The requirements for materials used in jet engine parts are necessarily very exacting. They must survive extremes of temperature and force, while being as light as possible and ultra-reliable. Contributed by Iscar
A turbojet engine can be divided simply into three sections – the compressor, the combustor and the turbine. The compressor pressurises the air flowing through the engine before it enters the combustion chamber, where the air is mixed with fuel, ignited and burnt. The compressor components are predominantly made from titanium alloys, while the combustor and turbine components are typically made of a nickel-based superalloy such as Inconel 718.
The excellent physical properties that characterise nickel-based high temperature alloys make them ideal for use in the manufacture of aerospace components.
Properties such as high yield strength and ultimate tensile strength, high fatigue strength, corrosion and oxidation resistance even at elevated temperatures enable the usage of nickel-based high temperature alloys in many applications and over a very wide temperature spectrum.
The aerospace industry accounts for about 80 percent of the nickel-based high temperature alloys used in manufacturing rotating parts of gas turbines, including disks and blades, housing components such as turbine casing, engine mounts, and components for rocket motors and pumps.
Nickel-based high temperature alloys contain 35-75 percent Ni and 15-22 percent Cr; they constitute about 30 percent of the total material requirement in the manufacture of an aircraft engine and are also used as structural material for various components in the main engine of space shuttles.
The very same properties that make nickel-based alloys such a great choice for jet engine parts also cause substantial machining difficulties.
The cutting forces and temperature at the cutting zone are extremely high due to the high shear stresses developed and the low thermal conductivity. This, coupled with the reactivity of nickel-based high temperature alloys with the tool material, leads to galling and welding of the chips on the work piece surface and cause excessive tool wear, which can limit cutting speeds and reduce useful tool life.
All these characteristics contribute to low material removal rates and short tool life, resulting in massive machining costs.
Due to their high strength to weight ratio and excellent corrosion resistance, titanium alloy parts are ideally suited for advanced aerospace systems. Titanium-based alloys which contain 86-99.5 percent Ti and 5-8 percent Al, are immune to almost every medium to which they would be exposed in an aerospace environment.
Very large quantities of titanium can be found in jet engines, where titanium alloy parts make up to 25-30 percent of the weight, primarily in the compressor. The high efficiency of these engines is obtained by using titanium alloys in components such as fan blades, compressor blades, rotors, discs, hubs, and other non-rotor parts—for instance inlet guide vanes.
Titanium’s superior properties and light weight allow aeronautical engineers to design planes that can fly higher and faster, with high resistance to extreme environmental conditions. However, titanium has historically been perceived as a material which is difficult to machine due to its physical, chemical and mechanical properties.
The material’s relatively high temperature resistance and low thermal conductivity do not allow generated heat to dissipate from the cutting tool, which causes excessive tool deformation and wear. Titanium alloys retain their strength at high temperatures, resulting in relatively high plastic deformation of the cutting tool resulting in depth of cut notches. During machining, the high chemical reactivity of titanium alloys causes the chips to weld to the cutting tool, leading to built-up cutting edges and chip breakage problems.
Over the past few years, Iscar has invested many resources in research and development to resolve these obstacles and optimise the machining of nickel-based and titanium high temperature alloys, with solutions that include the creation of customised grades and implementation of high pressure coolant technologies to develop cutting tools that will handle the heat issues.
For high material removal rates, Iscar developed ceramic grades to facilitate machining nickel-based alloys at cutting speeds of 200–400m/min:
IW7—Whisker-reinforced ceramic grade, provides high hardness with excellent toughness used for roughing and semi-finishing continues operations at 8-10 times faster cutting speeds when compared with carbide grades.
IS25—Reinforced SiAlON composite grade, Excellent for machining Ni based high temperature alloys at continuous and light interrupted applications.
IS35—Reinforced SiAlON composite grade, Excellent for machining Ni based high temperature alloys at light & heavy interrupted applications.
A series of carbide grades was developed specifically to create tools for machining nickel-based and titanium alloys:
IC806—A hard submicron substrate combined with a thin TiAlN PVD coating. The unique coating procedure which involves a special post coating treatment creates a thinner and smoother coating layer providing the insert with the best characteristics suitable for machining nickel-based and titanium alloys.
IC804— Same TiAlN PVD coating on a harder submicron substrate designed especially for machining Ni based alloys used in newly designed jet engine parts that feature very high hardness (40-47 HRC).
IC20—An uncoated carbide grade which is highly recommended for machining aluminum and titanium. IC20 provides very high performance and is mostly used for continuous cut applications.
High Pressure Coolant Tools
Although high pressure coolant features have been in existence for a long time in the metal removal world. Today high pressure coolant tools play an increasingly significant role in the machining process, facilitating enhanced productivity and chip control especially for hard to machine materials such as titanium and nickel-based alloys. Incorporating high pressure is the key to directing coolant to exactly where it is needed in order to flush the chips away from the cut.
Iscar was one of the first cutting tool producers to respond to market needs by developing and manufacturing tools for the optimal use of high pressure coolant in lowering high temperatures and regulating chip flow, including Jetcut custom high-pressure coolant tools.
While the aerospace parts OEM/PMA sector is under constant pressure to keep costs down, the quality and life expectancy of the parts produced cannot be compromised—and this represents an enormous challenge for all involved. Iscar’s enhanced cutting tools allow jet engine manufacturers to utilise the ideal materials for the production of high quality parts, with minimum wastage and maximum efficiency.