23 November 2021
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How EVs are changing carbody production

Spanish and UK vehicle body parts manufacturer Gestamp has developed a high-volume hot steel stamping process to form panels and structures. Ignacio Martin, R&D general director, body in white, explains to Will Dalrymple how the process is suitable for electric vehicles, and how electrification is affecting vehicle design

Worldwide, Gestamp operates more than 100 plants. In the UK, Gestamp is consolidating production to three manufacturing plants: Newton Aycliffe, Wolverhampton (opened in 2018) and Llanelli, and closing smaller plants in Washington and Cannock, West Midlands.

Body in white R&D general director Ignacio Martin explains the company’s approach and developments with particular reference to electric vehicles. “We try to look for the best lightweight safety solution to integrate batteries. Our approach is not the same as with internal combustion engines. We are seeing gradually a move of safety features for crash management into the body-in-white structure. The battery is more in the centre, separated from the crash, and Gestamp has invested a lot of innovation in this direction.

“Weight reduction is a big driver. The battery weight is very high: depending on the car, 500-600kg. One of the effects of its weight is that the rest of the body needs to be increased and reinforced for a crash. From a driving point of view, the lower centre of gravity of batteries [than their mounting of an internal combustion engine] helps, but for a crash, there is more mass, so we have to change the crash designs at front and rear. From the point of view of vehicle operation, less weight leads to higher range, and better dynamics, but less weight offers less crash improvement too.

CRASH SAFETY

“And due to the battery’s heavy weight, we increase the body’s side impact resistance. In design, we are trying to separate a crash event to take more of the side forces from the doorway into the side of the car. That opens up a wide new field.

“So-called extreme-size parts are big hot-stamped steel [assemblies] that integrate a door ring, the A pillar and B pillar in one piece using welding and overlap technology. You can put different material thicknesses and hardnesses in one piece. They are spot-welded and stamped all at once. This offers a huge part with a higher quality standard and tolerances at a lower cost, a lower weight and consequently lower CO₂. Customers have lower assembly costs because there are fewer parts to build a car. This is applied to sides, door, floor and rail system in front, crash structures – nearly all of the body structural parts. I believe this is a major step in electrification, while protecting the battery, which is the main problematic issue in a crash event; the forces and deformation of the battery.

“Extreme-size parts are made with hot stamping, and the joining technology is spot welding. It is very fast. We have developed the technology of hot stamping for aluminium products, but we can’t reach the same performance as steel, so it is an alternative for inner door sections, saving weight and improving performance. Multi-material hot stamping is not possible because of the different temperature requirements of each metal.”

Another production area for the company is battery protection, Martin explains. “In the first designs of battery boxes that we have been producing, there has been a massive use of aluminium extrusion, with internal aluminium structures to carry the cells. The assembly and joining technologies are mainly welding technologies: lasers, hybrid welding, steel friction welding. Because of thermal deformation of this type of product, we need to apply machining in most cases to get parts in tolerance.

“The battery and cooling system cannot have any leaks; this is the reason why manufacturing tolerances are extremely high, to guarantee no-leak performance. In addition, the boxes are closed with adhesive.

“Adhesive is being applied more and more. We see different applications for this. With structural parts, adhesives are facing their limits, so they are used to support rivets or welding. In a lot of our designs, adhesion to the base material is the limiting factor. Adhesives are being used more and more in combination with riveting and welding. Spot welding is still a main feature, but we enhance joint quality with an adhesive for high structural loads. In dissimilar materials, we use riveting in combination with adhesive. That supports the joint’s stiffness, and in the case of battery boxes also creates liquid tightness in thin units.

“Adhesive dispensing systems that are automatic are always used because adhesive is expensive, so the quantity needs to be very accurate: too little or too much is not good. It’s a complex to integrate adhesive dispensing into production lines, but it is applied because requirements are higher; more strength is put into the parts.”

MECHANICAL JOINTS

“Riveting is used for joining aluminium and steel. We fit aluminium extruded rockers into a steel body. That is coming more and more. It [riveting] is an optimal solution, but has a higher cost, and from the standpoint of the supplier, high investment. We use a steel rivet, but there is no standard design. We apply each to fit the right combination of thickness and material hardness. We try to optimise rivet thickness and hardness to get the best performance. It might be possible to standardise, but we prefer to have the right rivet for the solution, and some of the crash forces are [very extreme]. It is always an automatic riveting process, because we incorporate process controls to be sure of the parameters and make sure the forces are right. Most rivets are one-sided [blind]; sometimes they are two-sided.

“Hinges are mostly in steel, and developed within the Edscha subsidiary. New products entering the mainstream are power sliding doors, which are becoming more popular in EVs. And there is the ‘frunk’ – front trunk – where the engine used to be. That opening can be powered and automatic. To that there is a pedestrian safety angle too; right before a crash they can open to improve crash performance. There are electrical power systems as well in these doors, and conventional hinges. Even they are changing, as the normal hinge needs reinforcement. Batteries are in the bottom of the vehicle, and in a hard side impact there are higher forces on the doors. We are moving especially into powered systems, electronic motors, and the next step is CPU-related functions for powered systems, and a software solution.

“In conclusion, we have seen much aluminium coming into cars, in battery boxes, rockers and doors. Multi-materials have come into about half of cars, but we forsee a movement back to steel. In chassis for EV and body structural areas, we believe with new hot stamping and other technical solutions for EVs will move gradually back to steel. This is not only because of forming but also welding. The full process of hot stamping always goes best with spot welding because it is a huge cost saving for the customer. The moment you go into riveting, the cost is quite high, and future applications are much more linked to hot and cold stamping.”

BOX: New material

Tata Steel is introducing a new range of hot-rolled steels for chassis applications. It is said to combine the benefits of high-strength low-alloy (HSLA) and advanced high strength steels (AHSS) such as its FB-, CP- and XPF- range, with a protective zinc layer against corrosive influences. To meet the requirements of these new steels, Tata Steel in the Netherlands and Wuppermann Staal Nederland BV (WSN) are jointly increasing capabilities of producing thicker high-strength galvanized steels (2-4mm) while meeting automotive requirements. This was developed in a multi-year improvement programme, which has been completed.

Author
William Dalrymple

Related Companies

Gestamp Tallent Automotive

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