EP0051549A1 - Interrupted quenching method for aluminium alloys - Google Patents

Abstract

The invention relates to a method of interrupted quenching of aluminum alloys with structural hardening. It comprises, from the dissolution: a) rapid cooling, by quenching until the product reaches a temperature between 150 ° and 260 ° C. b) stopping quenching for a few seconds to a few tens of minutes (air cooling) c) resuming quenching to room temperature. Under these conditions, quenched tempered products are obtained which have mechanical characteristics close to the T6 state and superior to those of the conventional T73 state, with very good corrosion resistance (flaky or under tension) and a level of internal stresses divided substantially by 2 compared to the classic T6 state. This treatment is applicable to all Al alloys of the 2000, 6000 and 7000 series (according to the A.A.).

Description

The invention relates to a method of interrupted quenching of aluminum alloys with structural hardening.

It is known that obtaining the high mechanical characteristics associated with good corrosion resistance (intergranular, by pitting) of alloys with structural hardening depends, to a large extent, on the quality of the quenching operation; it must be quick and energetic. It is sought, in fact, to avoid any harmful precipitation between 400 ° C and 260 ° C by a transfer time between dissolution and the start of quenching as short as possible, followed by vigorous quenching (cold agitated water for example).

However, this practice leads to highly deformed products, with strong internal constraints; this results in expensive dressing operations and, frequently, additional distortions during final machining, for example.

On the other hand, in order to improve the resistance to corrosion under stress, it is common to use over-tempering techniques (double tempering, on alloys of the 7000 series). These techniques produce a significant drop in mechanical strength properties.

• a) un refroidissement rapide par trempe jusqu'à ce que le produit atteigne une température comprise entre 150 et 260°C,
• b) un arrêt de la trempe pendant quelques secondes à quelques dizaines de minutes (refroidissement à l'air),
• c) une reprise de la trempe jusqu'à la température ambiante

The method according to the invention makes it possible to improve the characteristics of resistance to corrosion and to reduce the level of internal stresses of the products treated without significant modification of the mechanical characteristics of resistance. It consists in subjecting the products, after the solution treatment, to an interrupted quench comprising:

• a) rapid cooling by quenching until the product reaches a temperature between 150 and 260 ° C,
• b) quenching for a few seconds to a few tens of minutes (air cooling),
• c) resumption of quenching to room temperature

The treatment is possibly supplemented by the conventional operations of traction or controlled compression (stockholding) and / or tempering (hardening).

The quenching is carried out by (or in) an appropriate fluid, preferably with cold water, by any known means (spraying, immersion, spraying, air-water mist, etc.).

The products thus treated have, compared to conventional direct quenching treatments, good resistance to corrosion, in particular to corrosion under stress, at the cost, possibly, of a slight decrease in the mechanical tensile characteristics. On the other hand, the level of residual stresses after quenching is greatly reduced.

Stopping of the quenching is obtained by closing the watering inlets in the case of quenching by spraying, or by emersion of the product out of the quenching bath, in the case of quenching by immersion.

Compared to the stepped quenching processes described in the technical literature, this method is distinguished by an interruption and a resumption of the quenching, while the stepped quenching comprises only one quenching operation at an intermediate temperature between the dissolution and ambient in various known environments (salt bath, oil bath, hot water).

In addition, in the method according to the invention, the average initial cooling rate [step a)] is, in general, high, and preferably greater than 3 ° C / sec. between the solution temperature and 260 ° C.

Likewise, the average final quenching speed [step c)] is preferably greater than 60 ° C / min. between the temperature reached at the end of [step b) jet 100 ° C.

It has also been observed that the duration and the position of the interruption of the quenching have a great influence on the optimal combination of the mechanical characteristics and of the corrosion resistance of the products.

In this text, we will call the duration of interruption (t) during step b), not the physical duration of the latter (T), but the duration which separates the instant when the temperatures of the product treated are substantially uniform. (temperature difference ≤ 5 ° C) from that of resumption of quenching [step c)]. The interruption temperature (θ) is the temperature substantially uniform and constant product during this last phase.

The effective surface temperature, when the quenching is interrupted (start of step b) and the effective duration thereof (T) which depend, among other things, on the nature of the alloy, the shape and of the size of the parts, etc. are easily accessible to those skilled in the art through experience, calculation or simulation.

For alloys of type 2214 which have, in the usual T6 state, a very marked susceptibility to intergranular corrosion, the durations and temperatures of interruption being located inside the ABDCEF perimeter, make it possible to improve the resistance to corrosion (fig. 2). Preferably, these times and temperatures will be within the CDEGH perimeter (fig. 2).

For alloys of the type 7475 having, in the usual state T6, a very high susceptibility to corrosion by laminating, with durations and temperatures of interruption being located inside the perimeter ABCDEG, the resistance to corrosion is improved flaky, losing only 5% of the hardness of the T6 state (fig. 3).

Preferably, the durations and temperatures of interruption located within the CDEF perimeter lead to the best results (fig. 3).

Polygonal perimeters, plotted in semi-logarithmic coordinates, have vertices whose coordinates are:

The stress relief can be carried out after quenching by plastic deformation of tension or compression and tempering is preferably practiced in the temperature range from 130 to 170 ° C for durations between 7 and 15 hours for alloys type 7075 and between 10 and 30h for alloys of type 2214.

• La figure 1 représente les courbes de refroidissement comparées d'un produit de 60 mm d'épaisseur lors d'une trempe étagée classique et suivant l'invention.
• Les figures 2 et 3 représentent les conditions d'interruption optimales du traitement de trempe (rappelées ci-dessus).

The invention will be better understood and illustrated by the following examples and figures.

• FIG. 1 represents the compared cooling curves of a product 60 mm thick during a conventional stepped quenching and according to the invention.
• Figures 2 and 3 show the optimal interruption conditions of the quenching treatment (recalled above).

FIG. 1 represents the thermal evolution of a 60 mm thick plate of 2214 alloy quenched from 500 ° C., on the one hand, according to the method of the invention by watering for 9 sec. with cold water, stop watering, around = 220/230 ° C, for 370 sec. (T), and resume watering, and, on the other hand, according to the conventional stepped quenching method in a salt bath heated to 250 ° C.

We can see the very different shape of the curves on the surface (S) or at the heart of the product (C) in cases 1 according to the invention or 2 according to the previous method.

The values of time (T, t) and temperature (8) defined above have also been reported in FIG. 1.

EXAMPLE 1

• - 5 minutes à 225-230°C
• - 8 minutes à 225-230°C
• - 10 minutes à 205-210°C et revenu de 24h à 150°C.

Sheets in 2214 (according to the specifications of the AA) of 60 mm thickness were treated, on the one hand according to the usual method with direct quenching in cold water and tempered (state T6), on the other hand , according to the invention, by quenching in cold water from 505 ° C. with interruption of:

• - 5 minutes at 225-230 ° C
• - 8 minutes at 225-230 ° C
• - 10 minutes at 205-210 ° C and 24 hour tempering at 150 ° C.

The results of mechanical characteristics of traction (in the long direction, long cross and short cross), of corrosion under tension (short cross direction) determined according to standard AIR 9048, of conductivity and residual stresses in the long direction (value of Rs ) are shown in Table I.

• - une bonne résistance à la corrosion sous tension,
• - des propriétés mécaniques de traction voisines du T6 classique,
• - une diminution de moitié du niveau de contraintes résiduelles.

In this example, we therefore obtain a product presenting simultaneously:

• - good resistance to corrosion under stress,
• - mechanical tensile properties close to that of conventional T6,
• - a halving of the level of residual stresses.

• I = corrosion intergranulaire )
• P = corrosion par piqûres suivant norme AIR 9050 C

Corrosion tests were carried out on 2214 alloy wafers of dimensions 40 x 80 x 5 mm. The largest dimension is parallel to the direction of rolling. After dissolving at 505 ° C, the wafers were cooled to the bearing temperature at a speed of 26 ° Clsec. Different temperatures and interruption times were applied, then the test pieces underwent 24-hour tempering at 150 ° C. On these samples, as well as on a control sample conventionally quenched and treated T6, the Vickers hardnesses were measured as well as the degree of laminating corrosion. The results corresponding to these tests are shown in FIG. 2. Above each experimental point, there is the ratio of the Vickers hardness of the test to the Vickers hardness of the usual T6 state. Below each point, the corrosion index is given, the meaning of which is as follows:

• I = intergranular corrosion)
• P = pitting corrosion according to AIR 9050 C standard

It can be seen that the DEGH domain corresponds to hardnesses almost equal to or greater than the T6 state and to immunity against intergranular corrosion.

EXAMPLE 2

A sheet of 7475 (according to the specifications of the A.A.) of 60 mm thickness was treated, on the one hand, according to the conventional process of quenching with cold water and tempered (states T6 and T73) and, on the other hand, following the invention, by quenching in cold water from 470 ° C and interruption for 6 minutes at 185 ° C and income 8 hours at 160 ° C. The results are reported in Table II.

• - des propriétés mécaniques de traction supérieures à l'état T73,
• - une résistance à la corrosion sous tension identique à l'état T73.

In this example, a product is obtained having both:

• - mechanical tensile properties superior to state T73,
• - resistance to corrosion under tension identical to state T73.

Corrosion tests were carried out on samples identical to those of Example 1 and treated in a similar manner, except as regards the final tempering carried out at 160 ° C. for 8 h.

These samples were subjected to a Vickers hardness and laminating corrosion test according to standard ASTM G34 / 78.

The results corresponding to these tests are given in FIG. 3. Above each experimental point is the ratio of the Vickers hardness of the test to the Vickers hardness of the control corresponding to the T6 state. Below each point, the index of laminating corrosion is indicated (EM: moderate laminating corrosion - EI: intermediate laminating corrosion).

It can be seen that there is a substantial reduction in laminating corrosion for a negligible loss of mechanical strength characteristics in the field under consideration.

The method according to the invention applies to the quenching of all aluminum curing alloys with structural hardening, in particular to alloys of the 2000, 6000 and 7000 series (according to the nomenclature of the Aluminum Association).

Claims (13)

1. A method of heat treatment of A1 alloys with structural hardening comprising dissolution, quenching, possibly plastic deformation, and one (or more) tempering (s) characterized in that the quenching is carried out under the conditions following: a) cooling of the product at an average speed greater than 3 ° C / sec. between the solution temperature and the range 150 ° C-260 ° C in a medium or with a cooling agent whose temperature is lower than the temperature (θ) defined in b). b) stopping the cooling so as to obtain a temperature (θ) of the product substantially uniform and included in the range 150 ^{0 -} 260 ° C for a period ranging from a few seconds to a few tens of minutes. c) resumption of product cooling to ambient temperature at an average speed greater than 60 ° C / min. between the temperature reached at the end of step b) and 100 ° C. 2. Method according to claim 1, characterized in that the cooling of step a) is obtained by cooling with water and that of step c) by cooling with water. 3. Method according to claim 1, characterized in that the cooling of step a) is obtained by cooling with water and that of step c) by cooling in air. 4. Method according to claims 2 or 3, characterized in that the water is cold (temperature below 40 ° C). 5. Method according to one of claims 1 to 4, characterized in that the quenched products subsequently undergo a controlled traction or compression and / or tempering. 6. Method according to one of claims 1 to 5, characterized in that for alloys of type 2214, the durations (t) and temperatures (θ) of quenching interruption (step b) are located inside the perimeter ABCDEF of figure 2. 7. Method according to claim 6, characterized in that the durations (t) and temperatures (θ) are located inside the CDEGH perimeter of Figure 2. 8. Method according to claims 6 or 7, characterized in that the products subsequently undergo income between 10h and 30h in a temperature range between 130 ° C and 170 ° C. 9. Method according to one of claims 1 to 5, characterized in that for the alloys of type 7075, the durations (t) and temperatures (θ) of quenching interruption are located within the perimeter ABCDEG of the figure 3. 10. Method according to claim 9, characterized in that the durations (t) and the temperatures (θ) are located inside the perimeter CDEF of FIG. 3. 11. Method according to one of claims 9 or 10 characterized in that the products subsequently undergo an income between 7h and 15h in a temperature range between í30 ° C and 170 ° C. 12. Product obtained by the process of one of claims 1 to 5 and 9 to 11, characterized in that it essentially comprises aluminum, from 4 to 8% of zinc, from 1.5 to 3.5 % of magnesium, from 1 to 2.5% of copper, and at least one element such as chromium, manganese or zirconium, from 0.05 to 0.30% having a resistance to corrosion under tension identical to that of l state T73 and a mechanical strength close to state T6. 13. Product obtained by the process of one of claims 1 to 8, characterized in that it essentially comprises aluminum, from 2 to 5% of copper, from 0.2 to 2.0% of magnesium, from 0.2 to 1.0 of manganese and from 0.1 to 1.0% of silicon having a resistance to corrosion under tension markedly improved compared to that of state T6, for an equivalent mechanical resistance.

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