Titanium and its machinability
Posted by Turnad Lenggo Ginta | May 12th 2008
Titanium and its alloys are used extensively in aerospace because of their excellent combination of high specific strength (strength-to-weight ratio) which is maintained at elevated temperature, their fracture resistant characteristics, and their exceptional resistance to corrosion. They are also being used increasingly (or being considered for use) in other industrial and commercial applications, such as petroleum refining, chemical processing, surgical implantation, pulp and paper, pollution control, nuclear waste storage, food processing, electrochemical (including cathodic protection and extractive metallurgy) and marine applications. They have become established engineering materials available in a range of alloys and in all the wrought forms, such as billet, bar, plate, sheet, strip, hollows, extrusions, wire, etc. Despite the increased usage and production of titanium and its alloys, they are expensive when compared to many other metals because of the complexity of the extraction process, difficulty of melting, and problems during fabrication and machining. Near net-shape methods such as castings, isothermal forging, and powder metallurgy have been introduced to reduce the cost of titanium components.
The titanium alloy machining is hindered basically due to its high chemical reactivity and its low thermal conductivity (7.3 W/m K) generating high temperature. Of this generated heat, about 80% it is retained in the tool and 20% in the chip.
However, most titanium parts are still manufactured by conventional machining methods. Virtually all types of machining operations, such as turning, milling, drilling, reaming, tapping, sawing, and grinding, are employed in producing aerospace components. For the manufacture of gas turbine engines, turning and drilling are the major machining operations, whilst in airframe productions, end milling and drilling are amongst the most important machining operations. The machinability of titanium and its alloys is generally considered to be poor owing to several inherent properties of the materials. Titanium is very chemically reactive and, therefore, has a tendency to weld to the cutting tool during machining, thus leading to chipping and premature tool failure. Its low thermal conductivity increases the temperature at the tool/workpiece interface, which affects the tool life adversely. Additionally, its high strength maintained at elevated temperature and its low modulus of elasticity further impairs its machinability. The poor machinability of titanium and its alloys have led many large companies (for example Rolls-Royce and General Electrics) to invest large sums of money in developing techniques to minimize machining cost. Reasonable production rates and excellent surface quality can be achieved with conventional machining methods if the unique characteristics of the metal and its alloys are taken into account.
Machinability of titanium alloys
Progress in the machining of titanium alloys has not kept pace with advances in the machining of other materials due to their high temperature strength, very low thermal conductivity, relatively low modulus of elasticity and high chemical reactivity. Therefore, success in the machining of titanium alloys depends largely on the overcoming of the principal problems associated with the inherent properties of these materials, as discussed
below:
High cutting temperature:
It is well known that high cutting temperatures are generated when machining titanium alloys and the fact that the high temperatures act close to the cutting edge of the tool are the principal reasons for the rapid tool wear commonly observed. A large proportion (about 80%) of the heat generated when machining titanium alloy Ti-6Al-4V is conducted into the tool because it cannot be removed with the fast flowing chip or bed into the workpiece due to the low thermal conductivity of titanium alloys, which is about 1/6 that of steels. About 50% of the” heat generated is absorbed into the tool when machining steel. Investigation of the distribution of the cutting temperature has shown that the temperature gradients are much steeper and the heat-affected zone much smaller and much closer to the cutting edge when machining titanium alloys because of the thinner chips produced (hence short chip-tool contact length) and the presence of a very thin flow zone between the chip and the tool (approximately 8 µm compared with 50 µm when cutting iron under the same cutting conditions) which causes high tool-tip temperatures of up to about 1100°C.
High cutting pressures:
The cutting forces recorded when machining titanium alloys are reported to be similar to those obtained when machining steels, thus the power consumption during machining is approximately the same or lower. Much higher mechanical stresses do, however, occur in the immediate vicinity of the cutting edge when machining titanium alloy.
This may be attributed to the unusually small chip-tool contact area on the rake face, which is about one-third that of the contact area for steel at the same feed rate and depth of cut, and partly to the high resistance of Ti-alloy to deformation at elevated temperatures, which only reduces considerably at temperatures in excess of 800°C.