The appropriate selection of welding methods and processes in the manufacturing of power lithium batteries directly affects the battery's cost, quality, safety, and consistency. The following is a summary of aspects related to power lithium battery welding.
1. Laser welding principle
Laser welding utilizes the excellent directionality and high power density of a laser beam. Through an optical system, the laser beam is focused into a very small area, creating a highly concentrated heat source zone at the welding point in a very short time. This melts the workpiece and forms a strong weld point and weld seam.
2. Laser welding type
Heat conduction welding and deep penetration welding
Laser power density of 10⁵~10⁶ W/cm² forms laser heat conduction welding, while laser power density of 10⁵~10⁶ W/cm² forms laser deep penetration welding.
Penetration welding and seam welding
Penetration welding eliminates the need for punching holes in the connecting pieces, making processing relatively simple. However, it requires a high-powered laser welding machine. The penetration depth of penetration welding is lower than that of seam welding, resulting in slightly lower reliability.
Compared to through-weld, seam welding requires only a lower-power laser welding machine. Seam welding also offers greater penetration depth and relatively better reliability. However, the connecting pieces require punching, making processing relatively difficult.
Pulse welding and continuous welding
1) Pulse mode welding
When laser welding, it is crucial to select an appropriate welding waveform. Commonly used pulse waveforms include square waves, spike waves, and double-peak waves. Aluminum alloy surfaces have a very high reflectivity; when a high-intensity laser beam strikes the material surface, 60%-98% of the laser energy is lost due to reflection, and the reflectivity varies with surface temperature. Generally, spike waves and double-peak waves are the optimal choices for welding aluminum alloys. These waveforms have a longer pulse width in the subsequent descent, effectively reducing the occurrence of porosity and cracks.
Pulsed laser welding samples
Because aluminum alloys have a high reflectivity to lasers, the welding head is usually deflected at a certain angle during welding to prevent vertical reflection caused by perpendicular laser beam incidence, which could damage the laser focusing lens. The diameter of the weld joint 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 weld joint and effective bonding surface are maximized. The weld penetration and effective penetration decrease with the laser tilt angle. When the angle is greater than 60°, the effective weld penetration drops to zero. Therefore, tilting the welding head to a certain angle can appropriately increase the weld penetration and width.
In addition, during welding, the laser weld spot should be welded to 65% of the cover plate and 35% of the shell, with the weld seam as the boundary. This can effectively reduce the explosion caused by the cover closure problem.
2) Continuous mode welding
Continuous laser welding, unlike pulsed laser welding which involves rapid heating and cooling, exhibits a less pronounced tendency to crack during welding. To improve weld quality, continuous laser welding is employed, resulting in smooth, uniform weld surfaces free of spatter and defects, with no internal cracks observed. In aluminum alloy welding, continuous laser welding offers significant advantages. Compared to traditional methods, it boasts higher production efficiency and eliminates the need for filler wire. Compared to pulsed laser welding, it addresses post-weld defects such as cracks, porosity, and spatter, ensuring excellent mechanical properties of the aluminum alloy. It also prevents post-weld dents, reducing polishing and reducing production costs. However, due to the smaller laser spot size, continuous laser welding demands higher assembly precision on the workpiece.
Continuous laser welding samples
In the welding of power lithium batteries, welding technicians will select appropriate lasers and welding process parameters based on the customer's battery materials, shape, thickness, tensile strength 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 the power lithium battery manufacturer.
3 Advantages of laser welding
The laser beam is highly concentrated, resulting in high welding efficiency, high processing precision, and a large weld depth-to-width ratio. It is easy to focus, align, and guide using optical instruments, allowing it to be placed at an appropriate distance from the workpiece and further guided by fixtures or obstacles around the workpiece. Other welding methods are limited by these spatial constraints.
It features low heat input, a small heat-affected zone, and minimal residual stress and deformation in the workpiece; the welding energy can be precisely controlled, resulting in stable welding effects and a good weld appearance.
Non-contact welding, fiber optic transmission, good accessibility, and high degree of automation. When welding thin materials or fine-diameter wires, it avoids the problem of back melting that is common in arc welding. Battery cells used in power lithium batteries, due to the principle of lightweight construction, typically use lighter aluminum materials and are made even thinner. Generally, the casing, cover, and bottom are required to be less than 1.0mm thick, with mainstream manufacturers currently using materials with a thickness of around 0.8mm.
It can provide high-strength welds for various material combinations, and is particularly effective when welding copper and aluminum materials. It is also the only technology that can weld electroplated nickel onto copper materials.
4. Challenges in Laser Welding Process
Currently, aluminum alloy battery casings account for over 90% of all power lithium batteries. The difficulty in welding them lies in the extremely high reflectivity of aluminum alloys to lasers and their high sensitivity to porosity during the welding process. Some defects are unavoidable during welding, the most important of which are porosity, hot cracking, and sparking.
Porosity is a common problem in the laser welding of aluminum alloys, mainly of two types: hydrogen porosity and porosity caused by the collapse of gas bubbles. Due to the rapid cooling rate of laser welding, the hydrogen porosity problem is exacerbated, and an additional type of hole, caused by the collapse of small holes, also occurs during laser welding.
Hot cracking is a problem. Aluminum alloys are typical eutectic alloys, which are prone to hot cracking during welding, including weld crystallization cracks and HAZ liquefaction cracks. Due to compositional segregation in the weld zone, eutectic segregation occurs, leading to grain boundary melting. Under stress, liquefaction cracks will form at the grain boundaries, reducing the performance of the welded joint.
Spatter (also known as splatter) is a problem. Many factors can cause spatter, such as the cleanliness, purity, and inherent properties of the materials. However, the stability of the laser is the decisive factor. Other issues include surface bulges, pores, and internal bubbles. These are primarily caused by an excessively small fiber core diameter or an excessively high laser energy setting. It's not true that better beam quality, as some laser equipment suppliers claim, guarantees superior welding results. Good beam quality is suitable for layered welding with large penetration depths. Finding the appropriate process parameters is the key to solving these problems.
Other difficulties
Welding of flexible electrode tabs requires sophisticated welding fixtures to ensure the tabs are firmly secured and the welding gap is maintained. High-speed welding of complex trajectories such as S-shapes and spirals is possible, increasing the weld joint area while enhancing weld strength.
Welding of cylindrical battery cells is primarily used for welding the positive electrode, as the negative electrode casing is thin and easily burned through. For example, some manufacturers currently use a solderless negative electrode process, while the positive electrode uses laser welding.
When welding prismatic battery packs, thick contamination on the terminals or connecting pieces can cause decomposition of contaminants during welding, easily forming welding spalls and creating holes. Batteries with thinner terminals or underlying plastic or ceramic structural components are prone to burn-through. Smaller terminals are also susceptible to misalignment during welding, leading to plastic burn-out and the formation of spalls. Avoid using multi-layer connecting pieces, as gaps between layers make welding difficult.
The most crucial step in the welding process of prismatic batteries is the encapsulation of the casing, which is divided into welding the top cover and the bottom cover, depending on their location. Some battery manufacturers, due to the small size of their batteries, use a deep drawing process to manufacture the battery casing, requiring only the welding of the top cover.
Square power lithium battery side-welded sample
Square battery welding methods are mainly divided into side welding and top welding. A key advantage of side welding is its minimal impact on the internal structure of the cell, preventing spatter from easily entering the inner casing. However, because welding may cause bulges, which can slightly affect subsequent assembly processes, side welding places extremely high demands on laser stability and material cleanliness. Top welding, on the other hand, welds to a single surface, requiring less sophisticated welding equipment integration and simplifying mass production. However, it also has two disadvantages: firstly, a small amount of spatter may enter the cell; and secondly, the high precision required for the front-end casing processing can lead to cost issues.
5. Factors affecting welding quality
Laser welding is currently a preferred method for welding high-end batteries. Laser welding involves irradiating the workpiece with a high-energy laser beam, causing a rapid increase in temperature, melting the workpiece, and rejoining it to form a permanent bond. Laser welding offers good shear strength and tear resistance. The conductivity, strength, airtightness, metal fatigue resistance, and corrosion resistance are typical welding quality evaluation criteria for battery welding.
Many factors influence the quality of laser welding. Some of these are highly volatile and quite unstable. Properly setting and controlling these parameters to maintain them within appropriate ranges during high-speed, continuous laser welding is crucial for ensuring weld quality. The reliability and stability of weld formation are critical issues related to the practical application and industrialization of laser welding technology. The key factors affecting laser welding quality can be categorized into three aspects: welding equipment, workpiece condition, and process parameters.
1) Welding equipment
The most important quality requirements for lasers are beam mode, output power, and their stability. Beam mode is a crucial indicator of beam quality; a lower beam mode order results in better focusing performance, a smaller spot size, higher power density at the same laser power, and a wider and deeper weld. Generally, a fundamental mode (TEM00) or a low-order mode is required; otherwise, it's difficult to meet the requirements for high-quality laser welding. Currently, domestically produced lasers still face challenges in beam quality and power output stability for laser welding. Internationally, laser beam quality and output power stability are already quite high and are no longer a problem for laser welding. The most significant factor affecting weld quality in the optical system is the focusing lens. The focal length used is generally between 127mm (5in) and 200mm (7.9in). A smaller focal length is beneficial for reducing the waist diameter of the focused beam, but too small a focal length makes it susceptible to contamination and spatter damage during welding.
The shorter the wavelength, the higher the absorption rate. Generally, materials with good conductivity have high reflectivity. For YAG lasers, silver has a reflectivity of 96%, aluminum 92%, copper 90%, and iron 60%. The higher the temperature, the higher the absorption rate, showing a linear relationship. Generally, coating the surface with phosphates, carbon black, graphite, etc., can improve the absorption rate.
2) Workpiece condition
Laser welding requires edge processing of the workpiece, high assembly precision, and strict alignment of the laser spot with the weld seam. Furthermore, the original assembly precision of the workpiece and the alignment of the laser spot must not change due to welding heat deformation during the welding process. This is because the laser spot is small, the weld seam is narrow, and generally no filler metal is added. If the assembly is not tight and the gap is too large, the laser beam can pass through the gap without melting the base material, or it may cause obvious undercut or indentation. Even a slight deviation in the laser spot alignment can lead to incomplete fusion or incomplete penetration. Therefore, the gap and laser spot alignment deviation for butt joints of sheet metal should generally not exceed 0.1 mm, and the misalignment should not exceed 0.2 mm. In actual production, sometimes these requirements cannot be met, making laser welding technology unsuitable. To achieve good welding results, the allowable gap and lap gap should be controlled within 10% of the sheet thickness.
Successful laser welding requires tight contact between the substrates being welded. This necessitates careful tightening of the parts for optimal results. This is particularly challenging on thin tab substrates, as they are prone to bending and misalignment, especially when the tabs are embedded in large battery modules or components.
3) Welding parameters
(1) Effects on laser welding mode and weld formation stability Among the welding parameters, the most important one is the power density of the laser spot. Its effects on the welding mode and weld formation stability are as follows: as the laser spot power density increases, the order is stable heat conduction welding, unstable mode welding and stable deep penetration welding.
The power density of a laser spot, given a fixed beam mode and focusing lens focal length, is primarily determined by the laser power and the position of the laser focal point. Laser power density is directly proportional to laser power. The focal point position, however, has an optimal value; the most ideal weld is achieved when the beam focal point is located at a certain position below the workpiece surface (within a range of 1-2 mm, depending on the plate thickness and parameters). Deviating from this optimal focal point position results in a larger laser spot on the workpiece surface, causing a decrease in power density. Within a certain range, this can lead to changes in the welding process.
The welding speed has less of an impact on the welding process and stability than laser power and focal position. Only when the welding speed is too high can a stable deep penetration welding process be impossible to maintain due to insufficient heat input. In actual welding, stable deep penetration welding or stable heat conduction welding should be selected according to the weldment's requirements for penetration depth, and unstable welding should be absolutely avoided.
(2) In the range of deep penetration welding, the influence of welding parameters on penetration depth: In the range of stable deep penetration welding, the higher the laser power, the greater the penetration depth, which is approximately the power of 0.7; while the higher the welding speed, the shallower the penetration depth. Under certain laser power and welding speed conditions, the penetration depth is the greatest when the focal point is in the optimal position. If it deviates from this position, the penetration depth decreases, and it may even become unstable mode welding or stable thermal conductivity welding.
(3) The effects of protective gas: The important uses of protective 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 during high-power laser welding; and to cool the workpiece and reduce the heat-affected zone.
The protective gas is usually argon or helium, but nitrogen can be used if the apparent quality requirements are not high. Their tendency to produce plasma differs significantly: helium, due to its high ionization rate and rapid thermal conductivity, is less prone to producing plasma than argon under the same conditions, thus allowing for a greater melting depth. Within a certain range, as the flow rate of the protective gas increases, the tendency to suppress plasma increases, thus increasing the melting depth, but this increase tends to plateau after reaching a certain range.
(4) Monitorability Analysis of Each Parameter: Among the four welding parameters, welding speed and shielding gas flow rate are easy to monitor and maintain stability, while laser power and focal position are parameters that may fluctuate during the welding process and are difficult to monitor. Although the laser power output from the laser is highly stable and easy to monitor, the laser power reaching the workpiece will change due to losses in the beam guiding and focusing system. This loss is related to the quality of the optical workpiece, usage time, and surface contamination, making it difficult to monitor and becoming an uncertain factor in welding quality. The beam focal position is one of the welding parameters that has a significant impact on welding quality and is the most difficult to monitor and control. Currently, in production, a suitable focal position needs to be determined by manual adjustment and repeated process experiments to obtain the ideal penetration depth. However, during the welding process, due to workpiece deformation, thermal lensing effect, or multi-dimensional welding of spatial curves, the focal position may change and exceed the allowable range.
Regarding the two situations mentioned above, on the one hand, high-quality and highly stable optical components should be used and regularly maintained to prevent contamination and keep them clean; on the other hand, it is necessary to develop real-time monitoring and control methods for the laser welding process to optimize parameters, monitor changes in laser power and focal position reaching the workpiece, achieve closed-loop control, and improve the reliability and stability of laser welding quality.
Finally, it's important to note that laser welding is a melting process. This means that both substrates melt during the laser welding process. This process is rapid, resulting in a relatively low overall heat input. However, because it is a melting process, brittle, high-resistivity 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 impact the short-term electrical and long-term mechanical properties of microelectronic device joints. The effects of these intermetallic compounds on the long-term performance of lithium-ion batteries are still uncertain.
