Aluminium Busbar Products

for Cell Contacting Systems and Busbars

Aluminium busbar products are used in manifold applications in batteries and battery systems due to their favourable structural, physical, and chemical properties. When it comes to cell contacting systems and busbars, electrical (and thermal) conductivity are decisive for balancing component resistance and cross-section to meet the designed space. However, often additional requirements such as strength, thermal stability, fatigue properties, bending radii, creep / relaxation and weldability interact and thereby, influence the material and temper selection.

The conductivity of aluminium and its alloys is predominantly defined by the chemical composition and the distribution of impurities and alloying elements in intermetallic phases and solid solution. Higher impurity and alloying element concentrations reduce the conductivity particularly if the elements are dissolved within aluminium in solid solution. Therefore, the degree of age-hardening is of central importance in conductors made from 6xxx alloys. Additionally, work hardening has an impact on conductivity, as conductivity decreases with increasing degree of deformation. Last but not least, the conductivity is sensitive to temperature.  

Based on the foregoing introduction it is evident that the conductivity and other properties need to be balanced individually for different applications. For aluminium products this is done by alloy selection and thermo-mechanical processing within casting, rolling or extrusion as well as heat treatments to provide the optimal fit.

1xxx – optimized for conductivity

The highest conductivity is achieved by high purity aluminium (purity of 99.9 wt% Al and higher) in soft temper. Nevertheless, high purity alloys are not commonly used in volume application due to cost and volume constraints. Instead of this, commercially pure aluminium products of 1xxx series, with minor levels of impurities and alloying elements (< 99.9 wt% Al), are rather typical for conductors as they balance cost, availability, and high electrical conductivity.

The most common 1xxx alloys are given below in table 1

It is evident from Table 1 that 1xxx provide high conductivity, but limited strength. Therefore, such materials are used where conductivity is the central design criteria and resistance to mechanical as well as thermal loads is of minor importance. Within batteries such grades of aluminium are often used for cell-to-cell connections as well as cell contacting systems, where mechanical loads are comparably low, while conductivity and excellent laser weldability are a must. However, when it comes to busbars joint by nut-and-bold connections the limited strength of 1xxx series aluminium introduces limitations and other aluminium alloy grades are a better fit.

6xxx – optimized strength to conductivity ratio

As highlighted in the introduction, high purity in conjunction with low element concentrations in solid solution are key for achieving high electrical and thermal conductivity. Hence, rather low-alloyed 6xxx (Al-Mg-Si) alloys with minimized impurity levels are common and frequently used when an excellent strength to conductivity ratio is needed. The most prominent alloy in this context is 6101B also known as E-AlMgSi, which has been used in electro-technical applications for decades. Within battery technology such alloys are used in battery cables and could substitute copper in module connectors.

6101B alloy contains between 0.30 and 0.6 wt% Si and between 0.35 and 0.6wt% Mg, allowing for effective strengthening via age-hardening while keeping the impurity concentration in tight control. Typically, high strength in 6xxx alloys is achieved by performing solution heat treatment at about 530°C, followed by air or water quench and artificial ageing within the temperature range from about 160°C to 220°C. During the solution heat treatment Mg and Si are dissolved in the aluminium lattice in solid solution. The quenching generates a supersaturated solution, where Mg and Si are kept in the aluminium lattice and the final age-hardening results in the formation of finely dispersed intermetallic phases, which generate a strengthening effect with very limited impact on the conductivity. The selected temperature and soaking time define the degree of artificial ageing and strengthening and are included in the temper designation. In case of peak-strength temper (T6), artificial ageing periods are shorter to achieve highest strength with good conductivity. Over-ageing (T7) can be applied to maximize conductivity while reducing the strength in a controlled manner.

Recently, modified thermo-mechanical processings⁽⁵⁾ deviating from the standard solution heat treatment followed by artificial ageing have been suggested to improve the strength to conductivity ratio. For instance, combinations of cold-working and age-hardening provide opportunities to improve strength, while maintaining excellent conductivity. Thereby, yield strength levels of up to 250MPa combined with electrical conductivity above 32 MS/m can be achieved. While such processing’s do not fit within the scope of T6 / T7 temper designations, they can provide for materials with superior materials property combinations.

While providing an excellent strength to conductivity ratio 6xxx alloys are more challenging in terms of bending operations and welding is likely to require additional filler wire, as the combination of Mg and Si introduces a certain susceptibility to hot cracking within the welding process. Within 6xxx series long-term exposure to elevated temperatures above 100°C can result in microstructural changes, having an impact on the strength of the material. Hence, thermal effects have to be considered and carefully evaluated, if long term temperature exposure is expected in service.

Solutions optimized for thermal stability

As mentioned above the use of 1xxx and 6xxx might be challenging when elevated temperatures are expected in service and in particular when elevated temperatures can introduce relaxation and creep. To address this topic, several materials have been developed providing improved thermal stability such as thermal resistant aluminium alloys. For instance, Al-Zr (TAL) alloys can generate improved thermal resistance and strength by the formation of thermally stable Al₃Zr phases, if suitable production processes (e.g. wire rod production) allow for high cooling rates during casting to generate a supersaturated solution of Zr, which is needed to form Al₃Zr phases with appropriate size and distribution.

Within flat rolled products range, 3xxx alloys with improved thermal stability and quite good electrical conductivity have been developed and are commercially available.

References:

1. Speira Datasheet AA 1050 acc. EN 485-2 & 573-3 (01.10.2021)
2. DIN EN 14121: 2009
3. Speira Datasheet VERSA Conduct 1070 acc. EN 485-2 & 573-3
4. DIN EN 40501-2:2005
5. A modified processing route for high strength Al-Mg-Si aluminum conductors based on twin-roll cast strip, M. Lentz et al., Journal of Materials Processing Technology, Volume 278, April 2020, 116463,  https://doi.org/10.1016/j.jmatprotec.2019.116463
6. Speira Datasheet Versa Connect 6101
7. Aluminium Taschenbuch 3, Aluminium-Verlag, 16th Edition, p. 702
8. Speira Datasheet 3105 H22

Aluminium Cell Housings for Cylindrical Lithium-ion Batteries

Thermal simulations reveal significant improvements in cooling performance at 3C fast-charging of the aluminium housing version compared to nickel-plated steel reference cell. The impact of the cell housing material is particularly pronounced in case of a sidewall cooling. In this case, simulation reveals differences in maximum temperature (hot spot) of 11°C after 10 minutes.