Note: Descriptions are shown in the official language in which they were submitted.
338
BACKGROUND OF THE INVENTION
The invention concerns a filler metal for welding aluminum
alloys, in particular for welding alloys of the AlZnMg type.
AlZnMg alloys have found wide application because of
thelr good weldability, in particular because the weld region
hardens at room temperature to the strength level Or the
parent metal. Initial difficulties, which were due to poor
- resistance to stress corrosion, were overcome by choosing the
appropriate alloy composition, for example, by having an
appropriate Zn/Mg ratio and by suitable heat treatment, for
example, multi-stage artificial aging.
It is also known that additions of copper in amounts up
to 2.0% to the AlZnMg type alloy raises the strength and to
a large extent prevents stress corrosion cracking from
occurring.
In using these alloys in welded construct~ons, howeYer,
lt has been ~ound that the welds meet the requirements
regarding stress corrosion and exfoliation corrosion
susceptibility only when the construction has been heat
treated as a whole. It has been found imposs~ble to comply
with this requirement, in particular in the case Or large
welded constructions.
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11C~1?338
Various efforts have been made to improve the corrosion
resistance of the weld by means of suitable filler metals.
Thus, for example, in the Aluminium Taschenbuch, 13th issue,
Page 551, non age-hardenable alloys of the type AlSi, AlMg and
AlMgMn have been suggested for welding AlZnMg 1. The corrosion
problems are indeed solved this way, but only low weld strengths
can be achieved with these materials. It is clear, therefore,
that the high strength values which can be reached with AlZnMg
alloys in welded constructions cannot be exploited with these
materials.
Attempts have already been made to use AlZnMg alloys as
filler metal. Thus, for example, in the German Patent DT-OS 22
34 111, an age-hardenable aluminum filler metal of the following
composition has been proposed: zinc from 2.0 to 6.0g, magnesium
from 1.5 to 5.0%, chromium from 0.1 to 0.7%, silver from 0.05 to
1.04%, bismuth from 0.001 to 1.0%, beryllium from 0.001 to 1.0%,
zirconium from 0.05 to 0.2%, less than 0.4% manganese, less than
0.2% silicon, less than 0.5% iron, and less than 0.08% copper.
The mechanical properties in the weld which can be achieved
with this filler metal are comparable with those of the parent
metal. This filler metal also allowed the requirements regarding
stress corrosion susceptibility to be satisfied to a large degree.
It has been found, however, that in spite of optimal heat
treatment of the welded construction, there is relatively large
2~ susceptibility to weld boundary corrosion. Therefore, although
adequate strength values are obtained with such welds, there are
risks involved in their use in corrosive surroundings.
3o
110(~338
The inventor set himself the task of developing a
filler metal which produces welds of the same strength as
the parent metal and at the same time having good corrosion
resistance in corrosive environments.
The object of the invention is fulfilled by way
of the novel aluminum base alloy of the present invention,
and specifically by way of a filler metal according to
DIN 1732, sheet 1 to which has been added copper in the
amount of 0.2 to 0.5%. Specifically, the alloys of the
present invention consist essentially of in weight %:
zinc from 0.05 to 0.25%, magnesium up to 5.5% and prefer-
ably from 0.05to 5.5%, copper from 0.2 to 0.5% and pre-
ferably from 0.25 to 0.5%, manganese from 0.05 to 2.5%,
titanium from 0.1 to 0.25%, chromium from 0.05 to 0.3%,
and balance aluminum.
The alloys of the present invention suitably
contain less than 0.3%, by weight, silicon and less than
0.4%, by weight, iron.
Copending Canadian patent application S.N. 305,738,
filed June 19, 1978, Miroslav Pirner et al claims the addi-
tion of copper to alloys of the AlMgZn type in order to
develop a filler metal which produces welds of the same
strength as the parent metal plus having good corrosion
resistance.
It was found, surprisingly, in accordance with the
present invention that the foregoing filler metals can be
used for welding AlZnMg alloys without the previously
mentioned disadvantages being encountered, when the fore-
going copper addition is made to a filler metal of the types
AlMg, AlMn, AlMgMn in the amounts according to the present
invention. It is assumed that above all
AS-1208-A
110~338
the copper addition prevents both the occurrence of stress
corroslon and weld boundary corrosion, and that the amounts Or
manganese, titanium and chromium are responsible for reducing
susceptlbility to weld cracking. Metallographic investigations
have shown that the copper addition influences the cast structure
during solidification of the weld bead, and consequently
influences the boundary between the weld bead and the parent
metal, in such a way that stress corrosion and in particular weld
boundary corrosion are to a large extent avoided.
Particularly surprising was that the filler metal with the
amount Or copper added in accordance with the present invention
raised the resistance of the weld to stress corrosion
considerably, without causing a corresponding increase in
susceptibility to exroliation corrosion in the heat affected zone
in the parent metal.
The filler metal of the invention has also been found to be
suitable for welding constructional parts of AlZnMg alloys to
parts made out of other types of alloys, such as, e.g, ~lMn or
AlMg alloys.
The advantages of the weld filler metal of the invention
will now be illustrated in some detail by means of the
following examples.
EXAMPLE I
This example shows the results o~ testing welds in 4 mm
thick sheet of an artificially aged AlZnMg 1 alloy, prepared
using filler metal according to DIN 1732 and a filler metal
according to the present invention and by means of various
welding methods. The compositions of the flller metals are
given in Tahle I. .
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110~338
TABLE I
Filler
Metal Mg Mn Cu Cr Zn Ti Fe Si
D 4.9 0.35 0.05 0.120.12 0.17 0.38 0.30
E 4.8 0.35 0.31 0.150.15 0.14 0.25 0.20
Filler metal D corresponds to DIN 1732; filler metal E
contains the copper content in accordance with the present
invention.
The results of' testing the welds f`or mechanical strength and
corrosion resistance are given in TableII~
TABLE II
Average Lif'e-
time of Jones
Weld Stre2ngth test pieces
Filler Metal Weldin~ Method(N/mm ) (days)
D TIG,DC CHelium) 336 24
E TIG,DC ~Heliu~ 337 90
D MIG-Pulsed-Arc 305 21
E MIG-Pulsed-Arc 305 52
The corrosion resistance of the welds prepared using the
filler metal composition of the present invention was markedly
superior to those prepared using the f`iller metal in accordance
with DIN 1732. This was particularly so in the case of the
TIG,DC-Helium weld.
This invention may be embodied in other ~orms or carried out
in other ways without departing f'rom the spirit or essential
characteristics thereof. The present embodiment is there~ore to
be considered as in all respects illustrative and not restrictive,
the scope of the invention being indicated by the appended claims,
3~ and all changes which come within the meaning and range of
equivalency are intended to be embraced therein.
