EP0391815B1 - Aluminium-based alloy with a high modulus and an increased mechanical strength and process for production - Google Patents

Abstract

Al-based alloys of the 7000 series which have a high modulus (E >/= 74 GPa), a high mechanical strength (R0.2 >/= 530 MPa in the lengthwise direction), a good tenacity (KIC, lengthwise direction > 20 MPa  2ROOT m) and a good corrosion resistance under (O pressure >/= 250 MPa in the short transverse direction, lifetime >/= 30 days - ASTM Standard G 38-73). <??>The alloy according to the invention corresponds to the following weight composition: from 5.5 to 8.45% of Zr from 2 to 3.5% Mg from 0.5 to 2.5% Cu up to 0.5% Fe up to 0.5% Si other elements </= 0.05% each and up to 0.15% in all with 0.1 </= Zr </= 0.5% 0.3 </= Cr </= 0.6% 0.3 </= Mn </= 1.1% <??>It is preferably produced by the following process: a) a massive body which has the composition claimed above is formed by spray-deposition, b) this body is converted into a wrought product between 300 and 450 DEG C and then optionally when cold c) the wrought product is heat-treated by dissolving, quenching and annealing to a T6 or T7 state. <IMAGE>

Description

The invention relates to Al-based alloys of the 7000 series, according to the nomenclature of Aluminum Association (AA), having a high Young modulus and high mechanical characteristics of resistance and toughness as well as their method of obtaining.
Among the most resistant of the 7000 series aluminum alloys generally have a Young E modulus of the order of 70 GPa but not exceeding 72-73 GPa.
However, in order to lighten the structures, in particular in the aeronautical and space fields, the need for light alloys with higher Young's modulus (E ≧ 74 GPa) and high resistance (R 0.2 ≧ 530 MPa in the long sense) is felt, these characteristics must be achieved without significant damage to other properties of use such as toughness (KIC, long sense ≧ 20 MPa √ m) or resistance to corrosion under tension (threshold of non rupture at 30 days ≧ 250 MPa in the short cross direction and in the test medium considered).

Admittedly, alloys based on Al containing Li are known, the elastic modulus and mechanical characteristics of which are high. However, these pose complex production problems, given the reactivity of the Li and this therefore requires special and costly production and casting installations. The alloys according to the invention can be produced using conventional installations known in the metallurgy of common Al alloys. In addition, the mechanical resistance characteristics of Al-Li are generally lower than those of 7000 alloys.

Alloys type 7000 much more loaded with alloying elements and obtained by powder metallurgy have high mechanical characteristics, good resistance to corrosion under tension but a modulus lower than 74 GPa.

Also known from US-A-3563814 is an alloy of the following chemical composition (by weight%):
Zn: 6.8; Mg: 2.8; Cu: 2.1, Mn: 1.0; Cr: 0.5 and Zr: 0.1
obtained by powder metallurgy and hot spinning.

Application EP-A-0375571 describes the obtaining of alloys of the 7000 series, by spray-deposition, but with a high Zn content (Zn> 8.5% by weight).

The invention therefore relates to the alloys with the following weight composition (in%), produced by spray-deposition: Zn: 5.5 - 8.45 Mg: 2.0 - 3.5 Cu: 0.5 - 2.5 Zr: 0.1 - 0.5 Cr: 0.3 - 0.8 Mn: 0.3 - 1.1 Fe: up to 0.5 Yes: up to 0.5 other elements each ≦ 0.05 total ≦ 0.15 Rest Al

A preferred composition is as follows: Zn: 7.0 - 8.4 Mg: 2.0 - 2.9 Cu: 0.8 - 2.0 Zr: 0.1 - 0.4 Cr: 0.3 - 0.6 Mn: 0.3 - 0.9
the rest being identical to the above compositions.

• 1- à former par pulvérisation-dépôt un corps massif ayant une composition rentrant dans les limites indiquées ci-dessus.
• 2- à transformer à chaud ce corps en un produit ouvré entre 300 et 450°C, et puis éventuellement à froid.
• 3- à le traiter thermiquement par mise en solution, trempe et revenu, en un état T6 ou, de préférence, T7 suivant la nomenclature de l'AA.

A process for obtaining this consists of:

• 1- to form by spray-deposition a solid body having a composition falling within the limits indicated above.
• 2- to transform this body hot into a processed product between 300 and 450 ° C, and then possibly cold.
• 3- to heat treat it by dissolving, quenching and tempering, in a state T6 or, preferably, T7 according to the nomenclature of AA.

By spray-deposition is meant a process in which the metal is molten, atomized by a jet of gas at high pressure in the form of fine liquid droplets which are then directed and agglomerated on a substrate so as to form a massive and coherent deposit, containing a low closed porosity. This deposit can be in the form of billets, tubes or plates whose geometry is controlled.

A technique of this type is known as "Spray Deposition" by the Anglo-Saxons and is also called "OSPREY process". This latter process is mainly described in the following patent applications (or patents): GB-B-1379261; GB-B-1472939; GB-B-1548616; GB-B-1599392; GB-A-2172827; EP-A-225080; EP-A-225732; WO-A-87-03012.

The hot transformation stage can be preceded by a homogenization treatment of the solid body, in one or more stages, between the temperatures of 450 and 520 ° C., generally between 2 and 50 hours.

The product thus obtained meets the characteristics referred to above.
These properties are attributed to a fine dispersion of standard phases (Al, Mn, Cr) and Al₃Zr - due to the combination of the composition of the alloy and to its production process; this structure makes it possible to achieve, among other things, good ductility, toughness and a high elastic limit.

The dissolution is generally carried out between 450 and 520 ° C and the T6 type treatment between 90 and 150 ° C, for a sufficient time to substantially obtain the hardness peak (from 2 to 25 hours).
The T7 treatment consists of a T6 type treatment supplemented by tempering at a higher temperature, for example between 150 and 170 ° C, for 0.5 to 20 hours.
The invention is also applicable to composite materials hardened by dispersed ceramic particles of the oxide, carbide, nitride, silicide, boride, etc. type introduced into the alloy according to the invention, which constitutes the matrix thereof, during operation 1 , for example by injecting the powder into the liquid stream.
These particles have a size between 1 and 50 μm and represent (relative to the metal) a volume fraction of between 3 and 12%.

The invention will be better understood using the following tests: alloys 1 to 4 are in accordance with the invention, alloys 5 and 6 are outside the invention and alloy 7 is a conventional alloy (7075) from the prior art for comparison; this was poured semi-continuously and processed hot and heat treated like the other alloys; FIG. 1 represents the mechanical characteristics E and R _0.2 of the alloys tested, FIG. 2 the characteristics of toughness as a function of R _0.2 and FIG. 3 the characteristics of corrosion under tension as a function of R _0.2 .

EXAMPLE

• température de coulée: 750°C
• distance atomiseur-dépôt: 600 mm, maintenue sensiblement constante pendant l'essai
• collecteur en acier inoxydable animé d'un mouvement de rotation
• oscillation de l'atomiseur par rapport à l'axe de rotation du collecteur
• débit gaz/débit métal de 2 à 3 m³/kg.

Different alloys marked 1 to 6 whose weight compositions (in%) are given in Table 1 were melted and produced by spray-deposition (OSPREY process) in the form of billets

• casting temperature: 750 ° C
• atomizer-deposit distance: 600 mm, kept approximately constant during the test
• stainless steel manifold rotated
• oscillation of the atomizer relative to the axis of rotation of the collector
• gas flow / metal flow from 2 to 3 m³ / kg.

• mise en solution 2 heures à une température comprise entre 460 et 485°C
• trempe à l'eau froide
• revenu bipalier: 24 heures à 120°C + 1 à 20 heures entre 155 et 170°C

Les propriétés mécaniques obtenues sont reportées au Tableau 2.After peeling at ⌀ 140 mm, the billets are homogenized for 8 hours at a temperature of 460 ° C.
The blanks are then hot-spun at 400 ° C in a press whose container has a diameter of 143 mm in the form of flats with a cross-section of 50 x 22 mm, ie a spinning ratio of 14.6.
The flats thus obtained undergo a T7 type heat treatment under the following conditions:

• solution for 2 hours at a temperature between 460 and 485 ° C
• cold water quenching
• two-bearing income: 24 hours at 120 ° C + 1 to 20 hours between 155 and 170 ° C

The mechanical properties obtained are given in Table 2.

Alloys 1 to 4 are in the claimed domain and have a module ≧ 74 GPa and a long-term elastic limit ≧ 530 MPa while having good long-term ductility (≧ 8%) and long cross-beam (≧ 6%), a toughness LT direction of at least 20 MPa √m and good resistance to corrosion under tension (measured according to standard ASTM G 38 73).

The alloy 5 is outside the invention due to its too high Cr and Mn content, and although having a high modulus and elastic limit, it is very little ductile and cannot be used for the manufacture of parts.

The alloy 6 is also outside the invention because of its too low Cr and Mn content, does not have the advantages of the alloys according to the invention, has a low modulus and elastic limit and therefore does not differ from the alloys classics like the 7075.

The composition and properties of a conventional 7075 alloy conventionally cast and then transformed and heat treated according to the same range as alloys 1 to 6 are reported for comparison.

It is found that the modulus and the elastic limit of this alloy are significantly lower than for the alloys of the invention. TABLE 1 COMPOSITION OF TESTED ALLOYS Alloy Zn Mg Cu Cr Mn Zr Fe Yes Rest 1 7.8 2.3 1.4 0.35 0.85 0.16 <0.1 <0.1 Al 2 8.0 2.4 1.35 0.45 0.50 0.17 <0.1 <0.1 Al 3 6.5 2.2 1.5 0.50 0.60 0.20 <0.1 <0.1 Al 4 7.0 2.3 1.4 0.35 0.40 0.18 <0.1 <0.1 Al 5 7.5 2.2 1.35 0.9 1.2 0.25 <0.1 <0.1 Al 6 6.0 2.2 1.5 0.15 0.18 0.12 <0.1 <0.1 Al 7075 classic 5.5 2.3 1.6 0.23 - - <0.05 <0.04 Al

The alloys according to the invention are mainly intended for the manufacture of profiles or parts of forged or stamped structures.

Claims (8)

1. An alloy based on aluminium, obtained by spray deposition, containing 5.5 to 8.45% (by weight) of Zn, 2 to 3.5% of Mg, 0.5 to 2.5% Cu, up to 0.5% of Fe, up to 0.5% of Si, 0.1 to 0.5% of Zr, 0.3 to 0.6% of Cr, and 0.3 to 1.1% of Mn, other elements: ≦ 0.05% each and up to 0.15% in total, remainder aluminium, and having the following mechanical properties:
   E (Young's modulus) ≧ 74 GPa
   R 0.2 (elastic limit, longitudinal direction) ≧ 530 MPa
   KIC (longitudinal direction) ≧ 20 MPa √m
   Resistance to corrosion under tension (30 days) in the short transverse direction σ ≧ 250 MPa.
2. An alloy based on aluminium containing 7.0 to 8.4% (by weight) of Zn, 2 to 2.9% of Mg, 0.8 to 2.0 of Cu, up to 0.5% of Fe, up to 0.5% of Si, 0.1 to 0.4% of Zr, 0.3 to 0.6% of Cr and 0.3 to 0.9% of Mn, other elements: ≦ 0.05% each and up to 0.15% in total, remainder aluminium.
3. A composite material, the matrix of which is constituted by the alloy according to one of Claims 1 or 2, and which contains a homogeneous dispersion of ceramic particles from 1 to 50µm in size and representing a fraction by volume (relative to the metal) of between 3 and 12%.
4. A process for obtaining an alloy according to one of Claims 1 to 3, comprising the following steps:

   a) a solid body of the composition claimed hereinabove is formed by spray deposition,

   b) the body is converted to a worked product between 300 and 450°C and then possibly converted cold,

   c) the worked product undergoes heat treatment comprising solution heat treatment quenching and annealing in a T6 or T7 state.
5. A process according to Claim 4, where, between steps a and b, the body is homogenised between 450 and 520°C for a period of between 2 and 50 hours.
6. A process according to one of Claims 4 to 5, where the solution heat treatment takes place at between 440 and 520°C.
7. A process according to one of Claims 4 to 6, where the annealing is carried out at between 90 and 150°C for 2 to 25 hours.
8. A process according to Claim 7, where the annealing is supplemented by a 2nd annealing operation at a higher temperature of between 150 and 170°C for a period of time of between 0.5 and 20 hours.

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