EP0756017A1 - Aluminium-copper-magnesium alloy with high creep resistance - Google Patents

Abstract

An alloy comprises 2.0-3.0, pref. 2.5-2.75, wt.% Cu, 1.5-2.1, pref. 1.55-1.8, wt.% Mg, 0.3-0.7 wt.% Mn, >0.3 wt.% Fe, >0.3 wt.% Ni, >1.0 wt.% Ag, >0.15 wt.% Zr, >0.15 wt.% Ti, and Si such that 0.3 < Si + 0.4 Ag < 0.6, the residue being Al with other elements amounting to less than 0.05% each and less than 0.15% in total. The alloy may be drawn, forged or rolled, then hardened and tempered, after which it exhibits a creep of less than 0.3% in 1000 hours under a load of 250 MPa at 150 degrees C.

Description

Technical area

The invention relates to aluminum alloys of the 2000 series according to the designation of the Aluminum Association of the United States, of the AlCuMg type, exhibiting, after transformation by spinning, rolling or forging, very low creep deformation and a time at high rupture for temperatures between 100 and 150 ° C., while retaining properties of use at least equivalent to those of alloys of this type usually used for applications of the same kind.

State of the art

It has been known for several decades that alloys of the AlCuMgFeNi type have a higher creep resistance than AlCuMg alloys with the same Cu and Mg content. First used in the form of molded, stamped or forged parts, such alloys have been adapted to the manufacture of high-strength sheets and used in particular for the fuselage of the Concorde supersonic aircraft. They correspond to designation 2618 of the Aluminum Association with the following composition ranges (% by weight):
Cu: 1.9 - 2.7 Mg: 1.3 - 1.8 Fe: 0.9 - 1.3
Ni: 0.9 - 1.2 Si: 0.10 - 0.25 Ti: 0.04 - 0.10

A variant, which may contain up to 0.25% Mn and 0.25% Zr + Ti, has also been registered under the designation 2618A.

The 2618 alloy, used now for more than 20 years, does indeed have a creep resistance compatible with the flight conditions of a supersonic aircraft, but its resistance to crack propagation is somewhat insufficient, which requires monitoring. increased fuselage.

In order to prepare a successor to Concorde, we sought to modify alloy 2618 to improve its resistance to the propagation of cracks. Thus, patent FR 2279852 by CEGEDUR PECHINEY proposes an alloy with reduced iron and nickel content of the following composition (% by weight):
Cu: 1.8 - 3 Mg: 1.2 - 2.7 Si <0.3 Fe: 0.1 - 0.4
Ni + Co: 0.1 - 0.4 (Ni + Co) / Fe: 0.9 - 1.3

The alloy may also contain Zr, Mn, Cr, V or Mo at contents of less than 0.4%, and optionally Cd, In, Sn or Be of less than 0.2% each, Zn of less than 8% or Ag less than 1%. With this alloy, a significant improvement is obtained in the stress concentration factor K _1c representative of the resistance to the propagation of cracks. On the other hand, the results of the creep tests at temperatures of 100 and 175 ° C are quite comparable to those of 2618.

Subject of the invention

As part of the study of a new civilian supersonic aircraft whose speed and operating conditions will lead to a higher skin temperature for the fuselage, but also for other applications such as plastics molds, wheels or de-icing structures of aircraft or rotating machine parts, it appeared necessary to have an alloy having a higher creep resistance than that of the alloys of the prior art, that is to say total deformation under very low stress between 100 and 150 ° C for a period greater than 60,000 h, and a limitation of the creep damage liable to initiate fatigue cracks, resulting in a high failure time, without good sure to deteriorate the other properties of use such as static mechanical characteristics or resistance to corrosion.

The subject of the invention is therefore an AlCuMg alloy making it possible to obtain, on a product wrought by spinning, rolling or forging, a creep deformation after 1000 h, at 150 ° C. and under a stress of 250 MPa, of less than 0, 3% and a time to breakage of at least 2500 h, composition (% by weight):
Cu: 2.0 - 3.0 Mg: 1.5 - 2.1 Mn: 0.3 - 0.7 Zr <0.15
If: 0.3 - 0.6 Fe <0.3 Ni <0.3 Ti <0.15
other elements <0.05 each and 0.15 in total Al balance. The alloy can also contain silver with a content of less than 1% and, in this case, this element can partially replace silicon and the sum If + 0.4Ag must be between 0.3 and 0.6%.

Cu is preferably between 2.5 and 2.75% and Mg between 1.55 and 1.8%.

Description of the invention

The alloy according to the invention differs from that described in patent FR 2279852 by an even lower content of iron and nickel and by a higher content of silicon.

Iron and nickel are kept below 0.3% instead of 0.4% and it is even possible to completely remove nickel, which is a definite advantage for the recycling of manufacturing waste into current second alloys fusion. This reduction was not suggested by the state of the art. Thus, D. ADENIS and R. DEVELAY studied the influence of iron and nickel on creep resistance in the article "Relation between creep resistance and microstructure of AU2GN" published in the Scientific Memoirs of the Revue de Métallurgie, n ° 10, 1969 and they showed that the creep resistance at 150 ° C of an alloy without Fe and Ni was rather worse than that of a 2618. The same article also studies the role of silicon and shows that the creep resistance is optimal for a silicon content of 0.25%.

Similarly, the studies carried out at ONERA by H. MARTINOD and J. CALVET on alloy 2618 ("On the hot stability of refractory aluminum alloys of the AU2GN type" Study ONERA 1961) conclude that a content of silicon between 0.15 and 0.25% is best suited for even very prolonged use at 200 ° C, higher silicon contents up to 0.5% bringing no improvement. On the other hand, the metallurgical role of silicon, present in the structure in the form of a solid solution or of Mg ₂ Si precipitates, does not seem to have to be different for 2618 and for an alloy with low iron and nickel. Thus, increasing the silicon content to values of the order of 0.5% was not at all suggested by the literature on the subject nor by metallurgical reasoning.

The favorable role of silver in the creep resistance of AlCuMg alloys has been known for many years, in particular for casting alloys, and it has been the subject of metallurgical studies, for example the work of I.J. POLMEAR and M.J. COUPER "Design and development of an experimental wrought aluminum alloy for use at elevated temperatures" Metallurgical Transactions A, vol. 19A, April 1988, pp. 1027-1035.

The Applicant has found that it is possible to substitute for silicon a quantity 2.5 times greater than silver, which, taking into account the cost of this metal, is not of great economic interest. It has also found, and surprisingly, that the simultaneous addition of silicon and silver at contents such as Si + 0.4Ag is greater than 0.6% has an unfavorable influence on the creep resistance, in especially on the break time.

The alloy according to the invention has a manganese content of between 0.3 and 0.7%. Manganese contributes to increase the mechanical characteristics. Alloy 2618 did not contain manganese (H. MARTINOD mentions in its article a content of 0.014% for an example of industrial alloy) undoubtedly so as not to disturb the formation of intermetallic compounds with iron and nickel Al ₉ FeNi. It is probably for the same reason as the patent FR 2279852, if it mentions the possibility of an addition of manganese of at most 0.4%, this element being only one of the 11 optional addition elements, does not give any example of a composition containing manganese. This addition, up to a content of 0.7% beyond which appear harmful precipitates, is made possible by the limitation of iron and nickel and it corresponds to that of the high-strength alloy 2024 used for fuselages of subsonic aircraft.

The combination of these various modifications, namely the limitation of iron and nickel, the increase in the silicon content and the presence of manganese, leads to an unexpected increase in the creep resistance compared to alloy 2618 and by compared to an alloy as described in patent FR 2279852. Thus, during tests on thin sheets 1.6 mm thick, with a duration of 1000 h under stress of 250 MPa at a temperature of 150 ° C, a deformation at 1000 h is obtained of less than 0.3% instead of 1%, a creep rate in secondary regime less than 10 ^-9 s ^- 1 instead of 2.5 10 ^-9 s ^-1 and a time at failure greater than 2500 h instead of less than 1500 h. However, the fine-grain recrystallized structure of the thin sheets represents the most unfavorable case for the creep behavior, in particular for the deformation under stress, because of the localized deformation at the grain boundaries.

This last result is particularly interesting, although it has rarely been taken into account in previous studies on the creep of aluminum alloys. Indeed, it is important, in the case of a structural part subjected to cyclic stresses, not only that the creep deformation is low, but that the rupture is as late as possible. We thus delay the entry of the strain-time creep curve in the so-called "tertiary" phase, that is to say the one where the slope of the curve starts to increase again and where the rupture begins, with l appearance of creep cracks leading, at this temperature, to a low resistance to fatigue.

The tenacity of the alloys according to the invention is quite similar to that mentioned in patent FR 2279852, that is to say that it represents, for the stress concentration coefficient K _1c, a gain of 20 to 40%. compared to alloy 2618.

The alloys according to the invention can be cast in the form of billets or plates by the conventional methods of casting alloys of the 2000 series, and transformed by spinning, hot and possibly cold rolling, stamping or forging, the semi-finished product thus obtained is usually heat treated by dissolution, quenching, optionally controlled traction to reduce residual stresses and tempering, to give it the mechanical characteristics required by the intended application.

Examples Example 1

Plates of alloy 2618, of alloy A according to patent FR 2279852, were cast in 4 alloys B, C, D and E according to the invention and 3 alloys F, G and H outside the invention. The chemical compositions of the alloys are indicated in Table 1. Alloy A contains manganese, unlike the alloys exemplified in the patent, which allows a better appreciation by comparison of the role of the other elements, in particular silicon. Alloys B, D and E contain silver. Alloy E is in accordance with the invention, but its Mg content is outside the preferred range. Alloy F is just below the lower limit for the sum Si + 0.4Ag and, moreover, outside the preferential zone for Mg. Alloy G is slightly above the upper limit for Si + 0.4Ag and alloy H is out of limits for Cu.

The plates were then homogenized 24 h at 520 ° C, hot rolled, then cold rolled to the thickness of 1.6 mm, having a recrystallized metallurgical structure with fine grains after dissolving for 40 min at 530 ° C, traction controlled at 1.4% deformation, quenching and tempering from 19 h at 190 ° C.

Creep tests were carried out according to ASTM E standard 139 and we measured, for a constraint of 250 MPa and a temperature of 150 ° C, the deformation after 1000 h, the minimum creep speed, that is to say the slope of the deformation curve in creep as a function time in the secondary creep zone, as well as the failure time, which is representative of the resistance to damage. The results are collated in Table 2.

It is found that the alloys according to the invention all have a creep deformation at 1000 h less than 0.30%, a minimum creep speed less than 0.6 10 ^-9 per second and a failure time greater than 2500 h, while these values are respectively, both for 2618 and for the alloy according to FR 2279852 with the addition of manganese, of the order of 0.9 to 1%, 2.5 10 ^-9 s ^-1 and 1400 h.

We also note the critical nature of the limits of the sum Si + 0.4Ag, the deformation and the break time being very degraded below the lower limit and the break time being also degraded above the upper limit of 0, 6%. We finally see the interest of the preferential ranges of composition for Cu and Mg.

Example 2

Plates were cast of alloy 2618, of alloy A of the previous example and of 3 other alloys according to the invention I, J and K, the chemical composition of which is given in Table 3. These alloys do not contain silver and alloy J does not contain nickel at all. Alloys I and J have a manganese content close to the lower limit of the range, while that of alloy K is close to the upper limit.

The plates were homogenized 24 h at 520 ° C, scalped and hot rolled to a thickness of 14 mm. Part of the sheets obtained was left at this thickness, and another part was cold rolled to 1.6 mm. The sheets were dissolved at 530 ° C. for 1 h for the 14 mm sheets and 40 min for the 1.6 mm sheets, then towed, soaked and returned 7 p.m. to 190 ° C.

The elastic limit at 0.2% R _0.2 , the breaking load Rm and the elongation at break A were measured on these sheets. These results are shown in Table 4. They show that the limit of elasticity and breaking load are practically the same for the 5 alloys, and that the elongation of the sheets of alloys according to the invention is slightly greater than that of the sheets of 2618 or of alloy A.

The minimum creep rate was then measured at 150 ° C (for 1.6 mm sheets only) and at 175 ° C at 250 MPa, as in the previous example. The results are reported in Table 5, which shows a very significant improvement in the creep resistance of the alloys according to the invention compared to those of the prior art, and particularly at 175 ° C. Finally, the tenacity of the sheets of alloys according to the invention (about 125 MPavm for the thickness 1.6 mm) is entirely comparable to that of alloy A.

Claims (6)

Aluminum alloy with high creep resistance presenting in the wrought state and treated by dissolution, quenching and tempering, a creep deformation at 1000 h at 150 ° C under 250 MPa of less than 0.3% and a time breakage of at least 2500 h of composition (% by weight):
Cu: 2.0 - 3.0 Mg: 1.5 - 2.1 Mn: 0.3 - 0.7
Fe <0.3 Ni <0.3 Ag <1.0 Zr <0.15 Ti 0.15
with Si such that: 0.3 <Si + 0.4Ag <0.6
other items <0.05 each and <0.15 in total. Alloy according to claim 1, characterized in that Cu is between 2.5 and 2.75%. Alloy according to either of Claims 1 and 2, characterized in that Mg is between 1.55 and 1.8%. Alloy according to one of claims 1 to 3, characterized in that it is wrought by spinning. Alloy according to one of claims 1 to 3, characterized in that it is wrought by forging. Alloy according to one of claims 1 to 3, characterized in that it is wrought by rolling.