Machinability of copper — select the additive that improves cutting performance Which alloying addition is commonly used to improve the machining characteristics (chip breaking and tool life) of wrought copper grades?

Introduction / Context:
Pure copper is tough and ductile but tends to produce continuous, stringy chips that make machining difficult. Free-machining copper grades intentionally add minor elements that promote chip breakage and reduce built-up edge, improving surface finish and tool life.

Given Data / Assumptions:

• Base metal is copper for general engineering use.
• Interest is in machinability, not primarily strength or corrosion resistance.
• Additions considered are in small percentages typical of free-machining alloys.

Concept / Approach:
Sulphur-bearing coppers (often called free-machining copper, e.g., UNS C14700) contain a controlled sulphur content that forms fine copper sulfide inclusions. These inclusions act as chip breakers and lubricants at the tool–chip interface. Lead can also be used in some copper alloys to improve machinability, but among the options provided, sulphur is the classic improvement for wrought copper without substantially harming conductivity at very low levels.

Step-by-Step Solution:

Verification / Alternative check:
Standards list free-machining copper compositions with S additions on the order of 0.02–0.1% producing improved machinability ratings compared with electrolytic tough pitch copper.

Why Other Options Are Wrong:

• Vanadium: microalloy in steels; not a common machinability aid in copper.
• Tin: forms bronzes; affects strength and corrosion, not free-machining behavior per se.
• Zinc: makes brass; machinability depends on phase structure, but the question targets copper itself.
• Lead: can help in some copper alloys, but the canonical answer for wrought free-machining copper grades is sulphur.

Common Pitfalls:
Confusing brass (Cu–Zn) machinability trends with those of pure copper; assuming conductivity constraints forbid any inclusions—small S additions are acceptable in designated grades.

Final Answer: