Titanium is one of the abundantly available materials in nature. Pure titanium has excellent corrosion resistance with low strength and is generally used for cryogenic applications. Very rarely pure titanium is used in engineering applications. Titanium alloys are more commonly used. Some of the examples in aerospace are components of jet engines such as combustion chambers, exhaust nozzles, blades of compressors and turbines, aircraft fuselage, etc. Because they cannot be exposed to temperatures higher than 595°C, their use in gas turbine engines is limited to cooler side such as the compressor, casing, high pressure blades and rotors. Some applications such as for structural components of aircrafts, a large amount (up to 90%) of material need to be removed to achieve the required strength to weight ratio. Another important application is for medical implants because of its excellent biocompatibility.
Titanium alloys have two types of crystal structures alpha (á) and beta (â). Pure titanium below 882°C retains alpha structure, while above it changes to beta, which is an allotropic transformation. This transformation temperature is raised by some alloying elements, such as aluminium, gallium, oxygen and nitrogen, while elements such as vanadium, molybdenum, niobium, iron, chromium and nickel lower the transformation temperature.
The most common alloying elements used are aluminium and vanadium. Alpha alloys contain a very small quantity of 1 to 2% beta stabilisers and are good for aerospace applications. With larger (4 to 6%) percentages of beta stabilisers alpha-beta alloys are formed, which can be used at a lower temperature in the range of 350 to 400°C. The most commonly used engineering material Ti-6Al-4V is part of this group. With higher percentages (10 to 15%) of beta stabilisers, beta alloys are formed, which will have high strength and toughness and are used for structural parts in aerospace applications.
Titanium machining principles
The titanium alloys are not very hard, but do not have good machinability. Titanium alloys are considered as hard-to-cut materials because of the very properties that make them suited for their wide applications. As a result of the poor thermal conductivity (about 15 W/m °C) of titanium, the heat generated in the machining zone is not easily dissipated. As high as 80% of this heat may get into the cutting tool, thereby, increasing the temperature in the machining zone to as high as 1100°C. This very high temperature in the machining zone is not good for the life of the cutting tool.
In view of its low modulus of elasticity titanium experiences higher strain for a given force compared to steels. This makes titanium workpiece to spring away from the cutting tool, thereby, affecting the depth of cut. Particularly slender parts deflect more from the cutting forces, thereby, promoting tool rubbing and causing chatter. Dimensional tolerance is going to be greatly affected by this. Also, though titanium is very inert at low temperatures, its reactivity goes high above 500°C, which affects its tool life. Titanium alloys react readily with many of the cutting tool materials, thereby, forming built-up-edge that is detrimental to the life of the cutting tool. As discussed earlier, the temperature in the machining zone goes to as high as 1100°C that allows the surface to get hardened by the diffusion of atmospheric nitrogen. Titanium retains its hardness even at higher temperatures thus making it difficult to machine. This reflects the low machinability rating of titanium, which is 0.3 and for titanium alloys it is 0.2 compared to 1.0 for free machining steels.
The right tool
The machine tool used for machining titanium should be rigid and should be able to absorb vibrations and cutting loads. The most commonly suggested cutting tool materials for machining titanium are either M42 HSS or straight tungsten carbides with 6% cobalt binder and a grain size in the range of 0.8 to 1.4 ìm. The cutting tools used should have positive rake angles to ensure lower cutting forces and temperatures. The tool bits utilised should have active chip breaking to ensure that the chips leave the machining zone. The cutting speeds are used rarely goes beyond 60 m/min during roughing. The feed rates employed need to be carefully calculated basing on the chip load, cutting speed employed and final surface finish desired, etc. The feed rate should be high enough to prevent work hardening. The speed and feed employed should start from the cutting tool manufacturer’s recommended values and experimented to get the best productivity.
The use of cutting fluid is generally recommended to keep the machining zone as cool as possible. A general preference is to have a little higher concentration of cutting fluid with copious quantity and high pressure pump to ensure that the chips are blasted away from the machining zone as fast possible. Use of cutting tools with through the tool cutting fluid application would greatly benefit the machining operation. It is very important that the workpiece when mounted on the machine tool table should remain closest to the strongest points of the fixture to absorb the cutting loads. Avoid any conditions that are likely to cause vibrations. The machine tool rigidity should not be compromised. When the cutting tool and workpiece are in contact, let the tool move continuously without any dwell as it is likely promote work hardening. This need to be taken care in CNC part programming.
Turning and milling
Turning and milling are by far the largest machining processes used with titanium machining. The machining principles that were mentioned earlier will certainly have to be followed in relation to turning. The following is an example where aircraft manufacturer Boeing’s machinability team has conducted turning tests on a range of popular titanium alloys. They found that all of the tested titanium variants could be turned at speeds greater than 50 m/min, for a carbide tool life of at least 15 minutes. Further, the test results revealed that alpha-dominated grades are machined more easily than beta-dominated grades. Milled titanium part geometries are more elaborate and complex. Also, many of the parts are machined from solid to remove bulk of the material to the tune of 90% to reduce the weight and at the same time to maintain the integrity and strength of the structural element. Depending on the process used to make the initial part such as casting, forging, etc they would have some built-in residual stresses. As the material is being removed layer-by-layer, these stresses get relieved and that would cause distortion of the parts, which will affect the uniformity of the cut.
Get the best out of machining
In order to maintain the accuracy, there are a number of precautions to be taken during the milling operation. Since the heat needs to be removed from the machining zone, reduce the radial engagement of the cutter so that heat in the cutter gets dissipated in that part of rotation when the cutter is not in contact with the workpiece. Otherwise, when there is large radial engagement such as cutting a slot with the same diameter as the cutter, the cutting speed need to be reduced, thereby, increasing the machining time. Use cutters with large number of flutes. The cutter should enter the work material smoothly to reduce the impact force. That will improve the life of the cutter. Similar approach has to be taken when cutter leaves the workpiece as well. It is a good idea to vary the axial depth of the cutter for different passes so that any surface damage to the cutter will be distributed along its length. Ideally tool diameter should be smaller than the pocket size. When machining pockets with thin walls, the springiness of titanium causes it to deflect under the cutting force, thereby, reducing the depth of cut. When the cutter passes that point then the workpiece springs back, thereby, not achieving the required dimension. To remedy such a situation, use sharp cutting tools, this will reduce cutting forces and, thereby, reduce deflection. Also, add some extra finishing passes to take care of any leftover material.
Titanium is a material that has got a lot of problems for machining. The methods used for steel machining are not applicable in this case. However, these problems can be solved by carefully analysing the requirements and controlling the process parameters accordingly. It is necessary to remember to use low cutting speed, higher feed, low depth of cut and good high pressure coolant supply to ensure good quality at low cost. ☐