What are the applications of laser in the manufacture of power lithium batteries?

Since 1990, because of its high energy density, high voltage, environmental protection and the advantages of long life and fast charging, attracted by 3 c digital, power tools and other industries, lithium battery has excellent contribution to the new energy automotive industry. As a new lithium battery industry to provide energy vehicle with power source, the market potential is tremendous. It is an important part of national strategy development. The industry scale is expected to exceed 160 billion yuan in the next 5-10 years.

Power battery as the core component of new energy vehicles, its quality directly decides the performance of the vehicle. Lithium battery manufacturing equipment is commonly front-end equipment, in the end, after three end equipment, the equipment accuracy and automation level will directly influence the production efficiency and consistency of the products. The laser processing technology as an alternative to the traditional welding technology has been widely used in li-ion battery manufacturing equipment.

In this paper, the laser in the application of power battery industry, expounds the process characteristics of laser welding, aluminum alloy laser welding were analyzed the difficulty on the quality of the welding and welding mode, enumerates the square power battery and battery PACK process characteristics and equipment development trend.

Laser welding process

From the manufacture of lithium-ion batteries to the battery PACK group, welding is an important manufacturing process, the conductivity of the lithium battery, strength, air tightness and metal fatigue and corrosion resistance, is a typical battery welding quality evaluation standard.

The selection of welding methods and welding technology, it will directly affect the cost of the battery, quality, safety and consistency of the battery. In many welding method, laser welding to advantage to stand out as follows: first of all, high energy density laser welding, small welding deformation, heat affected zone is small, can effectively improve the precision of fabrication, smooth weld without impurities, uniform density, without additional grinding work; Second, the laser welding can be precise control, focusing on small points of light, high precision positioning, combined with mechanical arm is easy to realize automation, improve the efficiency of welding, reduced working hours, reduce the cost; In addition, the laser welding of thin sheet or fine diameter wire, not as susceptible to melt back arc welding.

The structure of the battery usually contains a variety of materials, such as steel, aluminum, copper, nickel, etc., these could be made from metal electrodes, wires, or shell; Therefore, whether it is between a material or a variety of materials, welding, for welding technology put forward higher requirements. Laser welding technology advantage is that can wide variety of welding material, welding between different materials can be achieved.

Technological difficulties

The manufacture of power battery batteries due to follow the principle of "light", usually adopt a "light" aluminium, but also to do more "thin", generally the thickness of the shell, cover and bottom basic require below 1.0 mm, the thickness of some mainstream manufacturers of basic materials at about 0.8 mm. According to statistics, aluminum alloy material of battery shell account for more than 90% of the whole power battery.

Aluminum welding difficulty lies in the aluminum alloy high initial reflectivity of laser beam and its own high thermal conductivity, makes the absorptance of laser is prior to melting of aluminum alloy in low, due to the low ionization energy, aluminum plasma during the welding process performance to is not easy to spread, make welding stability. In addition, the welding process of alloy element loss, decrease the mechanical properties of aluminum alloy welded joint. Because of high stomatal sensitivity in welding process, welding defects inevitably appeared some problems, one of the most main is porosity and thermal crack. Occurring in the process of laser welding of aluminum alloy gas mainly include two categories: hydrogen porosity and keyhole burst of porosity. Due to the laser welding cooling speed too fast, hydrogen hole problem is more serious, and also in laser welding for a class of more holes produced due to the collapse of pores.

Hot crack problems. Aluminum alloy belongs to the typical type of eutectic alloy, easy to appear when welding hot crack, including crystallization weld cracks and crack, HAZ liquefied due to weld area composition segregation will appear a eutectic segregation and grain boundary melting, under the action of stress will be liquefied crack at grain boundary, degrade the performance of the welded joint.

Fried fire (also called splash) problem. Fried fire caused by many factors, such as the purity of the materials, the purity of material itself, the characteristics of material itself, and the decisive role is the stability of the laser. Bump, porosity, internal bubble shell surface, investigate its reason, mainly is the fiber core diameter is too small or caused by laser energy is set too high.

In view of the above problems, find the suitable process parameters is the key to solve the problem.

Welding mode analysis

(1) pulse welding mode

Pulse laser commonly used square wave pulse waveform, spike wave, bimodal wave, such as several, due to the aluminum alloy surface light reflectivity is too high, should be to choose the appropriate welding waveform. When the incident laser beam to the surface of a material, high intensity of metal surface will be 60% ~ 98% of the laser energy loss due to reflection, and reflectivity changes with the temperature of the object surface. In when the welding of aluminium alloy, the optimal choices pointed and bimodal wave, these two kinds of welding waveform behind slow down part of the pulse width is longer, can effectively reduce the pores and cracks.

Due to the high reflectivity of laser is aluminium alloy, in order to prevent vertical reflection and damage caused by the incident laser beam is focused on the mirror, the welding process will usually welding head deflection Angle. Solder joints and effectively combine the diameters, along with the increase of the laser Angle increases, when the laser Angle of 40 °, the largest solder joints and joint surface effectively. And the effective penetration depth of molten solder joints with laser Angle decreases, and when the laser Angle is greater than 60 °, the effective welding penetration to zero. So tilted welding head to a certain point of view, can be appropriately increased weld depth and weld width. In addition during welding to weld lines, laser spot welding must be partial shell cover 65%, 35% for welding, so can effectively reduce because of the close cover problems caused by fire.

(2) model of continuous welding

Continuous laser welding process because of its heat quench heat, unlike pulse welding crack tendency is not obvious, in order to improve the quality of weld, using continuous laser welding, weld surface smooth, uniform, no splash, no defects, no crack found inside the weld. In terms of aluminum alloy welding, continuous laser obvious advantages: compared with the traditional way of welding, high efficiency, without wire filling; Compared with the pulsed laser welding, can be solved in the produce after welding defects, such as crack, air hole, splash, etc., to ensure good mechanical properties of aluminium alloy after welding; Don't sag after welding, polishing grinding amount decreases, save the production cost, but because of the continuous laser spot is lesser, higher requirements so that the fitting precision of the workpiece.

In power battery during the welding process, welding technology and technical personnel will be according to the requirement of battery materials, shape, thickness, tension, and choose the appropriate laser welding process parameters, such as welding speed, waveform, peak, welding head tilt Angle to set up the reasonable welding process parameters, such as to guarantee the final welding results meet the requirements of power battery manufacturers.

Square battery welding

In the welding process of square cells, one of the most important working procedure is encapsulation shell cover, the square battery shell sealing method is usually on the top of the battery has a rectangular plate, plate with the anode input end, into the shell plate flush with the mouth, then between the plate and shell with a laser rectangular aperture by pulse or a continuous laser welding, welding good sealing.

Welding methods are divided into side of the square battery welding and welding, one side welding are the main benefits of less effect on the internal batteries, splash will not easily into the shell cover inside. Because may lead to raise after welding, the assembly will be a slight effects of subsequent process, thus side welding process on the stability of the laser, the material of high cleanliness requirements. Because top welding in a plane, low requirements for welding equipment integration.

At present, power battery set welding way is widely favored by the industry, vertical welding a convergent node, can significantly reduce side welding four convergent node sliding sideways risk, but also to mass production. Wuhan escape flying laser equipment co., LTD. "high speed laser welding equipment" battery shell, achieved more than 99.5% of the welding production efficiency of roa and 12 PPM.

The battery PACK process

(1) battery PACK

Battery batteries by adding protection circuit, shell, output, and form the application of battery production process called PACK. Battery PACK is an important step of battery application in different fields. With the continuous development of PACK process, connection mode also continue to improve, from the initial soldering to later resistance welding, development up to now, laser welding because of its welding precision, reliability and the advantages of high degree of automation, has become the current PACK process is the most widespread way of connection, and carrying the laser welding technology of intelligent automation equipment has become a square, cylindrical, soft bag, 18650 different types of batteries PACK groups of high-end manufacturing equipment.

(2) the development trend of intelligent equipment

The development of new energy automotive industry, has not the power battery and battery module used by the specifications of the standard stereotypes and standardization, numerous incompatible specification system problems, the current technological process and the production tempo and efficiency of manual operation restricted the enterprise, to effectively improve product quality and production capacity. Therefore, raising the level of automation in power battery module assembly is very necessary. Today, the realization of "the whole line equipment + + robot software control" of the intelligent solution, to solve the user focus on both the compatibility, the whole line tempo and efficiency, and to solve the user battery PACK orders, the problem of small batch and many specifications.

Management software. The whole MES system directly to the production line into unmanned production workshop, artificial only need to supplement the material online outside, can not only improve the safety, and reduce the human intervention. Welding process link, just need to laser welding process data integration in MES management software system, to facilitate the user calls, switch directly. From batteries to PACK group, the parameters of every working procedure, data and other incoming information, etc。, can quickly query and timely analysis through the MES system, not only to do process control, and to effectively guarantee the production efficiency, users have reserved by industrial communication interface to realize remote monitoring and management, fully embodies the intelligent automation manufacturing characteristics. With laser solutions products have been towards the trend of high intelligence, high automation direction.

Summary

Although laser welding technology in China has become increasingly mature, but the quality of the power battery manufacturer still need to design personnel and laser welding technicians work closely, from material, shape, thickness, process optimization design, the real-time detection and so on various aspects, to achieve the ideal effect of welding. Wuhan fly from laser equipment co., LTD. With more than 10-year-experience in the area of power battery welding, to create high precision, high efficiency, high reliability, unmanned, visualization and information of battery cell and module production line and PACK intelligent automation solutions.

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