X.1 Introduction
The long term strategic objectives ‘‘Flightpath 2050’’ [62] have taken the Advisory Council for Aviation Research in Europe (ACARE) development targets beyond the year 2020. By year 2050, the following goals for air traffic have to be achieved:
• 75% reduction in CO2 per passenger kilometer
• 90% reduction in NOx emissions
• 65% reduction in noise
This will require improvement in all three areas Airframe, Engine and Air traffic management (ATM) & operations. This means
• Light weight designs and materials (e.g. composites) on airframes and engines.
• Advanced aerodynamic performance and light weight designs
• Higher engine bypass ratio.
• Lean burn engine combustor technologies.
• Advanced turbine materials for aeroengines (discs and blades).
The need for enhanced aerodynamic performance requires improved accuracy and reduced part-to-part variation in manufacturing quality. 3D improved aerodynamics on integrally bladed rotors (IBR, blisk), blades and vanes lead to more complex free form geometries to be machined. Cost effective and validated high performance machining solutions and methods are required for advanced high temperature alloys. In this context, the present paper is to provide an overview of machining characteristics of advanced aerospace alloys for aeronautical applications. A critical assessment of machining behaviour, tool wear and surface integrity is presented. Further, advances in high performance machining technologies are reviewed and finally industrial perspectives are provided in the context of machining specific aerospace components where future challenges are discussed (M'Saoubi et al. 2015).
Titanium alloys have medium density, good strength to weight ratio, excellent oxidation resistance at intermediate temperatures, low coefficient of thermal expansion, high toughness and good weldability. Today, more and more advanced Ti alloys are used in Airbus A350XWB and Boeing B787 due to high materials compatibility with carbon fibre composites, for example, no risk of galvanic corrosion at material interfaces and low thermal expansion (half that of aluminium alloys), therefore, reduce thermally induced loads. The most commonly used alloys are alpha (a), beta (b), alpha–beta (a/b) Ti–6Al–4V. Beta tempered Ti–6Al–4V is chosen for damage tolerance performance and in gear beam, underwing fittings, centre wing box main frames, etc. Titanium sheet Ti–3Al– 2.5V, which has good formability are used in sub-structure for anti-crash frames. In the context of landing gear applications, the Ti–10V–2Fe–3Al has been used in replacement of high strength low alloy steel 4340M on the Boeing 777 aircraft and the next generation of high strength b Titanium alloy, Ti–5Al–5V–5Mo–3Cr (Ti-5553) is used in the Boeing 787. (M'Saoubi et al. 2015)
X.2 Machinability of Titanium alloys
Titanium alloys have received considerable interest recently within a wide range of applications in the aerospace, automotive, chemical, and medical industries due to their high specific strength, fracture resistance, and superior resistance to corrosion. The most common titanium alloy is Ti6Al4V, which belongs to the α + β alloy group and accounts for more than 50% of the titanium alloy production. This alloy displays very high tensile strength and toughness at high temperatures combined with low density and is increasingly employed within the aerospace industry. The mechanical and thermal properties of Ti6Al4V are reported in Table 1 [2]. This alloy has the same tensile strength of steel being 45% lighter, a modulus of elasticity 50% lower than steel, and a very low thermal conductivity (down to 1/5 of steel). Moreover, an allotropic transformation occurs in the titanium structure at 882 °C (beta transus), with change from alpha titanium (EC structure) to the more deformable beta titanium (CCC structure) [2-3].
Table 1. Comparison between main properties of Ti6Al4V and steel.
Properties Ti6Al4V Steel
Density (g/cm3) 4.43 7.86
Melting point (°C) 1649 1454
Thermal conductivity (W/mK) 7.2 11.2-36.7
Elastic modulus (GPa) 114 210
Tensile strength, ultimate (MPa) 1000 827
Tensile strength, yield (MPa) 880 552
Because of the material properties described above, two critical issues arising during machining of Ti6Al4V alloy parts are represented by the very high temperatures in the workpiece and at the tool cutting edge ( 1100°C) due to the material low thermal conductivity as well as excessive vibrations during machining. These phenomena are intensified when machining under dry conditions, as the high temperatures in the cutting zone cause rapid tool wear and changes in the workpiece structure close to the cut zone [4-7].
In order to achieve weight savings in diverse applications, with particular reference to the aerospace industry sector, new alloys are being developed. Boyer and Briggs (2004) analyzed the properties and different applications of the new beta titanium alloys, which are being used by aerospace companies because they provide an exceptional combination of fracture toughness, high strength, simpler processing, and deep hardenability. One of the latest developed alloys is the Ti555.3, which can be heat treated to high strengths, with minimum tensile strength values of above 1200MPa. This high tensile strength makes Ti555.3 a promising material for advanced structural and landing gear applications, compared with traditional titanium alloys such as Ti6Al4V. Titanium alloys can be classified based on the value of the Al and Mo equivalent parameters. The Al equivalent value indicates the capacity of the alloy to obtain a given hardness, whereas the Mo equivalent value indicates the capacity to obtain an ultimate tensile strength (UTS) and hardness in the aged condition. The near-beta alloy (Ti555.3) has a Mo-equivalent value of nearly 8 times higher than that of the Ti6Al4V alloy. This observation explains the higher mechanical properties of these near-beta titanium alloys compared with the traditional Ti6Al4V alloys. (Arrazola et al. 2009)
X.3 Sensor monitoring of Titanium alloys machining
When dealing with low machinability materials such as titanium alloys, the development of smart monitoring procedures can significantly increase productivity and reduce tool costs, optimizing tool life by implementing condition-based tool replacement strategies (i.e. by replacing tools only when they are close to end of life) instead of conservative time-based tool replacement strategies (in which the tool is replaced after a predetermined time independently of its real wear conditions) and they can help reduce machine and workpiece damage risk by allowing fast reaction when a tool breakage occurs. (Caggiano et al. 2016)
The typical machining process monitoring system operates according to the following rationale. In the cutting region there are several process variables, such as cutting forces, vibrations, acoustic emission, noise, temperature, surface finish, etc., that are influenced by the cutting tool state and the material removal process conditions. The variables that are prospectively effective for machining process monitoring can be measured by the application of appropriate physical sensors. Signals detected by these sensors are subjected to analogue and digital signal conditioning and processing with the aim to generate functional signal features correlated (at least potentially) with tool state and/ or process conditions. Sensor signal features are then fed to and evaluated by cognitive decision making support systems for the final diagnosis. This can be communicated to the human operator or fed to the machine tool numerical controller in order to suggest or execute appropriate adaptive/corrective actions (Teti et al. 2010).