Why is titanium alloy a difficult material to machine?

1 The "culprit" behind titanium alloy's difficulty to work is heat.

Titanium alloy machining cutting force may only slightly exceed that of steel at similar hardness levels; however, due to the complex physical phenomena involved with processing titanium alloy versus processing steel, titanium alloy machining faces many additional obstacles and difficulties.

Most titanium alloys have low thermal conductivities - only 1/7 that of steel and 1/16 that of aluminum - making the heat generated during cutting not quickly transferred to workpiece or carried away by chips, but instead concentrated in cutting area with temperatures reaching 1000 or higher resulting in tool quickly wearing, chipping, chip tumor formation and further heat generated in cutting area shortening its lifespan.

High temperatures generated during the cutting process also compromise the surface integrity of titanium alloy parts, leading to decreased geometric accuracy and work hardening that severely decreases their fatigue strength.

Titanium alloys' elastic properties may help improve part performance, yet elastic deformation of workpieces during cutting is an important source of vibration. Cutting pressure causes elastic deformation that results in more frictional force between tool and workpiece than cutting action alone - further exacerbating poor thermal conductivity of titanium alloys.

Machining thin-walled or ring-shaped parts that are easily deformed presents unique challenges when trying to meet desired dimensional accuracy; titanium thin-walled parts present particularly difficult challenges because as soon as they're pushed away from the tool, local deformation exceeds elastic range and plastic deformation occurs, leading to material strength and hardness at cutting point increasing substantially; at which point, machining at previously determined cutting speeds becomes too high, leading to drastic tool wear.

 

Process know-how for machining titanium alloys

Based on an understanding of titanium alloy machining mechanisms and prior experience, here is the core process know-how for cutting titanium alloy:

(1) Employ inserts with positive angular geometry to reduce cutting forces, cutting heat and deformation of the workpiece.

(2) Maintain a consistent feed rate to avoid hardening of the workpiece, with cutting tool in feed state at all times and 30% radial draft for milling operations.

(3) Apply high pressure and high flow cutting fluid in order to achieve thermal stability during machining operations and protect the workpiece surface from denaturing due to high temperature machining conditions as well as protect tools against sudden temperature spikes.

(4) Maintain a sharp blade edge to avoid heat build-up and wear, which could ultimately result in tool failure.

(5) To achieve optimal results when processing titanium alloys, machine them at their softest state possible; hardened material becomes increasingly challenging to machine while heat treatments increase strength while increasing insert wear.

(6）Utilizing a large tip radius or chamfered plunge to incorporate as much of the cutting edge into each cut will reduce cutting force and heat at each point, helping prevent local breakages. When milling titanium alloy, cutting speed has the greatest influence on tool life while radial draft (milling depth) comes second.

03 Make use of titanium processing inserts to resolve titanium processing issues

Titanium alloy machining insert groove wear refers to back and front in the direction of cutting depth local wear, usually as a result of processing hardening layers left from previous operations. Chemical reactions between tools and workpiece materials at temperatures exceeding 800 deg C may also contribute to its formation, contributing further to groove wear formation.

As part of its titanium machining, titanium molecules from the workpiece accumulate in front of an insert and "weld" to its cutting edge with high pressure and temperature, creating what is known as a chip tumor. When this tumor peels away from its cutting edge, it carries with it all of the carbide coating from an insert - thus necessitating special insert materials and geometries for its processing.

04 Tool Structure Appropriate for Titanium Machining

Heat is at the core of titanium machining, so large volumes of high-pressure cutting fluid must be used to cool the cutting edge in an accurate and timely fashion. There are milling cutters designed specifically for titanium machining available on the market that have unique configurations specifically tailored towards this task.