From year of 1950, titanium, as a kind of very important structure metal , developed very fast! Because titanium alloy has advantages of High intensity, Corrosion resistant, High temperature resistant, it is used for many areas. Many countries, for example USA, EU, Japan,  realized the importance of titanium, and they have been spending a lot to the titanium researching and developing to apply titanium and titanium alloy to many different industrials. From the 1950s to the 1960s, high temperature resistant titanium and structure titanium is mainly developed for aviation. In the 1970s, corrosion resistant titanium alloy was developed. From 1980s, corrosion resistant and high intensity titanium alloy has been developed a lot.
1. Characteristics of Tiantanium and Titanium Alloy
Titanium alloy density is about 4.51g/cubic centimeter, it is about 60% of steel. Pure titanium density is close to the density of steel. The high intensity titanium alloy is more strength than structral steel alloy. Therefore, the specific strength of titanium alloy is much bigger than other structural matel. Titanium alloy can be used to produce high strength, good ragidity and light weight components and parts.The airplane engine parts, skeleton, skin, fastener and undercart are all made of titanium alloy; Titanium alloy are also used for auto parts, medical industry, electronical (3C), and some other areas.
2.  The heat conductivity coefficient of titanium is 15.24W/(m.K), it is about a quarter of nickle, 1/5 of metal, 1/14 of aluminium. Titanium alloy heat conductivity coefficient is about 50% lower than pure titanium. Titanium alloy has a half of elasticity modulus than steel, so the regidity of titanium alloy is not good,  and it is easy to be deformation, titanium alloy is not good for making slender bar, or thin-wall parts.The resilience is big when titanium alloy is machining on its surface, the resilience is about 2-3 times than stainless steel, this is not good to cutter, can take a lot of friction and felted wearing to cutter.
3. Titanium Alloy Cutting Properties
If the titanium alloy hardness is more than HB350, cutting and machining is very difficult;  If the hardness is less than HB350, it is easy to have sticking phenomenon, cutting and machining is also difficult. About difficult cutting and machining for titanium alloy, hardness is one reason, the other important reason is that the titanium alloy has its own physical and chemical property, that also makes the cutting and machining difficult.
Titanium has below cutting and machining properties:
1>. Deformation Coefficient is small.
This is the prominent feature of titanium alloy cutting and machining, deformation coefficient is less than or equal to 1. Cutting friction is very big,
speed up cutter wearing.
2>. High Cutting Temperature
Because the titanium alloy thermal conductivity coefficient is very small( amount to 1/5 – 1/7 of 45# steel), the cutting fragment leaves the cutter very fast,.
so the cutting fragment can not sink much heat, the temperature is very high!  The cutting temperature is more than 2 times higher than cutting 45# steel.
3>. Cutting force is big on unit area
Cutting force is about 20% smaller than cutting steel, but because the cutting fragment leaves the cutter very quickly, cutting force on unit area is very big,
it is easy to have the blade cracking. At the same time, as elasticity modulus of titanium alloy is very small, it is easy to bend or out of shape, these can
cause shocking, the shocking can speed up cutter wearing. Therefore, the machining system must be with very good rigidity.
4. Cold hard phenomenon is serious
Because the chemical activity of titanium alloy is big, titanium alloy is easy to react with Oxygen and Nitrogen to form a hard and crisp surface when it is under machining with high temperature. At the same time, the plastic deformation that occured during processing can cause surface hardening. The cold hard phenomenon can decrease the parts fatigue strength, and it can increase the cutter wearing as well.  This is an important feature of cutting titanium alloy.
5. Cutter Wearing Easily
After stamping, forging, hot rolling, titanium alloy workblank will have a surface that is hard crisp and not even, very easy to have cutter blade broken, peeling off the hard surface is the most difficult work among the titanium alloy machining. In addition, because the titanium alloy has a very strong appetency to the cutter, if the cutting temperature is high, and the cutting force is big on the unit area, the cutter is easy to have adhesive wearing. When cutting titanium alloy by turning tool, sometimes the front turning tool has a more serious wearing than the back one. If f<1mm/r, the wearing is mainly on the back turning tool; If f>0.2mm/r, wearing is mainly on the front turning tool. If cutting with hard alloy cutter on precision lathe or half precision lathe, the appropriate wearing for back turning tool is VBmax<0.4mm.
During titanium alloy mitting processing, because the titanium thermal conductivity coefficient is low, and the cutting frament has a very shot contaction with front turning tool, the heat from turning process can’t sink efficiently, the turning tool will have a very high temperature, this high temperature can reduce the cutter life a lot.
Regarding titanium alloy Ti6AI4V, if the strength of the cutting tool and machine power is good enough, the cutting temperature is the key factor to the life of the cutter, but not cutting force.
3. Titanium alloy cutter tool material selection
When we select a titanium alloy cutter, we should consider that the material of the cutter can reduce the cutting temperature and bonding, select and use high strength, good bending strength, high thermal conductivity, and better appetency to titanium alloy cutter tool material. YG type hard alloy is a good choice. Because the heat resistent of the high speed steel is not good, we should use the hard alloy material cutter. The common materials for the hard alloy cutter has YG8, YG3, YG6X, YG6A, 813, 643, YS2T and YD15.
Coating cutter and YT type hard alloy cutter has very good appetency to titanium alloy, can intensify cutter adhesive wearing, so they are not good for cutting titanium alloy; About complex and multi cutting edge tool, we can select and use high-vanadium high speed steel, (For example W12Cr4V4Mo), high-cobalt high speed steel(for example W2Mo9cr4VCo8), or aluminium high speed steel(for example W6Mo5Cr4V2AI,M10Mo4Cr4V3AI) for cutter tool materials, these materials are good for making drill, reamer, end mill, broach, screw tap etc. There are all googd for cutting titanium alloy.
4. Titanium Alloy Cutting Notes
1)  Because titanium alloy elasticity modulus is small, workpiece clamp deformation and laoding deformation is big during processing, this can reduce the precision of the workpiece. The clamp force of the work piece shold not be too much, assistant tool is needed if it is necessary.
2)  If the cutting fluid contains hydrogen, the hydrogen mabye released by the high temperature during cutting, the hydrogen maybe absorbed by titanium and cause hydrogen embrittlement; the hydrogen maybe also can cause titanium alloy corrosion craze in the high temperature.
3) The chloride in the cutting fluid sometimes can be resolved or volatilize poisonous gas, safegard should be used during processing. After cutting, the parts should be washed by no chlorine clearners immediately.
4) Workpiece and fixture that made of lead or zinc base alloy is prohibited to contact with titanium alloy, as well as copper alloy, tin alloy and cadmium alloy.
5)  All the wrokpieces, fixtures or any other devices that contacts with titanium alloy must be clean. The clean titanium parts must be take care to prevent contaminated by oil or by fingerprint, otherwise, the contamination may cause  titanium alloy part salty stress corrosion.
6)  Generally, there is no ignition hazard during titanium alloy cutting processing, but for micro cutting, the small cuttings maybe on fire. In order to avoid fire disaster , except for pouring a lot of cutting fluid, the small cutting fragement also need to be removed. change the cutter if the cutter is blunt, or reduce the cutting speed. However,
if there is a fire happened, we should use talcum powder, limestone flour, sand, or other thing to put out the fire. Do not use carbon tetrachloride, carbon dioxide fire extinguisher, or water, because water can speed up burning, even cause exploding.
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