Machinability Technology of Titanium Alloy Material

Titanium alloy is an ideal manufacturing material for aircraft and engines because of its high specific strength, good mechanical properties and corrosion resistance, however, due to its poor machinability, its application has been restricted to a great extent for a long time. In recent year, with the development of processing technology, titanium alloy has been widely used in the compressor section of an aircraft engine, engine cover, exhaust device and other machined parts, as well as the manufacture of structural frames such as the girder frame.

Machinability and general principles of titanium alloy material

According to the metal structure, titanium alloy is divided into α phase, β phase, α+β phase, and represent with TA, TB, TC respectively. General casting, forging using TA series, rod material with TC series.

Characteristics and machinability of titanium alloy material

Titanium alloy has the following advantages over ordinary alloy steel materials：

1. High strength-to-weight ratio. Titanium alloy has a density of only 4.5g/cm3, much smaller than iron, and its strength is similar to that of ordinary carbon steel.

2. Good mechanical property. With a melting point of 1660℃, higher than iron, and have higher thermal strength. They can operate below 550℃, while generally showing better toughness at low temperatures, too.

3. Good corrosion resistance. It is easy to form dense oxide film on titanium alloy surface below 550℃, not easily oxidized further, so it has high corrosion resistance to atmosphere, seawater, steam and some acid, alkali and salt media.

The machinability of titanium alloy is relatively poor, the main reasons are:

• Poor thermal conductivity. Because of the high cutting temperature, the tool durability is reduced. When the temperature is above 600℃, oxidized hard layer is formed on the surface, which has strong abrasion effect on the knife.
• Low plasticity, high hardness. The shear angle increases, the contact length between the chip and the front cutter face is small, the stress of the front cutter face is great, and the blade is easy to be damaged.
• The elastic modulus is low and the elastic deformation is large. The workpiece surface near the back cutter face has a large amount of rebound, so the machined surface has a large contact area with the back cutter face, cause serious abrasion.

These characteristics make the machining of titanium alloy very difficult, resulting in low machining efficiency and large tool consumption.

General principles of machining

• As far as possible, choose hard alloy tools, such as tungsten cobalt hard alloy. Small chemical affinity with titanium alloy, good thermal conductivity, high strength.
• Small front Angle and large back Angle are used to increase the contact length between the chip and the front tool face, and reduce the friction between the workpiece and the back tool face. The arc transition edge is used to improve the strength of the tool tip, so as to avoid the burning and edge collapse of the sharp Angle.
• Keep the blade sharp to ensure smooth chip removal and avoid sticking chips.
• The cutting speed should be low to avoid excessive cutting temperature.
• The cutting depth can be large, so that the tip of the tool works below the hardened layer, help to improve tool durability.