How to avoid built-up edge and work hardening in titanium alloy semi-finished product processing?
Publish Time: 2025-03-31
In the process of titanium alloy semi-finished product processing, built-up edge (BUE) and work hardening are like twin problems, which seriously restrict processing efficiency and surface quality. These two phenomena are essentially closely related to the special physical and chemical properties of titanium alloys - low thermal conductivity leads to cutting heat accumulation, high chemical activity causes material and tool adhesion, and significant work hardening tendency makes subsequent cutting resistance increase exponentially. To break through these bottlenecks, it is necessary to build a systematic solution from multiple dimensions such as tool technology, cutting parameters, and cooling methods.
The selection and treatment of tools is the first line of defense to inhibit built-up edge. Because titanium alloys are prone to diffusion reaction with common tool materials at high temperatures, traditional carbide tools often fail due to crater wear within a few minutes. Ultra-fine-grained carbide tools with physical vapor deposition (PVD) coatings are often used in modern processing. Among them, AlCrN coatings are outstanding due to their excellent oxidation resistance, which can extend tool life by 3-5 times. The tool geometry needs to be carefully designed. The rake angle is usually controlled between 5°-10° to balance the cutting force and chip removal efficiency, and the back angle is maintained at 12°-15° to reduce the friction of the back face. The radius of the tool tip arc should not be too large, otherwise it will aggravate the work hardening. It is recommended to use a precision grinding edge of 0.4-0.8mm. Under continuous cutting conditions, the use of variable helix angle end mills can effectively disrupt the cutting vibration frequency and prevent the formation of periodic built-up edge.
The optimization of cutting parameters requires a breakthrough in the traditional metal processing mindset. There is a sensitive range for the cutting speed of titanium alloys. Usually, the turning speed is controlled within the range of 30-60m/min and the milling speed is controlled within the range of 40-80m/min, which can avoid material adhesion in the low-speed zone and avoid the risk of overheating in the high-speed zone. The feed per tooth should be maintained at a high level of 0.05-0.15mm/z. The unit cutting force is reduced by increasing the chip thickness. This strategy is completely opposite to ordinary steel processing. The axial cutting depth should be greater than 0.5mm to ensure that the cutting edge works in a stable stress state and avoid repeated friction of the tool tip on the hardened layer. In terms of programming strategy, modern path planning methods such as cycloidal milling or spiral interpolation can be used to keep the cutting edge working in the unhardened area of the material, fundamentally avoiding the negative impact of work hardening.
The innovation of cooling technology has brought breakthrough progress to titanium alloy processing. Traditional overflow cooling has limited effect in titanium alloy processing, while high-pressure jet cooling (pressure 70-300bar) can penetrate the steam barrier in the cutting area and achieve effective cooling. A more advanced method is to use low-temperature cold air (-30℃ to -50℃) combined with minimal lubrication (MQL). This combination can not only reduce the temperature of the cutting area, but also avoid environmental pollution of the cutting fluid. In special occasions such as deep hole processing, an internal cooling tool structure can be introduced to allow the cooling medium to act directly on the heat source area of the tool tip. Some military-grade processing even uses liquid nitrogen cooling to reduce the temperature of the cutting area to -196℃, completely inhibiting the chemical activity reaction of titanium alloys. The combination of these cooling schemes can reduce the incidence of built-up edge by more than 80%.
Reasonable arrangement of the process route can effectively control the accumulation of work hardening. A large cutting depth and fast feed strategy should be adopted during rough machining so that the cutting force mainly acts on the unhardened layer of the material, and a 0.3-0.5mm margin should be left for final finishing during fine machining. For workpieces that have already undergone work hardening, an intermediate annealing process (700-800℃ vacuum annealing) can be arranged, or sandblasting can be inserted between two cuts to break the surface hardened layer. When milling thin-walled parts, a symmetrical machining method is used to balance the residual stress to avoid workpiece deformation caused by unilateral hardening. The introduction of an intelligent monitoring system realizes real-time control. By detecting spindle current fluctuations or acoustic emission signals, the initial signs of built-up edge can be immediately detected, and the cutting parameters can be automatically adjusted for intervention.
The pretreatment of the material itself also affects the processing performance. For α+β duplex titanium alloys, the proportion of the primary α phase can be adjusted (controlled at 20%-50%) through solid solution aging treatment to obtain more balanced cutting performance. β-type titanium alloys can be pre-treated by cold deformation to increase dislocation density and reduce the hardening rate of subsequent processing. Ultrafine-grained titanium alloys (grain size <1μm) that have emerged in recent years have shown excellent resistance to built-up edge, and their uniform microstructure effectively suppresses local strain concentration. These innovations at the material level and advances in processing technology complement each other and jointly promote the development of titanium alloy processing technology in a more efficient and precise direction.
