The reasonable selection of welding methods and processes during the manufacturing process of power lithium batteries will directly affect the battery's cost, quality, safety, and consistency.
1. Laser Welding Principle
Fiber Laser welding machine uses the laser beam's excellent directivity and high power density to work. The laser beam is focused on a small area through the optical system, quickly forming a highly concentrated heat source in the welded area. area, so that the object to be welded melts and forms a strong welding point and welding seam.
Fiber Laser welding machine
2. Laser Welding Type
Thermal Conduction Welding and Deep Penetration Welding
The laser power density is 105~106w/㎝² to form laser heat conduction welding, and the laser power density is 105~106w/㎝² to form laser deep penetration welding.
Penetration and Seam Welding
Penetration welding: the connecting piece does not need to be punched, and the processing is relatively simple. Penetration welding requires a more powerful laser welder. The penetration depth of penetration welding is lower than that of seam welding, and its reliability is relatively poor.
Compared with penetration welding, seam welding requires a smaller power laser welder. The penetration depth of seam welding is higher than that of penetration welding, and its reliability is relatively good. However, the connecting piece must be punched, which is relatively difficult to process.
Pulse Welding and Continuous Welding
1) Pulse mode welding
Appropriate welding waveforms should be selected during laser welding. Commonly used pulse waveforms include square, peak, double-peak waves, etc. The reflectivity of the aluminum alloy surface to light is too high. When a high-intensity laser beam hits the material surface, the metal surface will lose 60%-98% of laser energy due to reflection, and the reflectivity changes with surface temperature. Generally, it is best to choose pointed and double-peaked waves when welding aluminum alloys. The pulse width of the slow-down part behind this kind of welding waveform is longer, which can effectively reduce the occurrence of pores and cracks.
Pulse laser welding samples
Due to the high reflectivity of aluminum alloy to laser, to prevent the laser beam from being vertically incident and causing vertical reflection and damaging the laser focusing mirror, the welding head is usually deflected at a certain angle during the welding process. The diameter of the solder joint and the diameter of the effective joint surface increase with the increase of the laser tilt angle. When the laser tilt angle is 40°, the largest solder joint and effective joint surface are obtained. The welding spot penetration and effective penetration decrease with the laser tilt angle. When it is greater than 60, the effective welding penetration drops to zero. Therefore, by tilting the welding joint to a certain angle, the weld penetration depth and width can be appropriately increased.
In addition, during welding, with the weld seam as the boundary, 65% of the laser welding spot partial cover plate and 35% of the shell need to be welded, which can effectively reduce explosions caused by cover closing problems.
2) Continuous mode welding
Because the heating process of continuous laser welding is unlike the sudden cooling and heating of pulse machines, the tendency of cracks during welding is not very obvious. Continuous laser welding is used to improve the quality of the weld. The surface of the weld is smooth and uniform, with no spatter, no defects, and no defects inside the weld. No cracks were found. In welding aluminum alloys, continuous laser's advantages are obvious. Compared with traditional welding methods, the production efficiency is high and no wire filling is required. Compared with pulse laser welding, it can solve the defects that occur after welding, such as cracks, Porosity, spatter, etc. ensure that the aluminum alloy has good mechanical properties after welding; there will be no dents after welding, and the amount of polishing and polishing after welding is reduced, which saves production costs. However, because the spot of the continuous laser is relatively small, the assembly accuracy of the workpiece is not high. The requirements are higher.
3. Advantages of Laser Welding
The automatic CNC laser welding machine with a fiber laser generator has concentrated energy, high welding efficiency, high processing precision, and a large weld depth-to-width ratio. The laser beam is easy to focus, align, and guide with optical instruments. It can be placed at an appropriate distance from the workpiece and redirected between fixtures or obstacles around it. Other welding methods cannot be used due to the above-mentioned space restrictions.
The heat input is small, the heat-affected zone is small, and the residual stress and deformation of the workpiece is small; the welding energy can be precisely controlled, the welding effect is stable, and the welding appearance is good;
Non-contact welding, optical fiber transmission, good accessibility, and a high degree of automation. There is no meltback problem like arc welding when welding thin materials or fine-diameter wires. Because the cells used in power lithium batteries follow the principle of being lightweight, they are usually made of lighter aluminum and are made thinner. Generally, the shell, cover, and bottom are basically required to be less than 1.0mm. The current basic materials of mainstream manufacturers have A thickness of around 0.8mm.
It can provide high-strength welding for various material combinations, especially when welding copper and aluminum. This is also the only technique to solder electroplated nickel to copper materials.
4. Difficulties In Laser Welding Process
Currently, aluminum alloy battery shells account for over 90% of the entire power lithium battery. The difficulty in welding lies in the extremely high reflectivity of aluminum alloy to laser and the high sensitivity of pores during welding. Some problems and defects will inevitably occur during welding, the most important of which are pores, hot cracks, and explosions.
Pores are prone to appear during the laser welding process of aluminum alloy. Two main types are hydrogen pores and pores caused by bubble bursting. Because the cooling rate of laser welding is too fast, the problem of hydrogen holes is more serious, and an additional type of hole appears due to the collapse of small holes in laser welding.
Thermal cracking problem. Aluminum alloy is a typical eutectic alloy. It is prone to hot cracks during welding, including weld crystal cracks and HAZ liquefaction cracks. Due to component segregation in the weld area, eutectic segregation and grain boundary melting will occur. Under stress, Liquidification cracks will form at grain boundaries, reducing the performance of welded joints.
Explosion (also known as splash) problem. Many factors cause explosions, such as the cleanliness of the material, the purity of the material itself, the characteristics of the material itself, etc. The decisive factor is the stability of the laser. Shell surface bulges, pores, 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 that the better the beam quality, the better the welding effect, as some laser equipment suppliers promoted. Good beam quality is suitable for superposition welding with larger penetration depth. Finding the right process parameters is the key to solving the problem.
Other difficulties
Welding of soft-packed tabs requires high welding tooling. The tabs must be pressed firmly to ensure the welding gap. It can realize high-speed welding of complex trajectories such as S-shaped and spiral shapes, increasing the joint area of the weld and strengthening the welding strength.
The welding of cylindrical battery cores is mainly used to fabricate the positive electrode. Since the shell of the negative electrode is thin, it is very easy to weld through. For example, some manufacturers currently use a welding-free process for the negative electrode, while laser welding is used for the positive electrode.
The poles or connecting pieces are thickly contaminated when the square battery combination is welded. When the connecting pieces are welded, the pollutants decompose and easily form welding explosion points and holes. Batteries with thin poles and plastic or ceramic structural parts underneath are easy to weld. wear. When the pole is small, it is easy to weld to the point that the plastic is burned, forming an explosion point. Do not use multi-layer connecting sheets. Holes between the layers make it difficult to weld them firmly.
The most important process in the welding process of square batteries is the packaging of the shell cover, divided into the welding of the top cover and the bottom cover according to the different positions. Because the batteries they produce are small, some manufacturers use a deep drawing process to manufacture the battery shell and only need to weld the top cover.
5. Factors Affecting Welding Quality
Laser welding is currently an important method recommended for high-end battery welding. Laser welding is a process in which a high-energy laser beam irradiates the workpiece, causing the working temperature to rise sharply, melting the workpiece, and reconnecting to form a permanent connection. The shear strength and tear resistance of laser welding are relatively good. The conductivity, strength, air tightness, metal fatigue, and corrosion resistance of battery welding are typical welding.
quality evaluation criteria
Many factors affect the quality of laser welding. Some of them are extremely volatile and have considerable instability. How can these parameters be correctly set and controlled within an appropriate range during high-speed and continuous laser welding to ensure welding quality? The reliability and stability of welding seam formation are important issues related to the practicality and industrialization of laser welding technology. The important factors affecting the quality of laser welding are divided into three aspects: welding equipment, workpiece condition, and process parameters.
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