Why is Titanium Alloy a Difficult-to-Process Material?

Titanium alloy refers to a new type of alloy prepared by adding single or multiple other elements based on titanium. Titanium is highly active, and most elements can interact with it to form continuous solid solutions, limited solid solutions, metal compounds, covalent compounds, ionic compounds, etc. Alloying elements can optimize their performance by changing their allotropic transition points and phase composition.

01 "Heat" is the main reason why titanium alloys are difficult to process

The cutting force during titanium alloy processing is only slightly higher than that of steel with the same hardness, but the physical phenomenon of titanium alloy processing is much more complicated than that of steel processing, which makes titanium alloy processing face great difficulties.

The thermal conductivity of most titanium alloys is very low, only 1/7 of steel and 1/16 of aluminum. Therefore, the heat generated during the cutting of titanium alloys will not be quickly transferred to the workpiece or taken away by the chips, but will accumulate in the cutting area. The temperature generated can be as high as 1,000°C or more, causing the cutting edge of the tool to quickly wear, crack and generate built-up edge, and the worn blade will quickly generate more heat in the cutting area, further shortening the life of the tool.

The high temperature generated during the cutting process also destroys the surface integrity of titanium alloy parts, resulting in a decrease in the geometric accuracy of the parts and work hardening that seriously reduces their fatigue strength.

The elasticity of titanium alloys may be beneficial to the performance of parts, 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 rebound, so that the friction between the tool and the workpiece is greater than the cutting action. The friction process also generates heat, which aggravates the poor thermal conductivity of titanium alloys.

This problem is even more serious when processing thin-walled or annular parts that are easy to deform. It is not easy to process titanium alloy thin-walled parts to the expected dimensional accuracy. Because as the workpiece material is pushed away by the tool, the local deformation of the thin wall has exceeded the elastic range and produced plastic deformation, and the material strength and hardness of the cutting point have increased significantly. At this time, processing according to the originally determined cutting speed becomes too high, further causing rapid wear of the tool.

02 Technological tips for machining titanium alloys

Based on the understanding of the machining mechanism of titanium alloys and previous experience, the main technological tips for machining titanium alloys are as follows:

(1) Use a blade with a positive angle geometry to reduce cutting force, 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 cutting depth ae during milling should be 30% of the radius.

(3) Use high-pressure and high-flow cutting fluid to ensure the thermal stability of the machining process and prevent surface deformation of the workpiece and tool damage due to excessive temperature.

(4) Keep the blade edge sharp. Blunt tools are the cause of heat accumulation and wear, which can easily lead to tool failure.

(5) Process titanium alloy in the softest state as much as possible, because the material becomes more difficult to process after hardening. Heat treatment increases the strength of the material and increases blade wear.

(6) Use a large tip radius or chamfer to cut in and put as much blade edge as possible into the cutting. This can reduce cutting force and heat at each point and prevent local damage. When milling titanium alloy, the cutting speed has the greatest impact on tool life vc among all cutting parameters, followed by radial cutting depth ae.

03Solve titanium processing problems by starting with the blade

The blade groove wear that occurs when titanium alloy is processed is the local wear of the back and front along the cutting depth direction, which is often caused by the hardened layer left by the previous processing. The chemical reaction and diffusion between the tool and the workpiece material at a processing temperature exceeding 800°C is also one of the reasons for the formation of groove wear. Because during the processing, the titanium molecules of the workpiece accumulate in front of the blade and "weld" to the blade under high pressure and high temperature to form a built-up edge. When the built-up edge is peeled off from the blade, the carbide coating of the blade is taken away. Therefore, titanium alloy processing requires special blade materials and geometries.

04 Tool structure suitable for titanium processing

The focus of titanium alloy processing is heat. A large amount of high-pressure cutting fluid must be sprayed onto the cutting edge in a timely and accurate manner to quickly remove the heat. There are unique milling cutter structures on the market specifically for titanium alloy processing.