FORGING, FORMING, JOINING, AND CASTING

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Titanium - a Review of Current

Forming and Fabrication Techniques

J W Brooks, P J Bridges and D Stephen

Inco Engineered Products Ltd UEPL)

28-30 Derby Road, Melbourne, Derbyshire, England DE7 ·lFE

Abstract

Titanium alloys are unique in t.erms of the wide range of forming and fabrication methods which can be used on them: This paper describes the modern methodS of making shapes and components from titanium, and outlines the potential and limitation of these techniques.

Topics covered include isothermal forging, casting and superplastic forming/diffusion bonding.

Introduction

The increased use of titanium alloys in the engineering industries has given rise to a number of advanced forming technologies. The processes discussed in this paper, isothen:rial forging, casting and superplastic forming are used to produce a wide range of components and some of these are discussed in the context of the manufacturing route.

Isothermal Forging

Many of the alloys used to make gas turbine components have characteristics which require special consideration in relation to their processing.

Firstly, by the nature of the engineering requirement placed on them, these materials retain high strength at elevated t.emperatures even up to the normal forging range. This obviously has implications with respect to the size and cost of the forging equipment used. Secondly there is uSually a stringent requirement for a particular distribution of microstructure and properties and a corresponding requirement for consist.ency between successive components and production batches. Thirdly, turbine materials are comparatively expensive because of the alloying elements used and the complex_ melting route needed to ensure a clean, homogeneous forging billet.

All these considerations have led to the development of. isothermal forging. This section describes the process, the equipment, and the techniques used to produce turbine discs by this method.

Cha.-acteristics of Isothermal Forging

There are three main features of isothermal forging which distinguish it from the conventional forging process.

Titanium '92 Science and Technology ·

Edited by F .H. Frees and I. Caplan The Minerals, Motels & Malarials Sacicly, 1993

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(1) The dies and work-piece are maintained at the same temperature throughout the forging operation.

(2) The forging operation is carried out at a relatively slow, controlled, strain rate, generally in the range 10-6 t.o 10·1 per second.

(3) The forging operation is done in a vacuum or an inert atmosphere.

These features produce several benefits. With the dies and the work-piece at the same temperature there is no die-chilling effect. This facilitates the generation of a uniform microstructure across the section of the work-piece.

The slow strain rate used enables advantage t.o be taken of the corresponding reduced flow stress. Figure 1 illustrates this effect with reference t.o IMI 834. Reducing the strain rate by a fact.or of ten reduces the flow stress by up t.o 50%. The implication of this is that, for an equivalent work piece, press loads are much less in isothermal forging than in conventional forging. Thus the amount of superstructure is less and there is no environmental pollution in terms of noise and vibration. The slow action is also much more controllabie and easier t.o record than in conventional processes.

In the development of specific microstructures in titanium based materials there is often a requirement t.o operate within a certain strain rate range during hot working. This is necessary t.o give the right balance between microstructure and properties.

Flow Stress of IM 1834 at 940 C.

l--o--- 005/s Strain Rate

-><- 0005/s. -"7- 00005/s

300 Stress M Pa

250

200

TOO

50

Strain. .7

Figure 1 - The effect of strain

rate on flow stress

The control of ram speed in isothermil.I forging is infinitely variable and can be pre00set t.o follow_ a specifi_c strain rate programme. Thus, for example, a constant compress1onil.I stram rate along the axis of a cylindricil.I billet necessitates a corresponding decrease in the ram speed as the axial length decreases.

A further benefit of a slow strain rate is that there is no adiabatic heating in the work­piece. Adiabatic heating would, in a faster process, be generated in a non-uniform manner and could thereby generate non-uniform structures.

The production rates from an isothermal press are generally lower than those from a conventional press. However this disadvantage is offset by the fact that isothermal forging is a one00shot operation and there is no need for a series of shaping stages with the associated reheating and die changes.

The slow squeezing operation also enables pieces t.o be forged t.o near net shape. This can give significant savings on material cost in the forging and on subsequent machining operations.

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The use of an inert atmosphere is for two reasons. The first is to prevent oxidation of the work piece during forging. This would normally be aggravated by the extended times that the piece is in the preheat furnace or in the press. However both these operations are done in an oxygen-free atmosphere and this results in a very clean,

Figure 3 - As forged near

net shape turbine disc

Figure 2 - View of the

IEP isothermal press

almost shiny, surface on the forging. The second reason for using this P.nvironment for forging relates to the fact that, for forging operations above about l000°C, it is necessary to use a molybdenum based material for the dies. Only this has sufficient strength for the stresses imposed. However at these temperatures molybdenum oxidises very quickly and hence an inert atmosphere is essential.

The IEP Airfoils Isothermal Press

IEP Airfoils installed their isothermal press in 1986. It has a maximum load capability of 3200 tonnes and, at the start, had a relatively simple air-lock device for introducing the forging billets into the dies. Modifications to this system have involved building a robot-serviced pre-heating chamber in which the billets are heated before introduction into the dies. The pre-heating chamber contains the same inert atmosphere as the forge and, after forging, the piece is removed, by the robot arm, and stored in the chamber. A view of the plant is shown Figure 2.

The forging operation is viewed through a window at the front of the press and the whole operation is monitored and recorded using appropriate load and movement sensors. In this way it is possible to ensure that the process is consistent from piece to piece and from batch to batch. Temperature control in the dies is maintained by

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appropriate sensors and control of the induction heating and it is possible to maintain the temperature to within •5•c throughout the dies and work-piece.

The maximum diameter of forging currently possible is 800mm and the maximum weight which the robot can handle is 230kg. Figure 3 illustrates the ability to produce complex near net shape components.

Modelling of the Isothermal Forging Process

The intrinsic cost both of the material being forged and of the molybdenum dies means that any failure during the process could be very expensive. Therefore mathematical modelling techniques are used to predict:-

(a)

(b)

~· LOAD

2000

1000

FE - • ACTUAL - o

1 O TIME (MINS) 20

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(c)

~

(d) Figure 4 - Typical finite element analysis output

(a) - (c) Meshes, (d) Total strain

Figure 5 - Comparison between the actual press load and the finit.e element simulation of a titanium alloy disc forging

(a) the maximum load on the press during a forging operation

(b) the local stresses both in work-piece and die

(c) the strain rate regime and total strain in each part of the forging and, hence, the microstructure.

The model used is one developed in-house and is based on finite element methods(!). Because of the proliferation of relatively fast and cheap computers these methods have been applied to a number of engineering problems in recent years. The present system is based on the assumption that the material being forged behaves in an elasto-viscoplastic manner. ·

The forging piece and dies are presented graphically with the piece divided into triangular segments (Figure 4) as the dies are moved together the mesh is forced out sideways as shown. The relationship between each node in the mesh depends on various properties of the material and on the amount of friction between work-piece and die. Therefore, prior to the modelling, a significant amount of small scale testing has to be done in order that the deformation behaviour can be described adequately. In addition various assumptions have to be made about the constitutive relationships used to describe the physical processes being simulated.

Once this background information is generated for any one alloy it is relatively easy to predict total loads on the 3,200T press for a particular·geometry of forging. The success of this technique can be illustrated by reference to Figure 5 which shows actual and predicted press loads for a titanium alloy IMI 834 disc forging.

A more challenging task is that of predicting microstructural development. The effective flow stress is often defined in material models as a power function of effective strain and strain rate. However this description is inadequate for many materials. In particular, dynamic recrystallisation can take place during hot deformation and this results in flow softening which is not described by the latter approach (2,4). As a consequence the constitutive relationship utilised incorporates a variable which folloWs the microstructural evolution that takes place during hot working. The primary use of this state variable is to allow aecurate prediction of flow stress. However it is possible, given the necessary microstructural information and an understanding of the deformation process, to correlate the structural development with the parameter.

Potential and Limitations of Isothermal Forging

Process costs generally restrict use of this forming technique to high value items. The bulk of production has been gas turbine discs but there is scope for forging non­axisymetric components such as large airfoils. The advantage of the isothermal route is the consistent achievement of accurate shape witll no residual stress.

A related application is for the forging of intermetallic materials which are very brittle below about 1000°C and for which conventional forging is not applicable. The reason for hot working is that the properties of many of these intermetallics are thereby enhanced.

Casting of titanium is now a well establish route for the manufacture of components both for commercial and aerospace applications. In the case of the latter, confidence in titanium castinge is !IUCh that casting factors approaching unity are now commonplace at. the design stage.

Casting Techniques

There are three major problems in the production of titanium castings ..

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(a) High melting point of the alloys, generally above 1700°C. (b) Low fluidity of the metal at pouring temperatures. (c) High reactivity with almost every gas, liquid or solid at temperatures above

5oo·c.

The usual method of melting titanium for the casting process is to use .consumable electrode, arc-melting in a vacuum. The melting crucible is a double walled copper container with water cooling, which causes a solid skull of titanium to be formed on the inner surface. This skull and the a)?sence of atmosphere protects the molten metal from contamination.

Once the titanium electrode has been consumed there is no more heat input tD the system and the metal must be poured into the moulds as quickly as possible before further solidification in the crucible occurs. The batch weight of the IEP furnace in Belgium (SETIAS) is 1 .tDnne of metal and a i:entrifugal casting arrangement is used tD move this metal intD the moulds. The melting and casting equipment is shown

. diagrammatically in Figure 6. The mould assembly has a diameter of up tD 3 metres and, when rotating, generates a centrifugal force of up tD 60G at the periphery. This force ensures fast movement of the metal into the moulds and also provides extra pressure for feeding during solidification. ·

Moulding Methods

The two principle methods for making. moulds are:

(a) Ceramic shells made by the lost wax process - investment casting. (b) Precision sand and graphite - rammed sand.

The choice between the two depends on the complexity of the cast piece, the required dimensional tDlerances and surface finish, and economic considerations.

The dimensional tDlerances which are obtainable from the two processes are given in Table I. The investment casting technique can be used tD make complex shapes with a surface finish of 3µ. However the cost of the wax pattern dies is high and therefore this method is better suited tD long runs. It is also amenable tD the use of robotics in the shell making process. Typical examples of castings made by the lost wax process are shown in Figure 7.

TABLE I COMPARISON OF MOULDING METHODS

Method Pattern Moulding Surface Diameter Thickness Material Finish Tolerance Tolerance

(µm) (mm) (mm)

Precision Wood, Resin Special 6 750 +/-2 2.5 +/-0.6 Sand Sand

Lost Wax Metal+ Wax Ceramic 3 750 +/-1 2.5 +/-0.3

The precision sand method is generally used for simpler shapes and the surface finish is slightly inferior tD that obtained on investment castings, typically about 6µ. However the tDoling used tD shape the sand/graphite mixture is relatively inexpensive and the technique can be used tD make large pieces. Typical examples of castings

made by . the precision sand method are shown in Figure 8. This technique is a standard production route for aerospace parts and, with the flexibility offered, it is a preferred route, during the development phase before the design is fixed.

Comparison of mechanical properties obtained from castings made by the two moulding methods are given in Table II. Both routes give material which meets the relevant ASTM specification.

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Figure 7 - Typical titanium investment castings

Figure 6 - Schematic diagram of a centrifugal casting furnace

Figure 8 - Examples of titanium precision sand castings

TABLE II MEHCANICAL PROPERTIES OF CAST Ti 6/4

(-summary of all casts 1986-91 with Oxygen Cont.ent in range 0.16-0.24% after HIP and anneal)

0.2% YS 'IS el (MP al (MP al (%)

Sand Castings 891±31 1021±26 10.3±1.8

Investment Castings 871±22 984±21 9.9±1.7

ASTM B 367-83 Grade CS 825 (min) 875 (min) 6 (min)

Casting Quality

There are several other factors which have to be considered when aiming for a sound titanium casting. The first is shrinkage control to which is applied the normal rules calculating the size and position of risers and in-gates. In the precision sand process there is the possibility of using various moulding mixtures to give a progressive chilling effect but this is not so easy with investment casting shells. There is an extra complication imposed by the centrifugal casting pro.ess. This gives an added

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compressive force in the radial direction of the chamber. This also leads on to the second consideration which is mould distortion. All the usual precautions need to be taken during mould making, dewa.xing, drying and firing to controi and monitor mould distortion and shrinkage. In addition, because of the extra hydrostatic and gravity forces during casting, particular care has to be taken in mounting and supporting the moulds in the chamber. There is also a need to ensure that the mould assembly is balanced in relation to the axis of rotation, both before, during and after filling, otherwise mechanical instability will result.

The third consideration in casting quality relates to surface contamination. Although titanium is cast under vacuum and in a mould made of an appropriate non-reactive material, it is quite difficult to avoid any contamination of the casting surface by oxygen, carbonaceous material or first coat oxides picked up by the molten metal from the mould surface at the time of pouring. The resulting layer of contamination, known as alpha-case, is very hard and can range in thickness from 0.6mm down to zero. The alpha-case, because it is hard and can easily initiate surface cracks, has to be removed and this is usually done by chemical milling in mixed acids. Obviously to achieve good dimensional tolerances and good surface finish in the casting, the alpha-case thickness and its removal .need to be controlled and monitored. Removal of the alpha-case has the added benefit that it generally further improves the surface finish.

Hot Isostatic Pressing

The final factor to be considered in relation to quality is internal microporosity which is inevitably present in any casting. Although this is minimised by appropriate foundry engineering and the imposition of centrifugal force there is still a need, particularly in. pieces for aerospace, to take further steps to ensure the absence of internal cavities. Hot isostatic pressing (HIP) is very effective in producing full densification of titanium and its alloys.

Trials have been done in order to define the effect of HIP on porosity level. Results are shown in the quantative metallography histogram in Figure 9. For this latter technique six micrographs from each sample, at a magnification of x50, were analysed in order to record a statistically significant number of pores.

Opportunities and Limitations

The maximum size of titanium castings is limited by the furnace capacity the largest of which is reported to be 3 tonnes (in CIS). The complexity is limited only by the skill of the tool designer and manufacturer, and the willingness of the customer to pay. Large and complex engine components are now appearing as titanium castings (Fig 10). In many of these, thin walls are called for and, although these cannot be cast directly, they can be sculptUred usin~ chemical milling.

Superplastic Forming and Diffusion Bonding

Twenty five years of development have brought the processes of superplastic forming and diffusion bonding to a state of maturity. These processes provide the designer with the opportunity to design components which are both cost and weight efficient. However, to achieve optimum performance, the designer needs to have an in-depth understanding of the freedoms and limitations provided by these processes.

A tradition has grown up over the years which limits the use of titanium to-

a)areas which demand its high temperature strength.

b)highly loaded, fatigue sensitive fittings.

This situation has arisen due, in the main, to the perceived high cost of material and the associated conventional manufacture of titanium. Designers need to be reminded

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r~igure 9 - Quantitative metallographic measurement of pore size and distribution in:-

.JO I .... -

(a) as - cast titanium (b) cast and HIP titanium

Figure 10 - Titanium 6-4 compressor casing

that the specific properties of titanium and it's corrosion and fatigue resistance make it an attractive structural material even for room temperature use. With the development of the combined processes of SPF/DB and their costsaving potential, there is now a growing body of evidence that titanium can compete with aluminium fabricated components in both cost and weight. In the light of this evidence, therefore, designers need to review their traditional views of this material.

Production Equipment

Early experimentation was conducted using primitive press/heated platen systems and in many cases using ''hard back" bolted up tooling systems. Today, however, purpose built heated platen presses of a type similar to that shown in Figure 11. have become the mainstay of SPF and SPF/DB manufacture.

Variations on this basic theme have now been produced in, for example, the "Shuttle" platen press developed by Aerospatials, the Gantry press system recently installed by McDonnell/Douglas and the "C-Frame" restraint system developed by Grumman.

Alongside the press development there has been automation of the welding processes for cutting and stitching together of the preforms, and of the subsequent non­destructive testing.

Design and Manufacturing Aspects

All of the SPF/DB structural types which are presented schematically in Figures 12-14 have specific structural and manufacturing aspects which should be understood by the designer. 1,327

Figure 11 - SPF/DB platten press

SUPERPLASTICALLY FORMED AND DIFFUSION --~~d B.ONDED ENGINE BAY NOULE PANEL .-o;;;;;.,...i;o~

Figure 15 - A 2 sheet SPF/DB part

Figure 16 - A 4 sheet SPF/DB part 1,328

P ... CK BONDINO USINO 0.AS PRESSURE

SP FORMING OF STRUCT\.'PE UStNO

OAS PRESSURE

Figure 12 - 2 sheet structure

[!) PACt( BONOINO

@FORMING

Figure 13 - 3 sheet structure

[!J CORE SHEETS

~ [!] P"CK BONDING

~FORMING

IQ] BONDING

_Jnprq_nnL l~I

Figure 14 - 4 sheet structure

2-Sheet Structure The two sheet structure of the type illustrated in Figure 12 represents the SPF/DB equivalent of a common conventional form of construction and therefore, with the exception of the use of DB as a means of joining the two sheets, the structural advantages and limitations should already be well understood. Although the formed sheet will be subject to thinning, generally the shapes demanded in such structures do not present difficulties for the prediction of the thickness distributions.

In effecting the DB of such structures, two methods of manufacture.are possible:-

a) pre bonding, either singly or in a pack, using gas pressure and delineating the bond areas using a bond-inhibitor (stop-om then subsequently expanding in the form tool as a separate operation.

b) bonding by mechanical means in the tool prior to forming, this is usually a continuous but sequential operation.

The former route requires good alignment between the bonded areas of the blank and the corresponding features in the tool to ensure a consistent and optimum transition between the swaged section and the bonded area.

Although the latter route will have a bond area which is defined and effected by the pressure generated by the tooling features, the quality of the bond is totally dependent upon the matching of tool faces, including the flatness and combined thickness of the component blank in the area to be bonded(6).

3-Sheet Structure Unlike the two sheet SPF/DB form, which is controlled directly by the tool in terms of its final structural definition, the three sheet structural form is largely controlled by the definition of the starting blank. The tool in this case, only provides a restraint to the blank around its periphery and in addition provides the envelope shape of the finished component. This, therefore, places responsibility on the designer to define both the starting blank and the finished component and to accomplish this it is necessary for the designer to model and understand the forming process. For simple structures, such as that illustrated in Figure 14, the modelling is relatively easy but for the more complex stiffening patterns such as sine wave or discontinuous stiffening, the establishment of a finite element technique as a basis of an SPF CAD system is highly desirable and will greatly reduce the iterative trial and error development which is currently a feature of SPF/DB technology.

The fact that the tooling can, to a large degree, be divorced from the component structural definition, provides the designer in turn with freedom to change the structural definition without affecting the tool. This is comforting to the designer bearing in mind the cost and lead time of provisioning tools.

One feature of the three sheet structure, is the fact that the skin thicknesses used need to be significantly greater than the core thickness to ensure skin stability during forming and to overcome external waviness on the finished component.

A wide variety of core stiffening patterns are possible with the three sheet structure. These include straight, sine wave, and discontinuous features. Orthogonal stiffening is possible for a three sheet structure but requires a considerable increase in the blank complexity.

4-Sheet Structure The four sheet structure illustrated in Figure 16 results in cellular stiffening which has the virtue of having design and stressing characteristics which, although not identical in detail, are familiar to the designer in their conventionally fabricated equivalent.

As for the three sheet structure, the designer has the responsibility of defining the details of the starting blank to achieve his finished component. Again this cannot be

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achieved without an in depth understanding of the manufacturing process and this has been greatly assisted by the development of a finite element CAD analysis system.

Opportunities and Limitations

The plan view area of an SPF/DB structure is limited by the size of the press. The depth of the structure can be accommodated by cell design which allows for the metal thinning. This latter aspect can be alleviated either by using thicker starting material or by selective metal removal on the blanks to leave re-inforcement'where needed on the blown piece. There is a limit to the increase metal thickness useable because in sheet above about 4mm thick it is not easy to achieve superplastic. properties. The main advantages of the SPF/DB process are economic, due to higher material utilisation and simple starting blank forms combined with significant weight savings.

Conclusion

The advances in the production methods for titanium alloy components discussed in this paper have provided engineers with the ability to specify parts with good mechanical properties, high integrity and complex forms which can be manufactured economically.

References

(1) P.S.Bate: AGARD Conf. Proc. 426 (1987) 22. (2) J.V.Bee, AR.Jones, P.R.Howell: J. Mat. Sci. 15 (1980) 337. (3) A.Y.Kandiel, J-P.A.lmmarigoon, W.Wallace, M.C.de Malherbe:

Met. Sci. 14 (1980) 493. (4) R.G.Menzies, J.W.Edington, G.J.Davies: J. Mat. Sci. 15 (1981) 217. (5) M.Ohsumi, M.Shimizu, A.Takahashi, T.Tsuzuku, Mitsubishi Heavy Industries -

Technical Review, 21 (1) (1984) 42.

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