SHINHOP laser

The application of laser in battery welding is a trend in the current market development, and it is determined by the technical advantages of the laser itself. It is one of the important welding technologies that will replace traditional welding in the future.
1. The advantages of laser welding for batteries are mainly reflected in the following points:
1. Energy concentration, high welding efficiency, high processing accuracy, and large weld depth-to-width ratio. The laser beam is easy to focus, align, and guided by optical instruments, and can be placed at an appropriate distance from the workpiece. It can also be guided around the fixtures or obstacles surrounding the workpiece, while other welding methods cannot perform due to the spatial limitations mentioned above.
2. Small heat input, small heat-affected zone, and low residual stress and deformation of the workpiece; welding energy can be precisely controlled, resulting in stable welding effects and good welding appearance.
3. Non-contact welding, good accessibility with fiber transmission, and high degree of automation. When welding thin materials or fine diameter wires, it does not have the re-melting issues that arc welding often encounters.
The battery cells used for power batteries usually follow the principle of "lightweight" and typically use lighter aluminum materials, while also needing to be thinner. Generally, the shell, cover, and bottom are required to be below 1.0mm. Currently, mainstream manufacturers have a basic material thickness of around 0.8mm, which can provide high-strength welding for various material combinations, especially effective when welding between copper and aluminum materials. This is also the only technology that can weld electroplated nickel to copper materials.

2. Difficulties in laser welding process
Currently, aluminum alloy battery shells account for more than 90% of the entire power battery. The difficulty in welding lies in the extremely high reflectivity of aluminum alloy to laser, high sensitivity to porosity during the welding process, and some inevitable defects during welding, the most significant of which are porosity, hot cracks, and spatter.
1. Porosity is easily generated during the laser welding of aluminum alloys, mainly of two types: hydrogen pores and pores generated from bubble collapse. Due to the rapid cooling speed of laser welding, the hydrogen pore issue is more severe, and there is also an additional type of pore generated from the collapse of small holes.
2. Hot crack issues. Aluminum alloys are typical eutectic alloys, and hot cracks are prone to occur during welding, including weld crystallization cracks and HAZ liquefaction cracks. Due to the segregation of components in the weld zone, eutectic segregation can occur, leading to melting at the grain boundaries, which can form liquefaction cracks under stress, reducing the performance of the welded joint.
3. Spatter (also known as flying sparks) issues. There are many factors that cause spatter, such as the cleanliness of the materials, the purity of the materials themselves, and the characteristics of the materials, while the decisive factor is the stability of the laser.
Surface bulges, porosity, and internal bubbles. The main reasons are due to the small core diameter of the fiber or the laser energy being set too high. It is not as some laser equipment providers promote that "the better the beam quality, the better the welding effect"; good beam quality is suitable for deep penetration overlay welding. Finding suitable process parameters is the key to solving the problem.

▲ Sample of laser welded battery cell components

3. Other difficulties
Welding of soft-pack tabs requires high demands on welding fixtures, which must firmly press the tabs to ensure welding gaps. It can achieve high-speed welding of complex trajectories such as S-shaped and spiral shapes, increasing the bonding area of the weld while enhancing welding strength.
The welding of cylindrical cells is mainly used for the welding of the positive electrode. Due to the thin shell at the negative electrode position, it is very easy to burn through. For example, some manufacturers currently use a no-welding process for the negative electrode, while the positive electrode uses laser welding.
When welding square battery assemblies, if the pole or connecting piece is heavily contaminated, the contaminants decompose during welding, easily forming welding spatter and causing holes; if the pole is thin and has plastic or ceramic structural parts underneath, it is easy to burn through. If the pole is small, it is also easy to misalign and burn the plastic, forming explosion points. Do not use multi-layer connecting pieces, as the gaps between layers make it difficult to weld securely.

▲ One of the cell components that require laser welding

The most important process in the welding of square batteries is the sealing of the shell cover, which is divided into top cover and bottom cover welding based on different positions. Some battery manufacturers, due to the small size of the batteries produced, have adopted a "deep drawing" process to manufacture the battery shell, requiring only the welding of the top cover.
The welding methods for square batteries are mainly divided into side welding and top welding, where the main advantage of side welding is that it has less impact on the inside of the cell, and spatter does not easily enter the inside of the shell cover. However, since welding may cause bulges, which can slightly affect the assembly of subsequent processes, the side welding process has very high requirements for the stability of the laser and the cleanliness of the materials. The top welding process, on the other hand, has lower integration requirements for welding equipment since it is welded on one surface, making mass production simpler, but it also has two disadvantages: one is that there may be slight spatter entering the cell, and the other is that high processing requirements for the front section of the shell can lead to cost issues.