Aluminum is the dominant material for electric vehicle (EV) battery enclosures for one simple but significant factor: lightweighting capability. All currently available long-range BEVs – those that can travel beyond 250 miles (400 km) – use aluminum as the main material for the battery enclosure for that very reason, Dr. Andreas Afseth, technical director for Constellium North America operations, said during a recent Center for Automotive Research (CAR) webinar.
“Aluminum continues to be the fastest-growing material in automotive application,” Afseth said. Growth is driven in part by the increasing market share of BEVs, including electric trucks and vans, which already employ a greater amount of aluminum than do conventional-powertrain models – more than 640 lb (290 kg) in BEV platforms compared to about 450 lb (205 kg) in non-BEVs.
“Electric trucks will require very large batteries, maximum payload and minimum energy consumption (operating costs),” he said. “There will continue to be a very high value of lightweighting, so I would expect aluminum to be the material of choice.”
Aluminum battery enclosures or other platform parts typically provide a weight savings of 40% compared to an equivalent steel design. The most-used and best-suited alloys for battery enclosures are of the 6000-series Al-Si-Mg-Cu family, Afseth shared, noting that these alloys are “very well compatible” with end-of-life recycling. The current state-of-the-art solution for bottom plates is high-strength 6111 alloy in peak aged temper, which reduces weight by 30% compared to the benchmark 5754 O-temper alloy.
Constellium has a 4xxx alloy in development with 80-GPa E-modulus and 350-MPa yield stress. A 40% weight reduction is “technically feasible” with the developmental 4xxx alloy. “You can think of this high-modulus 4000 series alloy as a ‘very excess’ silicon 6000 series alloy,” Afseth said, noting that gauges and widths will be similar to 6000. It is compatible with conventional cold forming and is cost-competitive, he said.
A 7075 T6 alloy also in development offers 500-MPa yield stress and 70-GPa E-modulus. 7000-series alloys of Al-Zn-Mg-Cu are not yet widely used in automotive application, he noted. 7000 could be considered for bottom plates where impact resistance is key, but a point of “diminishing returns” likely will hinder its use in this application. “At the moment, with the manufacturing processes needed, this [additional 10% weight savings] benefit probably doesn’t cover the cost of using 7000,” Afseth said.
The battery enclosure has a critical role in crash energy management, both in terms of preventing intrusion into the battery cells as well as absorbing energy to protect the passengers. A dual-frame prototype illustrated by Constellium employs two different advanced extruded alloys. The inner frame is made of strength-optimized 6000 from the HSA6 family, while the outer reinforcement is a ductile 6000 alloy of the HCA6 family.
“The inner frame’s main function is to prevent intrusion into the cells even if it leads to fracture of the metal,” he said. “The outer reinforcement is designed to crumple in a very controlled way without fracturing so the maximum amount of energy is absorbed.”
The concept of placing battery cells directly in the body-in-white (BiW) is “very interesting” and would remove the redundancy of having a “box within a box,” he said. “If the trend continues to eliminate the modules and then the enclosures and have cells directly integrated in the BiW, effective sealing and joining will grow in importance.”
If the industry shifts to solid-state batteries, the function of the enclosure likely will shift, too. “We may see some load-bearing function in the solid-state battery cells themselves and therefore less structural demand on the enclosure,” Afseth said. Another trend he is seeing is OEMs increasingly bringing the development of battery enclosures in-house rather than outsourcing to tier suppliers. “Most OEMs and start-ups developing BEVs are at least considering aluminum enclosures if not actively developing them,” he said.
Despite lightweighting and recyclability benefits, aluminum enclosures fare less favorably when thermal runaway occurs or if a vehicle catches fire. “Aluminum has very high thermal conductivity and the melting point is 630°C,” Afseth said. “A battery fire can reach 1200°C or more and the aluminum casing will last only a short time before the metal melts. So, for the top cover either a heavier steel sheet or a fire-retardant loaded polymer molding will resist longer and give the passengers more time to evacuate.”
Global Technical Regulation No. 20 on EV safety (GTR 20), which aims to protect occupants during thermal runaway for at least five minutes, has been adopted in China and is coming to Europe and elsewhere. An attendee of the CAR webinar asserted that “aluminum cannot sustain this regulation.” Afseth’s response: “I believe you might be right.”
Heat generated by the battery cells also can be a concern for aluminum enclosures, especially for parts that are in direct contact with the cells or other parts of the high-voltage system that gets heated during charging or discharging. “My main concern would be with alloys like 5182 which has more than 3.5 wt% Mg, as these may over time develop a film of beta-phase precipitates at the grain boundaries which can result in degraded properties,” Afseth explained.
For parts of the enclosure that are away from the cells, such as the bottom plate located below the cooling plate, heat is not a concern. Afseth said he does not see any issues regarding immersion cooling: “Aluminum alloys of the 3000, 5000 and 6000 series are very well compatible and completely resistant to common coolant liquids.”
Move to multi-materials
Justification for the over-cost of aluminum structures is found in the secondary mass and cost savings tied to being able to downsize the battery and the powertrain, Afseth stressed. But, as battery costs continue to drop, the value equation for aluminum may dissipate. In the past decade, battery cost has fallen by almost a factor of ten, he noted, from about $1,000 kWh in 2010 to below $150 kWh last year. Energy density has almost tripled over this same period, so batteries also weigh much less than before.
“Why we see all the long-range BEVs today using aluminum is because at the time they were making their engineering and material choices, this equation was super simple: You spend a few hundred dollars more on the body structure and you save thousands of dollars on downsizing the battery,” Afseth explained. “Today, at the current prices, it’s still strongly in favor of aluminum designs, especially for the larger vehicles like SUVs and trucks that target long range. But soon, it may no longer be economically beneficial to use aluminum, especially for the small cars that have moderate range and target the lowest possible price point.”
Afseth said he sees a transition to more mixed materials for battery enclosures in the coming years – but “very little” carbon fiber, which makes more sense in motorsports or ultra-luxury sports cars where cost is not an issue. “Other, cheaper fiber-reinforced plastics may grow more,” he added.
Some OEMs already have begun shifting to steel or mixed-material designs for their battery enclosures, Afseth acknowledged. Tesla is a prime example. The EV maker has reduced the amount of aluminum in the battery enclosure for the Model 3 and Model Y compared to what was used in its S and X models, according to Afseth. “Statements made public about the upcoming structural battery pack to be used first in Berlin [Gigafactory] also mention that the upper and lower covers are steel, not aluminum,” he added.Continue reading »