GB2070055A - Forging a Ti-base Alloy - Google Patents

Abstract

A method of forging a titanium- based alloy having a first metallurgical state (e.g. the a+b state) in which the alloy retains relatively larger amounts of strain energy in its grains after forging, and a second metallurgical state (e.g. the b state) in which the alloy is capable of relieving itself of at least some of its locked-in strain energy by recrystallisation. In the method three or more forging cycles are carried out, each cycle comprising one forging step with the workpiece in the first state followed by a second step (which may be a forging or a heat treatment step) with the workpiece in the second state. In this way in each cycle, work is put into the grains in the first step and causes recrystallisation in the second step which leads to grain refinement, repeating the cycles allow best use of the energy put in.

Description

SPECIFICATION Method of Forging a Titanium-based Alloy The invention relates to a method of forging titanium alloys. It is particularly applicable to the , near s2 and a+p microstructural types. One aim of such a forging process is to produce in the final article a p grain size which is as fine and uniform as possible.

It is known that the grain size of the workpiece is influenced to some extent by the forging route.

The invention is a result of our studies to understand more fully the influence of recovery and recrystallisation processes occurring during forging and heat treatment of titanium alloys.

Titanium alloys have an allotropic transformation from body centered cubic ss phase at high temperatures to close packed hexagonal a phase with small amounts of retained p at lower temperatures. The reactions occurring during forging are influenced to a large extent by whether the forging is carried out above or below the allotropic change point, termed the P transus.

Forging above the P transus is relatively easy.

The p phase deforms at fairly low stresses and deformation is distributed evenly through the section of the workpiece. Deformation introduces strain energy into the material in the form of dislocation arrays. The temperature is high enough for dislocation movement to be relatively easy and as soon as dislocations form they tend to rearrange themselves into regular networks to reduce strain energy. This process is termed recovery. If deformation occurs faster than it can be accommodated by recovery, then strain energy can build up to such a level that within the highest energy regions, typically grain boundaries, new grains will nucleate.

The new strain-free grains then grow preferentially into the deformed structure and replace it. This is termed P recrystallisation. By changing the forging temperature and strain rate it is possible to produce p recrystallised structures with grain sizes ranging from coarser than the starting stock (few nuclei) to considerably finer than the starting stock (large numbers of nuclei).

Such a range of grain sizes has been produced in the course of our investigation.

It is common practice to forge titanium alloys in the p field for the advantages of ease of metal movement, uniform deformation and good die filling. We find that by deliberate control of the p forging conditions, a significant amount of grain refinement can also be achieved. This forms one part of the overall grain refinement process described by the invention.

Forging below the ,8 transus is more difficult.

The microstructure is predominantly a phase with small amounts of retained p and forging operations below the transus are termed a+p forging. The a phase is relatively stiff and requires higher forging pressures for metal movement, and as a result deformation tends to be distributed unevenly through the section of the workpiece.

The lower temperature of the forging operation restricts dislocation motion and makes it more difficult for deformation induced strain energy to be accommodated by recovery or by recrystallisation of the phase. At the end of a typical a+p forging operation the material contains a high level of strain energy. Heat treatment in the ,3 field produces recrystallisation and since recovery during forging has been kept to a minimum the recrystallised ss grain size is finer than that produced by an equivalent amount of forging work in the P field.

The use of an a+p forging operation per se for grain refinement is well known and employed in several commercial forging routes for titanium alloys.

In some commercial forging routes several a+,B forging operations are carried out with the material being allowed to cool to room temperature between. This occurs for example when the final shape of a workpiece is achieved by forging in a succession of graduated shape dies. Since the main advantage of a+p forging is for grain refinement on p heat treatment, the widespread use of several successive a+p forging operations implies belief that each a+p forging operation contributes strain energy towards the final ss recrystallisation on heat treatment. This could be assumed to be confirmed by the optical micro-structure of the material which shows that successive a+p forging operations produce a cumuiative amount of deformation.

We recognise that only the final a+p forging operation contributes strain energy for grain refinement on p recrystallisation. The strain energy from the previous a+p forging operations is in fact almost completely removed by recovery and ss recrystallisation during cooling and reheating between a+p forging operations.

The present invention uses this understanding to define a forging method which may produce significantly finer and more uniform grain structures than those achieved by current forging routes.

Accordingly the present invention provides a method of forging a titanium base alloy having a first a+p metallurgical state or its equivalent and a second, higher temperature p metallurgical state or its equivalent in which three our more forging cycles are carried out, each comprising a first step of (cg+p) forging followed by a recrystallisation introduced either as part of a forging operation or as a separate ss heat treatment.

In some instances, the forging process may be considered to include the last process used to form the billet, thus a final a+p working operation on the billet may be followed by a recrystallisation heat treatment to form the first forging cycle in accordance with the invention.

A particular alloy with which the process of the invention is useful comprises nominally, 5,5% Aluminium, 3,5% Tin, 3% Zirconium, 0.25% Molybdenum, 0.35% Silicon and 1% Niobium, the balance being Titanium and all percentages being by weight.

The invention will now be particularly described, merely by way of example, with reference to the accompanying drawings in which, Figure 1 is a graph of grain size in a specimen disc forged in accordance with the prior art, Figure 2 is a graph similar to that of Figure 1 but of a specimen forged in accordance with the present invention, and Figure 3 is a graph of grain size in various parts of a further disc forged in accordance with the invention.

The discs to which Figures 1 and 2 relate were forged as part of the experimental investigation we carried out into the forging process. The material of the discs comprises a titanium based alloy obtained from Imperial Metal Industries Limited and referred to as IMI 829. The alloy consisted nominally of 5.5% Aluminium, 3.5% Tin, 3% Zirconium, 0.25% Molybdenum, 0.35% Silicon and 1% Niobium, the balance being Titanium plus impurities. The temperature at which the material changes state from a+,3 to (the p transus) is some 101 soy.

IMI 829 is designed to be used in the heat treated condition and for optimum mechanical properties and integrity in service operation it requires a fine uniform recrystallised XB grain structure.

The experimental discs had a final size of 500 mm diameter and 70 mm thickness. The forging stock was similar for both discs having a 200 mm diameter and with the final billet working operation being carried out in the a+p field. The forging techniques used are set out in detail bellow, but generally it will be seen that the second disc, produced by a method in accordance with the invention, has a smaller and consistent grain size through its thickness, compared with the prior art forged disc, see Figures 1 and 2.

The forging routes, and comments on the results are set out below; Disc 1 This disc was produced using conventional forging practice. The billet was preheated into the ,3 field to take advantage of the easier metal movement. As an incidental benefit the material recrystallised with an average recrystallised grain diameter of 1 mm. After P preheating the billet was hammer forged to produce a pancake.

Because of the hammer process used, the temperature fell below the jB transus during forging and before there was sufficient work for,3 recrystallisation to occur. The remainder of the forging operation was carried out in the a+p field.

The disc was then set down to cool. Preheating to the M+P field prior to the next forging operation provided opportunity for the strain energy from the previous a+, forging to be relieved by recovery and by recrystallisation of there phase.

Forging in the cg+p field re-introduced strain energy into the material but because of the high resistance of a phase to deformation the work tended to be concentrated in the centre of the disc section. On p heat treatment the material recrystallised to a grain size of 0.8 to 0.9 mm average diameter in the centre of the section and a considerably coarser grain size in the remaining envelope of material, see Figure 1.

Disc2 This disc was produced using the forging method defined by the invention. The billet had been formed by an a+p forging operation, and therefore the ss preheat for the first hammer forging operation provided a first p recrystallisation. This billet forming operation and the ss preheat together make up a first of the cycles in accordance with the invention. The first hammer forging operation was then carried out as for Disc 1. The subsequent forging operation for Disc 2, however, started from a temperature in the p region. Hence there was an additional p recrystallisation on preheating due to the previous a+p work.During the second hammer forging operation the temperature again fell below the p transus and therefore a satisfactory amount of work was put into the structure while in the a+,3 state. When the final heat treatment took place this strain energy gave rise to a further recrystallisation. These three steps of grain refinement gave the disc a finer grain size whilst the inclusion of p deformation in both hammer forging operations produced more uniform metal movement and hence greater uniformity of grain size, see Figure 2.

It will be noted that Disc 2 includes in its routes the subsequent steps of p recrystallisation of the a+p worked billet, cr+p working towards the end of forging, p recrystallisation during preheating, +,B working towards the end of forging and p recrystallisation on heat treatment. This provides the plurality of a+,il and ,3 forging and heat treatment cycles called for in the present invention.

In order to confirm the results achieved by these tests a further disc was forged in accordance with the invention. The disc was of the same material as the experimental discs and had a final diameter of 700 mm and 160 mm thickness. The forging process used was,,3 heat treatment to recrystallise the +,B worked billet, a+ preheating and forging, P preheating and forging, a+ preheating and forging and a final P heat treatment. This gave effectively four df the forging cycles required in the method of the invention and as a result the final grain size in the thinner parts of the disc was 0.3 to 0.7 mm average diameter which is remarkably fine and uniform for such a large and complex disc forging, see Figure 3.

It will be seen from the above that although the invention is defined as being a forging process it may also include as its first step recrystallisation of the billet structure. It should also be noted that although the tests described above were carried out on a particular alloy, it is quite clear that the method could be used for any titanium-based alloy having the allotropic change from a to P within the forging temperature range for the alloy, and probably for any titanium based alloy as long as it possessed an analogous phase transformation.

Claims (6)

Claims

1. A method of forging a titanium-based alloy having a first (cr+,B).metallurgical state or its equivalent and a second higher temperature ss metallurgical state or its equivalent in which three or more forging cycles are carried out, each cycle comprising a first step of (a+,8) forging followed by a p recrystallisation.

2. A method as claimed in claim 1 and in which said alloy has a nominal composition of 5.5% Aluminium, 3.5% Tin, 3% Zirconium, 0.25% Molybdenum, 0.35% Silicon and 1% Niobium, the balance being Titanium plus impurities and all percentages being by weight.

3. A method as claimed in claim 1 or claim 2 and in which one said forging cycle includes recrystallisation of the starting billet from its +p worked conditions of supply.

4. A method as claimed in any one of the preceding claims and in which one said forging step comprises the latter portion of a longer forging step in which the initial temperature in such that the material is in said second state but because of falling temperature during forging the material changes state during the longer step into the first stage.

5. A method substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

6. A titanium-based alloy object made by the method of any one of the preceding claims.