METHOD FOR MANUFACTURING A TITANIUM PART THROUGH INITIAL BETA FORGING
The invention relates to a method of fabricating a titanium alloy part, the method comprising: heating the part to a temperature T1 so that the temperature of the part is substantially uniform, performing an initial forging operation on the part, followed immediately by quenching the part down to ambient temperature; and heating the part to a temperature T2, followed by a final forging operation on the part at the temperature T2 followed immediately by quenching the part, the final forging operation being suitable for giving the part its final shape; the temperature T1 being higher than the β-transus temperature of the alloy, the temperature T2 being lower than the β-transus temperature, the only heating of the part to above the β-transus temperature being the heating to the temperature T1, the initial forging preceding the final forging, and the initial forging being performed as soon as the temperature of the part is substantially uniform, the method being characterized in that the quenching immediately following the initial forging is performed at a speed faster than 150° C./min, with the deformation ratio during the initial forging being greater than 0.7.
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The present invention relates to a method of fabricating a part out of titanium alloy. More particularly, it relates to a method comprising:
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- heating said part to a temperature T1 so that the temperature of said part is substantially uniform, performing an initial forging operation on said part with a deformation ratio that is greater than 0.7, followed immediately by quenching said part down to ambient temperature; and
- heating said part to a temperature T2, followed by a final forging operation on said part at said temperature T2 followed immediately by quenching said part, said final forging operation being suitable for giving said part its final shape.
Titanium alloys are used in high-tech applications, in particular for aviation turbines, in order to fabricate certain parts that are subjected to high levels of stress at high temperatures. Pure titanium exists in two crystallographic forms: the α phase, which is hexagonal and exists at ambient temperature, and the β phase, which is body-centered cubic and exists at temperatures above the so-called β-transus temperature, which is equal to 883° C. for pure titanium. On phase diagrams for titanium alloyed with other elements, the β phase is to be found above the β-transus temperature, and below that temperature there is equilibrium between the β phase and the α phase over an area that depends on the elements of the alloy. The αβ phase is constituted by a mixture of α phase and β phase. In particular, the alloying elements have the effect of causing the β-transus temperature to vary around 883° C. Developing a titanium alloy that possesses desired properties consists, in particular, in selecting alloying elements and in selecting the thermomechanical treatment to which the alloy is to be subjected.
For αβ or quasi-a alloys, such as the TA6V and Ti6242 alloys, the alloy is thus in the β phase above the β-transus temperature, and respectively in a state of equilibrium between the α and β phases, or essentially in the α phase at ambient temperature.
In the description below, the term “β domain” is used to designate the range of temperatures above the β-transus temperature, and the term “αβ domain” is used to designate the range of temperatures immediately below the β-transus temperature in which the α and β phases are in equilibrium.
By way of example, one present method of fabricating forged parts made of titanium alloys comprises a plurality of forging passes, all of which are performed in the αβ domain (the temperatures T1 and T2 are then both lower than the β-transus temperature). Such a forging range does not enable the macrostructure to be fully recyrstallized and refined. At the end of the forging, there remain large colonies of α phase nodules that are inherited from the alloy billet (semi-finished form). The term “colony of α nodules” is used to designate a group of one or more nodules presenting a preferred crystallographic orientation. These colonies contribute to reducing the ability of the part to withstand fatigue.
Another method of fabricating forged parts out of titanium alloys comprises a plurality of forging passes, these passes being performed in the αβ domain, with the exception of the large pass, which is performed in the β domain (the temperature T1 is then lower than the β-transus temperature, while the temperature T2 is higher than the β-transus temperature). This last pass at a higher temperature makes the part easier to shape. Nevertheless, this last forging pass takes place at a temperature higher than the β-transus temperature, so the entire microscopic structure of the part as obtained voluntarily during the earlier passes is erased. Furthermore, the alloy grains (microscopic structure) tend to become larger and the deformation ratio of the last forging pass is often not sufficiently great to encourage recrystallization, and thus refining, of the grains (since the part immediately prior to this last forging pass is already close to its final shape). Since the grains are larger, the mechanical properties of the part are diminished.
Furthermore, during the last forging pass, the dies that are used are complex in shape (in order to give the part its final shape), which gives rise to the part having a macrostructure that is not uniform (presence both of zones that are deformed little and of zones that are deformed considerably). This non-uniformity gives rise to large variations in mechanical behavior within the part.
The present invention seeks to remedy those drawbacks.
The invention seeks to propose a method of enabling a titanium alloy part to be obtained that possesses a structure that is more uniform and that possesses better mechanical properties, in particular in terms of ability to withstand fatigue.
This object is achieved by the facts that the temperature T1 is higher than the β-transus temperature of the alloy, that the temperature T2 is lower than the β-transus temperature, that the only time said part is heated above the β-transus temperature is when it is heated to the temperature T1, that the initial forging precedes said final forging, the initial forging being performed as soon as the temperature of said part is substantially uniform, and that the quenching is performed at a speed faster than 150° C./min.
By means of these arrangements, the high deformation ratio of the part due to forging at a temperature that is sufficiently high serves to refine the microstructure (to obtain β grains of smaller size) and to erase the heredity of the part. Below the β-transus temperature, the part is constituted by β phase grains that are substantially equi-axial, since the part has not yet been deformed, given this is the first forging operation (the thickness of the part at this stage is substantially constant). Forging deforms those grains, which recrystallize into fine β grains. These small β grains themselves recrystallize into a fine needled a phase during quenching after forging. The part therefore does not have undesirable nodules of α phase at ambient temperature. The facts of subsequently quenching the part sufficiently fast and of subsequently not going back into the β domain enable this refined microstructure to be conserved, and avoids the grains growing.
Consequently, the microstructure of the alloy is refined and more uniform. The ability of the part to withstand fatigue is thus improved.
Furthermore, while detecting metallurgical defects by ultrasound, background noise is reduced. Such background noise is generated by non-uniformities in the microstructure. Since the structure is generally more uniform, it follows that background noise is diminished, and thus that any metallurgical defects in the part can be detected more finely and more easily.
The invention also provides an aviation part in the form of a body of revolution fabricated by a method of the invention.
The invention can be well understood and its advantages appear better on reading the following detailed description of an implementation given by way of non-limiting example. The description refers to the accompanying drawings, in which:
The method of the invention applies in general to a billet obtained by one or more melts of a titanium alloy, casting said alloy as an ingot, and then forging using a given thermodynamic cycle.
The microstructure difference between a titanium alloy heated to above the β-transus temperature and the same alloy heated to below the β-transus temperature is shown by comparing
As explained above, it is necessary for the entire part to be at a temperature that is higher than the β-transus temperature during the forging operation, as happens once all of the zones of the part are substantially at the temperature T1. The part is then forged at a temperature that is substantially equal to T1 so as to give it an intermediate shape that approaches its final shape (step 1-2).
During this initial forging operation, the deformation ratio is greater than 0.7. The deformation ratio Td is defined as being the logarithm of the ratio of the thickness Hi of the part prior to deformation and its thickness Hf after deformation:
If the part is not deformed, (i.e. Hf=Hi), then the deformation ratio Td is equal to 0.
Advantageously, the deformation ratio is greater than 1. Preferably it is greater than 1.6. A higher deformation ratio gives rise to greater refining of the microstructure (reduction of grain size), thereby improving the resistance of the part to fatigue. These microstructure differences can be seen in
Ideally, the initial forging operation above the β-transus temperature should be implemented using dies such that the shape of the part after forging is as close as possible to the final shape of the part, so as to minimize the stresses generated by the subsequent final forging operation. Furthermore, care can be taken to use dies that are of simple shape (e.g. a frustoconical die, in a flat stack, or of a diabolo shape) so as to enable material to flow freely throughout the mold and prevent any material becoming trapped in cavities during the forging operation.
For example, immediately after this initial forging, the shape of the part is of the frustoconical or diabolo type.
Once the part has been subjected to the forging operation in the β domain, it is subjected to quenching (step 1-3) from the forging temperature T1 down to ambient temperature at a speed faster than 150° C./min (degrees Celsius per minute). This rapid quenching serves to conserve a fine microstructure for the part (fine grains) and thus to optimize the mechanical characteristics of the part, in particular its elastic limit, as has been verified during mechanical testing undertaken by the inventor.
Advantageously, the quenching is performed at a speed lying in the range 200° C./min to 400° C./min. Even more advantageously, the quenching is performed at a speed substantially equal to 250° C./min, where tests carried out by the inventors have shown that the mechanical characteristics are best optimized at this quenching speed. Quenching is preferably performed in water.
After quenching, the part is heated to a temperature T2 that is lower than the β-transus temperature (corresponding to step 2 in
This final forging is followed by quenching (step 2-3) down to ambient temperature TA. This quenching serves to optimize the mechanical characteristics of the part, and in particular its elastic limit.
Under certain circumstances, the method of the invention may include one or more intermediate forging passes, all in the αβ domain (and thus at a temperature lower than the β-transus temperature), which passes are performed after the initial forging and before the final forging.
Under certain circumstances, it may be advantageous for the final forging to be followed by a tempering operation in the αβ domain. This forging tempering (step 3 in
Solution annealing of the part between final forging and tempering (at a temperature lying in the range T2 and T3) is pointless (since the final forging is in the domain and is therefore less severe), or might even be harmful.
Various titanium alloys may be subjected to the above-described method of the invention. For example, the titanium alloy used is an alloy of the αβ or quasi-α titanium family. In particular, the alloy may be TA6V or Ti6242 (TA6Zr4DE). By way of example, these alloys are used in aviation turbines.
Tests performed by the inventors on Ti6242 alloys show that a part obtained by a method of the invention possesses better fatigue properties than does a part obtained by a prior art method.
The part fabricated by a method as described above may be a disk for an aviation turbine, for example. By way of example, the part may be a drum for an aviation turbine.
Under certain circumstances, depending on the nature of the titanium alloy and on the type of part being treated, a portion only of the part is heated to above the β-transus temperature and is subjected to the method of the invention. Such forging is then referred to as upset forging.
Claims
1. A method of fabricating a part out of titanium alloy, the method comprising:
- heating said part to a temperature T1 so that the temperature of said part is substantially uniform, performing an initial forging operation on said part, followed immediately by quenching said part down to ambient temperature; and
- heating said part to a temperature T2, followed by a final forging operation on said part at said temperature T2 followed immediately by quenching said part, said final forging operation being suitable for giving said part its final shape;
- said temperature T1 being higher than the β-transus temperature of said alloy, said temperature T2 being lower than the β-transus temperature, the only heating of said part to above the β-transus temperature being the heating to the temperature T1, said initial forging preceding said final forging, and said initial forging being performed as soon as the temperature of said part is substantially uniform, said method being characterized in that said quenching immediately following said initial forging is performed at a speed faster than 150° C./min, with the deformation ratio during said initial forging being greater than 0.7.
2. A method according to claim 1, characterized in that said deformation ratio is greater than 1.
3. A method according to claim 1, characterized in that said deformation ratio is greater than 1.6.
4. A method according to any one of claims 1 to 3, characterized in that said quenching is performed at a speed substantially equal to 250° C./min.
5. A method according to any one of claims 1 to 4, characterized in that said final forging is followed by an αβ phase tempering operation.
6. A method according to any one of claims 1 to 5, characterized in that said titanium alloy is an alloy of the αβ or quasi-α titanium family.
7. A method according to any one of claims 1 to 6, characterized in that said titanium alloy is selected from TA6V alloy and Ti6242 alloy.
8. A method according to any one of claims 1 to 7, characterized in that said shape of the part immediately after the initial forging is of the frustoconical or diabolo type.
9. A method according to any one of claims 1 to 8, characterized in that said part is a body of revolution for an aviation turbine.
10. A method according to any one of claims 1 to 8, characterized in that said part is a disk for an aviation turbine.
11. A method according to any one of claims 1 to 8, characterized in that said part is a drum for an aviation turbine.
Type: Application
Filed: Sep 22, 2009
Publication Date: Oct 6, 2011
Applicant: SNECMA (Paris)
Inventor: Philippe Gallois (Corbeil Essonnes)
Application Number: 13/120,243
International Classification: C22F 1/18 (20060101); C21D 9/00 (20060101);