U.S. patent application number 15/102019 was filed with the patent office on 2016-10-20 for high performance creep resistant magnesium alloys.
The applicant listed for this patent is DEAD SEA MAGNESIUM LTD.. Invention is credited to Boris Bronfin, Vladimir Kotlovsky, Nir Moscovitch, Nir Nagar.
Application Number | 20160304996 15/102019 |
Document ID | / |
Family ID | 50690349 |
Filed Date | 2016-10-20 |
United States Patent
Application |
20160304996 |
Kind Code |
A1 |
Bronfin; Boris ; et
al. |
October 20, 2016 |
HIGH PERFORMANCE CREEP RESISTANT MAGNESIUM ALLOYS
Abstract
The invention provides a magnesium based alloy consisting of at
least 94.8% magnesium, 2.5-4.6% neodymium, 0.05-0.40% yttrium, and
0.03-0.65% zirconium, exhibiting good castability, high strength,
high corrosion resistance and high creep resistance even at high
temperatures. The alloy is suitable for high pressure die casting,
sand casting, investment casting, permanent mold casting, twin roll
casting, or direct chill casting.
Inventors: |
Bronfin; Boris; (Beer Sheva,
IL) ; Nagar; Nir; (Beer Sheva, IL) ;
Kotlovsky; Vladimir; (Beer Sheva, IL) ; Moscovitch;
Nir; (Beer Sheva, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DEAD SEA MAGNESIUM LTD. |
Beer Sheva |
|
IL |
|
|
Family ID: |
50690349 |
Appl. No.: |
15/102019 |
Filed: |
July 17, 2014 |
PCT Filed: |
July 17, 2014 |
PCT NO: |
PCT/IL2014/050649 |
371 Date: |
June 6, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 23/06 20130101;
B22D 21/007 20130101; C22F 1/06 20130101 |
International
Class: |
C22C 23/06 20060101
C22C023/06; B22D 21/00 20060101 B22D021/00; C22F 1/06 20060101
C22F001/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 23, 2014 |
IL |
230631 |
Claims
1. A lightweight alloy for high-temperature applications,
consisting of i) at least 94.8 wt % magnesium, ii) 2.5 to 4.6 wt %
neodymium, iii) 0.05 to 0.40 wt % yttrium, iv) 0.03 to 0.65 wt. %
zirconium, and v) incidental impurities.
2. An alloy according to claim 1, further containing up to 0.02 wt
% calcium.
3. An alloy according to claim 1, comprising essentially no heavy
rare earth (HRE) elements with the atomic number from 61 to 71.
4. An alloy according to claim 1, comprising essentially no cerium,
lanthanum, and praseodymium.
5. An alloy according to claim 1, comprising essentially no
zinc.
6. An alloy according to claim 1, wherein the total content of Nd
and Y is higher than 4.3 wt %.
7. A lightweight alloy according to claim 1, for prolonged
operation at a temperature of up to 200.degree. C.
8. An alloy according to claim 1, usable for a process selected
from the group consisting of high pressure die casting (HPDC), sand
casting, investment casting, and permanent mold casting.
9. An alloy according to claim 1, usable for a process comprising
either twin roll casting with subsequent rolling, or direct chill
casting with subsequent forging, extrusion or rolling.
10. A lightweight alloy for high-temperature applications according
to claim 8 usable for HPDC.
11. An alloy according to claim 10, which contains 2.8 to 4.3 wt %
Nd, 0.06 to 0.25 wt % Y, 0.05 to 0.4 wt % Zr, and 0.0 to 0.02 wt %
Ca.
12. An alloy according to claim 10, exhibiting a castability of at
least 96%, when measured in relative units characterizing oxidation
resistance, fluidity, and dies sticking.
13. An alloy according to claim 10, exhibiting a tensile yield
strength (TYS) at 200.degree. C. of at least 153 MPa.
14. An alloy according to claim 10, exhibiting a compression yield
strength (CYS) at 200.degree. C. of at least 152 MPa.
15. An alloy according to claim 10, exhibiting a minimum creep rate
(MCR) of not more than 1.5.times.10.sup.-10/s at 200.degree. C.
under stress of 100 MPa.
16. An alloy according to claim 10, exhibiting a corrosion rate
under GM 9540 of not more than 2.7 mpy.
17. An article cast of an alloy according to claim 10, exhibiting a
superior combination of strength and ductility after T5 treatment
which includes direct aging at 150-250.degree. C. for 1-10 h.
18. An article according to claim 17, exhibiting a superior
combination of strength and ductility after T5 treatment which
includes direct aging at 175-225.degree. C. for 1-6 h.
19. An alloy according to claim 8 suitable for sand casting,
investment casting, permanent mold casting, and low pressure
modifications thereof, containing 2.7 to 3.4 wt % Nd, 0.15 to 0.40
wt % Y, 0.3 to 0.6 wt % Zr, and 0.0 to 0.02 wt % Ca.
20. An article cast of an alloy according to claim 19, exhibiting a
superior combination of performance properties after full T6 heat
treatment comprising solid solution heat treatment at
520-560.degree. C. for 1 to 16 hrs, followed by cooling in a
quenching medium and by subsequent aging at 200-270.degree. C. for
1 to 16 h.
21. An article according to claim 20, exhibiting a superior
combination of performance properties after full T6 heat treatment
comprising solid solution heat treatment at 535-545.degree. C. for
3 to 5 hrs, followed by cooling in a quenching medium and by
subsequent aging at 225-250.degree. C. for 3 to 6 h.
22. An alloy according to claim 9, suitable for forging, extrusion,
and rolling, which contains 2.8 to 3.8 wt % Nd, 0.20 to 0.40 wt %
Y, 0.35 to 0.60 wt % Zr, and 0.0-0.02 wt % Ca.
23. An article cast of an alloy according to claim 22, exhibiting a
superior combination of performance properties after T5 heat
treatment comprising aging at 200-250.degree. C. for 1 to 16 h.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to creep-resistant
magnesium-based alloys for applications at high temperatures which
exhibit good castability, particularly suitable for high-pressure
die casting, but advantageously used also in a processes comprising
sand casting, investment casting, permanent mold casting, as well
as direct chill casting or twin-roll casting.
BACKGROUND OF THE INVENTION
[0002] The magnesium industry is experiencing dramatic growth, in
part due to the demands of the transportation industry to improve
fuel economy and emissions. In addition, a great progress in weight
reduction has been made in consumer applications of magnesium
alloys such as power hand tools, lawn and garden equipment,
electronic and optical equipment, etc. In order to significantly
expand the above applications, new advanced alloys are
required.
[0003] High pressure die casting (HPDC) is the dominant form of
casting due to its productivity and suitability for mass
production. Currently, most common and new magnesium alloys that
are being used for HPDC process are Al containing alloys. However,
these alloys cannot serve at temperatures higher than
150-170.degree. C. under high stresses of 60-100 MPa. U.S. Pat. No.
6,193,817 describes magnesium-based alloys containing 0.1-2.0 wt %
Zn, 2.1-5.0 wt % RE elements (Ce based mischmetal) up to 0.4 wt %
of a combination of at least two elements chosen from the group
consisting of Zr, Hf and Ti, and optionally up to 0.5 wt % Mn and
up to 0.5 wt % Ca. High-pressure die casting of the alloys results
in low strength (TYS=120 MPa, UTS=165 MPa) and elongation (E=2%).
EP 1866452 discloses magnesium based alloys containing 1.5-4.0% RE
elements, 0.3-0.8% Zn, 0.02-0.1% Al, 4-25 ppm Be and optionally up
to 0.2% Zr, 0.3% Mn, 0.5% Y, and 0.1% Ca. The alloys, under die
cast conditions, exhibit low tensile strength (TYS=130 MPa, UTS=160
MPa) and elongation (E=1-3%).
[0004] WO 2009/086585 relates to magnesium based alloys containing
2-5% RE elements (primarily La and Ce, wherein La content is higher
than Ce content) and 0.2-0.8% Zn. In addition, the alloys contain
optionally Y, Gd, Zr, Mn, Ca, and Be. These alloys are also
designated for high-pressure die casting but exhibit very low
values of elongation, TYS, and UTS.
[0005] SU 1,360,223 discloses Mg-based alloys containing 0.1-2.5%
Zn, 0.3-1.0% Zr, 0.8-4.5% Nd, 0.5-5.0% Y, 0.8-4.5% Gd, and
0.01-0.05% Mn. The alloys are intended for sand casting process and
exhibit optimal properties after full T6 treatment.
[0006] U.S. Pat. No. 4,116,731 describes heat treated and aged
magnesium based alloys containing 0.8-6.0 wt % Y, 0.5-4.0 wt % Nd,
0.1-2.2 wt % Zn, 0.3-1.1 wt % Zr, up to 0.05% Cu, and up to 0.2%
Mn. Due to relatively wide concentration ranges claimed by the
above patent, the alloys exhibit very diverse properties; in
addition, they are designated only for sand casting process
[0007] EP 1329530 discloses magnesium-based casting alloys
containing 0.2-0.8 wt % Zn, 0.2-0.8 wt % Zr, 2.7-3.3 wt % Nd,
0.0-2.6 wt % Y, and 0.03-0.25% Ca. The alloys exhibit high strength
and high creep resistance after gravity casting and after full T6
heat treatment, as well as after extrusion and forging. However,
the HPDC is not addressed.
[0008] CN 1752251 describes magnesium alloys containing 0.35-0.8 wt
% Zr, 2.5-3.6 wt % Nd, 0.0-0.4 wt % Zn, 0.0-0.5 wt % Ca, and
0.0-0.02 wt % impurities. The alloys are prepared by a two-stage
process including a step of preparing intermediate master alloys
Mg--Nd, Mg--Ca, and Mg--Zr, and a step of alloying said master
alloys by Nd, Ca, and Zr. The complexity of the technology
significantly increases the cost of the final alloy product.
[0009] EP 1641954 describes creep resistant alloys containing
2.0-4.5% Nd, 0.2-1.0% Zr, 0.2%-7.0% HRE (Heavy Rare Earth Elements
of atomic numbers 62-71), optionally up to 0.4% of other RE
elements, up to 0.5% Y, up to 1.3% Zn, up to 0.5% Mn, and up to
0.4% Hf or Ti. The alloys are mainly designated for sand casting
and, in addition, they are expensive due to the use of heavy rare
earth elements, such as Gd in amounts of 1.0-1.6%.
[0010] US 2009/0081313 relates to biodegradable magnesium alloys
containing 1.5-5.0% Nd, 0.1-4.0% Y, 0.1-2.0% Ca, and 0.1-1.0% Zr.
The alloys are designated for manufacturing medical implants by
extrusion. The high Ca content results in increased porosity,
embrittlement and hot cracking in HPDC processes.
[0011] WO 2010/038016 relates to magnesium alloys containing
2.0-4.0% Y, 0.5-4.0% Nd, 0.05-1.0% Zr, 0.0-5.5% Gd, 0.0-5.5% Dy,
0-5.5% Er, 0.0-0.2% Yb, and 0.0-0.04% Sm. In addition, the total
content of Gd, Dy and Er is in the range of 0.3-12 wt. %. The alloy
is dedicated for sand casting, and it can also be used as a wrought
alloy. The alloy is unsuitable for HPDC process. Furthermore, the
high content of heavy rare earth elements leads to high cost of
these alloys.
[0012] WO 2011/117628 describes magnesium alloys containing
0.0-10.0% Y, 0.0-5.0% Nd, 0.00-1.2% Zr, 0.0-0.3% Gd, and 0.0-0.2%
Sm, wherein the total content of Ho, Lu, Tm and Tb is in the range
of 0.5-5.5%. The alloy is dedicated for manufacturing medical
implants. Due to very wide concentration ranges of Y, Nd, and heavy
rare earth elements Ho, Lu and Tm, the alloys exhibit very diverse
properties. The alloys are not suitable for HPDC process and are
expensive.
[0013] It is therefore an object of this invention to provide
magnesium alloys suitable for high pressure die casting (HPDC)
applications.
[0014] It a further object of this invention to provide
magnesium-based alloys allowing crack-free castings at HPDC
applications.
[0015] It is also an object of this invention to provide
magnesium-based alloys having a superior combination of strength
and ductility, as well as capability to operate at a temperature of
200.degree. C. for a long time.
[0016] It is another object of the present invention to provide
alloys which are also suitable for sand casting, investment
casting, and permanent mold casting, and which exhibit excellent
combination of castability, creep performance, and corrosion
resistance.
[0017] It is a still further object of this invention to provide
alloys which are also suitable for direct chill casting and twin
roll casting with subsequent plastic forming operations such as
rolling, forging and extrusion.
[0018] It is still another object of this invention to provide
alloys which exhibit the aforesaid behavior and properties, and
have an affordable cost.
[0019] Other objects and advantages of the present invention will
appear as the description proceeds.
SUMMARY OF THE INVENTION
[0020] The invention provides a lightweight alloy for high-pressure
die casting (HPDC) process, consisting of at least 94.8 wt %
magnesium, 2.5 to 4.6 wt % neodymium, 0.05 to 0.40 wt % yttrium,
0.03 to 0.65 wt. % zirconium, and incidental impurities. In one
embodiment, the alloy according to the invention further contains
up to 0.02 wt % calcium. The alloy according to the invention
comprises essentially no heavy rare earth (HRE) elements with the
atomic number from 61 to 70. The alloy according to the invention
comprises essentially no cerium, lanthanum, and praseodymium. The
alloy according to the invention comprising essentially no zinc. In
one embodiment, the alloy according to the invention contains Nd
and Y in an amount higher than 4.3 wt. %. Said incidental
impurities usually comprise Si, Fe, Cu, and Ni in an amount of up
to 0.02 wt %. The lightweight alloy according to the invention is
suitable for prolonged operations at high temperatures of up to
200.degree. C. The alloys for HPDC and other applications according
to the invention exhibit superior casting properties, high
strength, high creep resistance, high corrosion resistance, and the
articles manufactured from the alloys show superior performance at
high temperatures. The alloy according to the invention is usable
for high pressure dies casting (HPDC), but it may be also used for
a process selected from the group consisting of sand casting,
investment casting, and permanent mold casting. The alloy according
to the invention are also usable for a process comprising either
twin roll casting with subsequent rolling, or direct chill casting
with subsequent forging, extrusion or rolling.
[0021] In a preferred embodiment of the invention, the lightweight
alloy is advantageously used for HPDC. In one embodiment, the alloy
suitable for HPDC contains 2.8 to 4.3 wt % Nd, 0.06 to 0.25 wt % Y,
0.05 to 0.4 wt % Zr, and 0.0 to 0.02 wt % Ca. In a preferred
embodiment of the invention, the alloy used for HPDC exhibits a
tensile yield strength (TYS) at 200.degree. C. of at least 153 MPa,
a compression yield strength (CYS) at 200.degree. C. of at least
152 MPa, a minimum creep rate of not more than
1.5.times.10.sup.-10/s at 200.degree. C. under stress of 100 MPa,
and a corrosion rate of not more than 2.65 mpy. When measured in
relative units characterizing oxidation resistance, fluidity, and
dies sticking, the alloy according to the invention preferably
exhibits a castability of at least 96%.
[0022] The invention relates to an article cast of the alloy
containing 2.8 to 4.6 wt % Nd, 0.06 to 0.25 wt % Y, 0.05 to 0.4 wt
% Zr, and 0.0 to 0.02 wt % Ca, the article exhibiting a superior
combination of strength and ductility after T5 treatment which
includes direct aging at 150-250.degree. C. for 1-10 h. In one
embodiment, said article exhibits a superior combination of
strength and ductility after T5 treatment which includes direct
aging at 175-225.degree. C. for 1-6 h.
[0023] The alloy according to the invention is also suitable for
sand casting, investment casting, permanent mold casting, and low
pressure modifications thereof; in one embodiment, the alloy
contains 2.7 to 3.4 wt % Nd, 0.15 to 0.40 wt % Y, 0.3 to 0.6 wt %
Zr, and 0.0 to 0.02 wt % Ca. The invention relates to an article
cast of said alloy, the article exhibiting a superior combination
of performance properties after full T6 heat treatment comprising
solid solution heat treatment at 520-560.degree. C. for 1 to 16
hrs, followed by cooling in a quenching medium and by subsequent
aging at 200-270.degree. C. for 1 to 16 h. In one embodiment, said
article exhibits a superior combination of performance properties
after full T6 heat treatment comprising solid solution heat
treatment at 535-545.degree. C. for 3 to 5 hrs, followed by cooling
in a quenching medium and by subsequent aging at 225-250.degree. C.
for 3 to 6 h.
[0024] The alloy according to the invention may be advantageously
used for forging, extrusion, and rolling; in one embodiment, the
alloy contains 2.8 to 3.8 wt % Nd, 0.20 to 0.40 wt % Y, 0.35 to
0.60 wt % Zr, and 0.0-0.02 wt % Ca. The invention relates to an
article cast in said alloy, the article exhibiting a superior
combination of performance properties after T5 heat treatment
comprising aging at 200-250.degree. C. for 1 to 16 h.
[0025] The present invention provides creep-resistant
magnesium-based alloys designated for applications at temperatures
as high as 200-250.degree. C., which exhibit good castability and
low susceptibility to hot tearing, which are strong and are
corrosion resistant, and have excellent ductility.
[0026] The invention provides a process for manufacturing a
lightweight alloy for prolonged operation at high temperatures of
up to 200.degree. C., comprising steps of i) alloying magnesium
with neodymium and zirconium at 765-785.degree. C. under intensive
stirring; ii) settling the melt for 20-40 minutes to allow iron to
settle; iii) adding yttrium, while avoiding intensive stirring to
prevent the formation of Y--Fe intermetallic compounds; iv)
optionally adding calcium prior to settling; v) settling the molten
alloy for 30-60 minutes; and v) casting into desired form; wherein
the steps are performed under a protective atmosphere of
CO.sub.2+0.5% HFC134a till solidification; the amount of magnesium
in the alloy being at least 94.8 wt %, of neodymium from 2.5 to 4.6
wt %, of yttrium from 0.05 to 0.40 wt %, of zirconium from 0.03 to
0.65 wt. %, and of calcium 0.00 to 0.02%. The lightweight alloys
thus manufactured are particularly suitable for high pressure die
casting, but can be advantageously employed in sand casting,
investment casting, and permanent mold casting.
[0027] The alloys according to the invention contain more than 94
wt % magnesium, from 2.5 to 4.6 wt % neodymium, from 0.05 to 0.40
wt % yttrium, from 0.03 to 0.65 wt % zirconium, optionally calcium
up to 0.02 wt %, and incidental impurities. The alloys usually
contain up to 0.007 wt % iron, up to 0.001 wt % nickel, up to 0.003
wt % copper, up to 0.015 wt % silicon, and eventually other
incidental impurities. The alloys of the invention exhibit an
excellent combination of high tensile and compressive yield
strength, and high ductility. The great advantage of new alloys is
related to their high creep rupture stress, creep strength and low
minimum creep rate, as well as low corrosion rate measured under GM
9540 cyclic corrosion test. Thus, the alloys of the present
invention combine superior performance properties, good
castability, and relatively moderate cost. Articles according to
the invention are preferably subjected to T5 or T6 heat treatments
depending on preceding casting process and plastic forming
operations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The above and other characteristics and advantages of the
invention will be more readily apparent through the following
examples, and with reference to the appended drawings, wherein:
[0029] FIG. 1. is Table 1, showing chemical compositions of alloys
for HPDC according to the invention (Examples 1-7) and comparative
alloys (Comparative Examples 1-7);
[0030] FIG. 2. is Table 2, showing the results of die castability
evaluation for alloys of Table 1;
[0031] FIG. 3. is Table 3, showing mechanical and corrosion
properties of the alloys of Table 1;
[0032] FIG. 4. is Table 4, showing creep properties of the alloys
of Table 1;
[0033] FIG. 5. is Table 5, showing chemical compositions of alloys
for sand casting according to the invention (Examples 8-14) and
comparative alloys (Comparative Examples 8-14);
[0034] FIG. 6. is Table 6, showing mechanical and corrosion
properties of the alloys of Table 5;
[0035] FIG. 7. is Table 7, showing chemical compositions of alloys
according to the invention after forging (Examples 15-18) and
comparative alloys (Comparative Examples 15-18);
[0036] FIG. 8. is Table 8, showing mechanical properties of the
alloys of Table 7; and
[0037] FIG. 9. is a scheme showing GM 9540 cycling test procedure
for the corrosion evaluation.
DETAILED DESCRIPTION OF THE INVENTION
[0038] It was found that certain combinations of elements in
magnesium based alloys comprising neodymium, zirconium, yttrium,
and optionally calcium, impart to the alloys superior properties,
particularly for high pressure die casting. These properties
include excellent combination of high tensile and compressive
properties with high ductility, outstanding corrosion resistance,
and creep properties allowing to achieve service temperatures up to
250.degree. C.
[0039] The above combination of properties can be realized at
high-pressure die casting, at sand casting, as well as at
direct-chill casting or twin roll casting followed by plastic
forming processes such as forging, extrusion and rolling.
[0040] Magnesium-based alloys of the instant invention contain 2.5
to 4.6 wt % neodymium. It was found by the inventors that if the Nd
content is less than 2.5 wt %, the alloys have insufficient
strength at ambient and elevated, and their creep resistance is not
sufficient for serving at 250-300.degree. C. temperatures; Nd
content higher than 4.6 wt % results in low ductility due to
excessive amount of intermetallic compounds which are sources of
cracks initiation and propagation. An alloy according to the
present invention contains 0.05 to 0.40 wt % yttrium. It was found
that yttrium content less than 0.05 wt % makes the alloy prone to
oxidation and results in increased susceptibility to burning during
molten metal handling at 700-780.degree. C. On the other hand,
increasing the yttrium content to more than 0.40 wt % leads to
lower ductility, significantly deteriorated castability, while
increasing the alloy cost. Zirconium is mainly used to remove iron
in the case of high pressure die casting process. In the case of
gravity casting (sand casting, investment casting, and permanent
mold casting) it also serves as a grain refiner. It has been found
that 0.03 wt. % of Zr is sufficient to ensure low iron content in
the alloy, while at least 0.3 wt % of zirconium is required for
grain refining. The upper limit for the zirconium content is about
0.65 wt % due to its limited solubility in Mg--Nd--Y alloys.
[0041] The alloys of the present invention contain substantially no
zinc; it would deteriorate creep resistance and corrosion
performance due to the formation of Zn--Y--Nd--Zr coarse
intermetallics. Furthermore, the alloys of the instant invention do
not contain rare earth elements with low solubility in solid
magnesium such as Ce and La. The presence of those elements results
in deterioration of mechanical properties, and particularly of
ductility, due to the formation of coarse intermetallic compounds.
An admixture of calcium in the alloys of the invention of up to
0.02% may improve oxidation resistance.
[0042] The Ca content is limited to 0.02% because higher Ca content
leads to increased micro-porosity and embrittlement of the
alloys.
[0043] The alloys of instant invention also do not contain heavy
rare earth elements with atomic number higher than 60, they would
increase the alloy price without remarkably improving the alloy
performance.
[0044] Surprisingly simple alloys of the invention are suitable for
HPDC and other applications, while exhibiting superior casting
properties, high strength, high creep resistance, high corrosion
resistance, and the articles manufactured from the alloys show
superior performance at high temperatures.
[0045] The magnesium alloys according to the invention were
examined along with comparative alloys. The results show that the
new alloys exhibit better oxidation resistance and fluidity, as
well as lower susceptibility to die sticking than comparative
alloys. Neither burning nor oxidation was observed on the surface
of ingots made of alloys according to this invention. In contrast,
the preparation of comparative alloys was sometimes accompanied by
significant oxidation and undesirable losses of alloying elements.
The alloys according to the invention reached between 96 and 100%
on the relative castability scale, when evaluating oxidation
resistance, fluidity, and susceptibility to die sticking (see
Examples below), in comparison with 73 and 83% of comparative
alloys whose composition differed more or less from the composition
of the invention.
[0046] Part of the ingots of both the new and the comparative
alloys were then remelted and high pressure die cast to produce
different specimens for testing and examination. Other ingots were
remelted, grain refined and sand cast into different specimens for
testing. Tensile Yield Strength (TYS), Ultimate Tensile Strength
(UTS), percent elongation (% E), compressive strength (CYS) and
different creep characteristics such as Creep Strength, Creep
Rupture Strength, and Minimum Creep Rate were then determined.
Corrosion behavior was evaluated by the GM 9540 cyclic test. The
alloys according to the invention surpassed the comparative alloys
in creep stress to rupture, creep strength, and corrosion
resistance. They also exhibit better combination of strength and
ductility, characterized by elongation values, than comparative
alloys.
[0047] The alloys according to the invention are very suitable for
HPDC; it was found that they develop excellent properties after
direct T5 aging at 150-250.degree. C. for 1-10 h, preferably at
175-225.degree. C. for 1-6 h. As for wrought alloys, it was found
that the alloys according to the invention achieve very good
properties after direct aging at 200-250.degree. C. for 1-16 h. It
was found that the alloys according to the invention also provide
excellent mechanical properties on sand casting after full T6 heat
treatment; particularly, good results were obtained when the heat
treatment comprised solid solution heat treatment at
520-560.degree. C. for 1 to 16 hrs, followed by cooling in a
quenching media and by subsequent aging at 200-270.degree. C. for 1
to 16 h, preferably after solid solution treatment at
535-545.degree. C. for 3 to 5 hrs, followed by cooling in a
quenching media and by subsequent aging at 225-250.degree. C. for 3
to 6 h.
[0048] The invention will be further described and illustrated in
the following examples.
EXAMPLES
[0049] The alloys of the present invention were prepared in 150 l
crucible made of low carbon steel. The mixture of CO.sub.2+0.5%
HFC134a was used as a protective atmosphere. The raw materials used
were as follows:
[0050] Magnesium (Mg)--pure magnesium, grade 9980A, containing at
least 99.8% Mg.
[0051] Neodymium (Nd)--commercially pure Nd (less than 0.5%
impurities).
[0052] Zirconium (Zr)--Zr95 Tablets, containing at least 95%
Zr.
[0053] Yttrium (Y)--commercially pure Y (less than 1%
impurities).
[0054] Calcium (Ca)--pure Ca (less than 0.1% impurities).
[0055] Contrary to the alloying procedure described in CN1752251,
where intermediate Mg--Nd, Mg--Ca and Mg-- Zr master alloys were
used, the alloys of the present invention have been prepared using
pure Nd and pure Ca that significantly simplifies the process,
reduces its duration and markedly lowers the alloy cost. Neodymium
and zirconium were added typically at 770-780.degree. C. with
intensive stirring of the melt. After addition of zirconium, the
melt was held for 20-40 minutes to allow iron to settle. Yttrium
was added after the iron settling, without intensive stirring, to
prevent the formation of Y--Fe intermetallic compounds, which leads
to excessive loss of yttrium. A strict temperature control was
provided during the alloying in order to insure that the melt
temperature will not increase above 785.degree. C., thus preventing
an excessive contamination by iron from the crucible walls, and to
ensure that the temperature will not decrease below 765.degree. C.,
thus preventing an excessive loss of zirconium. Calcium was added
prior to settling. After obtaining the required compositions, the
alloys were held for 30-60 minutes for homogenization, and settling
of iron and non-metallic inclusions, and then they were cast into
the 15 kg ingots. The casting was performed with gas protection of
the molten metal during solidification in the molds by
CO.sub.2+0.5% HFC134a mixture. The die casting trials were carried
out using an IDRA OL-320 cold chamber die casting machine with a
345 ton locking force.
[0056] The castability was evaluated based on observed fluidity,
oxidation resistance and die sticking or soldering. The casting
temperature was 710.degree. C. Each of the properties (fluidity,
oxidation resistance, die sticking) was evaluated by assigning from
0 to 10 points on a relative scale, the higher the better (see
Table 2). The sum of the points for an alloy divided by 30 and
multiplied by 100 provides "castability coefficient", a relative
assessment having a value between 0 and 100%, which characterizes
the overall suitability of an alloy for die casting. The alloys
according to the invention had castability coefficient between 96
and 100%, while comparative examples, even if differing only
slightly from the new alloys of the invention, had castability
coefficient between 73 and 83%.
[0057] Tensile and compression testing at ambient and elevated
temperatures were performed using an Instron 4483 machine. Tensile
Yield Strength (TYS), Ultimate Tensile Strength (UTS), percent
elongation (% E) and Compression Yield Strength (CYS) were
determined. The SATEC Model M-3 machine was used for creep testing.
Creep tests were performed at 200.degree. C. and 250.degree. C. for
200 h or until rupture under various stresses. Creep resistance was
estimated by measuring rupture strength and creep strength. Creep
strength is usually defined as the stress, which is required to
produce a certain amount of creep at a specific time and
temperature. It is a common practice to report creep strength as
the stress, which produces 0.2% creep strain at a given temperature
for 100 hours. This parameter is used by design engineers for
evaluating the load-carrying ability of a material for limited
creep strain in prolonged time periods. Creep rupture stress is the
stress resulting in specimens rupture at a selected testing
temperature for a certain time, usually 100 h. In addition, minimum
creep rate at a steady state (MCR) was used to evaluate creep
performance.
[0058] Corrosion behavior was evaluated as per GM9540 cyclic test
for 40 days (FIG. 9). The test procedure includes three main
stages, combining both wet-dry transition and short sprays of light
electrolyte solution. In this test a gradual increase of
temperature is applied during the cycle. The die cast plates with
dimensions of 140.times.100.times.3 mm were used. The plates were
degreased in acetone and weighed prior to the test. Five replicates
of each alloy were tested. At the end of the test the corrosion
products were stripped in a chromic acid solution (180 g CrO.sub.3
per liter solution) at 80.degree. C. about three minutes and the
weight loss was determined. The weight loss was used to determine
the average corrosion rate in mpy (milli-inch per year).
[0059] Tables 1 to 4 illustrate chemical compositions, castability
parameters, and properties of alloys for HPDC according to the
invention and of comparative alloys. The new alloys of the
invention demonstrate significantly better die castability
evaluated by tendency to oxidation, fluidity and susceptibility to
die sticking (Table 2), reflected by a castability coefficient of
minimally 96%. As can be seen from Table 3, the new alloys are
superior in tensile yield strength (TYS) and compressive yield
strength(CYS) over the comparative alloys at both ambient and
elevated temperatures. The same is true for UTS values. For
example, TYS values of the new alloys according to the invention at
200.degree. C. are 150 MPa or more, usually 153 MPa or more,
whereas the comparative alloys have lower values. Furthermore, new
alloys exhibit much better combination of strength and elongation
than comparative alloys. Corrosion resistance of the new alloys
determined under GM 9540 cyclic test conditions (FIG. 9) also
surpasses that property of the comparative alloys; the corrosion
rate of the new alloys is less than 2.9 mpy, usually less than 2.7
mpy, such as 2.65 mpy or less (Tab. 3). In addition, new alloys
also exhibit excellent creep resistance in the temperature range
200-250.degree. C., outperforming the comparative examples (Table
4). The creep rupture strength of the new alloys for HPDC is
typically about 200 MPa or more at 200.degree. C., and about 105
MPa or more at 250.degree. C. The MCR values of the new alloys are
1.5.times.10.sup.-10/s or less at 200.degree. C. and 100 MPa,
usually 1.0.times.10.sup.-10/s or less; the comparative alloys have
higher values even if differing only slightly in composition from
the new alloys (Tab. 4).
[0060] The excellent combination of these properties along with low
susceptibility to hot tearing makes the alloys of the instant
invention the most promising candidates for high pressure die
casting of moving parts serving at high temperatures of
200-250.degree. C. where low moment inertia and correspondingly low
vibration are required.
[0061] Tables 5-6 demonstrate chemical compositions and properties
of alloys for sand casting according to the invention and of
comparative alloys subjected to full T6 heat treatment. The alloys
of the instant invention exhibit superior combination of TYS and
Elongation in comparison with comparative examples. The compressive
strength of new alloys is also higher both at ambient and elevated
temperatures. Furthermore a great advantage of the alloys of this
invention is that they combine excellent mechanical properties with
outstanding corrosion resistance which outperforms corrosion
resistance of comparative alloys.
[0062] Tables 7-8 illustrate chemical composition and mechanical
properties of forged alloys of instant invention. The alloys of
present inventions and comparative alloys were direct chilled cast,
homogenized, forged and T5 heat treated. The forged alloys of the
present invention exhibit higher TYS and UTS values than
comparative alloys at both ambient temperature and 200.degree. C.
It is important that they the alloys according to the invention
have also superior elongation and significantly higher compressive
yield strength.
[0063] While this invention has been described in terms of some
specific examples, many modifications and variations are possible.
It is therefore understood that, within the scope of the appended
claims, the invention may be realized otherwise than as
specifically described.
* * * * *