Introduction to welding methods and processes for 6F22 carbon battery

The reasonable selection of welding methods and processes in the manufacturing process of power lithium batteries will directly affect the cost, quality, safety and consistency of the batteries. Next, let's sort out the content of power lithium battery welding.

1. Principle of laser welding

Laser welding uses the excellent directionality and high power density of the laser beam to work. The laser beam is focused in a very small area through the optical system, and a heat source area with highly concentrated energy is formed at the weld in a very short time, so that the welded material is melted and a firm weld and weld are formed.

2. Types of laser welding

Heat conduction welding and deep fusion 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 fusion welding

Penetration welding and seam welding

Penetration welding, the connecting piece does not need to be punched, and the processing is relatively simple. Penetration welding requires a laser welder with higher power. The penetration depth of penetration welding is lower than that of seam welding, and the reliability is relatively poor.

Compared with penetration welding, seam welding only requires a lower power laser welder. The penetration depth of seam welding is higher than that of penetration welding, and the reliability is relatively good. However, the connecting piece needs to be punched, which is relatively difficult to process.

Pulse welding and continuous welding

1) Pulse mode welding

When laser welding, a suitable welding waveform should be selected. Common pulse waveforms include square wave, peak wave, double peak wave, etc. The reflectivity of the aluminum alloy surface to light is too high. When a high-intensity laser beam is shot to the surface of the material, 60%-98% of the laser energy on the metal surface will be lost due to reflection, and the reflectivity changes with the surface temperature. Generally, the best choice for welding aluminum alloy is the sharp wave and the double peak wave. The pulse width of the slow-down part of this welding waveform is longer, which can effectively reduce the appearance of pores and cracks.

Pulse laser welding sample

Due to the high reflectivity of aluminum alloy to laser, in order to prevent the vertical incidence of the laser beam from 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 weld spot and the diameter of the effective bonding surface increase with the increase of the laser tilt angle. When the laser tilt angle is 40, the largest weld spot and effective bonding surface are obtained. The weld 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 head to a certain angle, the weld penetration and width can be appropriately increased.

In addition, during welding, the laser welding spot needs to be welded by 65% of the cover plate and 35% of the shell with the weld as the boundary, which can effectively reduce the explosion caused by the cover problem.

2) Continuous mode welding

Because the heating process of continuous laser welding is not like the sudden cooling and heating of pulse machines, the crack tendency during welding is not very obvious. In order to improve the quality of the weld, continuous laser welding is used. The weld surface is smooth and uniform, without spatter and defects, and no cracks are found inside the weld. In the welding of aluminum alloys, the advantages of continuous lasers are obvious. Compared with traditional welding methods, it has high production efficiency and does not require wire filling. Compared with pulse laser welding, it can solve the defects that appear after welding, such as cracks, pores, spatter, etc., to ensure that the aluminum alloy has good mechanical properties after welding. There will be no dents after welding, and the amount of polishing and grinding 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 required to be high.

Continuous laser welding sample

In the welding of power lithium batteries, welding process technicians will select appropriate lasers and welding process parameters according to the customer's battery materials, shapes, thicknesses, tensile requirements, etc., including welding speed, waveform, peak value, welding head tilt angle, etc. to set reasonable welding process parameters to ensure that the final welding effect meets the requirements of power lithium battery manufacturers.

3 Advantages of laser welding

Energy concentration, high welding efficiency, high processing accuracy, and large weld depth-to-width ratio. The laser beam is easy to focus, align and guide by optical instruments. It can be placed at an appropriate distance from the workpiece and can be guided between fixtures or obstacles around the workpiece. Other welding methods cannot be used due to the above space limitations.

Small heat input, small heat-affected zone, small residual stress and deformation of the workpiece; 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 high degree of automation. When welding thin materials or thin-diameter wires, there is no problem of remelting like arc welding. The battery cells used for power lithium batteries usually use lighter aluminum materials in addition to being made thinner due to the principle of lightness. Generally, the shell, cover, and bottom are basically required to be less than 1.0mm. The current thickness of the basic materials of mainstream manufacturers is about 0.8mm.

It can provide high-strength welding for various material combinations, especially when welding between copper materials and aluminum materials. This is also the only technology that can weld electroplated nickel to copper materials.

4 Difficulties of laser welding process

At present, battery shells made of aluminum alloy materials account for more than 90% of the entire power lithium battery. The difficulty of welding is that aluminum alloy has extremely high reflectivity to lasers, and is highly sensitive to pores during welding. Some problems and defects will inevitably occur during welding, among which the most important are pores, thermal cracks and explosions.

Pores are prone to occur during laser welding of aluminum alloys. There are two main types: hydrogen pores and pores caused by bubble collapse. Because the cooling speed of laser welding is too fast, the hydrogen pore problem is more serious, and there is an additional type of pores caused by the collapse of small holes in laser welding.

Thermal crack problem. Aluminum alloy is a typical eutectic alloy. It is easy to have thermal cracks during welding, including weld crystallization cracks and HAZ liquefaction cracks. Due to the segregation of components in the weld zone, eutectic segregation will occur and grain boundary melting will occur. Under stress, liquefaction cracks will form at the grain boundary, reducing the performance of the welded joint.

Explosion (also known as spatter) problem. There are many factors that cause explosions, such as the cleanliness of the material, the purity of the material itself, the characteristics of the material itself, etc., and the stability of the laser is decisive. The shell surface is convex, pores, and internal bubbles. The main reason is that the fiber core diameter is too small or the laser energy is set too high. It is not the case that the better the beam quality, the better the welding effect as advertised by some laser equipment suppliers. Good beam quality is suitable for superposition welding with a large penetration depth. Finding suitable process parameters is the magic weapon to solve the problem.

Other difficulties

Soft-packaged pole ear welding has high requirements for welding tooling. The pole ear must be pressed firmly to ensure the welding gap. It can achieve high-speed welding of complex trajectories such as S-shaped and spiral shapes, increase the weld joint area and enhance the welding strength.

The welding of cylindrical battery cells is mainly used for the welding of the positive electrode. Because the shell of the negative electrode is thin, it is very easy to weld through. For example, some manufacturers currently use the negative electrode free welding process, and the positive electrode uses laser welding.

When welding square batteries, the poles or connecting pieces are contaminated. When welding the connecting pieces, the contaminants decompose, which can easily form welding explosion points and holes. Batteries with thin poles and plastic or ceramic structural parts underneath are easy to be welded through. When the poles are small, it is also easy to weld off-center and burn the plastic, forming explosion points. Do not use multi-layer connecting pieces, which have pores between the layers and are not easy to weld firmly.

The most important process of square battery welding is the encapsulation of the shell cover, which is divided into the welding of the top cover and the bottom cover according to the different positions. Some battery manufacturers use the deep drawing process to manufacture the battery shell because the battery they produce is not large in size, and only the top cover needs to be welded.

Square power lithium battery side welding sample

The main square battery welding methods are side welding and top welding. The important advantage of side welding is that it has less impact on the inside of the battery cell, and spatter will not easily enter the inside of the shell cover. Since bulges may occur after welding, which will have a slight impact on the assembly of subsequent processes, the side welding process has extremely high requirements for the stability of the laser and the cleanliness of the material. Since the top welding process is welded on one surface, the requirements for welding equipment integration are relatively low, and mass production is simple, but there are also two disadvantages. One is that welding may cause a little spatter to enter the battery cell, and the other is that the high requirements for the front processing of the shell will lead to cost problems.

5. Factors affecting welding quality

Laser welding is an important method currently 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, the workpiece to melt and reconnect to form a permanent connection. The shear strength and tear strength of laser welding are relatively good. The conductivity, strength, airtightness, metal fatigue and corrosion resistance of battery welding are typical welding quality evaluation criteria.

There are many factors that affect the quality of laser welding. Some of them are extremely easy to fluctuate and have considerable instability. How to correctly set and control these parameters so that they are controlled within a suitable range during high-speed and continuous laser welding to ensure welding quality. The reliability and stability of weld formation are important issues related to the practical application 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.

1) Welding equipment

The most important quality requirements for lasers are the beam mode, output power and stability. The beam mode is an important indicator of beam quality. The lower the order of the beam mode, the better the beam focusing performance, the smaller the spot, the higher the power density at the same laser power, and the greater the depth and width of the weld. Generally, the fundamental mode (TEM00) or low-order mode is required, otherwise it is difficult to meet the requirements of high-quality laser welding. At present, there are still some difficulties in the use of domestic lasers for laser welding in terms of beam quality and power output stability. From the perspective of foreign countries, the beam quality and output power stability of lasers are already quite high and will not become a problem for laser welding. The biggest factor affecting welding quality in the optical system is the focusing mirror. The focal length used is generally between 127mm (5in) and 200mm (7.9in). A small focal length is good for reducing the diameter of the waist spot of the focused beam, but too small a focal length is prone to contamination and splash damage during welding.

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 a linear relationship; generally, the surface coating of 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 spot alignment cannot change due to welding thermal deformation during the welding process. This is because the laser spot is small, the weld is narrow, and generally no filler metal is added. If the assembly is not strict and the gap is too large, the beam can pass through the gap but cannot melt the parent material, or cause obvious undercuts and depressions. If the deviation of the spot to the seam is slightly larger, it may cause incomplete fusion or incomplete penetration. Therefore, the gap between the assembly of the general plate and the deviation of the spot to the seam should not be greater than 0.1mm, and the misalignment 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 gap and overlap gap of the butt joint should be controlled within 10% of the thickness of the thin plate.

Successful laser welding requires close contact between the base materials to be welded. This requires careful tightening of the parts to achieve the best results. This is difficult to do on thin tab substrates because they are prone to bending and misalignment, especially when the tabs are embedded in large battery modules or components.

3) Welding parameters

(1) Impact on laser welding mode and weld formation stability The most important welding parameter is the power density of the laser spot, which affects the welding mode and weld formation stability as follows: as the laser spot power density increases from small to large, it is stable thermal conductivity welding, mode unstable welding and stable deep fusion welding.

The power density of the laser spot is mainly determined by the laser power and the beam focus position when the beam mode and the focal length of the focusing mirror are constant. The laser power density is proportional to the laser power. The influence of the focal position has an optimal value; when the beam focus is at a certain position below the workpiece surface (within the range of 1 to 2 mm, depending on the plate thickness and parameters), the most ideal weld can be obtained. Deviating from this optimal focal position, the spot on the workpiece surface becomes larger, causing the power density to decrease. To a certain range, it will cause changes in the form of the welding process.

The influence of welding speed on the welding process form and stable parts is not as significant as that of laser power and focal position. Only when the welding speed is too high, the stable deep fusion welding process cannot be maintained due to too little heat input. In actual welding, stable deep fusion welding or stable thermal conduction welding should be selected according to the requirements of the weldment for the fusion depth, and mode unstable welding should be absolutely avoided.

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

(3) The influence of shielding gas. The important uses of shielding gas are to protect the workpiece from oxidation during the welding process; to protect the focusing lens from metal vapor contamination and liquid droplet sputtering; to disperse the plasma generated by high-power laser welding; to cool the workpiece and reduce the heat affected zone.

Shielding gas usually uses argon or helium, and nitrogen can also be used if the apparent quality requirements are not high. Their tendency to form plasma is significantly different: Helium has a high ionization charge and fast thermal conductivity. Under the same conditions, it has a lower tendency to form plasma than argon, so it can achieve a greater penetration depth. Within a certain range, as the shielding gas flow rate increases, the tendency to suppress plasma increases, and thus the penetration depth increases, but it tends to stabilize after increasing to a certain range.

(4) Analysis of the monitorability of each parameter: Among the four welding parameters, welding speed and shielding gas flow rate are parameters that are easy to monitor and maintain stable, while laser power and focal position are parameters that may fluctuate during welding and are difficult to monitor. Although the laser power output from the laser is very stable and easy to monitor, the laser power reaching the workpiece will change due to the loss of the light guide and focusing system. This loss is related to the quality of the optical workpiece, the use time and the surface contamination. Therefore, it is not easy to monitor and becomes an uncertain factor in welding quality. The beam focal position is a factor that has a great impact on welding quality and is the most difficult to monitor and control. At present, in production, it is necessary to rely on manual adjustment and repeated process tests to determine the appropriate focal position to obtain the ideal penetration depth. However, during the welding process, due to workpiece deformation, thermal lens effect or multi-dimensional welding of spatial curves, the focal position will change and may exceed the allowable range.

Regarding the above two situations, on the one hand, high-quality and high-stability optical components should be used, and they should be maintained regularly to prevent contamination and keep them clean; on the other hand, it is required to develop real-time monitoring and control methods for the laser welding process to optimize parameters, monitor the changes in laser power and focal position reaching the workpiece, realize closed-loop control, and improve 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 very fast, so the overall heat input is low. But because this is a melting process, fragile high-resistance intermetallic compounds may be formed when welding different materials. The aluminum-copper combination is particularly prone to forming intermetallic compounds. These compoundsThe compounds have been shown to have negative effects on the short-term electrical and long-term mechanical properties of microelectronic device lap joints. The effects of these intermetallic compounds on the long-term performance of lithium-ion batteries are less certain.

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