Car manufacturing is a highly competitive industry, and modern customers considering buying a car value cost-effectiveness of the car, comfort and connectivity, while car manufacturers continue to develop car safety, fuel economy and vehicle performance to increase competitiveness. At the same time, many international regulations and obligations are driving the automotive industry to become increasingly safer, more economical and more environmentally friendly. There is a constant effort to reduce greenhouse gases and emissions from transport, and here the economy of the cars has a significant role to play.
The bodywork of the car has a large impact on both safety and also to fuel consumption of the car due to weight. Al has been used in car body structures for a long time, but its use is expected to grow significantly in the next few years. Another growing phenomenon is the optimization body structure of the car, which combines several different manufacturing materials and connection methods, making the body structure more complex, but overall increasing its quality and performance in terms of durability and lightness. (Summe 2019, pp. 39-40; Vadirajav, Abraham & Bharadwaj 2019, pp. 89-90.)
The main construction material of the car has traditionally been steel. Indeed, steel has accounted for roughly about 60 % of weight of the car in North America on average until recently. In 2015, Al accounted for an average of 10.4 % of weight of the car in North America. Al was projected to account for 16 % of the average car weight in North America as early as 2028. Because Al is a lightweight material, its volume share will be clearly more than its weight share. The growing popularity of electric cars is one of the biggest reasons for the rise in the popularity of Al as a structural material for cars. The energy efficiency is a critical metric in the comparison of electric cars and lightening a car is one of the biggest factors affecting energy efficiency. It is estimated that saving 100 kg of weight in a car will reduce fuel consumption of the car by up to 6-7 % on average. (Summe 2019, pp. 40-50;
Vadirajav, Abraham & Bharadwaj 2019, p. 91.) 3.4.1 Structure of car body
Modern car body structures consist of several pieces of different materials joined together for optimal durability and lightness. The lightness of the body structure brings large advantages in terms of fuel economy and therefore, where possible, light Al is used in the body. The durability of the body provides safety in crash situations and therefore ultra-strong
steel grades are used at suitable points. The car body has traditionally been made of the same material throughout to simplify the manufacturing process, for example high-strength steel.
Today, this manufacturing technique is seen as old-fashioned, and the modern car body is increasingly optimized and consists of ever smaller, individually designed parts. Each part is designed to meet the requirements of that little detail, and a large number of different fabrication materials and manufacturing methods are available. Modern car bodies use several different Al alloys, boron steels and other steel grades. When designers are free to develop the car body as a whole according to small different parts, the end result is the most optimal solution in terms of weight and durability. In the automotive industry, the Al alloys used as the body structure are mainly belonging to 6xxx and 7xxx series alloys. The 6xxx series alloys are used for applications requiring high strength and impact energy absorption, as well as to enhance the optimization of the closed structure. The 7xxx series alloys are used for very strong structures. In addition, other Al alloys are used in the surface structures of the body to improve design, joint structures, finish and durability. All of these can be used in a variety of body structures and weight savings compared to steel are 50 to 60 %, depending on the target. An example of weight savings in the car body is the difference between the Ford F150 model year 2015 and the model year 2014. In the 2015 car model, the body is made of Al and steel parts where possible. The weight savings of just over 200 kg have been achieved from the car body alone. Figure 6 shows the body structure of the Audi A8 in different material grades marked with colour codes. (Sierra et al. 2007, pp. 197-198; Summe 2019, pp. 47-52; Vadirajav, Abraham & Bharadwaj 2019, p. 94-95.)
Figure 6. Picture of Audi A8 (2017) car body structure (Mod. Green Car Congress, 2017a).
As it can be seen from Figure 6, there are several different material qualities on display. Audi A8 is expensive price class car but it is a suitable example to illustrate the many different materials a modern car body contains and how versatile their use is. The side parts of the body, marked in purple and grey, are still made of steel and are designed to increase crash safety, but otherwise the body of the car in question is mainly made of the Al.
An essential part of a functioning optimized body structure is the interconnection of smaller subassemblies. Al parts and subassemblies can be joined using conventional resistance spot welding, conventional and remote laser welding, self-pierce riveting and flow drill screws.
In addition, many of these methods use adhesive bonding to support the joints. Suitable connection methods must be considered in the design from the outset in order to create production as smooth and disturbance-free as possible. The structure of the bodies of modern cars is increasingly a hybrid body, which combines several different materials. Essentially, this denotes joining Al and steel parts together. Al and steel can be joined using methods such as mechanical clinching, FSW (friction stir welding), adhesive bonding and riveting.
When joining Al and steel, the different properties of the materials must be considered in order to perform the joint as durable and even as possible to fuse. Studies have shown that
using spot-welding for joints between Al and steel has stress level at its maximum and the rivet joint method has the lowest stresses at the joints (Long, Lan & Chen 2008, p. 491). Due to the slightly lower strength of Al, the thickness of the Al sheets frequently must often be dimensioned to be thicker than the steel sheets of the same joints. Figure 7 shows the amount of Al used in the surfaces of the 2013 Audi A8. (Summe 2019, p. 51; Long, Lan & Chen 2008, pp. 483-491.)
Figure 7. Picture of Audi A8 (2013) car body structure with all surface parts (Mod. Green Car Congress, 2017b).
As it can be seen from Figure 7, materials marked in light green are Al sheets, and virtually all surfaces on that car are made of this material. The car in question belongs to rather expensive price class, but the image conveys the idea of Al use in body structures, this trend will carry over to cheaper price class cars in the future as well.
4 ALUMINIUM LASER WELDING
This chapter presents laser welding of Al alloys in more detail. Basic principles of laser welding of Al and equipment for laser welding of Al are introduced. Strengths and limitations of laser welding of Al is considered, including the removal of the oxide layer and the bonding of different materials. The purpose for using of laser welding of Al is discussed, especially in scope of automotive industry.