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Aluminum - a popular material in automobile construction

In the automotive industry, aluminum has undergone significant changes in the recent past. High-quality product innovations, the growing demand for aluminum parts and rapid future developments are worth taking a look at this versatile material.

Fig. 1. The globally positioned Constellium group of companies produces hard and soft-alloyed aluminum rolled products - here you can see the production in the factory in Neuf-Brisach / F. Image: Constellium

Political guidelines for reducing CO2Emissions and the growing demand for more fuel-efficient vehicles require automobile manufacturers to make considerable efforts, including when it comes to the use of materials. For example, steel is increasingly being replaced by significantly lighter components made of aluminum. Starting with premium brands, aluminum is increasingly conquering the segment of compact vehicles in the upper middle class. Car bodies in particular are made partially or entirely of aluminum. For many, however, the use of this material is only just beginning: Experts assume that the use of aluminum in automobile construction will double by 2020. For manufacturers of aluminum parts for the automotive industry, this means that capacity increases must go hand in hand with continuous research and the development of new products in close collaboration with customers.

Renowned manufacturer supplies all OEMs in the automotive industry

As one of the market-leading suppliers of semi-finished aluminum products - i.e. aluminum goods at the production stage between raw material and end product (such as extruded products or rolled products, as shown in Figure 1) - Constellium supplies almost all original equipment manufacturers (OEMs) in the German automotive industry with two different types: "Automotive Structures" are extrusion profiles as well as ready-to-install components and systems for automobile manufacturers. "Automotive Body Sheets" include aluminum rolled semi-finished products - strips and sheets that are further processed by the automobile manufacturers in their own pressing plants.

Since the market for aluminum rolled semi-finished products is growing steadily due to the increasing demand from the automotive industry, the aluminum industry is currently required to expand its own production volume. For this reason, Constellium has invested significantly in the expansion of its own capacities in recent years.

Specific requirements for modern vehicles

The growing proportion of aluminum vehicle parts for bodies and structures requires the suppliers' research and development teams to continuously optimize their product portfolio. With the extended use, the demands on the material also change. For example, aluminum sheets from which doors are made must be more malleable than other vehicle parts. At the same time, safety expectations are growing: modern, high-strength crash management systems (CMS) made of aluminum at the front and rear of the vehicle protect the body in the event of a collision and thus increase the safety of the driver and passengers. Thanks to improved energy absorption, all other road users are better protected in the event of a collision. Experts predict that the aluminum share in the CMS area will continue to increase over the next five years, especially in European premium vehicles. By 2018, aluminum CMS systems are expected to account for 30% of the EU (European Union) market and 20% of the US market. The entire CMS production for China, Europe and North America is expected to reach more than 28 million units by 2018.

A special focus of the automotive division of Constellium is on the construction of new types of aluminum sheets, especially for the vehicle outer skin. The main focus here is a high level of malleability, aesthetic requirements / design and the desire for lightweight construction and safety requirements.

High quality novel alloy

Constellium has developed the "Surfalex" series for the outer skin of modern vehicles. It meets the growing demands on the design of premium vehicles, Figure 2.

Fig. 2. Bonnet of a premium vehicle made with the high-quality “Surfalex” alloy. Image: Constellium

A decisive challenge that this product line encounters are the "character lines" - tapering lines on the outer skin for a distinctive look. This design trend calls for sharp lines, which in turn means increased demands on the alloys.

The surface qualities of aluminum parts have also become more demanding. There is a trend towards thinner paintwork - the tolerance for unevenness in the surface structure ("roping lines") is decreasing. The answer to this challenge lies in optimized hot / cold phases during the rolling process.

Surfalex is an aluminum-magnesium-silicon alloy that is used on a large scale by leading automobile manufacturers in Germany and around the world for components such as bonnets, fenders, doors, side panels and tailgates, Figure 3.

The alloy begins its development as a large ingot made using the ingot casting process with tight tolerances in the casting conditions in order to obtain constant and reproducible microstructures. The proportion of the main alloy elements silicon and magnesium was chosen with a view to achieving the best possible balance of properties. Variations of these property-determining elements are kept within much tighter tolerances than with conventional alloys. The proportion of trace elements and impurities such as iron, copper, manganese, chromium or titanium is well-defined and kept constant in order to prevent variations in properties between batches, while at the same time maintaining compatibility with the closed recycling cycle.

After casting, the bars are thermally homogenized in large continuous furnaces. This ensures that all alloying elements are evenly distributed in the metal. The ingot is then rolled into a hot strip in a tandem hot rolling mill and wound up. During hot rolling, the temperature and the pass schedule with regard to the degree of rolling and speed are precisely controlled in order to control microstructural phenomena such as work hardening, recrystallization, precipitation and structural development. The microstructure generated during hot rolling determines the final product properties such as formability, anisotropy, flexibility and surface properties. The hot strip is rolled to the final thickness in a cold rolling mill, with special attention paid to purity and a surface free from dents, marks and scratches.

Finally, the cold strip is subjected to a heat treatment with solution annealing and quenching in a continuous strip furnace. The metal strip is heated with hot gas in a hovering furnace to a temperature at which the magnesium and silicon are completely in solution. At the exit of the furnace, the strip is rapidly cooled to room temperature in order to obtain the alloying elements as mixed crystals in the supersaturated, single-phase state. In this way, the metal in the delivery condition (T4) is soft and malleable, while hardening in the "paint-bake process" (the baking of the paintwork) at the automobile manufacturer achieves a significant increase in strength.

Thanks to the optimized alloy composition and precisely controlled process parameters, Surfalex sheets offer consistent, industry-leading properties in terms of formability, folding behavior, strength increase during paint-bake, corrosion resistance and completely uniform surface quality after forming, free of surface defects such as "orange peel" or "roping". The unique thing about Surfalex is the combination of all these properties in one product, whereas previous generations of products were based on the compromise of improving one property at the expense of another.

view in the future

Constellium continuously invests in research and development as well as the expansion of production capacities - between 2014 and 2016 alone, around 200 million euros flowed into increasing automotive sheet production capacities in Europe by 140,000 t. During the same period, the supplier company and UACJ Corporation, one of the leading aluminum producers in Japan, invested in production plants in Bowling Green, Kentucky / USA. Further investments in the Muscle Shoals plant - after the acquisition of Wise Metals - are planned for the automotive market. In addition, Constellium is continuously researching even more high-strength, more malleable and lighter aluminum solutions.

Assuming that aluminum manufacturers will expand their rolling and heat treatment capacities over the next few years in line with the increasing demand from automobile manufacturers, aluminum as a material can open up many other areas of application and, for example, replace steel with lighter solutions that can be recycled without any loss of quality. The potential of aluminum as a material in automobile construction and many other areas is far from being exhausted.

Aluminum for the world

Constellium is a global aluminum industry leader developing innovative, value-added aluminum products for the aerospace, packaging and automotive industries. In addition to the locations in Neuf-Brisach in France and Gottmadingen in the immediate vicinity, the plant in Singen in Baden-Württemberg (1,600 employees) is an important location in the international production network for automotive applications. Singen's production areas include both rolled products and pressed products. Constellium aluminum products enjoy an excellent global reputation thanks to constant further development and willingness to innovate.

Dieter Höll is Director Sales and Marketing, Automotive & Specialties at Constellium in Singen.