EP0008996B1 - Process for heat-treating aluminium-copper-magnesium-silicon alloys - Google Patents

Description

The present invention relates to a process for heat treatment of wrought products of aluminum alloys of the 2000 series (aluminum - copper - magnesium - silicon) intended to improve their resistance to intercrystalline corrosion and to corrosion under stress.

The process is applicable to all wrought aluminum alloy products having, in particular, 3.5% to 5% copper, 0.2% to 1.0% magnesium and 0.25 weight contents. % to 1.2% in silicon, such that the ratio of the Si / Mg content by weight is greater than 0.8. These alloys can also contain weight contents less than or equal to 1% manganese, 0.5% chromium and 0.3% zirconium.

The most characteristic aluminum alloy in this field of compositions is the alloy known as 2014 according to the designations of the Aluminum Association. This alloy and its variants, 2X14 (2214 etc ...), which differ from 2014 by lower iron contents, are widely used in the aeronautical industry.

The current practice of heat treatment of these alloys includes dissolving at a temperature generally below 510 ° C, as rapid quenching as possible, maturing for several days at room temperature (state T4) and simple tempering at a temperature generally between 150 ° C and 190 ° C for an isothermal holding time between 4 hours and 48 hours (state T6). This range of heat treatment is notably that of stamped products. The known practice of heat treatment of rolled, forged or extruded products also includes work hardening by plastic deformation of 1 to 5% of the raw quenching products before maturation and tempering, intended to stress the quenched products. This work hardening can be obtained by controlled traction or leveling of long products (states T351 after maturation or T651 after isothermal tempering) and by compression of forged products (states T352 or T625).

In the current state T6 or T651, the products have very high mechanical tensile properties (breaking load Rp and elastic limit at 0.2% of residual deformation Rp 0.2), but their resistance to intercrystalline corrosion and to corrosion under tension in the short-short direction is bad.

The resistance to intercrystalline corrosion is evaluated after immersion for a period of 6 hours in NaCI-H ₂ 0 ₂ reagent according to the French aeronautical standard AIR 9050 C of 1/12/1964.

The resistance to corrosion under stress is evaluated in the short-cross direction after air-immersion-emersion test in buffered NaCl solution, the composition of which appears in French standard NF 91-411 of October 1966 -reactive said A3-. It is characterized by the non-breaking stress in 30 days of testing ( _Q NR 30), often given as a percentage of the elastic limit Rp 0.2 in the short-cross direction.

Under these conditions, the 2014 alloy has, in the T6 (or T651) state, a non-breaking constraint in the cross-short direction of less than 100 MPa in 30 days of testing, and even in the absence of a constraint applied, is very sensitive to intercrystalline corrosion after the NaCI-H ₂ 0 _{2 test} .

The Applicant has discovered that it was possible to significantly improve the compromise between the mechanical characteristics and the corrosion resistance of the alloys which are the subject of the invention after treatment, without modifying the compositions as defined industrially and in satisfactory economic conditions, in particular as regards the duration of the heat treatment.

• 1) un revenu principal à une température supérieure à 225°C et inférieure à 280°C, d'une durée comprise entre 6 secondes et 1 heure, la température étant la température maximale atteinte par la partie la plus froide du produit à traiter et la durée du revenu étant comptée entre le moment où la température ainsi définie dépasse 225°C dans le sens ascendant et le moment où elle atteint 225°C dans le sens descendant.

The heat treatment according to the invention comprises dissolving, quenching, optionally hardening by plastic deformation of 1 to 5% after quenching, intended to tension the quenched products (for example by controlled leveling, traction or compression) ambient temperature of indefinite duration and a final income comprising at least two stages:

• 1) main tempering at a temperature above 225 ° C and below 280 ° C, lasting between 6 seconds and 1 hour, the temperature being the maximum temperature reached by the coldest part of the product to be treated and the duration of the income being counted between the time when the temperature thus defined exceeds 225 ° C in the upward direction and the time when it reaches 225 ° C in the downward direction.

The holding time above 225 ° C is shorter the higher the temperature reached.

2) additional income at a temperature between 120 ° C and 175 ° C lasting between 4 hours and 8 days.

• a) dans le cas où le produit a subi un écrouissage après trempe et avant revenu:
  
  
• b) dans le cas où le produit n'a pas subi d'écrouissage:
  
  

The main tempering may possibly precede a preheating of a duration less than or equal to 24 hours at a temperature less than or equal to 160 ° C. The temperatures and durations of the main income, as defined above, are preferably located, in a diagram of temperature-time coordinates, inside a quadrilateral having for vertices the following points:

• a) in the case where the product has undergone work hardening after quenching and before tempering:
  
  
• b) in the case where the product has not undergone work hardening:
  
  

For the main tempering, the rate of temperature rise and the rate of cooling of the product to be treated must be fast enough. In particular, between 175 ° C and 225 ° C, they must be higher on average than 1 ° C / min.

After the main tempering, the product must be cooled, either to room temperature or to the temperature of the supplementary tempering. It can then undergo work hardening by plastic deformation of 1 to 5% intended for its stress relieving, if this operation has not already been carried out between the quenching and the main tempering.

• a) dans le cas où le produit a subi un écrouissage après trempe et avant le revenu complémentaire:
  
  
• b) dans le cas où le produit n'a pas subi d'écrouissage avant le revenu complémentaire:
  
  

The temperatures and durations of the additional income are preferably located in a temperature-time coordinate diagram inside a quadrilateral whose vertices are as follows:

• a) in the case where the product underwent work hardening after quenching and before the additional income:
  
  
• b) in the case where the product has not undergone hardening before the additional income:
  
  

If the hardening takes place between the main income and the additional income, the temperature of the additional income will preferably be at least 70 ° C. lower than that of the main income. In this case, the work hardening can be carried out at an intermediate temperature between that of the main tempering and the ambient temperature.

• - figure 1: les domaines ABCD (produits écrouis) et EFGH (produits non écrouis) du revenu principal;
• - figure 2: les domaines IJKL (produits écrouis) et MNOP (produits non écrouis) du revenu complémentaire.

The conditions of the heat treatment according to the invention are illustrated in the appended figures which represent respectively, in semi-logarithmic temperature-time coordinates:

• - figure 1: the ABCD (hardened products) and EFGH (unhardened products) domains of the main income;
• - Figure 2: the IJKL (hardened products) and MNOP (unhardened products) areas of additional income.

An advantage of the present invention is the good reproducibility of the conditions of the main income, obtained by simple control of the temperature development, in the coldest part, of a control room. In addition, the main income may not include an isothermal bearing, at a temperature above 225 ° C. It can therefore be produced on products of all thicknesses and by means of the most diverse techniques allowing a sufficiently rapid rise in temperature, for example, a ventilated oven, passage oven, high frequency oven, oil, salt or of molten metal, or by Joule effect, depending on the nature of the products to be treated.

Knowing at all times the temperature of the coldest part of the room, especially when it exceeds 225 ° C, allows the main income to be interrupted so that the duration of keeping the room at temperature above 225 ° C or inside the duration range corresponding to the maximum temperature reached, range delimited by Figure 1.

• - des caractéristiques mécaniques de traction (charge de rupture Rm et limite élastique à 0,2% d'allongement rémanent Rp 0,2) au moins égales à 90% de celles obtenues à l'état actuelT6, T651 ou T652 selon la nature des produits, sans diminution de ductilité,
• - une résistance à la corrosion intergranulaire évaluée par le test NaCI-H₂0₂ (norme AIR 9050 C) très supérieure à celle des états T 6 (T661-T652),
• - une résistance à la corrosion sous tension très supérieure à celle des produits traités à l'état actuel T6 (ou T651, T652) puisque leur contrainte de non-rupture dans le sens travers-court est supérieure à 70% de la limite élastique Rp 0,2 en 30 jours d'essais immersion-émersion alternée en réactif A3 selon la norme NF A 91-411.

The products treated according to the invention have:

• - mechanical tensile characteristics (breaking load Rm and elastic limit at 0.2% of residual elongation Rp 0.2) at least equal to 90% of those obtained in the current state T6, T651 or T652 depending on the nature of the products, without reduction of ductility,
• - a resistance to intergranular corrosion evaluated by the NaCI-H ₂ 0 _{2 test} (AIR 9050 C standard) much higher than that of the T 6 states (T661-T652),
• - a resistance to corrosion under tension much higher than that of the products treated in the current state T6 (or T651, T652) since their non-breaking stress in the cross-short direction is greater than 70% of the elastic limit Rp 0.2 in 30 days of alternating immersion-emersion tests in reagent A3 according to standard NF A 91-411.

The method according to the invention applies to the thermal treatment of rolled, forged, stamped, extruded or other products, whatever the homogenization or solution treatment carried out before quenching and whatever the method of stress relieving by work hardening after quenching. However, it is particularly advantageous that the alloy, before working, has been homogenized at a temperature between the starting melting temperature of the metastable eutectics and the temperature of the equilibrium solidus of the alloy, as described in French patent no. 2,278,785.

The combination of such homogenization and of an income according to the invention gives the alloy, without the need to modify its composition, a set of improved characteristics since the elastic limit Rp 0.2 is at least equal to 95% of that obtained on an alloy of the same composition having undergone the same work hardening and the treatment of tempering T6 or T651 with an elongation (A%) greater than that of the current state T6.

In the particular case of alloy 2014 or 2214, the Applicant has found that the modification of the alloy by increasing the content of Cu and / or Mg and / or Si up to their limit of solubility in aluminum at the homogenization temperature (according to the certificate of addition no. 2,293,497 to French patent no. 2,278,785), associated with a homogenization carried out at a temperature between the starting melting temperature of the metastable eutectics and the temperature of the solidus balance of the alloy (as described in French patent no. 2,278,785) and a tempering treatment according to the invention makes it possible to achieve a compromise between mechanical properties of traction and resistance to corrosion under tension quite exceptional for alloys of the 2000 series and impossible to reach by other means in the current state of the art.

In fact, the 2014 alloy products with modified composition have, after special homogenization and tempering according to the invention, mechanical tensile characteristics (Rm and Rp 0.2) superior to those of the classic 2014 alloy, treated with state T6 (or T651 ° or T652) without reduction in elongation or toughness, with, in addition, a much higher corrosion resistance: the non-breaking stress is greater than 75% of the elastic limit Rp 0 , 2 and the alloy treated according to the invention is not sensitive to intercrystalline corrosion according to the AIR 9050C standard.

These advantages are illustrated by the following exemplary embodiments, given by way of non-limiting indication:

EXAMPLE 1

Sheets 60 mm thick in alloy 2214, of conventional composition (Cu = 4.4 - Mg = 0.4 - Mn = 0.6 - Si = 0.8) underwent, after conventional homogenization, dissolution conventional at 505 ° C. followed by quenching in cold water, a controlled traction of 2.2%, a maturation of duration 2 months at ambient temperature, and conventional tempering treatments T651 or according to invention, controlled by a thermocouple placed at the heart.

The main income (R.P.) was carried out in a nitrites-nitrates salt bath.

Table 1 below indicates the duration of keeping the part at temperature above 225 ° C and the maximum temperature reached by the product. The products were cooled with water after main tempering, and the complementary tempering (R.C.) was carried out in a ventilated stationary oven. Table 1 gives the mechanical tensile characteristics in the cross-long and cross-short direction, the stress at NR 30 of non-rupture in corrosion under tension in the cross-short direction (imposed stresses 100, 200 and 300 MPa) in reagent A3, and resistance to intercrystalline corrosion according to AIR 9050C standard.

All these characteristics were measured at mid-thickness of the sheets. This example shows the very significant improvement in the resistance to stress and intercrystalline corrosion of the products, obtained at the cost of a reduction in the mechanical tensile characteristics of less than 10% compared to the T651 state.

EXAMPLE 2

• - mise en solution 505°C-4. h
• - trempe eau 20°C suivie d'une traction contrôlée de 2%
• - maturation 5 jours
• - revenu classique T6 en four fixe ventilé ou revenu principal selon l'invention en four à bain de sel (montée en température instantanée)
• - refroidissement air
• - revenu complémentaire selon l'invention en four fixe ventilé.

Sample plates 2.5 mm thick in 2014 alloy underwent the following heat treatment:

• - dissolving 505 ° C-4. h
• - water quenching 20 ° C followed by a controlled traction of 2%
• - maturing 5 days
• - classic T6 tempering in a ventilated fixed oven or main tempering according to the invention in a salt bath oven (instantaneous temperature rise)
• - air cooling
• - additional income according to the invention in a ventilated stationary oven.

Table II below gives the Vickers hardness (under load 3 kg) and the sensitivity to intercrystalline corrosion (NaCI-H ₂ 0 _{2 test} ) on the surface of sheets.

This example shows that only the main income and additional income treatments carried out in the time and temperature domains (above 225 ° C.) claimed by the invention, make it possible to desensitize the 2014 alloy from intercrystalline corrosion with low decrease in hardness.

EXAMPLE 3

• - une homogénéisation comme décrit au brevet français n° 2.278.785 avant laminage,
• - une trempe à l'eau froide suivie d'une traction contrôlée de 2,5%, d'une maturation de 1 mois à température ambiante,
• - un revenu classique T651 en four fixe ventilé ou un revenu selon l'invention comportant un revenu principal en four à bain de sel nitrites-nitrates suivi d'un refroidissement à l'eau et d'un revenu complémentaire en four fixe ventilé. Le revenu principal a été précédé d'un préchauffage à 154°C (montée 8 heures - maintien 4 h) pour les tôles repérées A et C.

Sheets of thickness 60 mm in alloy 2214 with composition modified according to the certificate of addition n ° 2.293.497 to the French patent n ° 2.278.785 (Cu = 4,5 - Mg = 0,6 - Si = 0,8 - Mn = 0.6 - Fe = 0.2) underwent:

• - homogenization as described in French Patent No. 2,278,785 before rolling,
• - quenching in cold water followed by controlled traction of 2.5%, maturing for 1 month at room temperature,
• - a conventional income T651 in a ventilated stationary oven or an income according to the invention comprising a main income in an oven with a nitrites-nitrates salt followed by cooling with water and an income complementary in fixed ventilated oven. The main tempering was preceded by preheating to 154 ° C (rise 8 hours - hold 4 h) for the sheets marked A and C.

The wire table below gives the mechanical tensile characteristics and the stress α NR 30 of non-rupture in 30 days of corrosion test under tension in reagent A3 (constraints imposed 100, 200, 300 MPa) in the cross direction. short.

This example shows that the combination of the tempering treatment according to the invention with the aforementioned modification of composition and the homogenization according to French Patent No. 2,278,785, makes it possible to obtain a compromise between mechanical characteristics of traction and resistance to corrosion under tension, quite exceptional for this alloy.

EXAMPLE 4

• - mise en solution 505°C-12 h
• - trempe à l'eau à 65°C
• - maturation 3 jours
• - revenu classique T6 (20 h-160°C) en four fixe ventilé ou revenu selon l'invention en bain de sel nitrites-nitrates pour le revenu principal et en four fixe ventilé pour le revenu complémentaire.

Forged 2014 alloy blanks, dimensions 90 x 210 x 500 mm, underwent the following operations:

• - solution 505 ° C-12 h
• - water quenching at 65 ° C
• - maturation 3 days
• - conventional T6 tempering (20 h-160 ° C) in a ventilated fixed oven or tempering according to the invention in a nitrites-nitrates salt bath for the main tempering and in a ventilated fixed oven for the supplementary tempering.

Table IV gives the holding time of the blanks at a temperature above 225 ° C. and the maximum temperature reached by the blanks, measured by a mid-thickness thermocouple.

• - les caractéristiques mécaniques de traction à mi-épaisseur dans le sens des fibres (sens long) et perpendiculairement aux fibres (sens travers-court),
• - la contrainte _Q NR 30 de non-rupture en corrosion sous tension à mi-épaisseur dans le sens travers-court en 30 jours d'essais d'immersion-émersion alternée (selon norme AIR 9050 C).

We give:

• - the mechanical traction characteristics at mid-thickness in the direction of the fibers (long direction) and perpendicular to the fibers (cross-short direction),
• - the constraint _Q NR 30 of non-rupture in tension corrosion at mid-thickness in the cross-short direction in 30 days of alternate immersion-emersion tests (according to AIR 9050 C standard).

The table shows that the conditions of main income and additional income according to the invention make it possible to obtain good resistance to corrosion under tension with mechanical tensile characteristics (Rp 0.2 in particular) at least equal to 90% of those of the current T6 state.

Claims (7)

1. Process for the thermal treatment of wrought products made of aluminium alloys of 2000 series containing (by weight) from 3 to 5% copper, from 0,2 to 1 % magnesium, from 0,25 to 1,2% silicon, with a ratio Si/Mg >0,8 and optionally from 0 to 1% manganese, from 0 to 0,5% chromium and from 0 to 0,3% zirconium, said thermal treatment comprising at least a solution anneal, a quenching, a natural ageing and a tempering, characterized in that the tempering comprises at least two stages:

a) a main temper at a temperature greater than 225°C and lower than 280°C, for a period of between 6 seconds and 60 minutes

b) a complementary temper at a temperature comprised between 120°C and 175°C for a period comprised between 4 and 192 hours.

2. Process according to claim 1, characterized in that the point which is representative of the main temper is located, in a diagram time-temperature, inside a quadrangle having as peaks

and representative point of complementary temper is located inside of a quadrangle having as peaks:

3. Process according to claim 1, characterized in that the products are subjected after quenching and before the main temper to a cold working comprised between 1 and 5%, the point which is representative of the main temper being inside a quadrangle having as peaks:

and the representative point of complementary temper being inside a quadrangle having as peaks:

4. Process according to claim 1, characterized in that a cold working from 1 to 5% is applied to the product between the main and complementary tempers, the main temper being made in a domain on a time temperature diagram fixed by the following coordinates:

and the complementary temper in a domain fixed by the following coordinates:

5. Process according to one of the claims 1 to 4, characterized in that the main temper is preceded by a heating treatment for a period less than or equal to 24 hours at a temperature lower than or equal to 160°C.

6. Process according to claim 4, characterized in that the temperature of complementary temper is lower by at least 70°C than the temperature of main temper.

7. Process according to one of the claims 1 to 6, characterized in that, the alloy before being worked, is subjected to an homogeneization treatment at a temperature comprised between the incipient melting of metastable eutectics and the equilibrium solidus temperature.

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