Why Is Titanium Alloy Machining Difficult, and Which Tools Work?

Titanium is allotropic. Below 882°C it has a close-packed hexagonal alpha structure; above 882°C it has a body-centered cubic beta structure. Alloying changes the transformation temperature and phase balance, producing alpha, beta, and alpha-beta titanium alloys.

Properties That Influence Machining

Why Titanium Is Difficult to Cut

• The cutting deformation coefficient is close to or below 1, increasing chip sliding along the rake face and accelerating wear.
• Cutting temperature is high because heat is concentrated near a short tool–chip contact zone. Under comparable conditions, Ti-6Al-4V can run at more than twice the cutting temperature of 45-grade carbon steel.
• Although the main cutting force may be about 20% lower than for structural steel, contact pressure can reach 130–150% of that for 45-grade steel, concentrating stress at the edge.
• Gas pickup and reaction products create a hard, brittle, nonuniform surface layer that damages fatigue strength and promotes notching, chipping, and flaking.
• High affinity, temperature, and pressure cause severe adhesion and built-up material on the tool.
• Low stiffness permits workpiece deflection and springback, reducing the effective clearance angle and harming accuracy.

Choosing Tool Materials

Common choices are cobalt-, aluminum-, or vanadium-bearing high-speed steel; tungsten-cobalt cemented carbide, sometimes with small additions of other carbides; and selected PVD/CVD-coated carbide. Tungsten-titanium carbide grades and many coated inserts are generally unsuitable for this application.

Diamond and polycrystalline cubic boron nitride tools can perform well in finishing and ultra-precision work. Diamond offers hardness up to about 10,000 HV and excellent heat conduction but should be kept below roughly 700–800°C with adequate cooling and lubrication. PCBN has lower hardness but thermal stability above 1,200°C and high chemical inertness toward titanium, making it suitable for high-temperature dry cutting.