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

Recently, similar 6xxx series alloys have been used in Europe, North America, and Japan. Table 2.4 shows the chemical compositions of aluminum alloys for body panels. AA6016 contains less than 0.2% Cu content, and is used in Europe. AA6111 contains higher Cu content than AA6022. Both alloys are used in North America. Alloys similar to low Cu content AA6016 and AA6022, and high Cu content AA6111 are also used in Japan. KS6K21 and KS6K31 are alloy codes of Kobe Steel, which correspond to AA6016, AA6022, and AA6111 respectively. AA5022 and AA5023 are special Al-Mg alloys produced by using high purity primary aluminum. They contain optimum Cu content, and have high formability and medium strength after paint baking. KS5J30 and KS5J32 are corresponding Kobe Steel alloys. These alloys are still in use for body panels of severe complex shapes in Japan. For inner panels, the conventional 5xxx series AA5182 alloy has been used recently in Europe and Japan. Table 2.5 shows typical mechanical properties of aluminum alloys for body panels produced by Kobe Steel. KS6K21-1 has high strength, with a yield

strength of 200 MPa after paint baking, and is in use for many body panels in Japan. However, the formability of KS6K21-1 is inferior to that of KS5J32. On the other hand, KS5J32 has higher elongation than KS6K21-1.