Titanium alloy has the advantages of low density, high strength ratio (strength/density), good corrosion resistance, heat resistance, toughness and plasticity, etc. It is widely used in many fields such as aerospace, automobile, medicine, sports goods and electrolysis industry. However, its poor thermal conductivity, high hardness, low modulus of elasticity and other characteristics make titanium alloys become more difficult to process metal materials. Gr.5(Ti6AI-4V ) is the most commonly used titanium alloy grades. Today we will analyze why gr5 is difficult to machine.
- Low thermal conductivity
Grade 5 titanium alloy at 200 ℃ heat conductivity l = 16.8 W/m?℃, the coefficient of thermal conductivity is 0.036 ℃ / cm, is only a quarter of the steel, aluminum, 1/13 of 1/25 of the copper. Heat in the process of cutting of titanium alloy will not quickly be passed to the workpiece or taken away by chip, and agglomeration in cutting area, the temperature can be as high as 1 000 ℃ above, make the cutter blade quickly wear, split, produce more heat and the cutting area, further shorten the tool life. The high temperature in the cutting process destroys the surface integrity of the titanium alloy parts at the same time, which leads to the decrease of the geometrical precision of the parts and the machining hardening phenomenon which seriously reduces their fatigue strength.
- Low elastic modulus
The surface of the machined parts has great resilience, which leads to the increase of the contact area between the machined surface and the cutter surface, which not only affects the dimensional accuracy of the parts but also reduces the tool durability. The elasticity of titanium alloy may be beneficial to the performance of parts, but in the cutting process, the elastic deformation of the workpiece is an important cause of vibration. Cutting pressure causes the “elastic” workpiece to leave the tool and rebound, thus making the friction between the tool and the workpiece greater than the cutting effect. Friction process also produces heat, aggravating the poor thermal conductivity of titanium alloys. This problem is more serious when machining thin-walled or annular parts, which are easily deformed. It is not easy to process titanium alloy thin-walled parts to the expected dimensional accuracy. Because as the workpiece material is pushed by the tool, the local deformation of the thin wall has exceeded the elastic range and plastic deformation, cutting point material strength and hardness increased significantly. At this point, machining at the previously determined cutting speed becomes too high, further resulting in sharp tool wear.
- High hardness
When some titanium alloys with low hardness value are processed, the viscosity increases and the chip sticks to the cutting edge of the cutter to form a chip tumor, which affects the processing effect. The hardness of titanium alloy such as Grade5 titanium alloy machining is easy to make the tool edge collapse and abrasion. These characteristics result in a low metal removal rate of titanium alloy, which is only 1/4 of that of steel pieces while the processing time is much longer than that of steel pieces of the same size.
- Strong chemical affinity
Titanium can not only react with the main components in the air such as nitrogen, oxygen, carbon monoxide and other substances, forming TiC and TiN hardened layer on the surface of the alloy, but also react with the tool material under the high-temperature conditions generated by cutting, which reduces the tool durability.
- Poor safety performance in the cutting process
Titanium is a flammable metal. The high temperature and spark are extremely flammable and if ignited, are almost impossible to put out during the micro-cutting.
Compared with most other metal materials, titanium alloys have higher processing requirements and more restrictions. Satisfactory machining results for titanium alloys can also be obtained if the proper cutting tools are used and the machine tools and configurations are optimized to the best condition according to the machining requirements.