3) Laser welding method.
In order to obtain a stable and reliable welding effect, laser welding must not only consider the process parameters, but also need welding tooling to ensure good contact between the welding surfaces, and also consider the selection of welding materials. Generally, the welding surface should be cleaned to remove the oxide layer. If there is a little control that is not good, the welding effect will be uncontrollable.
(4) Precautions for laser welding.
To obtain reliable and stable welding results, it is necessary to control all aspects of production, from the aspects of materials, process parameters, tooling and fixtures, staff training, etc., to refine and implement. Only when the details are well controlled can it be well controlled. The following highlights some issues that need to be paid attention to:
① Welding parameters. Needless to say, the importance of welding parameters is unnecessary. To optimize the parameters, only through experiments, it is best to use the DOE method to find the best welding parameters.
② Welding fixture. When laser welding, the contact surfaces between the welded workpieces need to be tightly joined together, and there should be no gaps, so as to ensure the welding effect. At the same time, we need to pay attention to the prevention of welding slag. Do not let the welding slag splash into the module, otherwise there will be endless troubles.
③ Welding material. Do not change the material model or even the batch number that has been validated by the process. If it must be changed, it is recommended to verify before the change, and then change after the verification is passed. In addition, because aluminum material is easily oxidized in the air, if it is aluminum material, it is recommended to do passivation treatment in advance to prevent the aluminum material from being oxidized and affecting laser welding.
④Clean. Before the module is welded, the welding surface is generally cleaned to remove surface stains; in addition, when the aluminum pole is laser cleaned, the oxide layer on the welding surface needs to be removed to improve the welding effect and yield.