Aluminium casting metallurgy — effect of silicon on fluidity and castability Assess the statement: “Aluminium has low density, and adding silicon improves its melt fluidity and, therefore, its castability.”

Introduction / Context:
Cast aluminium alloys dominate automotive and consumer components because they fill thin sections readily and have good as-cast properties. Understanding how alloying elements modify fluidity and freezing range is central to alloy selection for sand, die, and investment casting.

Given Data / Assumptions:

• Base metal: aluminium (low density ~2.7 g/cm^3).
• Alloying addition: silicon in typical casting ranges (e.g., 5–13%).
• Casting processes where fluidity and feeding are important.

Concept / Approach:
Silicon greatly improves castability of aluminium by lowering the melting temperature and narrowing the solidification range near the Al–Si eutectic (approx. 12.6% Si). Eutectic or near-eutectic Al–Si melts are highly fluid, feed thin sections well, and show low hot tearing tendencies. These alloys (e.g., LM6/356/AlSi10Mg with modifiers) are workhorses of casting. The statement correctly links the physical property (low density) with the practical effect of silicon additions on fluidity and casting performance.

Step-by-Step Solution:

Verification / Alternative check:
Foundry manuals list Al–Si as “hypoeutectic/eutectic” casting alloys with superior fluidity compared to Al–Cu or Al–Mg systems; die casting grades are largely Al–Si based.

Why Other Options Are Wrong:

• “Incorrect” or temperature/wrought caveats: contradicted by standard casting practice.
• “Si reduces fluidity”: opposite to well-established behavior.

Common Pitfalls:
Confusing castability with weldability; assuming higher Si always better—excess Si can embrittle unless modified and heat treated appropriately.

Final Answer: