Note: Descriptions are shown in the official language in which they were submitted.
~19~
Oxidation resistant magnesium alloY
The invention generally relates to magnesium alloys
that contain beryllium and are sufficiently resistant to
oxidation in the molten condition to obviate the need for
the use of protective flux covers to prevent excessive
melt oxidation or burning when exposed to oxygen-containing
atmospheres. Beryllium functions to reduce the propensity
of molten magnesium alloys to oxidize when exposed to
oxygen-containing atmospheres such as air.
The elimination of the need to employ a protective
flux cover for molten magnesium alloys is advantageous
from at least several respects. First of all, the
elimination of flux covers results in a significant cost
reduction. In addition, the absence of flux covers means
that flux particles cannot become mixed into the molten
magnesium metal and then become trapped in the resultant
casting in the form of flux inclusions. The absence of
flux covers also results in increased magnesium yields
because entrapment and subsequent loss of molten magnesium
in the flux covering are eliminated.
It is known in the art to add beryllium to magnesium
base alloys for various purposes. United States Patents
Numbers 2,380,200; 2,380,201; 2,383,281; 2,461,229 and
3,947,268 as well as an article by F. L. Burkett entitled
"Beryllium in Magnesium Die Casting Alloys" which appeared
. .
.
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9~
--- 2
in AFS Transactions, Volum~ 62, pages 2-4 (]954) disclose
the addition o~ beryllium to magnesium base alloys. Of
the above cited information, United States Patents Numbers
2,380,200 and 2,380,201, and the surkett article teach
that beryllium reduces the propensity for molten magnesium
alloys to oxidize. These prior efforts to reduce oxidation
do not involve beryllium additives at the levels of the
invention and do not appear to involve the imposition of
a restriction of manganese content to permit increased
beryllium solubility in the magnesium alloy. Moreover,
the Burkett article suggests that higher beryllium levels
must be avoided.
According to one aspect of the invention there is
provided a magnesium alloy characterized by having good
resistance to oxidation in the molten state, good corro-
sion resistance and good tensile strength, said alloy
consisting essentially of up to 12 percent by weight of
aluminum, up to 1.5 percent by weight of zinc, up to 1.5
percent by weight of silicon, not more than 0.15 percent
by weight of manganese, and a given amount of dissolved
beryllium, the given amount constituting from 0.0025
percent to 0.0125 percent by weight of the alloy, balance
essentially magnesium, and wherein the manganese content
of the alloy is sufficiently low that it does not prevent
dissolution of the given amount of beryllium.
According to another aspect of the invention there
is provided a method of producing a magnesium alloy die
casting, comprising the steps of: a. providing a molten
pool of a magnesium alloy characterized by having good
resistance to oxidation in the molten state, good cor-
rosion resistance and good tensile strength, said alloy
consisting essentially of up to 12 percent by weight of
aluminum, up to 1.5~ by weight percent of zinc, up to 1.5
percent by weight of silicon, not more than 0.15 percent
by weight of manganese, and a given amount of dissolved
beryllium, the given amount constituting from 0.0025
)8
-- 2~ -
percent to n.Ol25 percent by weight of the alloy, balance
essentially magnesium, and wherein the manganese content
of the alloy is suf~iciently low that it does not prevent
dissolution of the given amount of beryllium; b. protecting
said molten pool by exposing it to a nitrogen-containing
atmosphere; and c. die casting said molten magnesium alloy
to form a die casting characterized by being essentially
free of flux inclusions.
It is preferred to restrict the manganese content to a
maximum of about 0.05% when the beryllium content ranges
between about 0.012% and 0.015% to increase the solubility
of beryllium in molten magnesium to an extent sufficient
to enable the above mentioned amount of beryllium to be
dissolved in the magnesium. For exampler about 0.15%
manganese will permit the dissolution of from about 0.007%
beryllium in molten magnesiu~.
It is preferred to maintain manganese from about 0.04%
to 0.15% and beryllium from about 0.005% to 0.0125% in the
magnesium alloys of the invention to enhance corrosion
resistance of the alloy. It is further preferred to
restrict manganese from about 0.08% to 0.15% and beryllium
from about 0.006% to 0.01% to further enhance corrosion
resistance of the magnesium alloys.
The principles of the invention are readily adaptable
. ~ ,,
0~
for use in the production of magnesium alloy die casting.
Magneslum die casting alloys typically contain from 2% to
12% aluminum, up to 1.5% zinc, up to 1.5% silicon, from
0.2% to 1.0% manganese, balance essentially magnesium.
The manganese content of the alloys of the invention
is important because of its influence upon the solubility
and ease of alloying of beryllium in molten magnesium.
Because this influence was not heretofore recognized,
AZ9lB, a widely used die casting alloy having a nominal
composition of 9% aluminum, 0.7~ zinc, 0.2% manganese,
0.5% silicon maximum, 0.3% copper maximum, 0.03% nickel
maximum, balance essentially magnesium has contained less
than 0.001% beryllium. It has been discovered that ber-
yllium is soluble in AZ91B magnesium alloys to an extent
greater than previously believed. In any event, a beryl-
lium level of on the order of 0.001% is considered to be
inadequate for purposes of achieving good protection of
the molten magnesium. Rather it has been determined that
about 0.0025~ to about 0.015% beryllium should be dissolved
in molten magnesium or its alloys to inhibit burning, with
the amount of beryllium being increased with increasing
oxygen content of the atmosphere. Accordingly, the mang-
anese content should not exceed more than about 0.18%,
preferably no more than about 0.15%. When nitrogen atmos-
pheres and short exposure times are involved, additions of
from about 0.0025% to 0.005% beryllium are sufficient to
provide protection of molten magnesium. However, when
longer exposure times or significant air leakage into the
nitrogen atmosphere occurs, beryllium contents on the order
of from about 0.005% to 0.01% are recommended. On the
other hand, should it be desired to inhibit the burning of
molten magnesium or magnesium alloys held in air, a beryl-
lium content of about 0.012% to 0.015% is preferred. Such
beryllium contents require manganese to be restricted to
no more than about 0.05%.
19~8
-- 4
The beryllium level used depends upon the
a~ount o~ oxygen in the atmosphere over the melt. For
exam~le, if the molten magnesium is exposed to air with-
out a cover, the oxygen content of the atmosphere willre~ain at about 20~, and, accordingly, high beryllium
levels, on the order of 0.01% to 0.015%, will be needed
to avoid excessive oxidation or burning. Should the
molten magnesium be ex~osed for prolonged periods, it
may be desirable to periodically add beryl'ium to ccm-
pensate for beryllium that is oxidized or to add larger
amounts of beryllium; e.g., 0.02% in order that the
excess above the solubility limit will gradually dis-
solve to compensate for oxidation losses and thereby
maintain the beryllium at or close to the saturation
level in the molten magnesium.
T~ reduce the berylliu~ level reouired for
good melt protection it is desirable to J;eep the oxygen
level as low as is practical. Placement of a lid or
hood over the molten magnesium is helpful in this regard.
Reaction of the molten metal with oxy~en in the enclosed
air will lower the o~ygen content of the atmosphere. If
the system is very tight and the resultant oxygen content
becomes very low, beryllium levels as low as 0.0025~
will provide adequate protection. If the system is not
tight or is periodically opened for brief periods for
operations such as ladling, it may be desirable to intro-
duce sufficient nitrogen or other inert gases to main-
tain the low oxygen contents. In such situations an
intermediate beryllium level, e.g., 0.005% to 0.01~, may
be used. Other protective gases such as SF2, ~2~ and
various inert gases may also be used, although nitrogen
is preferred due to its relative availability.
Impurities such as iron tend to for~ insoluble
intermetallic compounds with beryllium and accordingly
should be minimized. Because manganese, when in the
presence of aluminum contents on the order of 1~ to 12%,
f orms a relatively insoluble phase with iron which then
settles to the bottom of the melt, small quantities of
~0 manganese such as 0.1% may be included in die casting
X
19~8
-- 5 --
alloys for puriflcatlon purposes. However, the manga-
nese level should not be hiqh enough to precipitate
beryllium. ~ypically, man~anese contents should be
decreased from 0.18~ to O.Q5% as the beryllium level
increases from 0.0025~ to 0.015~ in magnesium alloys
containing about 9% al~minum.
The following experimental results il]ustrate
certain o~ the principles of the invention.
A magnesium test alloy containing about 9%
aluminum, about 0.7~ zinc, and about 0.0025% berylli~n
was held under a hood for 8 hours without burning or
excessive oxidation.
A 130 lb. batch of an alloy containing 7.1%
aluminum, 0.71~ zinc, 0.05~ manganese, balance magnesi~l
was melted, covered with a flux and held under a hood at
1250 F. Followirg removal of the flux by skimming,
burning of the molten alloy occurred after 1 minute.
The burning was then extinguished with the establish-
ment of a flux cover. The hood was closed and nitrogen
was flooded over the surface of the flux-covered molten
bath at a rate of 30 cfh for about 5 minutes. The hood
was closed, the flux cover removed, and nitrogen flow
was continued at a rate of 30 cfh. After 30 minutes,
blooms (localized areas of high oxidation) began to
form and increase in size. After 51 minutes the blooms
began to burn slowl~ and emit a bright light. The hood
door was then hriefly opened periodically to permit
ladling and casting of test bars. Burning became more
vigorous after 5 minutes of casting and very intense
after 15 minutes.
Additional tests were conducted by adding
various amounts of beryllium to the molten ma~nesium
test alloy descri~ed in the preceeding paragraph. In
general, the tests indicated that beryllium additions
decrease the tendency of the molten alloy to burn. I~hen
on the order of 0.008% beryllium was incorporated, the
alloy was held satisfactorily under a 30 cfh nitrogen
flow and then die cast into test bars. This alloy was
~0 also held in air without burning for approximately 15
.
-- 6
minu~es. As the beryllium content was increased during
the various tests, it was noted that the oxidation re
sistance of the molten ma~nesiu~ alloy increased and
tha~ lessened rates of nitrogen flo~ were re~uired for
satisfactory operation. I^~hen about 0.01].% to 0.013~
berylli~m was incorporated into the molten alloy, the
surface of the alloy became silvery in apnearance and
was satisfa~torily held under exposure to air and then
die cast. I~hen the silvery protective surface fil~ wa
deliberately disrupted, a new film formed instantly,
indicatinq that the protective function of berylli~m was
still operative. Following exposure to air for about
1 hour, however, oxide blooms began to form and grow
slowlv.
t~hen 0.00~5~ beryllium was alloyed into the
magnesium test alloy, the melt was satisfactorily held
under a nitrogen flow of 30 cfh with door closed and
then was cast into test bars. Pollowing 15 minutes, the
molten maqnesium alloy was heavily bloomed and commencing
to burn. ~?hen 0.007~ to 0.01~ beryllium was alloyed,
the castin~ run was successfully completed without the
occurrence of bloo~ins with 60 cfh nitrogen. ~he door
of the hood was then held open for 15 minutes without
bloon fo mation. ~itrogen flow was then stopped and the
molten alloy was held for an additional 15 minutes with-
out bloom formation. After the alloy was saturated with
ab~ut 120-130 ppm berylliu~ at 1200 - 1300 F, it was
held in air with the door o~en for over 30 minutes with-
out bloom formation and was then successfully cast ~:ith-
out a nitro~en atmosphere. ~xtended holding, however,
finally led to bloo~ for~ation.
To determine the compatibility of manganese
and beryllium in magnesium alloys, two AZ9lB ingots con-
taining about 0.2~ manganese were added to the melt.
.his addition reduced the beryllium content to about0.008~ an~ increased the manganese content to 0.12~.
~he molten alloy was successfully die cast with a flow
of 6Q cfh nitrogen and the hooA door opened only as re-
~0 quired. ~ portion of the melt was poured in air into a
;~
.
large ingot mold. ~o discoloration h~as noted on themetal surface as it slowly solidified.
Ano~her AZ9lB ingot was added to the molten
alloy with a resultant lowering of the beryllium con-
tent to about 0.007% and an increase in the man~anese
level to about 0.1S%. Test bars were again cast under
60 cfh of nitrogen. Several blooms had formed at the
end of the run.
The variations in manganese and beryllium
level had no apparent effect upon the castability of
the magnesium test alloy. Some improvement in fluidity
and surface appearance appears to result from increas-
ing beryllium content because of less oxidation of the
molten material.
Five die cast bars of each alloy were tested
in tension to determine the effect of beryllium and man-
ganese. ~he results set forth in ~able I indicate that
lower manganese and higher beryllium function to
increase both ductility and tensile strength of the
magnesium test alloy.
Sanded test bars of each alloy were also
immersed in salt water (3% NaCl) for 3 days to de-
termine corrosion resistance. The bars were sanded
to remove the cast surface. The results in Table II
indicate that beryllium additions reduce the salt water
corrosion rate of the magnesium test alloy to the same
low level obtained by manganese additions. Small
amounts of manganese, e.g., 0.12%, reduce the amount
of beryllium re~uired for good corrosion resistance.
The improvement effected by beryllium can be attributed
to a reduction in iron content.
:'........ .
t~08
Ta~le I
6 Be % ~ ~ E T~S TS
0 0.05 6 21,50036,300
0.00~5 0.05 7 22,90038,900
0.0086 0.05 6 22,70036,800
~0 0.0113 0.04 7 21,00038,200
0.0125 0.04 5 22,00037,800
0.0081 0.12 6 22,70039,000
O.OQ71 0.15 8 21,90040,500
0.0006** 0.2 4 21,70034,600
Pounds per 5quare Inch
(AZ9lB)
~y .
191~8
Table I I
% Be % Mn % Fe Corrosion Rate-IPY*
-- 0.05 0.015 1.30
0.0025 0.05 O.OlS 0.95
0.0086 0.05 0.008 0.17
0.0113 0.04 0.005 0.03
0 . 0125 0 . 04 0 . 005 0 . 03
0 . 00~1 0 . 12 0 . 006 0 . 03
0 . 0071 0 . 15 0 . 007 0 . 03
0 . 0006** 0 . 2 0 . 003 0 . 03
* i nches per year
* * (A Z 9 lB )
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~UPPLEMENTARY DI~CLOSUR~
The principal. discl.osure describes and claims a mag-
nesium alloy characterized by having good resistance to
oxidation in the mo]ten state, good corrosion resistance
and good tensile strength, said alloy consisting essen-
tially of up to 12 percent by weight of aluminum, up to
1.5 percent by weight of zinc, up to 1.5 percent by weight
of silicon, not more than 0.15 percent by weight of man-
ganese, and a given amount of dissolved beryllium, the
given amount constituting from 0.0025 percent to 0.0125
percent by weight of the alloy, balance essentially mag-
nesium, and wherein the manganese content of the alloy is
sufficiently low that it does not prevent dissolution of
the given amount of beryllium.
It has now been determined that a magnesium alloy of
the same characteristics can be obtained which has a zinc
content up to 30~. Thus, the present invention in its
expanded form provided a magnesium alloy characterized by
having good resistance to oxidation in the molten state,
good corrosion resistance and good tensile strength, said
alloy consisting essentially of up to 12 percent by weight
of aluminum, up to 30 percent by weight of zinc, up to 1.5
percent by weight of silicon, not more than 0.15 percent
by weight of manganese, and a given amount of dissolved
beryllium, the given amount constituting from 0.0025
percent to 0.0125 percent by weight of the alloy, balance
essentially magnesium, and wherein the manganese content
of the alloy is sufficiently low that it does not prevent
dissolution of the given amount of beryllium.
In another form the invention provides a method of
producing a magnesium alloy die casting, comprising the
steps of: a. providing a molten pool of a magnesium alloy
characterized by having good resistance to oxidation in
the molten state, good corrosion resistance and good
tensile strength, said alloy consisting essentially of up
to 12 percent by weight of aluminum, up to 3O% by weight
- IOa -
percent of zinc, Up to ~.5 percent by weight o~ ~ilicon,
not more than O.l5 percent by weight of manganese, and a
given amount of dissolved beryllium, the given amount
constituting from 0.0025 percent to 0.0125 percent by
weight of the alloy, balance essentially magnesium, and
wherein the manganese content of the alloy is sufficiently
low that it does not prevent dissolution of the given
amount of beryllium; b. protecting said molten pool by
exposing it to a nitrogen-containing atmosphere; and c.
die casting said molten magnesium alloy to form a die
casting characterized by being essentially free of flux
inclusions.
All other features of the invention as described
in the principal disclosure also relate to those alloys
containing between 1.5~ and 30% zinc.
