The special properties of titanium alloys make it more and more widely used. The high strength/weight ratio, excellent toughness and excellent corrosion resistance make titanium alloys useful for making medical human implants, golf club heads, and military armors.
However, titanium cnc turning parts is very difficult, making mechanical technologists intimidated. They think that the super performance of titanium alloy greatly weakens its machinable “ability”, making it extremely challenging to cut. Although this view is reasonable, it is not comprehensive. This article discusses the turning strategy of titanium alloys, which will be helpful for mechanical technologists to process this kind of difficult-to-machine materials with a wide range of uses (such as racing parts, underwater breathing devices, etc.) and to apply new cutting technologies.
Although many processing workshops regard titanium alloy processing as a daunting way, in fact, titanium alloys include a wide range of varieties, and you must know which one you are processing. There are many grades of titanium alloy, some of which are extremely difficult to process, while others are not. Commercial pure titanium (CP grade) is a non-alloy material, usually used in the manufacture of medical parts, heat exchangers and spectacle frames. CP grades have excellent corrosion resistance and are relatively easy to process. But compared with other titanium alloys, its strength is very low, and it is sticky and soft.
After adding an alloy to pure titanium, its crystal phase (crystal structure) is changed, and the properties and machinability of the material are also changed. Alpha titanium alloys and quasi-alpha titanium alloys contain additives such as nickel, aluminum, and vanadium. The machinability of these intermediate grade titanium alloys is quite good. Alpha-beta titanium alloy grades may contain more aluminum and vanadium. The mainstream industrial titanium alloy Ti6Al4V is an α-β titanium alloy grade, which contains about 6% aluminum and 4% vanadium. Ti6Al4V and its variants account for about 50%-70% of the titanium alloys currently used.
β-grade titanium alloy with iron and chromium added is one of the most difficult grades to process. Due to its high fracture toughness and excellent resistance to high cycle fatigue, the machinability of β grades is similar to Hastelloy nickel-based alloys and similar materials. A typical application example is the manufacture of light springs for triggering underwater launch of tactical missiles. Foldable tail wing.
Various titanium alloys show different cutting performance. Some people think that the time required to process a Ti6Al4V workpiece is usually three times that of processing a steel part; while some people say that the time required to process a Ti5553 hard-to-machine β brand workpiece is twice that of a Ti6Al4V workpiece.
In turning processing, the most important feature of titanium alloy is poor thermal conductivity. Because the high temperature generated during cutting is difficult to be absorbed by the workpiece and concentrated on the cutting edge of the tool, excessive heat promotes a chemical reaction between the cutting edge and the chips and produces crater wear.
Titanium alloy also has a work hardening tendency, so it is very important to remove metal by shearing rather than extrusion. In addition, although titanium alloy has high strength, it also has a low modulus of elasticity, which means that compared with other materials, titanium alloy is relatively more elastic and easier to leave the cutting edge (especially in light-load cutting Time). Comprehensively considering these characteristics of titanium alloys, in order to successfully realize the turning of titanium alloys, the key is to achieve the balance of cutting speed, feed rate and cutting depth.
Cutting speed is the primary factor affecting cutting heat generation. Stefan Gyllengahm, a turning expert at Sandvik Coromant, spent three and a half years developing tool grades for tool manufacturers. During this period, he conducted Ti6Al4V cutting tests in the laboratory, and the results showed that the choice of cutting speed must be very careful: in some cases, when the cutting speed increases by 10%-15%, the tool life will decrease from 40 pieces It is 6 pieces, which indicates that the adjustment range of the cutting speed is too large. He also found that when turning at a cutting speed that does not shorten the tool life, if the feed rate is increased, it will reach a critical temperature that impairs the tool life, due to the existence of an excessive heat limit.
The geometry of the tool plays a key role in controlling the shape of the chip to dissipate heat. The wider and thinner chip enlarges the contact area between the chip being formed and the cutting edge, thereby reducing the accumulation of heat on the cutting edge. If the chip is thinner and generates less heat, the cutting speed can be faster.