PROCESS FOR MANUFACTURING A NICKEL-BASED ALLOY PRODUCT

Abstract

Method for manufacturing a nickel-based alloy product, including: a step of supplying an ingot of a nickel-based alloy including at least one intermetallic phase, a first forging step of forging the ingot, during which the true strain rate is less than 1 s−1, a second forging step of forging the ingot, during which the true strain rate is greater than 1 s−1, and a step of static recrystallisation, after the second forging step, during which the ingot is exposed to a temperature of between 1000 and 1055° C. for a duration of between 30 minutes and 10 hours.

Description

TECHNICAL FIELD

The present disclosure relates to a method for manufacturing a nickel-based alloy product, more particularly in the field of aeronautics. Such a method makes it possible to obtain improved tensile and fatigue properties.

PRIOR ART

In the field of aeronautics, the parts most subjected to mechanical stress and requiring tensile and fatigue dimensioning, as in the case of turbine discs for example, are generally produced from nickel-based alloy with intermetallic hardening, in other words from nickel-based alloy comprising at least one hardening intermetallic phase, such as Ni₃Al or Ni₃Ti for example.

In general, these materials are produced by remelting before being forged in order to obtain billets or other semi-finished products such as bars. This forging conventionally includes a first forging step at low strain rates, using a hydraulic press for example, then a second forging step at higher strain rates, using hammer machines for example.

However, at the end of such a manufacturing process, in some cases an incomplete or inhomogeneous recrystallisation or restoration is observed between the core and the periphery of the billet. FIGS. 1 and 2 illustrate such an imperfect microstructure. More specifically, the presence of non-recrystallised elongated grains is observed, the length of which can exceed 1 mm.

These heterogeneities lead to deteriorated mechanical properties. In particular, tensile and fatigue tests on test specimens of these materials reveal a significant dispersion in the results, the zones which are not totally recrystallised being responsible for the minimum values measured during these tests. Moreover, poor recrystallisation has an impact on the non-destructive inspection of parts and, in particular, on the level of background noise during ultrasonic inspection of parts. Thus, ultrasonic background noise exceeding the relevant thresholds prevents the detection of faults in the part.

There is therefore a real need for a method for manufacturing a product made of nickel-based alloy which is devoid, at least in part, of the disadvantages inherent in the aforementioned known method.

DISCLOSURE OF THE INVENTION

The present disclosure relates to a method for manufacturing a nickel-based alloy product, comprising:

• • a step of supplying an ingot of a nickel-based alloy comprising at least one intermetallic phase,
  • a first forging step of forging the ingot, during which the true strain rate is less than 1 s⁻¹,
  • a second forging step of forging the ingot, during which the true strain rate is greater than 1 s⁻¹,
  • a step of static recrystallisation, after the second forging step, during which the ingot is exposed to a temperature of between 1000 and 1055° C. for a duration of between 30 minutes and 10 hours.

Through this static recrystallisation step, carried out under these particular conditions of temperature and duration, it is possible to finalise the recrystallisation of the entire product obtained, including its core. More specifically, this takes place in this case at a temperature greater than the temperature for the start of static recrystallisation, and at a temperature less than the solvus of the intermetallic phase.

It is then observed that this static recrystallisation step makes it possible to obtain test results having higher means and lower dispersions. This manufacturing method therefore results in nickel-based alloy products benefiting from improved tensile and fatigue properties. Moreover, the ultrasonic background noise is reduced, which then makes it possible to detect lower-amplitude echoes.

In some embodiments, said at least one intermetallic phase is a γ′ phase combining nickel with aluminium and/or titanium.

In some embodiments, said at least one intermetallic phase is present in the alloy of the ingot at a level of 30% to 50% by weight, preferably at a level of 35% to 45% by weight. In particular, it can involve a γ′ phase present at a level of 40% by weight in the ingot.

In some embodiments, before the supplying step, the ingot has undergone a vacuum remelting step.

In some embodiments, the vacuum remelting step comprises at least one vacuum arc remelting step (VAR).

In some embodiments, the vacuum remelting step comprises, in this order, a step of vacuum induction melting (VIM), a step of electro slag remelting (ESR) and a step of vacuum arc remelting (VAR).

In some embodiments, the product obtained is a semi-finished product, preferably a billet or a bar, for example cylindrical.

In some embodiments, the first forging step is carried out using a hydraulic press.

In some embodiments, the first forging step is carried out at a temperature between 1050 and 1150° C. Thus, it takes place above the solvus of the intermetallic phase so that the grain boundaries can move. Furthermore, the temperature is sufficiently low as not to cause overheating and deterioration of the grain boundaries.

In some embodiments, the first forging step leads to a strain of the ingot greater than 1. This strain ε is measured relative to the start of this first forging step.

In some embodiments, the second forging step is carried out using a rolling mill and/or one or more hammers or power hammers. In particular, a forging machine with 4 hammers can be used.

In some embodiments, the second forging step is carried out at a temperature between 900 and 1055° C. Thus, it takes place at a sufficient temperature to enable forging, without however exceeding the solvus of the intermetallic phase.

In some embodiments, the second forging step leads to a strain of the ingot greater than 0.2, preferably greater than 0.5. This strain ε is measured relative to the start of this second forging step.

In some embodiments, the recrystallisation step is carried out at a temperature between 1000° C. and 1025° C., preferably between 1000 and 1010° C., and for a duration between 1 hour and 10 hours, preferably between 4 hours and 6 hours. In particular, it can be carried out at 1000° C. for 5 hours.

In some embodiments, the recrystallisation step is carried out at a temperature between 1025° C. and 1055° C., preferably between 1045 and 1055° C., and for a duration between 30 minutes and 2 hours, preferably between 50 minutes and 70 minutes. In particular, it can be carried out at 1052° C. for a period of 1 hour.

In some embodiments, the recrystallisation step is followed by a step of air cooling.

In the present disclosure, a nickel-based alloy is an alloy for which the nickel is the element having the largest mass fraction, preferably greater than 40%.

The above-mentioned features and advantages, and others, will become apparent on reading the detailed description which follows, of exemplary embodiments of the method for manufacturing a nickel-based alloy product. This detailed description refers to the attached drawings.

BRIEF DESCRIPTION OF THE FIGURES

The attached drawings are schematic and primarily aim to illustrate the principles of the disclosure.

In these drawings, from one figure to another, identical elements (or parts of elements) are identified by the same reference signs.

FIG. 1 is a first micrograph of a billet according to the prior art.

FIG. 2 is a second micrograph of a billet according to the prior art.

FIG. 3 is a flow diagram representing the various steps of an exemplary manufacturing method according to the present disclosure.

FIG. 4 is a first micrograph of a billet manufactured by this method.

FIG. 5 is a second micrograph of a billet manufactured by this method.

FIG. 6 is a graph representing comparative fatigue test results.

DESCRIPTION OF THE EMBODIMENTS

In order to make the disclosure more concrete, an example of a method for manufacturing a nickel-based alloy product is described in detail below, with reference to the attached drawings. It is recalled that the invention is not limited to this example.

FIG. 3 schematically represents the various steps of this method. This method starts with the supply of an alloy ingot 10 and results in the obtaining of a billet 20, a semi-finished product ready to be transformed into a particular part by forging operations.

In the present example, the ingot 10 is a nickel-based alloy ingot comprising γ′ intermetallic phases combining nickel with aluminium and/or titanium. These γ′ intermetallic phases are present in the ingot at a level of 40% by weight.

This ingot 10 can have been obtained by a VIM-ESR-VAR method comprising, successively, a step of vacuum induction melting (VIM), a step of electro slag remelting (ESR) and a step of vacuum arc remelting (VAR).

The vacuum induction melting step (VIM) enables the raw materials to be mixed by vacuum induction melting. The electro slag remelting step (ESR) is applied to the ingot coming from the VIM melting and makes it possible to refine the microstructure of the alloy and to improve its degree of inclusions (inclusional cleanliness). The vacuum arc remelting step (VAR) is applied to the ingot coming from the ESR remelting and makes it possible to refine the microstructure of the alloy a little further and to again improve its degree of inclusions.

The ingot 10 thus obtained then undergoes a first forging step E1 carried out at true strain rates less than 1 s⁻¹. In particular, this first forging step E1 can be carried out using a hydraulic press. It is carried out at a temperature between 1080 and 1110° C. and results in a strain ε greater than 1.

A second forging step E2 is then carried out at true strain rates greater than 1 s⁻¹. In particular, this second forging step E2 can be carried out using a forging machine with 4 hammers. It is carried out at a temperature between 1000 and 1055° C. and results in a strain ε greater than 0.5.

At the end of these two forging steps E1 and E2, an as-cast billet 19 is obtained. This as-cast billet 19 then undergoes a static recrystallisation step E3 by heating the as-cast billet 19 to a temperature of 1052° C. for 1 hour, the billet obtained 20 is then air cooled.

FIGS. 4 and 5 are micrographs illustrating the microstructure of the billet 20 thus obtained. It should be noted in these figures that the microstructure is homogeneous and no longer has large non-recrystallised grains, as was the case in the micrographs according to the prior art of FIGS. 1 and 2. The areas outlined in these figures correspond to the ALA grains.

Some test results will now be presented. Samples were taken in an as-cast billet 19, before the static recrystallisation step E3, in a final billet 20, after the recrystallisation step E3, and in a comparative billet obtained from the same as-cast billet 19 after a heat treatment step carried out at 1066° C. for 1 hour. For each of these billets, samples were taken at the half radius (“R/2”) as well as at the periphery.

For each sampling, two populations of grains, having significantly different grain sizes, were observed, forming a duplex structure within the meaning of standard ASTM E1181. The tables below then give, for each location, the size of the primary grains, having a smaller size, and of the secondary grains, having a larger size. In these tables, “ALA” means “As Large As”, within the meaning of standard ASTM E930: it is therefore the largest size of the population.

Table 1 below gives the measurement results for the as-cast billet 19. These results reveal a duplex structure within the meaning of ASTM E1181 having, in particular, proportions of up to 70% secondary population with coarser grains and deviations of up to 12 ASTM between the two populations observed.

	TABLE 1
	R/2	Periphery
Primary	85% ASTM Number 11.5	30% ASTM Number 12,
Secondary	15% ASTM Number −1	70% ASTM Number 0.5

Table 2 below gives the measurement results for the final billet 20 obtained at the end of the method according to the present example. These results reveal reduced proportions (down to almost 0% in certain cases) of the secondary population. Furthermore, the secondary population displays finer grains than before the invention (billet 19), reducing the deviations between the primary and secondary populations.

	TABLE 2
	R/2	Periphery
Primary	ASTM No. 11.5	80% ASTM Number 12,
Secondary	ALA ASTM No. 4.5 (<1%)	~20% ASTM Number 7.5 +
		ALA ASTM Number 2.5 (<1%)

Furthermore, FIG. 6 illustrates results of fatigue tests carried out on specimens from as-cast billets 19 and final billets 20, hence before and after static recrystallisation step E3. These fatigue tests were carried out by subjecting specimens to cyclic tensile loading with Cmax maximum stress: several Cmax were tested in order to note, for each Cmax, the number of cycles N before rupture of the specimen. A large number of specimens were tested for each billet 19, 20, in order to obtain statistical data for each material and, in particular, the mean and the standard deviation of this number of cycles at rupture N.

In FIG. 6, curve 31 thus corresponds to the mean of the number of cycles at rupture N for the final billet 20 after the static recrystallisation step E3 as a function of the maximum stress Cmax of the cyclic loading. Curve 32 corresponds, for its part, to the minimum value of N as a function of Cmax, still for the final billet 20.

By way of comparison, curve 33 corresponds to the minimum value of N as a function of Cmax for the as-cast billet 19. It should thus be noted that the carrying out of a static recrystallisation step E3 greatly increases the fatigue resistance of the nickel-based alloy.

Moreover, ultrasound tests have been performed. These were carried out with billets having a diameter between 150 and 290 mm, in ultrasonic inspection format. These tests were carried out in a tank of water, with a 2-mm flat bottom hole calibration; 80% of screen height. All other things being equal, a 20 to 50% reduction in the background noise is obtained relative to the reference without the static recrystallisation treatment.

Although the present invention has been described by referring to specific exemplary embodiments, it is obvious that modifications and changes can be made to these examples without going beyond the general scope of the invention as defined by the claims. In particular, the individual features of different embodiments illustrated or mentioned can be combined in additional embodiments. Consequently, the description and the drawings should be considered as illustrating rather than limiting.

It is also obvious that all the features described in reference to a method can be transposed, alone or in combination, to a device, and inversely, all the features described in reference to a device can be transposed, alone or in combination, to a method.

Claims

1. A method for manufacturing a nickel-based alloy product, comprising: a step of supplying an ingot of a nickel-based alloy comprising at least one γ′ intermetallic phase combining nickel with aluminium and/or titanium, said at least one γ′ intermetallic phase being present in the alloy of the ingot at a level of 30% to 50% by weight,a first forging step of forging the ingot, during which the true strain rate is less than 1 s−1, the first forging step being carried out at a temperature between 1050 and 1150° C.,a second forging step of forging the ingot, during which the true strain rate is greater than 1 s−1, anda step of static recrystallisation, after the second forging step, during which the ingot is exposed to a temperature of between 1000 and 1055° C. for a duration of between 30 minutes and 10 hours.

2. The method according to claim 1, wherein said at least one γ′ intermetallic phase is present in the alloy of the ingot at a level of 35% to 45% by weight.

3. The method according to claim 1, wherein, before the supplying step, the ingot has undergone a vacuum remelting step.

4. The method according to claim 1, wherein the first forging step leads to a strain of the ingot greater than 1.

5. The method according to claim 1, wherein the second forging step is carried out at a temperature between 900 and 1055° C.

6. The method according to claim 1, wherein the second forging step leads to a strain of the ingot greater than 0.2.

7. The method according to claim 1, wherein the recrystallisation step is carried out at a temperature between 1000° C. and 1025° C. and for a duration between 1 hour and 10 hours.

8. The method according to claim 1, wherein the recrystallisation step is carried out at a temperature between 1025° C. and 1055° C. and for a duration between 30 minutes and 2 hours.

9. The method according to claim 1, wherein the second forging step leads to a strain of the ingot greater than 0.5.

10. The method according to claim 1, wherein the recrystallisation step is carried out at a temperature between 1000° C. and 1010° C. and for a duration between 4 hours and 6 hours.

11. The method according to claim 1, wherein the recrystallisation step is carried out at a temperature between 1045° C. and 1055° C. and for a duration between 50 minutes and 70 minutes.