Steel Versus Aluminium: Who’s Winning the Lightweighting Battle In Cars?
23 June 2017 by Goran Djukanovic
Much like the metals industry, automobile manufacturers are continually and significantly investing in research and development, from developing new alloys and materials with improved properties to perfecting the latest techniques in the welding and shaping of various materials and alloys. These efforts have one shared goal: lightweighting vehicles and reducing emissions. Since the automobile industry will need to meet strict emission standards, the best way to achieve it will undoubtedly be by reducing the weight of its cars. Achieving this goal will require a mix of various aluminium alloys and high strength steels, magnesium, composite materials (carbon fibre) and various plastic materials.
Anticipating the need for higher production volumes and greater efficiency, the rolling industry in particular (both steel and aluminium) has been investing heavily in improving technologies in recent years. Alcoa’s Micromill continuous casting technology, for instance, is the latest breakthrough which could significantly reduce costs and narrow the gap between high-strength steel and aluminium auto body sheet (ABS). According to the company, this process is also much faster, turning molten metal into an aluminium coil in 20 minutes instead of the 20 days needed by the conventional rolling process. Such a development could be a game-changer for the sector, and incentivise manufacturers to switch to ABS.
The main battle for a leading position in future automobiles is between aluminium and steel. The latest outcome is that aluminium is advancing and increasing its content in cars – while steel, with its advanced high strength steel (AHSS) variations is trying to defend its dominant position.
Apart from a few critical areas related to the safety of the passenger cabin (such as: roofrail, longitudinal front and upper), it is now possible to make an entire body-in-white from aluminium, resulting in significant weight reductions and improved performance.
This explains why all leading producers of aluminium sheet for the automotive industry (Novelis, Aleris, Amag, Constellium etc.) are developing new 5xxx and 6xxx aluminium alloys for automotive applications with higher tensile strength (up 2-3 times compared to previously used alloys) and other improved mechanical properties (ductility, elongation), critical for applications in automobiles. The combination of strength with ductility is the key performance parameter desired for automotive applications. These new alloys may serve as an alternative to high-strength 7xxx aluminium alloys (up to 700 MPa tensile strength) in most cases and at much lower costs.
For the car body (body-in-white, hoods/bonnets, doors, etc.) producers mainly use work-hardening AlMg(Mn)-alloys of 5xxx series and heat-treatable AlMgSi-alloys of 6xxx series. Sheets of heat-treatable 6xxx alloys like AA6016 or AA6111 obtain their material strength thanks to a heat treatment process subsequent to rolling.
Some latest trends
The Center for Automotive Research (CAR), in a recent 2017 report, estimated that the increased use of high strength steels (HSS) is expected to peak at around 15 % of total vehicle weight composition by 2020, before gradually falling to roughly 5 % by 2040 as other lightweight materials gain ground. At the same time, mild steel content will fall from historic highs of 55 % of vehicle weight to about 5 %. In terms of lighter materials, UHSS steel and aluminium use will grow steadily, especially in safety-cage parts and components (e.g. frames and rails). The use of third generation steels with better formability properties will grow significantly. Use of magnesium will also grow, particularly in applications such as instrument panel crossbeams. The use of carbon fibre reinforced polymer (CFRP) will grow slowly with most applications in reinforcements rather than panels. CAR predicts that by 2030, 96 % of vehicle programs will consider aluminium for body-in-white applications.
Ferrari, for example, has developed unique forms of aluminium, such as reinforcing it with ceramics to reduce the weight by additional 15-20 %. Ferrari favours aluminium over CFRP since in a crash, aluminium is safer and easier to repair – aluminium crumples while carbon fibre shatters.
For car body and closure applications, AHSS currently costs between US$ 2 and 2.5 for each kg of weight reduction, mostly, depending of type and grade. Aluminium alloys are in the range of US$ 3-6/kg, magnesium between US$ 5-12/kg, and carbon fibre starts at about US$ 16/kg. Apart from cost, each material has unique processing characteristics.
The prices that OEMs are willing to pay for lighter weight will probably increase in the future, as regulations become tighter, other material options are exhausted, and the number of electric vehicles on sale grows. While in the past vehicle manufacturers would willingly pay US$1-2 per kg of weight reduced, currently they pay US$4-12, depending on segment. In the future, analysts project that they would be ready to pay up to US$20 per kg. This upper limit applies primarily to luxury and performance cars. Acceptable costs for mid-level premium models will probably range between US$5-15 per kg, and for smaller segment cars about US$3 per kg. Other than segment, the willingness to pay is a function of the position of the weight saved in the car and on other factors not always related to pure economic factors.
Enter the 3rd generation of AHSS
The long awaited the 3rd generation of AHSS (3rd Gen AHSS), known also as NanoSteel, named after the eponymous company, is finally ready to enter the automotive industry. It provides the high strength required by automotive engineers (over 1,000 MPa) in combination with significant ductility and elongation. This can provide geometric freedom, or the ability to create ultra complex shapes from sheet.
Typically, conventional mild steels offer superior elongation with less strength, whereas previous generation of AHSS offered superior strength with lower elongation and ductility. The 3rd Gen AHSS is bridging that gap, producing steel that is both strong and ductile at the same time.
ArcelorMittal has launched recently the first product in a brand new family of 3rd Gen AHSS for cold stamping. Named HF1050, the new steel is the first in a series of highly formable (HF) 3rd Gen AHSS grades ArcelorMittal will release in 2017. The tensile strength of the material is approximately 1200 MPa and its ductility is measured as 50% elongation.
The new material leads to weight savings of between 10 and 20% in vehicle parts, compared to existing dual phase (DP) grade steels, according to ArcelorMittal. Kia Motors has recently unveiled four new models at the 2017 Geneva International Motor Show, with the main highlight being the world debut of the all-new Kia Picanto, which uses 3rd Gen AHSS.
However, despite the obvious advantages of the 3rd Gen AHSS for lightweighting and formability, where it can compete successfully with the previous generations of AHSS, the latest aluminium alloys still get to keep their crown. Especially when applied in bigger vehicles, aluminium offers a 10-15% advantage in lightweighting for most shapes and designs. For the time being, the 3rd Gen AHSS and UHSS will probably maintain a strong grip only on smaller cars.
What’s more, while the 3rd Gen of AHSS is still in the R&D phase, the latest aluminium alloys are already in use. Finally, with excellent safety grades awarded to aluminium intensive cars, it is irrelevant whether the steel industry offers metals with even higher tensile strengths than existing AHSS-s.
Possible loosening of fuel standards
Despite the fact that U.S. President Donald Trump ordered recently a review of the fuel efficiency standards for vehicles implemented by the Obama administration, it is debatable how much it would benefit US carmakers. It is also up for debate how this would be reflected in job creation, knowing that many new workshops and repair shops entered the business specifically in order to support the implementation of new, lightweighting technologies in vehicles. Moreover, automakers will continue developing hybrid and electric vehicles program – regardless of fuel emission standards, they’ll feel pressured to reduce the weight of those vehicles. Lighter vehicles will burn less fuel, and will thereby reduce costs for consumers. Indeed, an analysis published by the EPA indicated that current emission standard targets, if realised, would result in savings of between US$1,460 and US$1,620 over the lifetime of a vehicle. The Obama administration said that while stricter rules would cost the auto industry US$200 billion over 13 years, but would save motorists US$1.7 trillion over the life of the vehicles.
Finally, even if fuel emission standards are lowered, it is not excluded that the next US president would reinstate previous emission standards, a fact that automakers must also factor in their risk assessments. The only actor who would undoubtedly profit from lower emission standards is the steel industry, since steel would maintain its comparative advantage to aluminium and other lightweighting materials.
Aluminium use in automobiles is gathering steam, while the automotive industry is racing to reduce the weight of vehicles in order to meet proposed emission standards. Over the long term (beyond 2025), cars will most likely use a multi-material mix, combining various types of AHSS steel, aluminium alloys, carbon fibre, magnesium, plastics, mild steel and other materials to achieve the weight, cost and performance targets. The main difference however, will be that no material will achieve the dominance that mild steel enjoyed in the past. The long awaited 3rd Gen AHSS, which will be rolled out commercially in vehicles in late 2017/early 2018, will only replace older AHSS – not aluminium.
What is clear is that aluminium and steel (AHSS, 3rd Gen AHSS & UHSS) will be dominant in the future, with each making up between 20 and 30% of a vehicle’s weight (on average). The rest will be made up by a mix of composite materials, magnesium, plastics and others, with each material fitting best according to its purpose and characteristics. Finally, aluminium will be dominant material in big vehicles (SUV-s, trucks, pick-up-s), while AHSS will maintain dominance in smaller cars.
Goran Djukanovic will be a speaker on Aluminium vs. steel in automobiles at the 4th Danieli Innovaction Meeting in October, in Buttrio, Italy.