Aluminum-based alloy and procedure for its heat treatment

Details Aluminum-based alloy and procedure for its heat treatment Aluminum-based alloy and procedure for its heat treatment

2001-11-26

2002-10-08

King, Roy (Department: 1742)

Alloys or metallic compositions

Aluminum base

Copper containing

C420S541000, C420S543000, C420S545000

Reexamination Certificate

active

06461566

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. Pat. 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.


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