Quality control of CNC milling for rounded corners of thin-walled titanium alloy parts
Titanium alloy is widely used in the aerospace field because of its high specific strength, good thermal stability, strong corrosion resistance and many other advantages. However, due to the need of weight reduction, titanium alloy parts are mostly designed for thin walls, which brings great problems to machining, especially the quality of rounded CNC milling is difficult to control. The radial cutting depth and the real feed have sudden changes in the process of the round corner cutting from the straight section to the curve section, and the change has great influence on the cutting force, which makes the machining process can not proceed smoothly. The cutting force of cavity fillet has been studied in related literature.
General method of CNC milling for rounded corners
Now the most used round corner milling is equal radius tool method. For example: the final round corner of R6 will be processed, generally using a φ20mm rigid tool to process the side wall to the final size or leave a small amount of finishing allowance, and then respectively change the smaller tool to round the corner, the number and size of the required tool according to the actual situation. The advantage of the equal radius tool method is that most of the metal materials are removed through the large diameter tool with better rigidity, and the part processed by the weak tool is only the smaller part at the rounded corner, avoiding the processing deformation and cutting vibration phenomenon of the direct use of the slender tool in the case of large cutting amount. This method solves the broach problem of rounded corner machining to a certain extent, but frequent tool change and tool matching will cause problems such as prolonged processing cycle, and more importantly, this method does not completely solve the processing problems such as broach and vibration.
Analysis of the difficulties in CNC milling of thin-walled titanium alloy parts
Thin-walled titanium alloy parts are widely used in the aerospace field, with side walls and web are typical characteristics of such parts, complex structure, relatively low stiffness, so the processing performance is poor. Machining deformation and low machining efficiency have become important problems restricting the machining of thin-walled titanium alloy parts. Under the action of cutting force, clamping force, cutting chatter and other factors, the machining deformation is easy to occur, especially the machining quality and precision at the rounded corner are not easy to control. The research found that after the cutting tool is cut into the rounded corner from the straight edge, there is an obvious excess cutting force mutation phenomenon at the rounded corner, often undercut, overcut, chatter and other phenomena, so there are obvious overcut, undercut marks or vibration marks, which not only seriously affects the processing quality of the parts, reduces the service life of the tool, but also reduces the production efficiency. There are the following processing difficulties in CNC milling of thin-wall parts of titanium alloy.
(1) The thermal conductivity of titanium alloy is low, and the thin-walled parts are deformed by heat before and after rounded processing.
The thermal conductivity of titanium alloy material is only 1/15 of aluminum and aluminum alloys, 1/5 of steel, and less than stainless steel and superalloy. Low thermal conductivity makes titanium alloy thin-walled parts produce large temperature difference and thermal stress in the cutting process, resulting in cutting heat is not easy to dissipate, resulting in tool bond wear, thin-wall heat deformation, this phenomenon is particularly serious when rounded CNC milling.
(2) The stress of the blade part in the cutting process of titanium alloy is large, which affects the fillet processing quality of thin-walled parts.
Although the cutting force of titanium alloy is only 1/3 to 1/2 of 45 steel, the contact area of titanium chips and the front cutter surface is small, only 1/2 to 2/3 of 45 steel, the cutting force on the unit contact area is greatly increased, so the stress on the cutting edge is 1.3 to 1.5 times that of carbon steel, and the stress concentration of the cutting edge is very easy to cause the phenomenon of falling edge. In the process of rounded CNC milling, the numerical control program generally adopts a constant feed speed, but the actual instantaneous feed speed has a sudden change, and the tool is more likely to break the edge at the rounded corner of the cutting titanium alloy thin-wall parts, once the tool breaks the edge, it is likely to scratch the machining surface, seriously affecting the processing quality.
(3) Titanium alloy has high chemical activity during high temperature cutting, which greatly reduces the fatigue strength of thin-walled parts.
The chemical activity of titanium alloy is large, under the conditions of high cutting temperature and large cutting force per unit area, it is easy to absorb oxygen and nitrogen in the air to form a hard and brittle skin, while the cutting process will produce plastic deformation, will also form a very hard oxide layer, resulting in surface hardening, due to the sudden change of cutting force in round corner CNC milling, and the instantaneous cutting amount is large. The resulting cutting temperature is also relatively high. The research shows that the fatigue strength at the rounded corners is greatly reduced under this chilling phenomenon, and there are huge safety risks in the future use of parts.
(4) Sudden change of cutting force in CNC milling of thin-walled titanium alloy parts.
For the rounded corner machining of thin-walled titanium alloy parts, the general programmer will consider the use of equal radial cutting, that is, the radial cutting depth is a certain value in the process of a cutting tool, when the tool transitions from straight cutting to circular cutting, the contact area between the tool and the workpiece increases due to the increase in the cutting Angle, which causes the excess mutation of the cutting force and is easy to induce cutting vibration. It will also cause the machining deformation of the tool and the workpiece to increase or broach the phenomenon, and the cutting vibration will produce vibration at the rounded corner, affecting the processing quality of the part. When cutting with the same radius of the round corner, the cutting area is much larger than when cutting in a straight line, which will cause a sharp increase in cutting force. In addition, when machining small radius rounded corners of deep cavities, the use of slender tools with relatively large diameters and the reduction of the rigidity of thin-walled parts are also one of the reasons for inducing cutting vibration.
The instantaneous cutting force can be obtained by substituting the instantaneous radial cutting depth and instantaneous real feed into the empirical formula of cutting force. The relevant literature has carried out in-depth research on this, and confirmed that the sudden change of cutting force will occur in the process of round corner CNC milling.
