Note: Descriptions are shown in the official language in which they were submitted.
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Magnesium alloy
The invention relates to a corrosion-resistant magnesium alloy, in particular
a
secondary alloy melted from scrap metal, to its use and to a process for its
production.
In the field of magnesium metallurgy, no secondary alloys have existed so far
such
as they exist in the case of other metals such as in the field of aluminium
metallurgy,
for example. Secondary alloys are smelted from scrap metal. They are then
reprocessed into products.
It is known that magnesium alloys are corrosion-resistant if the copper,
nickel and
iron contents are very low. In the widely used alloys of the groups AZ
(magnesium
with aluminium and zinc), AM (magnesium with aluminium and manganese), AS
(magnesium with aluminium and silicon) and AJ (magnesium with aluminium and
silicon) and AJ (magnesium with aluminium and strontium) the limits of
tolerance are
usually fixed at 250 ppm copper, 10 ppm nickel and 50 ppm iron. If these
contents
are exceeded, strong corrosion, particularly by pitting, occurs, as indicated
by Bakke,
P., Sannes, S., Albright, D.: Soc. Automotive Engineers, paper 1999-01-0926,
(1999), pp. 1-10 and Kammer, C. (editor): Magnesiumtaschenbuch (Magnesium
handbook), Aluminium-Verlag Dusseldorf, 2000, 1st edition, ISBN 3-87017-264-9.
As
a result, non-coated parts such as gearboxes or crank cases of magnesium
alloys
become unfit for use.
Coated parts, such as mobile phone, computer or chainsaw cases are exposed to
attack by corrosion if the surfaces exhibit even slight damage. The magnesium
alloys
must consequently be produced from pure precursor products which must be free,
above all, from copper and nickel since these elements cannot be removed from
magnesium and its alloys without major effort.
The need to save energy and protect the environment requires recycling of
products
consisting of magnesium alloys. This becomes obvious if the energy expenditure
of
kWh/kg of magnesium (disregarding the degrees of effectiveness for energy
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conversion) during the primary production of magnesium is compared with the
energy requirement of 1 kWh/kg for recycling new scrap metal.
Recycling of old scrap metal and in particular of shredder fractions by simple
re-
smelting with an advantageous energy requirement that is only insignificantly
higher
than for fresh scrap metal has so far not been possible. Shredder fractions
always
contain scrap aluminium and consequently also copper. Even dismantled parts
sorted according to type cause the same problems since nickel and copper pass
from the coatings into the re-smelted alloys. As a result of this situation,
no
secondary alloys exist, contrary to the case of aluminium alloys which have
been
accepted for some considerable time. Known magnesium alloys with increased
contents of aluminium and zinc are listed in table 1.
All the percentage data used in the following mean percentages by mass as is
common practice in this field of specialisation.
Table 1: Composition of magnesium alloys with increased aluminium and zinc
contents, in percent by mass, ASM Handbook, ISBN 0-87170-657-1, 199.
Alloy Al Zn Mn max. max. max. max.
max.
Cu Ni Fe Si
others
AZ92 8.3-9.7 1.6-2.4 0.1-0.35 0.35 0.01 0.02 0.3
0.3
AZ125 11-13 4.5-5.5
0.3
AM100 9.3-10.7 0.3 0.1-0.5 0.1 0.01 0.023 0.18
0.3
AM90 7-9.5 0.3-2 0.15 0.35 0.02 0.05 0.5
(SIS144640)
The alloys AZ92, AM100 and AM90 from Table 1 have either high aluminium
contents and low zinc contents or vice versa. With increased contents of
copper,
nickel and iron, the corrosion properties of these alloys are poor in
comparison with a
pure alloy as described below by way the figures and examples. Although the
alloy
AZ125 has high contents of aluminium and zinc, the sum total of the other
components amounts to only 0.3%. However, a secondary alloy ought to be
characterised by higher defined contents of copper, nickel and iron which are
always
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present in secondary alloys, being tolerable regarding the corrosion
resistance.
Consequently, these alloys cannot be regarded as secondary alloys.
In the Drawings:
Figure 1 shows the network structure of the beta phase Mg17A112;
Figure 2 shows that the pure alloy AZ91 does not exhibit the network structure
of the
beta phase; and
Figure 3 compares corrosion of a contaminated alloy, a pure alloy and a new
secondary alloy of the present invention.
The invention was based on the object of developing a magnesium alloy which,
in
spite of higher defined contents of copper and nickel, possesses corrosion
properties
comparable with or better than a highly pure magnesium primary alloy.
Such a magnesium alloy can be obtained from scrap metal or impure precursors
containing in particular copper or nickel by adjusting the components during
smelting
and be reused for structural parts.
The magnesium alloy according to the invention contains 10 to 20% by mass of
aluminium, 2.5 to 10% by mass of zinc, 0.1 to 2% by mass of manganese, 0.3 to
2%
by mass of copper or up to 2% by mass, preferably 0.001 to 2% by mass of
nickel,
essentially made up to 100% with magnesium, i.e., the alloy consists
essentially of
the above-mentioned components, it being possible for the fluxes mentioned
below
and, optionally, (further) impurities to be present in a small quantity.
In most cases, at least one of the elements of copper, nickel, cobalt, iron,
silicon,
zirconium and beryllium will additionally be contained in the alloy to a total
content of
up to 2% (copper in addition to nickel or nickel in addition to copper). It
will be
possible to speak of the element concerned "being contained" as a rule if it
is present
in a minimum quantity of approximately 0.001% by mass.
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,
Surprisingly enough, it has been found that, in spite of frequently higher
contents of
copper, nickel, iron and silicon in the new magnesium secondary alloy in
comparison
with the limit contents of the alloys previously used for structural parts,
the corrosion
behaviour is just as good as that of the highly pure alloys.
Preferably, the impurity-tolerant magnesium alloy consists of 11 to 18% by
mass
aluminium, preferably 12 to 16% by mass aluminium, 3 to 8% by mass zinc,
preferably 3 to 5% by mass zinc, 0.3 to 1.5% by mass manganese, preferably 0.5
to
1% by mass manganese, 0.3 to 2% by mass copper, preferably 0.45 to 0.8% by
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mass copper and, if necessary, at least one of the elements of nickel, cobalt,
iron,
silicon, zirconium and beryllium to a total content of 1.5%, preferably a
total content
of up to 1%, the remainder being magnesium.
The subject matter of the invention is consequently a magnesium alloy which
contains the indicated contents of aluminium, zinc and manganese as well as
additionally 0.3% to 2% copper and/or the elements nickel, cobalt, iron,
silicon,
zirconium and beryllium to a total content of up to 2%, preferably up to 1.5%,
further
preferably up to 1%, the elements of copper, nickel, cobalt, iron and silicon
generally
being introduced into the alloy by contaminated alloy starting materials
and/or scrap
metal.
Additional small contents of other elements which the expert would not
consider to be
alloy components may be present. Such contaminants are present within orders
of
magnitude of up to maximum 0.1% and in particular maximum 0.01%. Contaminants
of this order of magnitude may in turn have been entrained by the use of
impure
precursor materials or scrap metal.
The nickel content of the magnesium alloy is preferably at least 0.001%,
further
preferably at least 0.003%. These nickel contents can be balanced by higher
aluminium, zinc and manganese contents in the inventive alloys in the sense
that, in
spite of the higher nickel content, it has not been possible to detect
increased
corrosion properties.
Moreover, the magnesium alloy preferably contains at least 0.4% copper.
In a preferred further development of the invention, the magnesium alloy
moreover
contains up to 2% of at least one of the elements of calcium and strontium and
in a
further preferred embodiment up to 2% of at least one of the elements of the
group of
elements of rare earths, yttrium and scandium. According to a further
embodiment of
the invention, the magnesium alloy preferably contains up to 2% and further
preferably at least 0.1% of cerium mixed metal. Cerium mixed metal is
available
commercially and well known to the expert. A typical composition for cerium
mixed
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metal would be for example: rare earths at least 99.00%, cerium maximum
57.12%,
lanthanum maximum 36.19%, praseodymium maximum 4.33%, neodymium
maximum 2.36%, iron maximum 0.54%, magnesium maximum 0.14%, silicon
maximum 0.051%, sulphur maximum 0.01%, phosphorus maximum 0.01% (from
5 Handbook of Extractive Metallurgy, vol. III, 1997).
Surprisingly enough it has been found that by adding strontium, calcium, rare
earths,
yttrium and scandium individually or in mixture in contents of up to 2%, the
corrosion
properties were further improved. The rates of corrosion of these alloys,
determined
according to the salt spray test according to DIN 50021, are indicated below
in Table
4, the compositions in Table 5.
The magnesium alloy according to this invention is characterised preferably by
the
beta-phase having a network structure.
The magnesium alloy obtained preferably has a corrosion rate of less than 1.2
mm/year, measured by means of a salt spray test according to DIN 50021, as
indicated below in further detail in the examples.
The object of the invention is also achieved by the magnesium alloy being a
secondary alloy which was obtained by smelting scrap metal or impure precursor
materials containing copper and/or nickel.
Such a magnesium secondary alloy can be produced in a cost-effective manner
and
is particularly suitable for the production of structural parts.
Such a magnesium alloy is also particularly suitable for the production and
use of
corrosion protection anodes in the fresh water sector.
The invention comprises also a process for the production of a magnesium alloy
from
precursor materials contaminated with copper and/or nickel, in particular from
scrap
magnesium, which process is characterised in that the scrap metal or the
impure
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precursor materials are smelted and the alloy is adjusted to a content of
components
corresponding to an inventive magnesium alloy as described above.
The comparative corrosion investigations were carried out by immersion in a
3.5 and
5% sodium chloride solution and according to the salt spray test according to
DIN
50021. During the immersion measurements, the rate of corrosion was determined
by measuring the amount of hydrogen developed and/or by titration with
hydrochloric
acid. In the salt spray test, the loss of mass is determined. In Table 2, the
rates of
corrosion of a fresh secondary alloy, a pure alloy and a comparative alloy
with similar
contents of copper and nickel are compared. The composition of the alloys
listed in
Table 2 is given in Table 3.
Table 2: Rates of corrosion of the magnesium alloys in mm/year
Immersion pH = 6 Salt spray test
mm/year mm/year
New secondary alloy 6.57 1.00
Pure alloy 5.73 1.07
Comparative alloy 35.27 23.79
Table 3: Composition of the alloys indicated in Table 2, in percent by mass
Al Zn Mn Cu Ni Fe Si
New
11.7 3.04 0.48 0.47 0.0032 0.0087 0.39
secondary
alloy
Pure alloy 8.65 0.67 0.20
0.0081 0.00061 0.0022 0.054
Comparative 8.17 2.84 0.21 0.0085 0.0026 0.023
0.18
alloy
The conditions for the immersion test were as follows:
3.5% by weight of aqueous NaCI solution
pH = 6 (constant)
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Volume of the solution: 1.9 I
Sample size: Diameter 25mm, thickness 4mm
Sample treatment: ground with granulation 1200, rinsed with water and ethanol.
After immersion for at least 100 hours, a rate of corrosion of less than
10mm/year is
obtained for the alloys according to the invention and after at least 400
hours, a rate
of corrosion of less than 20mm/year. The salt spray test according to DIN
50021
showed that the magnesium alloys according to the invention exhibit rates of
corrosion of less than 1.2mm/year and consequently the highly pure magnesium
alloys are at least comparable.
The microstructure of the new secondary alloy is determined by a very small
grain
size and a change in the beta phase Mg17A112. In this case, the beta phase
forms a
network structure according to Figure 1, which slows down the corrosion attack
caused by the local element generators copper, nickel, cobalt and iron. The
microstructure of the pure alloy AZ91, on the other hand, does not exhibit the
network structure of the beta phase, figure 2. The new alloy is consequently
tolerant
vis-à-vis high contents of copper, nickel, cobalt and iron.
In Figure 3 and Table 2, an alloy is also given which shows how a contaminated
alloy
with only a slightly raised nickel and iron content is corroded when the
composition
does not correspond to a secondary alloy according to the invention. This
alloy is
referred to as "comparative alloy".
Table 4: Rates of corrosion of the magnesium alloys with strontium, calcium
and rare
earths, in mm/year.
Magnesium secondary alloy Salt spray test mm/year
With strontium 0.67
With calcium 0.25
With rare earths 0.33
Table 5: Composition of the alloys indicated in Table 4, in percent by mass
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Magnesium Al Zn Mn Cu Ni Fe Si
secondary alloy
With 0.0034% 10.97 3.28 0.61 0.47
0.0037 0.0043 0.12
strontium
With 0.30% 9.84 2.38 0.40 0.27 0.0025 0.0014 0.10
calcium
With rare earths*) 10.47 3.00 0.60 0.47 0.0025 0.0047 0.15
*) 0.16% Ce, 0.13% La, 0.028% Pr, 0.039 Nd
The magnesium secondary alloys according to the invention are excellently
suitable
for use as structural parts in the cast alloy field and the semi-solid casting
process,
e.g. New Rheo Casting. They are also suitable for use as corrosion protection
anodes in the fresh water field.
The invention improves the state of the art regarding the following aspects:
- secondary alloys for structural parts can be introduced on the market which
are
more cost effective than the highly pure magnesium alloys and which have the
same
rate of corrosion.
- the new alloys permit the cost and energy effective recycling of old scrap
metal with
the aim of producing new structural parts.
- more impure precursor materials containing copper and nickel can be used for
the
new alloys. Consequently, refining steps can be omitted during the production
of
magnesium oxide and magnesium chloride.
- downcycling such as the use of shredder material in the aluminium sector or
as
desulphurising agent in the steel industry is avoided.
The invention will be explained in further detail by way of practical
examples.
Example 1
A magnesium alloy with the composition 11.7% Al, 3.04% Zn, 0.48% Mn, 0.47% Cu,
0.0032% Ni, 0.0087% Fe and 0.39% Si in the alloy is smelted. The production of
the
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alloy took place at 760 C. The melt was cast into a copper chill mould
preheated to
200 C. In the same way, a highly pure reference alloy with the composition of
8.65%
Al, 0.67% Zn, 0.20% Mn, 0.0081% Cu, 0.00061% Ni, 0.0022% Fe and 0.054% Si
was produced. The reference alloy corresponds to a highly pure magnesium alloy
AZ
91 with a particularly high resistance to corrosion.
From the alloy rod, discs with a diameter of 25 mm and a thickness of 4 mm
were cut
and subjected to the corrosion test. The corrosion test was carried out by
immersing
the discs into a 3.5% aqueous NaCl solution at a constant pH. According to the
reaction
Mg + 2H20 = Mg(OH)2 + H2
one mol of hydrogen is formed per atom of magnesium. As a result, the rate of
corrosion can be determined from the volume of hydrogen formed. The corrosion
behaviour of the new secondary alloy is compared in Figure 3 with the
reference
alloy as well as a comparative alloy contaminated with Ni and Fe and having
the
composition 8.17% Al, 2.84% Zn, 0.21% Mn, 0.0085% Cu, 0.0026% Ni, 0.023% Fe
and 0.18% Si. Figure 3 shows the hydrogen development in 3.5% NaCI solution at
a
constant pH = 6. After a varying initial hydrogen development in Figure 3, the
corrosion behaviour is characterised by the linear regions in Figure 3. The
rates of
corrosion calculated therefrom are indicated in Table 2. In Table 2, the rates
of
corrosion determined from the salt spray test according to DIN 50021 are also
recorded.
From Table 2 it can be seen that the new secondary alloy has the same rates of
corrosion as the highly pure alloy.
Example 2
A magnesium secondary alloy with the composition of 9.84% aluminium, 2.38%
zinc,
0.40% manganese, 0.27% copper, 0.0025% nickel, 0.0014% iron, 0.10% silicon and
0.30% calcium, the remainder being magnesium, was smelted. The rate of
corrosion,
determined by the salt spray test according to DIN 50021, amounted to
0.25mm/year.
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The rate of corrosion is consequently considerably below that of the highly
pure alloy
AZ 91 of 1.07mm/year.
Example 3
5
A magnesium secondary alloy with the composition of 10.97% aluminium, 3.28%
zinc, 0.61% manganese, 0.47% copper, 0.0037% nickel, 0.0043% iron, 0.12%
silicon
and 0.0034% strontium, the remainder being magnesium, was smelted. The rate of
corrosion, determined by the salt spray test according to DIN 50021 amounted
to
10 0.67mm/year. The rate of corrosion is consequently considerably below
that of the
highly pure alloy AZ 91 of 1.07mm/year.
Example 4
A magnesium secondary alloy was smelted with an addition of cerium mixed metal
with the composition of 10.47% aluminium, 3.00% zinc, 0.60% manganese, 0.47%
copper, 0.0025% nickel, 0.0047% iron, 0.15% silicon and 0.16% cerium, 0.13%
lanthanum, 0.028% praseodymium and 0.039% neodymium, the remainder being
magnesium. The rate of corrosion, determined by the salt spray test according
to DIN
50021, amounted to 0.33mm/year. The rate of corrosion is consequently
considerably below that of the highly pure alloy AZ 91 of 1.07mm/year.
