Lightweight construction as the supreme discipline

PG_Leichtbaustrategien_QuelThat was the subject of the VDI Symposium “Lightweight construction strategies for the automotive industry”, which took place in Wolfsburg (Germany) in July.  As an inveterate in polyurethane I really felt strange amongst all those ‘metal people’ – apart from that, there were only a few CFK people in attendance.
Lightweight –a key theme in our industry, particularly during the current season of exhibitions!

The car of the future must be lighter: This not only reduces fuel consumption, but CO2 emissions too – ok so far so good – we’ve known this for a long time and it is equally valid in the field of plastics where it is replacing or complementing metals.  However the press conference during this event was one of the most interesting I had attended for some time – maybe due to the fact that lightweight metal construction is really alien to me.

Basically, the following statements are fundamental:
1. Steel is the heaviest material in bodywork construction – aluminium is 40 % lighter. On the other hand magnesium is 15 % and CRP 40 % lighter than aluminium.
2. Compared to plastics, metals have a clear recycling advantage – with virtually endless possibilities for re-use.

Essentially, light weight construction with metals means reducing density (e.g. alloys), reducing thickness (if possible) and/or a material mix.
VW maintains that steel is the best, most cost-efficient route to light weight construction in mass production and demonstrated this in the development of the Golf 6 into the Golf 7. The new model would normally have been 30 kg heavier with all its additional fittings – but the use of the weight reducing steel body, means that the Golf 7 is – depending on the specific configuration, between 23 and 36 kg lighter.
Aluminium light weight construction, such as Audi’s „Alu Space Frame“, is the way to achieve a weight reduction up to 230 kg. The pricing of this however, is more suited to high end cars – up to now they have only used it in the A8 series.
Magnesium seems to be facing a revival – the first important application of magnesium alloys was in the frame of the zeppelin air ships.
As I had never thought about magnesium, I decided to google it: “The most important property of magnesium alloys, making them very strong competitors of aluminium and its alloys, is its possibilities in light weight construction. The density of around 1.74 g/cm3 is considerably lower than the density of aluminium at 2.75 g/cm3. In addition, a melting range between 430 and 630 °C, is lower, allowing energy savings. “
But then I found another point which contradicts the theme “Reduction of CO2 emissions“:  ”Between 18-45 kg CO2 are generated during the production of 1 kg magnesium (for comparison: Only 1 kg CO2 is produced for 1 kg steel).

CRPs have fantastic potential – but if you’re not in the Lamborghini and Co. price bracket – they are simply too expensive. And in the case of car body damage, you have to completely replace the part since the material disintegrates during a crash.

But this doesn’t really matter, because even in normal cars, dented aluminium bumpers, wings and side doors are also only replaced.

The cost of CRP production is expected to drop by about 90 % in around 10 years, but this is still double the price of aluminium.

Statements during the press conference: “Every light weight construction technology is valuable and the future lies in a hybrid construction method.”
“E-cars in big and mega cities make sense. The weight of batteries in these cars will not change significantly over the next 20 years, so only light weight construction will work to increase the travelling distance.”

Research is currently underway on how crash energy can be transferred into other energy during an accident – there is a possibility that in around 15 years time, sensors will be able to identify whether the car has hit concrete or just an apple crate – allowing the behaviour of the material during the crash to be influenced.
Birgit Harreither

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