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Processing Technology of Power Battery Box of Aluminum Alloy: Casting, Welding and Extrusion Forming

In electric vehicles, the weight of the power battery pack accounts for about 30% of the total vehicle mass. The lightweight of automobiles and the inexhaustible pursuit of the energy density of power battery systems all require the lightweight design of the battery pack structure. Inside the battery pack system, the cabinet as a maximum structural member, its weight cut down, the energy density can be improved not be ignored. Under the premise of structural optimization and re-optimization, the use of new materials is the fundamental way to reduce the weight of the battery box. Not to mention the cost, the cost of small batches of new products is relatively high, which is a problem that needs to be solved in the follow-up, not a reason to prevent people from considering the possibility of its application.

Based on the experience gained from the whole vehicle, it is considered to be used as a new material for steel substitutes in automobiles. Common materials are:
Aluminum alloy, magnesium alloy, carbon fiber composite material, today's protagonist aluminum alloy material, is the relatively mature technology among the three materials. At present, a large part of the body can use aluminum, such as heat exchangers, wheels and body, aluminum alloy can achieve a good weight reduction effect. This paper organizes the main types of aluminum alloys, as well as the main processing methods of aluminum alloy boxes.
Power Battery Box of Aluminum Alloy

Type and performance of aluminum alloy
The aluminum element is the most abundant metal element in the earth's crust, accounting for about 8.13%. The atomic number of aluminum is 13, the atomic weight is 27, the melting point is 660 ° C, and the density is 2.7 g/cm^3 . The actual density of aluminum alloy structural parts varies according to the processing technology and varies in a small range. The die-casting is about 2.6-2.63 g/cm^3, the extrusion is 2.68-2.7 g/cm^3, and the forging is 2.69-2.72 g/cm^3.
Typical aluminum alloy sheet mechanical parameters found on the Internet, typical 6-series aluminum sheet, tensile strength 310 MPa, yield strength 276 MPa; The mechanical properties of the 5 series are lower than the 6 series, and the 7 series is higher than the 6 series. Common steel Q235 characteristic parameters, tensile strength 375-500 MPa, yield strength 235 MPa. Comparing steel and aluminum, tensile strength and yield strength, aluminum is slightly lower.

Type of aluminum alloy
first, Application of aluminum alloy casting.
Cast aluminum alloys are widely used in automobile manufacturing and can provide different casting methods according to different automobile production requirements. In the original market, cast aluminum alloys were mainly used for engines, hubs, and anti-collision beams. Cast aluminum alloy battery case, the history of use is also relatively long. However, the original mainstream products adopt the traditional casting method, the surface of the box is rough, the precision is low, and the shape is simple, and the wall thickness of the box body cannot be too thin.

second, Application of deformed aluminum alloy.
Compared with cast aluminum alloy, deformed aluminum alloy has greater arbitrariness and strength advantage, and its alloy content is relatively low. It is generally used in automotive trim parts, structural parts, heat dissipation systems and body panels. The deformed aluminum alloy consists of a series of aluminum alloy sheets, which have high strength and good weldability, and have been used to manufacture battery cases and modules.

Third, The application of aluminum matrix composites.
The kind of aluminum alloy material having good dimensional stability, and has a lower density, high strength, in automotive production applications capable of producing anti-fatigue, fracture resistance and other advantages.

Typical aluminum alloy box processing technology
Large aluminum alloy box molding process, mainly including casting and welding. Among them, precision casting (or net size casting) can be realized, that is, the inner cavity and shape of the casting often require one-time forming, so that its shape is close to the final shape of the part or part, with less processing or no processing. There are three main types: anti-gravity casting, investment casting, and gypsum casting.

3.1 Casting
3. Casting has always been the main process for mass production of aluminum alloy boxes. When net size casting has been widely used, casting is the gospel of large size parts processing.

Anti-gravity casting
A casting method in which the alloying liquid is filled and solidified from the bottom in the opposite direction to the gravity by the applied pressure. The counter gravity casting process has the main characteristics of stable filling, controllable filling rate, reasonable temperature distribution, solidification under pressure and conducive to solidification and feeding of castings. The anti-gravity castings have good mechanical properties, compact structure and few casting defects.
According to different processes, anti-gravity casting is divided into low pressure casting, differential pressure casting and pressure regulating casting. During World War II, low-pressure casting technology was invented and used to manufacture aircraft air-cooled engine block castings; On the basis of low-pressure casting, a differential pressure casting process combining low-pressure casting and autoclave casting has been developed for the manufacture of large, complex, thin-walled parts. The pressure-regulating casting process was developed on the basis of differential pressure casting. The biggest difference between pressure regulating casting and differential pressure casting is that it can not only realize positive pressure control, but also realize negative pressure control. At the same time, the control accuracy of the control system is also higher.

Precision-investment casting
Investment casting has the following advantages:
Investment castings have high dimensional accuracy and surface finish. The dimensional accuracy is generally up to CT4-6 (the sand casting is CT10-13 and the die casting is CT5-7); Flexible design allows casting of highly complex castings; Clean production, no chemical binder in the molding sand, the mold material is harmless to the environment at low temperature, and the recovery rate of the old sand is over 95%.
Explain "CT4-6", CT is the dimensional tolerance level of the casting, the higher the number followed by the lower the accuracy, that is, the larger the allowable range of casting size.

Gypsum casting
The gypsum type can be used to make castings with high dimensional accuracy and low surface roughness and low residual stress, which has many features that other castings do not have: Can accurately replicate the pattern, the surface roughness of the aluminum alloy casting can reach 0.8~3.2μm; The thermal conductivity is low, the thin-walled part is easy to form completely, and the thinnest can be cast into a thin wall of 0.5 mm; Castings with complex shapes can be manufactured.
There are three main types of casting gypsum:
Non-foaming gypsum mould, foaming gypsum mould and investment casting gypsum mould. The non-foamed gypsum type has poor gas permeability, and mainly uses low-pressure casting to produce castings with lower performance requirements. The foamed gypsum type has a certain gas permeability and can be used to produce thin-walled (thinest 0.5 mm) aluminum alloy castings with curved shapes.

3.2 welding
At present, there are many welding methods for aluminum and its alloys. Its welding methods usually include TIG welding, MIG welding, laser welding, seam welding, resistance welding, electron beam welding, friction stir welding and induction welding. The first two kinds of welding are widely used: TIG welding and MIG welding.
Tungsten argon arc welding is the most common welding method for aluminium products. Especially suitable for welding aluminum and aluminum alloy with a thickness of less than 5mm, mainly due to heat concentration during welding. The arc is stable in combustion, the weld metal is dense, the forming is good, the surface is bright, the strength and plasticity of the welded joint are high, and the quality is good; The erosion of argon gas to the welding zone accelerates the cooling of the welded joint and improves its structure and properties. The joint form is unrestricted and suitable for all-position welding. But this method is not suitable for operation in open air environment.

Compared with argon tungsten arc welding, TIG welding (MIG welding) in addition to the above characteristics. It also has high welding efficiency, easy to achieve automatic welding and semi-automatic welding, and is suitable for welding of various thicknesses of aluminum and its alloys. However, due to the limitation of wire feeding system, the wire diameter should not be too large, and the porosity sensitivity of the weld is relatively high.

3.3 Extrusion molding
Extrusion forming is to exert strong pressure on the metal blank placed in the die cavity (or extrusion cylinder). A plastic working method for forcing a metal blank to produce a directionally plastic deformation, which is extruded from a die hole of an extrusion die, thereby obtaining a part or a semi-finished product having a desired sectional shape and size and having a certain mechanical property. Extrusion moulding is usually used with other technological means in the process of battery case processing.
During the extrusion process, the extruded metal can obtain a more intense and uniform three-direction compressive stress state in the deformation zone than the rolling forging, which can fully exert the plasticity of the metal to be processed; The precision of the extruded product is high, the surface quality of the product is good, and the utilization rate and yield of the metal material are also improved; The process flow of extrusion is short and the production is convenient. One extrusion can obtain integral structural parts with larger area than hot forging or forming rolling.
Light metal and light alloy have good extrusion properties, especially suitable for extrusion processing. Such as aluminum and aluminum alloys, can be processed through a variety of extrusion processes and a variety of mold structures. Extrusion also has obvious limitations. It is only suitable for equal-section products, and the shape cannot be too complicated.

4  Defects in several processing techniques
4.1 Defects prone to occur in foundry
Investment casting has the following disadvantages: The raw materials are expensive and the casting costs are high; The process is complicated, the process is long, the production cycle is long, and the casting performance is generally not high.

Gypsum casting also has its drawbacks: The chilling effect of gypsum mold is poor. When the wall thickness of castings varies greatly, the defects such as shrinkage porosity and shrinkage hole are easy to occur in the large part of the thickness. The gypsum type is extremely poor in gas permeability, and the casting is prone to defects such as blowholes and bonfires.
Implemented to the specific types of casting defects, the current trend of consensus is that. At the end of solidification, the solidification shrinkage produced by the isolated liquid phase between the dendrites cannot be effectively compensated for in the liquid phase, resulting in major casting defects, pores and thermal cracking.

The formation of voids in the paste zone of alloy solidification, with the formation of more solid phases, the gas concentration in the liquid phase at the solidification front gradually reaches supersaturation state. At the same time, due to the capillary action between the dendrites, the local pressure drop in the high solid fraction region is lowered. When the partial pressure of the supersaturated gas in the liquid phase is greater than the pore forming pressure, the pores will adhere to the dendrites, inclusions or cracks in the mold, and nucleation at the grooves. Then grow up and eventually form a hole.
The formation of thermal cracking, thermal cracking is one of the most common casting defects in production. External cracks often occur at corners, sudden changes in cross-section thickness, slow local condensation and where tensile stress is sustained during solidification. Internal cracks occur in the final solidification part of the castings and often occur near shrinkage holes.

4.2 Welding difficulties
Aluminum is easily oxidized
Aluminum and its alloys are highly susceptible to oxidation during the soldering process, forming a dense Al2O3 film on the surface of the material. The melting point of Al2O3 is as high as 2050 ° C, which is much higher than the melting point of aluminum and aluminum alloy (660 ° C pure aluminum, 595 ° C aluminum alloy). Al2O3 is very stable and difficult to remove, which hinders the fusion of the base metal during the welding process. Since the melting point of the Al2O3 film is nearly three times that of aluminum and aluminum alloy, and the density is much higher than that of aluminum and aluminum alloy, defects such as unmelting and inclusion are easily formed during the soldering process. In addition, the oxide film has good hydrophilicity, which causes the weld to generate pores during welding. Therefore, in order to ensure the quality of aluminum alloy welding, it is necessary to strictly clean the oxide film on the surface before welding, and prevent it from re-oxidizing or removing its newly formed oxide film during the welding process.

High thermal conductivity and large specific heat capacity
The specific heat capacity and thermal conductivity of aluminum alloys are larger than those of steel. When welding, the heat of arc diffuses easily to all sides. Therefore, it is necessary to use a heat source with concentrated energy and heat input. For thicker aluminum alloys, it is sometimes necessary to preheat the workpiece. The higher heat input often forms overheating, and if it is slightly careless, it is prone to sagging of the weld bead, causing the workpiece to burn through.

Large linear expansion coefficient and large thermal cracking tendency
Aluminum and aluminum alloys have a coefficient of expansion that is about twice that of steel. The volume shrinkage at solidification is large (up to 6.5%, compared to 3.5% for steel). The deformation and stress of the weldment are large, and shrinkage, hot cracking and high internal stress are easily generated during welding. In the production, the occurrence of hot cracks can be prevented by adjusting the composition of the welding wire, selecting reasonable process parameters and welding sequence, and suitable welding tooling.

BMW Power Battery Cast Aluminum Box

Sensitive to hydrogen
Pores are easy to occur in aluminium welding. Since liquid aluminum can dissolve a large amount of hydrogen, and solid aluminum hardly dissolves hydrogen, when the temperature of the molten pool is rapidly cooled and solidified, hydrogen does not overflow, and it is easy to agglomerate and form pores in the weld. The hydrogen element in the weld mainly comes from the moisture in the arc column atmosphere, the welding material and the moisture adsorbed by the oxide film on the surface of the base metal; Aluminum has a large thermal conductivity. Under the same process conditions, the cooling rate of the aluminum fusion zone is 4-7 times that of steel, which is not conducive to the escape of bubbles, which is also an important factor in the formation of pores. Compared with steel, aluminum produces 40 times more hydrogen bubbles than steel. Therefore, the source of hydrogen should be strictly controlled to prevent the formation of pores; At the same time, it is necessary to clean the base material and the welding wire before welding.
There are no published cases of aluminum alloy box design. Let's enjoy a BMW cast aluminium battery case for the time being.
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