The search for ultra-light, super-strong automotive material

As automobile companies are always fishing for stronger and lighter materials to make vehicles, exciting research is happening in this area. India is no laggard here, going by indications from its labs.

Few realise how advancements in material science have contributed to the affordability and comfort levels of modern automobiles. For instance, the growing use of ultra high-strength steel (UHSS) and advance high-strength steel (AHSS) has made Maruti’s third-generation Swift (New Swift) lighter than the first generation by 125 kg (New Swift weighs 855 kg).

Material research is central to automotive research and development. Today we have interesting alternatives such as carbon fibre or glass fibre-reinforced composites, but as CV Raman, Senior Executive Director at Maruti points out, steel is still the “conscious” choice for a vehicle body due to reasons of cost, familiarity and local availability. Carbon fibre composites are expensive. Moreover, the alternatives have limited use — for example, engineered polymers are preferred over aluminium for fuel tanks.

Still, for vehicles, steel is king (although challengers abound — but more about that later). Research, as such, is on developing better steels.

The two key properties of steel are strength [tensile strength, measured in megapascals (MPa)] and flexibility (ductility or elongation, usually expressed as a percentage). There is a trade-off between the two — you can make very strong steel by putting in more carbon, but you’ll make it less plastic. The better steels are those that have more of both. Ductility is particularly important for safety, as flexible steel can absorb more impact energy by deforming itself. These properties depend on the micro-structure of the steel — the different ways in which the atoms are arranged, which gives us different kinds of steels such as ferrite, bainite, austenite and martensite.

Plenty of research is happening in India. Dr Arunansu Haldar of the Indian Institute of Engineering Science and Technology, Shibpur, is developing a “nano-structured, multiphase AHSS”, an austenite steel, targeting strength of 1,400 MPa and 20 per cent elongation. (Conventional UHSS is about 550 MPa, with elongation of 10 per cent.) Austenite is tricky, because it retains that micro-structure only at around 900 degrees Celsius, and researchers are trying to achieve it at room temperature.

The project is supported by a ₹1.15-crore grant of the Department of Science and Technology (under the IMPRINT-2 programme, a technology development initiative), with Tata Steel as the industrial partner.

A similar project is underway at IIT-Roorkie. With a ₹48-lakh grant, also under IMPRINT-2, Prof Sourav Das is confident of developing a bainitic steel with ‘ultimate tensile strength’ of 1,400 MPa and 15 per cent elongation (he told Quantum his aim is 1,500 MPa and 25 per cent elongation).

The aim is to develop a novel nano-structured UHSS with good ductility at a cheaper cost, says Das. Offering better passenger safety, it can potentially substitute some of the steels currently used for automotive components, he says. Das’s steel awaits testing, with the pandemic delaying the effort. However, once validated, it would be at a ‘technological readiness level of 4’— ripe for industry to take it for a pilot plant. JSW Steel is the industrial partner.

Is there scope beyond steel? Yes, magnesium (more than aluminium) is slowly emerging as a challenger, because of its extreme light weight. Steel’s density is 7.8 gm/cubic metre, aluminium 2.7 and magnesium 1.7.

With magnesium instead of steel, a car would be 55 per cent lighter, says Prof Sushanta Panigrahi of IIT-Madras, who has developed a magnesium alloy for automotive applications.

In its natural state, magnesium is brittle, it cannot be ‘deformed’. It also suffers from an undesirable property called ‘high yield asymmetry’ [high difference between its ability to take tensile (pulling) and compression (pressing) forces — which could lead to fracture].

Prof Panigrahi’s team made an alloy of magnesium with three rare elements — gadolinium, yttrium and zirconium — and engineered its micro-structure to make it more useful. With a strength of 500 MPa and 11 per cent elongation at room temperature, the alloy is comparable to conventional UHSS, but when heated to high temperatures it becomes super-plastic, with elongation of 1,300 per cent. Super-plasticity is a property that vehicle manufacturers love.

Admittedly, this material is expensive; but being ultra-strong and ultra-light, the payback is in the form of fuel efficiency. Dr Jose Immanuel of IIT-Bhilai calls it “a leap rather than a step in magnesium alloy research”.

Vehicle manufacturers Maruti and Mahindra have shown interest in this new material.

Maruti’s Raman observes that it “offers the best combination of strength-ductility among all magnesium alloys researched till date”, but also stresses the need for “local and cost-effective manufacturing”. In a statement to Quantum , Mahindra noted that “developing wrought magnesium alloys could be an interesting area for Mahindra in future”.

But truly, the ultra-light, super-strong material delivers a bigger bang for the buck in electric mobility, as there is less burden on the batteries, making way for higher power and range.

You see an emerging combination of lightweight materials and more powerful batteries, helping the e-mobility revolution.

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