Research On Titanium Alloy Cutting And Machining Process

Titanium alloy is a typical difficult-to-machine material, and its machining characteristics are mainly reflected in the following aspects:

(1) Titanium alloy has high strength and high hardness, so the processing equipment is required to have high power, and the tool should have high strength and hardness.

(2) The contact area between the chip and the front tool face is small, and the tip stress is large.

(3) Titanium alloy has high friction factor and low thermal conductivity. The contact length between the tool and the chip is short, and the cutting heat accumulates in a small area close to the cutting edge, which is not easy to distribute. These factors make the titanium alloy cutting temperature higher, resulting in faster tool wear and affecting machining quality.

(4) Due to the low modulus of elasticity of titanium alloy, the workpiece has a large rebound in the cutting process, which is easy to cause the wear of the tool back face and the deformation of the workpiece.

(5) Titanium alloy is chemically active at high temperatures, and it is easy to react chemically with hydrogen, oxygen and other gaseous impurities in the air to form a hardened layer, which further exacerbates the wear of the tool.

(6) Titanium alloy cutting process, the workpiece material is easy to bond to the tool surface, the cutting temperature is high, so the tool is prone to diffusion wear and adhesion wear.

2, cutting process analysis in the production of pipe fittings

2.1 Tube parts processing technology

Figure 1 is a tube type structure, its material selection TA2M titanium alloy steel pipe, material specifications for φ63 × 3.5 × 130, each blank can be made into a piece.

In the machining process, according to the requirements of the parts, it can be completed by turning and milling. Its machining process specification is shown in Figure 2.

The final molded parts are thin-walled parts. When machining, in order to ensure that the geometric dimensions of the part meet the requirements of use, the machining is not deformed. Therefore, the mandrel must be clamped during turning and milling. In the case of milling, a second clamping is performed. In order to minimize the cut marks after milling, the clearance when the mandrel is clamped should be as small as possible. After milling is completed, knock off the cutter with a knife and sand.

2.2 Selection of tool material

The tool material for machining titanium alloy should have the following properties: chemical stability at high temperatures; sufficient strength and toughness; good thermal conductivity. When processing titanium alloy, the tool material with small affinity to titanium alloy should be selected as much as possible. The tool is made of YG8 cemented carbide material, not YT-type cemented carbide material.YT-type cemented carbide material also contains Ti, so the affinity between the same elements will be generated, and the phenomenon of adhesion will occur. When the cutting temperature is high, the coefficient of friction is higher, which increases the wear of the tool. The geometry of the tool is shown in Table 3.

2.3 Optimization of tool parameters

3.3.1 Turning process

Due to the poor thermal conductivity of titanium alloy, in order to reduce the heat generated by the friction between the back face of the tool and the machined surface, the back angle of the tool should be chosen to be larger. The tool back angle for processing general materials is α0=6°-8°; the tool back angle α0=14°-17°; the front angle is larger, γ0=8°-12°, to ensure that the cutting edge is sharp, reduce the machining deformation, and improve the surface quality; the main deflection angle Kr=90°, to reduce the radial force and prevent vibration; the cutting edge arc rε ≤ 0.8mm is appropriate.

3.3.2 Milling

Three notches are machined on the outer surface of Figure 1. Geometric parameters such as the front angle and back angle of the milling cutter have a great influence on improving cutting accuracy and efficiency and extending tool life. Therefore, the shape of the milling cutter must be improved with specific measures:

(1) Reduce the overhang of the milling cutter. If the overhang of the milling cutter is too large, the rigidity is difficult to ensure. Under the premise of ensuring the normal processing of the workpiece, shorten the overhang as much as possible.

(2) Reduce the chip depth of the cutting edge of the milling cutter. This will increase the service life of the milling cutter.

2.4 Cutting parameter optimization

For cutting processing, it is necessary to ensure both the quality of the parts and the processing efficiency, but also to ensure that the tool has a certain life.

(1) Rough machining of the outer circle, it is necessary to ensure that the tool life, cutting speed v < 80m/min, tool life T = 480min. retracting amount t < 1mm, feed rate S < 0.15mm / r. Rough machining generates increased heat, the flow rate of the coolant is increased.

(2) For the finishing of parts, the main task is to ensure surface quality, machining accuracy and appropriate tool life. Cutting speed v ≤ 80m/min, retraction t ≤ 0.3mm, tool life T = 1080min, feed S ≤ 0.15mm/r.

2.5 Measures to improve the stability of the processing system

In order to obtain satisfactory machined parts and efficiency, the system must be highly stable.

(1) Reduce the overhang of the tool to avoid breaking the tool when it is subjected to a large impact force.

(2) Select a machine with high spindle stiffness to avoid the occurrence of process system chatter caused by high cutting volume.

(3) Select clamping strength fixture to increase the rigidity of the system.

3, parts color difference problem analysis

In actual production, due to parts inventory, parts are usually used in batches. However, there is a slight color difference between the surfaces of the two batches. This seriously affects the quality requirements of the batch product. Through analysis, the main reasons for the color difference are as follows:

(1) Different material batches, due to the titanium alloy is added to the industrial pure titanium alloy elements, the composition of different batches of materials have different differences, processing will appear different color differences;

(2) Titanium alloy after processing is highly active, easy to react with substances in the air, so that the surface of the parts produce different color differences;

(3) Two batches of processing, from different operators, make the surface roughness inconsistent, due to the principle of diffuse reflection of light, high roughness looks brighter, low roughness looks darker;

(4) Due to high lathe speed or sharp tools during processing, turning generates a lot of heat, which can not be emitted in time, and due to the high activity of titanium alloy, it is easy to have chemical reactions with substances in the air, resulting in different color differences on the surface. This is for different batches of materials;

(5) The operator of the two batches of processing, the tool material used, or the geometric parameters of the tool are different, so that the two batches of material surface changes are different, resulting in two batches of different color differences;

(6) After the two batches of parts are machined and left unused for a long time, the titanium alloy will be slightly different. Due to the activity of titanium alloy, it will react with substances in the air, resulting in different color differences on the surface.

4, the method of eliminating color difference

The quality of the surface of the parts directly affects the appearance of the product quality. By observing the surface of the parts of the two batches, the same batch of material processing after the color difference is the same. There is only a slight color difference between different batches. It can be seen that the stability of the same batch of material is better, so the following methods can be used to eliminate the color difference:

(1) for the same batch of products, using the same batch of material processing parts, the use of zero inventory, that is, the parts in this batch of one-time use of processing out, which is also in line with the principles of lean management: to eliminate unnecessary waste.

(2) The parts can not be idle for a long time after processing, and require the same batch to use the same processing method, so that the surface roughness is uniform. This will ensure that the surface of the same batch of parts will have the same color difference.

(3) Select the correct tool material for machining, and require the tool material to have high stability at high temperatures. It must have sufficient strength and toughness. The tool material should also have thermal conductivity. Choose a tool material that has a small affinity for titanium alloys. Generally, YG-based carbide tool materials are used for turning titanium alloys. This material will not react chemically during machining, and the same batch of parts will remain consistent.

(4) Correctly select the tool geometry parameters when machining. Due to the poor thermal conductivity of titanium alloy, in order to reduce the heat generated by the friction between the back face of the tool and the machined surface, the back angle of the tool should be selected larger. The front angle should be larger to ensure that the edge is sharp, reduce the machining deformation, and improve the machined surface quality. The main offset angle is 90° to reduce radial force and prevent vibration. The tool tip arc is not greater than R0.8.

(5) After the tool material and the geometric parameters of the tool are selected, adequate cooling is required to ensure heat dissipation in the cutting zone, which can improve the surface quality of the part, make the surface of the part consistent in color, and extend the service life of the tool.

5 Conclusion

Through the analysis of turning and milling of titanium alloy parts, the batch processing of titanium alloy materials can be realized to meet the requirements of use. The analysis of the color difference problem in production found that the color difference between the two batches of parts processed may be a material problem, or it may be a problem of operator processing, that is, the two batches of parts operators are different. As long as each batch of products are selected from the same batch of materials, processing methods are correct, you can completely eliminate the color difference problem.