Overcoming resistance with aluminium

Major cost advantages compared to copper

Copper established itself as the material of choice for current-conducting applications on the basis of its high specific electrical conductance, low thermal coefficient of expansion and good corrosion resistance. However, following continuous material optimizations, aluminium now presents a better alternative with regard to lightweight design and commercial considerations. [1] Copper has a density of 8.96 g/cm³ - more than three times the density of aluminium at 2.70 g/cm³. [2] At the same time, the price of copper is almost four times that of aluminium (as of July 2024). This means, despite its lower electrical conductivity, the use of aluminum achieves significant cost and weight savings. [3] Consequently, aluminium conductors require a sligthly larger cross-section than copper conductors to achieve the same electrical current flow density.

Alloy families

Two aluminium alloy families are particularly well suited to use in bus bar applications. These are 1xxx alloys, which are made from almost entirely pure aluminium, and 6xxx alloys made from Al-Mg-Si.Although pure 1xxx aluminium alloys can provide higher electrical conductivity, they can sometimes be too soft for certain processing steps. And, due to their high purity, they also require a higher proportion of primary aluminium, which in turn means a larger carbon footprint. [4]

Another important factor for bus bar applications is the processability and good bendability to save space in the compact battery composite. Aluminum has an further advantage over copper due to a improved strength-to-weight ratio in Al–Mg–Si alloys. [5]

Influencing factors for electrical conductivity

The alloy EN AW-6101 is the most widely established choice for applications requiring high electrical conductivity. Its chemical composition is detailed in Table 1.The electrical conductivity of a material is essentially controlled by two influencing variables: On the one hand, by the chemical composition and, on the other hand, the solution state of the alloying elements in the material. [6] Due to their high specific electrical resistance, the elements chromium, lithium, titanium, vanadium and manganese significantly reduce the conductivity of the material.Manganese is particularly important for final use, as it is a typical by-element in 6xxx alloys. [2]

The electrical conductivity is increasing if the alloying elements are present in the structure in the precipitated state. This allows the electrons to move through the structure more freely. When in solution and with alloying elements more evenly distributed, the entire aluminium matrix is distorted and electron passage is impeded, which reduces the alloys conductivity. In contrast, the formation of coarse Mg₂Si precipitates reduces the material’s strength, which is why it is important to achieve a balance between these properties. The material is aged starting from initial temper T4 over the hardened temper T6 and finally transferred to over-aged temper T7 before delivered to the customer. [7, 8] The T7 temper is soft and has excellent processing properties.

Tempering kinetics of 6xxx alloys

The mechanism for increasing the strength of 6xxx alloys relies primarily on the formation of Mg₂Si precipitates and their interim stages. Precipitations impede the sliding movement of dislocations on the slip planes. Dislocations are one-dimensional lattice defects that determine the strength of a material by their ability to slide in the microstructure. After the rolling process, the alloy is subjected to solution heat treatment and subsequent quenching at room temperature to progress the alloying elements into a super-saturated solution. This temper is defined as T4. The material is soft and easily formable but, because the alloying elements are in solution, they also impede electron flow, which means lower electrical conductivity. [6] For this reason, the T4 temper is not suitable for the production of electrical conductors. Annealing processes are therefore applied to form the interim stages of precipitates, which makes it possible to achieve the desired strength and electrical conductivity in the material. The best compromise between strength and conductivity is usually achieved with the over-aged T7 temper. At this point, the material’s strength is well past its peak and has decreased noticeably. The finely distributed precipitates merge and form smaller Mg₂Si precipitates. Therefore the T7 temper has lower strength than the T6 temper. [7, 9] Figure 1 illustrates the impact of time and temperature on material tempers.

Production processes

In principle, aluminium bus bar products can be manufactured using either extrusion or rolling. Rolled materials have significant advantages:

• Heat dissipation: The generally flatter part geometries result in a larger specific surface area, allowing heat generated in the final application to dissipate more effectively.
• Tolerances: Considerably smaller dimensional tolerances can be achieved.
• Homogeneity: A more uniform microstructure enables more consistent material behavior.

AMAG AL4® ED 6BB-150

AMAG has developed a new product group to meet customer demand for electrical conductor materials as effectively as possible: AL4® ED 6BB-150. The development process for these new products is a prime example of the synergy effects generated by AMAG’s extensive portfolio. In the beginning, a similar product was already in serial production and used for lightning conductors in aviation. On this basis, a suitable material could be produced quickly and efficiently. An overview of different product properties is provided below.

Product facts (sheet thickness 3.0 mm):

• Electrical conductance: up to 58.1% IACS (33.7 MS/m)
• Yield tensile strength Rp_0.2: at least 150 MPa
• Grain size: 50 μm
• Bending capacity of 180° in accordance with ASTM290 (radius N=1)

The final over-aging annealing has a significant influence on the ratio of strength to electrical conductance, which makes it possible to create a new variant with a tailored property window in line with specific customer requirements. In the product example shown here, the focus is on a minimum strength of 150 MPa, which achieved a conductance of 58.1% IACS. Higher conductance can be achieved if a material is subject to lower strength requirements. In contrast, higher strength requirements lead to material with lower conductivity. In keeping with AMAG’s DNA, sustainability is an essential criterion in this development process, so the product can also be produced in an AL4® ever variant with a guaranteed and reduced carbon footprint.