Titanium alloys have the advantages of light weight, high specific strength, and good corrosion resistance, so they are widely used in the automobile industry. The most common use of titanium alloys is in automobile engine systems. There are many benefits to using titanium alloys to make engine parts. However, titanium alloys have poor process performance and low wear resistance, which makes cutting difficult and the production process complex. During the processing, it is very easy to absorb impurities such as hydrogen, oxygen, nitrogen, and carbon, which affects the quality of the workpiece. The hardness of titanium alloy is only one aspect that makes it difficult to machine. The key lies in the comprehensive impact of the chemical, physical and mechanical properties of the titanium alloy itself on its machinability. The following is a brief introduction to the issues that need to be paid attention to when processing titanium alloys:
1. Characteristics of titanium alloy cutting processing
(1) Small deformation coefficient: This is a significant feature of titanium alloy cutting processing. , the deformation coefficient is less than or close to 1. The sliding friction distance of chips on the rake face is greatly increased, accelerating tool wear.
(2) High cutting temperature: Since the thermal conductivity of titanium alloy is very small, the contact length between the chip and the rake face is extremely short, and the heat generated during cutting is not easily transmitted and is concentrated in the cutting area and cutting edge. In a nearby smaller range, the cutting temperature is high. Under the same cutting conditions, the cutting temperature can be more than twice as high as when cutting stainless steel.
(3) Large cutting force per unit area: Since the contact length between the chip and the rake face is extremely short, the cutting force per unit contact area is greatly increased, easily causing edge chipping. At the same time, due to the small elastic modulus of titanium alloy, it is prone to bending deformation under the action of radial force during processing, causing vibration, increasing tool wear and affecting the accuracy of parts. Therefore, the process system is required to have good rigidity.
(4) The cold hardening phenomenon is serious: Due to the high chemical activity of titanium, at high cutting temperatures, it can easily absorb oxygen and nitrogen in the air to form a hard and brittle skin; at the same time, during the cutting process Plastic deformation can also cause surface hardening. The cold hardening phenomenon will not only reduce the fatigue strength of parts, but also aggravate tool wear, which is a very important feature when cutting titanium alloys.
(5) Tools are easy to wear: After the blank is processed by stamping, forging, hot rolling and other methods, a hard and brittle uneven skin is formed, which can easily cause chipping and make the hard skin become titanium alloy. The most difficult process in processing. In addition, due to the strong chemical affinity of titanium alloys to tool materials, tools are prone to adhesive wear under conditions of high cutting temperatures and large cutting forces per unit area. When turning titanium alloys, sometimes the wear of the rake face is even more severe than that of the flank face.
(6) The thermal conductivity of titanium alloy materials is low: the contact length between the chip and the rake face is extremely short, and the heat generated during cutting is not easy to escape, and is concentrated in the cutting deformation zone and smaller areas near the cutting edge. Within the range, extremely high cutting temperatures will be generated at the cutting edge during processing, which will greatly shorten tool life. For titanium alloy cutting, as long as the tool strength and machine tool power allow, the cutting temperature is the key factor affecting tool life, not the cutting force.
2. Precautions during the cutting of titanium alloy:
(1) Due to the small elastic modulus of titanium alloy, the workpiece will be clamped, deformed and stressed during processing. Large deformation will reduce the machining accuracy of the workpiece; the clamping force should not be too large when installing the workpiece, and auxiliary supports can be added if necessary.
(2) If hydrogen-containing cutting oil is used, it will decompose and release hydrogen at high temperatures during the cutting process, which will be absorbed by titanium and cause hydrogen embrittlement; it may also cause high-temperature stress corrosion cracking of titanium alloys.
(3) Chlorides in cutting oil may decompose or volatilize toxic gases when used. Safety precautions should be taken when using them. After cutting, parts should be thoroughly cleaned with chlorine-free cleaning agents to remove chlorine-containing chemicals. Chlorine residue.
(4) It is prohibited to use tools and fixtures made of lead or zinc-based alloys to come into contact with titanium alloys. The use of copper, tin, cadmium and their alloys is also prohibited.
(5) All tools, fixtures or other devices in contact with titanium alloy must be clean; cleaned titanium alloy parts must be prevented from being contaminated by grease or fingerprints, otherwise salt (sodium chloride) may be caused in the future ) stress corrosion.
(6) Under normal circumstances, there is no danger of fire when cutting titanium alloy. Only in micro-cutting, the small chips cut will ignite and burn. In order to avoid fires, in addition to pouring a large amount of cutting oil for cooling, you should also prevent chips from accumulating on the machine tool. Replace the tool immediately after it is blunt, or reduce the cutting speed and increase the feed to increase the chip thickness.
The above are the precautions for the titanium alloy cutting process. Formulating a rigorous process flow operation plan will help improve production efficiency and reduce the overall production cost of the enterprise.