Titanium alloy, mainly used to make aircraft engine compressor parts, followed by structural parts of rockets, missiles and high-speed aircraft. The density of titanium alloy is generally about 4.51g / cubic centimeter, only 60% of steel, the density of pure titanium Close to the density of ordinary steel, some high-strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength/density) of titanium alloys is much larger than other metal structural materials, which can produce high unit strength, good rigidity and light weight. 1. The engine components, skeleton, skin, fasteners and landing gear of the aircraft are made of titanium.
In order to process titanium alloys, it is necessary to have a thorough understanding of its processing mechanism and phenomena. Many processors believe that titanium alloy is a material that is extremely difficult to process, because it is not enough to understand it. Today, it is a small series for everyone to analyze. Analysis of the processing mechanism and phenomenon of titanium alloy.
The first thing to talk about is the physical phenomenon of titanium alloy processing. Although the cutting force of titanium alloy is only slightly higher than that of the same hardness, the physical phenomenon of processing titanium alloy is much more complicated than machining steel, which makes titanium alloy The difficulty of processing rises linearly.
Most titanium alloys have very low thermal conductivity, only 1/7 of steel and 1/16 of aluminum. Therefore, the heat generated during the process of cutting titanium alloy is not quickly transferred to the workpiece or taken away by the chips, but is concentrated. In the cutting area, the temperature can be generated up to 1 000 °C, which causes the cutting edge of the tool to wear rapidly, crack and form built-up edge, quickly wear the cutting edge, and generate more heat in the cutting area, further shortening the tool. life.
The high temperature generated during the cutting process also destroys the surface integrity of the titanium alloy parts, resulting in the geometric accuracy of the parts and the work hardening phenomenon which seriously reduces the fatigue strength.
The elasticity of titanium alloys may be beneficial to the performance of the part, but during the cutting process, the elastic deformation of the workpiece is an important cause of vibration. The cutting pressure causes the 'elastic' workpiece to leave the tool and bounce, thus making the tool and the workpiece The inter-friction phenomenon is greater than the cutting effect. The friction process also generates heat, which aggravates the problem of poor thermal conductivity of titanium alloy.
This problem is even more serious when machining thin-walled or toroidally deformable parts. It is not an easy task to machine titanium alloy thin-walled parts to the expected dimensional accuracy. Because the workpiece material is pushed by the tool. When opened, the local deformation of the thin wall has exceeded the elastic range and plastic deformation occurs. The material strength and hardness of the cutting point increase significantly. At this time, the machining speed is too high according to the originally determined cutting speed, which further causes the tool to wear sharply. It is said that 'hot' is the 'root of disease' that makes titanium alloy processing difficult.
As a leader in the tool industry, Sandvik Coromant has carefully crafted a process for processing titanium alloys, which is shared with the whole industry. Sandvik Coromant said that in understanding the processing mechanism of titanium alloys, plus the past The experience, the main process of processing titanium alloys is as follows:
(1) Blades with positive-angle geometry to reduce 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 the feed state during the cutting process. The radial ae amount during milling should be 30% of the radius.
(3) High-pressure and high-flow cutting fluid is used to ensure the thermal stability of the process and prevent the surface of the workpiece from being denatured and the tool from being damaged due to excessive temperature.
(4) Keep the blade edge sharp, the blunt tool is the cause of heat build-up and wear, which can easily lead to tool failure.
(5) As far as possible in the softest state of titanium alloy, because the material becomes harder to process after hardening, heat treatment increases the strength of the material and increases the wear of the blade.
(6) Use a large tool nose radius or chamfer to cut in, and cut more cutting edges into the cutting as much as possible. This can reduce the cutting force and heat at each point and prevent local damage. When milling titanium alloy, each cutting parameter The cutting speed has the greatest influence on the tool life vc, and the radial cutting amount (milling depth) ae is second.
When the hardness of titanium alloy is higher than HB350, the cutting process is particularly difficult. When it is less than HB300, the sticking phenomenon is easy to occur and it is difficult to cut. Therefore, the problem of titanium processing can be solved from the blade. The blade groove wear which occurs during the processing of titanium alloy is the back and front. In the local wear along the depth of cut, it is often caused by the hardened layer left by the previous processing. The chemical reaction and diffusion of the tool and the workpiece material at a processing temperature of over 800 °C is also one of the causes of groove wear. Because during the processing, the titanium molecules of the workpiece gather in the front area of the blade, and 'weld' to the blade at high pressure and high temperature to form a built-up edge. When the built-up edge is peeled off from the blade, the cemented carbide of the blade is coated. The layers are taken away, therefore, titanium alloy processing requires special blade materials and geometries.
It is worth mentioning that because titanium alloys produce high heat during processing, a large number of high-pressure cutting fluids must be sprayed onto the cutting edge in time to accurately remove heat. Today, there are also special titanium used on the market. The unique structure of the alloy machining cutter can be better applied to titanium alloy processing.
At present, all countries are developing new low-cost and high-performance titanium alloys, and strive to make titanium alloys enter the civil industry with huge market potential. China has spared no effort to move forward in this field. I believe that through the joint efforts of all industry players, In the future, the processing of titanium alloys will no longer be a problem. Instead, it will become a sharp blade for the development of China's manufacturing industry, and it will be a breakthrough for the development of the entire industry.