Calculating the density of aluminum for lightweight EV battery packs

Abstract

Today’s increasing demand for battery energy for the ever-growing electromobility industry leads to an engineering paradox. Higher energy performance often requires larger batteries, which results in higher vehicle weights and, in turn, shorter battery lifetime. This ‘weight spiral’ can only be prevented by means of battery protection through a smart, lightweight design of the battery housing and the cooling system. The core of the considerations is the density of aluminum, which will serve as the benchmark for optimizing the gravimetric energy performance. Based on the crystal structure of the body-centered cubic (FCC) lattice and from the mass of the alloy of the material density 6061 aluminum, a theoretical clarification can be developed on laser joining processes of aluminum alloys from a thermodynamic point of view. This opens up for Tier-1 suppliers in the automotive industry all necessary options and methods for the precise calculation of the required aluminum density in g cm−3 and the corresponding aluminum density in lb/in³, thus enabling a correspondingly precise calculation of the target value for the desired ±0.05 mm hermetic battery sealing reliability while at the same time ensuring the desired Wh/kg value of the battery.

Overcoming the EV weight spiral through the density of aluminum

Range anxiety is currently the major obstacle preventing the widespread adoption of Electric Vehicles (EVs). While there have been modest advancements in the lithium-ion cell chemistry, the major factor behind the high weight of the battery pack, structural and accessory components such as the tray, cover, cooling manifolds, is still determined by the relatively low cost and high yield strength of deep-drawn structural steel. Unfortunately, the high iron metal density of approximately 7.87 g/cm3 directly counterbalances the battery cell capacity.
Automotive Original Equipment Manufacturers (OEMs) have identified one key challenge: the weight spiral. As battery packs grow in size, they must be paired with larger, heavier suspension systems, braking systems, and structural components – all of which add weight to the vehicle. Using aluminum-intensive structures is one way to counteract this trend. By considering the low density of the aluminum material (approximately one-third that of steel), engineers are able to re-allocate the saved mass (as a result of the lower material density) to either additional battery cells or to reinforce the passive safety structures of the vehicle. However, a detailed knowledge of the variation in aluminium density between different material categories and how this affects complex manufacturing processes such as laser welding and CNC machining is necessary.

Understanding the density of aluminum in metric and imperial units

In order to mass balance in a Computer-Aided Design (CAD) environment, engineers need to start with a host of physical properties. What is the density of al? The density of the elemental form of aluminum at 20∘C is about 2.699 g/cm3. In today’s global marketplace, density is often reported in several units.
The aluminium density g/cm³ value is the international standard for scientific and metallurgical analysis. For North American automotive manufacturing, the industry often uses the aluminum density in lb/in³, and that is calculated as approximately 0.0975 lb/in³.
The density of aluminum is a property that arises from the arrangement of atoms in the solid metal. In this state, the aluminum has a face-centered cubic (FCC) crystal structure. This is a cubic structure in which each of the eight corners of the unit cell is occupied by an atom, and each of the six faces is occupied by an atom at the center of the face. The resulting packing efficiency is 74%. This, together with the relatively low atomic mass of the element (26.98 u) give a low density. The FCC structure that is responsible for the relatively high ductility of aluminum at ambient temperature is also retained at cryogenic temperatures, and this is an increasingly important factor for the storage of liquefied gases and for vehicles that travel in the very low temperature conditions of extreme northern latitudes.

Comparing the density 6061 aluminum and 5000-series structural grades

Pure aluminum is not used extensively for the EV battery enclosures because it does not possess the required mechanical yield strength. As a result, the common materials selected are the 5000-series (Al-Mg) and the 6000-series (Al-Mg-Si) alloys. The added alloying elements not only strengthen the alloy by solid-solution or precipitation hardening but also result in an increase in volumetric mass.
When designing parts for density, engineers have to consider that the specification for 6061 aluminum includes additions of Magnesium 0.8%−1.2% and Silicon 0.4%−0.8%. Magnesium is lower in density (1.74 g/cm³) than aluminum, and Silicon is slightly higher in density (2.33 g/cm³) than aluminum. The density of 6061 aluminum is still reported close to the pure element at 2.70 g/cm3 or 0.0975 lb/in³.
The base plates of battery trays are often made of 5000-series alloys, particularly 5083. These alloys have a higher Magnesium content and, therefore, slightly lower aluminium density at about 2.66 g/cm³ (compared to 2.70 g/cm³ in the 2000-series). Although a difference of 0.04 g/cm³ may seem extremely small, when you calculate it for a large-format battery tray that is 2.5 m2 in size, you are talking about a weight reduction of several kilos. Calculating the aluminium density to such a fine degree is what differentiates a high-efficiency EV platform from a simple conversion.

How the density of aluminum influences thermal management

Thermal management in EV battery packs is crucial for battery safety. It is necessary to protect the lithium-ion cells from hotspots that can cause premature aging or thermal runaway. The cells have to be kept within a defined window of operating temperatures (15°C to 35°C). Cold plates are made of aluminum due to their high thermal conductivity and low density.
SThan Thermal diffusivity, α, is defined as the ability of a material to dissipate heat. It can be calculated by using the following formula:

Where:
k is thermal conductivity.
ρ is the density of aluminum.
C_p is the specific heat capacity.
Since aluminum is a low-density material, it has a lower volumetric heat capacity compared to copper or steel. While aluminum may have a higher temperature rise for the same quantity of energy applied, that energy is dissipated very efficiently to the liquid coolant due to the high thermal conductivity of aluminum. In our modern “cell-to-pack” designs, where the structural tray is also the cooling manifold, aluminum’s density of approximately 2.7 g/cm³ makes it possible to design extremely thin, multi-channled extrusions for effective cooling of the battery with a minimum amount of weight to the pack.

Laser welding parameters based on the density of aluminum

One of the main challenges in the assembly of EV batteries is the hermetic sealing of the housing. In order to protect against humidity and also in case of internal venting of the battery, the seal between the battery tray and the cover has to be gap-free. Fiber laser welding is therefore used for this purpose, but this method is only reliably applicable in the case of high-density materials such as aluminum.
Iron metal density in the typical automotive steel is high. Solid material properties like welding are quite easy to manage due to the wide processing window. Aluminium is a highly reflective material. In the solid state, the reflection of more than 90% of the energy provided by a 1070 nm fiber laser is observed. Upon reaching the melting point of the material after a certain amount of energy has been delivered by the laser, the absorption of the laser increases because of the keyhole (vapor-filled capillary) that has been generated.
The stability of the keyhole is determined by the balance between the vapor pressure and the hydrostatic pressure of the molten metal. Aluminum has a low density, which results in a low hydrostatic pressure of the melt pool; thus, the keyhole easily collapses, resulting in porosity and lack of penetration. In order to ensure a tolerance of ±0.05 mm, high-speed galvanometric scanners (wobble heads) are required. The oscillation of the laser beam stirs the melt pool and helps in maintaining stability of the keyhole, which is necessary for the low al density, and in order to get gas tight seal of the keyhole, which is necessary for the dynamic pressure variations of the battery pack during charge and discharge cycles.

The impact of what is the density of al on international shipping

The density of the aluminum we use in our products has a huge impact on the cost of our end product. When we talk about foreign trade and global manufacturing, the density of the aluminum is one of the many factors that can increase the final cost of a product. For a Tier-1 supplier exporting battery enclosures from Asia to Europe and North America, the shipping cost is charged based on a volumetric weight or a chargeable weight that combines both the volume and weight of the shipment.

The low aluminium density g/cm³ makes aluminum components “volume-intensive” as opposed to “weight-intensive”. In other words, a container full of steel components does not necessarily equal a heavy container of battery trays made of aluminum. Even though a container full of aluminium battery trays might still have plenty of space left before it is fully loaded in terms of weight, in practice, it will have already reached full volume. Hence, the possibility of utilizing so-called “High Cube” containers for a much more efficient shipping.

The density of aluminum is also very relevant to the environmental performance of the whole production and supply chain. As aluminum is 66% less dense than steel, using the lighter metal will result in a significant reduction in fuel consumption for transportation from mine to production plant, for overland and maritime transport to the vehicle manufacturing plant, and also for intra-plant transportation. This material selection aspect, based on the density of aluminum, is therefore a very important component in the sustainability reporting of the global OEMs in the case of “Scope 3” reductions of greenhouse gas emissions. And so, what is the density of al then?

High-velocity impacts and the density of aluminum

A significant number of EV design considerations center on battery cell protection during a side-impact event. To minimize potential risks, the battery enclosure must be a “survival cell.” We utilize 6000-series EV extrusions in a very complex inner configuration – referred to as multi-hollow profiles. These parts will collapse in a predetermined manner to take the impact forces in a safer fashion.
Energy absorption capacity of materials is described by their specific energy absorption (SEA) quantity and stands for the energy that is absorbed per unit of mass. Aluminum materials have a low density that also leads to a high specific energy absorption. An aluminium crash rail will then always absorb twice the energy as a steel crash rail of the same mass.
The engineer has to consider the density of the 6061 aluminum to determine its impact resistance for a given wall thickness. Modeling the deformation zones using the aluminum density in lb/in³ ensures that there will be no intrusion into the battery cells even in the worst case scenario of a high-velocity impact. That the battery cells can be contained within ±0.05 mm dimensional stability in the cell-holding structure, under all conditions of motoring and vibration, further illustrates the benefits of using the density of aluminum in structural design.

Quality control: Maintaining ±0.05 mm tolerances in high-density production

In a high-volume production environment, quality tolerances are extremely tight. For the EV battery trays we are fabricating, the sealing surface must be flat to tolerances of less than 0.05 mm to ensure that the gasket does not leak.
The density of aluminum is an important factor in determining the material’s behavior under mechanical stress during the CNC milling process. Aluminum has a lower modulus of elasticity than that of steel, which causes the material to deflect under the pressure of the cutting tool. This means that the density of the material must be taken into account when designing the workholding of the part in order to meet the ±0.05 mm tolerance. Vacuum chucks and hydraulic clamping systems are required to support the thin-walled aluminum extrusions to prevent vibration and deflection caused by the high-speed machining required for the density of 6061 aluminum.
We should also be mindful of thermal expansion. As mentioned above, the CTE for aluminium is quite high, and the low density of aluminium also means that there can be considerable expansion of the material with temperature increase. An increase of only 5°C in the factory can see an expansion of several tenths of a millimetre in a large aluminium tray, a change which would prove very difficult to manage and contain in an open factory. Hence, a precision factory needs to be air-conditioned in order to provide a stable aluminium density g/cm³ and exact volumetric measurements that are no greater than ±0.05 mm.

What is the role of the density of Al in solid-state battery housings

Looking ahead to the next decade of battery design, lithium solid-state batteries (SSBs) could deliver even higher energy densities. SBSs need a higher external pressure between the solid electrolyte and the electrodes to ensure they are in contact with each other. So, battery casings will need to be even sturdier.

High-density aluminum is a growing concern in light of planned developments in HPDC Aluminum-Scandium and Aluminum-Lithium materials. Lithium is the only known metal that is less dense than aluminum and still stable enough to be used as an alloying element. With the addition of lithium, the density of aluminum can be decreased by as much as 3 percent, while still increasing the Young’s modulus of the material.

The new materials will also be based on the aluminum density (g/cm³) and aluminum density (lb/in³) principles that we know today. The basic idea is the same: the lowest possible mass of the inactive parts, so that the battery cells deliver the longest possible range.

Read More： The Ultimate Guide to Laser Cutting Lightweight Metals

Conclusion:

Data-driven material selection: Physics, not aesthetics, should always be considered when selecting the material for critical applications in automotive designs. With the increasing popularity of battery-electric vehicles, the optimum density of a material is considered to be close to 2.70 g/cm³. Accurate determination of the density of 6061 aluminum and other 5000 series alloys will enable Tier-1 suppliers to produce materials that are within tolerances of ±0.05 mm to ensure the highest levels of vehicle safety and performance. Whether the application is weld surface distortion and stabilization of the laser weld melt pool, or optimizing the volumes of materials that can be transported internationally, the density of Al will be a focal point in the pursuit of the ultimate lightweight, long-range battery-electric vehicle. And to understand the density of Al, one must know how it relates to more than just another number on a datasheet – but to the engineering of the safe, high-performance, and sustainable vehicle of the future and the many long journeys it will take.

References

1. Bäuerle, D. (2011). Laser Processing and Chemistry. Springer-Verlag Berlin Heidelberg. (Focus on thermal diffusivity and density in laser-matter interaction).
2. Callister, W. D., & Rethwisch, D. G. (2014). Materials Science and Engineering: An Introduction. Wiley. (Crystallographic derivation of the density of aluminum).
3. Kou, S. (2003). Welding Metallurgy. John Wiley & Sons. (Analysis of melt pool dynamics and al density in welding).
4. Polmear, I. (2005). Light Alloys: From Traditional Alloys to Nanocrystals. Elsevier. (Detailed data on the density of 6061 aluminum and other series).
5. Sanders, R. E. (2001). Technology Innovation in Aluminum Alloys. JOM Journal of the Minerals, Metals & Materials Society.

Frequently Asked Questions (FAQs)

1. What is the density of al in standard units?
   For most industrial and commercial purposes, the density of aluminum is 2.70 g/cm³ or 0.0975 lb/in³.
2. Does the density of 6061 aluminum differ from that of pure aluminum?
   Aluminum alloy 6061 has several alloying elements, including Magnesium and Silicon in order to obtain improved mechanical properties. The density of 6061 aluminum did not change. Typical value is 2.70 g/cm³.
3. Why is aluminium density g/cm³ important for EV batteries?
   Aluminium density g/cm³ is important as the battery pack is the heaviest part of an EV. Using lighter aluminium material instead of steel can save up to hundreds of kilograms of total vehicle weight. The lighter weight of an EV leads to better range and efficiency.
4. How does the density affect laser welding?
   One important reason why laser welding can be more difficult than resistance spot welding is the low aluminum density. This results in lower hydrostatic pressure in the liquid metal. When pressure drops, the capillary stability of the “keyhole” can be reduced. In order to achieve precise tolerances of ±0.05 mm, stabilizing the melt pool with laser “wobble” is needed.
5. What is the density of al compared to steel?
   Lightweighting is often a primary benefit of using aluminum as a material. Why? Because aluminum is about one-third the density of steel. Iron is the major component of steel, and iron has a density of about 7.87 g/cm³. The density of aluminum is 2.70 g/cm³. Although these are only slightly different numbers, the effect on mass is quite dramatic. Thus, the use of aluminum is a very common method of reducing the mass of cars and airplanes.