Power battery laser welding quality influencing factors

Laser welding is currently the main method of high-end battery welding. Laser welding is the process of irradiating the workpiece with a high-energy beam of laser light, so that the working temperature rises sharply and the workpiece melts and reconnects to form a permanent connection. Laser welding has good shear and tear strength.battery pilot line The quality of battery welding is characterized by its electrical conductivity, strength, gas tightness, metal fatigue and corrosion resistance.

There are many factors that affect the quality of laser welding. Some are extremely volatile and very unstable. How to correctly set and control these parameters in the high-speed continuous laser welding control process to ensure the quality of the weld. The reliability and stability of weld shaping is an important issue related to the practicalization and industrialization of laser welding technology. The main factors affecting the quality of laser welding are welding equipment, workpiece condition and process parameters.

1. Welding equipment

The most important requirements for laser quality is the beam pattern, output power and its stability. Beam mode is the main indicator of beam quality. The lower the order of the beam mode, the better the beam focusing performance, the smaller the spot under the same laser power, the higher the power density, the greater the depth and width of the weld.laser welding machine for lithium ion batteries Generally require basic mode or low-order mode, otherwise it is difficult to meet the requirements of high-quality laser welding. At present, domestic lasers are still difficult to use for laser welding in terms of beam quality and power output stability. From the foreign situation, the laser beam quality and output power stability has been quite high, will not be a problem for laser welding. Optical system in the biggest factor affecting the quality of welding is the focusing mirror, the use of the focal length is generally between 127mm (5in) to 200mm (7.9in). Small focal lengths are good for reducing the waist spot diameter of the focused beam, but they are susceptible to contamination and spatter during welding.

The shorter the wavelength, the higher the effect of absorbance; materials with good electrical conductivity generally have a high reflectance, for YAG laser technology the reflectance is 96% for silver, 92% for aluminum, 90% for copper and 60% for iron.ESS lithium battery machine The higher the temperature level, the higher the absorption rate, a linear correlation; generally through the surface coating phosphate, carbon black, graphite, etc. can effectively improve the classroom absorption rate.

2.Workpiece condition

Laser welding on the edge of the workpiece processing, assembly precision requirements are very high, strict alignment of the weld point and weld seam, and welding thermal deformation in the welding process can not change the original assembly precision and point alignment of the workpiece. This is because the laser spot is small, the weld seam is narrower, generally not filled with metal, such as assembly gap is not strictly too large, the beam can pass through the gap can not melt the base material, or cause obvious biting edges, depressions, if the spot on the weld seam deviation is slightly larger may result in non-fusion or non-fusion. Therefore, the assembly gap and spot to the general plate deviation should not be greater than 0.1 mm, the wrong side should not be greater than 0.2 mm. In actual production, sometimes these requirements can not be met, and laser welding technology can not be used. In order to obtain good welding results, the allowable butt gap and lap gap should be controlled within 10% of the thickness of the sheet.

Successful laser welding requires close contact between the weld substrates. This requires careful tightening of the parts for best results. This is difficult to do well on thin jointed substrates because it is easy to bend out of alignment, especially when the joint is embedded in a large cell module or component.

3. Welding Parameters

First, the laser spot power density is the most important factor affecting the laser welding mode and weld seam forming stabilizer welding parameters, which has the following effects on the welding mode and weld seam forming stability: As the laser spot power density gradually increases from small to large, the stable heat conduction welding sequence of stable thermal conductivity welding, mode instability welding and stable deep melt welding.

When the beam pattern and the focal length of the focusing mirror are fixed, the power density of the laser spot is mainly determined by the laser power and the focal position of the beam. The laser power density is proportional to the laser power. There exists an optimum value for the effect of the focal position; the most optimal weld seam is obtained when the beam focal point is at a certain position below the surface of the workpiece. Deviation from this optimal focus position, the spot on the surface of the workpiece becomes larger, resulting in a decrease in power density, to a certain extent, will cause changes in the form of welding process.

Welding speed on the main form of welding work process and stabilize the influence of the piece itself is not as laser power and focal position as a significant, only through the welding speed is too large, due to the heat input is too small and the problem of not being able to maintain the stability of the deep fusion welding process. The actual welding, should be based on the requirements of the weldment on the depth of fusion we choose to stabilize the deep fusion welding or stabilize the heat conduction welding, and to be absolutely can avoid the management mode is not stable welding.

Second, in the deep melt welding range, welding parameters on the depth of fusion: in the stable deep melt welding range, the higher the laser power, the greater the depth of penetration, the correlation coefficient is about 0.7, the faster the welding speed, the shallower the depth of penetration. In a certain laser power and welding speed conditions, when the focus is in the optimal position, the maximum depth of penetration, if the focus deviates from this position, the depth of penetration will be reduced, or even into the mode of unstable welding or stable heat conduction welding.

Third, the influence of shielding gas, the main role of the shielding gas is to protect the workpiece from oxidation in the welding process; to protect the focusing lens from metal vapor contamination and droplet sputtering; to eliminate the plasma generated by high-power laser welding; cooling the workpiece to reduce the heat-affected zone.

The shielding gases are usually argon or helium, or nitrogen if the apparent quality is not required. Their tendency to produce plasma is significantly different: under the same conditions, helium has a lower tendency to produce plasma than argon because of its high ionization, fast heat conduction and greater penetration depth. Within a certain range, the tendency to inhibit plasma increases as the protective gas flow increases, and therefore the penetration depth increases, but stabilizes when it reaches a certain range.

Fourth, the monitoring and analysis of each parameter:Among the four welding parameters, the welding speed and shielding gas flow rate are easy to monitor and keep stable, while the laser power and focus position are difficult to monitor due to possible fluctuations in the welding process. Although the laser power output from the laser is stable and easy to monitor, the laser power reaching the workpiece will change due to the loss of the light guide and focusing system, which is related to the quality of the optical workpiece, the use of time and surface contamination, and is therefore difficult to monitor and becomes an uncertainty factor for welding quality. Beam focus position is one of the welding parameters that has the greatest impact on welding quality and is the most difficult to monitor. Currently in production, it is necessary to determine the appropriate focus position through manual adjustment and repeated process tests to obtain the ideal penetration depth. However, during the welding process, due to workpiece deformation, thermal lens effect or multi-dimensional welding of space curves, the focal point position will change and may exceed the allowable range.

For the analysis of the two different situations through the above, on the one hand, we must use high-quality, high-stability optical components, and often maintenance, to prevent environmental pollution, keep clean; another important aspect of the laser welding work process can be required to real-time monitoring and control of the research method, in order to optimize the design parameters, monitoring to the workpiece to reach the changes in the laser power and the focus position, to achieve a closed-loop management and control, to improve the laser welding quality of the Reliability and stability of laser welding quality.

Finally, it should be noted that laser welding is a melting process. This means that the two substrates will melt during the laser welding process. This process is fast, so the overall heat input is low. However, because it is a melting process, brittle, high-resistance intermetallic compounds may form when welding different materials. Aluminum-copper combinations are particularly susceptible to the formation of intermetallic compounds. These compounds have been shown to negatively affect the short-term electrical and long-term mechanical properties of lap joints in microelectronic devices. The effect of these intermetallic compounds on the long-term performance of lithium-ion batteries is uncertain.

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