Free-machining aluminium — which addition most increases machinability? The machinability (chip breaking, tool life, surface finish) of aluminium is most improved by adding which of the following?

Introduction / Context:
While aluminium machines easily, some grades produce long, continuous chips and built-up edge, hurting productivity. “Free-machining” compositions introduce inclusions or low-melting phases that act as chip breakers and lubricants at the tool interface.

Given Data / Assumptions:

• Objective: enhance machinability of aluminium, not necessarily its strength or corrosion resistance.
• Commercial practice includes adding small amounts of Pb and Bi to certain wrought or cast aluminium grades.

Concept / Approach:
Lead and bismuth additions in small percentages segregate to chip interfaces, reducing friction and promoting chip segmentation. This yields improved surface finish and tool life. In contrast, copper, magnesium, and silicon mainly influence strength, hardenability, and castability. They do not target machinability the way Pb/Bi do, and can even make chips more ductile and continuous if used without Pb/Bi modifiers.

Step-by-Step Solution:

Verification / Alternative check:
Standards for free-machining aluminium (e.g., 2011, 6262) specify Pb and Bi; machinability ratings for these grades are markedly higher than unmodified 6061 or 1100.

Why Other Options Are Wrong:

• Copper/magnesium/silicon: primarily alter strength, precipitation response, or casting fluidity; not the main machinability enhancers.
• Nickel: uncommon in aluminium for machinability purposes.

Common Pitfalls:
Confusing castability (improved by Si) with machinability; overlooking environmental/health considerations of Pb (some regions restrict leaded alloys).

Final Answer: