Laser welding process of power battery

Laser welding process of power battery??

Laser welding uses the excellent directivity and high power density of the laser beam to work. The commonly used pulse waveforms are square wave, peak wave, double peak wave, etc.

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

1 Principle of laser welding

Laser welding uses the excellent directivity and high power density of the laser beam to work. The laser beam is focused in a small area through the optical system, and a heat source with high energy concentration is formed at the welded part in a very short time. zone, so that the welded material melts and forms a solid welding spot and seam.

2 types of laser welding

â–¼Thermal conduction welding and deep penetration welding

Heat conduction welding: The laser beam will melt together on the surface of the workpiece along the seam, and the melt will flow together and solidify to form a weld. Mainly used for relatively thin materials, where the maximum weld depth of the material is constrained by its thermal conductivity, and the weld width is always greater than the weld depth.

Deep penetration welding: When the high-power laser is concentrated on the surface of the metal, the heat cannot be dissipated in time, and the welding depth will be sharply deepened. This welding technology is deep penetration welding. Because deep penetration welding technology is extremely fast, has a small heat affected zone, and minimizes distortion, this technology can be used for deep welding or welding of several layers of material together.

The main difference between thermal conduction welding and deep penetration welding is the power density applied to the metal surface per unit time, and the lower critical value of different metals is different.

â–¼Through Welding and Seam Welding

Through penetration welding, the connection piece does not need to be punched, and the processing is relatively simple. Penetration welding requires a powerful laser welder. 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 requires less power laser welding machine. 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.

â–¼Pulse welding and continuous welding

1) Pulse mode welding

The appropriate welding waveform should be selected during laser welding. The commonly used pulse waveforms include square wave, peak wave, double peak wave, etc. The reflectivity of the aluminum alloy surface to light is too high. When the high-intensity laser beam hits the surface of the material, the metal surface will 60%-98% of the laser energy is lost due to reflection, and the reflectivity varies with the surface temperature. Generally, sharp wave and double peak wave are the best choices when welding aluminum alloys. The pulse width of the slow-down part behind this welding waveform is longer, which can effectively reduce the generation of pores and cracks.

Due to the high reflectivity of the aluminum alloy to the laser, in order to prevent the vertical reflection of the laser beam from causing vertical reflection and damage to the laser focusing mirror, the welding head is usually deflected by 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 inclination angle. When the inclination angle of the laser is 40°, the largest solder joint and the effective joint surface are obtained. The welding point penetration and effective penetration decrease with the laser inclination angle, and when it is greater than 60°, the effective welding penetration decreases to zero. Therefore, by tilting the welding head to a certain angle, the penetration depth and width of the weld can be appropriately increased.

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

2) Continuous mode welding

Because the heating process of continuous laser welding is not like the sudden cooling and sudden heating of pulse machines, the tendency of cracks during welding is not very obvious. In order to improve the quality of the weld, continuous laser welding is used. The surface of the weld is smooth and uniform, no spatter, and no defects. No cracks were found. In the welding of aluminum alloys, the advantages of continuous lasers are obvious. Compared with traditional welding methods, the production efficiency is high, and no wire filler is required; compared with pulsed laser welding, it can solve the defects generated after welding, such as cracks, Air holes, splashes, etc., ensure that the aluminum alloy has good mechanical properties after welding; it will not sag after welding, and the amount of polishing and grinding after welding is reduced, which saves production costs, but because the spot of the continuous laser is relatively small, so the workpiece assembly accuracy Higher requirements.

In power battery welding, the welding process technicians will select the appropriate laser and welding process parameters according to the customer's battery material, shape, thickness, tension requirements, etc., including welding speed, waveform, peak value, welding head inclination angle, etc. to set a reasonable Welding process parameters to ensure that the final welding effect meets the requirements of power battery manufacturers.

3 Advantages of laser welding

The energy is concentrated, the welding efficiency is high, the machining accuracy is high, and the welding seam aspect ratio is large. The laser beam is easy to focus, align, and guided by optical instruments. It can be placed at an appropriate distance from the workpiece, and can be redirected between fixtures or obstacles around the workpiece. Other welding laws cannot be used due to the above-mentioned space constraints.

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 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 or thin wire rods, there is no problem of melting back like arc welding. Due to the principle of "lightness", the cells used for power batteries are usually made of "lighter" aluminum materials, and also need to be made more "thin". Generally, the shell, cover and bottom are basically required to be less than 1.0mm. Mainstream manufacturers currently have a basic material thickness of about 0.8mm.

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

4 Difficulties of laser welding process

At present, the battery shell made of aluminum alloy material accounts for more than 90% of the entire power battery. The difficulty of its welding lies in the extremely high reflectivity of the aluminum alloy to the laser, the high sensitivity of pores during the welding process, and some problems and defects will inevitably occur during welding, the most important of which are pores, hot cracks and explosions.

There are two main types of pores in the laser welding process of aluminum alloys: hydrogen pores and pores generated by bubble bursting. Because the cooling rate of laser welding is too fast, the hydrogen hole problem is more serious, and there is another type of hole caused by the collapse of the small hole in the laser welding.

Thermal crack problem. Aluminum alloys are typical eutectic alloys, which are prone to hot cracks during welding, including weld crystallization cracks and HAZ liquefaction cracks. Due to the segregation of components in the weld area, eutectic segregation occurs and grain boundary melting occurs. Under the action of stress, it will be Liquefaction cracks form at grain boundaries, reducing the performance of welded joints.

Blast (also known as splash) problem. There are many factors that cause the explosion, 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 plays a decisive role. The surface of the shell has bulges, pores, and internal air bubbles. The main reason is that the core diameter of the fiber is too small or the laser energy is set too high. It is not that "the better the beam quality is, the better the welding effect" as advertised by some laser equipment providers. Good beam quality is suitable for superposition welding with larger penetration depth. Finding the right process parameters is the magic weapon to solve the problem.

other difficulties

Soft-package tab welding requires high welding tooling, and 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-shaped, and increase the welding area while strengthening the welding strength.

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

When the square battery is welded, the poles or connecting pieces are polluted and thick. When the connecting pieces are welded, the pollutants decompose, and it is easy to form welding explosion points and cause holes; batteries with thin poles and plastic or ceramic structural parts are easy to weld. Put on. When the pole is small, it is easy to be welded to the point of burning of the plastic, forming an explosion point. Do not use multi-layer connecting pieces, there are pores between the layers, and it is not easy to weld.

The most important process of the welding process of the square battery is the packaging of the shell cover, which is divided into the welding of the top cover and the bottom cover according to the different positions. Due to the small size of the batteries produced by some battery manufacturers, the "deep drawing" process is used to manufacture the battery case, and only the top cover needs to be welded.

The welding methods of square batteries are mainly divided into side welding and top welding. The main advantage of side welding is that the impact on the inside of the cell is small, and the spatter will not easily enter the inside of the shell cover. Since it may cause bumps after welding, which will have a slight impact on the assembly of the subsequent process, the side welding process has extremely high requirements on the stability of the laser and the cleanliness of the material. Since the top welding process is welded on one surface, the integration requirements of welding equipment are relatively low, and mass production is simple, but there are also two disadvantages. One is that welding may have a little splash into the cell, and the other is the front section of the shell. High processing requirements can lead to cost issues.

5 Factors Affecting Welding Quality

Laser welding is currently the main method of high-end battery welding. Laser welding is a process in which a high-energy beam laser irradiates a workpiece, so that the working temperature rises sharply, and the workpiece is melted and reconnected to form a permanent connection. The shear strength and tear strength of laser welding are relatively good. The quality of battery welding, its electrical conductivity, strength, air tightness, metal fatigue and corrosion resistance are typical evaluation criteria for welding quality.

There are many factors that affect the quality of laser welding. Some of them are extremely volatile and have considerable instability. How to correctly set and control these parameters so that they can be controlled within a suitable range in the high-speed continuous laser welding process to ensure the welding quality. The reliability and stability of weld formation are important issues related to the practicality and industrialization of laser welding technology. The main 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 requirements for the quality of the laser are the beam mode and output power and its stability. The beam mode is the main indicator of the 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 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, domestic lasers still have certain difficulties in 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 quite high and will not become a problem for laser welding. The most important factor affecting the welding quality in the optical system is the focusing lens, the focal length used is generally between 127mm (5in) and 200mm (7.9in). Contamination and splash damage during the process.

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, coating the surface with 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 alignment of the spot cannot be changed during the welding process due to welding thermal deformation. This is because the laser spot is small and the welding seam is narrow. Generally, no filler metal is added. If the gap is too large if the assembly is not strict, the beam can pass through the gap and cannot melt the base metal, or cause obvious undercut and depression, such as the deviation of the spot to the seam. If it is too large, it may cause incomplete fusion or incomplete penetration. Therefore, in general, the butt assembly gap of the plates and the deviation of the spot alignment should not be greater than 0.1mm, and the wrong side 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 butt gap and overlap gap should be controlled within 10% of the thickness of the sheet.

Successful laser welding requires intimate contact between the substrates being welded. This requires careful tightening of the parts for best results. This is difficult to do well on thin tab substrates, which are prone to bending misalignment, especially if the tabs are embedded in large battery modules or assemblies.

3) Welding parameters

(1) Influence on the laser welding mode and welding seam forming stable parts The most important welding parameter is the power density of the laser spot, which has the following influence on the welding mode and the welding seam forming stability: as the laser spot power density increases from small to large The order is stable thermal conduction welding, mode unstable welding and stable deep penetration welding.

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

The effect of welding speed on the welding process form and stable parts is not as significant as the laser power and focus position. Only when the welding speed is too high, the heat input is too small to maintain a stable deep penetration welding process. In actual welding, stable deep penetration welding or stable thermal conduction welding should be selected according to the requirements of the weldment on the penetration depth, and unstable mode welding should be absolutely avoided.

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

(3) Influence of shielding gas, the main function of shielding gas is to protect the workpiece from oxidation during the welding process; to protect the focusing lens from metal vapor contamination and the sputtering of liquid droplets; to dissipate the plasma generated by high-power laser welding; workpiece, reducing the heat-affected zone.

The protective gas is usually argon or helium, and nitrogen can also be used if the apparent quality is not high. Their propensities to generate plasma are significantly different: helium has a lower tendency to generate plasma than argon due to its high ionizing body and fast thermal conductivity, and thus can obtain greater penetration depth. Within a certain range, with the increase of the protective gas flow rate, the tendency to suppress the plasma increases, so the penetration depth increases, but it tends to be stable when it increases to a certain range.

(4) Monitor ability analysis of each parameter: Among the four welding parameters, welding speed and shielding gas flow are parameters that are easy to monitor and maintain stable, while laser power and focus position are the ones that may fluctuate during the welding process and are difficult to monitor. parameter. Although the laser power output from the laser is highly stable and easy to monitor, the laser power reaching the workpiece will vary due to losses in the light guide and focusing systems, which are related to the quality, age, and surface contamination of the optical workpiece. It is not easy to monitor and becomes an uncertain factor of welding quality. The position of the beam focus is one of the welding parameters that has the greatest impact on welding quality and is the most difficult to monitor and control. At present, manual adjustment and repeated process tests are needed to determine the appropriate focus position in production to obtain the ideal penetration. However, during the welding process, due to workpiece deformation, thermal lens effect or multi-dimensional welding of spatial curves, the focus position will change and may exceed the allowable range.

For the above two cases, on the one hand, high-quality, high-stability optical components should be used, and frequent maintenance should be performed to prevent pollution and keep clean; on the other hand, it is required to develop real-time monitoring and control methods for the laser welding process to optimize parameters and monitor The change of the laser power and focus position of the workpiece realizes closed-loop control and improves the reliability and stability of laser welding quality.

Finally, be aware that laser welding is a melting process. This means that the two substrates will melt during the laser welding process. The process is fast, so the overall heat input is low. But because this is a melting process, brittle, high-resistance intermetallic compounds can form when welding dissimilar materials. Aluminum-copper combinations are particularly prone to forming intermetallic compounds. These compounds have been shown to negatively affect the short-term electrical and long-term mechanical properties of microelectronic device lap joints. The effect of these intermetallic compounds on the long-term performance of lithium batteries is uncertain.