Patent Application
20100180991
The present invention relates to a Ti 5-5-5-3 titanium alloy (meaning 5% aluminum, 5% vanadium, 5% molybdenum, 3% chromium on a titanium base) and more particularly a heat treatment for said alloy, the object of which is to improve the standard and uniformity of its mechanical properties.
The Ti 5-5-5-3 alloy is a quasi-beta type titanium alloy which at ambient temperature, has two phases, an alpha phase (hereinafter “α”) and a beta phase (hereinafter “β”), and which has a β transition (hereinafter “β-transus”) between a domain where the α and β phases coexist and the pure β phase domain. The temperature at which the β-transus is found varies between 840° C. and 860° C. depending on the composition of the Ti 5-5-5-3 alloy. The Ti 5-5-5-3 alloy has both low density and high mechanical strength. This is why it is highly prized in applications in the aeronautical field, for example to produce landing gear parts and structural parts. However, this alloy is very sensitive to microstructural flaws. Parts made of Ti 5-5-5-3 are usually obtained after thermomechanical transformation steps followed by heat treatment steps.
The thermomechanical transformation steps are carried out in the beta phase domain, in other words at temperatures which are higher than the β-transus temperature of the alloy and at which the beta phase grains form the matrix of the alloy, then in the alpha-beta phase domain, in other words at temperatures which are lower than the β-transus temperature of the alloy.
The half-finished products obtained after the thermomechanical transformation steps have, at ambient temperature, a microstructure comprising primary alpha phase in the form of globular particles and elongated particles, secondary alpha phase in the form of lamellar particles, and beta phase. The primary alpha phase represents 10 to 30% of the structure. In the rest of this description, when a percentage of the structure represented by a given phase is discussed, it should be understood that, as is conventional, this percentage is measured by optical micrograph image analysis: the extent of the area occupied by said phase on the micrograph is measured by comparison with a reference grid.
After the thermomechanical transformation steps, the parts made of Ti 5-5-5-3 alloy are subjected to conventional heat treatments to obtain the desired mechanical properties.
However, after these heat treatments a large dispersion in the mechanical properties of parts made of the Ti 5-5-5-3 alloy persists, particularly with regard to the properties of ductility, toughness, tensile strength and resistance to fatigue, which remain very anisotropic in the alloy.
A common heat treatment of the Ti 5-5-5-3 alloy consists of carrying out in succession:
The dispersion of the mechanical properties of parts made of Ti 5-5-5-3 alloy obtained after conventional heat treatments is due to heterogeneity of the microstructure of the alloy, which itself is the result of the initial texture of the alloy following the thermomechanical transformation steps. In particular, after a conventional heat treatment, the Ti 5-5-5-3 alloy has a heterogeneous alpha phase distribution within the microstructure. Moreover, after a conventional heat treatment, the alpha phase appears in the form of particles elongated in a dominant orientation resulting from the forging or rolling direction during the last thermomechanical transformations. This dominant orientation of the alpha phase particles leads to mechanical properties which, measured in a direction parallel to that of the alpha particles, are acceptable, but which are very inadequate in a direction transverse to that of the alpha particles.
The object of the invention is to improve the standard and uniformity of the mechanical properties of a part made of Ti 5-5-5-3 alloy while avoiding the above-mentioned drawbacks of the prior art.
Accordingly, the invention relates to a heat treatment process for the Ti 5-5-5-3 titanium alloy, which has the following composition, in weight percent:
The heat treatment process according to the invention comprises the following successive steps:
The above-mentioned heat treatment stages are carried out at temperatures below the β-transus temperature of the Ti 5-5-5-3 alloy.
As explained earlier, the microstructure of the alloy after the thermomechanical transformations (forging or rolling, for example) is heterogeneous. The first stage according to the invention allows the microstructure of the alloy which has been affected by the preceding thermomechanical transformations to be homogenised. The first stage temperature is slightly lower than the β-transus temperature of the Ti 5-5-5-3 alloy, so as to solution treat as much alpha phase as possible without however eliminating this phase which remains necessary to avoid an excessive increase in the size of the grain. In fact, without a minimum quantity of alpha phase, the beta phase grains would grow uncontrollably leading to a significant reduction in the mechanical properties, particularly tensile strength. Preferably, the temperature and duration of the first stage are determined so as to obtain a quantity of alpha phase of between 2 and 5% at the end of the first stage.
The second stage according to the invention is defined so as to precipitate an equiaxial globular primary alpha phase. Because of the first stage in which the microstructure of the alloy was homogenised, the new alpha phase nuclei appear in a homogeneous distribution in the microstructure of the alloy and their growth occurs equiaxially during the second stage to form globular primary alpha phase particles.
Thus, at the end of the second stage, the microstructure of the alloy is homogeneous and the first two heat stages carried out according to the invention have allowed a homogeneous globularisation of the primary alpha phase within the microstructure and an adequate proportion of said primary alpha phase to be obtained.
Because of the heat treatment according to the invention, the Ti 5-5-5-3 alloy has homogeneous and improved mechanical properties (ductility, toughness, tensile strength and resistance to fatigue). More specifically, the presence of homogeneously distributed globular primary alpha phase markedly improves the ductility of the alloy.
The inventors have been able to demonstrate that the compromise between the tensile strength and ductility of the alloy was optimal when the quantity of globular primary alpha phase at the end of the second stage was between 10 and 15%. The temperature and duration of the second stage of the heat treatment according to the invention are therefore preferably determined to obtain a quantity of globular primary alpha phase at the end of the second stage of between 10 and 15% in a beta phase matrix. Preferably, the temperature of the second stage is between 770° C. and 790° C.
The first stage is preferably carried out at a temperature between the β-transus temperature less 20° C. and the β-transus temperature less 30° C. and the second stage is carried out at a temperature of between 770 and 790° C.
The first and second stages are preferably performed successively.
The cooling speed between the first stage and the second stage is preferably between 1.5° C. and 5° C. per minute, and cooling at the end of the second stage is carried out to ambient temperature at a speed of between 5° C. and 150° C. per minute.
The third stage is known as ageing as is standard practice for this type of alloy.
The titanium alloy is maintained at the temperature of the third stage for six to ten hours, preferably about eight hours.
The invention also relates to a part made of Ti 5-5-5-3 alloy, characterised in that it has been obtained from a half-finished product obtained by the above heat treatment process.
Other advantages and characteristics of the invention will emerge on reading the following description given as an example and with reference to the accompanying drawings, in which:
The Ti 5-5-5-3 alloy heat treatment process according to the invention applies to parts that have, as is standard, been shaped following one or more thermomechanical transformation steps performed in the beta phase domain, followed by steps performed in the alpha-beta phase domain. These may be thermomechanical transformation steps through rolling, forging or extrusion.
The parts obtained after such thermomechanical transformation steps have, at ambient temperature, a microstructure comprising primary alpha phase in the form of globular particles and elongated particles, secondary alpha phase in the form of lamellar particles, and beta phase. Following the thermomechanical transformation steps, the texture of the alloy is affected (orientation of the different alpha phase morphologies), and the microstructure of the alloy is very heterogeneous. In particular, the alpha phase particles are in the form of needles which are distributed in particular in the region of the beta phase grain boundaries. The alpha phase particles may be contiguous and form ribbons which have a detrimental effect on the toughness and resistance to fatigue and ductility of the alloy.
Heat treatments to improve the mechanical properties of the Ti 5-5-5-3 alloy have been intensively studied. However, the so-called conventional treatments do not result in a homogeneous microstructure of the alloy, such that the mechanical properties are anisotropic across the alloy and inadequate to meet the stricter requirements demanded for certain applications, such as landing gear parts.
In fact, as shown in the micrograph in
One of the objects of the heat treatment according to the invention is therefore to homogenise the microstructure of the Ti 5-5-5-3 alloy.
The inventors have developed an optimised Ti 5-5-5-3 alloy heat treatment as shown diagrammatically in
The first stage 4 situated between 810° C. and 840° C. and a little lower than the β-transus temperature of the alloy, according to the invention, allows the microstructure of the alloy affected by the previous thermomechanical transformation steps to be homogenised, and as much alpha phase as possible to be solution treated, without however completely eliminating said alpha phase. Preferably, the temperature and duration of the first stage 4 are determined to obtain a quantity of alpha phase of between 2 and 5% at the end of the first stage 4. A minimum content of 2% ensures that the beta phase grains do not grow uncontrollably, which would have the consequence of considerably reducing the mechanical characteristics of the alloy particularly the tensile mechanical properties. In addition, an alpha phase content of less than 5% is preferable to allow good homogenisation of the microstructure of the alloy, and in particular to break the alpha phase ribbons which formed following the thermomechanical treatments.
As explained earlier, the β-transus temperature varies depending on the exact composition of the Ti 5-5-5-3 alloy. To obtain the required quantity of alpha phase, the temperature of the first stage 4 is determined depending on the exact composition of the Ti 5-5-5-3 alloy and its β-transus temperature. To achieve the alpha phase quantity preferably required, the first stage 4 is carried out at a temperature between the β-transus temperature less 20° C. and the β-transus temperature less 30° C., regardless of the Ti 5-5-5-3 composition.
The duration of the first stage 4 is between one and three hours and is a function in particular of the geometry and bulk (diameter, thickness) of the part. The bulkier the part, the longer the stage lasts.
The second stage 6, between 760° C. and 800° C. according to the invention, is defined to allow the precipitation of globular primary alpha phase. Because of the first stage which allowed a homogeneous structure of the alloy to be obtained, the new alpha phase nuclei appear in the course of the second stage 6, in a homogeneous distribution in the beta matrix of the alloy, and the growth of the alpha nuclei occurs equiaxially during the second stage 6 to form globular primary alpha phase particles 11, as shown in
Thus at the end of the second stage 6, the microstructure of the alloy is homogeneous and the heat treatment according to the invention allows, moreover, a homogeneous globularisation of the primary alpha phase 11 to be obtained within the microstructure (see the micrograph in
The inventors have been able to demonstrate that the compromise between the tensile strength and ductility of the alloy was optimal when the quantity of globular primary alpha phase 11 at the end of the second stage 6 was between 10 and 15%. Preferably the second stage temperature is between 770° C. and 790° C. to obtain a quantity of globular primary alpha phase of between 10 and 15% at the end of the second stage 6.
The duration of the second stage 6 is between two and five hours and is also a function of the geometry and bulk (diameter, thickness) of the part. The bulkier the part, the longer the stage lasts.
Typically, for a part of complex form made of Ti 5-5-5-3 titanium alloy with a composition of:
At the end of the first stage 4, cooling to ambient temperature or to a temperature lower than that of the second stage 6 would not be in accordance with the invention. In fact, such cooling, which would have to be followed by reheating to the temperature of the second stage 6, would lead to the formation of Widmanstätten-type alpha phase (slender secondary alpha phase), at the expense of the formation, during the second stage 6, of a minimum quantity of equiaxial globular primary alpha phase required to obtain good ductility characteristics of the alloy after heat treatment.
The cooling speed 5 between the first stage and the second stage is preferably between 1.5° C. and 5° C. per minute and is, for example, carried out without removing the part from the treatment furnace. The part therefore cools progressively in a controlled manner inside the furnace, the set temperature of which has been lowered progressively or immediately until it reaches the temperature of the second stage 6.
A cooling speed of over 1.5° C./min is preferred to avoid a change occurring in the distribution of primary alpha phase during cooling speeds that are too low which could be detrimental to obtaining good mechanical properties. On the other hand, a cooling speed of more than 5° C./min could lead to the precipitation of needle-type alpha phase which is detrimental to obtaining good mechanical properties such as elongation at rupture. In fact, an excess of needle-type alpha phase in the structure of the material increases the risk of brittle fracture.
Cooling performed in the open air is not usually advisable, as its speed is difficult to control and, in many cases, the temperature of the part drop too low, requiring reheating to the second stage temperature. Such reheating must be avoided, for the reasons already stated, and cooling inside the furnace is an advantageous solution for implementing the invention. Moreover, carrying out air cooling by removing the part from the furnace requires handling the part at high temperature, which is difficult to perform.
The first 4 and second 6 stages are, preferably, carried out successively.
What is meant by “successively” is that the move from the first treatment stage 4 to the second treatment stage 6 is achieved by progressively reducing the temperature during cooling 5 to pass from the first stage 4 to the second stage 6, without maintaining an intermediate temperature which would be lower or higher than that of the first stage 4.
This successive performance of the two stages is preferred, thus separating them by a progressive cooling to avoid any change occurring in the primary alpha phase distribution during an intermediate stage, which could be detrimental to obtaining good mechanical properties.
The cooling 7 following the second stage 6 is carried out to ambient temperature at a speed preferably of between 5° C. and 150° C. per minute. This is for example air cooling carried out after having removed the part from the treatment furnace.
It is preferable that the cooling speed following the second stage should be less than 150° C. per minute to avoid too heterogeneous a hardening between the surface and the core of the part and avoid the risk of contraction cracking (superficial cracking) during cooling.
A speed of at least 5° C. per minute is preferable to anticipate a homogeneous response to the subsequent tempering treatment during which hardening precipitation occurs.
The third stage 9 is known as the ageing stage as is standard practice for this type of alloy, the object of which is to harden the alloy by alpha phase precipitation.
The titanium alloy is maintained at the temperature of the third stage 9 for six to ten hours, preferably about eight hours. The microstructure obtained after this third stage 9 is illustrated in
At the end of the heat treatment according to the invention, the mechanical properties of the Ti 5-5-5-3 alloy are isotropic, and have been improved compared with those of parts made of Ti 5-5-5-3 alloy obtained by conventional heat treatments. Because of the heat treatment according to the invention, it has been possible in particular to improve the tensile strength and ductility of parts made of Ti 5-5-5-3. On the parts tested, Rm values of more than 1290 MPa, elongation values “A” of more than 5% and reduction in area values “Z” of more than 15% have in fact been obtained.
As a comparison, after conventional treatments very dispersed RM values of between 1230 MPa and 1360 MPa are obtained on the same part. The elongation values are also very dispersed, between 0.7 and 6.8%. The treatment according to the invention allows high and far less dispersed Rm values of between 1260 and 1300 MPa, and similarly high and far less dispersed elongations, of between 5 and 7.5%. Generally, the treatment according to the invention guarantees a minimum Rm value of 1260 MPa and a minimum A value of 5%, whereas conventional treatments do not guarantee these minimum values.
The effects of the invention are particularly remarkable on bulky parts, in other words parts with a thickness or diameter of more than 100 mm.
Once the alloy has been treated according to the invention, finishing operations continue as usual in the prior art to obtain the final part.
| Number | Date | Country | Kind |
|---|---|---|---|
| 08 59071 | Dec 2008 | FR | national |
| Number | Date | Country | |
|---|---|---|---|
| 61140748 | Dec 2008 | US |
