Titanium alloy parts have high specific strength among metal structural materials. It is as strong as steel, but only 57% of the weight. In addition, titanium alloy has the characteristics of small specific gravity, high thermal strength, good thermal stability and corrosion resistance, but titanium alloy material is difficult to cut, and the processing efficiency of CNC machine tools is low. Therefore, how to overcome the difficulty and low efficiency of titanium alloy processing has always been an urgent problem to be solved.

Reasons for the difficulty of processing titanium alloys

The thermal conductivity of titanium alloy is small, and the cutting temperature is very high during processing. Under the same conditions, the cutting temperature of TC4 is more than twice that of 45 steel, and the heat generated during processing is difficult to release through the workpiece; the specific heat of the alloy is small, and the local temperature rises rapidly during the processing. As a result, the tool temperature is high, the tool tip wears severely, and the service life is reduced.

The low elastic modulus of titanium alloy makes the machined surface easy to spring back, and the machining springback of thin-walled parts is more serious. Strong friction is easily generated between the flank and the machined surface, which wears the tool and chips.

Titanium alloys have strong chemical activity and are easy to interact with oxygen, hydrogen and nitrogen at high temperatures, thereby increasing strength and reducing plasticity. The oxygen-rich layer formed during heating and forging makes machining difficult.

Cutting principle of titanium alloy materials

In the processing of titanium alloys, the selection of tool materials, cutting conditions and cutting time will affect the cutting efficiency and economy of titanium alloys.

1. Select reasonable tool materials

According to the properties, processing methods and processing conditions of titanium alloy materials, the tool materials should be reasonably selected. Tool materials should be more commonly used, less expensive, with good wear resistance, high hot hardness and sufficient toughness.

2. Improve cutting conditions

The stiffness of the CNC machine tool clamping system is better. The clearance of each part of the machine tool is well adjusted, and the radial runout of the main shaft is small. The clamps should be strong enough. The cutting part of the tool should be as short as possible, and the thickness of the cutting edge should be increased as much as possible when the chip volume is sufficient to increase the strength and rigidity of the tool.

3. Appropriate heat treatment of the processed material

The properties and metallographic structure of titanium alloy materials are changed by heat treatment to achieve the purpose of improving material machinability.

4. Choose a reasonable cutting amount

The cutting speed should be low. Since the cutting speed has a great influence on the cutting edge temperature, the higher the cutting speed, the sharper the cutting edge temperature rises. The temperature of the cutting edge directly affects the service life of the tool, so choose the appropriate cutting speed.

Machining Technology

1. Turning

Turning titanium alloy products 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 view of these characteristics, the following measures are mainly taken in terms of tools and cutting parameters:

Tool material: choose YG6, YG8, YG10HT according to the situation.

Tool geometry parameters: appropriate front and rear angles of the tool, and the corners of the tool nose.

The cutting speed is low, the feed rate is moderate, the cutting depth is deep, and the cooling is sufficient. When turning the outer ring, the tool tip should not be higher than the center of the workpiece, otherwise it is easy to stick to the tool. When finishing turning and turning thin-walled parts, the leading angle of the tool should be larger, generally 75-90 degrees.

2. Milling

The milling of titanium alloy products is more difficult than turning, because milling is intermittent cutting, and the chips are easy to bond with the cutting edge. When the sticky teeth cut into the workpiece again, the sticky chips are knocked off and a small piece of tool material is carried away. Debris greatly reduces tool durability.

Milling method: Down milling is generally used.

Tool material: high speed steel M42.

Down milling is generally not used for machining alloy steels. Due to the influence of the clearance between the screw and the nut of the machine tool, when milling, the milling cutter acts on the workpiece, and the component force in the feeding direction is the same as the feeding direction, which is easy to cause intermittent motion of the workpiece table, resulting in tool collision. For down milling, the tool teeth hit the housing at the start of the cut, causing the tool to break. However, since the up-milling chips vary from thin to thick, the tool is prone to dry friction with the workpiece during the initial cutting process, which increases tool sticking and chips. In order to make the titanium alloy milling process go smoothly, it should be noted that the rake angle should be reduced and the clearance angle should be increased compared with ordinary standard milling cutters. The milling speed should be low, try to use sharp-toothed milling cutters, and avoid using spade-toothed milling cutters.

3. Tapping

When the titanium alloy product is tapped, due to the small chip, it is easy to bond with the insert and the workpiece, resulting in a larger surface roughness value and a larger torque. When tapping, improper tap selection and improper operation can easily cause work hardening, extremely low processing efficiency, and sometimes tap breakage.

Skip-tooth taps must be used first, and the number of teeth should be less than that of standard taps, generally 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 evacuation, a negative inclination can also be ground on the cutting cone. Try to choose short taps to increase the rigidity of the taps. The reverse taper part of the tap should be appropriately larger than the standard to reduce the friction between the tap and the workpiece.

4. Reaming

When the titanium alloy is reamed, the tool wear is not serious, and the cemented carbide and high-speed steel reamers can be used. When using carbide reamers, a rigid process system similar to drilling should be used to prevent chips from being generated by the reamer. The main problem with titanium reaming is the poor finish of the reaming. Narrow width whetstone reamer blades must be used to prevent the blade from sticking to the hole wall, but sufficient strength must be ensured. The blade width is generally 0.1 to 0.15mm.

The transition between the cutting edge and the calibration piece should be a smooth arc, and it should be sharpened in time after wear, and the arc size of each tooth must be the same; if necessary, the back taper of the calibration piece can be enlarged.

5. Drilling

Titanium alloy processing is difficult, and the phenomenon of knife burning and drill breaking often occurs during the processing of CNC machine tools. This is mainly due to poor sharpening of the drill bit, delayed chip removal, poor cooling, and poor rigidity of the process system. Therefore, when drilling titanium alloys, we should pay attention to reasonable drill sharpening, increase the apex angle, reduce the rake angle of the outer edge, increase the rear angle of the outer edge, and increase the reverse taper to 2 to 3 times that of the standard drill bit. Take out the tool frequently, take out the chips in time, and pay attention to the shape and color of the chips. During the drilling process, if the chips appear feathery or change in color, the drill bit is blunt, and the tool should be replaced and sharpened in time.

The drill die should be fixed on the workbench, and the guide surface of the drill die should be close to the processing surface. Use as short a drill as possible. Another noteworthy issue is that when using manual feed, the drill should not advance or retreat in the hole, otherwise the drill blade will rub against the machined surface, causing work hardening and dulling the drill.

6. Grinding

A common problem when grinding titanium parts is sticky debris causing wheel clogging and part surface burn. The reason is that the thermal conductivity of the titanium alloy is poor, resulting in a high temperature in the grinding zone, which makes the titanium alloy bond and diffuse with the abrasive and undergo a strong chemical reaction. Sticky chips and clogging of the grinding wheel result in a significant reduction in the grinding ratio, burn the workpiece surface, and reduce the fatigue strength of the part. This is more pronounced when grinding titanium alloy castings.

To solve this problem, the measures taken are:

Choose the right grinding wheel material: Green Silicon Carbide TL. Wheel hardness is slightly lower: ZR1.

The CNC machine tool cutting processing of titanium alloy materials must be controlled from the aspects of tool material, cutting fluid, process parameters, etc., in order to improve the overall efficiency of titanium alloy material processing.