Since its inception in 1990, it has been favored by 3C digital and power tools industries for its advantages of high energy density, high voltage, environmental protection, long life and fast charging, and its contribution to the new energy automotive industry is particularly outstanding.As a lithium battery industry providing new energy automotive power, it has huge market potential and is an important part of national strategic development. It is expected that the industry scale will exceed 160 billion yuan in the next 5-10 years.

　　 As the core component of new energy vehicle, the quality of power battery directly determines the performance of the vehicle.Lithium battery manufacturing equipment generally includes front-end equipment, middle-end equipment and back-end equipment. Its equipment accuracy and automation level will directly affect the productivity and consistency of products.Laser processing technology, as an alternative to traditional welding technology, has been widely used in lithium electrical manufacturing equipment.

　　 Through the application of laser in power battery industry, this paper describes the process characteristics of laser welding, analyses the difficulties of laser welding of aluminium alloy and the influence of welding mode on welding quality, and lists the process characteristics and equipment development trend of square power battery and battery PACK.

　 Laser welding process

　　 Welding is an important manufacturing procedure from the manufacture of lithium battery cells to the PACK group of batteries. The conductivity, strength, air tightness, metal fatigue and corrosion resistance of lithium batteries are typical criteria for evaluating the welding quality of batteries.

　 The selection of welding method and welding process will directly affect the cost, quality, safety and uniformity of the battery.Among many welding methods, laser welding distinguishes itself by the following advantages: firstly, laser welding has high energy density, small welding deformation and small heat-affected zone, which can effectively improve the precision of the parts. The welding seams are smooth, impurity-free, uniform and dense, and no additional grinding work is required;Secondly, laser welding can be precisely controlled, with small focus point and high precision positioning. It is easy to realize automation with mechanical arm, so as to improve welding efficiency, reduce labor and reduce costs.In addition, laser welding of thin plates or thin wires is not as susceptible to melting back as arc welding.

　 The battery structure usually consists of a variety of materials, such as steel, aluminium, copper, nickel, etc., which can be made into electrodes, wires or enclosures;Therefore, high requirements are put forward for welding process whether between one material or between different materials.The technological advantage of laser welding is that there are a wide range of materials that can be welded to achieve welding between different materials.

　　 Process difficulties

　 Due to the principle of "handiness", the manufacturing of power battery cell usually uses lighter aluminum material, but also needs to be thinner. Generally, the thickness of shell, cover and bottom should be less than 1.0mm. At present, the basic material thickness of some mainstream manufacturers is about 0.8mm.According to statistics, aluminum alloy battery case accounts for more than 90% of the total power battery.

　　 The difficulty of aluminium welding lies in the high initial reflectivity of aluminium alloy to laser beam and its high thermal conductivity, which makes the absorbance of aluminium alloy to laser low before melting. Due to the low ionization energy of aluminium, the photoinduced plasma is not easy to diffuse during welding, thus the welding stability is poor.In addition, the mechanical properties of aluminium alloy welded joints are degraded due to the loss of alloying elements during welding.Due to the high porosity sensitivity during welding, some problems and defects will inevitably occur during welding, the most important of which are pore and hot crack.There are two main types of pore produced during laser welding of aluminium alloy: hydrogen pore and pore produced by key hole breakdown.The hydrogen pore problem is more serious due to the rapid cooling rate of laser welding, and there are also a kind of holes caused by the collapse of small holes in laser welding.

　　 Hot crack problem.Aluminum alloy is a typical eutectic alloy. Hot cracks are easy to occur during welding, including weld crystallization cracks and HAZ liquefaction cracks. Due to eutectic segregation in the weld zone, grain boundary melting occurs. Under stress, liquefaction cracks will form at the grain boundary, which reduces the performance of welded joint.

　 Explosion (also known as splash) problems.There are many factors causing 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 is the decisive factor.Surface bulge, pore and internal air bubbles of the housing are mainly caused by small diameter of the optical fiber core or high laser energy setting.

　 To solve the above problems, it is the key to find the proper process parameters.

　　 Welding mode analysis

　　（1） Pulse mode welding

　　 The common pulse waveforms of pulse laser are square wave, peak wave and double peak wave. Because the surface of aluminium alloy has too high reflectance to light, the appropriate welding waveform should be selected when welding.When high-intensity laser beam is incident on the material surface, 60%~98% of the laser energy on the metal surface will be lost due to reflection, and the reflectivity changes with the temperature of the object surface.Spike wave and bimodal wave are the best choices for welding aluminium alloy in general. The pulse width of slow-drop part behind these two welding waveforms is longer, which can effectively reduce the generation of pore and crack.

　　 Due to the high reflectivity of aluminum alloy to laser, the welding joint is usually deflected to a certain angle during welding in order to prevent vertical reflection of laser beam caused by vertical incidence and damage the laser focusing mirror.The diameter of the weld joint and the effective joint surface increase with the increase of the laser inclination angle. When the laser inclination angle is 40 degrees, the maximum weld joint and the effective joint surface are obtained.The effective penetration and effective penetration of the weld decrease with the inclination angle of the laser. When the inclination angle of the laser is more than 60 degrees, the effective penetration decreases to zero.Therefore, tilting the weld joint to a certain angle can appropriately increase the penetration and width of the weld.In addition, when welding, with the weld as the boundary, 65% of the offset cover plate and 35% of the shell of laser spot need to be welded, which can effectively reduce the explosion caused by cover closure problems.

　　（2） Continuous mode welding

　　 Because the heating process of continuous laser welding is different from that of pulse quench and abrupt heat, the tendency of cracks is not obvious when welding. In order to improve the quality of welding seam, continuous laser welding is used. The surface of welding seam is smooth and even, without spatter and defects, and no cracks are found inside the welding seam.In the aspect of aluminium alloy welding, the advantages of continuous laser are obvious: compared with traditional welding method, it is more efficient and does not need filling wire;Compared with pulse laser welding, it can solve the defects after welding, such as cracks, pore, spatter, etc., and ensure good mechanical properties of aluminium alloy after welding.There is no depression after welding, less polishing and grinding, which saves production costs. However, because of the small light spot of continuous laser, the assembling accuracy of workpiece is required to be high.

　　 During the welding process of power battery, welding technicians will select appropriate laser and welding parameters according to the material, shape, thickness and tension requirements of the battery, including welding speed, waveform, peak value, inclination angle of welding head, etc. to set reasonable welding parameters, so as to ensure the final welding effect meets the requirements of power battery manufacturers.

　　 Square Battery Welding

　 In the welding process of square battery, the most important procedure is the encapsulation of the shell cover. The closure method of the square battery shell is usually a rectangular cover plate with positive input on the top of the battery. The cover plate is inserted into the shell and the opening is flush. Then the rectangular gap between the cover plate and the shell is welded by pulse or continuous laser welding to achieve the seal.

　　 The welding methods of square cell are mainly divided into side welding and top welding, in which the main advantage of side welding is that it has little influence on the inside of the core and spatter does not easily enter the inside of the shell cover.Side welding requires a high level of laser stability and material cleanliness because it may cause bumps which may have a slight effect on subsequent process assembly.The top welding process requires less integration of welding equipment because it is welded on one side.

　　 At present, vertical welding of power battery is the most popular welding method in the industry. Vertical welding only requires one closure joint, which can greatly reduce the risk of side leakage of four closure joints and is beneficial to mass production."Laser Vertical Welding Equipment for High-Speed Battery Housing" of Wuhan Yifei Laser Equipment Co., Ltd. has achieved over 99.5% welding yield and 12PPM production efficiency.

　　 Battery PACK Process

　　（1） Battery PACK

　 The production process of an applied battery pack, which consists of a protective circuit, a housing, and an output, is called PACK.Battery PACK is an important process to realize the application of batteries in different fields.With the continuous development of PACK process and the continuous improvement of connection mode, from the initial soldering to the later resistance welding, laser welding has become the most widely used connection mode for PACK process due to its high welding accuracy, reliability and automation. The intelligent automatic equipment carrying laser welding process has become square, cylinder, soft package, etc.High-end manufacturing equipment in groups of different types of PACK cores, such as 18650.

　　（2） Intelligent Equipment Development Trend

　 The development of new energy automotive industry has not standardized and finalized the specifications of power batteries and battery modules used in it. Many incompatible specifications systems have arisen. Current process flow and manual operation restrict the production pace and efficiency of enterprises, which can not effectively improve product quality and productivity.Therefore, it is necessary to improve the automation level of power battery module assembly.Nowadays, realizing the intelligent solution of "whole line equipment + robot + software control" not only solves the compatibility, whole line rhythm and efficiency problems focused by users, but also solves the problems of small batch and large specifications of PACK orders.

　　 Manage software aspects.The whole MES system directly turns the production line into quasi-unmanned workshop, and only on-line material replenishment is required by manual, which not only improves safety, but also reduces human intervention.In welding procedure, only the data of laser welding process need to be integrated in MES management software system to facilitate users to call and switch directly.From the core to PACK group, parameters, data and other incoming information of each process can be queried and processed quickly by MES system. It is necessary to control the process and guarantee the production efficiency effectively. Users also realize remote monitoring and management through reserved industrial communication interface, which fully reflects the manufacturing characteristics of intelligent automation.Products with laser solutions are moving toward high intelligence and automation.

　 Summary

　 Although China's laser welding technology is becoming more and more mature, the high-quality power battery still needs close cooperation from manufacturers'designers and laser welding technicians to optimize the design of material, shape, thickness, process, real-time detection and other aspects in order to achieve the ideal welding effect.Wuhan Yifei Laser Equipment Co., Ltd. has more than ten years of experience in power battery welding field, and is committed to creating high-precision, high efficiency, high reliability, unmanned, visualized and informationized production line solutions for battery cells, modules and PACK intelligent automation manufacturing.