Titanium alloy processing is difficult? That is the principle that you didn't understand it~

- Oct 23, 2018-

Titanium alloy processing is difficult? That is the principle that you didn't understand it~

Titanium alloys are mainly used to make aircraft engine compressor components, followed by structural parts for rockets, missiles and high-speed aircraft. The density of titanium alloy is generally about 4.51g / cubic centimeter, which is only 60% of steel. The density of pure titanium is 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 the titanium alloy is much larger than that of other metal structural materials, and parts with high unit strength, good rigidity, and light weight can be produced. Titanium alloys are used for aircraft engine components, skeletons, skins, fasteners and landing gear.


In order to process titanium alloys, it is necessary to understand its processing mechanism and phenomena. Many processors believe that titanium alloy is a very difficult material to process because it is not well understood. Today, I will be a small editor for everyone to analyze and analyze the processing mechanism and phenomena 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 steel of the same hardness, the physical phenomenon of processing titanium alloy is much more complicated than that of processing steel, which makes the difficulty of processing titanium alloy straight up.


Most titanium alloys have very low thermal conductivity, only 1/7 of steel and 1/16 of aluminum. Therefore, the heat generated during the cutting of the titanium alloy is not quickly transferred to the workpiece or carried away by the chips, but accumulates in the cutting area, and the temperature generated can be as high as 1 000 ° C or more, causing the cutting edge of the tool to rapidly wear, crack and The formation of built-up edges, the rapid emergence of worn edges, and the production of more heat in the cutting area, further shortening the life of the tool.


As a leader in the tool industry, Sandvik Coromant has carefully crafted a process for processing titanium alloys, which is shared with the entire industry. Sandvik Coromant said that based on the understanding of the processing mechanism of titanium alloys and the previous experience, the main processes for processing titanium alloys are 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 amount of radial tool ae should be 30% of the radius during milling.


(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, since the material becomes harder to process after hardening, the 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 as much as possible into the cutting. This can reduce the cutting force and heat at every point and prevent local damage. When milling titanium alloy, the cutting speed of each cutting parameter 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 greater than HB350, the cutting process is particularly difficult. When it is less than HB300, the sticking phenomenon is likely to occur and it is difficult to cut. Therefore, it is possible to solve the problem of titanium processing from the blade. The blade groove wear that occurs during the processing of titanium alloys is the local wear of the back and front in the direction of the cutting depth, which 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 exceeding 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 coating of the blade is carried away, so titanium alloy processing requires special blade materials and geometries.


It is worth mentioning that because titanium alloys produce high heat during processing, a large amount of high-pressure cutting fluid must be sprayed onto the cutting edge in time to quickly remove heat. There is also a unique structure on the market that is specifically designed for the machining of titanium alloys, which is better suited for titanium machining.


At present, all countries are developing new low-cost and high-performance titanium alloys, and strive to make titanium alloys enter the civilian industry with huge market potential. China has spared no effort to move forward in this field. It is believed that through the joint efforts of all industry players, the processing of titanium alloys in the future will no longer be a problem, but instead 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.


The high temperatures generated during the cutting process also destroy the surface integrity of the titanium alloy parts, resulting in a decrease in the geometric accuracy of the parts and a work hardening phenomenon that severely reduces the fatigue strength.


The elasticity of titanium alloys may be beneficial to the performance of the part, but the elastic deformation of the workpiece during the cutting process is an important cause of vibration. The cutting pressure causes the "elastic" workpiece to leave the tool and bounce, so that the friction between the tool and the workpiece is greater than the cutting action. The friction process also generates heat, which increases the problem of poor thermal conductivity of the titanium alloy.

 

This problem is exacerbated when machining thin-walled or toroidally deformable parts. It is not an easy task to machine titanium alloy thin-walled parts to the desired dimensional accuracy. Because the local deformation of the thin wall has exceeded the elastic range and plastically deformed as the workpiece material is pushed away by the cutter, the material strength and hardness of the cutting point are significantly increased. At this time, the machining becomes too high according to the previously determined cutting speed, which further causes the tool to wear sharply. It can be said that "heat" is the "root of disease" that causes the difficulty in processing titanium alloys.