Abstract: Titanium alloy has excellent properties such as low density, high strength and high temperature resistance. It has been widely used in aerospace and other fields, but its application is difficult due to its difficult processing, low cutting efficiency and short tool life. In this paper, through the analysis of the characteristics and cutting performance of titanium alloy materials, through the examples in production, the machining research of the turning and milling process was carried out, and the problem of color difference in different batches of material processing parts was analyzed. .
Key words: titanium alloy, example, color difference analysis
Titanium alloy is a typical difficult-to-machine material, and its processing 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 cutter should have high strength and hardness.
(2) The contact area between the chip and the rake face is small, and the tool tip stress is large.
(3) Titanium alloy has large friction factors and low thermal conductivity. The contact length of the tool with the chip is short, and the cutting heat accumulates in a small area near the cutting edge and is not easily dissipated. These factors make the titanium alloy have a high cutting temperature, which causes the tool to wear faster and affect the processing quality.
(4) Due to the low elastic modulus of the titanium alloy, the workpiece rebounds greatly during the cutting process, which tends to cause the wear of the tool flank and the deformation of the workpiece.
(5) Titanium alloy has high chemical activity at high temperature, and it is easy to chemically react with gas impurities such as hydrogen and oxygen in the air to form a hardened layer, which further aggravates the wear of the cutter.
(6) In the titanium alloy cutting process, the workpiece material is easily bonded to the surface of the tool, and the cutting temperature is high, so the tool is prone to diffusion wear and adhesive wear.
2. Analysis of cutting process for tube parts in production
2.1 Processing technology of pipe parts
Figure 1 is a structural view of a pipe type, the material of which is selected from the titanium alloy steel pipe of TA2 M. The material specification is φ63×3.5×130, and each blank can be made into one piece.
In the process of machining, according to the requirements of the part, it can be completed by turning and milling. The process specification for its processing is shown in Figure 2.
The final forming of the part belongs to the thin-walled part. When processing, in order to ensure that the geometrical size of the part meets the requirements of use, it is not deformed during processing. Therefore, the core rod must be clamped during turning and milling. In the case of milling, the second clamping is performed. In order to reduce the cutting marks after milling, the gap should be as small as possible when performing mandrel clamping. After the milling is completed, the knives and sand are used to knock off the knives.
2.2 Selection of tool materials
Tool materials for processing titanium alloys should have the following properties: chemical stability at high temperatures; sufficient strength and toughness; good thermal conductivity. When processing titanium alloys, the tool material with a small affinity to the titanium alloy should be selected as much as possible. The tool is made of YG8 cemented carbide material, and YT type hard alloy material is not used. The YT type hard alloy material also contains Ti, so that the affinity between the same elements will occur and the sticking phenomenon will occur. When the cutting temperature is high, The large friction coefficient increases the wear of the tool. The geometry of the tool is shown in Table 3.
2.3 Tool parameter optimization
3.3.1 Turning processing
Due to the poor thermal conductivity of the titanium alloy, in order to reduce the heat generated by the friction between the flank and the machined surface of the tool, the back angle of the tool should be selected to be larger. The back angle of the tool for processing general materials is α0=6°-8°; The back angle of the tool is α0=14°-17°; the front angle is larger, γ0=8°-12°, to ensure sharp edge, reduce machining deformation and improve the surface quality; the main declination Kr=90°, Reduce the radial force and prevent vibration; the cutting edge arc rε≤0.8mm is suitable.
Three notches are machined on the outer surface of Fig. 1. The geometric parameters such as the rake angle and the back angle of the milling cutter have a great influence on improving cutting accuracy and efficiency and prolonging tool life. Therefore, we must improve the shape of the milling cutter, 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 guarantee. Under the premise of ensuring the normal processing of the workpiece, the overhang is shortened as much as possible.
(2) Reduce the chip depth of the cutting edge of the milling cutter. This will increase the life of the milling cutter.
2.4 Optimization of cutting parameters
For the cutting process, it is necessary to ensure the quality of the parts and the processing efficiency, and to ensure that the knife has a certain life.
(1) For the roughing of the outer circle, the service life of the tool must be guaranteed, the cutting speed is v<80m/min, and the service life of the tool is T=480min. The amount of back-feeding knife is t<1mm, and the feed rate is S<0.15mm/r. The amount of heat generated by roughing is increased, and 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 proper tool life. The cutting speed is v≤80m/min, the amount of back-feeding knife is t≤0.3mm, the service life of the tool is T=1080min, and the feed rate is S≤0.15mm/r.
(3) In the milling process, in order to improve the service life of the tool, in addition to reducing the cyclic stress of the milling cutter, the amount of backing and the thickness of the cutting should be increased. At the same time, the milling speed has a great influence on the surface roughness of the part. As shown in Fig. 4, as the milling speed increases, the crushing and shedding of the reinforcing particles increases during the milling process, so that the milling force increases the milling vibration and the surface of the workpiece. The roughness also increases.
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 subjected to large impact forces.
(2) Select a machine tool with a large spindle stiffness to avoid the occurrence of chattering in the process system caused by large cutting amounts.
(3) Select a fixture with a large clamping strength to increase the rigidity of the system.
3. Analysis of the problem of color difference in parts
In the actual production, due to the inventory of parts, there are usually cases where parts are used in batches. However, there is a slight chromatic aberration on the surface of the two batches. This seriously affects the quality requirements of batch products. Through analysis, the main reasons for the chromatic aberration are as follows:
(1) Different batches of materials, because titanium alloy is added with alloying elements in industrial pure titanium, the composition of different batches of materials has different differences, and there will be different color differences during processing;
(2) After processing, the titanium alloy has high activity and is easy to react with substances in the air to cause different chromatic aberrations on the surface of the part;
(3) Two batches of processing, from different operators, make the surface roughness inconsistent, due to the principle of diffuse reflection of light, the roughness is high to look brighter, and the roughness is low, it seems darker ;
(4) Due to the high rotation speed of the lathe or the sharpness of the cutter during machining, a large amount of heat is generated during turning, which cannot be dissipated in time, and because the titanium alloy has high activity, it is easy to chemically react with substances in the air, resulting in different chromatic aberration on the surface. This is for different batches of materials;
(5) The operator of the two batches of machining, the tool material used, or the geometric parameters of the tool are different, so that the surface of the two batches of materials changes differently, resulting in different color difference between the two batches;
(6) After the two batches of parts are processed, they will be left unused for a long time, and the titanium alloy will be slightly different. Due to the activity of the titanium alloy, it will react with the airborne substances, causing different chromatic aberrations on the surface.
4, the method of eliminating chromatic aberration
The quality of the surface of the part directly affects the appearance quality of the product. By looking at the surface of the parts of the two batches, the color difference of the same batch of materials is the same after processing. It is only a slight color difference between different batches. It can be seen that the stability of the same batch of materials is good, so the following methods can be used to eliminate the chromatic aberration:
(1) For the same batch of products, the same batch of materials is used to process the parts, and the zero-inventory method is adopted, that is, the parts are processed out of one-time use in this batch, and this is also in line with the principle of lean management: eliminating unnecessary Waste.
(2) After the parts are processed, they cannot be left unused for a long time, and the same batch is required to use the same processing method to make the surface roughness uniform. This will ensure that the surface of the same batch of parts will have the same color difference.
(3) Selecting the correct tool material during processing requires high stability of the tool material at high temperature. It must have sufficient strength and toughness. The tool material should also have thermal conductivity. Choose a tool material with a small affinity for titanium alloy. In general, YG-based carbide tool materials are used when turning titanium alloys. This material does not undergo a chemical reaction during processing, and the same batch of parts will remain consistent.
(4) The geometric parameters of the tool are correctly selected during processing. Because the thermal conductivity of the titanium alloy is poor, in order to reduce the heat generated by the friction between the flank and the machined surface of the tool, the back angle of the tool should be selected larger. The front corner should be larger to ensure sharp edges, reduce machining distortion and improve the quality of the machined surface. The main declination is 90° to reduce the radial force and prevent vibration. The arc of the tool tip is not more than R0.8.
(5) After the geometric parameters of the tool material and the tool are selected, sufficient cooling is required to ensure the heat dissipation in the cutting zone, which can improve the surface quality of the parts, make the surface color difference of the parts consistent, and extend the service life of the tool. .
Through the analysis of the turning and milling of titanium alloy parts, the batch processing of titanium alloy materials can be realized and the requirements for use can be met. Analysis of the chromatic aberration problems in production found that the chromatic aberration after processing of the two batches of parts may be a material problem or an operator's processing, that is, the two batches of parts operators are different. As long as each batch of products is selected from the same batch of materials and the processing method is correct, the chromatic aberration problem can be completely eliminated.