1.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, which makes titanium alloys machining face great difficulties.
Most of the thermal conductivity of titanium alloys is very low, only 1/7 of steel, aluminum 1/16. Therefore, the heat generated in the process of cutting titanium alloys will not be quickly transferred to the workpiece or by the chip away, 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, crumbling and generating chip tumors, the rapid emergence of the edge of the blade wear, and the cutting area to generate more The rapid wear of the cutting edge, and the cutting area produces more heat, further shortening the life of the tool.
The high temperatures generated during the cutting process also destroy the surface integrity of the titanium alloy part, leading to a reduction in the geometric accuracy of the part and the emergence of work-hardening phenomena that severely reduce its fatigue strength.
The elasticity of titanium alloys may be beneficial to part performance, but the elastic deformation of the workpiece during the cutting process is a significant 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 workpiece than cutting. The friction process also generates heat, exacerbating the problem of poor thermal conductivity of titanium alloys.
This problem is exacerbated when machining deformable parts such as thin-walled or toroidal shapes. Machining titanium alloy thin-walled parts to the desired dimensional accuracy is not an easy task. Because as the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range and produce plastic deformation, the cutting point of the material strength and hardness increased significantly. At this point, machining at the previously determined cutting speed becomes too high, further leading to sharp tool wear.
“Heat” is the “main culprit” of titanium alloy difficult to machine!
2.Process know-how for machining titanium alloys
Based on the understanding of the machining mechanism of titanium alloy and the past experience, the main process know-how for machining titanium alloy is as follows:
(1) Use inserts with positive angular geometry to minimize cutting forces, cutting heat and deformation of the workpiece.
(2) Maintain a constant feed to avoid hardening of the workpiece. The tool should always be in feed during the cutting process and the radial draft ae should be 30% of the radius when milling.
(3) Use high-pressure high-flow cutting fluid 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 excessively high temperature.
(4) Keep insert edges sharp. Dull tools are the cause of heat buildup and wear, which can easily lead to tool failure.
(5) Machine titanium alloys in their 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 insert.
(6) Use a large tip radius or chamfer cut to put as much of the cutting edge into the cut as possible. This reduces cutting forces and heat at each point and prevents localized breakage. When milling titanium alloys, cutting speed has the greatest effect on tool life vc among the various cutting parameters, with radial draft (depth of milling) ae second.
3.Solving titanium machining problems from the blade point of view
Insert groove wear that occurs during titanium machining is localized wear at the back and front in the direction of the depth of cut, and it is often caused by the hardened layer left by the previous machining. Chemical reaction and diffusion between the tool and the workpiece material at a 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 the front of the blade accumulation, in the high pressure and high temperature “welding” to the cutting edge, the formation of chip tumors. When the chip-accumulator 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.
