Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2000-07-13
2002-05-28
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S702000
Reexamination Certificate
active
06395111
ABSTRACT:
The invention relates to an aluminum-based alloy, preferably from the Al—Li—Mg system, which contains lithium, magnesium, zinc, zirconium and manganese, and relates to the metallurgy of alloys used as a construction material in aeronautics and aerospace engineering, in shipbuilding and mechanical engineering of earthbound means of transportation, including welding structures.
Known in the art are alloys of the system Al—Li—Mg that exhibit a reduced density and relatively high strength, but have a low ductility and diminished fracture toughness. The alloy according to U.S. Patent Specification NO. 4,584,173 dated Apr. 22, 1986 has the following chemical composition, % w/w:
Aluminum
Base
Lithium
2.1-2.9
Magnesium
3.0-5.5
Copper
0.2-0.7
and one or more elements from the group containing zirconium, hafnium and niobium:
Zirconium
0.05-0.25
Hafnium
0.10-0.50
Niobium
0.05-0.30
and
Zinc
0-2.0
Titanium
0-0.5
Manganese
0-0.5
Nickel
0-0.5
Chromium
0-0.5
Germanium
0-0.2
If this alloy is quenched at a temperature of 530 ° C. and then stretch-adjusted with a ductility of 2% and artificially aged at 190 ° C. for 4-16 h, the disadvantage is that the alloy exhibits low ductility in the heat-treated state (relative elongation 3.1-4.5%) and low corrosion resistance.
The alloy according to International Patent Application WO No. 92/03583 has the following chemical composition in % w/w:
Aluminum
Base
Lithium
0.5-3.0
Magnesium
0.5-10.0
Zinc
0.1-5.0
Silver
0.1-2.0
At a max. 12% total content of these elements and, when they measure 7.0-10.0% in sum, lithium cannot exceed 2.5%, and zinc 2.0%; in addition, the alloy can contain up to 1.0% zirconium.
This alloy exhibits a strength of 476-497 MPa, an apparent yield point of 368-455 MPa, a relative elongation of 7-9% and a density of 2.46-2.63 g/cm
3
. The alloy is recommended as a structural material for products in aeronautics and aerospace. The disadvantages to this alloy are as follows:
The high strength can be ensured:
by a high lithium content, but this reduces the ductility and fracture toughness of the alloy, diminishes its cold formability, and difficulties are encountered during the manufacture of thin sheets required for flying devices;
by a high zinc content; this increases the alloy density to values of 2.60-2.63 g/cm
3
, which significantly diminishes the savings in weight for the product;
by stretching the quenched material prior to an artificial ageing with a degree of ductility of 5-6%, which diminishes the fracture toughness parameters.
The alloy is alloyed with silver, which increases the product costs, from semi-finished to finished products. Alloys with a high zinc content and added copper exhibit a diminished corrosion resistance; during fusion welding, they show an increased tendency to form defects and a distinct loss of cohesion.
A comparable alloy for the entire area of application is known from U.S. Pat. No. 4,636,357. This alloy has the following composition, % w/w:
Aluminum
Base
Lithium
2.0-3.0
Magnesium
0.5-4.0
Zinc
2.0-5.0
Copper
0-2.0
Zirconium
0-0.2
Manganese
0-0.5
Nickel
0-0.5
Chromium
0-0.4
The alloy is hardened via heat treatment:
Quenching at a temperature of 460° C., stretching with a stretching degree of 0-3% and a two-stage heat treatment:
Stage 1 at 90° C., 16 h and stage 2 at 150° C., 24 h.
This alloy exhibits a sufficiently high level of strength of 440-550 MPa and an apparent yield point of 350-410 MPa.
The disadvantages to this alloy include the low level of relative elongation of the alloy (1.0-7.0%) and the low fracture toughness, inadequate corrosion resistance and limited strength of welds in comparison to the strength of the base material.
Therefore, the object of the present invention is to achieve an increased ductility for the alloy in a heat-treated state while retaining a high strength and ensuring a high corrosion resistance and weldability, at the same time ensuring sufficiently high parameters for fracture toughness and thermal stability after warming at 85° C. over the course of 1000 h.
This object is achieved according to the invention by an alloy from the Al—Li—Mg system with the following chemical composition, % w/w:
Lithium
1.5-1.9
Magnesium
4.1-6.0
Zinc
0.1-1.5
Zirconium
0.05-0.3
Manganese
0.01-0.8
Hydrogen
0.9 × 10
−5
-4.5 × 10
−5
and at least one element selected from the following group:
Beryllium
0.001-0.2
Yttrium
0.01-0.5
Scandium
0.01-0.3
Aluminum
Remainder
As solid, finely distributed lithium hydride particles form, the hydrogen content reduces the contraction during solidification, and prevents the formation of porosity in the material.
The magnesium content ensures the necessary level of strength characteristics and weldability. If the magnesium content drops below 4.1%, strength will decrease, and the tendency of the alloy to form hot cracks both during casting and welding will rise. Increasing the magnesium content in the alloy to over 6.0% diminishes processability during casting, hot and cold rolling, and the plasticity parameters of completed semi-finished products and articles made from them.
Maintaining the lithium content is important to ensure the required processability, in particular during them manufacture of thin sheets, the necessary level of mechanical and corrosion characteristics, and sufficient fracture toughness and weldability. A drop in lithium content to below 1.5% increased the alloy density, diminished the level of strength characteristics and the modulus of elasticity. A lithium content exceeding 1.9% was associated with diminished processability via cold forming, weldability, plasticity parameters and fracture toughness.
0.05-0.3% zirconium is a modifier during the casting of ingots, and together with manganese (0.01-0.8%) ensures a structural solidification in the semi-finished products due to the formation of a polygonized or fine-grained structure.
In particular adding one or more of the elements beryllium, yttrium and scandium yields the formation of a homogeneous, fine-grained structure in semi-finished products comprised out of the alloy according to the invention, and an increased ductility during cold-rolling.
The invention also relates to a procedure for heat-treating aluminum-based alloys, preferably from the Al—Li—Mg system.
The object of such a heat-treatment procedure is to increase the ductility of the alloy while retaining its high strength, and simultaneously achieve high parameters for corrosion resistance and fracture toughness, but in particular to preserve these characteristics when exposing the material to an elevated temperature over a prolonged time.
Known from U.S. Patent Specification 4,861,391 is a procedure for heat treatment, which involves quenching with rapid cooling, stretching and two-stage ageing as follows:
Stage 1 at a temperature not to exceed 93 ° C., from several hours to several months; preferably 66-85° C., at least 24 h.
Stage 2 at a max. temperature of 219° C., from 30 minutes to several hours, 154-199 ° C., max. 8 h.
While the strength parameters and fracture toughness are increased, this procedure does not ensure stability with respect to the characteristics of lithium-containing aluminum alloys after low-temperature warming at 85° C. over the course of 1000 h, which simulates heating by the sun during the prolonged operation of flying devices. After warming to 85° C. over 1000 h, the relative elongation and fracture toughness of the lithium-containing alloys treated according to this method drop by 25-30%.
According to the invention, a procedure for achieving the set task encompasses the following steps:
heating the material to a temperature of 400 to 500° C.
quenching in water or air, stretch-adjusting the material with a ductility of up to 2%, and
artificial ageing, wherein artificial ageing takes place in 3 stages, of which the third ageing stage takes place at 90 to 110° C. over the course of 8 to 14 h.
As an alternative to executing the third ageing stage at a constant temperature, the latter ca
Chertovikov Vladimir Mikhajlovich
Davydov Valentin Georgijevich
Fridlyander Iosif Naumovich
Galkin Dmitrij Sergejevich
Kablov Evgenij Nikolajevich
Combs Morillo Janelle
Eads Deutschland GmbH
Ladas & Parry
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