Note: Descriptions are shown in the official language in which they were submitted.
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Thi~ invention relates to magnesium ba~e alloyY.
Ma$nesium alloys have a very low weight in
comparison with alloy~ of other metals and accordingly
find application~, particularly in the aerospace indu~try,
where a low weight is important. Exi~ting magnesium
alloys having advantageous mechanical properties~ in
particular a high proof stres~, are described in British
Patent Specification 875~929.
These alloy~ depend largely for their mechanical
properties o~ the presence of a considerable proportion of
silver~ which is typically present in an amount from 2 to
3% by weight. This makes the alloy very expensive. - -
Moreover the marXet price of silver is liable to fluctuate
violently for reasonY associated with its u~e as a currency
and as the cost of the silver presents a major part of the
cost of the alloy the latter also fluctuates.
In these alloys the mechanical properties improve
with an increasing content of silver. It ha~ now been
- found that part of the silver can be replaced with copper
without signlficant loss of properties.
According to one aspect of the invention there i~
provided a masnesium bAse alloy of the following
composition (other than iron and other impurities):
Magne~ium at least 88%
Silver from 1 to 3% by weight
Copper from 0.05 to 0.15% by weight
Rare earth metal~ qf
which at lea~t 60% i~
neodymium from 0.5 to 3-% by weight
Zirconium nil to 1% by weight
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Manganese nil to 2% by weight
Zinc nil to 0.5% by weight
Cadmium nil to 1.0,/o by weight
Lithium nil to 6.o% by weight
Calcium nil to o.8% by weight
-~ Gallium nil to 2.0% by weight
Indium nil to 2.0% by weight
Thallium nil to 5.0% by wei~ht
Lead nil to 1.0% by weight
~- 10 Bismuth nil to 1.0% by weight
the maximum quantities of zirconium and manganese being
limited by the quantity of the other.
In a preferred embodiment of the invention the
content of silver i9 from 1 to 2%, advantageously 1 to
4,
L75% by weight.
Neodymium, being a rare earth metal, is a material
in the pure state but it may conveniently be added in the
form of a mixture of rare earth metals. The mixture
preferably contains at least 60~o by weight of neodymium
and not more than 25% by weight of lanthanum and cerium
together. Such mixtures are currently available
commercially. It should be noted that yttrium is not n
~rare earth metal~.
Zirconium may be present in the alloy in an amount
of up to 1% by weight for grain refining purposes. It i~ ;
desirable to incorporate at least 0.4% zirconium by weight
to obtain satisfactory castings. It is possible to !i
replace part of the zirconium with manganese, but the
content of manganese is limited by its mutual solubility
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with zirconium.
Other elements soluble in magne~ium may be present
provided that they do not, by forming compounds, interfere
with the beneficial effects of the other alloy constituents.
Thus, zinc, cadmium, lithium, calcium, gallium, indium, ~s
- thallium, lead and bismuth may be present in the abo~e- mentioned proportions.
~leat treatment is required in order to develop the
optimum mechanical properties for the alloys of the
invention. This treatment normally comprises solution
heat treatment at an elevated temperature followed by
quenching and ageing at a lower temperature. The higher
temperature solution treatment is designed to give the
maximum practical solubility of the alloying elements such
as silver, neodymium and copper; the rapid quench maintains
these elements in solution and the ageing allows the
` required degree of precipitation hardening to occur. It
has been found that a temperature of at least 520 C is
required for the higher temperature solution treatment;
the upper llmit on the solution treatment temperature ia
the solidus of the alloy. A high temperature treatment
time of at least 2 hours is generally required.
Ageing may be carried out at A temperature from
100C to 275C for a period of at least -~ an hour~ longer
; times being required for lower temperatures in this range.
Typical heat-treatment conditions are holding for 8 hours
at 5ao - s2sc for solution treatment, quenching and then
holding for 16-hours at 200 C for precipitation
treatment.
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- The above-mentioned treatrnent conditions are
suitable for alloys containing up to 0.1% by weight copper.
I~hen the copper content exceedct this amount a copper-rich
eutectic may be forrned having a lower melting point and
melting of this phase during the solution treatment can ?
cause cracking during subsequent quenching. In order to
prevent incipient melting of this copper-rich phase the
solution treatment can be initially carried out at a lower
temperature, advantageously fro~ 400 to 485 C, followed by
solution treat~uent at 485 C or abo~e. The initial lower
temperatures solution treatment may be carried out for
at least one hour. Typical treatment conditions for an
alloy containing 0.1 - 0.15% copper are 16 hours at 470C
followed by ô hours at 520C, quenching and precipitation
treatment for 16 hours at 200C.
Particular alloys according to the invention will
be described by way of illustration in the following
Examples.
EXAMPLES
- Alloys having the composition given below were
prepared by melting magnesium under a conventional flux,
raising its temperature to 800C~ adding all alloy
materials~ puddling tho melt and casting the melt into
specimens of quitable shape and size at 780C. The
specimens were heat treated as shown below.
The mechanical properties of the alloy specimens
were measured at ambient temperatures i~ accordance with
; British Standard 18 and at elevated temperatures in
accordance with British Standard 3688. In tests at 200
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or 250C a soak time of 15 minute~ or 1 hour wa4 used.
Corrosion re~istance of the ~ample~ wa~ te~ted by
the ~oyal Aircraft Establishment ~eawater spray te~t in j
which sample~ are expo~ed but sheltered from precipitation ~ -
and sprayed 3 times per working day with natural seawater
over a period of 2 month~. The weight losse~ were
determined and the average corro~ion rate calculated.
The ca~tability of the alloys was measured by -~
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ca~ting plate~ 18 mm thick with and without chilling along
the extreme edge, machining the plate~ on both faces and
.
radiographing the plate~
The results of the room temperature mechanical tests
are ~hown in Figure 1, which iY a graph of ultimate ten~ile
~tre~ and 0.2% proof streqs, mea~ured at room temperature,
again~t ~ilver content for magnesium alloys containing 2.0
or 2.5% of neodymium and o. 6% of zirconium. The point~
marked ~ith different symbol~ reIate to alloys containing
different amo-mts of copper; the pointR indicated by open
~ ~quare~ relate to "control~ alloy-~ containing no copper and
are given for compari~on.
It will be seen that in alloy~ colltnining above
2.0% of ~il~er the pre~enco of copper has a marginal effect
on the mechanical propertie~. Ilowever with a silver range
from 1.0 to 2.0% the addition of copper hac a considerable
effect ~uch that both ultimate and 0.2% proof ~tre~ for
alloys containing from 1.5% to 1.75% silver are
~ub~tantially the same as for alloy~ containing up to 3%
~ilver. The de~irable minimum 0.2% proof stre~s for
alloy~ of thi~ type i~ 175 N/mm and it can be ~een from
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Figure 1 that~ whereas an alloy containing 1% of silver
and no copper has a value welL helow this Figure, addition
of copper gives values above this figure. The copper-
containing alloys al~o give ultimate tensile stress above J
240 N/mm which is the desirable minimum for these alloys.
The effect of copper addition on mechanical
properties at a high temperature (250 C) is sho~m in Table
l together with room temperature results. It is seen
that at both high and low temperatures the addition of
copper to low-silver alloys gives properties as good as or
even better than those of the high-silver alloys.
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The results of porosity tests are shown in Table 2
below:
TABLE 2
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RADIOGRAP~IC ANALYSIS OF POROSITY ;
ANALYSIS % PLATES
~3
AS RE Zr Cu Unchilled Chilled
2.7 1.9 o-55 _ Porous for 4" rating 7 Very slight
general 3
rating O ~3l
2.6 1.9 -59 0.09 NONE NONE
1.69 1.84 o-55 _ rating 5 NONE
1.62 1.71 o.58 0.07 Porous for 2~t' NONE
_ _ rating 3
It can be seen from these results that the addition
of 0.1% Cu gives a marked improvement in unchilled porosity
and some improv~ement in chilled porosity. The porosities
are rated on an arbitrary scale~ the value increasing with
increasing porosity.
The results of corrosion tests are shown in Table
3 belo~. They show that the low silver alloy~ containing
copper have a reduced corrosion rate. The invention thus
provide~ alloys hnving mechanical properties as ~ood as
those already known but wlth a lower tendency to corrode.
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TABLE 3
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ANALYSIS % Corrosion Rate Average Corrosion
_ _ (mg/cm2/day) Rate (mg/cm2/day)
Ag RE Zr Cu ~i
~ o5~ ~7 ~
1.04 1.77 0.57 o.o8 2.75 2.83 ~1~
_ . _ 2.91 _ ~ :
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2.6 1.9 0.59 0.093 96 3.95 ;~
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