In electric vehicles, the weight of the power battery pack accounts for about 30% of the gross vehicle weight. The endless pursuit of energy density in automobiles and power battery systems all requires a lightweight design with a battery pack construction. Inside the battery pack system, the cabinet as the largest structural member, its weight reduction and increased energy density cannot be overlooked. Under the premise of structural optimization and re-optimization, the use of new materials is a basic 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 resolved in follow-up and is not a reason to prevent people from considering its applicability.
Based on the experience gained from the entire vehicle, it is believed to be used as a new material for iron substitutes in automobiles. Common materials are:
Aluminum alloys, magnesium alloys, carbon fiber composites, and today’s leading aluminum alloy materials are relatively mature technologies among the three materials. Currently, most of the body can use aluminum for heat exchangers, wheels, bodies, etc., and aluminum alloys can achieve excellent weight reduction effects. This paper organizes the main types of aluminum alloys and the main treatment methods for aluminum alloy boxes.
Types and performance of aluminum alloys
Aluminum element is the most abundant metallic element in the earth’s crust, accounting for about 8.13%. Aluminum has an atomic number of 13, an atomic weight of 27, a melting point of 660 ° C, and a density of 2.7 g / cm ^ 3. The actual density of aluminum alloy structural parts depends on the processing technique and varies to a small extent. Die casting is about 2.6-2.63 g / cm ^ 3, extrusion is 2.68-2.7 g / cm ^ 3, and forging is 2.69-2.72 g / cm ^ 3.
Mechanical parameters of typical aluminum alloy sheets found on the internet, typical 6 series aluminum sheets, 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. General steel Q235 characteristic parameters, tensile strength 375-500 MPa, yield strength 235 MPa. Comparing steel and aluminum, and tensile strength and yield strength, aluminum is slightly lower.
Types of aluminum alloy
First, the application of aluminum alloy casting.
Cast aluminum alloys are widely used in automotive manufacturing and can offer a variety of casting methods to meet different automotive production requirements. In the original market, cast aluminum alloys were mainly used in engines, hubs, and anti-collision beams. Cast aluminum alloy battery case, has a relatively long history of use. However, the original mainstream products use traditional casting methods, the surface of the box is rough, the precision is low, the shape is simple, and the wall thickness of the box body cannot be too thin.
Second, the application of deformed aluminum alloys.
Compared to cast aluminum alloys, deformed aluminum alloys have a greater advantage in intention and strength, and their alloy content is relatively low. It is commonly used in automobile trim parts, structural parts, heat dissipation systems and body panels. The deformed aluminum alloy is made up of a series of aluminum alloy sheets, which have high strength and good weldability, and are used in the manufacture of battery cases and modules.
Third, the application of aluminum-based composites.
A type of low-density, high-strength aluminum alloy material for automotive production applications that has excellent dimensional stability and can produce fatigue resistance, fracture resistance, and other benefits.
Large aluminum alloy box forming process mainly including casting and welding. Among them, precision casting (or net size casting) can be realized. That is, the internal cavities and shapes of the casting often require one-time molding. Its shape is close to the part or the final shape of the part, and there is little or no processing.
There are three main types. Anti-gravity casting, investment casting, plaster casting.
Casting has always been the main process of mass production of aluminum alloy boxes. When net-sized casting is widely used, casting is the gospel of large part machining.
A casting method in which an alloying liquid is filled from the bottom in the direction opposite to gravity and solidified by applying pressure. The anti-gravity casting process is characterized by stable filling, controllable filling rate, reasonable temperature distribution, solidification under pressure, and facilitation of solidification and supply of castings. Anti-gravity castings have good mechanical properties, a compact structure, and few casting defects.
According to various processes, anti-gravity casting is divided into low pressure casting, differential pressure casting, and pressure adjustment casting. During World War II, low-pressure casting technology was invented and used to manufacture air-cooled engine block castings for aircraft. Based on low pressure casting, a differential pressure casting process that combines low pressure casting and autoclave casting has been developed for the production of large and complex thin wall parts. The pressure-adjusted casting process was developed on the basis of differential pressure casting. The biggest difference between pressure-adjusted casting and differential pressure casting is that not only can positive pressure control be achieved, but negative pressure control can also be achieved. At the same time, the control accuracy of the control system is also high.
Precision investment casting
Investment casting has the following advantages
Investment casting has high dimensional accuracy and surface finish. Dimensional accuracy is usually up to CT4-6 (CT10-13 for sand casting and CT5-7 for die casting). The flexible design allows the casting of extremely complex castings. Clean production, no chemical binders used in casting sand, mold material is cold and harmless to the environment, old sand recovery rate is over 95%.
Please explain “CT4-6”. CT is the dimensional tolerance level of the casting, and the higher the number, the lower the accuracy. This means a wider tolerance for casting size.
The gypsum type can be used to create castings with high dimensional accuracy, low surface roughness and low residual stress. It has many features not found in other castings. The pattern can be accurately duplicated and the surface roughness of aluminum alloy castings can reach 0.8-3.2 μm. It has low thermal conductivity, thin walls are easy to form perfectly, and the thinnest parts can be cast to 0.5 mm thin walls. We can manufacture castings with complex shapes.
There are three main types of cast gypsum.
Non-foaming plaster mold, foaming plaster mold and investment casting plaster mold. The non-foaming gypsum type has low gas permeability and mainly uses low pressure casting to produce castings with low performance requirements. The foamed gypsum type has specific gas permeability and can be used for the production of curved thin-walled (thinnest 0.5 mm) aluminum alloy castings.
Currently, there are many welding methods for aluminum and its alloys. The welding methods usually include TIG welding, MIG welding, laser welding, seam welding, resistance welding, electron beam welding, friction stirring welding, and induction welding. The first two types of welding are widely used: TIG welding and MIG welding.
Tungsten argon arc welding is the most common welding method for aluminum products. Especially suitable for welding aluminum and aluminum alloys less than 5 mm thick, mainly due to heat concentration during welding. The arc is stable in combustion, the weld metal is dense, the molding is good, the surface is bright, the strength and plasticity of the welded joint is high, and the quality is good. Erosion of argon gas into the weld accelerates the cooling of the weld and improves its structure and properties. The joining form is not limited and is suitable for all-position welding. However, this method is not suitable for operation in an outdoor environment.
In addition to the above properties, compared to argon tungsten arc welding, TIG welding (MIG welding). In addition, it has high welding efficiency, can easily realize automatic welding and semi-automatic welding, and is suitable for welding aluminum and its alloys of various thicknesses. However, due to restrictions on the wire feeding system, the diameter of the wire should not be too large and the porosity of the weld is relatively sensitive.
3.3 Extrusion molding
Extrusion is the application of strong pressure to a metal blank placed in a die cavity (or extrusion cylinder). A plastic working method for obtaining a part or semi-finished product having a desired cross-sectional shape and size and having specific mechanical properties by causing a directional plastic deformation in a metal blank and extruding it from a die hole of an extrusion die. Property. Extrusion is typically used in the process of battery case processing along with other technical means.
During the extrusion process, the extruded metal can obtain a stronger and more uniform three-way compressive stress state in the deformation zone than rolling forging, which can fully demonstrate the plasticity of the metal to be machined. The precision of extruded products is high, the surface quality of the products is good, and the utilization and yield of metallic materials are also improved. The extrusion process flow is short and production is convenient. With a single extrusion, you can obtain monolithic parts with a larger area than hot forging or forming rolling.
Light metals and light alloys have excellent extrusion properties, especially suitable for extrusion. Aluminum, aluminum alloys, etc. can be processed in different extrusion processes and different mold structures. There are also obvious restrictions on extrusion. Only suitable for products with the same cross-sectional area, the shape is not too complicated.
4.1 Defects that are likely to occur in foundry
Gypsum casting also has its drawbacks. The cooling effect of the gypsum type is poor. If the wall thickness of the casting changes significantly, defects such as shrinkage pores and shrinkage holes are likely to occur in most of the thickness. The gypsum type has very low gas permeability, and castings are prone to defects such as blow holes and bon bonfires.
That is the current trend of consensus implemented for certain types of casting defects. At the end of solidification, the liquid phase cannot effectively compensate for the solidification shrinkage caused by the isolated liquid phase between the dendrites, resulting in major casting defects, pores, and thermal cracks.
As voids form in the paste region of the alloy solidification and more solids form, the gas concentration in the liquid phase before solidification gradually becomes supersaturated. At the same time, the capillary action between the dendrites reduces the local pressure drop in the high solids region. If the partial pressure of the supersaturated gas in the liquid phase is greater than the pore formation pressure, the pores will attach to dendrites, inclusions or cracks in the mold, and nucleation in the grooves. It then grows and eventually forms a hole.
Thermal crack formation, thermal cracking is one of the most common casting defects in production. External cracks often occur at corners, sudden changes in cross-sectional thickness, slow local condensation, and where tensile stress is maintained during solidification. Internal cracks occur in the final solidified part of the casting and often occur near shrink holes.
4.2 Welding problems
Aluminum is easily oxidized
Aluminum and its alloys are 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 alloys (660 ° C pure aluminum, 595 ° C aluminum alloy). Al2O3 is very stable and difficult to remove, preventing the base metal from melting during the welding process. The melting point of the Al2O3 film is almost three times that of aluminum and aluminum alloys, and the density is much higher than that of aluminum and aluminum alloys, so defects such as unmelted and inclusions are easily formed during the soldering process. Will be done. In addition, the oxide film has good hydrophilicity, which causes the weld to create pores during welding. Therefore, to ensure the quality of aluminum alloy welding, it is necessary to thoroughly clean the oxide film on the surface before welding to prevent the newly formed oxide film from being reoxidized or removed during the welding process.
High thermal conductivity and large specific heat capacity
The specific heat capacity and thermal conductivity of aluminum alloys are higher than those of steel. When welding, the heat of the arc is easily diffused to all sides. Therefore, it is necessary to use a heat source with a concentrated energy and heat input. For thicker aluminum alloys, it may be necessary to preheat the workpiece. If the amount of heat input is large, it will often overheat, and if you are not careful, the weld bead will sag and the workpiece will burn out.
Large coefficient of linear expansion and large thermal crack tendency
The coefficient of expansion of aluminum and aluminum alloys is about twice that of steel. Volumetric shrinkage during solidification is large (up to 6.5% compared to 3.5% for steel). Deformation and stress of welds are large, and shrinkage, high temperature cracking and high internal stress are easily generated during welding. In production, the composition of welding wires can be adjusted, rational process parameters and welding sequences, and the selection of suitable welding tools can prevent the occurrence of high temperature cracks.
BMW power battery cast aluminum box
Sensitive to hydrogen
Pore is likely to occur in aluminum welding. Liquid aluminum can dissolve a large amount of hydrogen, and solid aluminum hardly dissolves hydrogen, so when the temperature of the molten pool is rapidly cooled and solidified, the hydrogen does not overflow and aggregates in the weld and pores. It becomes easier to form. The hydrogen element in the weld is mainly derived from the moisture in the arc column atmosphere, the welding material, and the moisture adsorbed on the oxide film on the surface of the base metal. Aluminum has a high thermal conductivity. Under the same process conditions, the cooling rate of the aluminum melting zone is 4-7 times that of steel, which does not facilitate the escape of air bubbles, which is also an important factor in the formation of pores. Compared to steel, aluminum produces 40 times more hydrogen bubbles than steel. Therefore, the hydrogen source must be tightly controlled to prevent the formation of pores. At the same time, the base metal and welding wire must be cleaned before welding.
There are no published examples of aluminum alloy box designs. For the time being, please enjoy the BMW cast aluminum battery case.
Link to this article：Complex aluminum alloy casing processing technology: casting, welding, extrusion
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