Today, we share the application of laser welding technology in electric vehicle-power battery, mainly including battery electrode ear-bus welding, battery shell welding, etc., involving Al-Fe, Al-Cu, Cu-Fe and other different-material welding.
01
Application background
In the context of global warming and greenhouse gas emission reduction, new energy vehicles, especially electric vehicles, are rising rapidly. The manufacturing of its core component-battery system, requires strict welding technology, mainly because the battery system has complex structure, including a variety of materials and complex connection. Traditional welding methods, such as ultrasonic welding and resistance spot welding, have limitations in dealing with the connection of battery electrode materials (such as aluminum, copper and steel). For example, ultrasonic welding is not suitable for the common battery structure of electric vehicles, and the resistance spot welding is difficult to weld due to the high conductivity of aluminum and copper.
Laser welding technology is an ideal choice for its non-contact, high energy density, precise thermal input control and easy automation. It can meet the welding needs of different materials of battery system, such as aluminum steel, copper aluminum, copper steel welding between battery electrode and bus, and aluminum / steel battery shell welding, which plays a key role in ensuring the reliability of connection, and improving battery performance and safety.
02
Power battery type
Power battery type
① Small cylindrical battery (e. g. model 18650), standardized size, safety and relatively cheap cost;
② Large prism batteries, which perform well in terms of energy density and stability;
③ Soft-coated polymer battery, prone to geometry when charging.
* Battery type
The battery pack is composed of multiple batteries in series or in series and parallel, which are connected through busbars. The working environment is complex, and the connection reliability directly affects the performance and safety of the battery system.
* Battery pack structure a) cylindrical battery b) prismatic battery
Limitations of common welding techniques
supersonic welding
Mainly uses high frequency vibration (usually 20 kHz and above) to make the material form a solid bonding under pressure to achieve the connection.
① This method is suitable for welding thin foil, different materials or high conductive materials, mainly applied to strip batteries.
② Electric vehicle batteries are usually cylindrical or prismatic batteries, which may destroy the integrity of the battery structure under the combination of pressure and vibration, so ultrasonic welding is not suitable for the battery welding of electric vehicles.
resistance spot welding
The working principle is mainly to apply pressure on the contact surface of the workpiece, and to use a large current to melt the parts locally. However, the common materials of electric vehicle batteries are aluminum and copper, which have the characteristics of high electrical conductivity and thermal conductivity, making it difficult to weld the resistance spot welding.
03
Battery pole ear is welded with the bus
Welding characteristics
Material combination: the battery pole ear material is often aluminum, copper or steel, the bus material is mostly copper or aluminum, forming aluminum-copper, aluminum-steel, copper-steel and other combinations.
High performance requirements: the welding site should ensure low resistance, high conductivity and good mechanical strength, in order to ensure the battery charge and discharge efficiency and long-term stability.
* Pole ear and bus of soft pack / cylindrical battery
Aluminum-steel laser welding
Welding difficulties:
① Aluminum and steel thermal properties are very different, welding will form brittle metal intermetallic compound (IMC), such as Fe₂Al₅, Fe₄Al₁₃, etc., affect the microstructure, electrical performance and thermal performance of the joint, increase the internal resistance of the battery, shorten the service life.
② IMC generation should be controlled during welding.
① Control the thermal input: adjust the laser power, welding speed and pulse parameters (frequency, duty ratio), balance the melting depth and the size of the thermal influence area, and reduce the IMC generation.
② Optimize pulse waveform: special pulse waveform is used to change the thermal cycle characteristics, such as slow rise and slow drop waveform to reduce the temperature gradient and thermal stress, and inhibit the rapid cooling leading to a large number of brittle IMCs generation.
① Composition: nickel selection, silicon based alloy and other intermediate layer materials, because of its reaction with aluminum steel. Nickel improves wetting and element diffusion, and the toughness of nickel-containing IMCs is better than that of aluminum steel. Si in the Al-Si compounds) affects the growth of Fe-Si compounds optimizes the mechanical properties of the joint, and the Si content is fine-adjusted according to the material and process requirements.
② Thickness: μ m to tens of μ m thickness range can effectively adjust the formation of IMMC, improve joint performance and reliability.
① Magnetic field direction: The vertical magnetic field inhibits the macroscopic diffusion of elements in the melt pool, changes the convection and crystallization morphology, and reduces the excessive fusion of Fe and Al to form brittle IMCs; the parallel magnetic field affects the micro-diffusion of solute and grain boundary migration, and refines the grains and optimizes the distribution and orientation of IMCs.
② Multi-field collaboration: combined with magnetic field and ultrasound, magnetic field collaboration and ultrasonic vibration to refine grains, remove stomatal inclusions, and improve the IMC structure. Ultrasonic vibration is helpful to break dendrites and uniform composition, magnetic field guides the direction of metal liquid flow and crystal growth, improve the solidification behavior and tissue uniformity of the molten pool, reduce the brittleness of IMC, and improve the toughness and electrical conductivity of joints.
* Laser swing welding aluminum-steel (swing amplitude 0.2-1.2mm)
* Micromorphology of stainless steel / aluminum alloy joint a) Ni foil b) no Ni foil
Copper-aluminum laser welding
Welding difficulties:
Copper and aluminum have different melting points, thermal conductivity and thermal expansion coefficient, forming Cu ₂ Al and Cu₄Al₃ IMC by welding, which affect the microstructure and mechanical properties of welds, and need to inhibit their formation and growth.
① Matching high welding speed with low laser power can reduce the thermal input time and strength, and inhibit the formation of a large number of IMC. For example, during high-speed welding, CuAl ₂ and other compounds are significantly reduced.
② Optimize the pulse frequency and duty cycle, change the atomic diffusion and reaction dynamics conditions of Cu and Al, make the IMC grow orderly and evenly distributed, and improve the joint performance.
For example, containing tin filling material, welding to form Cu₆Sn₅ and Cu₃Sn phases, change the tissue form of the joint, reduce the overall brittleness, improve the strength and toughness.
* Cu-Al SEM diagram (1500W, 30 mm/s)
Copper-steel laser welding
Welding difficulties:
The physical properties of copper and steel are very different, and the liquid phase separation and thermal cracks are prone to occur during laser welding, such as Cu infiltration into the steel grain boundary, leading to thermal cracks.
The welding quality can be effectively improved by deflecting the laser to the copper side.
Under the condition of annular beam oscillation of pure copper and stainless steel, the crack resistance of weld can be effectively improved. During the solidification process, the increase of the grain boundary significantly reduces the stress concentration and effectively controls the joint strength and deformation.
* Unoscillating with an oscillating SEM
04
Battery shell welding
* Tesla 4680 Battery
Laser welding of aluminum battery shell
Due to its high thermal conductivity and large thermal expansion coefficient, aluminum alloy welding is easy to show cracks and pore defects; the surface oxide film and impurities are easy to decompose at high temperature, making the gas difficult to escape and cause pores.
Laser welded 1060 aluminum alloy uses vertical oscillation to optimize the weld surface, reducing the porosity by 91% at a radius of 0.45mm.
* Laser beam focus rotation and vertical oscillation SEM
The light spot shaping is four-beam welding, which increases the size of the small hole in the molten pool, stabilizes the metal vapor, reduces the splatter and pores, and improves the weld quality.
* Schematic diagram of four beams
Steel battery shell by laser welding
The welding of austenitic stainless steel is prone to thermal cracking, which is related to the alloy composition and impurity content. The problem of thermal cracking can be effectively solved by adjusting the process parameters.
arouse:
① At present, the commonly used laser welding wavelength is mostly 1064nm, using blue laser / green laser welding heterogeneous materials may have a good effect.
Contact Person: Ms. Coco
Tel: +86 13377773809
