The reasonable selection of manufacturing process, welding method and process of new energy power lithium battery directly affects the cost, quality, safety and battery consistency of the battery. Next, the phase of power battery welding will be introduced
1Principle of laser welding
Laser welding uses the excellent directivity, high power, high density and other characteristics of the laser beam to concentrate the laser beam in a small area through optical path shaping, and form a heat source area with high energy concentration in the welded part in a very short time to melt the welded object, so as to produce a solid welding
2Laser welding type
Heat conduction welding
The laser power density is 105 ~ 106W / cm2 to form laser heat conduction welding, and the laser power density is 105 ~ 106W / cm2 to form laser deep penetration welding.
Penetration welding method and seam welding method
Through welding, the connecting piece does not need drilling, and the processing steps are relatively simple. The penetration depth of penetration welding is shallower than that of seam welding, and the reliability is relatively reduced.
Penetration welding requires a high-power laser welding machine.
Seam welding requires only a low-power laser welding machine. The penetration depth of seam welding is higher than that of penetration welding, and the reliability is relatively better. However, the connecting piece needs to be perforated, and the processing difficulty is relatively complex.
Pulse welding and continuous welding
1)Pulse mode welding
Proper welding waveform shall be selected during laser welding. Common pulse waveforms include square wave, spike wave, bimodal wave, etc. The reflectivity of aluminum alloy surface to light is too high. After the high-intensity laser beam reaches the material surface, 60% – 98% of the laser energy on the metal surface will be lost due to reflection, and the reflectivity will change with the change of surface temperature. Generally speaking, sharp wave and bimodal wave are the best choice when welding aluminum alloy. The pulse width behind the welding waveform is relatively long, which can effectively reduce pores and cracks.
Pulsed laser welding sample
The reflectivity of aluminum alloy to laser is very high. In order to prevent the laser beam from damaging the laser focusing lens after vertical incidence, the welding head will be offset by an angle during welding. The diameter of solder joint and effective joint surface increase with the increase of laser inclination angle. When the laser inclination angle is 40 degrees, the maximum solder joint and effective joint surface are obtained. The weld penetration and effective penetration decrease with the laser inclination angle. When it is greater than 60 degrees, the effective weld penetration decreases to 0. Therefore, tilting the welding head at a specific angle can appropriately increase the welding penetration and width.
In addition, when welding, take the weld as the boundary, and weld the laser welding spots to 65% of the cover and 35% of the shell, which can effectively reduce the explosion caused by the closure problem.
2)Continuous mode welding
The process of continuous laser welding is different from that of pulse laser, and there will be no sudden cold and heat, so the crack tendency is not obvious during welding. In order to improve the welding quality, continuous laser welding is adopted. The welding surface is smooth and uniform without splash and defects. No cracks are found in the welding. In aluminum alloy welding, the advantages of continuous laser welding are obviously higher than the production efficiency of traditional welding methods. Compared with pulsed laser welding, it can solve the defects such as cracks, pores and spatter after welding and ensure the mechanical properties of aluminum alloy after welding. There is no subsidence after welding, and the polishing amount after welding is reduced, which can reduce the production cost. Due to the small spot of the continuous laser, the assembly accuracy of the workpiece is required to be high.
Continuous laser welding sample
In the welding of new energy power battery, the welding process technicians select the appropriate laser and welding process parameters according to the customer’s battery material, shape, thickness and stress requirements. Including reasonable setting of parameters such as welding speed, waveform, peak value and inclination angle of welding joint, so as to ensure that the final welding effect meets the requirements of new energy power battery manufacturers.
3Advantages of laser welding
Energy concentration, welding efficiency, high machining precision and large welding depth. The laser beam can easily focus and aim the optical instrument, place it at an appropriate distance from the workpiece, and can be re guided between fixtures or obstacles around the workpiece, which cannot be achieved by other welding methods.
The heat input is small, the heat affected zone is small, and the residual stress and deformation of the workpiece are small. The welding energy can be accurately controlled, the welding effect is stable and the welding quality is good.
Non contact welding, transmission through optical fiber, good accessibility and high degree of automation. When welding thin materials or thin diameter wires, there is no problem of remelting like arc welding. The power battery follows the principle of “portability”, so generally, in addition to using lighter aluminum materials, it is also required to be thin. Generally, the basic requirements of shell, cover and bottom plate are less than 1.0 mm, and the thickness of aluminum materials currently used by mainstream power battery manufacturers is about 0.8 mm.
Easily realize high-strength welding of various materials. In particular, the welding between copper materials and aluminum materials is more effective. This is also the only technology for spot nickel welding to copper materials.
4Laser welding process difficulties
At present, the aluminum alloy battery shell accounts for more than 90% of the aluminum material of the whole power battery. The difficulty of welding is that the reflectivity of aluminum alloy to laser is very high, and the porosity sensitivity is very high during welding. Some problems will inevitably occur during welding. The most important ones are pores, thermal cracks and explosion.
Aluminum alloy is easy to produce pores in the process of laser welding, mainly hydrogen pores and pores caused by bubble rupture. Because the cooling speed of laser welding is too fast, the problem of hydrogen pores becomes more serious. There are more holes caused by hole collapse in laser welding.
Hot crack problem. Aluminum alloy is a typical eutectic alloy, which is easy to produce thermal cracks during welding. Including weld crystallization crack and HAZ liquefaction crack. The composition segregation of the welding zone leads to eutectic separation and grain boundary melting. Under the action of stress, the grain boundary forms liquefied cracks, which reduces the performance of the welded joint.
Splash problem. There are many factors causing splash, such as the cleanliness of the material, the purity of the material itself, the characteristics of the material itself, etc. the stability of the laser plays a decisive role. The surface of the shell is uneven, with air holes and internal bubbles. The main reason is that the fiber core diameter is too small or the laser energy setting is too high. It is not the “better beam quality, better welding effect” advocated by some laser equipment suppliers. Good beam quality is suitable for overlay welding with large penetration. Using appropriate process parameters is the best choice to solve the welding problem.
For welding of soft package pole lug, the requirements for welding process are high. The pole lug must be pressed to ensure the welding gap. High speed welding with complex tracks such as S-shape and spiral shape can be used to increase the welding joint area and improve the welding strength at the same time.
The welding of cylindrical core is mainly used for the welding of anode (positive pole). The shell of cathode (negative pole) is thin and easy to be welded through. At present, some laser R & D manufacturers adopt the scheme of welding only the positive electrode and not the negative electrode.
During the combined welding of square battery, after the pole column or connecting piece is polluted, the pollutants decompose when welding the connecting piece, which is easy to produce welding explosion points and holes. The battery with thin pole and plastic or ceramic structural parts under it is easy to weld. When the pole is small, it will be welded to the plastic and burned, which is easy to form explosion point. It is recommended not to use multi-layer connecting pieces, because there are gaps between layers, which is difficult to weld firmly.
The most important step in the welding process of square battery is the packaging of shell cover, which is divided into the welding of top cover and bottom cover according to the position. Some battery manufacturers produce small batteries. The battery shell can be made by “deep drawing” process, and only the top cover needs to be welded.
Side welding sample of square power battery
The welding methods of square battery are mainly divided into side welding and top welding. The main advantage of side welding is that it has little impact on the interior of the cell, and it is difficult for spatter to enter the interior of the shell cover. As the bulge may be caused after welding, which will have a certain impact on the subsequent assembly process, the side welding process has very high requirements for the stability of the laser and the cleanliness of the material surface. The top welding process only welds one surface, which has low requirements for welding equipment integration and simple mass production, but it has two disadvantages. One is that spatter will enter the cell during welding, and the other is that the high processing requirements of the shell will increase the cost.
5 factors affecting welding quality
Laser welding is the main method of high-end battery welding. Laser welding is a process of high-energy beam laser irradiating the workpiece. It is a process in which the working temperature rises sharply, the workpiece melts and reconnects to form a permanent connection. The shear strength and tear strength of laser welding are very good. The quality of battery welding is a typical welding quality evaluation standard, including conductivity, strength, air tightness, metal fatigue and corrosion resistance.
There are many factors affecting the quality of laser welding. Some of them are easy to fluctuate and have considerable instability. How to correctly set and control these parameters to ensure the welding quality in high-speed continuous laser welding, and the reliability and stability of welding forming are important issues related to the practicability and industrialization of laser welding technology. The main factors affecting the quality of laser welding are divided into three aspects: welding equipment, workpiece state and process parameters.
1) Welding equipment
The most important quality requirements of laser are beam mode, output power and stability. Beam mode is the main index of 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 welding depth. Basic mode (TEM00) or low order die is usually required. Otherwise, it is difficult to meet the requirements of high-quality laser welding. At present, it is difficult for domestic lasers to be used in laser welding in terms of beam quality and power output stability. In foreign countries, the beam quality and output power stability of laser are quite high, which will not become a problem of laser welding. The focal lens is the most important factor affecting the welding quality in the optical system. The common focal length is between 127 mm (5 inches) and 200 mm (7.9 inches). A small focal length helps to reduce the waist spot diameter of the focused beam, but if it is too small, it is vulnerable to pollution and splash damage during welding.
The shorter the wavelength, the higher the absorptivity. Generally, it is a material with good conductivity and high reflectivity. For YAG laser, the reflectivity is 96% for silver, 92% for aluminum, 90% for copper and 60% for iron. The higher the temperature, the higher the absorption rate, which has a linear relationship. Coating phosphate, carbon black and graphite on the common surface can improve the absorption rate.
2) Workpiece status
Laser welding needs to process the edge of the workpiece and assemble it with high precision. The spot and weld are strictly matched. The original component accuracy and spot pair of the workpiece cannot be changed according to the welding thermal deformation during the welding process. The reason is that the laser spot is small, the weld is narrow, and there is usually no filler metal. For example, because the assembly is not strict and the spacing is too large, the beam cannot melt the masterbatch through the gap, or the deviation between the light spot and the joint is slightly large, it cannot be fused or welded. Therefore, the butt installation gap and spot butt joint deviation of ordinary seats shall not be greater than 0.1mm, and the wrong edge shall not be greater than 0.2mm. In actual production, sometimes these requirements can not be met, and laser welding technology can not be used. In order to obtain good welding effect, the allowable butt gap and overlapping gap must be controlled within 10% of the plate thickness.
Successful laser welding requires close contact between welding substrates. To do this, the parts must be carefully secured for best results. It is difficult to do well on thin polar ear equipment. In particular, if the pole lug is embedded in a large battery module or assembly, it is easy to bend.
3) Welding parameters
(1) The most important of the welding parameters is the power density of the laser spot, which will affect the welding mode and welding forming stability. With the increase of laser spot power density in small scale, they are stable heat conduction welding, mode unstable welding and stable penetration welding.
The power density of laser spot is mainly determined by the laser power and beam focus position when the beam mode and focal length of focus lens are constant. The laser power density is directly proportional to the laser power. The influence of focus position has the best value. When the beam focus is located at a specific position below the workpiece surface (in the range of 1 ~ 2mm according to the plate thickness and parameters), the best welding can be obtained. After deviating from this optimal focus position, the spot on the workpiece surface increases, the power density decreases, and the welding process shape changes to a certain range.
The influence of welding speed on the form and stability of welding process is less than that of laser power and focus position. Only when the welding speed is too high, the heat input is too small to maintain a stable deep welding process. During actual welding, stable pass through or stable heat conduction welding shall be selected according to the melting requirements of weldments, and unstable mode welding shall be absolutely avoided.
(2) In the range of deep fusion welding, the influence of welding parameters on fusion: in the stable fusion welding range, the higher the laser power, the greater the fusion, and the relationship is about 0.7 square. The higher the welding speed, the shallower the melt. Under the conditions of specific laser power and welding speed, when the focus is in the optimal position, the melt is the largest. When leaving this position, the melt decreases, the mode is unstable or the heat conduction welding may be stable.
(3) The main function of gas protection is to protect the workpiece from oxidation during welding. Protect the focus lens from metal vapor pollution and droplet sputtering. Disperse the plasma produced by high-power laser welding. Cool the workpiece and reduce the heat affected area.
Helium or helium is usually used as the shielding gas, and nitrogen can also be used if the surface quality requirements are not high. The tendency to produce plasma is very different. Helium has high ionization electricity and fast heat conduction, so it has less tendency to produce plasma under the same conditions than argon, so it can get greater dissolution. With the increasing tendency of shielding gas flow in a certain range, the tendency of inhibiting plasma increases and the dissolution increases, but it tends to calm after increasing to a certain range.
(4) Monitoring analysis of parameters: among the four welding parameters, welding speed and shielding gas flow are easy to monitor and stable parameters. Laser power and focus position may fluctuate in the welding process, which are difficult to monitor. The laser power output by the laser is stable and easy to monitor, but the laser power reaching the workpiece will change due to the loss of transmitted light and focus system. This loss is related to the quality, service time and surface pollution of optical workpiece, so it is not easy to monitor and become the uncertainty of welding quality. The beam focus position has a great influence on the welding quality in the welding parameters, which is the most difficult factor to monitor and control. In the current production, it is necessary to determine the appropriate focus position through manual adjustment and repeated process tests in order to obtain the ideal dissolution. However, due to workpiece deformation, thermal lens effect or multidimensional welding of spatial curve in the welding process, the focus position may change beyond the allowable range.
In both cases, high-quality and stable optical elements should be used and maintained frequently to prevent pollution and keep clean. On the contrary, it is necessary to develop real-time monitoring and control methods for laser welding process, optimize parameters, monitor the changes of laser power and focus position reaching the workpiece, realize closed-loop control and improve the reliability and stability of laser welding quality.
Finally, please note that laser welding is a melting process. That is, during laser welding, the two substrates will melt. This process is fast because the total heat input is very low. However, since this is a melting process, fragile high resistance intermetallic compounds can be formed when welding other materials. The combination of aluminum and copper is particularly easy to form intermetallic compounds. These compounds have proved to have a negative impact on the short-term electrical and long-term mechanical properties of microelectronic equipment connectors. The effect of these intermetallic compounds on the long-term performance of lithium batteries has not been determined.