Clever metal

The continual search for lighter and stronger materials is never as simple as it seems. Justin Cunningham reports on the metal innovations that aim to answer the challenge.

While some engineers are drawn to the new and exotic, others prefer to go back to basics in search of elegance. This is a divide that exists across the engineering fraternity and is reflected in material selection, where ever more choice has meant that there are ever more trade-offs to consider.

For some time, interest in using aluminium, plastics and more exotic composite materials to replace conventional steel has been on the increase. The automotive industry has been a case in point, with some OEMs claiming over 50% by weight of cars are made up of non-metallic materials. Even some structural parts are starting to be made with alternatives.

However, this endeavour, driven by a desire to lose weight, is causing headaches. It is difficult. Designing with alternative materials often requires a different mantra and set of principles. It can require retooling in the factory and re-skilling by engineers. And these alternatives are often more energy intensive to produce and dispose off. This, of course, will offset any 'in use' saving of CO2.

Aluminium, for example, is extremely energy intensive to produce from ores; substantially more so than steel. Though recycled aluminium requires as little as 5% of the energy it initially takes to refine, steel is still more favourable in terms of embedded CO2 for like-for-like recycled or virgin material. Recycling plastic is also a topic of debate, and while many can be recycled, deterioration in properties from 'virgin' materials could create future stockpiles of recycled plastics that have very limited practical use and no real value.

The point is that factors such as energy costs, considerations of manufacture and disposal, the 'embodied CO2' in a material and all these issues that accompany material selection are seldom included in the overall equation of product. While a vehicle manufacturer is happy to quote a figure in g/km of CO2, it is rarely with the proviso that manufacture and disposal was 'x' times more energy intensive and therefore will take 'y' thousand miles to offset. Is anyone actually doing these sums?

Greg Ludkovsky, vice-president of global research and development at Arcelormittal says: "As a representative of the steel industry this is frustrating, but it also frustrates me as a citizen. It is disingenuous and is misrepresenting facts.

"I don't mind having a fair fight. What I don't want is these things that serve little purpose in addressing the actual problem. If you overdo it in one part of the world [by producing more aluminium], all it means is that in a very short period of time any 'in use' CO2 saving is going to get equalised anyway because of these smelting plants. So the whole notion is misleading. We need to influence the decision makers with facts and stop playing games."

Ludkovsky is passionate about the use of steel and is a firm believer that it has a significant role to play as an engineering material in the future. By working closely with its customers Arcelormittal has been able to find competitive solutions in terms of both weight and cost versus other materials such as aluminium and composite.

"The supposed quick and simple solution of going to a lighter material to solve the problem is not necessarily the right solution," he says. "We have proven on a number of projects that we can satisfy weight reduction targets without resorting to other materials. And we have found that engineers are taking note of this, but the environmental consideration is only one reason why. The other is cost."

Arcelormittal has seen a resurgence in its steel sales which coincides with improved physical properties accomplished by the nano-manipulation of the steel structures resulting in stronger, stiffer steels. But it is the cost of steel that is its main selling point; not just in terms of purchase cost, but the embedded costs. Engineers know how to work with it, factories are already tooled for it and it can easily be repaired when in service.

"What drives people into looking at alternative materials is bad legislation and that drives bad design," says Ludkovsky. "While engineers try to take weight out, they are increasing their costs a lot because of the time it takes to design with numerous materials, the interface and joining between them, and the time and cost of manufacturing. And working with composites opens another can of worms.

"Using aluminium in a car can often lead to complicated design and especially manufacture. And that's not to mention the cost to the consumer who will have to pay for repairs. It will be twice or three times more expensive in the case of aluminium and I can't even imagine what it will cost for composite. This is always totally removed from the dialogue which is totally unfair to the consumer as no one is giving them the information."

Arcelormittal's internal design team works closely with its customers to ensure they are getting the most that steel has to offer. So confident is the company in the material it often guarantees its solutions will offer the same reduction in material and weight as alternatives, but at a neutral energy and financial cost.

This Ludkovsky refers to as 'intelligent design', in which CO2 generation of the whole process is thought about. Again, using an automotive example, the body-side-outer is usually stamped out from a large steel sheet. This removes around two-thirds of the material, which, although it is recycled, needs to be cut, shredded, re-melted, re- rolled, and annealed.

"Imagine the additional CO2 created to do that," says Ludkovsky. "But, if you do the same thing from a laser-welded blank you already have your openings; there is virtually no scrap. So by incorporating that in to the design of the body-side-outer you immediately make a profound impact on the CO2 generation of that vehicle from the outset.

"The saving of scrap has never been related to CO2 content in manufacturing, but all that material has to be reprocessed. This is another way we are decreasing cost and CO2. No matter what it is you are producing, in automotive or not, this kind of intelligent design can have a direct and profound impact on CO2 generation."

However, when cost is less of an issue and the main driver is performance, developing complex design and manufacturing processes is much more acceptable. Ferrari, for instance, believes it will cut 15-20% of the weight from its cars with no sacrifice to stiffness using aluminium.

Ferrari body systems manager Patrizio Moruzzi, says: "We will continue to develop composite technologies for our ultimate cars where the overriding requirement is extreme performance. But when we look at the requirements for our V8 cars, produced in volumes of typically two to ten a day for owners who use them regularly, we see that aluminium is clearly the better solution and offers much greater potential for further improvement."

In the run-up to the Ferrari California and 458 Italia, investment for body-in-white technologies was increased by 50% which allowed engineers to begin taking greater advantage of aluminium's properties.

For example, the torque reaction box at the base of the A-pillar has traditionally been made from an extrusion reinforced by welded plates. However, it is now heat formed from a single piece, reducing the amount of material used and increasing consistency, allowing a greatly improved shape that better matches the space available.

Careful materials choice, combined with developments in joining technology allowed similar improvements across the vehicle. Five different aluminium alloys were selected for extrusions and three for body panels, allowing material characteristics to be optimised for the specific requirements of each component.

The latest high-strength alloys allowed the resistance of some components to be increased by up to 80% without any increase in weight. In total, the California and 458 use more than 20 different aluminium alloys, many of which are also specified in a range of different treatments to further optimise their properties.

Chief engineer from Ferrari, Roberto Fedeli, says: "The next car will have a structure that is much more optimised to take advantage of what we now know is possible with advanced alloys. For those who wish to compare it with a composite structure, I can tell you it will be much lighter."

The process started with the move to aluminium lithium for the anti-intrusion beam in the doors of the 458. Lithium atoms replace aluminium atoms within the aluminium crystal lattice, but as they are much lighter it reduces the material weight by up to 10%. Introducing another size of atom also helps to block the movement of dislocations during deformation, making the material stronger.

The company is also using aluminium foam, allowing a section the size of a brick to weigh just a few grams. Typically, 75-95% of the volume is just space and adjusting this ratio and some other parameters allows the material to be tailored for a wide range of applications.

Franco Cimatti, Ferrari's technical director, says: "We are moving from simple materials substitution, where a better material replaces a traditional one, to a point where the structure is designed specifically to allow the characteristics of new materials to be fully exploited."

One of the biggest challenges is how to join new materials together. Very thin panels, aluminium and very hard alloys cannot be welded. Mechanical fixings concentrate stresses and lightweight metal foams create a new set of challenges. The answer was found in structural adhesives.

This approach eliminates stress concentrations by distributing the load across a much bigger area. It is likely that the next-generation cars will see joints in which adhesives work in combination with enhanced riveting technologies.

Another joining innovation being trialled is Cold Metal Transfer (CMT) welding, which allows edge-to-edge joining like today's Metal Inert Gas (MIG) welding systems, but with so little heat that the joint can be touched almost as soon as it is made.

CMT welding reduces the energy input by around 30%, allowing welds close to adhesives. The process is well proven, but there is still work to do on the size of the CMT tool which is currently too large to allow the robot welding heads to reach the most tightly-packaged locations.

Another challenge is how to repair crashed cars. For the next-generation cars, Ferrari is developing repair systems alongside the new materials technologies. These include an improved two-component adhesive that can be used by body shops to provide a bond that has similar properties to the adhesive used in the factory.

Author
Justin Cunningham

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