Which number indicates the primary alloying agent of 2024-T3?

The correct choice indicates that the primary alloying agent of 2024-T3 is copper, which is represented by the number 2 in the Aluminum Association designation system. In this system, the first digit of the alloy designation signifies the principal alloying element. For 2000 series aluminum alloys, the main alloying element is copper.

Understanding this classification is crucial as it helps define the mechanical properties, workability, and applications of the alloy. For instance, 2024-T3 is known for its high strength-to-weight ratio and fatigue resistance, making it suitable for aerospace applications. In contrast, the other numbers correspond to different principal alloying elements, such as magnesium, manganese, or silicon, which lead to distinct properties and uses not attributed to the 2024-T3 alloy. This classification under lines the importance of knowing alloying components when working with different aluminum types in structural applications.