New energy vehicle power batteries, due to the contradiction between their own weight defects and energy density requirements, in the vehicle parts subsystem, lightweight requirements are particularly urgent.
In the power battery, the tray takes up 20 to 30% of the weight of the battery system, which is the main structural component. Therefore, under the premise of ensuring the safety of the battery function, the weight reduction of the tray becomes one of the main improvement targets of the battery structural member.
From the material comprehensive index evaluation, the aluminum alloy material can first meet the structural requirements of the vehicle components, including the battery system, and is still a substitute for part of the steel structure.
However, the high-strength steel plate itself is also taking the road of lightweight technology. Therefore, the aluminum alloy material and the lightweight high-strength steel plate have always been in a state of being glued on the road of material selection.
Adhesive aluminum and steel
Due to the trend of energy saving, environmental protection and light weight development of products, aluminum is generally the main solution for enterprises to achieve lightweight. However, lightweighting is not the only consideration when selecting a vehicle. The cost is also.
Undoubtedly, the lightweight effect of aluminum is obvious, so it will be more and more widely used in the future. Although the cost of aluminum alloy is high, its excellent workability, low density (density of aluminum alloy is 2.7g/cm), corrosion resistance, high recyclability and other characteristics, the advantages are obvious, and it is still electricized. An important symbol of the lightweighting process of new energy vehicles.
Dak Global Consulting conducted a survey and forecast of the average aluminum consumption in North America. They found that since 1996, the application of aluminum in vehicles has shown a trend of increasing year by year, and since 2012, it has been climbing. In 2015, the aluminum content of the car has reached 400 lbs/unit (about 181 kg/vehicle), and by 2020 it has exceeded 450 lbs/unit (about 204 kg/vehicle), and by 2028 it has exceeded 550 lbs/unit (about 249 kg). /car)
Of course, depending on the cost factor, the application of aluminum alloys on various models is not the same.
The early Tesla should be a radical for lightweight applications. At the beginning, the Model S has a large proportion of aluminum materials from the body to the battery system structure. Because Model S's consumer group positioning at the time was aimed at luxury customers.
The Tesla Model S is a high aluminum alloy.
However, the other components of other popular models use high-strength steel with cost advantages. For example, Nissan Leaf, Volkswagen Golf, and Toyota Prius are more inclined to work on high-strength steel and profiled steel.
It can be seen that although the application trend of aluminum alloy lightweight development is clear and clear, the cost factor still restricts its development. This is in turn beneficial to low-cost high-strength steel, which is characterized by the application of resurgence.
Tesla is not a technical madman. Considering the cost factor, adjusting the amount of aluminum is also a reasonable technical behavior. In the Model 3 design, the design idea changed the previous “radical” and “luxury”, and the body structure was made of steel-aluminum mixed metal, which reduced the proportion of aluminum application.
Even the designers of the popular MEB platform, which is famous for its reputation, have also indicated that they should choose low-cost steel plates and that new energy vehicles are not just “rich fashions”.
In fact, one material cannot completely replace another material. Any kind of material, whether from the perspective of cost and performance, has its own strengths and develops in parallel. It can only be said that a material, in one aspect, can better meet the needs of technology or market development.
The application of aluminum materials in new energy sources is still demanding for lightweight and energy-saving needs. At present, taking a 40KWh battery system as an example, if a steel structure is used, the cost can be controlled within 1,000 yuan; if the aluminum profile is welded, the shell structure is between 3 and 5,000 yuan. The cost ratio, aluminum alloy is still 3 to 5 times that of steel plate material.
In the promotion and application of aluminum in new energy, the cost factor is still a roadblock. However, this does not hinder the advancement and development of technology.
But what we need to be clear is, at this stage, what are the design differences brought about by the differences in steel and aluminum properties?
The design of the battery tray structure needs to be taught in accordance with the materials.
Steel and aluminum materials have very large differences in strength, fatigue resistance, elastic modulus, tensile strength, compression resistance, shear resistance, and bending resistance. The use of metal alloy technology does achieve a very significant improvement in some aspects, such as strength properties, compared to pure aluminum. However, the enhancement of a single characteristic does not mean the transfer of essential characteristics and complete changes. Especially in vehicle engineering, under dynamic and static loads, the difference in characteristics is more obvious.
Therefore, in the structural design, although the functions are identical parts, the aluminum alloy structure cannot be equated with the steel structure design.
Domestic new energy vehicles have not been designed for a long time. The body structure or platform is a transition from a fuel car. The body structure did not make too many adaptive changes and designs. At this time, the design of the battery tray and the fixed position and form of the body can only be followed.
However, with the enlargement and popularization of the new energy market, the functional safety of the battery system has been paid more and more attention. This structural design cannot meet the new functional requirements.
For the pre-production of new energy products, during the customer's use, the product is cracked by the ear, the IP is invalid, the internal module structure is invalid, and the electrical performance is invalid. The layout of the pallet hanger is unreasonable, and it is directly or indirectly. One of the main reasons.
The density of the battery body is very high, and as a battery tray or housing that carries the battery module, it is always in a heavy load state. The fatigue performance of aluminum is only half that of steel, and the modulus of elasticity is only one-third of that of steel.
If the load on the lifting lugs of the pallet is over-limit, or the difference in the force of different lifting lugs is large and uneven, the dynamic performance is even worse in the face of complicated road conditions of the vehicle. In the case of high vibration and high stress concentration, the aluminum material is more prone to fatigue, resulting in cracking and deformation.
Therefore, it is not surprising that the tray is in the position of the lifting lug, the inner frame beam structure, cracking and other failure phenomena, and even the fixed point of the module is detached.
The aluminum lifting lugs of the pallet should be fixed in a large number and evenly arranged.
Not only that, it is not an easy task to integrate the battery module and the loaded pallet. It can stand the test of vibration experiments and is also a good way to test the design results. During the experiment, cracking of the inner frame and the tray welding and cracking of the inner frame supporting beam body are often encountered.
Preliminary analysis of the causes of cracking:
From the analysis of material properties, the stress at the fault point exceeds the stress or stress concentration that the material itself can carry.
From a process perspective, the resulting burnout during material welding changes or weakens the parametric properties of the material.
From a structural point of view, whether the cracked support beam is integral with the inner frame structure. The overall structure is more conducive to stress dispersion and uniform stress, and the vibration frequency is consistent.
Audi's battery tray design is a good example. The yellow arrow is in a state of being stressed. The internal frame passes through a uniform frame to allow the stress to be released reasonably. At the same time, it corresponds to the outer frame lifting ear hole, so that the inner and outer structures are integrated. At the same time, it can also resist damage from external collisions.
Tray design soul: aluminum outer frame beam strength design
As mentioned above, the inside and outside of the tray structure design are integrated, and the outer frame design is also very important.
From the point of view of material properties, the yield strength and tensile strength of aluminum are lower than those of steel.
The yield strength and tensile strength of aluminum and its alloys are 30-500 N/sq mm and 79-570 N/sq mm, respectively. The yield strength and tensile strength of the steel are in the range of 250-1000 N/sq mm and 400-1250 N/sq mm, respectively.
This factor must be considered in relation to the position or structural design of the pallet hanger.
At the same time, the modulus of elasticity of aluminum is inferior to that of steel. This property is also very important, related to the fatigue or life of the material of the structure.
The automotive aluminum alloy application mainly includes a 5××× system (Al-Mg system) 6××× series (Al-Mg-Si system) and the like. It is understood that the aluminum tray mainly uses 6 series aluminum profiles (the application of materials, further analysis and exploration).
Several types of structures commonly used in battery aluminum trays
Aluminum battery trays are generally available in several forms due to their light weight and low melting point: die-cast aluminum trays, extruded aluminum frame and aluminum plate tailored trays (housing), molded top cover.
The structural characteristics of the die-cast aluminum tray are more represented by one-time die-casting, which reduces the material burning loss and strength problems caused by the welding of the tray structure, and the overall strength characteristics are better.
The structure of the tray, the frame structure features are not obvious, but the overall strength can meet the battery bearing requirements. Common in small energy battery system structures.
The extruded aluminum tailor welded frame structure is more common and is a more flexible structure. Through the tailoring and processing of different aluminum profiles, it can meet the needs of various energy levels. At the same time, it is easy to modify the design and it is easy to adjust the materials used.
From the perspective of cost, extruded aluminum framed frame structure has certain advantages compared to die-cast aluminum trays. Of course, with the difference in the quantity of mass production, whether this cost advantage exists or not.
The frame structure is a structural form of the pallet, which was described in detail in the previous article "Three +6". The frame structure is more conducive to lightweight, and is more conducive to the strength guarantee of different structures.
The structure of the aluminum battery tray also follows the frame structure design form: the outer frame body mainly completes the carrying function of the whole system of the battery; the inner frame body mainly completes the bearing function of the sub-modules such as the module and the water-cooled plate; in the middle of the inner and outer frames The protective surface mainly completes the isolation and protection of the battery pack from the outside world, for example, sand impact, waterproof, heat insulation and the like.
As an important material for vehicle lightweighting, aluminum must be based on the global market and pay attention to its sustainable development for a long time. At the same time, we must also correctly consider the difference between the cost factors and technological progress of steel and aluminum in vehicle applications.
The correct application of aluminum in the design requires a deeper understanding of the material properties. Especially for heavy-duty battery tray applications, you need to constantly explore, do your best, and continue to accumulate application experience, in order to be more effective in lightweight applications.