2013 Mercedes SL550 embraces aluminum

  • 24-Jan-2012 01:42 EST
2013 SL-Class (4).jpg

Although the 2013 Mercedes-Benz SL550 is both longer and wider than its predecessor, the new car is 275 lb (125 kg) lighter thanks to a new aluminum design.

In a bid to improve the dynamic response of its flagship roadster, Mercedes-Benz parent company Daimler AG Chairman Dieter Zetsche introduced the 2013 SL500 at the North American International Auto Show featuring the car’s first use of aluminum construction.

Naturally, the new SL also enjoys an improved engine and new comfort-related gadgets, but the 90% aluminum structure is literally the car’s foundation.

The 429-hp (320-kW) 4.6-L twin-turbo V8 seen previously in other Mercedes models arrives in the SL, replacing the old 382-hp (285-kW) 5.5-L naturally aspirated V8. The engine is matched to a seven-speed automatic transmission.

Electronic upgrades include the addition of the Magic Sky Control glass roof panel with the ability for the driver to switch between clear and nearly opaque. That debuted last year on the SLK roadster.

Magic Vision Control (no shortage of hyperbole in the Mercedes marketing department these days) is a windshield washer system that mounts the nozzles on the wiper arm to reduce splash onto the occupants when the roof is down. While we’ve seen this on plenty of cars in the past, Mercedes says this is the first execution with nozzles on both sides of the wiper blade. So that’s what makes it magic.

The SL has done a poor job recently of living up to its name: the letters stand for Super Lightweight. Perhaps due to this situation, the company does not list official curb weight numbers for either the old SL550 or the new one, but it does say the new car is 275 lb (125 kg) lighter than before. Of that, 242 lb (110 kg) has been cut from the mass of the body-in-white, even though the new car is 1.97 in (50 mm) longer and 2.24 in (57 mm) wider than before.

As we have seen in other aluminum models, the SL is a mixture of materials, manufacturing techniques, and fastening methods. There is stamped sheet aluminum, vacuum die casting, high pressure casting, and extrusions. The parts are robot MIG welded [62 m (203 ft) of welds], friction stir welded, screwed, riveted, bolted, and bonded together.

Additionally, there are magnesium castings and high-strength steel in the A-pillars and windshield header. “The hybrid body includes a variety of materials and joining features,” noted Chief Engineer Gunter Fischer in a briefing at the show. “The object is to optimize between weight, safety, and cost.”

This combination produces a body that is 20% torsionally stiffer than its predecessor, with a rigidity of 20,000 N·m (14,750 lb·ft) per degree.

The SL incorporates several very large castings for the front and rear shock towers and front and rear bulkheads tying those together. The benefits of using such large castings include simplifying assemble through the integration of parts and the ability to precisely distribute loads, Fischer explained.

Modern casting techniques permit engineers to vary the casting wall thickness precisely to minimize weight where possible but provide strength where needed, he said.

Adding ribbing to the castings gives localized reinforcement as needed. “We have increased the dynamic stiffness in those areas where we connect the body to the powertrain for reduced noise and vibration,” Fischer said.

The car’s door sills feature castings fore and aft connected by aluminum tubing from a tailor-welded blank that is thicker at the ends, where it meets the castings, and is thinner in the middle for reduced mass.

But Fischer said his favorite innovation in the SL’s aluminum chassis is the car’s use of extruded aluminum panels, rather than simple sheet metal, to form the floor. This confers greater strength in the floor, which is critically needed in a car with no roof, he said.

So in place of the usual sheet metal, the SL has 23-mm (0.9-in) sections of extruded aluminum. However, it is very difficult to precisely extrude the correct thickness of a panel wide enough to cover the floor on either side of the driveshaft tunnel. So instead of using a single piece, Mercedes welds together three sections to form each side.

MIG welding these flat sections would be a problem. “With MIG welding, you bring heat in and the panel wants to warp,” he said. The extrusions would “warp endlessly.” Instead, Mercedes employs the industry’s first application of friction stir welded aluminum, a process that only heats the metal very locally and only heats to the point of placticization, which is much cooler than when MIG welding, so there is no warpage.

The long, flat slabs of extruded aluminum are ideal because friction stir welding tools present access problems for some parts and aren’t able to adjust for complex surfaces. “To use friction stir welding in an area with a 3D seam, it is impossible!” Fischer exclaimed.

The SL550 arrives in dealer showrooms in the spring, though pricing was not announced.

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