Fabrication of Monofilament Reinforced Titanium

3.24 Fabrication of Monofilament Reinforced Titanium C. M. WARD-CLOSE, J. G. ROBERTSON, and S. P. GODFREY Defence Evaluation and Research Agency, Farnborough, UK 3.24.1 INTRODUCTION

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3.24.1.1 Summary of Fabrication Routes

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3.24.2 MATRIX SOURCE

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3.24.2.1 Foil 3.24.2.2 Powder 3.24.2.3 Wire

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3.24.3 FIBER MANAGEMENT

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3.24.3.1 Weaving 3.24.3.2 Filament Winding

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3.24.4 CONSOLIDATION

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3.24.4.1 Processing Cycles 3.24.4.2 Consolidation Modeling

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3.24.5 SELECTED AREA REINFORCEMENT

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3.24.6 SHAPE CONTROL

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3.24.7 FABRICATION ROUTES 3.24.7.1 Foil-fiber Process 3.24.7.2 Wire-fiber Process 3.24.7.3 Powder Routes 3.24.7.4 Matrix Coated Fiber Process 3.24.7.5 Spray Deposition 3.24.7.6 Liquid Metal Infiltration

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3.24.8 CONTROL OF DEFECTS

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3.24.9 CHOICE OF FABRICATION ROUTE

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3.24.10 CONCLUSIONS

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3.24.11 REFERENCES

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3.24.1

INTRODUCTION

larly in the aerospace field. Table 1 shows mechanical properties for a typical titanium alloy MMC and its constituent matrix and fiber. The reinforcement is invariably silicon carbide (SiC) monofilament with a carbon-based multilayer coating, of which there are three commercially available types: Textron SCS

Titanium alloy metal matrix composites (MMCs), reinforced with continuous ceramic fibers, offer attractive combinations of strength, stiffness, and elevated temperature performance and are currently being considered for a range of advanced applications, particu1

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Fabrication of Monofilament Reinforced Titanium

Figure 1 Ti MMC production methods based on hot pressing and diffusion bonding.

Table 1 Longitudinal strength and stiffness of a titanium alloy MMC and its constituents. Tensile strength (MPa)

Young's modulus (GPa)

1690

186

Titanium alloy Ti±6Al±4V

950

110

Sigma 100 mm SiC fiber

3200

400

Ti±6A1±4V, 35 vol.% SiC MMC

series (USA; 140 mm diameter), DERA Sigma (UK; 100 mm diameter), and Trimarc (USA; 140 mm diameter). Because of the high chemical reactivity of titanium, fabrication methods based on liquid metal infiltration are not normally applicable for titanium fiber reinforced MMCs, and fabrication is by solid-state diffusion bonding.

3.24.1.1

Summary of Fabrication Routes

Figure 1 illustrates a number of the diffusion bonding lay-up methods which have been used for titanium-based MMCs. The various methods are essentially different ways of combining the titanium alloy and ceramic fibers prior to hot consolidation (typically in the 850±950 8C range). (i) The well known ªfoil-fiberº method is the best established one. Here, alternate layers of

metal foil and ceramic fiber are consolidated to give a fully dense product. The fibers are generally held in place during this process either by a titanium ribbon cross-weave or an organic binder which is later removed by vacuum degassing. (ii) The ªpowder clothº method, where alloy powers are mixed with organic binder and rolled into a cloth-like material as a substitute for foil. In a variant of this method, the ªtape castingº method, parallel fibers are coated with a mixture of powder and organic binder to form a precursor tape. (iii) The ªplasma sprayº method, where metal is sprayed onto an array of fibers to form monolayer tapes, now seems unlikely to become a commercially viable route. Experience has indicated fiber damage due to thermal shock and problems due to high porosity in the matrix and consequently very rough surfaces which interfere with the accurate location of the next layer of fibers. High plasma power tends to result in lower porosity and improved surface quality, but also increased fiber damage. (iv) The ªwire-fiberº method, where fiber and small diameter titanium wires are consolidated to form an MMC. Here the fiber sits in grooves made by the supporting wire mat but it is difficult to ensure that the complex lay-up maintains registry during consolidation processing. Also, the fine titanium alloy wires are difficult to make and are expensive. (v) The ªmatrix coated fiberº method, where fiber is precoated with the matrix alloy, usually by vapor deposition, to form a precursor material which is laid-up and consolidated into a finished MMC.

Fiber Management (vi) Liquid infiltration has also been considered, but the high reactivity of titanium makes this process very difficult. As few titanium MMC components have yet reached production, it is unclear which of these fabrication methods, if any, will become commercially viable. The choice is likely to be based on overall component cost, the availability of a robust supply chain, and product quality. 3.24.2

MATRIX SOURCE

Most Ti MMC fabrication techniques use titanium in the form of either foil, powder, or wire, which differ widely in terms of cost and availability. In the case of vapor coating the starting stock is alloy bar, which is relatively cheap, but the coating process is inefficient and adds to the cost. 3.24.2.1

Foil

MMC fabrication requires foil typically in the thickness range 70±200 mm, which can be produced in most titanium alloys. However, the higher strength alloys require dedicated processing equipment which is costly for low volume requirements. Foil provides a clean, easy to handle source of titanium and has therefore been the most widely used method for Ti MMCs. Apart from cost and availability, its main problems are shape limitation and the low packing efficiency of unconsolidated composite. The production of titanium alloy foil is not a simple process. The alpha/beta alloys are difficult to roll, requiring frequent interstage annealing, pack rolling, or very controlled processing. The near alpha alloys and even titanium intermetallic alloys such as gamma titanium aluminide based alloys can also be produced as foil but are even more difficult to process. Other problems in using these hightemperature brittle alloys mean their use is likely to be restricted. The beta titanium alloys are easier to roll to foil but interest in these alloys for Ti MMCs had diminished with the demise of the US hypersonic space plane (NASP) skin application. Many techniques for foil production have been investigated, including plasma spraying, powder processing, chemical milling, and the more conventional thermomechanical processing techniques. Foil production volumes are low and therefore there is little incentive for the titanium producer to invest in what for them is a very small market. Titanium alloy foil will never be cheap but at increased production volumes it should be an economically viable matrix source. Foil has the advantages of a

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controlled microstructure, accurate alloy chemistry, and low impurity levels. 3.24.2.2

Powder

Titanium powder can be produced in most alloys and if there is a reasonable volume requirement the costs are acceptable. However, the large surface area of suitably sized powder (typically 25±100 mm) and its susceptibility to contamination make powder unsuitable for highly critical parts. In addition, most powder processing routes for Ti MMC components use a binder to hold the powder in place during layup and for larger structures the subsequent removal of this binder cannot always be guaranteed. 3.24.2.3

Wire

Titanium alloy wire is commonly available as welding wire in diameters greater than 1 mm. It is held as a stock item by numerous titanium product manufacturers at about $80 kg71. However, to be suitable for use in Ti MMCs, much smaller diameters are required to obtain an acceptable fiber volume fraction. Fine Ti± 6Al±4V wire is manufactured as spinal support material for biomedical applications. This is mostly as Ti±6Al±4V ELI (extra low interstitial) for extra ductility, with a diameter of about 60 mm. The price of titanium alloy wire is highly dependent on the diameter required. The cost of wire down to 150 mm is reasonable, but there is an approximately 10-fold increase in price with a decrease in diameter from 150 mm to 50 mm. This is due to the labor-intensive nature of the drawing process (1 kg at 150 mm is about 13 km long and 1 kg at 50 mm is about 115 km long). To produce these lengths, dedicated equipment would be in use for long periods of time, especially as slower processing speeds are required for the finer wire to improve yield and reduce downtime due to breakages. For applications requiring continuous lengths of wire, even higher prices could be expected, particularly for fine wire where breaks may occur during the drawing or cleaning stages. The price of fine wire is relatively independent of volume as the manufacturing cost is dominated by labor charges.

3.24.3

FIBER MANAGEMENT

Fiber management is the control of fiber position during MMC processing. The two most common techniques when using foil as

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Fabrication of Monofilament Reinforced Titanium

Figure 2 Schematic diagram of an SiC woven mat with Ti ribbon cross-weave (not to scale).

the matrix source are weaving and filament winding, although other techniques such as grooving foil, tape collimation, and co-winding can be used. 3.24.3.1

Weaving

Unlike conventional weaving of yarn, the monofilaments must be handled individually, leading to problems of scale and minimum bend radius. It is impossible to weave large diameter monofilaments to produce two-dimensional structures suitable for Ti MMCs due to the monofilament's large minimum bend radius and problems of fiber±fiber contact. However, unidirectional woven material can be produced where the monofilaments are held in place using a cross-weave of a different (small diameter) material. The cross-weave material must be small enough to give the target fiber spacing but strong enough to cope with the stresses during the weaving process. This severely limits the choice and availability of cross-weave materials. Various refractory wires have been tried but can lead to undesirable microstructural interactions, and now the cross-weave is usually

Figure 3

titanium wire or ribbon. An example of a ª2-2 twillº woven structure is shown schematically in Figure 2. In principle, weaving can benefit from economies of scale, however, the brittle nature of the monofilament and the high cost and limited availability of cross-weave material makes these cost savings difficult to achieve. There are two types of loom suitable for weaving monofilament: a shuttle loom which yields a long thin tape product with a wire cross-weave, and a rapier loom, which produces wide product with flat titanium ribbons holding together cut widths of monofilament. Both products have their limitations, the products can have poor fiber spacing, lack of flexibility, low packing density, and may also suffer from fiber damage at crossover points. 3.24.3.2

Filament Winding

Filament winding offers a cheaper alternative to weaving but the inclusion of a fugitive binder and its subsequent removal leads to the possibility of contamination and fiber movement. These problems can be largely eliminated with good process control. The use of a fugitive binder is crucial to the success of this process. The binder, which is subsequently driven off in a bake-out and vacuum degassing stage (usually in the range 250± 600 8C), holds the fibers in their filament wound positions during the lay-up and degassing stages. The organic medium consists of a polymeric binder dissolved in an organic solvent such as toluene or acetone and the binder may be polycarbonate, polystyrene, acrylic, polyisobutylene or PMMA. A number of different filament winding and binder coating options are possible. Figure 3 shows filament winding onto a metal drum using a sprayed organic binder. The fiber mat is cut and removed from the drum to give a self-supporting precursor mat for subsequent

Filament winding fiber with fugitive binder.

Consolidation

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Figure 4 Production of binder-coated ªtackyº fiber.

Figure 5 Fiber preform ring-binder coated fiber.

diffusion bonding operations. To facilitate removal and handling, a disposable backing film is first attached to the drum. An alternative method is to coat an individual fiber with binder, producing a ªtackyº fiber for subsequent filament winding of different shapes (Berthelemy et al., 1995) and apparatus for carrying out this process is shown in Figure 4. Figure 5 illustrates a method for producing a spiral fiber preform using binder coated tacky fiber. For either drum winding or the tacky fiber method the ªfiberº can be standard SiC for use with the foil-fiber or powder methods, or can be matrix coated fiber for subsequent consolidation with or without foil. 3.24.4

CONSOLIDATION

Solid-state diffusion bonding is the conventional way to fabricate Ti MMC components. Heat and pressure are normally applied using either hot isostatic pressing (HIP) or vacuum hot pressing (VHP).

With vacuum hot pressing the pressure is applied between heated platens inside a vacuum furnace. While this is relatively simple in concept, it becomes complex in practice. The consolidation temperatures and pressures required for Ti MMC components require special molybdenum alloy (TZM) or ceramic tooling. Such tooling is difficult to work with and expensive to replace. On a small scale, VHP is a useful tool to help understand the consolidation process. For larger parts the press requirements become impractical. For example, a 500 ton press would be needed to press a flat panel 300 mm 6 300 mm square. Even then, it is difficult to ensure that all areas of the component receive a uniform load sufficient for consolidation. In addition, VHP does not lend itself to processing more complex shapes such as rings or shafts. Hot isostatic pressing applies the consolidation pressure via a gas acting on a metal can. With careful tooling design, complex shapes are possible, tooling can be made of a cheap material such as mild steel, and loads can be applied evenly over large areas. Many components can

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Figure 6

Fabrication of Monofilament Reinforced Titanium

Temperature/time consolidation map for Ti±6Al±4V matrix coated SCS6 SiC fiberÐeffect of pressure.

be processed during the same HIP cycle which can lead to economies of scale. The largest HIP in the UK has a chamber over 2 m 6 1 m and can operate up to 100 MPa at temperatures up to 1420 8C. HIP services are available commercially throughout the world and this has become the consolidation method of choice. 3.24.4.1

Processing Cycles

The optimum processing cycle for consolidation of a fiber reinforced titanium MMC is dependent on the component design, tooling mass, lay-up method, alloy and, to a lesser extent, the fiber and its coating. The object of the consolidation is to give a guarantee of 100% density while avoiding excessive movement of the fiber (fiber swimming), mechanical damage to the fiber, or excessive chemical reaction between the fiber and the alloy. It is well established that titanium alloy reacts with the carbon coating and the SiC fiber above about 700 8C, forming a reaction layer initially of TiC and then of TiC and Ti3Si5. Thus it is essential to keep exposure time above this temperature to a minimum. If attack reaches the stage where the carbon coating has been removed, the fiber strength is severely degraded and for Textron SCS6 fiber this corresponds to a reaction zone thickness of about 2 mm. In order to determine the optimum processing window for Textron SCS6/Ti±6Al±4V matrix coated fiber, Ward-Close and Loader (1995) carried out a parametric study of time, temperature, and pressure and plotted the results in the form of consolidation maps, an example of which is given in Figure 6. It is seen that increasing pressure has a substantial effect on reducing processing times in the temperature range 800±900 8C, but below 800 8C

the times for full consolidation increase very sharply. On the basis of this, suitable consolidation conditions might be 50 MPa at 900 8C for 30 min, which gives a safety margin of twice the minimum time for full consolidation. The disadvantage of increasing the pressure in order to reduce the process time is the increased danger of fiber damage. More ductile titanium alloys such as Timetal Beta 21S or the new Japanese low-temperature superplastic alloy SP700 may allow processing at lower temperatures (Yamada et al., 1997), but it is not normally possible to produce reliable diffusion bonds in titanium alloys below approximately 750 8C. 3.24.4.2

Consolidation Modeling

Process modeling has the potential to greatly reduce the time and cost of determining the optimum consolidation conditions for the solid-state consolidation of titanium MMC parts. The process of determining a consolidation map as described in the previous section is very labor intensive and does not necessarily provide all the information required for optimum quality and performance of the MMC. For example, to be complete, consideration must be given not only to final density, but also such critical factors as fiber fracture and matrix±fiber chemical attack, fiber movement, and component shape control. The processes taking place during consolidation are complex, and the effect of adjusting a process parameter is not always obvious and the effects on the consolidation processes are often contradictory. For example, control of matrix±fiber chemical reaction is particularly important, as it determines the strength of the matrix±fiber bond and can also affect the fiber strength. The interface

Consolidation

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Figure 7 Finite element model prediction of metal flow superimposed on Ti±6Al±4V/SCS6 SiC fiber MMC with yttrium marker layers.

has to be strong enough to transfer load between the matrix and the fiber, but weak enough to debond during fracture and provide energy absorbing failure modes. Generally, for titanium, excessive chemical attack is the more likely problem, and it is known that if the protective carbon coating on the fiber is consumed during the processing cycle then chemical attack of the SiC fiber will dramatically reduce its tensile strength. This would indicate that minimum temperature and time exposure would be preferred in order to reduce fiber±matrix reaction, however, the high pressure required to achieve this may result in excessive fiber fracture. Modeling of the metal flow during consolidation can help to define the time/temperature/ pressure window and reduce the number of trials required to produce the final defect-free part. A number of analytical and numerical models have been investigated for the foil± fiber process (Guo and Derby, 1993, 1994), powder-fiber processes (Semiatin et al., 1996; Newell et al., 1995), spray processing (Elzey and Wadley, 1994; Wadley and Vacheeswaran, 1995; Elzey et al., 1995), and the matrix coated fiber process (Schuler et al., 1996, 1997a, 1997b). Guo and Derby (1993) used an analytical model based on plastic flow and creep to predict consolidation behavior in the foil-fiber method. Their model considered pore closure and final fiber distribution and they concluded that fiber distribution was a function of both the initial geometric parameters and the con-

solidation method, which determined the extent of lateral constraint on fiber spreading. The model also predicted high initial contact stresses between the fiber and the matrix, and the authors suggested that therefore care should be taken with initial temperature/pressure rampup in order to avoid fiber damage. Subsequent experience has confirmed this, and generally a ªsoft startº (pressure rising after consolidation temperature has been reached) is preferred over a ªhard startº (temperature and pressure rising together). However, there are other considerations, such as deformation of the container and movement in the lay-up pack. For the modeling of consolidation behavior, numerical methods such as finite element modeling (FEM) have a number of advantages over analytical methods. They give information on stress±strain distribution, including residual stress, and are more easily applied to different materials, by substituting basic materials data. Schuler et al. (1996, 1997a, 1997b) used a finite element model of plastic flow and power-law creep to predict the consolidation behavior of Ti±6Al±4V matrix coated Textron SCS6 SiC fiber. The model was tested against experimental results using special matrix coated fiber containing thin marker layers of yttrium. The marker layers were too thin to significantly influence the deformation behavior of the titanium alloy and they revealed a close correspondence to the predictions of the finite element model (Figure 7). It is noticeable that in the

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Fabrication of Monofilament Reinforced Titanium cells in which fiber segments experience bending forces as a result of surface roughness aspirates (Figure 9). Figure 10 shows the resulting prediction of the effect of densification rate and temperature (Figure 10(a)) and fiber strength and strength distribution on the number of fiber fractures per meter (Figure 10(b)) for a Ti±24Al±11Nb/Textron SCS6 composite and Figure 10(c) shows the detrimental effect of fiber breaks on the tensile stress±strain performance of the composite. The prevalence of this type of damage is a particular problem for the plasma spray route.

Figure 8 Consolidation map for Ti±6Al±4V/35% SiC matrix coated fiberÐFEM prediction (solid symbols) compared with experimental results (open symbols).

matrix coated fiber technique for a hexagonal array of fibers, very little matrix strain occurs adjacent to the fiber. The model was used to predict pressure/temperature consolidation maps for matrix coated fiber and again good agreement was found with experimental data (Figure 8). It is anticipated that, with further refinement, a finite element model of this type should be capable of predicting local consolidation phenomena such as fiber damage mechanisms and also macroscopic behavior such as component shape change. Wadley and co-workers developed a micromechanics model to predict the occurrence of fiber fractures in the consolidation of plasma spray laminates (Wadley and Vacheeswaran, 1995; Elzey et al., 1995). Their model considered the composite as an assemblage of unit

3.24.5

SELECTED AREA REINFORCEMENT

Unlike unidirectionally reinforced polymeric composites, where the matrix has poor mechanical properties, uniaxial MMCs can have relatively good transverse strength. The transverse strength of a typical fiber reinforced titanium MMC is about half the normal matrix strength, and this has important consequences for component design. For many potential applications the transverse properties are good enough for titanium MMCs to be used in a unidirectionally reinforced mode, taking maximum advantage of the exceptional strength and stiffness of the fiber. In addition, because of the good mechanical performance of unreinforced titanium alloy, it is not necessary, or in most cases desirable, for the whole part to be manufactured from composite. Thus, most applications of titanium composite will be as selectively reinforced regions within larger titanium alloy components. In this way expensive titanium MMC material will be used only in the most critical

Figure 9 Modeling of fiber bending fracture during consolidation of plasma spay laminates (reproduced by permission of TMS from `Symposium ProceedingsÐRecent Advances in Titanium Metal Matrix Composites', Rosemont, 1995, eds. F. H. Froes and J. Storer, 1995, pp. 101±116).

Selected Area Reinforcement

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Figure 10 Predicted frequency of fiber fracture during consolidation of a plasma spray laminate as function of: (a) temperature and densification rate, (b) fiber strength and strength distribution (Weibull modulus), and (c) the effect of pre-existing fiber breaks on stress±strain behavior (reproduced by permission of TMS from `Symposium ProceedingsÐRecent Advances in Titanium Metal Matrix Composites', Rosemont, 1995, eds. F. H. Froes and J. Storer, 1995).

Figure 11 Processing ring structures using filament-wound MCF.

regions, and damage to fibers will be avoided by carrying out all machining and joining operations, e.g., welding and weld repair, in the unreinforced regions of the component. The ability of titanium alloys to diffusion bond easily provides an effective route for small areas of MMC to be incorporated into larger titanium parts and this is illustrated for a reinforced ring structure in Figure 11. Here matrix coated fiber is filament wound into a channel in an inner titanium ring, this is then joined to a close fitting outer ring using electron beam welding, and the whole assembly is hot isostatically pressed (HIPed) to produce a single solid part. All voids and interfaces in the MMC and the joints in the tooling are eliminated by plastic flow and diffusion bonding. An alternative to one-step consolidation and diffusion bonding of MMCs and components is to incorporate preconsolidated (or partially consolidated) pieces of MMC into the compo-

nent in a second operation, commonly by diffusion bonding using VHP or HIPing. Because of the damaging effect of excessive chemical reaction between the fiber and the titanium matrix it is particularly important to keep the combined thermal exposure associated with the two processing operations to a minimum. Hence the initial MMC preform is sometimes only partially consolidated, sufficient for lay-up purposes, and full density is only achieved in the second processing step. It is also possible to incorporate some secondary shape forming into the MMC at this stage, and this is illustrated by the example shown in Figure 12 (Henshaw, 1994), where a shaped die is used to form a reinforced aerofoil section using the well-known multisheet superplastic forming and diffusion bonding method (SPFDB). Although it is possible to carry out hot forming or bending about the axis parallel to the fibers to produce curved or corrugated uniaxially

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Fabrication of Monofilament Reinforced Titanium

Figure 12 SPFDB manufacturing sequence for an aerofoil with internal ribs and fiber reinforced top and bottom MMC skins (reproduced by permission of ASME from `Flight-Vehicle Materials, Structures, and DynamicsÐAssessment and Future Directions', eds. A. K. Noor and S. L. Venneri, 1994, vol. 1, pp. 338±347).

Figure 13 GE Bi-cast fan frame-machined Ti MMC insert in titanium alloy casting (reproduced by permission of TMS from `Proceedings of Seventh World Titanium Conference', San Diego, CA, 1992, eds. F. H. Froes and I. L. Caplan, 1993, pp. 2495±2501).

reinforced panels, deformation at right angles to the fibers is limited by the high stiffness and lack of ductility of the ceramic reinforcement. Howmet and General Electric Aircraft Engines developed a casting method to incorporate selective areas of Ti MMC reinforcement into a prototype fan-frame strut, which they called the ªbicast method,º illustrated in Figure 13 (Veeck and Colvin, 1993). The Ti MMC insert was produced by the foil-fiber diffusion bonding method, inserted in a lostwax mold, and titanium was cast around it. A

number of technical questions must be addressed in order to use this method. For example, if the volume of cast metal is large compared to the volume of the preform, the MMC may experience excessive thermal exposure during the casting operation. This can result in erosion of the preform and damage to the fiber±matrix interface. Alternatively too small a volume of liquid metal will cause freezeoff and prevent proper filling of the mold. In addition, it is necessary to control the positioning of the preform in the casting mold. Not

Fabrication Routes

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Figure 14 Debulking associated with different Ti MMC processing methods.

withstanding these difficulties, in view of the increasing interest in titanium castings for aerospace applications, this appears to be a promising technique.

3.24.6

SHAPE CONTROL

Debulking, or shrinkage, associated with the hot pressing stage of Ti MMC manufacture is a critical factor in determining both the position of the individual fibers, the position and shape of the area of reinforcement, and the shape and surface finish of the MMC part. The choice of manufacturing route has a major effect on debulking and this can range from over 50% for the foil fiber method to as little as 10% for filament wound matrix coated fiber or preconsolidated monosheets (Figure 14). Generally, even with HIPing, shrinkage is not uniform. Careful can and tooling design is used to ensure accurate shape formation without fiber damage. For many component shapes it is essential to restrict consolidation to one direction only and at the same time control certain dimensions precisely. This is illustrated for ring and tube shapes in Figure 15, which also indicates the most favorable processing routes for the different shapes. For example, in a tube, consolidation must be restricted to the radial direction, as axial movement might result in buckling and severe distortion. But it may also be necessary to prevent movement of the outside dimension of the tube in order to avoid excessive hoop stressing of the fiber during consolidation, or, depending on the size and

shape of the tube, it may be necessary to prevent changes to the internal diameter. In many ways these problems are similar to those found in the manufacture of graphite fiber composite parts, where issues such as dimensional control, fiber uniformity, and surface finish are addressed through tool design and choice of curing cycles. For titanium MMC components the required constraint can often be provided by titanium tooling which is incorporated into the component by diffusion bonding. In the example shown in Figure 11 the inner ring will prevent movement in the radial direction and consolidation of the MMC area will be mainly in the axial direction, thus preventing either bucking or overstressing of the fibers. For other component shapes steel tooling can be used, which will have a high resistance to deformation at the consolidation temperature. Reusable steel tooling can often be utilized, with a coating of BN or Y2O3 powder to prevent bonding between the steel and the titanium, or if this is not possible, the tooling can be removed by machining or acid pickling.

3.24.7 3.24.7.1

FABRICATION ROUTES Foil-fiber Process

Processing of titanium MMCs using the foil and fiber (F/F) lay-up technique is the best established approach to the problem of combining the high strength and stiffness of monofilaments with the high temperature capability of reactive titanium alloys (Cornie, 1981;

12

Fabrication of Monofilament Reinforced Titanium

Figure 15 Effect of ring aspect ratio on preferred consolidation method.

Figure 16 Large demonstration airframe structures fabricated from Ti MMC for NASP (US National Aerospace Plane) using the foil-fiber route.

MacKay et al., 1991). For example, demonstration structures for NASP (the, now canceled, US National Aerospace Plane) made extensive use of foil-fiber processing routes to produce skin structures that would be capable of withstanding the re-entry temperatures of such a vehicle (Figure 16) (Wilson, 1993). Titanium foil is ideally suited for use in flat panels and other essentially two-dimensional shapes and it is now possible to produce high-quality composite with good fiber management, minimal matrix contamination, and no fiber damage. More complex curved shapes have been manufactured using foil and an example of a fiber

reinforced aerofoil shape is described in Section 3.24.5. The fiber distribution attained when using filament wound fiber mats in the foil-fiber method can be very good (Figure 17). By carefully controlling the process, this quality of fiber distribution can be maintained even for shaped components. However, the method cannot guarantee that fibers will not touch. The quality of fiber distribution depends to some extent on the diameter of the fiber. Large fibers can be handled more easily; the accuracy of each fiber position is not so critical, as neighboring fibers are further away.

Fabrication Routes

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Figure 17 Ti±6Al±4V/Sigma SiC fiber MMC produced by the filament wind/fugitive binder foil-fiber method; note good fiber distribution.

Figure 18 Foil/fiber method (a) a simple lay-up for a flat panel (b) a reinforced disc made using fiber spirals.

The relatively high debulking factor or low lay-up packing density means that many voids must be removed during consolidation. This makes it difficult to use foil for thick structures requiring a shape change during consolidation. There is a significant risk that fibers may move out of position, break due to excessive strain, or that tooling failure may occur. The inability of the foil-fiber technique to guarantee that no fibers will touch in the composite has led Ti MMC producers to look at alternative matrix sources and for research to be carried out producing foils with fine individual grooves that locate and hold the fibers in place.

Figure 18(a) shows the basic steps required for producing flat Ti MMC plates using a foilfiber technique. The process of filament winding the fiber is described in Section 3.24.3.2. The array of aligned fibers are cut to the required size and then alternately stacked with foils to give the desired number of plies. The lay-up is then positioned between simple disposable tooling which is coated with a release agent and sealed in an air-tight stainless steel can. The can is then heated under vacuum to remove any organic matter and other contaminants while maintaining fiber positioning. This ªdegassingº stage is completed by sealing the can, ensuring that a vacuum is maintained

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Fabrication of Monofilament Reinforced Titanium

Figure 19 Options for using titanium wire in the production of MMCs.

inside. The can is then HIPed to achieve full composite densification. The can is removed to reveal the composite which is then surface treated to remove any tooling release agent. Figure 18(b) shows a similar process for a more complex ring shape, using foil and preformed fiber spirals. Although the foil fiber method is best suited to flat shapes, various lay-up techniques have been devised for round shapes such as rings and tubes. For a hoop reinforced ring structure the large debulking associated with the consolidation stage of a foil-fiber lay-up leads to the possibility of fiber buckling or fiber breakage. One method of reducing this risk, developed by Rolls Royce, is to have intermittent rather than continuous reinforcement in the circumferential direction (Doorbar, 1990; Doorbar and Studds, 1990). This is achieved by laying-up cut lengths of preconsolidated foil-fiber mats interspersed with short sections of unreinforced titanium alloy foil of the same thickness. The position of these gaps in the reinforcement are staggered in each successive layer such that the effect on mechanical performance is minimal, but the unreinforced sections provide compliance during the consolidation stage and prevent unwanted tensile or compressive loading of the fibers. 3.24.7.2

Wire-fiber Process

In this fabrication method silicon carbide fiber is combined with titanium alloy wire and

consolidated into an MMC (Hanusiak et al., 1996). Figure 19 illustrates two lay-up options for the wire and fiber. The simpler method, illustrated in Figure 19(a), uses alternate layers of fiber and wire of the appropriate diameter in a way analogous to the foil-fiber method. In order to better control the positioning of the fibers and prevent fibers touching, a second option is to use a finer wire, and alternate each silicon carbide fiber with metal wire within the fiber layers, as illustrated in Figure 19(b). As in the foil-fiber method the titanium alloy flows around the fibers and diffusion bonds to produce a fully dense composite. Development of the wire-fiber process has been hindered by the difficulty of producing sufficiently fine alloy wire at reasonable cost. One potential advantage of the wire-fiber method is that it may lend itself to automation by established filament winding technology and if the diameter of the wire is chosen carefully, the valleys or grooves between the titanium wires will act as guides for fibers and will help maintain the fiber spacing during processing. Figure 20 shows a component produced using a wire-fiber method which is claimed to be the first commercial application of a titanium matrix composite. The part is a uniaxially reinforced piston for the engine in the F22 US fighter aircraft, 12 in. long, 2 in. diameter barrel, and 4 in. diameter head, and is 40% lighter than the stainless steel piston it replaces. The part was produced by Atlantic Research Corp. (ARC) using a method which involved wrapping layers of titanium fiber and titanium wire.

Fabrication Routes

15

Figure 20 An axially reinforced Ti MMC piston for the F22 fighter aircraft made by the wire-fiber method (reproduced by permission of Aviation Week and Space Technology from Aviation Week and Space Technology, 1997, May 12, 74±75).

Bi-layer sheets of wire and fiber were produced separately using a filament winding method and held together with a fugitive organic binder. The fiber-wire sheets were rolled into a bundle on an axis parallel to the fiber direction, inserted into a titanium tube, vacuum degassed and sealed by electron beam welding, HIPed, and machined to final size (Lavitt, 1997; Hanusiak et al., 1996). In order to avoid damaging fibers and exposing fiber ends on the surface, machining was confined to areas of unreinforced titanium, such as the surface skin. Lay-up instability during degassing and HIPing is likely to be a particular problem in the wire-fiber process as once the fugitive binder has been removed the wires and fibers will both be free to move. Careful design of tooling and lay-up configuration may alleviate this. 3.24.7.3

Powder Routes

Titanium powder has the potential to become a relatively inexpensive source of matrix for Ti MMCs. A particular advantage of a powder route for Ti MMC is that it is suitable for any titanium alloy, whereas the hard creepresistant titanium alloys and newer intermetallic based titanium alloys are difficult and expensive to process into foil or wire. A convenient way of handling the powder is to make the titanium powder into a sheet or cloth (Stephens, 1988; Truckner and Edd, 1995). The ªpowder clothº is manufactured by mixing appropriate sized powder with fugitive organic binders followed by casting onto a suitably flat surface such as glass sheet with wiping or ªdoctorº blades controlling the thickness of the

cast cloth. Plasticizers may also be added to the mixture to improve the flexibility of the cloth. A variation of this process allows fibers to be incorporated into the cloth as it is being cast, producing a prepreg material analogous to kevlar or carbon epoxy sheets used extensively for making polymer matrix composites (sheet molding compound), and this process is illustrated in Figure 21. Once cast into sheets, the cloth may be trimmed to size, laid-up with fiber mats, encapsulated, hot degassed to remove the binder, and finally hot isostatically pressed to achieve consolidation. By controlling the powder size, binder characteristics, and burnout cycle, undesirable settling of the powder can be avoided. Recent attention on cost reduction within the TMCTECC program (Titanium Matrix Composite Turbine Engine Components Consortium) has led both Textron Specialty Materials and the Atlantic Research Corporation to develop powder cloth. The main issues currently associated with this process are: (i) Clean binder burnout (ii) Availability of suitable powder. The choice of binder and use of degas cycle is critical if complete binder removal is to be achieved. The burnout stage may take considerably longer compared with degassing fiberfoil, fiber-wire, or matrix coated fiber lay-ups. It is particularly difficult to achieve complete binder burnout with powder cloth due to the tortuous path between the powder that the binder decomposition products have to take. In order to get the right properties from the composite, the powder used to make the cloth has to meet three main criteria. First, the powder has be fine enough to be cast into cloths that

16

Fabrication of Monofilament Reinforced Titanium

Figure 21 In situ collimation and roll bonding of fiber tape intermediate.

will give the right fiber spacing in the final composite. For 100 mm fiber the cloth and hence powder needs to be below 120±140 mm. The second criteria is that it has to be low in interstitial oxygen, nitrogen, and carbon, which are all readily absorbed by titanium at elevated temperature and above a certain level degrade mechanical properties. Third, the cost has be acceptable. The selection of a suitable powder is further complicated by the fact that size, interstitial content, and cost requirements are generally competing factors as, in general, the finer the powder the more expensive and difficult it is produce and the more likely to be higher in interstitial impurity content. Obtaining acceptable fiber distribution using titanium powder as the source of the matrix is extremely dependent on the relative sizes of the powder and fiber. Some good results have been obtained using 140 mm diameter SiC fiber but good quality, low oxygen titanium alloy powder is not commercially available in the size required for 100 mm diameter SiC fiber.

3.24.7.4

Matrix Coated Fiber Process

In this process the fibers are precoated with a thick concentric layer of matrix alloy and the coated fibers are then laid-up and hot pressed into the finished MMC (Dudek et al., 1985; Ward-Close and Partridge, 1990). The coating deforms to fill the interstices and diffusion bonds to form the matrix of the MMC. Several methods have been identified for coating the fibers, including liquid metal coating (Doorbar, 1990), ribbon wrapping (Holmes and Silgman, 1994), and powder coating (Hanusiak et al., 1996). But the most promising method, and the one receiving the most attention, is physical vapor deposition (PVD), using

either argon sputter coating (Dudek et al., 1984) or electron beam evaporation (WardClose and Partridge, 1990; McCullough and Storer, 1995). Figure 22 shows examples of fibers metal coated by different methods. Of the two PVD coating processes, sputter deposition has the advantage of being able to deposit any alloy and the coating quality and composition control is normally very good. However, deposition rates for sputtering are generally less than 10 mm h71 which makes this process uneconomical for production. Higher deposition rates have been obtained for hollow cathode sputter coating but using this process to coat fibers efficiently has yet to be demonstrated. Electron beam evaporation is capable of very much higher deposition rates than sputtering, typically 1 mm h71 or more, and the production of matrix coated fiber (MCF) by electron beam evaporation is in the process of being scaled-up to pilot plant level in both the UK and USA. In the UK, at the Defence Evaluation and Research Agency (DERA), 2000 m lengths of Ti±6Al±4V coated SiC fiber, suitable for 35 vol.% fiber MMC, have been produced on a continuous, electron beam evaporation, fiber coating machine. The DERA coater, illustrated schematically in Figure 23, uses a multipass system to build up the metal coating to the required thickness (typically 35±50 mm). Coating is carried out in a vacuum chamber, with the fibers entering and exiting the chamber through a series of small diameter holes via separately pumped intermediate vacuum chambers. In this way the leaks associated with the fiber entry and exit ports are controlled to an acceptably low level. In addition, the outer intermediate chamber in each case is maintained with a slight overpressure of argon such that the controlled leak into the main chamber is predominantly inert gas and not oxygen or nitrogen which are both

Fabrication Routes

17

Figure 22 Different examples of matrix coated fiber: (a) ribbon wrap, (b) powder coating, (c) vapor coating ((a) and (b) reproduced by permission of ASME from `Flight-Vehicle Materials, Structures, and DynamicsÐ Assessment and Future Directions', eds. A. K. Noor and S. L. Venneri, 1994, vol. 1, pp. 307±318).

Figure 23 Apparatus for continuous vapor coating of SiC fiber with titanium alloy.

readily absorbed by hot titanium and are detrimental to the mechanical performance of the alloy beyond a certain level. The PVD titanium alloy coating has a fine equiaxed microstructure which adheres well to the fiber with no tendency to crack or spall off. The coated fibers are consolidated by either vacuum hot pressing or hot isostatic pressing (HIPing) and the finished MMCs have very

regular fiber distributions. Residual stresses caused by differences in thermal contraction between the fiber and the matrix on cooling from the processing temperature have been shown to give rise to matrix and fiber cracking (MacKay et al., 1991; Thomas et al., 1998). Wood and Ward-Close (1995) showed that radial matrix cracking was most likely to occur at the point of closest approach of the fibers and

18

Fabrication of Monofilament Reinforced Titanium

that prevalence of cracking increased with decreasing fiber spacing. The volume fraction of reinforcing fiber in the finished MMC is determined by the thickness of the matrix coating and examples have been produced with volume fractions in the range 15±80%. Electron beam evaporation from a single alloy bath is possible if the vapor pressures of the constituent elements are relatively close to each other at the bath temperature, and this is the case for Ti±6Al±4V. For more complex titanium alloys, containing low vapor pressure refractory elements such as niobium or zirconium, multiple source evaporation must be used. The DERA fiber coater has multiple evaporation sources in line with the direction of travel of the fiber, and thus, when evaporating different master alloys in order to produce a particular alloy, the repeated translation of the fiber above the sources results in a microlayered coating. As the layers are very thin, it is anticipated that the constituents will diffuse during the hot pressing stage to give a uniform alloy composition in the finished MMC. The potential advantages of the matrix coated fiber process compared with alternative fabrication methods are summarized as follows: (i) Ideally suited for filament winding of product forms such as rings, disks, shafts, and tubes. For such shapes, the foil-fiber or monoply tape methods are likely to be prohibitively expensive and may leave large numbers of fiber ends embedded in the structure to act as points of chemical attack or stress raisers for crack initiation. (ii) Excellent fiber distribution with no touching fibers. (iii) The consolidation requirements are likely to be less severe for matrix coated fiber than for either foil-fiber or monotapes. (iv) Little or no disturbance of the fiber± matrix interface region. (v) Low debulking. (vi) Almost any matrix alloy can, in principle, be applied. (vii) No foils or powders are required. (viii) Very high volume fractions of fiber are possible (up to 80% has been demonstrated, but 35±50% is the preferred range for most projected applications). (ix) The metal coating protects the ceramic fiber from damage, both during handling and in the consolidation process. It should be noted that technical difficulties associated with PVD coating of large diameter ceramic fibers are considerable and have not been fully overcome. Alloy utilization is low and maintaining consistent alloy chemistry in the coating is difficult, particularly for complex alloys containing refractory elements. How-

ever, the simplicity of the matrix coated fiber method and the advantages this brings make the process very attractive for the fabrication of complex parts.

3.24.7.5

Spray Deposition

This potentially cost-effective method uses a powder-fed plasma gun to deposit titanium onto an array of silicon carbide fibers (Chou et al., 1985; Freeman et al., 1994; Zhao et al., 1995; Baker et al., 1997). The result is a monoply sheet which is stacked and hot pressed in the normal way. Alternatively, multilayer reinforcement can be produced by either repeated or simultaneous plasma spraying and filament winding. Figure 24 shows a schematic illustration of a spray winding system for simultaneous filament winding and spray deposition (Fan et al., 1996). In order both to prevent outgassing contamination and to protect the mechanism from metal dust particles, the filament winding apparatus is housed in an argon-filled box within the plasma spray chamber. Vacuum or low-pressure plasma spraying with argon is required in order to prevent excessive contamination, and for a well-designed system with good gas and powder feed control contamination from oxygen and other interstitial elements is minimal. The plasma sprayed material always contains a certain amount of porosity, and so HIPing is required to produce a fully dense product. The major concerns relating to quality control in the plasma spray process are fiber damage, porosity, surface roughness, and fiber misalignment. Figure 25 shows a typical plasma spray deposited MMC laminate. Zhao et al. (1995) showed that the use of an organic binder to hold the fibers in position during plasma spraying was best avoided. Due to the short duration of the thermal exposure they found that removal of the binder was incomplete, leading to increased porosity due to trapped binder vapor and reduced penetration of the metal droplets around the fibers. However, without the organic binder they found that the fibers were more likely to move out of position during spraying. The authors also report that titanium droplet infiltration of the SiC preform increased with decreasing spray distance, decreasing atomization gas pressure, and increasing arc current, due to the higher droplet temperatures and therefore fluidity on impact with the fiber preform. As well as improving infiltration, a high droplet temperature also results in a smoother top surface of plasma sprayed preform, which helps to maintain fiber alignment during filament

Fabrication Routes

19

Figure 24 Spray/wind system for multiply Ti MMC (reproduced by permission of Woodhead Publishing from `Proceedings of ICCM-7, Seventh European Conference on Composite Materials', London, 1996, pp. 413±424).

winding of successive layers of fiber and during subsequent hot consolidation. However, the high velocity of semimolten powder particles hitting the fiber surface, coupled with the thermal shock as the particle cools on contact with the fiber, has been shown to damage the fiber± coating interface and this is worse for high droplet temperatures (Baker et al., 1997). General Electric developed a plasma spray technique which used secondary radiofrequency heating to ensure full melting and enable the plasma power and therefore particle velocity to be reduced (Freeman et al., 1994). However, even with this technique, the roughness and porosity of the preform meant that further processing was difficult.

3.24.7.6

Figure 25 Typical plasma sprayed Ti MMC monoply (reproduced by permission of ASME from `Flight-Vehicle Materials, Structures, and DynamicsÐAssessment and Future Directions', eds. A. K. Noor and S. L. Venneri, 1994, vol. 1, pp. 319±328).

Liquid Metal Infiltration

The liquid metal infiltration route has been used successfully for a number of years for the production of fiber reinforced aluminum alloys, but the high chemical reactivity of liquid titanium in contact with ceramics makes the application of this route to titanium-based MMCs extremely difficult. A patent (Doorbar, 1990) refers to liquid metal coating of ceramic fiber for Ti MMCs but no further information has been published. For conventional titanium alloys with melting temperatures typically

above 1700 8C contact times between ceramic fiber and liquid metal of only a few seconds result in very severe attack or even complete dissolution of the fiber (Figure 26). Limited success has been achieved with liquid metal infiltration using very low melting point titanium alloys with both SiC and graphite fibers. Toloui (1985) used liquid infiltration with binary titanium copper alloy (up to 35 wt.% Cu) to make graphite fiber reinforced composites. The method used RF heating of packed

20

Fabrication of Monofilament Reinforced Titanium

Figure 26 Severely damaged fibers in a Ti±6Al±4V/ SCS6 SiC MMC produced by rapid IR heating and liquid infiltrationÐapproximately 5 s liquid/fiber contact (reproduced by permission of TMS from `Symposium ProceedingsÐRecent Advances in Titanium Metal Matrix Composites', Rosemont, 1995, eds. F. H. Froes and J. Storer, 1995, pp. 45±53).

bundles of graphite fibers, titanium rods, and copper ribbon, and the fabrication temperature was maintained close to the liquidus temperature for periods of 2±12 min. Wetting of the fibers was complete and the composites showed 100% density, but severe chemical reaction was observed, with a TiC reaction zone approximately 5 mm thick formed around each fiber. Tensile strengths were approximately in the range 169±469 MPa and tensile ductilities were less than 2%. Warrier and Lin (1993) used IR rapid heating to infiltrate titanium alloy/graphite fiber and titanium alloy/SiC fiber composites and claim liquid metal contact times of less than 30 s at 1250±1350 8C with an unspecified low melting point titanium±nickel based alloy. The authors report good fiber wetting, no void formation and reaction zone thicknesses and, in the case of titanium/SiC composites, mechanical properties comparable to conventional diffusion bonded titanium alloy/SiC fiber composites. Figure 27 shows a three-ply titanium±nickel alloy/Textron SCS6 SiC fiber composite made by the IR rapid heating liquid metal infiltration method. Unfortunately, the low-temperature alloys used in these experiments are of limited commercial interest and conventional engineer-

Figure 27 Ti±Ni alloy MMC produced by rapid IR heating and liquid infiltration (140 mm fiber): (a) three-ply laminate, (b) single fiber showing limited interfacial reaction (reproduced by permission of TMS from JOM, 1993, 45, 24±27).

ing titanium alloys all have melting temperatures in excess of 1700 8C. In subsequent work Warrier and Lin (1995) used their rapid IR heating method to produce Ti±6Al±4V/Textron SCS6 SiC fiber reinforced MMCs. They found that even with processing times as low as 5 s the fibers suffered severe damage (see Figure 26). However, a combination of a 2 mm TiC layer applied to the Textron SCS6 fiber and an addition of 0.5 wt.% C to the alloy prevented dissolution of the SiC and good composite mechanical properties were reported. In addition to the chemical attack problem, as with liquid infiltration of aluminum alloy MMCs, accurate fiber placement and control of fiber spacing will be very difficult and for these reasons the commercial exploitation of this process for titanium-based fiber reinforced MMCs is unlikely in the foreseeable future.

Control of Defects

21

Figure 28 Examples of fabrication defects in Ti MMCs ((a)(b) 100 mm, (c)(d) 140 mm): (a) poor fiber distribution; (b) plasma spray damage (fibers extracted by acid pickling); (c) poor diffusion bonding between foils; (d) thermal cracking in a brittle gamma titanium aluminide alloy matrix.

3.24.8

CONTROL OF DEFECTS

The following is a summary of the main types of defects that can occur in titanium fiber reinforced MMCs and an indication of the measures required to control them. Figure 28 shows some of the defect types. (i) Poor titanium±titanium diffusion bonding. Particularly in the foil-fiber method, leading to delamination under load, and caused by contamination, poor tooling design, uneven load distribution, poor fiber distribution, or insufficient consolidation time/temperature. For most common titanium alloys at least 850 8C is required to ensure good diffusion bonding. RemedyÐfoil pretreatment, clean handling, and lay-up practice or correct consolidation treatment. (ii) Excessive fiber movement (ªswimmingº). Leading to uneven fiber distribution, variations in local reinforcement volume fraction and, in the extreme, touching fibers. Fiber movement can lead to poor composite mechanical performance, particularly fatigue response. If fibers are too close together this can promote residual stress cracking in the gaps between the fibers, and if the fibers are touching these defects can initiate cracking under load. Variations in the fiber volume fraction can also cause long-range residual stresses and possible distortion of the component. Excessive movement may also cause fiber damage.

RemedyÐuse a low ªdebulkingº method, such as matrix coated fiber, good HIP-can and tooling design controls deformation and metal flow and also avoids kinking or excessive tensile loading of fibers during consolidation. (iii) Fiber damage (a) Carbon coating damage. Mechanical damage caused by handling, fibers touching during consolidation, transient shear or tensile stresses at the metal±fiber interface causing the coating to detach from the fiber (observed occasionally in matrix coated fiber method), or impact/thermal shock (plasma spray method) RemedyÐgood fiber management to prevent touching fibers, good filament winding and fiber position control of matrix coated fiber to prevent excessive matrix shear deformation during the consolidation cycle, and lower plasma power for plasma spray method (but this can cause excessive porosity and surface roughness). (b) Fiber fracture. Caused by excessive fiber±fiber point loading associated with touching fibers, or excessive tensile loading caused by poor tooling or HIP-can design, or excessive fiber bending during consolidation (a particular problem with crossing fibers, or for plasma sprayed material due to rough surfaces). RemedyÐgood tooling design to prevent fibers experiencing excessive stress during con-

22

Fabrication of Monofilament Reinforced Titanium

Figure 29 Ti MMC parts made by foil fiber method, including flat panels, tubes, rings, shafts, and aerofoil shapes.

solidation and good process cycle design to avoid stress concentration. For example, for a flat reinforced ring use steel or titanium tooling to restrict the deformation principally to the axial direction. (c) Excessive fiber/matrix chemical attack. Caused by excessive thermal exposure. The SiC fiber has a 2 mm thick sacrificial carbon coating, and during consolidation titanium reacts with the coating to form a reaction zone of TiC. This has little or no effect on mechanical properties providing the reaction layer is less than about this thickness. If the reaction layer is beyond this thickness the SiC fiber will have been attacked by the titanium to form TiC and Ti3Si5 and the longitudinal tensile strength of the fiber, and therefore of the composite, will fall drastically. Transverse composite strength is not affected by reaction zone thickness, unless the degradation of the fiber is very severe. RemedyÐoptimize consolidation process to reduce total thermal exposure time or temperature (service temperatures are normally low enough for fiber/matrix chemical reaction during service not to be a problem). (iv) Matrix chemical inhomogeneity. Caused by contamination in any of the fabrication processes. Titanium is very sensitive to interstitial absorption of oxygen, nitrogen, carbon, and hydrogen, all of which cause hardening and therefore brittleness of the matrix if present in excess. Of these hydrogen can be removed by vacuum degassing, but the others cannot be removed under normal circumstances. In the matrix-coated fiber process perturbation of the

evaporation bath can result in variations in the coating chemistry, or splashing from the bath can cause alloy droplets, with a different chemical composition from the coating, to adhere to the fiber and these may persist as microstructural defects in the finished MMC. Molybdenum wire used to weave fiber mats in early work using the foil fiber technique was found to degrade the matrix properties and titanium wire is now preferred. (v) Matrix cracking. Normally caused by the CTE mismatch between the fiber and the matrix giving rise to tensile residual stresses. As the composite cools from the consolidation temperature, the metal, having a much higher CTE than the ceramic fiber, contracts onto the fiber giving both axial and hoop tensile stresses in the matrix (and compressive stresses in the fiber) and this can cause both normal and radial cracks in the matrix. Hoop stresses are higher than axial stresses and therefore radial cracks are more numerous, usually occurring at the position of closest approach of the fibers. Matrix cracking on cooling from the consolidation cycle is not normally a problem for most titanium alloys, but can occur in high-temperature creep-resistant alloys such as Timetal 834 (Thomas et al., 1998) and gamma titanium aluminide, TiAl. RemedyÐimprove fiber management, reduce fiber volume fraction, cool slowly from the consolidation temperature, or use a more ductile matrix alloy.

3.24.9

CHOICE OF FABRICATION ROUTE

Flat panels are relatively easy to make using foil fiber processing and this process is ideally suited for the fabrication of skin structures, such as those proposed for the, now canceled, NASP. Although single stage to orbit (SSO) vehicles like NASP may eventually create demand for foil-fiber fabricated skins, for the present interest is focused on aeroengine parts such as rings, disks, blades, struts, and shafts. Foil fiber processing can be used for all of these shapes (Figure 29) but may not offer the best option in terms of cost, quality, and fiber architecture. For high integrity, safety-critical parts, the matrix coated fiber process is the only one that can guarantee no touching fibers. A major factor in determining the ease of fabrication of each component shape is the amount of void that must be removed during consolidation. The matrix coated fiber method offers the low debulking which is most important in ring-shaped structures. For the foil fiber

References method, grooving the foil reduces the amount of debulking while improving the fiber management, but the cost may not be justified by the improvements for particular applications. It is not clear which, if any, of the different Ti MMC processing routes will become established for commercial production. At present the foil-fiber, matrix-coated fiber, powder cloth, and wire-winding processes are all receiving attention. For all of these methods advances in the use of fugitive binder during preform and component production is leading to reliable fiber placement and consistent product quality. The wire-winding process is being applied successfully for a nonrotating part in the engine for the F22 US fighter aircraft (see Figure 20) but it is not yet known whether this process can be used economically to make high-quality Ti MMC.

3.24.10

CONCLUSIONS

(i) Used selectively, and in relatively small quantities, titanium MMC can have dramatic effect on the performance and weight of critical components. (ii) The current high cost of titanium fiber reinforced MMC is the result of a high fiber cost combined with labor and skill-intensive fabrication processes. (iii) A number of options exist for reduction of fabrication costs and these include the production of robust and convenient intermediate materials such as matrix-coated fiber and fiber/ powder-cloth prepregs and automation in fiber handling and placement. (iv) Several manufacturing routes have been identified as viable, and choice of route is likely to depend on specific component design. (v) Future use of titanium MMC will depend on quality, availability of a robust supply chain and, above all, cost.

3.24.11

REFERENCES

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23

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Fabrication of Monofilament Reinforced Titanium

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Comprehensive Composite Materials ISBN (set): 0-08 0429939 Volume 3; (ISBN: 0-080437214); pp. 655±678