MACHINABILITY OF TITANIUM

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Titanium alloys are widely used in the engineering field, because of their high specific strength and exceptional corrosion resistance. However, the machinability of titanium alloys is difficult due to their low thermal conductivity and elastic modulus, high hardness at elevated temperature, and high chemical reactivity.
Titanium is an expensive and challenging material to machine. Although not as popular in machine shops as steel or aluminum, due to the high material and manufacturing cost, it’s an important material to master. The machinability ranges from 15% to 45%.

What Is a Titanium Alloy?

Titanium is a chemical element with the symbol Ti. Titanium alloy is usually made from: about 88% of Ti with alloying elements, mostly vanadium (V) and aluminum (Al).

Titanium alloys have good features such as:

High specific strength

Good high temperature mechanical properties

It has an excellent strength-to-weight ratio.

Excellent corrosion resistance

On the one hand, it is almost as light as aluminum, and on the other hand, it has a higher strength than most steel alloys. On top of that, it has superb corrosion resistance. This combination makes it popular in aerospace components and transportation, power generation and chemical industries.

Titanium Machinability

Titanium alloys still suffer from a poor machinability compared to other metals owing to several inherent mechanical and material properties. The main reasons of relative low machinability of titanium alloys are due to their:

Low thermal conductivity → (This causes heat generating from the cut to transfer mainly to the cutting tool, instead of flowing to the chips and the workpiece).

High chemical reactivity

Low elasticity modulus.

Titanium alloys are characterized by a high cutting temperature, a short tool life and a high level of tool vibration.

Titanium has high flexibility, thus bending easily by the cutting forces.
Some of the popular titanium alloys have high hardness. For example, the popular Ti-6Al-4V alloy is 32 HRC, and other alloys can reach 40 HRC.

Pure Titanium alloys is the easiest to machine, they are made from 99.5%-99.8% of titanium with a small addition of Iron (Fe) and carbon (C) so they are not exactly pure. They are classified according to the titanium content as grade 1 through grade 4. (The most popular being grade 2).

Machinability of Pure Titanium

Material	Hardness	Machinability	SAE	DIN
Ti-99.5(Grade 1)	100 HRB	46%	Grade 1 B381F4	Ti-99.5
Ti-99.6(Grade 2)	90 HRB	40%	Grade 2 B381F3	Ti-99.6
Ti-99.7(Grade 3)	80 HRB	35%	Grade 3 B381F2	Ti-99.7
Ti-99.8(grade 4)	70 HRB	28%	Grade 4 B381F1	Ti-99.8

The Excellent mechanical properties: the combination between alloys that contain alloying elements up to 15% weight content, on top of that, most of the alloys are heat-treated to a hardness of 30-40 HRC. Also makes them much tougher to machine compared with pure titanium.