CA1066923A

CA1066923A - Magnesium alloys

Info

Publication number: CA1066923A
Application number: CA242,645A
Authority: CA
Inventors: William Unsworth; John F. King; Stephen L. Bradshaw
Original assignee: Magnesium Elektron Ltd
Current assignee: Magnesium Elektron Ltd
Priority date: 1974-12-30
Filing date: 1975-12-29
Publication date: 1979-11-27
Also published as: FR2296697B1; DE2558545C2; NL7515191A; JPS594497B2; DE2558519A1; IL48762A0; IT1052035B; CH611650A5; NO142582B; CH608833A5; NO754345L; JPS5192708A; SE7514711L; SE421635B; NO142582C; SE7514709L; AU8781575A; DE2558545A1; BE837170A; SE448241B

Abstract

ABSTRACT
Magnesium alloys having favourable tensile properties contain silver, copper and neodymium. The alloys are subjected to a solution heat treatment followed by ageing to give optimum properties.

Description

` ` ~0669Z3 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 .

-2-.

.. .

`^` `~ 10669Z3 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 . ', '. ~ .
, . , .
, ;"~ - . .

1~.
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.
_4_ .
- -:
, ~ 6~9Z3 i - 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 ~0669Z3 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 -~
'~
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 .
: :' . - :

. .~ ~ , .

~ -` lQ669~3 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.

~` ' '' .
:. .
.

_7_ ;

.; , :

- ~ ,: , : ,: ~ , . . : . , ' :' o~ ~. _ ___ '',''''' ~ f C ,l~e ~1 _1 _1 ,_~ _1 ~1 . - .

.'' ~ C~t . ' ~ , ~1 ~1 _~ ~ C~t . ~ ~ u~ E ~O ~- u~ u~ ~
H P Z _ ~1 ~1 _ _ _ . i h ~c~3 ~ ~ 1~ O ~ co ~ -:

1 ~ ~2; 1_~ ~ ~_1 _~ _t . . .
.''. ' _ ~ _ .:''.
~ :r ~ ~ c~t ~ ~ .' . :.
:' ~3 _ _ .` ":.
.. C~t . . - . . ~ .
U~ l oo t' t' t' ~t O
~1 fn E t' ~D ~ ~ ~o ~o H ~ ;Z N C~t t~t e~ C`~ C~t . h AC`1 . . .
. ~ t t~ a~ t~ u~ O u~
h ~ ~i H O r- c~ a~ ~
/ fX H~ E ~ ,/ ~ __ __ ..', ~ _ -o- oO 00 .

3 ~ ~ s o s :, ~ ;~; ~ ~
`.; ~ hH I` + C`~oco~ : :
, E!~l P Ir~ ~ Ir~u~
.; j,,,, ~ .C ~: ~
a, o ~ ~D ~ CO CO
_ U~ C~ .1 __ ~ ~ ~ ,Oo O l _ O

., ~ ~ ~ r~ U~ CO
. ~` f'LI U\ U~' U~ U~ U\ U~
'., ?il O O O O O O
~. ~ _ _ H ~ N cr~ 1~ U~ CO H
N ~i H H H ri . ~ ~ . -- N
.", . ~ ~ I~ ~O, O ~ ~O ~D
. ~. _ _ ~ _ C\i----'` .
_.. _.. _.. .
. ,, ~ .

.. .

: . . ,: . : ,- ~ - : : , ~ `~

The results of porosity tests are shown in Table 2 below:

. .
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.
:

' ' . . .: , ~ ~ ~ . ' ' .
.,' ' ' ' ' ' " . , ~ ~

` 10669Z3 _ .
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 _ ~ :
. __ _ _ ,: :
2.6 1.9 0.59 0.093 96 3.95 ;~

, ~ .

-10- .

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OF PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cast magnesium base alloy which when heat treated has a .2% proof stress of at least 175 N/mm2 and an ultimate tensile strength of at least 240 N/mm2 at ambient temperature consisting of a magnesium base alloy of the following compositions other than iron and other impurities:
Magnesium at least 88%
Silver from 1 to 2% by weight Copper from 0.05 to 0.15% by weight Rare Earth Metals of which at least 60% by weight are Neodymium from 0.5 to 3.0% by weight Zirconium nil to 1% by weight Manganese nil to 2% by weight Zinc nil to 0.5% by weight Cadmium nil to 1.0% by weight Lithium nil to 6.0% by weight Calcium nil to 0.8% by weight Gallium nil to 2.0% by weight Indium nil to 2.0% by weight Thallium nil to 5.0% by weight Lead nil to 1.0% by weight Bismuth nil to 1.0% by weight the maximum quantites of zirconium and manganese being limited by the quantity of the other, the balance being magnesium.

2. An alloy according to Claim 1, containing from 1.0 to 1.75% by weight of silver.

3. An alloy according to claim 1, which contains at least 0.5% by weight of zirconium.

4. An alloy according to Claims 1, 2 or 3, in which the neodymium is added as a mixture of rare earth metals containing at least 60% by weight of neodymium and not more than 25% by weight of lanthanum and cerium taken together.

5. An alloy according to Claims 1, 2 or 3, containing up to 0.1% copper.

6. An alloy according to Claims 1, 2 or 3, containing up to .1% copper in which the neodymium is added as a mixture of rare earth metals containing at least 60% by weight of neodymium and not more than 25% by weight of lanthanum and cerium taken together.

7. An alloy according to claim 1 containing less than 1.5% by weight silver.

8. An alloy according to claims 1, 2 or 3, containing more than .10% by weight of copper.

9. A method of making a magnesium base alloy product comprising holding the alloy according to claim 1, at a temperature from 485° C to the solidus of the alloy for at least 2 hours and then quenching and aging the product at a temperature from 100°C to 275°C for at least half an hour.

10. A method of making a magnesium base alloy product which comprises holding an alloy according to claim 8 at a temperature from 400°C to 485°C for at least one hour follow-ed by holding at a temperature from 485°C to the solidus of the alloy for at least 2 hours then quenching and aging the product at a temperature from 100° to 275°C for at least half an hour.

11. A method of making a magnesium base alloy product which comprises holding an alloy according to Claim 5 at a temperature from 485°C to the solidus of the alloy for at least 2 hours and then quenching and aging the product at a temperature from 100°C to 275°C for at least half an hour.

12. A method of making a magnesium base alloy product comprising holding the alloy according to Claim 7, at a temperature from 485°C to the solidus of the alloy for at least 2 hours and then quenching and aging the product at a temperature from 100°C to 275°C for at least half an hour.

13. A method of making magnesium base alloy product according to claims 1, 2 or 3 including the steps of controlling the copper content in the product to achieve a copper content from .05% to .15% by weight, solution heat treating the product at a temperature from 485°C to the solidus of the alloy for at least 2 hours and then quenching and aging the product at a temperature from 100°C to 275°C
for at least half an hour.