Nio uses carbon-fibre, aluminium in EV bodies
Material choices and welding techniques help China-based auto manufacturer produce lighter cars.
Today, China leads the world in electric vehicle (EV) purchases, with 1.1 million models sold in 2018 in that country alone, while worldwide sales totalled two million. Although the fall in the price of battery technology has contributed to this boom, the weight of batteries remains an issue.
In general, battery packs and electric motors weigh more than twice as much as conventional internal combustion engines (ICEs). And given that a lighter vehicle is inherently more efficient than a heavier one, that is a challenge for EV manufacturers trying to win over customers, who may be happy to propagate the environmental benefits of emission-free electric mobility, but still fear a limited driving range.
While lightweight materials are desirable in an ICE, in an EV they are essential. This was understood by electric vehicle and technology company Nio, China, when it launched in 2014. Headquartered in Shanghai, the company’s research and development facilities produce three all-electric vehicles – the EP9 supercar, and the ES8 and ES6 SUVs. Lightweighting is a core part of the company’s technology roadmap.
The smaller ES6 pushes the use of lightweight technology with the application of composite material. Carbon-fibre is used for the rear floor and the battery enclosure system. The main carbon-fibre rear floor measures nearly 2m2 and the total carbon-fibre components weighs 3.7kg, compared with the-body-in white’s (the shell once fully welded) total weight of 305kg.
A challenge was to ensure the floor could perform a number of structural functions. It had to support the seat pan and the occupants, along with acting as a crossbeam structure to mount the seat belt anchorage point. Extending into the boot space, it also acted as the floor and needed to be capable of withstanding the user standing on it.
Validating mechanical properties included testing both tension and compression at 0°, 90° and +45°/-45° against active standards ASTM D3039, ASTM D6641, ASTM D3518 and ASTM D6641 respectively. Bolt pull-out tests were further examples of physical tests before computer-aided engineering (CAE) was used to optimise the results.
Working with engineering software development company, Altair, UK, the first stage was to combine all structural load cases into a single digital model. Examples included occupant pressure load, ISOFIX bar loading, rear floor panel operator step-on loading, seatback luggage retention loading, seatback attachment stiffness, seatbelt anchorage loading, and linearised crash loads.
From here, CAE was used to optimise fibre orientation, find local requirements for fibre quantities and highlight hotspots and weaknesses in the design. CAE improved both the manufacturing process as well as the design. This included determining the quantity of fibre required for each ply and the optimum stacking sequence. Looking to the design, Nio improved the topology of the part to obtain a more uniform layup and tailor the nut carrier brackets to reinforce the carbon-fibre panels.
After each CAE iteration, loads or boundary conditions of linear model could be updated loads and re-optimised. The result was a highly efficient and functional composite part that delivered multiple benefits.
SGL Carbon, Germany, helped develop the carbon-fibre battery enclosure system. ‘Commercial battery enclosures for electric vehicles are mainly made of aluminium and steel,’ SGL Carbon Head of Segment Automotive, Sebastian Grasser, said.
‘In comparison, the CFRP [carbon-fibre reinforced plastic] battery’s enclosure is around 40% lighter. Other benefits include the enclosures’ stiffness, and the approximately 200 times lower thermal conductivity of CFRP compared to aluminium, which better shields the battery from heat and cold.’
The battery enclosure combines a sandwich core with multiple layers of carbon-fibre non-crimped fabrics. Nio battery packs can be swapped at roadside stations. A fully automated system enables a depleted battery to be removed and a fully charged replacement inserted in less time than it takes to fill the tank of an ICE with fuel. Using carbon fibre provides a lighter unit and robust protection during this process.
Advanced composite manufacturing
State-of-the-art fibre 50K, high-strength CF, textiles 150/300gsm UD-NCF and the fast curing resin system 2-K EP were selected for the best compromise of economic efficiency and mechanical properties. Multi-level validation was completed from the material and coupon level, over the component and sub-system to the system and vehicle level.
The processing and assembly solution is cost-competitive due to continuous process optimisation and a fully automated process for part production and vehicle assembly, lower labour and energy costs and the efficiency of an AI manufacturing platform that continuously improves the process using the production data. The main benefits include 50% tooling cost savings compared with metal stamping, 30% process cost savings, near-net-shape due to the high material utilisation rate, low cycle time at 2.5 minutes and a job per hour level of 20 units/hour.
Joined up success
One of the key challenges the company has successfully overcome is joining aluminium together during the vehicle assembly. Along the more widely used self-piercing riveting used at the factory in Hefei, China, manufacturing engineers have also adopted novel welding processes for the aluminium. The company has developed an aluminium laser wire welding for the roof and deep penetration laser welding for the metal on side doorframes. Laser welding uses a laser beam with a lower power than that used to cut metal, melting the material without vaporising it, and forming a contiguous solid structure after cooling. The energy of the laser welding process is concentrated. The material is formed in one pass, with little joint deformation and no pressure.
The facility uses welding torches designed for aluminium plate connection, and aluminium spot welding is used to connect materials of different thicknesses. With good applicability, it can be used in combination with structural gluing to obtain a greater strength than that of the base material itself. The riveting and gluing technique gives joint strength two to three times greater than that of the riveting-only method. For joining the carbon-fibre floor to aluminium, rivets are not
required. Instead, a pure bonding process proves sufficient.
Two types of adhesive are applied robotically to glue the carbon-fibre to the aluminium structures. Challenges such as different thermal elongations between carbon-fibre and aluminium and contact corrosion were managed through CAE testing.
To improve anti-corrosion, a zirconium and silane-based pretreatment that replaces the more widely used and thicker zinc phosphating alternative. The coating process, whereby silanol groups are chemically bonded to metal, gives excellent corrosion protection to the aluminium surface and promotes adhesion for the e-coat, while also offering reductions in water consumption, effluent and energy usage compared to traditional pre-treatments.
During the coating process, reactive silanol groups are chemically bonded to the metal surface, as well as to the paint coating. Heat treatment, which follows the curing of a subsequent cathodic e-coat, further cross-links the polysiloxanes, creating a coating layer in the range of 100nm, thereby reducing material consumption and pre-treatment times, as well as increasing productivity. This thin layer is still sufficient to achieve the same degree of corrosion protection as the traditional zinc-phosphate coating, which is 10 times thicker.
Selective use of materials for key storage areas, particularly around battery packs, can help the overall weight of an EV and in turn, help maximise the car’s range. While carbon-fibre and aluminium use might not be necessarily new, their novel application in a road-going commercial vehicle, particularly in an EV application, help drive the car’s intentions as a more efficient mode of personal transport, not just through its power source but also in their structure.