Steeled for an ultra light body

The Ultra Light Steel Auto Body project, unveiled at the Geneva Show, hammered home the messsage that steel remains the pre-eminent material for car bodies.

Companies involved say the project has achieved its triple aim of hitting an ambitious weight reduction target while improving structural performance with no cost penalty. That result is important, as the initiative comes in response to car makers’ growing interest in aluminium, aware that from a car industry which knows the trend towards heavier cars will have to be reversed to meet increasingly strict fuel economy targets.

In the three years since the concept design was revealed, the consultant to the 35 steel companies in the Ulsab consortium, Porsche Engineering, has completed the detailed design of the body. Last week the body was shown in physical form for the first time.

Seven demonstration bodies are about to be whisked on a world tour to show car design engineers the results of the study. Another dozen or so are being made by some of the individual steel companies involved, including British Steel.

The bodies, says Frank Walker, British Steel technical representative on the project, are not prototypes but pre-production examples put together using pre-production techniques: in other words ‘they are deemed to be ready for production’, he says. This is in line with the project’s aims to investigate ways to exploit new materials and manufacturing and assembly techniques that would be credible for the car industry to adopt.

Ulsab set out to achieve ambitious improvements in weight, cost and performance not just compared with standard practice at the start of the project, but over standard practice extrapolated to 2000 (see box).

It has achieved this through a combination of technologies: the use of high and ultra high strength steel; laser welding; hydroforming; and the use of tailor-welded blanks. These were brought together in a design process which optimised the car body as a holistic system, rather than optimising specific parts at the expense of the whole. Adjoining parts have been designed with shapes that ‘flow’ into each other, avoiding discontinuities and hence stress concentrations.

One byproduct has been a reduction in the parts count of a typical car body from 140 to 96, simplifying assembly and making substantial savings in stamping machinery, ‘more than enough to cover investment in new technologies’, says Walker.

Around 90% of the lightweight body structure is in higher strength steels with a yield strength of between 220 and 550MPa. The highest proportion in a production car is currently around 50%, in the new Vauxhall Astra. Much of this stronger steel is concentrated at the front of the Ulsab structure, where it has been used not so much to reduce weight as to improve crash performance without adding weight. Computer simulations predict the body would perform well in the Ulsab standard tests for frontal and offset impact, including the NCAP programme frontal impact test, which is at a speed 35% higher than the mandatory standard.

The body includes 18m of laser welded seams, replacing spot welds which are reduced by a third and contributing to better load flow and increased stiffness. The required quality of weld could be achieved by robots working at 5m/min, says Walker.

Two parts are hydroformed. The roof rails on either side run the length of the roof and attach to the A and B-pillars and to the rear body rails with 12 significant changes in profile along their length. They are made from a 2.5m long tube, pre-bent before being forced into the required profile by hydraulic pressure from inside.

The roof panel is hydro-mechanically formed: it is first stretched the wrong way by hydraulic pressure, work hardening it, then stamped into shape by a mechanical press. The result is a skin stretched tight like a drum, says Walker, allowing it to be made thinner without the risk of denting.

Tailor welded blanks are fabricated from different grades of steel before pressing to provide strength where needed without additional reinforcing panels. They have been used most notably in the body side outers, made up of five separate pieces and ranging from 350 grade steel at the front to 220 at the rear.

Sandwich panels of tinplate gauge steel bonded to a 0.65mm polypropylene core have been used in part of the dashboard structure and for the spare wheel well, both areas where the fact that sandwich material has to be bonded into place is not a disadvantage.

Advocates of aluminium for car bodies argue that even without going to special grades of the material it can reduce the weight of a body by 50%, twice as much as achieved by Ulsab. They add that the disadvantages cited by steel’s proponents that aluminium is an unfamiliar material to the car industry, needing unfamiliar techniques both at the manufacture stage and also when repairs are necessary apply equally to techniques such as laser welding and hydroforming.

Walker reels off a list of manufacturers who are using at least some laser welding or higher strength steels already; the lightweight body merely uses them more extensively.

He admits that hydroforming presents some challenges: fully automating the roof rail procedure is one, for example. The basic tube used is 96mm diameter with 1mm wall thickness in 280MPa steel. ‘You would find that hard to get,’ he says but believes the tube industry will rise to this challenge.

Walker dismisses claims that body repairers will find it any more difficult working with high-strength steels, or that the mixture of steel grades will affect the ability of the lightweight body to be recycled. Scrap will be added to the charge of new iron in the standard basic oxygen steelmaking process as it is now, with no effect on the final product, he says.

Some of the processes take longer to perform: the roof rail takes around a minute to hydroform. Normally it would be made of several parts, each individually stamped in seconds. But they would then have to be assembled and spot welded together, with added weight because of the need for overlaps to allow for welding, and resulting in a more variable end product.

Walker is relaxed about the fact that aluminium looks set to break into volume production for the first time with Audi’s small car based on the Al2 concept, due to go into production in 2000.

Styling freedom is constrained by the limitations on profiling aluminium sheet, he argues. And the aluminium industry does not have the capacity to take a large chunk of demand for car bodies. Even if the demand were there, he asks, where would the considerable amounts of electricity needed to smelt the required amount of aluminium come from?

But most important, he says, no other material has met Ulsab’s ‘three-cornered challenge’: to achieve the weight savings with no compromise to safety or performance and with no extra cost. ‘No-one has demonstrated a weight reduction without cost penalty at the same time as the improvements that Ulsab has achieved,’ he says.