Duralumin (Al–Cu Alloy) — Identifying the Incorrect Statement Which of the following statements about duralumin is incorrect?

Introduction / Context:
Duralumin refers to a family of age-hardenable aluminium–copper alloys (typically with Mg and Mn), long used in aerospace for their excellent strength-to-weight ratio and good fabrication characteristics. Knowing their key characteristics helps in materials selection and design.

Given Data / Assumptions:

• Duralumin contains significant Cu (and minor Mg, Mn), enabling precipitation hardening.
• We compare qualitative statements about properties and processing.
• Density comparison is made against pure aluminium.

Concept / Approach:
Age hardening (natural or artificial) is a hallmark of Al–Cu alloys: solution treat, quench, and age to form finely dispersed precipitates, increasing strength. Duralumin can be forged and generally machines well. However, adding copper and other alloying elements slightly increases density compared to pure aluminium; therefore, it is not “lighter” than pure aluminium. The strength benefits come from microstructural changes, not reduced density.

Step-by-Step Solution:
Identify true attributes: age hardening capability → true.Forming routes: forging and machining are common → true.Density logic: alloying additions raise density relative to pure Al → the statement “lighter than pure aluminium” is incorrect.

Verification / Alternative check:
Typical densities: pure Al ≈ 2.70 g/cm^3; Al–Cu alloys slightly higher due to heavier alloying additions, confirming the incorrectness of option (d).

Why Other Options Are Wrong (as the “incorrect” choice):
Options (a), (b), and (c) are established characteristics of duralumin.

Common Pitfalls:
Equating “high strength-to-weight” with “lower density.” Duralumin’s advantage is higher strength at about the same or slightly higher density, not lower mass density.

Final Answer:
It is lighter than pure aluminium