EP0157711A1 - Process for the manufacture of objects from Al-Li-Mg-Cu alloys with high ductibility and isotropy properties - Google Patents

Abstract

The invention concerns a process for producing products of Al-base alloys essentially containing Li, Mg and Cu as main alloy elements and having a high level of ductility and isotropy. The process comprises subjecting the homogenized product to a "tepid" transformation operation at from 100 DEG to 420 DEG C. The intermediate annealing operations, if required, are carried out in the range of from 200 DEG to 550 DEG C. The rate of cooling after the intermediate annealing operations is generally kept at from 1500 DEG C./second to 30 DEG C./hour. The isotropy and ductility of the resulting products are considerably improved in comparison with the prior art processes.

Description

The invention relates to a process for obtaining Al-based alloy products containing essentially Li, Mg and Cu as main alloying elements and having high ductility and isotropy.

The ranges of conventional manufacturing which include in particular a homogenization of the initial products obtained either by casting or by powder metallurgy, possible scalping, hot transformation, cold transformation with one or more intermediate anneals if necessary, dissolution and tempering, with a possible strain hardening between these last two operations, lead, on alloys of the Al-Li-Mg-Cu family, to mechanical tensile characteristics (breaking load Rm, elastic limit Rp 0.2 and ductility A%) varying very significantly depending on the direction of sampling compared to the main direction of hot and cold processing (long sense) of the products.

This is particularly true for flat products such as plates, strips, strips or sheets. This results in well-known drawbacks during subsequent shaping, for example, when stamping a sheet (formation of horns, local ruptures, etc.) or during end use.

The method according to the invention makes it possible to considerably improve the isotropy of the mechanical characteristics of products of alloy type Al-Li-Mg-Cu, while also improving their ductility.

It consists in imposing on a homogenized product a hot transformation at a temperature below 420 ° C, generally between 100 and 400 ° C and preferably between 300 and 350 ° C until the desired final dimensions or up to intermediate dimensions. When the final dimensions are directly reached, this "lukewarm" transformation is followed by the conventional solution treatment (generally between 500 ° and 550 ° C depending on the alloy) and thermomechanical hardening treatments structurally.

The intermediate product is generally brought to the final dimensions by cold processing, using one or more intermediate anneals, if necessary. These can be done just before cold processing and / or during it. In this case, at least one of these anneals is carried out in a temperature range between 200 and 550 ° C. The holding times are of the order of a few minutes to several hours, the shortest times being generally associated with highest temperatures.

It has been noted that the cooling rate after this (or these) intermediate annealing (s) is an important parameter of the process and should preferably be between 1500 ° C / sec and 30 ° C / hour. The speeds higher than 1500 ° C / sec are difficult to reach industrially, except on very thin product, and the speeds lower than 30 ° C / hour do not bring an appreciable improvement to the isotropy.

The intermediate annealing (s) can be conducted, as is known, either in salt bath furnaces (nitrite-nitrate, nitrate-nitrate), or in static ventilated air ovens, or in passage ovens.

After cold processing, the products undergo the usual solutions for dissolving, quenching and thermomechanical treatment for structural hardening, the dissolving constituting ipso facto the final annealing.

The invention will be better understood using the following examples, relating to sheets, it being understood that it is applicable to other forms of wrought products such as forged, stamped or spun products.

FIGS. 1 and 2 represent in polar coordinates the variations in the mechanical traction characteristics obtained on sheets according to the direction of removal according to conventional ranges (marks A to F) or according to the invention (marks 1 and 2).

EXAMPLE 1

An alloy containing (by weight) 2.76% Li - 1.32% Cu - 1.04% Mg - 0.11% Zr - 0.02% Fe - 0.02% Si, remains Al, was poured under shape of a 300 x 100 mm plate, homogenized at 533 ° C, 24 hours, scalped and hot rolled at 470 ° C - 420 ° C up to 5 mm thick.

From this blank, the hot and cold transformations up to 1.6 mm thick were carried out under the conditions shown in Table I, in accordance with a conventional range, the hot rolling (LAC) having taken place. between about 470 and 420 ° C, and cold rolling (LAF) at room temperature.

These sheets were then placed in solution at 533 ° C for 30 minutes (ventilated oven), quenching in cold water, controlled traction of 3.5%, then a 24-hour tempering at 190 ° C.

Tensile samples were taken in the directions 0 ° (long direction L), 30 °, 60 ° and 90 ° (long direction long TL) relative to the rolling direction.

.Range 1

• Annealing in salt bath 533 ° C - 7 min
• Tempering: cold water
• Lamination: cold up to 1.6 mm
• Solution: 30 'at 533 ° C in a salt bath
• Tempering: cold water
• Traction: 3.5% controlled
• Income: 24 h at 190 ° C

.Range 2

• Annealing in air oven 533 ° C - 7 min
• Cooling: slow (25 ° C / hour)
• Lamination: cold up to 1.6 mm
• Solution: 30 'at 533 ° C in a salt bath
• Tempering: cold water
• Controlled traction: 3.5%
• Income: 24 h at 190 ° C

.Range 3

• Annealing in air oven 350 ° C - 1 h 30
• Slow cooling (25 ° C / hour)
• Lamination: cold up to 1.6 mm
• Solution: 30 'at 533 ° C in a salt bath
• Tempering: cold water
• Traction: 3.5% controlled
• Income: 24 h at 190 ° C

The tensile test pieces were taken under the same conditions as above.

The results obtained are reported in Table II and shown graphically in Figures 1 and 2.

EXAMPLE 2

An alloy containing (% by weight) 2.10% Li - 2.38% Cu - 1.30% Mg - 0.11% Zr - 0.02% Fe - 0.02% Si, remains Al, has been cast in a plate (300 x 100 mm), homogenized at 526 ° C for 24 hours, laminated between 350 and 300 ° C up to 3.2 mm thick, annealed at 350 ° C for 1 h 30 in an air oven, slow cooling (20 ° C / hour), cold rolled up to 1.6 mm thick, dissolved at 526 ° C for 30 minutes, quenched in cold water, 2% tensioned and returned 20 hours at 190 ° C.

The results of tensile tests obtained in different directions are given in Table III.

EXAMPLE 3

An alloy containing (% by weight) 2.7% Li - 1.6% Cu - 1.0% Mg - 0.11% Zr-0.04% Fe - 0.03% Si, remains Al, has been cast and transformed in accordance with the conditions relating to Example 2, except as regards the controlled traction after quenching brought to 5%.

The traction results obtained are reported in Table IV.

The examples according to the invention clearly show that the range applied leads to highly isotropic products whose elongation is generally greater than 7%.

Claims (5)

1 - Process for obtaining products in Al-based alloys containing essentially Li, Mg and Cu as main alloying elements comprising the preparation, homogenization, hot transformation, possibly cold transformation with intermediate annealing if necessary, dissolving, quenching, possible controlled cold deformation and tempering, characterized in that the hot transformation takes place in the range of temperatures between 100 and 420 ° C. 2 - Process according to claim 1, characterized in that at least one of the intermediate anneals performed before the cold transformation and / or during this is carried out in the temperature range between 200 and 550 ° C. 3 - Method according to claim 1 or 2, characterized in that the cooling rate after the (or them) annealing (s) 'intermediate. (S) is between 1500 ° C / sec and 30 ° C / hour. 4 - Method according to one of claims 1 to 3, characterized in that the field of hot transformation extends between 100 and 400 ° C. 5 - Method according to claim 4, characterized in that this range extends between 300 and 350 ° C.

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