Why do we think titanium alloy is a difficult material to machine? Because there is a lack of deep understanding of its machining mechanisms and phenomena.
- Physical phenomena of titanium processing
The cutting force when machining titanium alloys is only slightly higher than that of steel of the same hardness, but the physics of machining titanium alloys is much more complex than that of machining steel, thus making titanium alloys machining a huge difficulty.
The thermal conductivity of most titanium alloys is very low, only 1/7 of steel and 1/16 of aluminium, therefore, the heat generated in the process of cutting titanium alloys will not be quickly transferred to the workpiece or taken away by the chips, but gathered in the cutting area, the resulting temperature can be as high as 1,000 ℃ or more, so that the cutting edge of the tool is quickly worn out, chipping and generating chip tumors, the rapid emergence of a worn edge, and the cutting area generates more heat. The rapid wear of the cutting edge and the generation of more heat in the cutting area further shorten the life of the tool.
The high temperatures generated during the cutting process simultaneously destroy the surface integrity of the titanium alloy part, leading to a reduction in the geometric accuracy of the part and the development of work-hardening, which severely reduces its fatigue strength.
The elasticity of titanium alloys may be beneficial to part performance, but during the cutting process the elastic deformation of the workpiece is an important cause of vibration. Cutting pressure causes the ‘elastic’ workpiece to move away from the tool and bounce back, resulting in more friction between the tool and the workpiece than cutting. The friction process also generates heat, adding to the problem of poor thermal conductivity of titanium alloys.
This problem is even more serious when machining deformable parts such as thin-walled or ring-shaped, and machining titanium alloy thin-walled parts to the desired dimensional accuracy is not an easy task. This is because as the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range and plastic deformation occurs, and the strength and hardness of the material at the cutting point increases significantly. At this point, machining at the originally determined cutting speed becomes too high, further leading to sharp tool wear.
‘Heat’ is the “main culprit” for the difficult processing of titanium alloys!
2.Process know-how for machining titanium alloys
Based on the understanding of the machining mechanism of titanium alloys and the past experience, the main process know-how for machining titanium alloys is as follows:
- Use inserts with positive angular geometry to reduce cutting forces, cutting heat and workpiece deformation.
- Maintain a constant feed to avoid hardening of the workpiece, the tool should always be fed during the cutting process, and the radial draft ae should be 30% of the radius when milling.
- High-pressure, high-flow cutting fluid is used to ensure the thermal stability of the machining process and to prevent surface denaturation of the workpiece and damage to the cutting tool due to high temperatures.
- Keep blade edges sharp; dull tools are the cause of heat build-up and wear, which can easily lead to tool failure.
- Machining of titanium alloys in the softest state possible, as the material becomes more difficult to machine after hardening, and heat treatment increases the strength of the material and increases wear on the inserts.
- Use a large tip radius or chamfer cut to put as much of the blade into the cut as possible. This reduces the cutting forces and heat at each point and prevents local breakage. When milling titanium alloys, cutting speed has the greatest effect on tool life vc of all cutting parameters, with radial draft (depth of milling) ae second.
3.Solving titanium machining problems from the blade perspective
Insert groove wear that occurs during titanium machining is localised wear at the back and front in the direction of the depth of cut, which is often caused by the hardened layer left by the previous machining. Chemical reaction and diffusion between the tool and the workpiece material at the machining temperature of more than 800 ℃ is also one of the reasons for the formation of groove wear. Because in the machining process, the titanium molecules of the workpiece in front of the insert accumulation, in the high pressure and high temperature ‘welding’ to the cutting edge, the formation of chip tumour. When the chip-accumulation tumour peels away from the cutting edge, it carries away the carbide coating of the insert, so titanium machining requires special insert materials and geometries.
4.Tool construction suitable for titanium machining
The focal point of titanium machining is heat, and large quantities of high-pressure cutting fluid have to be sprayed onto the cutting edge in a timely and accurate manner in order to remove the heat quickly. There are unique configurations of milling cutters on the market specifically for titanium machining.
