Improving castability of aluminium — effect of alloying The casting ability (fluidity and soundness) of aluminium improves most significantly when which element is added?

Introduction / Context:
Aluminium casting alloys must fill thin sections and complex moulds while resisting hot tearing and excessive shrinkage. Alloying additions tailor fluidity, freezing range, and feeding characteristics to enhance castability.

Given Data / Assumptions:

• Base metal is aluminium intended for sand, die, or permanent-mould casting.
• Goal is to increase fluidity and overall casting ability.

Concept / Approach:
Silicon dramatically increases the fluidity of aluminium and lowers its melting range, leading to superior mould filling and reduced hot tearing. Al–Si casting alloys (eutectic ~12–13% Si) such as A356 (Al–Si–Mg) are industry standards because they combine excellent castability with good mechanical properties after heat treatment.

Step-by-Step Solution:
Identify the alloying element most associated with castability → silicon.Recall that Al–Si eutectic composition provides high fluidity and good feeding.Select silicon as the correct choice.

Verification / Alternative check:
Foundry references show Al–Si alloys dominating casting applications, with documented improvements in fluidity and reduced hot cracking.

Why Other Options Are Wrong:

• Copper: improves strength, reduces corrosion resistance, and does not primarily drive castability.
• Magnesium: used for precipitation hardening (with Si) rather than fluidity alone.
• Lead and bismuth: improve machinability in some alloys, not castability for aluminium.
• Nickel: enhances high-temperature strength in some systems, marginal effect on castability.

Common Pitfalls:
Confusing strength-improving additions (Cu, Mg) with those that predominantly enhance fluidity and casting behaviour (Si).

Final Answer:
silicon