Aluminum Alloys for Body Panels

Automobile body panels consist of a double structure with an outer panel and an inner panel. For the outer panels, higher strength materials are especially required to provide sufficient denting resistance. For the inner panels, higher deep drawing capacity materials are especially required to allow the manufacture of more complex shapes. In other words, different properties are required for the outer and inner panels, as shown in Table 2.3.Research and development of aluminum body panels began in the 1970s. Aluminum alloys for body panels developed in different ways in Europe, North America, and Japan because of the different requirements of the automobile manufacturers. In Japan, higher formability alloys were required the automobile manufacturers. Therefore, special 5xxx series Al-Mg alloys, such as AA5022 and AA5023, were developed first. On the other hand, high strength alloys after paint baking were required in Europe and North America. Consequently, 2xxx series Al-Cu-Mg alloys, such as AA2036, and 6xxx series Al-Mg-Si-(Cu) alloys, such as AA6016, AA6111, and AA6022, were developed. The mechanism of paint bake-hardening of 6xxx seriesalloys is due to precipitation hardening of Mg2Si or a Cu-containing derivative. Figure 2.2 shows the transition of aluminum alloys for body panels

Increasing Aluminum Body Panel Usage

In order to promote the adoption of aluminum body panels, it is necessary to provide for the potential panel shapes and the low-cost materials required by automobile manufacturers. It is important to improve material properties as well as forming and joining technologies, so as to be able to manufacture suitable body panel shapes. On the other hand, it is necessary to minimize the number of manufacturing processes, and to be able to use recycled aluminum alloys to ensure a low-cost material.

Aluminum Alloys
The important properties required for body panels are as shown in Table 2.3. Especially, it is necessary to improve the formability to enable, for example, hem flanging and stretch-forming for outer panels, and deep drawing for inner panels. It is important to be able to decrease strength before forming, and then redevelop high strength after paint baking under conventional baking conditions.
Figure 2.3 shows a study of the bake-hardening properties of 6xxx alloys after pre-aging.  The specimens are solution heat treated at a high Temperature of 530°C and then water quenched, a conventional manufacturing process for aluminum body panels. Pre-aging is conducted at 50°C to 100°C immediately after water quenching. After one week at room temperature, the specimens are then heat treated using several different baking conditions