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Busbar Welding For EV Batteries--The selection of battery and busbar laser welding methods(Jade-busbar)

• 2023-11-28

Busbar Welding For EV Batteries--The selection of battery and busbar laser welding methods (JADE)

The reasonable selection of battery and busbar welding methods and processes in the manufacturing process of power batteries will directly affect the cost, quality, safety and consistency of batteries. Next, JADE will sort out the content of laser welding of power batteries. Hope it is helpful to readers.

1. Principle of laser

 

aser welding is to use the excellent directionality and high power density of the laser beam to work, through the optical system to focus the laser beam in a small area, in a very short time to form a highly concentrated heat source area of the welded place, so that the welded object melts and forms a firm weld joint and weld.

2. Types of laser welding

Thermal conduction welding & deep penetration

 

weldingThermal conduction welding, in which the laser beam melts the surface of the workpiece along the seam, and the melt flows together and solidifies to form a weld. Mainly used for relatively thin materials, the maximum welding depth of the material is constrained by its thermal conductivity, and the weld width is always greater than the welding depth.
 Deep penetration welding, when the high-power laser gathers on the surface of the metal, the heat does not have time to dissipate, and the welding depth will deepen sharply, this welding technology is deep penetration welding. Because of its extremely fast processing speed, small heat-affected area and minimal distortion, it can be used for deep welds or several layers of material together.
The main difference between heat conduction welding and deep penetration welding is the power density applied to the metal surface per unit time, and the critical value is different under different metals.
Penetration welding and seam welding, the connecting piece does not need to be punched, and the processing is relatively simple. Penetration welding requires a laser welder with high power. 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 a lower power laser welder. The penetration of seam welding is higher than that of penetration welding, and the reliability is relatively good. However, the connecting piece needs to be punched, and it is relatively difficult to process.

Pulse welding and continuous welding

1) Pulse mode welding laser welding should choose the appropriate welding waveform, commonly used pulse waveforms are square wave, spike wave, double peak wave, etc., the reflectivity of the aluminum alloy surface to light is too high, when the high-intensity laser beam to the surface of the material, the metal surface will have 60%-98% of the laser energy lost due to reflection, and the reflectivity changes with the surface temperature. Generally, when welding aluminum alloy, it is best to choose sharp wave and bimodal wave, which has a longer pulse width in the slowly descending part behind the welding waveform, which can effectively reduce the generation of porosity and cracks.

Due to the high reflectivity of the aluminum alloy to the laser in the pulsed laser welding sample, the welding
head is usually deflected at a certain angle during the welding process in order to prevent the vertical reflection caused by the vertical incidence of the laser beam and damage to the laser focusing mirror. The diameter of the solder joint and the diameter of the effective bonding surface increase with the increase of the laser inclination angle, and when the laser inclination angle is 40°, the largest solder joint and effective bonding surface are obtained. The penetration depth and effective penetration of the solder joint decrease with the inclination angle of the laser, and when it is greater than 60°, the effective welding penetration decreases to zero. Therefore, tilting the welding head to a certain angle can appropriately increase the penetration depth and width of the weld.
In addition, during welding, it is necessary to weld 65% of the laser weld spot and 35% of the shell with the weld as the boundary, which can effectively reduce the explosion caused by the problem of closing the cover. 

2) Continuous mode welding continuous laser welding because its heating process is not like pulse machine sudden cooling and sudden heating, the crack tendency is not very obvious during welding, in order to improve the quality of the weld, the continuous laser welding is adopted, the surface of the weld is smooth and uniform, no spatter, no defects, and no cracks are found inside the weld. In the welding of aluminum alloy, the advantages of CW laser are obvious, compared with the traditional welding method, the production efficiency is high, and there is no need to fill the wire; compared with pulse laser welding, it can solve the defects generated after welding, such as cracks, porosity, spatter, etc., to ensure that the aluminum alloy has good mechanical properties after welding; it will not sink after welding, and the amount of polishing and grinding after welding is reduced, which saves production costs, but because the spot of CW laser is relatively small, the assembly accuracy of the workpiece is required to be higher.

 In the welding of power batteries, welding process 

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

3. The advantages of laser welding are energy concentration, high welding efficiency, high processing accuracy, and large weld depth-width ratio. The laser beam is easily focused, aligned and guided by optical instruments, and 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 space constraints mentioned above.  The heat input is small, the heat affected zone is small, the
residual stress and deformation of the workpiece are small, the welding energy can be accurately controlled, the welding effect is stable, the welding appearance is good, the non-contact welding, the optical fiber transmission is good, the accessibility is good, and the degree of automation is high. When welding thin or thin diameter wires, it is not as easy to remelt as arc welding. Due to the principle of "lightness", the battery cells used for power batteries usually use lighter aluminum materials, and they also need to be made more "thin", and the general shell, cover, and bottom are basically required to reach less than 1.0 mm, and the thickness of the basic materials of mainstream manufacturers is about 0.8 mm. Provides high-strength welding for a wide range of material combinations, especially when welding between copper and aluminum. It is also the only technology that can be used to weld nickel plating to copper materials.

4 Difficulties in laser welding processAt present, the battery shell made of aluminum alloy material accounts for more than 90% of the entire power battery. The difficulty of welding lies in the extremely high reflectivity of aluminum alloy to laser, high sensitivity to porosity during welding, and some problems and defects will inevitably occur during welding, the most important of which are porosity, thermal cracks and explosions.
Pores are easy to occur during laser welding of aluminum alloys, and there are two main types: hydrogen pores and pores caused by bubble bursting. The problem of hydrogen porosity is exacerbated by the rapid cooling rate of laser welding, and there is an additional type of hole in laser welding due to the collapse of small holes.

Thermal crack problem. Aluminum alloy is a typical eutectic alloy, which is prone to thermal cracks during welding, including weld crystalline cracks and HAZ liquefaction cracks, due to the segregation of the components of the weld zone, eutectic segregation will occur and the grain boundary will melt, and liquefaction cracks will be formed at the grain boundaries under the action of stress, reducing the performance of the welded joint.

Exploding fire (also known as splashing) problems. 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. Protrusions, pores, and internal bubbles on the surface of the shell. The main reason is that the core diameter of the fiber is too small or the laser energy is too high. It is not the "better beam quality, better welding effect" as advertised by some laser equipment providers, and good beam quality is suitable for superimposed welding with large penetration depth. Finding the right process parameters is the key to solving the problem.

Other difficult soft package  tab welding has high requirements for welding tooling, and the tab must be pressed firmly to ensure the welding gap. It can realize high-speed welding of complex trajectories such as S-shape and spiral, and strengthen the welding strength while increasing the joint area of the weld.  The welding of
cylindrical cells is mainly used for the welding of the positive electrode, and it is easy to weld through due to the thin shell of the negative electrode. For example, the negative electrode weld-free process used by some manufacturers at present, and the positive electrode is laser welding.
When the prismatic battery is welded together, the pole or connecting piece is polluted thickly, and when the connecting piece is welded, the contaminants decompose, which is easy to form welding explosion points and cause holes, and the battery with thin pole and plastic or ceramic structural parts under it is easy to weld through. When the pole is small, it is also easy to weld to the point of plastic burning, forming a burst point. Do not use multi-layer connecting pieces, there are pores between the layers, it is not easy to weld firmly.
The most important process in the welding process of square batteries is the encapsulation 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, they use the "deep-drawing" process to manufacture the battery case, and only need to weld the top cover.

Prismatic power battery side welding
samplesPrismatic battery welding methods are mainly divided into side welding and top welding, among which the main advantage of side welding is that the impact on the inside of the battery cell is less, and the spatter will not easily enter the inside of the shell cover. Because it may cause protrusions after welding, which will have a slight impact on the assembly of the subsequent process, the side welding process has extremely high requirements for the stability of the laser and the cleanliness of the material. The top welding process has low requirements for the integration of welding equipment due to welding on one surface, and the mass production is simple, but there are also two disadvantages, one is that there may be a little splash in the welding into the cell, and the other is that the high processing requirements of the front section of the shell will lead to cost problems.

5 Influencing factors of 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, causing the working temperature to rise sharply, and the workpiece is melted and reconnected to form a permanent connection. The shear strength and tear resistance of laser welding are relatively good. The conductivity, strength, air tightness, metal fatigue and corrosion resistance of battery welding are typical welding quality evaluation standards.
There are many factors that affect the quality of laser welding. Some of them are highly volatile and quite unstable. How to correctly set and control these parameters to ensure the welding quality in the appropriate range in the high-speed continuous laser welding process. The reliability and stability of weld forming are important issues related to the practical application 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 lower the order of the beam mode, the better the beam focusing performance, the smaller the spot, the higher the power density and the greater the depth and width of the weld under the same laser power. Generally, the basic mold (TEM00) or low-order mold is required, otherwise it is difficult to meet the requirements of high-quality laser welding. At present, it is still difficult for domestic lasers to be used for laser welding in terms of beam quality and power output stability. Judging from the foreign situation, the beam quality and output power stability of the laser are quite high and will not become a problem for laser welding. The biggest 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), the small focal length is good for reducing the diameter of the waist spot of the focused beam, but too small is easy to be polluted and spatter damage during the welding process.

 

The shorter the wavelength, the higher the absorption, and generally the materials with good conductivity have high reflectivity, for YAG laser, the reflectivity is 96% for silver, 92% for aluminum, 90% for copper, and 60% for iron. The higher the temperature, the higher the absorption rate, which is a linear relationship, and the absorption rate can be improved by coating phosphate, carbon black, graphite, etc. on the surface.

2) The condition of the workpiece: laser welding requires the edge of the workpiece to be processed, the assembly has a high precision, the spot is strictly aligned with the weld, and the original assembly accuracy of the workpiece and the spot alignment can not be changed due to the thermal deformation of the weld during the welding process. This is because the laser spot is small, the weld is narrow, and the metal is generally not filled, if the assembly is not tight, the gap is too large, the beam can pass through the gap and cannot melt the base metal, or cause obvious undercuts and depressions, such as the deviation of the spot to the seam is slightly larger, it may cause no fusion or no weld penetration. Therefore, in general, the plate butt assembly gap and spot to seam deviation should not be greater than 0.1mm, and the wrong edge should not be greater than 0.2mm. In practice, sometimes laser welding technology cannot be used because these requirements cannot be met. In order to obtain good welding results, the allowable butt gap and lap gap should be controlled within 10% of the thickness of the thin plate.
Successful laser welding requires close contact between the substrates to be welded. This requires careful tightening of the parts to achieve the best results. And this is difficult to do well on thin tab substrates, as it is prone to bending misalignment, especially if the tabs are embedded in large battery modules or components.

3) Welding parameters (1) The most important of the welding parameters is the power density of the laser spot, which has the following effects on the welding mode and the stability of the weld forming: with the small to large power density of the laser spot, it is stable thermal conduction welding, mode unstable welding and stable deep penetration welding. The power density of the laser spot, in the case of a certain beam pattern and focal length of the focusing mirror, is mainly determined by the laser power and the focal position of the beam. The laser power density is proportional to the laser power. When the beam focus is in a certain position below the surface of the workpiece (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 spot on the surface of the workpiece will become larger, resulting in a smaller power density, and to a certain range, it will cause a change in the form of the welding process.

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

(2) In the range of deep penetration welding, 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, about 0.7 power relationship, and the higher the welding speed, the shallower the penetration depth. Under the condition of a certain laser power and welding speed, the penetration depth is the largest when the focus is in the optimal position, and when it deviates from this position, the penetration depth decreases, and even becomes unstable mode welding or stable thermal conduction welding.

(3) The main function of the shielding gas is to protect the workpiece from oxidation during the welding process, to protect the focusing lens from metal vapor pollution and sputtering of liquid droplets, to disperse the plasma generated by high-power laser welding, to cool the workpiece and reduce the heat-affected zone.

Argon or helium is usually used as the shielding gas, and nitrogen can also be used if the apparent quality is not high. Their propensity to produce plasma is significantly different: helium has a lower tendency to produce plasma than argon under the same conditions due to its high ionization and fast thermal conduction, allowing for greater penetration. In a certain range, with the increase of the shielding gas flow, the tendency to inhibit the plasma increases, so the penetration depth increases, but when it increases to a certain range, it tends to be stable.

(4) Monitoring: Among the four welding parameters, the welding speed and shielding gas flow are the parameters that are easy to monitor and maintain stability, while the laser power and focus position are the 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, due to the loss of the light guide and focusing system, the laser power reaching the workpiece will change, and this loss is related to the quality, use time and surface contamination of the optical workpiece, so it is not easy to monitor and becomes an uncertain factor in the welding quality. The beam focus position is one of the most important factors in the welding parameters that has the greatest impact on the weld quality and is the most difficult to monitor and control. At present, in production, it is necessary to rely on manual adjustment and repeated process testing methods to determine the appropriate focus position to obtain the ideal penetration depth. However, during the welding process, due to the deformation of the workpiece, the thermal lensing effect or the multi-dimensional welding of the spatial curve, the focus position will change and may exceed the allowable range.
 For the above two situations, on the one hand, it is necessary to adopt high-quality, high-stability optical components, and maintain them frequently, prevent pollution, and keep them clean; on the other hand, it is required to develop real-time monitoring and control methods for laser welding process to optimize parameters, monitor the changes of laser power and focus position of the workpiece, realize closed-loop control, and improve the reliability and stability of laser welding quality.

Finally, it is important to note that laser welding is a melting process. This means that the two substrates will melt during the laser welding process. This process is fast, so the overall heat input is low. However, because this is a melting process, brittle, high-resistance intermetallic compounds can be formed when different materials are welded. Aluminum-copper combinations are particularly susceptible to the formation of intermetallic compounds. These compounds have been shown to have negative effects on the short-term electrical properties and long-term mechanical properties of lap joints in microelectronic devices. The impact of these intermetallic compounds on the long-term performance of lithium batteries is uncertain.

 

Summarized By Jade-Busbar.(www.jade-busbar.com)