Exploring the Difficulties in Working with Titanium Alloys
Since the discovery of titanium in 1790, humans have been exploring its extraordinary properties for over a century. In 1910, titanium metal was first produced, but the journey toward using titanium alloys was long and challenging. It wasn’t until 1951 that industrial production became a reality.
Titanium alloys are known for their high specific strength, corrosion resistance, high-temperature resistance, and fatigue resistance. They weigh only 60% as much as steel at the same volume yet are stronger than alloy steel. Due to these excellent properties, titanium alloys are being increasingly utilized in various fields, including aviation, aerospace, power generation, nuclear energy, shipping, chemicals, and medical equipment.
Reasons why titanium alloys are difficult to process
The four main characteristics of titanium alloys—low thermal conductivity, significant work hardening, a high affinity for cutting tools, and limited plastic deformation—are key reasons why these materials are challenging to process. Their cutting performance is only about 20% that of easy-to-cut steel.
Low thermal conductivity
Titanium alloys have a thermal conductivity that is only about 16% of that of 45# steel. This limited ability to conduct heat away during processing leads to a significant rise in temperature at the cutting edge; in fact, the tip temperature during processing can exceed that of 45# steel by more than 100%. This elevated temperature easily causes diffuse wear on the cutting tool.
Severe work hardening
Titanium alloy exhibits a significant work hardening phenomenon, resulting in a more pronounced surface hardening layer compared to stainless steel. This can lead to challenges in subsequent processing, such as increased wear on tooling.
High affinity with cutting tools
Severe adhesion with titanium-containing cemented carbide.
Small plastic deformation
The elastic modulus of 45 steel is approximately half, leading to significant elastic recovery and severe friction. Additionally, the workpiece is susceptible to clamping deformation.
Technological tips for machining titanium alloys
Based on our understanding of the machining mechanisms for titanium alloys and previous experiences, here are the main technological recommendations for machining these materials:
- Use blades with a positive angle geometry to minimize cutting forces, reduce cutting heat, and decrease deformation of the workpiece.
- Maintain a constant feed rate to prevent workpiece hardening. The tool should always be in feed during the cutting process. For milling, the radial cutting depth (ae) should be 30% of the tool’s radius.
- Employ high-pressure and high-flow cutting fluids to ensure thermal stability during machining, preventing surface degeneration and tool damage due to excessive temperatures.
- Keep the blade edge sharp. Dull tools can lead to heat accumulation and increased wear, significantly raising the risk of tool failure.
- Machine titanium alloys in their softest state whenever possible. CNC machining processing becomes more difficult after hardening, as heat treatment increases the material’s strength and accelerates blade wear.
- Use a large tip radius or chamfer when cutting to maximize the contact area of the blade. This strategy can reduce cutting forces and heat at each point, helping to prevent local breakage. When milling titanium alloys, cutting speed has the most significant impact on tool life, followed by the radial cutting depth.
Solve titanium processing problems by starting with the blade.
The wear of the blade groove that occurs during the processing of titanium alloys is localized wear that happens along the back and front of the blade, following the direction of cutting depth. This wear is often caused by a hardened layer left over from previous machining processes. Additionally, at processing temperatures exceeding 800°C, chemical reactions and diffusion between the tool and the workpiece material contribute to the formation of groove wear.
During machining, titanium molecules from the workpiece can accumulate in front of the blade due to high pressure and temperature, leading to a phenomenon known as a built-up edge. When this built-up edge detaches from the blade, it can remove the carbide coating on the blade. As a result, processing titanium alloys necessitates the use of specialized blade materials and geometries.
Tool structure suitable for titanium processing
The processing of titanium alloys primarily revolves around managing heat. To effectively dissipate heat, a significant amount of high-pressure cutting fluid must be accurately and promptly applied to the cutting edge. Additionally, there are specialized milling cutter designs available that are specifically tailored for titanium alloy processing.
Starting from the specific machining method
Turning
Titanium alloy products can achieve good surface roughness during turning, and the work hardening is not severe. However, the cutting temperature is high, which leads to rapid tool wear. To address these characteristics, we primarily focus on the following measures regarding tools and cutting parameters:
Tool Materials: Based on the factory’s existing conditions, YG6, YG8, and YG10HT tool materials are selected.
Tool geometry parameters: appropriate tool front and rear angles, tooltip rounding.
When turning the outer circle, it’s important to maintain a low cutting speed, a moderate feed rate, a deeper cutting depth, and adequate cooling. The tool tip should not be higher than the center of the workpiece, as this can lead to it getting stuck. Additionally, when finishing and turning thin-walled parts, the tool’s main deflection angle should generally be between 75 and 90 degrees.
Milling
Milling of titanium alloy products is more difficult than turning, because milling is intermittent cutting, and the chips are easy to stick to the blade. When the sticky teeth cut into the workpiece again, the sticky chips are knocked off and a small piece of tool material is taken away, resulting in chipping, which greatly reduces the durability of the tool.
Milling method: generally use down milling.
Tool material: high-speed steel M42.
Down milling is not typically used for processing alloy steel. This is mainly due to the influence of the gap between the machine tool’s lead screw and the nut. During down milling, as the milling cutter engages with the workpiece, the component force in the feed direction aligns with the feed direction itself. This alignment can lead to intermittent movement of the workpiece table, increasing the risk of tool breakage.
Additionally, in down milling, the cutter teeth encounter a hard layer at the cutting edge, which can cause tool damage. In reverse milling, the chips transition from thin to thick, making the initial cutting phase prone to dry friction between the tool and the workpiece. This can exacerbate chip adhesion and chipping of the tool.
To achieve smoother milling of titanium alloys, several considerations should be taken into account: reducing the front angle and increasing the back angle compared to standard milling cutters. It is advisable to use lower milling speeds and opt for sharp-tooth milling cutters while avoiding shovel-tooth milling cutters.
Tapping
When tapping titanium alloy products, small chips can easily stick to the blade and the workpiece. This leads to increased surface roughness and torque. Improper selection and use of taps can cause work hardening, result in very low processing efficiency, and occasionally lead to tap breakage.
To optimize tapping, it is advisable to prioritize using a one-thread-in-place skipped tap. The number of teeth on the tap should be fewer than that of a standard tap, typically around 2 to 3 teeth. A larger cutting taper angle is preferred, with the taper section generally measuring 3 to 4 thread lengths. To aid in chip removal, a negative inclination angle can also be ground onto the cutting taper. Using shorter taps can enhance the rigidity of the taper. Additionally, the reverse taper should be slightly larger than standard to reduce friction between the taper and the workpiece.
Reaming
When reaming titanium alloy, tool wear is generally not severe, allowing for the use of both carbide and high-speed steel reamers. When using carbide reamers, it is essential to ensure the process system’s rigidity, similar to that used in drilling, to prevent chipping of the reamer.
The main challenge in reaming titanium alloy holes is achieving a smooth finish. To avoid the blade sticking to the hole wall, the width of the reamer blade should be carefully narrowed using an oilstone while still ensuring sufficient strength. Typically, the blade width should be between 0.1 mm and 0.15 mm.
The transition between the cutting edge and the calibration section should feature a smooth arc. Regular maintenance is necessary after wear occurs, ensuring that the arc size of each tooth remains consistent. If required, the calibration section can be enlarged for better performance.
Drilling
Drilling titanium alloys presents significant challenges, often causing drill bits to burn or break during processing. This primarily results from issues such as improper drill bit grinding, insufficient chip removal, inadequate cooling, and poor system rigidity.
To effectively drill titanium alloys, it’s essential to focus on the following factors: ensure proper grinding of the drill bit, use a larger top angle, reduce the outer edge front angle, increase the outer edge back angle, and adjust the back taper to be 2 to 3 times that of a standard drill bit. It is important to frequently retract the tool to remove chips promptly, while also monitoring the shape and color of the chips. If the chips appear feathery or if their color changes during drilling, it indicates that the drill bit is becoming blunt and should be replaced or sharpened.
Additionally, the drill jig must be securely fixed to the workbench, with the guide blade close to the processing surface. It is advisable to use a short drill bit whenever possible. When manual feeding is employed, care should be taken not to advance or retreat the drill bit within the hole. Doing so can cause the drill blade to rub against the processing surface, leading to work hardening and dulling the drill bit.
Grinding
Common issues encountered when grinding CNC titanium alloy parts include grinding wheel clogging due to stuck chips and surface burns on the parts. This occurs because titanium alloys have poor thermal conductivity, which leads to high temperatures in the grinding zone. This, in turn, causes bonding, diffusion, and strong chemical reactions between the titanium alloy and the abrasive material.
The presence of sticky chips and clogged grinding wheels significantly reduces the grinding ratio. Additionally, diffusion and chemical reactions can result in surface burns on the workpiece, ultimately reducing the fatigue strength of the part. This problem is particularly pronounced when grinding titanium alloy castings.
To solve this problem, the measures taken are:
Choose the appropriate grinding wheel material: green silicon carbide TL. Slightly lower grinding wheel hardness: ZR1.
The cutting of titanium alloy materials must be controlled through tool materials, cutting fluids, and processing parameters to enhance overall processing efficiency.
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