Turning of titanium alloys
Titanium alloys are widely used in aerospace, medical devices, and other fields due to their high strength (tensile strength 400-1400 MPa), low density (4.5 g/cm³), and excellent corrosion resistance. However, turning them is challenging, earning them the nickname “difficult-to-machine.” Titanium alloys have low thermal conductivity (approximately one-fifth that of steel), concentrating heat in the cutting zone during cutting, where temperatures can reach 800-1000°C, leading to severe tool wear. Furthermore, their high chemical activity makes them susceptible to chemical reactions with tool materials at high temperatures, causing tool sticking. For example, when machining TC4 titanium alloy, tool life is only one-fifth to one-tenth that of machining 45 steel, and surface roughness easily exceeds tolerances (Ra > 3.2 μm). Therefore, turning titanium alloys requires specialized processes encompassing tool selection, cutting parameters, and cooling systems to ensure both quality and efficiency.
The choice of tool material is crucial for turning titanium alloys. Ordinary high-speed steel and carbide tools are inadequate for these applications. Materials with excellent wear resistance and chemical stability are essential. Ultrafine-grained carbide (such as WC-TiC-TaC-Co alloy, with a grain size of 0.5-1μm) is suitable for rough and semi-finish turning. Its bending strength ≥1800MPa and hardness ≥HRA90 allow it to withstand high cutting forces. Polycrystalline diamond (PCD) tools are suitable for finish turning, achieving surface roughness up to Ra0.2μm, but are more expensive. Cubic boron nitride (CBN) tools are suitable for machining heat- treated titanium alloys (hardness HRC35 and above), offering wear resistance 5-10 times that of carbide. AlCrN coatings (3-5μm thick) are ideal for tool coatings, as they offer superior high-temperature oxidation resistance compared to TiAlN coatings and effectively reduce tool sticking and wear. An aircraft engine factory used ultra-fine grain carbide tools to process TC11 titanium alloy, extending the tool life from 20 minutes to 60 minutes and increasing the processing efficiency by 2 times.
The optimization of cutting parameters requires a balance between efficiency and tool life. The cutting speed of titanium alloys needs to be strictly controlled, 80-120m/min for rough turning and 100-150m/min for fine turning. Too high a speed will cause a sudden temperature rise in the cutting zone, aggravating tool wear; too low a speed will increase the contact time between the tool and the workpiece, causing tool sticking. The feed rate is 0.1-0.3mm/r, 0.2-0.3mm/r for rough turning, and 0.1-0.15mm/r for fine turning. Excessive feed rate will increase the cutting force and cause workpiece deformation; too small a feed rate will form thin chips and aggravate tool friction. The back cutting depth is determined according to the allowance, 1-3mm for rough turning and 0.1-0.5mm for fine turning. Intermittent cutting (such as machining grooves or steps) should be avoided, otherwise it will easily cause tool edge breakage. Experimental data show that when the cutting speed of TC4 titanium alloy is 100m/min, the feed rate is 0.2mm/r, and the back cutting depth is 2mm, the cutting force is about 800N and the tool wear is 0.01mm/min, which is within a reasonable range.
A robust cooling and lubrication system is crucial for titanium alloy turning. Due to the high temperatures in the cutting zone, a high-pressure, high-flow cooling system is required. A cooling pressure of ≥10 MPa and a flow rate of ≥50 L/min are required. Dual nozzles precisely spray cutting fluid onto the cutting edge and the machined surface, rapidly removing heat. The cutting fluid is an extreme-pressure emulsion (containing chlorine and sulfur additives) at a concentration of 10%-15%. This forms a chemical lubricating film at high temperatures, reducing the coefficient of friction (from 0.3 to 0.1). For deep-hole turning, internally cooled tools are required. The cutting fluid flows directly through internal channels within the tool to the cutting zone, improving cooling efficiency by 50% compared to external cooling. Inadequate cooling can lead to thermal cracking of the tool and burns on the workpiece surface. Therefore, regular inspection of the cooling system pressure and flow rate is crucial to ensure stability and reliability. A medical device manufacturer implemented high-pressure cooling, reducing the surface burn rate of titanium alloy bone screws from 15% to 1% and the surface roughness from Ra3.2μm to Ra0.8μm.
The clamping method and process must be adapted to the characteristics of titanium alloys. Titanium alloys have a low elastic modulus (approximately 110 GPa, half that of steel), making them prone to deformation during clamping. Therefore, a rigid fixture is required: a three-jaw chuck with soft jaws (aluminum or copper), with a clamping force controlled at 3-5 kN to avoid workpiece flattening. For thin-walled parts (< 2 mm thick), an expansion clamp or vacuum cup is used for uniform clamping. The process follows the principle of "rapid stock removal + low-temperature finish turning": After rough turning, stress relief annealing (600-650°C for 2 hours, air cooling) is performed to eliminate machining stresses. A 0.5-1 mm stock is reserved for semi-finish turning. For finish turning, low cutting parameters and PCD tools are used to ensure dimensional accuracy (IT7 grade). Tool stops must be avoided during machining to prevent dimensional errors caused by thermal deformation. An aerospace company reduced the machining deformation of titanium alloy drive shafts from 0.1mm to 0.02mm by optimizing clamping and processes, meeting assembly requirements.
Quality control and defect prevention must be implemented throughout the entire titanium alloy turning process. Common defects include surface scratches (caused by chip scratches), dimensional deviations (caused by deformation or tool wear), and microcracks (caused by excessive cutting temperatures). Surface scratches can be addressed by improving chip removal (such as using a chip separator) and cleaning. Dimensional deviations require regular machine calibration (first-piece inspection of each shift) and compensation for tool wear (0.01mm compensation for every 50 pieces machined). Microcracks should be detected using penetrant testing and, if detected, ground out (depth ≥ 0.1mm). Inspections include measuring dimensions with an outside micrometer (accuracy ±0.001mm), roundness with a dial indicator (≤0.005mm), and surface roughness testing with a surface roughness gauge (Ra value ≤1.6μm). Titanium alloy parts for aerospace applications also require mechanical property testing (such as tensile strength and elongation) to ensure they meet design requirements. Through comprehensive quality control, one aircraft manufacturer increased the pass rate for titanium alloy parts from 80% to 99%, significantly reducing production costs.
