Drive Cone Cavity

INTRODUCTION

The drive cone cavity is one of the hottest un-cooled components in the engine. Operating around 900k at 10,000 rpm, the material used in making the drive cone is operating at edge of its safe working temperature changes at these high temperatures. A 10 K rise in shaft temperature can reduce the life of the shaft. The temperature must therefore be predicted to within 10 K or better to guarantee accurate stress predictions. It the thermal model cannot guarantee the 10 K accuracy required, a much shorter component life would have to be declared or alternative materials must be found. This report contains the different type of the materials which can be used to enhance the performance of the drive cone cavity and in order to do so the criteria is sub-divided into four group as shown in figure 1

Key drivers for the selection of the material Fig 1
(Ref: http://www.mtu.de/en/technologies/engineering_news/others/Smarsly_Materials_komp.pdf)

Trends in aero-engine materials use

As shown in Fig 2 the trends in increase of high temperature materials for gas turbine part. Although there are many monolithic ceramics materials show evidence of fundamental properties, but the main issue is relative to their application in aero engines has been their flaw sensitivity and brittle fracture modes. In addition fibre CMCs are very appealing materials due to (i) their high temperature performance as compared with other super alloys and (ii) their higher fracture toughness relate with monolithic ceramics in aero engines, in which structural reliability is most required. For that reason, CMCs are potential materials to meet these requirements in drive cone cavity.

[pic]
Trends in aero-engine materials use Fig 2
(http://www.azom.com)

Nickel-based super alloys

Most of the improvement in material for gas turbine component has been associated with the nickel base alloy system since of the ability to achieve better strength with this system. These alloys form gamma-prime second phase particles in heat treatment, which impart very high strengths to the alloy. Gamma-prime has the common composition of X3Z, where X is mainly Ni, and Z is mostly Al and Ti. (Gamma-prime is generally written as Ni3 (Al,Ti)). Ta and Cb can replace with Al and Ti, and Co can substitute for Ni. As a result, a more correct formula would be (Ni, Co)3 (Al, Ti, Ta, Cb).
The gamma-prime alloys can be either cast or wrought. The cast forms are more common because of the economies of casting difficult shapes, the capability to uphold very high mechanical properties by vacuum casting, and the complications appear when forging metals having exceptional mechanical properties at high temperatures. In addition to the structure of gamma-prime particles, which is the principal strengthening mechanism, these alloys also incorporate strengthening by solid solution hardening and carbide formation.

The gamma-prime super alloys are composed of many alloying elements. Chromium is used for resistance to environmental attack. Aluminium and tantalum assist in the resistance to environmental attack. Cobalt is used to stabilise the microstructure. Aluminium, titanium, tantalum and columbium are elements that form gamma- prime. Refractory elements, such as tungsten, molybdenum, tantalum and columbium are used for solid solution hardening. (Note: Chromium and cobalt also contribute to solid solution hardening.) These same elements, along with chromium, form carbides with the carbon that is added to the alloy. These carbides primarily strengthen the grain boundaries. In addition to these major elements, there are several elements added in minute quantities (sometimes called fairy dust) that strengthen the grain boundaries. These elements include boron, hafnium and zirconium. The microstructure of a common gamma-prime alloy, IN-738.
Nickel base superalloys can be classified into solid solution alloys, and gamma-prime (or precipitation hardened) alloys. The solid solution alloys, which can be either cast or wrought, contain few elements that form gamma-prime particles. Instead, they are solid solution strengthened by refractory elements, such as tungsten and molybdenum, and by the formation of carbides. They also contain chromium for protection from hot corrosion and oxidation, and cobalt for microstructural stability. Because these alloys are not precipitation hardened, they are readily weldable. Common examples of these alloys are Hastelloy X, Nimonic 263, IN-617, and Haynes 230. The microstructure of IN-617 is shown in Figure 3.

The microstructure of IN-617 Fig 3
(Ref: MATERIALS ISSUES FOR USERS OF GAS TURBINES by Henry L. Bernstein)
Furthermore, the superalloys are relatively expensive, heavy and difficult to fabricate and machine. In light of these limitations, other materials approaches are being pursued.

Titanium alloys

Titanium is a plentiful, low density (4.5 gm/cm3) [4] element having a high melting temperature (1668°C) [4] and a long record of successful aerospace applications. Titanium and its alloys usually display good ductility, in-between elastic modulus, outstanding corrosion and fatigue resistance, good fracture toughness and moderate tensile strength [4]. In addition, they can be handled and machined via a number of conventional methods. Titanium alloys are on the other hand, quite reactive, dissolve large quantities of interstitial elements such as O, N and H and they are expensive compared to other common metals. The creep resistance of most conventional titanium alloys is significantly less than that of the nickel-based super alloys and reduced fracture toughness with increased strengthening is often observed. Compared with the nickel-based super alloys though, their often enhanced strength to density ratio has concluded in a range of uses variety from an aircraft engines to prostheses [4]. Of these, Ti-6Al-4V (wt %) has been the most widely used. It is recommended for use at -210°C to 400°C [5]. Pure titanium undergoes an allotropic transformation from α (hcp) to β (bcc) crystal structure at ~882°C (the β transus) [4]. By adding elements which stabilise the β phase at lower temperatures, a variety of microstructures can be obtained.
This allotropic quality of Titanium alloys are categorised alpha, alpha-beta or beta according to their general room temperature microstructure after processing. By adding α stabiliser which includes Al, Ga, Ge or interstitials C, N and O, one can elevate the fraction of α phase in an alloy. By adding b stabilisers which include the b isomorphous group comprised of elements such as Mo, Nb, Ta and V, one can elevate the fraction of b phase in an alloy and reduce the b transus temperature [4]. Although not strongly promoting phase stability, both Sn and Zr are quite soluble in the α and β phases and are frequently added in part to retard transformation rates and also act as solid solution strengthening agents [4]. Si is an often used addition for creep resistance [4]. If large fractions of Al are added to titanium, the α phase is replaced by γ (ordered fct) TiAl or (ordered hcp) Ti3Al intermetallic phases. If large fractions of Al and Nb are added, Ti2AlNb alloys are possible [6]. For high temperature applications, systems based on TiAl and Ti3Al (titanium aluminides) have received considerable attention because of their improved performance over conventional titanium alloys [7,8,9,10] which approaches that of nickel-based superalloys at a fraction (45-55%) of the density. A comparison of high temperature alloy properties is given in Table 1.

Table 1 High temperature alloy properties
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(Ref: http://www.ipm.virginia.edu/process/Con/Pubs/thesis19/chapter1.pdf)

The properties of titanium aluminides are attractive for aerospace applications. There are however, concerns with environmental embrittlement at high temperatures [6] and a lack of ductility at low temperatures which limits workability and contributes to low fracture toughness. To combat the latter problem, researchers have explored alloying [7 and 8]. In particular, isomorphous b stabilizing elements such as Nb have been added to produce two phase α2 + β microstructures with enhanced ductility [9,10,11 and 12]. When combined with improved processing techniques, some of the alloys produced thus far have shown a favourable balance of ductility, stiffness and strength while also ensuring adequate fatigue, oxidation and creep resistance [8].To further increase strength and stiffness, a significant effort has been directed towards development of titanium matrix composites (TMC) which utilise continuous ceramic fibers for reinforcement.

Titanium matrix composites

In recent years, the exceptional tensile properties of SCS-6 have been utilised by combining them with titanium matrices to form composites. In particular, intermetallic matrix composites (IMC) having titanium aluminide matrices [18] have attracted considerable interest because of the very high specific stiffness, strength and creep resistance they exhibit over a wide range of temperatures [19]. These experimental materials may lead to dramatic improvements in performance at operating temperatures once thought to be attainable with advanced nickel-based super alloys. Since the density of IMC’s can be less than half that of their super alloy counterpart, it is not surprising that significant resources have been invested toward their success. There are however, a number of issues which need to be resolved before the IMC’s make their way into the marketplace. These centre around environmental degradation, severe reactions at the fibre-matrix interface and problems associated with large linear expansion coefficient mismatches between the fibre and the matrix [1]. Processing difficulties and high cost (especially for the fibres) are also a major concern. Furthermore, there is a need to understand damage evolution since it controls mechanical performance and is the precursor to failure.
One IMC which has received significant attention contains continuous aligned SCS-6 fibres in a matrix of two phase Ti-14Al-21Nb (wt.%) (α2 + β alloy) [1]. Developed around the binary α2 (Ti3Al) phase, this intermetallic alloy includes Nb to enhance the β phase and improve low temperature ductility. At room temperature for example, neat (fibreless) Ti-14Al-21Nb can exhibit several percent deformation prior to failure. While optimization of matrix chemistry and processing techniques is still incomplete, much information has been obtained about the Ti-14Al-21Nb / SCS-6 system and mechanical properties in the fibre direction are good. For fibre volume fractions in the neighbourhood of 0.30, room temperature tensile strengths ranging from 840 to 1496 MPa and Stage I moduli of 172 to 229 GPa have been observed. At 650°C, tensile strengths greater that 950 MPa and 600 MPa creep lives approaching 1000 hours have been reported [18]. Although the measured tensile strengths of Ti-14Al-21Nb / SCS-6 systems are high, they are often less than predicted. It will be shown in Chapter 4 for example, that the average tensile strength for 0.25 volume fractions SCS-6 in Ti-14Al-21Nb should be in the neighbourhood of 1450 MPa. This is only occasionally achieved in practice and the “premature” failure associated with damage has generated a need for understanding the chain of events leading to failure. Studies have indicated that this involves a combination of fibre-matrix interface cracking, matrix plasticity and cumulative fibre α2 + β failure. Some findings however, concluded that fiber stress levels upon failure were not high enough to damage them and the researchers were puzzled why numerous fibre breaks were so often observed (especially near fracture surfaces) after testing. Metallography combined with optical and electron microscopy was the methods of choice for inspecting the damage post facto.

Monitoring damage evolution in composites

While metallography provides one method of obtaining information about evolving damage, these techniques are time consuming, laborious and the results obtained may be very sensitive to the skill and expertise of the investigator rather than the science it seeks to reveal. Furthermore, test pieces get destroyed during the inspection, cracks may tightly close when loads are removed and experiments must be interrupted at a variety of load levels if one seeks “the complete picture”. Even rare damage events (e.g. fiber breaks) may be difficult to detect since metallography only allows inspection of small regions within the sample. Since damage processes in composites are important, other methods enabling researchers to gain quantitative insight into evolving damage are needed. Many failure processes are accompanied by detectable acoustic emission (AE) [14,15,16]. Recently, a number of investigations have reported AE in metal matrix composites (MMC) [15,16]. Some have even attempted to locate and/or differentiate one source type from another. Using recorded AE signals, parameters such as source rise-time, threshold counts, signal amplitude, energy, duration, frequency spectra, etc. have been proposed to ‘‘characterise’’ the AE events and to differentiate one source ‘‘type’’ from another. To date however, many of these studies have relied upon ad-hoc empirical methods using instrumentation which could not faithfully reproduce important characteristics of the AE signal. Combined with an absence of models that establish fundamental relationships between damage micro mechanisms and AE signals, the conclusions of these studies are highly questionable. If however, such problems could be overcome, an AE approach based upon fundamental principles promises to reveal much new insight into damage processes.

REPAIR OF HOT SECTION COMPONENTS

Because of their excessive price, every effort is made to fix hot components before scraping them. In gas turbine most parts can be fixed just one time, some more than once and some can be done number of times. As repair technologies continue to improve, and new repair methods are developed, components previously thought to be non-repairable have now become repairable. Because of this possibility, some users save their scrapped hardware to await the improvement of better repair technique. As drive cone cavity is the one of the hottest part of the aero-engine, they are designed to repaired once but they are repaired more than once and operated successfully, this required a careful inspection and maintenance is very important.
The selection of cost-effective alloys for high temperature service depends on knowledge of service requirements and materials capability. Each temperature regime offers several alloy options, depending on mechanical property and environmental resistance requirements.

Discussion

High temperature alloys have reached a state of maturity in which tailoring existing alloys to specific needs takes precedence over alloy development. Alloy processing techniques and controls have advanced dramatically, leading to new thresholds of alloy performance.
Although CMCs are potential materials to meet these requirements in drive cone cavity, but it may need few more year to come in application as there is more development needed. Advanced engine and airframe concepts under development by NASA and DoD in programs such as Integrated High Payoff Rocket Propulsion
In spite of their otherwise excellent properties, nickel super alloys have unsatisfactory wear characteristics. Thus, surface treatments that can improve the tribological behaviour without adversely effecting corrosion resistance have high potential to improve performance and expand the field of applications of super alloys. Disadvantage of conventional diffusion treatment (performed at a temperature of 1000-1050C), is that substrates could distort because of exposure at high temperature. Plasma Aluminizing Process basically involves deploying of aluminium coating by sputtering and simultaneous diffusion at high temperature under plasma to prepare aluminide coatings in-situ. Plasma aluminising process eliminates the limitations of conventional treatments.
Corrosion resistance properties of titanium and its alloys are generally excellent. Titanium's resistance to seawater and other chloride-based solutions is very good. Generally, titanium is more corrosion resistant than stainless steel.
Working with titanium can present problems. Milling and drilling of titanium requires special care to be taken and the cutting tool has to be kept sharp. Welding titanium cannot be carried out in air, it must be done using TIG welding because molten titanium reacts with oxygen, nitrogen and hydrogen causing the metal to become more brittle. Casting titanium requires the use of a special vacuum furnace to ensure that the molten metal doesn't react with the atmosphere.

Conclusion

Although Nickel super alloy and Titanium alloy have their advantage and disadvantages but because of this characteristic, titanium aluminide is a candidate material for the drive cone cavity. Aluminide base alloys offer superior high temperature performance with low weight and non-burn. Gamma aluminide turbine blade based on Ti47Al2Cr2Ni have been tested and are strong as nickel based alloys up to 1200°C and half the weight. Aluminides have generally proved difficult to process, have limited heat treatability, and generally low ductility at room temperature. Work continues however in an attempt to optimise these characteristics and deliver useful alloys for specific applications.

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