EP0287486B1 - Process for making a titanium alloy component, and component obtained - Google Patents

Description

The invention relates to a method for manufacturing a titanium alloy part with high characteristics, intended for example for compressor disks for aircraft propulsion systems, as well as the parts obtained.

The document FR 2 144 205 (GB 1356734) describes a titanium alloy of composition by weight: AI 3 to 7 - Sn 1 to 3 - Zr 1 to 4 - Mo 2 to 6 - Cr 2 to 6 and up to approximately 0 , 2% 0.6% V, 0.5% Si, Ti complement and impurities.

• charge de rupture = 1204 MPa; limite élastique à 0,2% = 1141 MPa; résistance à la propagation des criques = 88 x 34,8/ V1000 = 96,9 MPa. νm; fluage à 425°C sous 525 MPa = allongement de 0,2% en 7,2 h et de 0,5% en 55 h. L'allongement de rupture n'est pas indiqué. En pratique on a constaté que les pièces obtenues à partir de ce type de composition et de ce procédé présentaient souvent des ségrégations importantes, se traduisant par des pertes de ductilité et de résistance à la propagation des criques (ténacité) et par ailleurs leurs résistances au fluage ont été trouvées insuffisantes. On a déterminé notamment que les ségrégations précédentes correspondaient à des zones enrichies en Cr, causant alors une fragilisation, et qu'un abaissement de Cr conduisait à des propriétés mécaniques trop faibles.

Preferably: AI 4.5 to 5.5-Sn 1.5 to 2.5-Zr 1.5 to 2.5-Mo 3.5 to 4.5-Cr3.5 to 4.5 - and up '' at about 0.12% O. The corresponding forgings were subjected to a double heat treatment of the solid solution between 730 and 870 ° C then between 675 and 815 ° C, followed by "thermal aging" or tempering between 595 and 650 ° C. Sample "4" (AI 5 - Sn 2 - Zr 2 - Mo 4 - Cr 4 - 0 0.08) has the following mechanical characteristics:

• breaking load = 1204 MPa; 0.2% yield strength = 1141 MPa; resistance to crack propagation = 88 x 34.8 / V1000 = 96.9 MPa. νm; creep at 425 ° C under 525 MPa = 0.2% elongation in 7.2 h and 0.5% in 55 h. The elongation at break is not indicated. In practice, it was found that the parts obtained from this type of composition and from this process often exhibited significant segregation, resulting in losses of ductility and of resistance to the propagation of cracks (toughness) and moreover their resistance to creep were found to be insufficient. It has been determined in particular that the preceding segregations corresponded to zones enriched in Cr, thus causing embrittlement, and that a lowering of Cr leads to mechanical properties which are too weak.

The Applicant has tried to obtain parts of the same alloy having a regular structure and without segregation, and having high mechanical characteristics at 20 ° C (Rm - R _p0,2 - K _ic ) with sufficient elongation as well as a creep resistance at 400 ° C significantly improved.

Exhibition of the invention

According to the invention, the above problem is solved by means of new composition limits and a new transformation process, these composition limits and the conditions of hot working and heat treatment then being inseparable.

• a) on élabore un lingot de composition (% en masse):
  AI 3,8 à 5,4 - Sn 1,5 à 2,5 ― Zr 2,8 à 4,8-Mo 1,5 à 4,5 ― Cr inférieur ou égal à 2,5 et Cr + V = 1,5 à 4,5 - Fe < 2,0 - Si < 0,3 - O < 0,15 - Ti et impuretés: le solde;
• b) on effectue un corroyage à chaud du lingot, comprenant un corroyage de dégrossissage de ce lingot donnant une ébauche à chaud, suivi d'un corroyage final d'une portion au moins de cette ébauche précédé d'un préchauffage dans le domaine bêta, ce corroyage final donnant une ébauche de la pièce;
• c) on effectue un traitement thermique de mise en solution solide de l'ébauche de pièce corroyée à chaud, en la maintenant à une température comprise entre ("transus béta" réel - 40°C) et ("transus béta réel - 10°C) puis en la refroidissant à l'ambiante;
• d) on effectue ensuite sur l'ébauche de la pièce ou sur la pièce obtenue à partir de cette ébauche un traitement thermique de revenu de 4 à 12 h entre 550 et 650°C.

The first object of the invention is a process for manufacturing a titanium alloy comprising the following steps:

• a) an ingot of composition (% by mass) is produced:
  AI 3.8 to 5.4 - Sn 1.5 to 2.5 - Zr 2.8 to 4.8-Mo 1.5 to 4.5 - Cr less than or equal to 2.5 and Cr + V = 1 , 5 to 4.5 - Fe <2.0 - Si <0.3 - O <0.15 - Ti and impurities: the balance;
• b) a hot-working of the ingot is carried out, comprising a roughing-up of roughing of this ingot giving a hot draft, followed by a final working of at least a portion of this blank preceded by preheating in the beta domain, this final working giving a rough outline of the part;
• c) a heat treatment for dissolving the hot-worked part blank is carried out, keeping it at a temperature between (real "transus beta" - 40 ° C.) and ("real beta transus - 10 ° C) then by cooling it to room temperature;
• d) is then carried out on the blank of the part or on the part obtained from this blank a heat treatment of income from 4 to 12 h between 550 and 650 ° C.

With regard to step b) the expression "hot working" (= "hot working" = "Warmverformung") relates to any hot deformation operation (s), consisting of or comprising forging for example , rolling, stamping, or spinning (extrusion).

The limits of the contents of additives have been adjusted, according to the observations made, so as to provide the desired high mechanical characteristics, while avoiding possible segregation on the transformed parts. These content ranges are discussed below, with an indication of preferred ranges which can be used individually or in any combination. These preferential intervals correspond to an increase in minimum characteristics and in the case of iron and oxygen to increased security with respect to possible brittleness or lack of ductility.

The alphagenic elements AI and Sn respectively give, in combination with the other addition elements, insufficient hardnesses when they are in lower contents than the minimum values chosen, and random or frequent precipitations when they are in higher contents than the maximum values set; they preferably have contents of between 4.5 and 5.4% for AI, and between 1.8 and 2.5% for Sn.

prise comme référencce dans FR 2 144 205 via à vis de la tendance du comosé Ti₃AI à se former, est égale à 7 pour leurs teneurs maximales.Zr has an important hardening role, and an embrittling effect above 5%, the Zr content is preferably between 3.5 and 4.8% and more preferably between 4.1 and 4.8%. The three elements AI, Sn and Zr do not entail fragility together, and it can be noted that the sum:

taken as reference in FR 2 144 205 via with respect to the tendency of the comosed Ti ₃ AI to form, is equal to 7 for their maximum contents.

Mo, slightly hardening, has a significant effect of lowering the temperature of transformation of the alpha-beta structure into a fully beta structure, hereinafter called "transus beta". The lowering of the "transus beta", for example by about 40 ° C thanks to 4% Mo, has an influence on the hot working in the vicinity of this temperature. Mo is preferably between 2.0 and 4.5%. V has substantially the same role as Mo and is hardening beta by precipitation like Cr, it is optionally added, (Cr + V) being maintained between 1.5 and 4.5%. Cr is limited to 2.5% maximum with regard to segregation risks which, at the level of Cr = 3.5 to 4.5% recommended by FR 2 144 205 (for example segregations called "beta flecks" enriched in Cr + Zr), have very unfavorable effects on the tenur in service, and it is preferably maintained above 1.5% in favor of hardness.

Fe causes a dircitation by precipitation of intermetallic compounds, it is known as lowering the resistance to hot creep at high temperature (about 550 to 600 ° C) because of these precipitates which thus cause a certain brittleness. The Fe content is maintained in all cases below 2%, and is preferably adjusted between 0.7 and 1.5% because it then surprisingly results in a very improved creep resistance at 400 ° C. , which is interesting for example for the parts used in the "medium temperature" stages (typically 350 to less than 500 ° C.) of aeronautical compressors.

The increase in the 0 content increases, as is known, the mechanical resistance and slightly decreases the toughness (K _ic ), it is therefore limited to a maximum of 0.15% and preferably maintained less than or equal to 0, 13%. A small addition of Si improves the creep resistance at 50 ^G -550 ^D C, it is limited to 0.3% maximum in the context of obtaining sufficient ductility.

It has been found that significantly superior properties are obtained by terminating the hot working by a final working, by rolling or more often by forging or forging forging, preceded by preheating in the beta domain, that is to say at least started in beta.

The "S / s" wrought ratio (initial section / final section) of this final wrought is preferably greater than or equal to 2.

We also found, and this goes against the usual, that it was preferable to know with good precision, for example better than + or -10 at 15 ° C, the actual "transus beta" temperature of the hot-wrought alloy. For this, samples are typically taken from the hot preform obtained by roughing up roughing (forging or rolling) and they are brought to and maintained at different staggered temperatures, then they are quenched with water and their structures micrographically. The "transus beta", possibly appreciated by intrapolation, is the temperature at which all traces of the alpha phase disappear. The actual "beta transus" specific to the hot-wrought alloy, thus determined experimentally, can be very different from the transus temperature estimated by a calculation (first series of tests).

The consequences of this knowledge of the real "transus beta", designated thus or simply by "transus beta", on the choice of the final beta wrought temperature (step b)) then on the adjustment of the temperature of the setting treatment. solid solution of the roughly hot-worked part blank (step d)) are important: it is indeed highly preferable for obtaining the desired structure and properties to carry out this solution treatment in the high range of alpha-beta temperatures, just below the "transus beta" determined experimentally or as it could be determined for example as above or by successive forging tests followed by quenching, and examinations of the structures obtained. More specifically, this solution treatment is usually carried out at a temperature chosen between ("transus beta" -40 ° C) and ("transus beta -10 ° C) with maintenance at a temperature of chosen duration usually between 20 min and 2 h and most often between 30 min and 1 h 30 min and this dissolving is followed by ambient cooling with water or more usually with air. between 550 and 650 ° C, so as to improve the elongation at break A% and the creep resistance at 400 ° C while retaining sufficient mechanical strength and toughness (R _m - Rp _o , ₂ and K _1C ).

Superior results, especially with regard to the elongation A% and the creep resistance at 400 ° C, were surprisingly obtained by organizing the final hot working, if necessary by further spacing its successive deformation passes, by so that it begins in beta at a temperature at least 10 ° C higher than this "transus beta" and ends in alpha-beta, all this work being done at a temperature close to plus or minus 60 ° C of said "transus beta" ". In practice, it is preferable to start the working at a temperature between ("transus beta" + 20 ° C) and ("transus beta" + 40 ° C), and finish it at a temperature below "transus beta" and at least equal to ("transus beta" -50 ° C) or even better at a temperature between ("transus beta" -10 ° C) and ("transus beta" -40 ° C). A fine needle-like structure of the alpha beta type is thus reproducibly obtained, corresponding to a particular state of homogeneity and fine precipitation and contributing to obtaining remarkable properties.

It is preferable to carry out at least the end of the hot roughing of the ingot, before the final heat treatment which has just been described, in alpha-beta between ("transus beta" -100 ° C.) and (" transus beta "-20 ° C). A better prior refining of the microstructure is thus obtained, with a favorable effect on the quantity of parts obtained in the end. The temperature at the end of the hot working which is considered here is the core temperature of the product, assessed for example by prior study of the microstructures obtained by varying the conditions of final hot working.

Finally, in the case where the final hot working is carried out in the preferred manner, the durations and tempering temperatures are typically chosen between 6 and 10 h and between 570 and 640 ° C.

A second object of the invention is the process for transforming a part into a titanium alloy, typically for use at temperatures not exceeding 500 ° C., corresponding to the preferential conditions described above, with Fe = 0.7 to 1, 5%, Zr = 3.5 to 4.8% and preferably 4.1 to 4.8%, at least the end of the roughing-up of roughing comprising forging at a temperature between ("beta transus" -100 ° C) and ("transus beta" -20 ° C), this forging producing a wrought of at least 1.5 and the income being typically from 6 h to 10 h between 580 and 630 ° C. A third object of the invention is also the remarkable parts obtained with the preceding process, the second object of the invention, with Cr = 3.5 to 4.8% and the following mechanical properties:

• obention de façon reproductible d'une structure fine aiguillée, sans ségrégations d'aucune sorte;
• élimination des risques de fragilité;
• obtention simultanée de toutes les caractéristiques souhaitées: structure et caractéristiques mécaniques précédentes.

Rm ≥ 1200 MPa - R _p0.2 ≥ 1100 MPa - A% ≥ 5 - toughness (= resistance to crack propagation) K _lc at 20 ° C ₅ : 45 MPa. Vm - creep at 400 ° C under 600 MPa: 0.5% in more than 200 h. The advantages of the process of the invention are as follows:

• reproducibly obtaining a fine needle structure, without segregation of any kind;
• elimination of the risks of frailty;
• simultaneous obtaining of all the desired characteristics: structure and previous mechanical characteristics.

Essays First series of tests (Tables 1 to 6)

Six ingots A-D-E-H-J-K were produced in a consumable electrode oven, by double fusion, the compositions obtained are given in Table 1.

80 mm. Puis, pour une portion de chacun, un deuxième dégrossissage d'affinage de la structure en alpha-béta par forgeage en méplat de 70 x 30 mm, à température du préchauffage) égale à 50°C de moins que la température de transus estimée pour chacun des six alliages (Tableau 2). Cette estimation était faite par une règle d'approche interne tenant compte des teneurs en éléments d'addition.Each ingot has undergone a first roughing in beta at 1050 ° / 1100 ° C of the initial diameter 0 200 mm squared

80 mm. Then, for a portion of each, a second rough refinement of the structure in alpha-beta by flat forging of 70 x 30 mm, at preheating temperature) equal to 50 ° C less than the estimated transus temperature for each of the six alloys (Table 2). This estimate was made by an internal approach rule taking into account the contents of addition elements.

The samples taken at this stage were then subjected to 30 min heatings at different temperatures staggered by 10 at 10 ° C, each followed by water quenching, and the micrographic structures were examined. The temperature of disappearance of the real alpha phase or "transus beta" was thus determined for each hot-worked alloy (Table 2).

The temperature of second roughing in alpha-beta actually went according to the alloy of ("transus beta" -170 ° C) (mark H) to ("transus beta" -40 ° C) (mark E) or ("transus beta "-60 ° C) (item K).

• 1° gamme (Tableau 3): après le forgeage alpha-béta précédent constituant alors le forgeage final, mise en solution 1 h à ("transus béta" -50°C) (Tableau 2) et mesure des caractéristiques mécaniques à l'ambiante dans l'état obtenu; essais de fluage en traction sous 600 MPa à 400°C après revenu complémentaire de 8 h à température indiquée pour chaque alliage dans le Tableau 2.
• 2° gamme (Tableau 4): on a repris des portions des carrés de 80 mm, sauf le carré H, issus de premier dégrossissage en béta, et on leur a appliqué un deuxième dégrossissage en alpha-béta en carré
  
  65 mm, à témperature ajustée à 50°C de moins que le "transus béta" réel déterminé précédemment (Tableau 2).

Three variants were then prepared corresponding to different ranges of transformation and heat treatment and the mechanical characteristics were measured, along the longitudinal (L) and possibly transverse (T) directions:

• 1st range (Table 3): after the previous alpha-beta forging then constituting the final forging, solution for 1 hour at ("beta transus" -50 ° C) (Table 2) and measurement of the mechanical characteristics at ambient in the state obtained; creep tests in tension under 600 MPa at 400 ° C after additional tempering of 8 h at temperature indicated for each alloy in Table 2.
• 2nd range (Table 4): we took portions of the 80 mm squares, except square H, from the first roughing in beta, and we applied a second roughing in alpha-beta to the square
  
  65 mm, with a temperature adjusted to 50 ° C less than the actual "transus beta" determined previously (Table 2).

Then, a final forging of a 70 x 30 mm plate was made on this square, starting from a preheated state for 30 min at ("transus beta" + 10 ° C) and ending in alpha-beta, fine needle point structures -beta being obtained. The parts were then subjected to a solution for 1 h at real "beta transus" (Table 2) real -30 ° C and an income of 8 h either at 550 ° C (A2), or at 500 ° C (D2- E2-J2-K2). The mechanical characteristics at 20 ° C and the creep resistance at 400 ° C are measured in this tempered state.

3rd range (Table 5): a portion of the dishes of 70 x 30 mm obtained in the second range was applied an additional final forgeae at 60 x 30 mm starting from ("transus beta" + 30 ° C) and also ending in alpha-beta (needle structures with alpha phase lines were observed micrographically).

Then, for each of the alloys, the same heat treatments (dissolution then tempering) were carried out as in the second range.

• les classements des alliages respectivement en résistance mécanique et en tenue au fluage en traction à 400°C sont les suivants, pour les 1° et 2° gammes:
  

The study of these results leads to the following comments:

• the classifications of the alloys respectively in mechanical resistance and creep resistance in tension at 400 ° C are the following, for the 1 ° and 2 ° ranges:
  

These classifications are very different for the two ranges. The samples of the 1st range have a final forging at a lower temperature than the samples of the 2nd range, and in addition this forging was carried out at a temperature offset variably with respect to the real "transus beta" of the alloy. : for example 110 ° C less than this transus for AI, and 40 ° C less for E1.

K is a control centered in the analysis recommended by FR 2 144 205 - H is another control without Sn and without Zr, which in this first series gives insufficient mechanical strength and creep resistance.

the comparison of the results of the 1st and 2nd ranges shows the importance of a final forging beginning in beta. The comparison of the results of the 2nd and 3rd ranges shows that the increase in the temperature at the start of this final forging above the "transus beta", resulting here in better homogenization during preheating and a greater proportion of the final working in the beta domain, causes a significant increase in mechanical strength, with consequently the possibility of obtaining a more interesting compromise of characteristics after adjustment of the tempering conditions. This also shows the importance of a precise adjustment of the final forging temperature compared to the real "transus beta" of the alloy.

alloys D, J and E appear particularly interesting (mechanical resistance and creep resistance observed for the 2nd range), subject to a setting above 550 ° C of the tempering temperature. The first two contain 2, 1 and 1.9% iron, respectively.

Second series of tests (Tables 7 to 9)

New ingots have been developed, with AI contents close to 5% and higher Zr contents than in the first series of tests. The compositions of the five ingots chosen in this example are given in Table 7. Only one ingot marked FB contains iron, with a content of 1.1%.

Each ingot has first sbi a first roughing in the beta press at 1050 ° C of the initial diameter 0 200 mm squared rb 40 mm.

The actual "transus beta" of the fifty alloys were determined at this stage, according to the method described for the first series of tests.

The 140 mm squares were then forged into 80 mm squares from preheating to ("beta transus" -50 ° C), then resumed in final forging in a 70 x 30 mm dish starting from ("transus beta "real + 30 ° C).

According to the structures obtained, the end of this forging was in alpha-beta, at more than ("transus beta" -80 ° C), for all the alloys except for KB. A fully beta structure was observed in KB micrography, with contours of the unmodified beta grains.

After the final forging, the hot wrought blanks obtained were heat treated: solution for 1 h at ("transus beta" of the alloy -30 ° C) followed by air cooling, then returned to 8 h at temperature (Table 8) chosen by a special procedure.

This procedure consisted of processing small samples at staggered temperatures, followed by microhardness measurements H _" 30 g and plotting the hardness curve as a function of the triat temperature, the temperature chosen for the income then corresponding to the minimum of hardness + 10%.

The temperatures of final forging and heat treatments are collated in Table 8. The results of the mechanical tests appear in Table 9.

The KB alloy has a catastrophic A% elongation, which shows the importance of finishing the final forging in alpha-beta (needle structure with alpha edges), to have sufficient ductility. This alloy could be of interest if its final forging had been slowed down so as to end in alpha-beta.

Among the samples obtained, FB and GB show the best compromises of the various properties including A% and the creep resistance at 400 ° C. FB which is the better of the two, especially in creep (384 h for 0.5% elongation) contains 5.4% AI -4.2% Zr and 1.1% Fe. AB2 present on micrograph segregation ("beta flecks") linked to its 4.1% Cr content, which means that Cr contents at most equal to 2.5% are preferred, without this condition preventing good properties from being obtained (results from FB).

Claims (16)

1. Process for the production of a titanium alloy part involving the following stages:

a) the production of an ingot of composition (% by weight): AI 3.8 to 5.4, Sn 1.5 to 2.5, Zr 2.8 to 4.8, Mo 1.5 to 4.5, Cr equal to or below 2.5 and Cr + V = 1.5 to 4.5, Fe < 2.0, Si < 0.3, 0 < 0.15, Ti and impurities constituting the residue;

b) the ingot undergoes hot working, involving a rough-shaping working of said ingot giving a hot blank, followed by the final working of at least a portion of said blank preceded by preheating in the beta range, said final working giving a blank of the part;

c) the hot worked part blank is solid solution heat treated, whilst maintaining it at a temperature between (real "beta transus" -40°C) and (real "beta transus" -10°C), followed by cooling it to ambient temperature;

d) an ageing heat treatment of 4 to 12 h at between 550 and 650°C is then performed on the blank of the part or on the part obtained from said blank.

2. Process according to claim 1, characterized in that, at the latest before stage c), the real "beta transus" of the hot worked alloy is experimentally determined on the basis of samples taken during or after hot working.

3. Process according to claim 1, characterized in that AI = 4.5 to 5.4, Sn = 1.8 to 2.5 and Zr = 3.5 to 4.8.

4. Process according to claim 3, in which Zr = 4.1 to 4.8.

5. Process according to any one of the claims 1, 3 or 4, characterized in that Mo = 2.0 to 4.5 and Cr = 1.5 to 2.5.

6. Process according to claim 1, characterized in that Fe < 1.5.

7. Process according to claim 1, characterized in that 0 = 0.07 to 0.13.

8. Process according to any one of the claims 1 and 3 to 7, characterized in that Fe = 0.7 to 1.5.

9. Process according to any one of the claims 1 to 8, characterized in that the final hot working of the blank or blank portion is started by using a temperature higher by at least 10°C than the real "beta transus" and is ended at a temperature below the "beta transus", all said working taking place at ±60°C of said "beta transus".

10. Process according to claim 9, characterized in that the final hot working of the blank or blank portion is carried out by starting at a temperature between the real "beta transus" +20°C and the real "beta transus" +40°C and is ended at a temperature below said "beta transus" and at least equal to the real "beta transus" -50°C.

11. Process according to claim 10, characterized in that the final hot working is carried out at a temperature between the real "beta transus" -10°C and the real "beta transus" -40°C.

12. Process according to any one of the claims 1 to 11, characterized in that at least the end of the rough-shaping of the ingot takes place at a temperature between the real "beta transus" -100°C and the real "beta transus" -20°C.

13. Process according to claim 11, characterized in that ageing is performed for between 6 and 10 hours at between 570 and 640°C on the blank of the part or on the part obtained from said blank.

14. Process for the production of a titanium alloy part comprising the following stages:

a1) an ingot of the following composition is produced (% by weight):

Al 4.5 to 5.4, Sn 1.8 to 2.5, Zr 3.5 to 4.8, Mo 2.0 to 4.5, Cr 1.5 to 2.5 and Cr + V = 1.5 to 4.5, Fe 0.7 to 1.5,0 0.07 to 0.13 and Ti and impurities constitute the residue;

b1) a rough-shaping of the ingot takes place giving a final hot blank, whereof the end at least comprises forging at a temperature between the real "beta transus" -100°C and the real "beta transus" -20°C, the working ratio of said forging being at a minimum 1.5;

c1) said real "beta transus" temperature of the hot worked alloy is experimentally determined on the basis of samples taken from the forged hot blank;

d1) a final working of said blank takes place by forging and/or die forging, starting at a temperature between the real "beta transus" +20°C and the real "beta transus" +40°C and is ended at a temperature between the real "beta transus" -40°C and the real "beta transus" -10°C;

e1) the blank of the thus obtained hot worked part is solid solution heat treated, the temperature being maintained at between the real "beta transus" -40°C and the real "beta transus" -10°C and then cooling to ambient temperature takes place;

f1) an ageing heat treatment for 6 to 10 hours at a temperature between 580 and 630°C is then performed on the blank of the part or on the part obtained from said blank.

15. Process according to claim 14, characterized in that Zr = 4.1 to 4.8.

16. Titanium alloy part having the mechanical characteristics and structure given below:

A) fine and regular alpha-beta structure;

B) composition (% by weight): AI 4.5 to 5.4, Sn 1.8 to 2.5, Zr 3.5 to 4.8, Mo 2.0 to 4.5, Cr 1.5 to 2.5, Cr + V 1.5 to 4.5, Fe 0.7 to 1.5, 0 0.07 to 0.13, the remainder being Ti and impurities,

C) Rm ≥ 1200 MPa

R_p0,2 ≥ 1000 MPa

A % ≥ 5

K_lc at 20°C ≥ 45 MPa·√m

creep at 400°C under 600 MPa: 0.5% in more than 200 h.