Why Is Titanium Difficult To Machine?

The four characteristics of titanium alloys, including low thermal conductivity, severe work hardening, high affinity with cutting tools, and small plastic deformation, are the essential reasons why titanium alloys are difficult to process. Its cutting index is only equivalent to 20% of free-cutting steel.

Low thermal conductivity

The thermal conductivity of titanium alloy is only about 16% of that of 45# steel. The heat cannot be conducted out in time during processing, causing local high temperatures on the cutting edge (the tool tip temperature during processing is more than twice that of 45# steel), which can easily cause diffusion wear of the tool.

Severe work hardening

The work hardening phenomenon of titanium alloy is obvious, and the surface hardening layer is more severe than that of stainless steel, which will cause certain difficulties in subsequent processing, such as increased damage to the tool boundary.

High affinity with knives

Severe bonding with titanium-containing carbide.

Small plastic deformation

It is about 1/2 of the elastic modulus of 45 steel, so the elastic recovery is large and the friction is serious. At the same time, the workpiece is also prone to clamping deformation.

Process know-how for processing titanium alloys

On the basis of understanding the processing mechanism of titanium alloys and adding past experience, the main process know-how for processing titanium alloys is as follows:

(1) Use inserts with 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 must always be in the feed state during the cutting process. The radial tool engagement amount 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 workpiece surface degeneration and tool damage caused by 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 the titanium alloy in the softest state as much as possible, because the material becomes more difficult to process after quenching, and heat treatment increases the strength of the material and increases blade wear.

(6) Use a large tool tip arc radius or chamfer to cut as much as possible into the cutting edge. This reduces cutting force and heat at every point and prevents local breakage. When milling titanium alloy, among the cutting parameters, cutting speed has the greatest impact on tool life, followed by radial tool engagement (milling depth).

Start with blades to solve titanium processing problems

The groove wear of the blade that occurs when machining titanium alloys is the local wear of the back and front along the cutting depth direction. It 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 causes of groove wear. Because during the machining process, the titanium molecules of the workpiece accumulate in front of the blade and are "welded" to the blade under high pressure and high temperature, forming built-up edge. When built-up edge peels off the cutting edge, it takes the insert's carbide coating with it, so titanium machining requires special insert materials and geometries.

Tool structure suitable for titanium machining

The focus of titanium alloy processing is heat. A large amount of high-pressure cutting fluid must be sprayed on the cutting edge promptly and accurately to remove the heat quickly. There are unique structures of milling cutters on the market specifically for titanium alloy processing.

Start with specific mechanical processing methods

turning

When turning titanium alloy products, it is easy to obtain better surface roughness and work hardening is not serious, but the cutting temperature is high and the tool wears quickly. In response to these characteristics, the following measures are mainly taken in terms of tools and cutting parameters:

Tool material: YG6, YG8, YG10HT are selected according to the existing conditions of the factory.

Tool geometric parameters: suitable tool front and rear angles and tool tip rounding.

Lower cutting speed, moderate feed rate, deeper cutting depth, sufficient cooling. When turning the outer circle, the tool tip cannot be higher than the center of the workpiece, otherwise it is easy to tie the tool. When finishing turning and turning thin-walled parts, the tool main deflection The angle should be large, usually 75 to 90 degrees.

Milling

Milling of titanium alloy products is more difficult than turning, because milling is intermittent cutting, and the chips are easy to bond with the blade. When the chip-sticking teeth cut into the workpiece again, the chips are knocked off and take away a small piece of tool material, forming a The chipping blade greatly reduces the durability of the tool.

Milling method: Generally, down milling is used.

Tool material: high speed steel M42.

Generally, down milling is not used in the processing of alloy steel. Due to the influence of the clearance between the machine tool screw and nut, during down milling, the milling cutter acts on the workpiece, and the component force in the feed direction is the same as the feed direction, which easily causes the workpiece table to collapse. Intermittent movement occurs, resulting in knife beating. For down milling, the cutter teeth hit hard skin when they start to cut in, causing the cutter to break. However, since the up-milling chips change from thin to thick, the tool is prone to dry friction with the workpiece during the initial cut, which aggravates the chip sticking and edge chipping of the tool. In order to mill titanium alloy smoothly, it should also be noted that compared with general standard milling cutters, the rake angle should be reduced and the relief angle should be increased. The milling speed should be low. Try to use sharp tooth milling cutters and avoid using shovel tooth milling cutters.

Tapping

When tapping titanium alloy products, the chips are small and easy to bond with the blade and workpiece, resulting in large surface roughness and high torque. Improper tap selection and improper operation during tapping can easily lead to work hardening, extremely low processing efficiency and occasional tap breakage.

It is necessary to give priority to the jump-thread tap with the right thread. The number of teeth should be less than that of the standard tap, usually 2 to 3 teeth. The cutting taper angle should be large, and the taper part is generally 3 to 4 thread lengths. To facilitate chip removal, a negative inclination angle can also be ground on the cutting cone. Try to use short taps to increase tap rigidity. The reverse taper part of the tap should be appropriately larger than the standard to reduce the friction between the tap and the workpiece.

reaming

The tool wear is not serious when reaming titanium alloy, and both carbide and high-speed steel reamers can be used. When using carbide reamers, process system stiffness similar to drilling must be adopted to prevent the reamer from chipping. The main problem that occurs when reaming titanium alloys is that the finish of the reaming is not good. The width of the reamer blade must be narrowed with a whetstone to prevent the blade from bonding with the hole wall. However, sufficient strength must be ensured. Generally, the blade width is 0.1~0.15mm. as well.

The transition point between the cutting edge and the calibration part should be a smooth arc. It must be ground in time after wear and tear, and the arc size of each tooth must be consistent; if necessary, the back taper of the calibration part can be increased.

Drilling

Drilling titanium alloys is difficult, and burnout and drill breakage often occur during the machining process. This is mainly due to several reasons such as poor sharpening of the drill bit, untimely chip removal, poor cooling, and poor rigidity of the process system. Therefore, when drilling titanium alloys, attention must be paid to reasonable drill bit sharpening, large vertex angle, reduced outer edge rake angle, increased outer edge relief angle, and increased back taper to 2 to 3 times that of standard drill bits. Retract the tool frequently and remove the chips promptly, paying attention to the shape and color of the chips. If the chips appear feathery or change color during drilling, it indicates that the drill bit is dull and the tool should be changed and sharpened in time.

The drilling die should be fixed on the workbench, the guiding surface of the drilling die should be close to the processing surface, and a short drill bit should be used as much as possible. Another issue worth noting is that when manual feeding is used, the drill bit should not advance or retreat in the hole, otherwise the drill edge will rub against the machined surface, causing work hardening and making the drill bit blunt.

grinding

Common problems in grinding titanium alloy parts are clogging of the grinding wheel due to stuck chips and burns on the surface of the parts. The reason is that the thermal conductivity of titanium alloy is poor, which causes high temperature in the grinding zone, causing bonding, diffusion and strong chemical reaction between titanium alloy and abrasive. Sticky chips and clogging of the grinding wheel lead to a significant decrease in the grinding ratio. As a result of diffusion and chemical reactions, the workpiece is burned on the grinding surface, resulting in a reduction in the fatigue strength of the part. This is more obvious 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 from the aspects of tool materials, cutting fluids, and processing process parameters in order to improve the overall efficiency of titanium alloy material processing.