Materials science plays a decisive role when it comes to both the quality and cost of a car. Hence, the right selection of materials is key in enabling the automotive industry to meet quality and emission goals.
During the last 100 years of development and applications, steel has proven itself to be an effective material of choice for vehicle body structures. Over that time, the steel industry has spent years on extensive research and development of advanced steels that are stronger, safer, greener, more fuel efficient and cost competitive. Today, over 200 grades of steel are available on the market .
New safety and fuel regulations have pushed automakers to turn to alternative lightweight materials and new technologies. Maybe alternative materials are a good option for mass reduction, but only if you can afford them.
The steel industry continues to innovate and alternative materials and steel are locked in a battle for the attention of the automotive industry. As this battle continues, what are the main challenges that the automotive industry is facing today?
The global automotive industry is under constant pressure from environmental and customer demands . The industry is facing issues regarding fuel economy, gas emissions, safety and affordability. Moreover, the competitive pressures on cost, quality, performance and manufacturability of the vehicles today are bigger than ever.
The response of the automobile industry is mainly centred around the selection of lightweight materials that meet these performance and cost requirements, thus improving the efficiency and fuel economy of vehicles . However, current EU safety and economy regulations are affecting materials selection for automotive applications, particularly the vehicle mass and performance .
One of the major issues facing the automotive industry in the 21st century is the environmental change resulting from rising CO2 emissions . Vehicles produce greenhouse gas emissions during their entire life cycle, with CO2 emissions from the transport sector accounting for about 20 % of total CO2 emissions in the EU .
Most vehicle-related CO2 emissions result directly from the use of the vehicles (85 %), while the other 15 % is a result of the manufacture of the automobiles, including material production .
In addition to directly influencing fuel economy, auto manufacturers have been focused on ‘lightweighting’ to achieve the fuel efficiency requirements .
Moreover, in 2017 the average CO2 emission performance of new EU passenger cars was investigated. The CO2 emissions of nine major manufacturer groups were grouped together and analysed for how well they meet CO2 standards. The data is presented in Figure 1.
With approximately 100 g/km and an average vehicle weight of 1350 kg, in 2017 Toyota had the lowest CO2 emissions out of all manufacturers. On the contrary, the Fiat Chrysler Automobiles (Alfa Romeo, Fiat, Jeep, Maserati) group was in the worse position to meet its 2020/21 target. The average CO2 emissions for a vehicle will have to decrease by approximately 24 g/km (20 %) to meet 2020/21 CO2 emission targets.
The introduction of new regulations for safety and crash improvement has challenged the steel industry to develop steels with unique microstructural combinations and higher strengths such as DP, CP, TRIP and TWIP, which aid in reducing component size and weight.
Read more about these various forms of AHSS along with its current development in the article “Advanced High Strength Steel (AHSS): The new generation of steel that makes your car stronger, lighter and safer”.
When it comes to lightweight materials, the first thought often starts with low-density materials such as aluminium, magnesium or carbon-fibre-reinforced materials, resulting in a negative perception of traditional steels because of their high density.
However, the introduction of new steel grades with higher strength has allowed designers to achieve amazing crash performance without increasing mass . This is achieved by using AHSS in the body structure and closures, where each load-bearing dominant part is replaced by AHSS. The mass reduction is then accomplished because a smaller, thinner part made from AHSS can bear a greater load [4,5].
In 2009, a study evaluated the mass reduction for a Toyota Venza vehicle by replacing the mild steel mainly with AHSS grades. A mass reduction of 25 % was achieved. Simultaneously, the study also evaluated the mass reduction for the same vehicle using low-density alternative materials. In this case, a mass reduction of 29 % was achieved . However, a cost-benefit analysis showed that steel parts are both stronger and cheaper than parts made from the other alternative lightweight materials [3, 5].
The 2016 Hyundai Tucson and the 2016 Kia Optima are excellent examples of improved vehicle performance and mass reduction through using AHSS, making their cars lighter and stiffer. The Ford Edge is another example of efficient design, using different AHSS generations for optimising performance [3, 5].
Steel vs. aluminium in automotive production
There has always been competition between materials in the automotive industry, especially between aluminium and steel [1, 5]. A major advantage of steel over aluminium is steel’s formability, strength and affordability. Steels have been widely known for their higher yield strength and ductility compared to aluminium. For example, the body design of Cadillac cars would not be possible with aluminium, as creating complex shapes from aluminium sheets is difficult to achieve with aluminium’s low formability .
Aluminium has been used in the automotive industry since the 1920s, however, forming aluminium sheets has always been expensive and difficult. Also, its lower tensile strength means that aluminium sheets need to be of a greater thickness to meet safety standards. Thus, the use of aluminium can offer weight reduction in comparison to standard automobile steel and AHSS, yet at a greater price [5,6].
When addressing relative manufacturing costs, the selection of material becomes the crucial factor and alternative materials such as aluminium and fibre-reinforced composites are at a cost disadvantage compared to AHSS. Table 1 shows a detailed comparison.
Composite materials have a high purchase price, but viewed from an economic point of view, they often still make sense. They have significantly low weight, good strength and rigidity, good corrosion resistance and are very fuel-efficient . Despite these advantages, fibre composite materials will remain expensive due to their high manufacturing cost and so are as yet viable only for certain applications [3, 8].
Speed of production is certainly also an issue when compared with steel materials. For example, the cycle time for manufacturing composite parts is between 1 and 5 minutes, whereas stamping steel parts only takes 10 seconds . Extensive research on reducing the manufacturing time is underway and the composite industry continues to explore processes and manufacturing solutions to develop lower-cost materials that can meet automakers’ needs.
Automakers are today using alternative materials in order to reduce the mass of vehicles. Cost being the major driver for automakers, any reduction in weight must ideally also save on manufacturing costs.
Despite the advantages of many of these alternative lightweight materials, they are still too expensive to make it into mainstream usage. Therefore, in the next few years, the mechanical and weight advantages of thin AHSS sections compared to aluminium will continue to make it the material of choice over other alternative lightweight materials.
Designers must balance structural performance and life-cycle cost with the material costs. While AHSS proves to be a low-cost material, the resulting maximum weight reduction from replacing mild steel by AHSS grades is limited to 25 %. However, with the rapid adoption of new technology such as the upcoming 3rd generation of AHSS, weight savings are expected to improve even further.
 G. Davies, Materials for Automobile Bodies. Elsevier Science, 2012.
 J. Anderton, “CO2 emissions from new passenger cars in the EU: Car manufacturers’ performance in 2017,” July 2015, [Online], [Accessed Apr. 30, 2019].
 M. Y. Demeri, Advanced High-strength Steels: Science, Technology, and Applications. ASM International, 2013.
 I. Maw, “The battle of bodies: Steel vs. Aluminium in Automotive,” Feb, 5 2018, [Online], [Accessed Apr. 30, 2019].
 R. Rana and S. B. Singh, Automotive steels: design, metallurgy, processing and applications. Woodhead Publishing is an imprint of Elsevier, 2017.
 J. Anderton, “Is the New Aluminium the Death of Automotive Steel?,” Aug 18 2015, [Online], [Accessed Apr. 30, 2019].
 J. Anderton, “Aluminium Versus Steel: Ferrous Fights Back with AHSS,” Aug 24 2015, [Online], [Accessed Apr. 30, 2019].
 M. P. Todor, I. Kiss, “Systematic approach on materials selection in the automotive industry for making vehicles lighter, safer and more fuel-efficient,” Dec 30 2016, [Online], [Accessed Apr. 30, 2019].
 A. I. Taub. and A. A. Luo, “Advanced lightweight materials and manufacturing processes for automotive applications,” MRS Bulletin, vol. 40, pp. 1045-1054, 2015 [Accessed Apr. 30, 2019]
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