In the four-digit aluminum index system number 2024, the first digit indicates

• A. The major alloying elements
• B. The finish type
• C. The heat treatment code
• D. The base alloy group

Answer: (A)

The leading digit designates the major alloying element that defines the alloy family. In the four-digit aluminum index, alloys are grouped by the primary element added to aluminum, so the first digit quickly tells you which element is driving the alloy’s properties (for example, the 2xxx series is aluminum-copper, the 3xxx series is aluminum-manganese, the 6xxx series is aluminum-magnesium-silicon, and so on). The remaining digits then refine which specific alloy within that family it is, while temper designations (not shown in the four-digit code) indicate the heat treatment. This is why the first digit is the best clue for identifying the major alloying element.