New Materials and Bonding Challenges for the Electric Vehicle Age

Posted on 7/22/2019 1:29:22 PM By George Ritter

Several automakers throughout the world have made it clear they are planning for significant increases in electric vehicle production within the next two decades. Some have gone so far as to say they will be all-electric. This thrust represents significant challenges in the development and selection of lightweighting materials and ways to put them together at assembly-line speeds.

The real driver for mileage is not fuel economy; it is carbon dioxide emissions (CO2). The derived world standard of 100g/km of CO2 (162 g/mile in the US) results in a back-calculation of fuel economy that equates to 54.5 mpg (9~ 23 km/liter). The attained mpg value can be lower based on “credits” for things like electric power steering, electric air-conditioning, idle start-stop engine controls, and – weight. The “real value” may be down in the mid-40’s mpg, after credits.

For mileage improvements, about 50% would come from drivetrain, about 25% from design, and 25% from weight. Many hybrid vehicles – and some conventional vehicles -- easily approach that now. Some believe the most composite-intensive vehicles will be small, affordable electric vehicles.

Petro-power is not going away. Increasing the electric propulsion fleet content removes pressure from petro-propulsion to meet the fleet averages for uses where they are essential. Electro-power substantially reduces strain on urban pollution and improves the prospects for self-driving vehicles, including delivery vehicles and people-movers, which are more likely to be found in urban settings.

A significant factor in this realization is increased battery energy density with a goal of reaching 300-400 miles per charge, including allowances for operating accessories and HVAC. When those batteries are available, they will require new structures to enclose and to support them. There are issues of crash protection and survivability for the battery and the occupants. Light-weighting must also maintain structural stability, safety, and crash protection of occupants, and acceptable cost.

Occupant protection is already being addressed with increased use of composites in the “safety cage” including crash barriers in rocker panels, doors, pillars, and roll-over protection. These substructures are typically joined into the remaining structure using mechanical fasteners and adhesives (rivet-bonding) or by having molded-in metal inserts that are welded into the frame, also potentially using adhesives in combination (weld-bonding).

Judicious inclusion of metal crumple zones will increase to optimize a combination of properties and materials that, among other things, promises to vastly reduce torque forces in the event of a car accident. A potentially large design difference is the floor pan – a crash-protection feature that must be integrated into the rest of the structure for assembly and will become the containment shelf for the battery bank. Increased composite use is likely in that application. Composites have the added benefit of electrical insulation.

There will be increased use of reinforced thermoplastics, based on cost, formability, and recycling concerns. Structures using glass-reinforced polypropylene have long been in production and carbon-reinforced polyamides have been introduced. Those materials are now mechanically combined, but there will be increased interest in welding, bonding, and weld-bonding of those structures.

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Automotive has been borrowing from aerospace applications of reinforced thermosets, such as carbon-epoxy. That has taken decades to happen. However, aerospace is flying many reinforced thermoplastics and those adoptions into automotive will likely come more quickly. Cost is always a factor in automotive structures, suggesting more reinforced polypropylene, reinforced polyamides, and continued incursions of reinforced urethanes/RIMs. There will also be increased use of aluminum alloys and high strength steels in combination with polymer materials.

Everything must be combined into an assembly. Welding of compatible materials and fasteners are the preferred methods for joining dissimilar materials. Adhesives are being included using either weld bonding or rivet bonding. However, structural adhesives and sealants could play a larger role in subassemblies that are only bonded. Early adoptions would be safety cages, battery compartments, and secondary structure in interiors and trunk areas.

The big question is “when?”. Design cycles are typically 3-4 years, although there can be overlapping improvements and upgrades introduced into a model design. For a radical change, like an electric vehicle launch, the materials and structures will be locked in early. OEMs stand to benefit by communicating strategic information to suppliers as to what is planned and when. If there is to be a major reliance on bonded, rivet-bonded, or weld bonded structures, integrating multi-material lightweighting options, that will be determined earlier rather than later in the structure’s development. For OEMs and their suppliers, the time for discussion is now.

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