In the 4-digit aluminum index system, what does the first digit indicate? (restate concept)

• A. the major alloying element
• B. the fineness of grain
• C. the surface coating
• D. the heat treatment state

Answer: (A)

The first digit in the four-digit aluminum index system identifies the major alloying element, placing the alloy into its family based on the dominant element and largely shaping its properties. That’s why this option is the best: it directly describes the role of the first digit. The remaining digits specify the exact alloy and its variations, while the heat treatment state is shown separately as a temper designation after the code, so the first digit does not indicate grain fineness, surface coating, or the heat treatment state.