Machinability and general principles of titanium alloys
Titanium alloy is divided into a phase, b phase and a+b phase according to the metal structure. TA, T and TC represent their brand and type respectively.Our company USES two materials for a new engine: T and TC.Generally, TA series is used for casting and forging, TC series is used for bar materials.
Features and machinability
Titanium alloy has the following advantages over general alloy steel:
Specific strength: titanium alloy density is only 4.5g/cm3, much smaller than iron, and its strength is similar to ordinary carbon steel.Good mechanical properties: titanium alloy melting point of 1660℃, higher than iron, has a higher thermal strength, can work under 550℃, at the same time at a low temperature usually shows good toughness.
Good corrosion resistance: under 550℃ titanium alloy surface is easy to form a dense oxidation film, so it is not easy to be further oxidation, the atmosphere, seawater, steam and some acid, alkali, salt medium have higher corrosion resistance.
On the other hand, the machinability of titanium alloy is poor.The main reasons are as follows:
Poor thermal conductivity, resulting in high cutting temperature, reducing tool durability.When the temperature is above 600℃, the oxidized hard layer is formed on the surface, which has a strong wear effect on the knife.Low plasticity, high hardness, make the shear Angle increase, chip and front knife surface contact length is very small, front knife surface stress is very large, the blade is prone to damage.
The elastic modulus is low, the elastic deformation is large, and the workpiece surface near the back cutting surface has a large amount of rebound, so the contact area between the processed surface and the back cutting surface is large, and the wear is serious.
These characteristics make the machining of titanium alloy very difficult, resulting in low machining efficiency and large tool consumption.
General principles of machining
According to the properties of titanium alloy and the characteristics of the cutting process, processing should consider the following aspects:
Use hard alloy cutter as far as possible, if tungsten cobalt kind hard alloy and titanium alloy chemical affinity is small, heat conduction is good, intensity is high also.Ultra-fine grain carbide with impact resistance can be used for intermittent cutting at low speed. High speed steel with good high temperature performance can be used for forming and complex cutting tools.
A smaller front Angle and a larger back Angle are used to increase the contact length between the chip and the front cutting surface, reduce the friction between the workpiece and the back cutting surface, and an arc transition edge is used for the tip to improve the strength and avoid the burning of the sharp corner and the blade.Keep the blade sharp to ensure smooth chip discharge and avoid chip breakage.
Cutting speed should be low to avoid high cutting temperature;Moderate feed, too large easy to burn the knife, too small because the blade in the work hardening layer and wear too fast;The cutting depth can be larger, making the tip work below the hardened layer, which is beneficial to improve the durability of the tool.Cooling fluid must be added during processing to allow sufficient cooling.
When cutting titanium alloy, the tool resistance is large, so the process system must ensure that there is enough rigidity.Because titanium alloy is easy to deformation, so the cutting clamping force can not be large, especially in some finishing processes, when necessary can use a certain auxiliary support.
These are the general principles to be considered in titanium alloy processing, in fact, with different processing methods and under different conditions there are different contradictions prominent points and focus to solve the problem.