As commercial aircraft OEMs seek to make airplanes lighter and more fuel-efficient, the technologies deployed by the aerospace industry have undergone a radical transformation. One of the major factors that have influenced the performance of commercial aircraft is the evolving dynamics of material science – yielding changing composition of metals used. Comprising high strength to weight ratio, corrosion resistance, fatigue resistance and capability to work in high temperature zones, titanium is one of the primary materials for aerospace structures and engines. Yet, its low modulus of elasticity, poor thermal conductivity, high chemical reactivity and hardening properties, together, make it one of the toughest materials to machine. Therefore, an appropriate selection of cutting tool design and machining strategies is extremely important for machining titanium and its alloys along with proper machine, coolant and work holding capabilities in order to maximise productivity & optimise costs.
To design the right machining solutions, it is important to understand the metallurgical properties of titanium.
Titanium alloys & their properties
Titanium undergoes various phases, from α to ß and depending on these phases, titanium alloys are classified into four main groups:
Unalloyed/untreated titanium – excellent corrosion resistance, but lower strength properties
Alpha structure (α alloys) – great creep resistance and low to medium strength
Beta structure (β alloys) – high strength and high density, but the most difficult to machine
Alpha beta structure (α-β alloys) – medium to high strength and includes alpha and beta stabiliser elements
The commercially viable alloys are ß and (α-ß). ß alloys contain transition metals, such as V, Nb, Ta and Mo that stabilise the ß-phase. Examples of commercial ß alloys include Ti11.5Mo6Zr4.5Sn, Ti15V3Cr3Al3Sn and Ti5553. Beta alloys are readily heat-treatable, generally weldable and have high strength. Excellent formability can be expected in the solution treated condition. However, they are prone to ductile-brittle transition and thus, are unsuitable for cryogenic applications. Beta alloys have a good combination or properties for sheet, heavy sections, fasteners and spring applications.
In comparison, α-ß alloys contain both α and ß stabilisers. The simplest and most popular alloy in this group is Ti6Al4V, which is primarily used in the aerospace industry. Alloys in this category are easily formable and exhibit high room temperature strength and moderate high temperature strength. The properties of these alloys can be altered through heat treatment.
Some challenges associated with machining comprise high-chemical reactivity of titanium alloys, which causes the chip to weld to the tool, leading to cratering and premature tool failure. In addition, the chip-tool contact area is relatively small, resulting in large stress concentration due to these higher cutting forces and temperatures, resulting in premature failure of the cutting tool.
The machining of flight critical components (engine rotatives) at optimal production rates requires reliable, high precision tooling for semi-finishing and finishing turning operations. The entire process needs to be secured to avoid any damage to the part that requires advanced chip breaker technologies.
The requirements include:
Grade toughness (chipping resistance)
Heat resistant coating
Chemical wear resistance
Micro geometry that promotes low cutting forces
Stable machines and conditions that promote higher Metal Removal Rates (MRR)
Stages of titanium machining and choice of the right tooling solutions
Titanium is typically produced in the shape of plates and bars, as well as forgings and castings. The buy-to-fly ratio is typically between 10 and 16, meaning a raw material of 10 kg titanium ends up being a finished part with the weight of one kg.
Typically, titanium machining is carried out in three stages – pre-machining, roughing and finishing, as illustrated in the diagram on the next page.
Flat bottom drilling is ideal for the pre-machining of high temperature alloy airframe structural parts. Quickly removing large amounts of material remains a challenge for these types of components. Traditionally, the first step of the process is to enter the material by using ramping techniques, which is time-consuming and leads to low metal removal rates.
Speeding up the machining process requires a tool that combines the advantages of a flat bottom drill and a z-axis plunge mill. The flat bottom design eliminates radial forces, while four effective cutting edges provide increased feed & speed rates, leading up to 200% higher metal removal rates than traditional ramping techniques and freeing up capacity for aerospace manufacturers.
Once the drill has shaped the basic structure of the component, it progresses for the roughing and finishing steps.
The Buy-To-Fly ratio requires efficient removal of the excess material achieved through titanium roughing applications, which causes the cutting edge to be exposed to extremely high temperatures for a long time. A recommended roughing solution for a large structural part is helical shoulder milling that addresses the high metal removal needs in aerospace machining. To achieve lowest possible cost per edge, tool makers have come with up to eight cutting edges per insert in Helical Milling Cutters. Such designs can reduce power consumption up to 50% and can deliver the highest MRR, especially in high-temperature alloys.
If the work holding is weak and tool overhang is high, another technique that can be used is High Feed Milling (HFM). With low entering angles, these cutters provide optimum cutting forces with maximum stability. Also, in materials like titanium, which has an extremely low thermal conductivity (thermal conductivity of Ti6Al4V is ~ 7.2 W/m K), it is prudent to work with higher feeds than speeds.
An array of new generation PVD coatings with alloyed substrates are available, which provide resistance to thermal cracks, least co-efficient of friction and prevent chip welding in roughing applications. The cutter body should be constructed of higher-quality steel for improved stiffness and rigidity to withstand the high cutting forces. The variable helix design helps to break the harmonics that lead to chatter, further improving tool life, part quality and throughput. Positive cutting action reduces vibration, minimising the cutting zone temperature and prevents work hardening tendencies.
Finishing operations are mostly performed with solid endmills. Some unique design features include unequal flute spacing, variable helix configuration to minimise chatter & vibration, eccentric reliefs, twisted end face for increased stability & ramping along with the most optimised geometry and flute design for titanium machining. In some advanced designs, endmills come with positive clamping technologies (safe lock) which prevent tool pull-out and ensure high process security. High performance endmills will also have features like chip gashers for smooth chip evacuation and lower cutting forces.