Adding lightness

ONE OF the tricks old hands used to play on young graduate engineers learning shop-floor lore was to get them to weld a couple of sheets of aluminium alloy together. Along with being sent to the stores department for some glass nails and a rubber hammer, taking a welding torch to aluminium would cause peals of laughter among the work-benches, as the alloy melted without warning and holes were burned through the sheet metal as if it were tissue paper.

Aluminium alloys are extraordinary engineering materials. Kilo for kilo, they are up to two-and-a-half times stronger than steel—and can absorb twice the energy in a crash. But working them can be tricky. They melt at temperatures as low as 463ºC compared with steel’s 1,425ºC and up. And they do so without first glowing red—hence your correspondent’s youthful embarrassment.

The good side is that their low melting point makes them easy to cast, draw or extrude. A third the density of steel and impervious to corrosion, aluminium alloys are widely used in the aircraft industry. Now the motor industry wants to use them in a big way, too.

In December, Ford will roll out an aluminium-bodied version of its F-150 pickup truck—its most popular model, accounting for 50% of the company’s earnings in North America. Being 15% lighter than its steel predecessor, the new F-150 is reckoned to be up to 20% more fuel efficient and able to haul 11% heavier loads. Such performance figures ought to make it a bigger seller than ever.

The question now is whether Ford can build the new lightweight version of its breadwinner without a litany of costly recalls. The company’s recent replacement for its Escape small SUV, a relatively conventional vehicle by comparison, has been recalled a dozen times so far.

In a bid to minimise such risks, Ford has spent $3 billion developing the aluminium-bodied truck and learning how to make it. It is not, by far, the first vehicle to incorporate aluminium bodywork. Carmakers have tinkered with the metal since the dawn of the automotive age, mainly to add lightness—and thus speed and agility.

But the cost of the metal, plus the difficulty of welding seams and pressing panels into complex shapes, has restricted its use to components that can be cast easily, such as engine blocks, cylinder heads, gearbox casings, wheels and suspension components. Only a handful of vehicles have used pressed or extruded aluminium alloys to make car bodies. Examples include Honda’s Acura NSX sportscar of the early 1990s; the Audi A8, launched in 1994; the Jaguar XJ from 2003 onwards; and, more recently, the Range Rover and the Tesla Model S.

All of the above were luxury vehicles costing well over $80,000 and built in modest numbers. By contrast, the base model of the 2015 Ford F-150 will be priced at a shade over $26,000 and built in huge volumes. Last year, Ford sold 510,000 of its F-150 pickup trucks.

Confronted by increasingly aggressive fuel economy and emission standards, motor manufacturers have to find ways to eke out yet more miles from a gallon of fuel. In America, the mandated Corporate Average Fuel Economy (CAFE) standard requires auto-makers selling cars and light trucks there to achieve a fleet-average fuel economy of 35.5 miles per US gallon (6.63 litres/100km) by model year 2016, and 54.5 mpg by model year 2025. Breaking out individual models based on their foot print on the road, the F-150 has a CAFE target of 24.5 mpg by 2016 rising to 30.2 mpg by 2025.

Half those fuel savings are expected to accrue from improvements in engines and transmissions—ie, turbo-charging, inter-cooling, cylinder deactivation, dual-clutch gearboxes, stop-start mechanisms and kinetic-energy recovery systems. The other half has to come from savings in weight. Automotive engineers reckon a 10% reduction in vehicle weight yields a 6% improvement in fuel efficiency. But trimming the fat from a vehicle's bodywork, components and accessories does not come cheap—as a general rule, around $2 a pound.

Still, weight reduction remains the biggest single improvement the motor industry can make. At present, aluminium accounts for around 10% of a typical vehicle’s weight—mostly in the form of castings and forgings. Vehicle makers have gone about as far as they can go in that direction. For further weight reductions, they have to start using aluminium sheets and extrusions for body panels and sub-frames. And to do that, they need different methods of joining parts together.

One of the biggest problems when working with aluminium alloys is their sensitivity to heat. Extra care has to be taken when welding them, as your correspondent learned aeons ago. With its high thermal conductivity, aluminium readily absorbs the heat from a welding torch and spreads it throughout the part, causing the metal to harden and warp while reducing its fatigue strength.

Back in the early 1950s, the aircraft industry learned the hard way about fatigue failure in aluminium alloys, when three De Haviland Comet aircraft—the world’s first passenger jets—broke up unexpectedly in mid-flight. Ever since, plane-makers have found ways of designing around likely sources of fatigue cracks. They also have better aluminium alloys at their disposal. Above all, though, they stick strictly to their long-standing practice of joining parts with rivets or structural adhesives instead of welds.

With care, most of the aluminium family of alloys can be welded using conventional tungsten-arc or metal-arc procedures. The main exceptions are high-strength alloys that include either copper (2000 series) or zinc (7000 series). But both welding approaches are relatively slow and require considerable skill. For faster and more consistent throughput, the motor industry has long preferred spot welding. This concentrates heat onto a small spot of metal by passing a large electric current through parts clamped above and below by copper electrodes. Most of the robots on automated assembly lines are spot welders.

Though it works well with steel, spot welding is less successful when used to join aluminium. The element's high thermal conductivity causes heat from the welding electrodes to spread rapidly away from the joint, preventing the metal from pooling properly. That means using more powerful welding transformers to generate the higher currents needed, so still more heat can be poured into the spot being welded. That adds considerably to the cost.

One answer is to do what the aircraft industry does—rely more extensively on rivetting and gluing parts together. However, by motor industry standards, aircraft are made in minuscule numbers, with sky-high costs caused by having to meet the most stringent of safety standards. Carmakers, by contrast, operate under an accounting environment where everything is costed to a tenth of a penny. Hence their preference for manufacturing methods that can be fully automated and run flat out.

Laser welding fits the bill perfectly, especially where aluminium is concerned. This uses a highly concentrated beam of infrared radiation to heat a spot a millimetre or less in diameter. The result is a continuous weld, which is extremely strong and can be almost invisible. Being continuous, laser welding allows loads to be transferred between jointed parts much more efficiently than spot welding does. That gives a vehicle better crash resistance, as well as increased durability and greater torsional stiffness. In turn, that means thinner materials can be specified—and thus further weight saved—without jeopardising the vehicle’s structural integrity.

Ford has used laser welding on a number of vehicles, including the outgoing F-150 truck. But the switch from steel to aluminium is not merely a matter of replacing one set of welding robots with another. To take full advantage of laser welding means redesigning many of the vehicle's parts.

For instance, not having to grip both sides of a joint, as a spot welder does, a laser welder needs access to only one side of the workpiece. That allows parts to be designed more for structural rigidity than for ease of manufacture. Also, because the weld is narrower and deeper, the flanges on aluminium panels can be made smaller than their spot-welded steel equivalents. While it may sound trivial, reducing flange widths on panels and other components saves a surprising amount of material, and thus weight. In the search for fuel efficiency, every scrap of weight-reduction counts.

No question, Ford’s new aluminium truck is an impressive feat of engineering. By all accounts, though, a number of niggling production problems remain. The stamping shop, for instance, is said to have had trouble preventing aluminium panels from cracking while being pressed into shape. And wrinkles still have to be ironed out in the laser-welding stages. Meanwhile, repair shops across the country are having to be retrained and equipped to handle aluminium trucks that have been involved in collisions. What will happen to vehicle insurance rates for commonplace aluminium trucks, as opposed to the odd luxury aluminium car, is anyone’s guess.

For Ford, everything is riding on the new F-150’s better fuel economy and hauling capacity, thanks to its wholesale use of aluminium. Will enough customers pay the premium needed to cover the truck’s higher material and manufacturing costs—and still provide the gross margin (said to be around $8,000 per unit) that has helped Ford stay out of the red and free from government interference?

With the aluminium F-150 having been designed during an era of rising fuel costs, the question is whether truck-drivers will care all that much about three or four more miles per gallon now those costs have fallen sharply, and are seemingly set to stay low for the foreseeable future? The next couple of years could be the making or breaking of Ford.