Turning of Titanium Alloy Deep Hole Sleeve
Turning titanium alloy deep-hole sleeves is a challenging process in machining. While titanium alloy offers advantages such as high strength, low density, and excellent corrosion resistance, it also suffers from low thermal conductivity, high chemical activity, and a low elastic modulus. These characteristics lead to rapid tool wear, significant machining deformation, and difficult chip removal. Titanium alloy deep-hole sleeves are widely used in aerospace, medical devices, and chemical equipment, such as fuel pipe sleeves for aircraft engines and implant sleeves for artificial joints. Their aspect ratio (the ratio of hole depth to hole diameter) is typically greater than 5, and for some precision parts, even exceeds 10. Machining accuracy requirements reach IT7-IT8, with a surface roughness Ra below 1.6μm. Processing data from an aviation company shows that the turning efficiency of titanium alloy deep-hole sleeves is only 30%-50% of that of 45 steel, yet the tooling costs are 3-5 times higher.
The selection of turning tools for titanium alloy deep-hole bushings requires optimization based on material properties. Due to the high high-temperature strength of titanium alloys, cutting temperatures can reach 800-1000°C. Therefore, tool materials must possess excellent heat and wear resistance. Commonly used tools include cemented carbide (such as WC-Co alloys), ceramic tools (such as Al2O3-TiC), and cubic boron nitride (CBN) tools. Cemented carbide tools are suitable for medium cutting speeds (30-80 m/min). Fine-grained alloys containing TaC or NbC (such as YG8N and YT798) are recommended to improve the tool’s thermal shock resistance. Ceramic tools are suitable for high-speed cutting (100-200 m/min), but they are more brittle and require a rigid machine tool. CBN tools are suitable for machining quenched titanium alloys, but they are more expensive. Tool geometry parameters include a rake angle of 5°-10° (to prevent oversharpening and chipping), a clearance angle of 8°-12° (to reduce flank friction), and a lead angle of 75°-90° (to reduce radial cutting forces). A medical device manufacturer, when machining TC4 titanium alloy deep-hole sleeves, used YG8N carbide tools, coupled with a dedicated cooling system, which extended tool life by 40%.
The turning process for titanium alloy deep-hole sleeves requires staged control of deformation and chip removal. The typical process route is: forging the blank → annealing → rough turning the outer diameter and end face → drilling or boring a pre-hole → rough boring the deep hole → semi-finish turning the outer diameter → finish boring the deep hole → finish turning the outer diameter and end face → stress relief. When rough boring deep holes, a larger cutting depth (1-3mm) and a lower feed rate (0.1-0.2mm/r) should be used to quickly remove machining allowances while avoiding workpiece deformation due to excessive cutting forces. When finish boring, a smaller cutting depth (0.1-0.3mm) and a higher feed rate (0.05-0.1mm/r) are used to ensure the dimensional accuracy and surface quality of the hole. Chip removal methods for deep hole machining include external chip removal (such as gun drilling) and internal chip removal (such as BTA deep hole drilling). Internal chip removal is recommended for titanium alloy machining, using high-pressure cutting fluid to expel chips from the hole. The cutting fluid must have excellent cooling and lubricating properties. Extreme-pressure emulsions or specialized titanium alloy cutting fluids are recommended, with a pressure controlled between 3 and 10 MPa. An aerospace machinery plant used a BTA deep-hole boring system, coupled with a cutting fluid flow rate of 50 L/min, to successfully resolve chip clogging issues when machining a titanium alloy sleeve with a depth-to-diameter ratio of 12.
Clamping and vibration control for titanium alloy deep-hole sleeves are critical to ensuring machining accuracy. Due to the poor rigidity of deep-hole sleeves, especially as the hole depth increases during machining, workpiece rigidity decreases further, leading to vibration, which can cause out-of-tolerance hole roundness and cylindricity. Highly rigid fixtures should be used for clamping, such as a three-jaw self-centering chuck in conjunction with a steady rest or center rest. For workpieces with an aspect ratio greater than 8, auxiliary supports should be added in the center to minimize bending deformation during machining. Wear-resistant cast iron or bronze is recommended for the steady rest support block. The contact area with the workpiece outer diameter should be as large as possible, with moderate contact pressure to avoid surface damage. A precision machining shop, when machining a 1000mm long, 100mm diameter titanium alloy deep-hole sleeve, employed a clamping method of “two-end positioning + center auxiliary support,” achieving hole cylindricity tolerances within 0.015mm, far exceeding the design requirement of 0.03mm.
The quality inspection and process improvement of titanium alloy deep hole sleeves need to be carried out throughout the entire processing process. Key inspection items include: hole diameter size (using an internal diameter dial indicator or a three-coordinate measuring machine), hole roundness and cylindricity (using a roundness meter), straightness of the hole axis (using a laser interferometer), surface roughness (using a roughness meter), etc. For deep hole sleeves with sealing requirements, a water pressure test or an air tightness test is also required to ensure that there is no leakage. In terms of process improvement, a materials research institute improved the cutting performance of the material by adding a small amount of rare earth elements to titanium alloys, reducing tool wear during turning by 25%; the adaptive deep hole processing system developed by a machine tool factory can monitor cutting force and vibration signals in real time, automatically adjust cutting parameters, and increase the processing efficiency of titanium alloy deep hole sleeves by 30%, and reduce the scrap rate to below 0.5%. With the increasing application of titanium alloys in high-end equipment, the turning technology of deep hole sleeves is also constantly innovating. The application of new technologies such as low-temperature cold air cutting and ultrasonic vibration assisted cutting has provided new ways to solve the problems of titanium alloy processing and promoted the development of titanium alloy deep hole sleeve processing towards higher precision and higher efficiency.
