With the rapid development of the aviation industry, our company currently uses a large number of high-tech and high-tech materials in production, especially in the processing of A320 various new materials and new parts are constantly increasing. In processing, such as high-hardness parts and high-tech material parts, when the fitter cannot guarantee the accuracy of the parts due to materials, tools, and accuracy, it seriously affects the smooth progress of scientific research and production. With the wide application of high-tech materials, combined with existing equipment and technology, the use of existing tool materials, and the use of new processing methods have been fully utilized in mechanical processing. Kaiyite New Materials has also produced many new processing techniques and processing methods under the existing production conditions through exploration. Improvements can not only improve product quality, increase production efficiency, and reduce production costs, but also reduce labor intensity of workers. And processing time, so that there is a solution to the technical problems in the processing of such parts.
1 Part structure and processing difficulties
1.1 Status analysis of parts
This part is a part of an Airbus model. From the appearance and structure of this part, the processing is not complicated. The appearance and main processing dimensions of the part are shown in Figure 1. The material of the part is specified in the drawing as T-A6V (annealed) ASNA3304, which is what we often call TC4 material.
Figure 1 Part shape and main processing dimensions
According to the previous analysis of the parts, it can be known that the processing difficulties of this part are mainly as follows:
①The material is titanium alloy T-A6V which is difficult to process;
②The position of the parts is required to be high, and the perpendicularity to the bottom surface and the position of the hole are required to be 0.05mm; ③The tolerance of the distance between the two holes is strictly required, and the requirement is ±0.05mm; ④The accuracy of the hole itself is required to be high, All are 7 grades of accuracy, respectively φ9.525H7 (0.015/0) and φ6.5H7 (0.015/0).
1.2 Material characteristics analysis
Titanium alloy is a typical difficult-to-process material with high specific strength, poor thermal conductivity, and serious work hardening; due to its high chemical activity, it is prone to chemical reactions with surrounding media at a certain temperature, resulting in a brittle and hard outer skin. Plasticity and impact toughness drop drastically, tool wear and damage are serious, durability is low, machining accuracy and surface quality are difficult to guarantee, and machining efficiency is very low.
Among the many properties of titanium alloys, the thermal conductivity, elastic modulus, work hardening, chemical activity, alloy type and microstructure play a major role. Therefore, the cutting of titanium alloys has the following limitations:
(1) The thermal conductivity of titanium alloy is low, about 1/3 of that of iron, which hinders the dissipation of heat generated in the machining process, and its low specific heat makes the temperature rise in the cutting zone too fast. The formation of an oxidized hard layer accelerates the wear of the tool; (2) The low modulus of elasticity of titanium alloy makes the machined surface easy to produce great net elasticity, especially the processing rebound of thin-walled parts is more serious, which is easy to cause back knife The surface and the machined surface produce strong friction, thereby wearing the tool and chipping. And under the action of resilience, the parts will deviate from the tool during machining.
(3) The hardness of titanium alloy is low, and the chemical activity is very strong, which leads to the occurrence of seizure between titanium and the tool. In addition, titanium easily interacts with oxygen, hydrogen, and nitrogen at high temperatures, increasing its hardness and decreasing plasticity. It is difficult to machine the oxygen-rich layer formed during heating and forging.
1.3 Traditional processing technology
Due to the fact that there are few titanium alloy parts in our unit, there is a lack of research on materials, corresponding processing technology, etc., using traditional processing methods, as shown in Figure 1, processing φ9.525H7 (0.015/0), φ6.5H7 ( 0.015/0) The two holes are processed by fitters relying on assembled drill dies, and the corresponding size requirements are guaranteed. As the assembly fixture has 99% versatility and interchangeability, it also brings some drawbacks to the assembly fixture. Due to the extremely poor material processing performance of such parts, and the combined assembly drill die is composed of multiple parts, the rigidity of this type of fixture is poor. During processing, the components between the assembly fixtures will produce small relative displacements, resulting in the parts The relative position is guaranteed. In addition, there are many dead corners of this combined fixture, iron filings are not easy to clean, and chip removal is difficult, which is also an important reason for the out-of-tolerance and scrapping of parts.
In view of the above problems, this article closely integrates the production of titanium alloy parts, focusing on research and discussion of titanium alloy material processing methods and practical technologies, including the design of special fixtures, the reasonable selection of tool geometry, and the optimization of cutting parameters.
2 Design of special fixture
In order to improve the quality of parts, increase productivity, and solve the processing bottleneck of our branch on titanium alloy parts, the following improvements have been made to the processing method.
According to the characteristics of the parts, the special drill clamps are designed and manufactured by ourselves. The main purpose of self-made drill clamps is to ensure the accuracy of the shape and position tolerance of the processed parts, improve the rigidity of the processing fixture, and increase the clamping speed.
In order to increase the clamping speed, the clamping and positioning considered when designing the fixture must be simple and reliable, so the positioning method of two sides and one pin is adopted; in order to ensure the accuracy requirements of the shape and position tolerance of the processed parts, the fixture positioning datum must be consistent with the design of the part The benchmarks coincide; to improve the rigidity of the machining fixture, the overall structure is adopted when designing the fixture, and the drill and reamer should be as close as possible to the part and the fixture when processing the part. The designed drill clamp is shown in Figure 2:
Figure 2 Designed fixture
The selected material of the clamp is 30CrMnSiA, and the clamp is made of a square steel plate with a length, width and height of 100×75×60, and a 48×27 slot is milled in the middle. The part is installed in the middle of the groove, the workpiece is clamped by the pressing plate and the screw, and two holes with an outer diameter of φ18 (+0.02/0) are made on the drill clamp according to the size requirements of the drawing, and the outer diameter is φ18 (0/-0.01) )Six drill sleeves with inner diameters of φ9.2, φ9.4, φ9.53, φ6.2, φ6.4, φ6.5, so that it can be completed by replacing the drill sleeve more quickly in the case of one clamping The two-hole rough drilling, rough reaming, and fine reaming processes ensure the size requirements of the parts. The characteristic of the drill clamp is that the number of clamping times is small, the positioning error caused by multiple clamping of the parts is avoided, it is difficult to ensure the requirements of the size and position of the parts, and the auxiliary processing time is saved, and the production efficiency is improved.
3 Analysis and improvement of the geometric parameters of the tool
The processing characteristics of titanium alloy determine that there are many problems in the traditional standard twist drill drilling and processing of titanium alloy, mainly from the following aspects:
(a) The tip angle of the drill is small, the cutting edge is long, and the cut chips are wide, so the torque of the drill is large, and the axial resistance is also large. At the same time, the chip curls into a spiral shape to a large extent, and the space occupied by the chip is also large, and the chip evacuation is not smooth, which affects the cooling.
The top angle of the drill bit determines the width of the chip and the size of the rake angle of the drill bit. When the drill diameter and feed rate are constant, increasing the top angle will narrow the chip and reduce the load on the unit chip edge. At the same time, the nose angle at the outer circle of the drill is reduced. The wear rate of the tool nose angle is reduced, and at the same time, it is beneficial to heat dissipation and the durability is also improved. The apex angle has a great influence on the rake angle, and the corresponding increase in the apex angle is beneficial to improve the cutting conditions at the core. The top angle affects the direction in which the chips flow out. The apex angle is larger, the degree of the chip curling into a spiral is reduced, and the chip is relatively straight and easy to remove, that is, the chip removal performance is improved. Through analysis and experimentation, when processing titanium alloys, the method of increasing the tip angle of the drill is adopted. Generally, the drilling effect is better when the tip angle is in the range of 135°～140°.
(b) The thickness of the drill core is small. Because the drill bit bears a lot of torque and axial resistance when drilling titanium alloy. If the core thickness is small, the strength of the drill bit will be low, especially for small diameter drill bits, the drill bit is prone to breakage, and the core thickness needs to be increased to increase the strength of the drill bit. Therefore, the core thickness should be increased appropriately. The core thickness is generally K=(0.45-0.32)D, where K is the core thickness and D is the diameter of the drill bit.
(c) The helix angle of the drill bit is small. The helix angle directly affects the rake angle of the main cutting edge. The larger the helix angle, the sharper the cutting edge and the lighter the cutting, otherwise it will cause serious work hardening and make the blade wear quickly. From the shape characteristics of the twist drill, it can be seen that the helix angle of each point on the cutting edge changes. The closer to the outer circle, the larger the helix angle, the larger the rake angle, the sharper the cutting edge, and the better the cutting performance. The helix angle near the drill core is the smallest and the cutting performance is the worst. You can grind it into a spherical arc shape to improve the cutting conditions. As the helix angle increases, the cutting edge strength weakens, wears quickly, and even the cutting edge burns. Therefore, a reasonable choice of helix angle to suit titanium alloy drilling has become a key issue.
(d) The clearance angle at the outer edge of the drill is small, which affects the rake angle of the cutting edge at the drill core. The relief angle at each point of the cutting edge of the drill is also different, the closer to the center, the larger the relief angle. Therefore, the marking and requirements of the drill bit’s clearance angle are based on the outer edge of the drill bit. Increasing the drill bit’s clearance angle at the outer edge can sharpen the cutting edge and improve the cutting performance, especially for the drilling process at the core. . Therefore, it is very important to appropriately improve the geometric parameters of the drill bit to suit the drilling process of titanium alloy.
(e) Selection of drill bits: For drills with a diameter greater than 5mm, it is best to use cemented carbide YG8 as the tool material; when machining holes less than 5mm, a high-speed steel drill with a hardness greater than 63HRC (such as M42 or B201) can be used; when the hole depth is less than the diameter When doubled, use a chute (short type) drill; when the hole depth is greater than twice the diameter, use a twist drill. The geometric parameters of the drill: λ=0°～3°, αc=13°～15°, 2φ= 120°～130°.
In order to facilitate the formation of chips, reduce friction and improve the cutting ability of the drill, the width of the guide land can be reduced to 0.1～0.3mm according to the diameter of the drill, the chisel edge can be sharpened to 0.1D, and the double sharpening angle 2φ=120°～130 °, 2φ=70°～80°. Table 1 lists the geometric parameters of the dual-angled drill bit, and Figure 3 is a typical dual-angled drill bit for drilling titanium alloys.
Figure 3 Double-edge angle drill bit
Table 1 Geometric parameters of dual-angle drill bits
4 Selection of cutting amount
Choosing an appropriate feed rate is also very important in titanium alloy processing. In the drilling process of titanium alloy, a lower cutting speed should be used to avoid excessive cutting temperature; the feed rate should be moderate, and the feed rate is too large to cause blade burns. According to practical experience, the feed rate is usually f=0.05mm/r～0.15mm/r, and the cutting speed=10～30m/min.
5 Selection of cutting fluid
In order to increase the heat dissipation speed, it is advisable to use water-based cutting fluid. Now use Telijie C008 synthetic grinding fluid. This cutting fluid has the advantages of good fluidity, fast heat dissipation and reduced friction. After using the cutting fluid, the smoothness is improved, the cutting is smooth, and the size is stable.
Through the above improvements, the production efficiency in parts processing has been greatly improved, the service life of the tool is prolonged, and the clamping is stable, fast and accurate. Product quality has made a qualitative leap.
Link to this article：Exploration of Process System for Machining Titanium Alloy Parts
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