TECHNICAL FIELD
[0001] The present invention relates to an aluminum (Al) alloy, and more particularly, to
an Al alloy having improved oxidation resistance, corrosion resistance, or fatigue
resistance, and a die-cast material and an extruded material prepared by using the
alloy.
BACKGROUND ART
[0002] Currently, magnesium (Mg) is regarded as one of main alloying elements in an aluminum
(Al) alloy. Addition of Mg allows an Al alloy to have a high strength, to be favorable
to surface treatment, and to have improved corrosion resistance. However, due to Mg
having a chemically high oxidizing potential, an oxide or another inclusion may be
mixed into molten Al during Mg is alloyed into the molten Al and thus the quality
of molten metal may deteriorate. In order to prevent an oxide or another inclusion
from being mixed into molten Al, for example, the surface of molten metal may be covered
with a protective gas such as SF
6 when Mg is added.
[0003] However, due to properties of a process of preparing an Al alloy, it may not be easy
to completely protect Mg, which is added in large amounts, with a protective gas.
Furthermore, since SF
6 used as the protective gas not only is expensive gas but also causes an environmental
problem, the use of SF
6 is now being gradually restricted all over the world.
DETAILED DESCRIPTION OF THE INVENTION
TECHNICAL PROBLEM
[0004] The present invention provides an aluminum (Al) alloy prepared environment friendly
and having improved chemical-mechanical properties such as oxidation resistance, corrosion
resistance, or fatigue resistance, and a die-cast material and an extruded material
prepared by using the Al alloy. The above problem to be solved is provided as an example
and the scope of the present invention is not limited thereto.
TECHNICAL SOLUTION
[0005] According to an aspect of the present invention, there is provided an aluminum (Al)
alloy casted by adding a magnesium (Mg) master alloy, in which a calcium (Ca)-based
compound is distributed in an Mg matrix, into molten Al, wherein an Al matrix includes
the Ca-based compound, and wherein the Al alloy has superior oxidation resistance,
corrosion resistance against salt water, or fatigue resistance to a corresponding
Al alloy not including the Ca-based compound.
[0006] In this case, the Ca-based compound may include at least one of an Mg-Ca compound,
an Al-Ca compound, and an Mg-Al-Ca compound, and the Mg-Ca compound may include Mg
2Ca, the Al-Ca compound may include at least one of Al
2Ca and Al
4Ca, and the Mg-Al-Ca compound may include (Mg,Al)
2Ca.
[0007] Also, the Mg master alloy may be prepared by adding a Ca-based additive into molten
parent material including pure Mg, or an Mg alloy including Al, as a parent material.
[0008] Furthermore, the Ca-based compound may be formed by dispersively adding a Ca-based
additive onto a surface of an upper part of molten Mg, and then exhausting at least
a portion of the Ca-based additive in the molten Mg.
[0009] In this case, the Ca-based compound may be formed by exhausting the Ca-based additive
in the molten Mg in such a way that the Ca-based additive does not substantially remain
in the Mg master alloy. For this, the upper part of the molten Mg may be stirred and
the stirring may be performed at the upper part which is within 20% of a total depth
of the molten Mg.
[0010] Meanwhile, the Ca-based additive may include at least one of calcium oxide (CaO),
calcium cyanide (CaCN
2), and calcium carbide (CaC
2).
[0011] Also, at least a portion of the Ca-based additive may be exhausted in molten parent
material, and the Ca-based compound may be formed due to reaction between Ca supplied
from the Ca-based additive and Mg or Al of the parent material.
[0012] In this case, the Mg master alloy may be added by 0.0001 parts by weight to 30 parts
by weight based on 100 parts by weight of Al, and the Ca-based additive may be added
by 0.0001 parts by weight to 30 parts by weight based on 100 parts by weight of the
parent material.
[0013] Furthermore, Mg may be dissolved in the Al matrix within a range of 0.1wt% to 15wt%.
[0014] If a content of the Ca-based compound is increased, a weight gain of the Ca-based
compound due to oxidation under the same oxidation condition may be reduced.
[0015] The superior fatigue resistance may refer to a larger cycle number leading fatigue
fracture if a cyclic load is applied at a predetermined frequency under stress conditions
of 40% to 80% of a tensile strength.
[0016] According to another aspect of the present invention, there is provided an aluminum
(Al) alloy extruded material prepared by extruding the above-described Al alloy, and
having a higher strength in comparison to an Al alloy extruded material prepared under
the same condition except that the Ca-based compound is not included.
[0017] According to another aspect of the present invention, there is provided an aluminum
(Al) alloy die-cast material prepared by using molten metal of the above-described
Al alloy, and having a higher strength in comparison to an Al alloy die-cast material
prepared under the same condition except that the Ca-based compound is not included.
[0018] According to another aspect of the present invention, there is provided a method
of preparing an aluminum (Al) alloy extruded material, the method including preparing
molten Al including magnesium (Mg); preparing an Al alloy by casting the molten Al;
and extruding the Al alloy, wherein the molten Al is prepared by melting Al together
with an Mg master alloy in which a calcium (Ca)-based compound combined with at least
one of Mg and Al is included in an Mg matrix.
[0019] In this case, the method may further include performing heat treatment on the Al
alloy extruded material after the Al alloy is extruded.
[0020] According to another aspect of the present invention, there is provided a method
of preparing an aluminum (Al) alloy die-cast material, the method including preparing
molten Al including magnesium (Mg); and casting the molten Al; and wherein the molten
Al is prepared by melting Al together with an Mg master alloy in which a calcium (Ca)-based
compound combined with at least one of Mg and Al is included in an Mg matrix.
ADVANTAGEOUS EFFECTS
[0021] If an aluminum (Al) alloy according to the present invention is used, even when a
protective gas conventionally used to prevent oxidation of molten Al, for example,
SF
6, is greatly reduced in amount or is not used, a die-cast product may be stably prepared
and an extruded material having excellent mechanical properties may be prepared by
performing an extruding process.
[0022] Also, in the Al alloy according to an embodiment of the present invention, since
a calcium (Ca)-based compound added when a magnesium (Mg) master alloy is added is
distributed in an Al matrix so as to achieve a distribution strengthening effect and
a grain refinement effect, mechanical properties of the Al alloy, for example, strength
and fatigue resistance, may be remarkably improved. Due to the improvement in casting
properties and/or mechanical properties, oxidation resistance and corrosion resistance
of the Al alloy may be improved.
[0023] The effects of the present invention are not limited to the above-described effects
and other effects not described above may be understood by one of ordinary skill in
the art from the following detailed description of the invention.
DESCRIPTION OF THE DRAWINGS
[0024]
FIG. 1 is a flowchart of a method of preparing a magnesium (Mg) master alloy to be
added into molten aluminum (Al) so as to prepare an Al alloy, according to an embodiment
of the present invention.
FIGS. 2A through 2D are images showing electron probe microanalysis (EPMA) results
of microstructures and components of an Mg master alloy, according to an embodiment
of the present invention.
FIG. 3 is a flowchart of a method of preparing an Al alloy, according to an embodiment
of the present invention.
FIGS. 4A and 4B are images showing the surfaces of a molten Al alloy prepared by adding
an Mg master alloy including calcium oxide (CaO), according to an embodiment of the
present invention, and a molten Al alloy prepared by adding pure Mg.
FIGS. 5A and 5B are images showing the surfaces of a cast material of an Al alloy
prepared by adding an Mg master alloy including CaO, according to an embodiment of
the present invention, and a cast material of an Al alloy prepared by adding pure
Mg.
FIGS. 6A and 6B are images showing analysis results on components of an Al alloy prepared
by adding an Mg master alloy including CaO, according to an embodiment of the present
invention, and components of an Al alloy prepared by adding pure Mg.
FIG. 7A is an EPMA image of a microstructure of an Al alloy prepared by adding an
Mg master alloy including CaO, according to an embodiment of the present invention,
and FIGS. 7B through 7E are EPMA images showing component mapping results of Al, calcium
(Ca), Mg, and oxygen (O), respectively.
FIGS. 8 through 10 are images comparatively showing microstructures according to Experimental
Examples 2 to 4 and Comparative Examples 2 to 4.
FIGS. 11 through 13 are images comparatively showing microstructures according to
Experimental Examples 5, 6, and 7 and Comparative Examples 5, 6, and 7.
FIG. 14 is a graph showing oxidation resistance of Al alloys based on the content
of CaO added to prepare an Mg master alloy.
FIG. 15 is a graph showing oxidation resistance of Al alloys according to comparative
examples and Al alloys according to embodiments of the present invention, based on
the content of Mg.
FIGS. 16A through 16G are images showing oxidation resistance of Al alloys according
to comparative examples and Al alloys according to embodiments of the present invention,
based on the content of Mg.
FIG. 17 is a graph showing corrosion resistance of an Al alloy according to a comparative
example and an Al alloy according to an embodiment of the present invention.
FIG. 18 is an image showing corrosion properties of an Al alloy according to a comparative
example.
FIG. 19 is an image showing corrosion properties of an Al alloy according to an embodiment
of the present invention.
FIG. 20 is a graph showing mechanical properties of an Al alloy used in a fatigue
test, according to an embodiment of the present invention.
FIG. 21 is a schematic diagram showing decomposition of CaO at an upper part of molten
Mg when CaO is added in to the molten Mg.
BEST MODE
[0025] The present invention will now be described more fully with reference to the accompanying
drawings, in which exemplary embodiments of the invention are shown. The invention
may, however, be embodied in many different forms and should not be construed as being
limited to the embodiments set forth herein; rather, these embodiments are provided
so that this disclosure will be thorough and complete, and will fully convey the concept
of the invention to one of ordinary skill in the art.
[0026] According to an embodiment of the present invention, an aluminum (Al) alloy is prepared
by preparing a master alloy into which a predetermined additive is added, and then
adding the master alloy into Al. Here, any of pure magnesium (Mg) and an Mg alloy
may be used as a parent material of the master alloy, and is denoted as an Mg master
alloy. Pure Mg refers to Mg into which no alloying element is intentionally added,
and is substantially defined to include an impurity that is inevitably added during
Mg is prepared.
[0027] An Mg alloy is an alloy prepared by intentionally adding another alloying element
such as Al into Mg. The Mg alloy including Al as an alloying element may be referred
to as an Mg-Al alloy. In this case, the Mg-Al alloy may further include another alloying
element other than Al.
[0028] FIG. 1 is a flowchart of a method of preparing an Mg master alloy, according to an
embodiment of the present invention. Referring to FIG. 1, the method of preparing
an Mg master alloy includes forming molten Mg (S1), adding an additive (S2), stirring
or holding (S3), and casting (S4).
[0029] In operation S1, pure Mg or an Mg alloy is put into a crucible and is heated to form
the molten Mg. In this case, a heating temperature may be, for example, in a range
of 400°C to 800°C. Although pure Mg may melt at a temperature higher than or equal
to 600°C, since a melting point is lowered due to alloying, the Mg alloy may melt
at a temperature lower than or equal to 600°C and higher than or equal to 400°C. Here,
if the temperature is lower than 400°C, the molten Mg may not be easily formed. If
the temperature is higher than 800°C, the molten Mg may be sublimated or ignited.
[0030] The Mg alloy used in operation S1 may include one selected from the group consisting
of AZ91 D, AM20, AM30, AM50, AM60, AZ31, AS141, AS131, AS121X, AE42, AE44, AX51, AX52,
AJ50X, AJ52X, AJ62X, MRI153, MRI230, AM-HP2, Mg-Al, Mg-Al-Re, Mg-Al-Sn, Mg-Zn-Sn,
Mg-Si, Mg-Zn-Y, and equivalents thereof. However, the Mg alloy is not limited thereto.
Any Mg alloy that is commonly used in the industrial field may be used.
[0031] Meanwhile, a small amount of a protective gas may be optionally provided in order
to prevent the molten Mg from being ignited. The protective gas may typically use
SF
6, SO
2, CO
2, HFC-134a, Novec™612, an inert gas, an equivalent thereof, or a gas mixture thereof,
and may suppress the molten Mg from being ignited.
[0032] Then, in operation S2, a calcium (Ca)-based additive is added to the molten Mg. In
this case, the added Ca-based additive may include at least one of calcium oxide (CaO),
calcium cyanide (CaCN
2), and calcium carbide (CaC
2). The Ca-based additive may improve oxidation resistance in the molten Mg and thus
a protective gas required to melt Mg may be greatly reduced in amount or may not be
used. As such, if an Mg master alloy is prepared according to an embodiment of the
present invention as described above, a problem caused by the use of a protective
gas such as SF
6 that is restricted due to an environmental reason may be solved.
[0033] Also, since oxidation resistance of the molten Mg is improved, ignition resistance
may be increased and thus an oxide or another inclusion may be suppressed from being
mixed into the molten Mg. Accordingly, the cleanliness of molten metal may be significantly
improved and thus mechanical properties of an Mg alloy casted by using the molten
metal may be improved.
[0034] At least a portion of the Ca-based additive may be exhausted in the molten Mg. Under
an appropriate condition, substantially all of the Ca-based additive may be exhausted
in the molten Mg. For example, the Ca-based additive may be reduced in the molten
Mg so as to be decomposed to Ca. For example, as a Ca-based additive, CaO may be decomposed
into Ca and O. In this case, the decomposed oxygen may be discharged from the molten
Mg into the air in the form of a gas or may float on the molten Mg in the form of
dross or sludge.
[0035] Meanwhile, Ca decomposed from CaO may form a compound due to various reactions in
molten metal. The compound may be an intermetallic compound formed due to a chemical
reaction between metals. The reduced Ca may react with another element(s) in a parent
material, e.g., Mg and/or Al, and thus may form a Ca-based compound.
[0036] Accordingly, the Ca-based additive is a source of Ca used to form a Ca-based compound
in the Mg master alloy and is an additive element added into a molten parent material
formed to prepare a master alloy. The Ca-based compound is a compound newly formed
due to a reaction between Ca supplied from the Ca-based additive and another element(s)
in a parent material. Although Ca has solubility with respect to Mg, it is uncovered
that Ca, which is reduced from the Ca-based additive in the molten Mg as in the present
invention, is only partially dissolved in the molten Mg and mostly forms the Ca-based
compound.
[0037] If a parent material of the Mg master alloy is pure Mg, a formable Ca-based compound
may be an Mg-Ca compound, e.g., Mg
2Ca. Also, if a parent material of the Mg master alloy is an Mg alloy, e.g., an Mg-Al
alloy, a formable Ca-based compound may include at least one of an Mg-Ca compound,
an Al-Ca compound, and an Mg-Al-Ca compound. For example, the Mg-Ca compound may be
Mg
2Ca, the Al-Ca compound may include at least one of Al
2Ca and Al
4Ca, and the Mg-Al-Ca compound may be (Mg,Al)
2Ca.
[0038] The decomposition and reaction of the Ca-based additive may be more inactivated due
to stirring, and a detailed description thereof will be provided below in relation
to operation S3.
[0039] The Ca-based additive is favorable to have a large surface area in order to improve
reactivity and thus is favorable to have a form of powder. However, the Ca-based additive
is not limited thereto and may have a form of pellets or masses formed by allowing
powder to agglomerate in order to prevent scattering of powder.
[0040] The Ca-based additive may have a size of 0.1
µm to 500
µm, and more particularly, 0.1
µm to 200
µm. If the Ca-based additive has a size less than 0.1
µm, the size is too small and thus the Ca-based additive may be scattered due to hot
air of sublimated Mg and thus may not be easily put into a crucible. Also, the Ca-based
additive may agglomerate and thus may not be easily mixed with molten metal having
a form of a liquid. Such agglomerates reduce a surface area for reaction and thus
are not preferable. If the Ca-based additive has a size greater than 500
µm, a surface area for reaction may be reduced and, furthermore, the Ca-based additive
may not react with the molten Mg.
[0041] The Ca-based additive may be added by 0.001wt% to 30wt%, and more particularly, by
0.01wt% to 15wt%. If the Ca-based additive is added by less than 0.001wt%, mechanical
properties of the Mg alloy are slightly or hardly improved. Also, if the Ca-based
additive is added by more than 30wt%, intrinsic properties of Mg may not appear.
[0042] The Ca-based additive may be added into the molten Mg all at once or separately with
time intervals. Also, a required amount of the Ca-based additive may be added all
at once or the Ca-based additive may be divided into appropriate amounts and may be
added separately with time intervals. If the Ca-based additive has a form of powder
having fine particles, the Ca-based additive may be added separately with time intervals
so as to reduce the possibility that the Ca-based additive agglomerates and to accelerate
reaction of the Ca-based additive.
[0043] In order to accelerate decomposition and reaction of the Ca-based additive, stirring
may be performed in operation S3. In this case, the stirring may be started at the
same time when the Ca-based additive is added or after the added Ca-based additive
is heated to a certain temperature in molten metal.
[0044] In a typical metal alloying process, molten metal and an alloying element are actively
stirred by using, for example, a convection method, in order to cause a reaction in
the molten metal. However, in the current embodiment, if an active reaction of the
Ca-based additive is induced, the Ca-based additive reacts less efficiently and thus
is greatly remains in the ultimate molten metal in an un-decomposed state. If the
Ca-based additive remains in the ultimate molten metal as described above, the Ca-based
additive may be included in a casted Mg alloy and thus mechanical properties of the
Mg alloy may deteriorate.
[0045] FIG. 21 is a schematic diagram showing decomposition of CaO at an upper part of molten
Mg when CaO is added in to the molten Mg. Referring to FIG. 21, CaO is decomposed
into O and Ca at the upper part of the molten Mg. In this case, the decomposed oxygen
may be discharged outside in the form of a gas (O
2) or may float on the molten Mg in the form of dross or sludge. Meanwhile, the decomposed
calcium may react with another element(s) in molten metal to form various compounds.
[0046] Accordingly, in the current embodiment, rather than mixing the Ca-based additive
into the molten Mg, forming a reaction environment for allowing the Ca-based additive
to react on the surface of the molten metal is more critical. For this, in order to
allow the added Ca-based additive to stay on the surface of the molten metal as long
as possible and to be exposed to the air, the upper part of the molten Mg may be stirred.
[Table1]
|
|
Addition of 5wt% CaO |
Addition of 10wt% CaO |
Addition of 15wt% CaO |
CaO Residues in Alloy |
No Stirring |
4.5wt% CaO |
8.7wt% CaO |
13.5wt% CaO |
|
Stirring of Inside of Molten Metal |
1.2wt% CaO |
3.1wt% CaO |
5.8wt% CaO |
|
Stirring of Upper Part of Molten Metal (Present Invention) |
0.001wt% CaO |
0.002wt% CaO |
0.005wt% CaO |
[0047] Table 1 shows results of measuring CaO residues according to a stirring method when
CaO is added into molten Mg of AM60B. In this case, the added CaO is 70
µm in size, and CaO is added by 5wt%, 10wt%, and 15wt%. As the stirring method, upper
part stirring, internal stirring, and no stirring of the molten Mg are selected. As
shown in Table 1, when the upper part of the molten Mg is stirred, unlike the other
cases, the most part of the added CaO is reduced to Ca.
[0048] The stirring may be performed at the upper part which is within 20%, and desirably,
within 10%, of a total depth of the molten metal from the surface thereof. If the
stirring is performed at a depth greater than or equal to 20%, the decomposition of
the Ca-based additive does not easily occur at the surface of the molten Mg.
[0049] In this case, a stirring time may differ according to the state of an added powder
and the temperature of the molten metal, and it is preferable to stir the molten metal
sufficiently until the added Ca-based additive is completely exhausted in the molten
metal as long as possible. Here, the exhaustion means that decomposition of the Ca-based
additive is substantially completed. Decomposition of the Ca-based additive in the
molten Mg due to the stirring and Ca formed due to the decomposition may further accelerate
a reaction for forming various compounds.
[0050] After operation S3 is completed, operation S4 for solidifying the molten Mg in a
mold is performed to prepare the Mg master alloy. In operation S4, the temperature
of the mold may be in a range of room temperature (for example, 25°C) to 400°C. Also,
a master alloy may be separated from the mold after the mold is cooled to room temperature.
However, if the master alloy is completely solidified, the master alloy may be separated
even before the temperature reaches room temperature.
[0051] Here, the mold may use any one selected from a metallic mold, a ceramic mold, a graphite
mold, and equivalents thereof. Also, the casting method may include sand casting,
die casting, gravity casting, continuous casting, low-pressure casting, squeeze casting,
lost wax casting, thixo casting, or the like.
[0052] Gravity casting may denote a method of pouring a molten alloy into a mold by using
gravity, and low-pressure casting may denote a method of pouring a molten alloy into
a mold by applying a pressure onto the surface of the molten alloy by using a gas.
Thixo casting is a casting method performed in a semisolid state and is a combination
method adopting advantages of typical casting and forging. However, the present invention
is not limited to a mold type and a casting method or process.
[0053] The above prepared Mg master alloy has a matrix having a plurality of domains divided
from each other by boundaries therebetween. In this case, the plurality of domains
divided from each other may typically be a plurality of grains divided by grain boundaries
therebetween and, as an another example, may be a plurality of phase regions defined
by two or more different phase boundaries therebetween.
[0054] Meanwhile, a Ca-based compound formed during the Mg master alloy is prepared may
be dispersed and exist in the matrix of the Mg master alloy. In this case, the Ca-based
compound may be one formed due to reaction between the Ca-based additive added in
operation S2 and another element(s), e.g., Mg and/or Al, in the Mg parent material.
[0055] That is, the Ca-based additive is reduced to Ca during the Ca-based additive is added
into the molten Mg and is stirred or held. In general, since the Ca-based additive
is thermodynamically more stable than Mg, Ca is not expected to be separated from
the Ca-based additive due to reduction in the molten Mg. However, according to experiments
by the present inventors, it is uncovered that the Ca-based additive is reduced in
the molten Mg. The reduced Ca may react with another element(s), e.g., Mg and/or Al,
in the parent material, thereby forming a Ca-based compound.
[0056] Accordingly, the Ca-based additive is a source of Ca used to form a Ca-based compound
in the Mg master alloy and is an additive element added into a molten parent material
formed to prepare a master alloy. The Ca-based compound is a compound newly formed
due to a reaction between Ca supplied from the Ca-based additive and another element(s)
in a parent material.
[0057] Although Ca has solubility with respect to Mg, it is uncovered that Ca, which is
reduced from the Ca-based additive in the molten Mg as in the present invention, is
only partially dissolved in the molten Mg and mostly forms the Ca-based compound.
[0058] In the case, if a parent material of the Mg master alloy is pure Mg, a formable Ca-based
compound may be an Mg-Ca compound, e.g., Mg
2Ca. Also, if a parent material of the Mg master alloy is an Mg alloy, e.g., an Mg-Al
alloy, a formable Ca-based compound may include at least one of an Mg-Ca compound,
an Al-Ca compound, and an Mg-Al-Ca compound. For example, the Mg-Ca compound may be
Mg
2Ca, the Al-Ca compound may include at least one of Al
2Ca and Al
4Ca, and the Mg-Al-Ca compound may be (Mg,Al)
2Ca.
[0059] In this case, it is highly probable that the Ca-based compound is distributed at
grain boundaries, i.e., boundaries between grains, or phase boundaries, i.e., boundaries
between phase regions. This is because such boundaries are further open and have a
relatively high energy in comparison to the inside of the grains or the phase regions,
and thus may provide a favorable site for nucleation and growth of the Ca-based compound.
[0060] FIGS. 2A through 2D are images showing electron probe microanalysis (EPMA) results
of an Mg master alloy prepared by adding CaO as a Ca-based compound into an Mg-Al
alloy, according to an embodiment of the present invention.
[0061] FIG. 2A shows a microstructure of the Mg master alloy observed by using back scattered
electrons. As shown in FIG. 2A, the Mg master alloy includes regions surrounded by
a compound (bright part), that is, a microstructure having a plurality of grains.
In this case, the compound (bright part) is formed along grain boundaries. FIGS. 2B
through 2D are EPMA images showing component mapping results of Al, Ca, and O, respectively,
in a region of the compound (bright part).
[0062] Al and Ca are detected in the compound (bright part in FIG. 2A) as shown in FIGS.
2B and 2C but O is not detected as shown in FIG. 2D. As such, it may be understood
that an Al-Ca compound, which is formed due to reaction between Ca separated from
CaO and Al included in the parent material, is distributed at grain boundaries of
the Mg master alloy. The Al-Ca compound may be Al
2Ca or Al
4Ca, which is an intermetallic compound.
[0063] Meanwhile, the above EPMA results show that an Al-Ca compound is mainly distributed
at grain boundaries of the Mg master alloy because the Ca-based compound is probably
distributed at grain boundaries rather than the inside of grains due to properties
of the grain boundaries having open structures. However, these analysis results do
not mean that all Ca-based compounds are distributed only at the grain boundaries.
In some cases, the Ca-based compound may exist inside grains.
[0064] The above prepared Mg master alloy is used to be added into an Al alloy. As described
above, an Mg master alloy includes a Ca-based compound, which is formed due to reaction
between Ca supplied from the Ca-based additive during an alloying process, and Mg
and/or Al. All Ca-based compounds are intermetallic compounds and have a melting point
higher than the melting point (658°C) of Al. For example, the melting points of Al
2Ca and Al
4Ca as Al-Ca compounds are 1079°C and 700°C, respectively, which are higher than the
melting point of Al.
[0065] Accordingly, if a master alloy including the above-described Ca-based compound is
added into molten Al, the Ca-based compound may mostly remain without being melted
in molten metal. Furthermore, if an Al alloy is prepared by casting the molten metal,
the Ca-based compound may be included in the Al alloy.
[0066] A method of preparing an Al alloy, according to an embodiment of the present invention,
will now be described.
[0067] An Al alloy according to an embodiment of the present invention may be prepared,
in order to form molten metal in which an Mg master alloy and Al are melted, melting
Al to form molten Al and then adding and melting the Mg master alloy including a Ca-based
compound, into the molten Al. As another method, Al and the Mg master alloy may be
put into a melting apparatus such as a crucible, and then may be heated together to
be melted.
[0068] FIG. 3 is a flowchart of a method of preparing an Al alloy, according to an embodiment
of the present invention, and more particularly, a method of preparing an Al alloy
by forming molten Al and then adding and melting an Mg master alloy prepared by using
the above-described method, into the molten Al.
[0069] As illustrated in FIG. 3, the method of preparing an Al alloy includes forming molten
Al (S11), adding an Mg master alloy (S12), stirring or holding (S13), and casting
(S14).
[0070] Initially, in operation S11, molten Al is formed by putting Al into a crucible and
heating Al at a temperature ranging from 600°C to 900°C. In operation S11, Al may
be any one selected from pure Al, an Al alloy, and equivalents thereof. The Al alloy,
for example, may be any one selected from 1000-series, 2000-series, 3000-series, 4000-series,
5000-series, 6000-series, 7000-series, and 8000-series wrought Al, and 100-series,
200-series, 300-series, 400-series, 500-series, and 700-series casting Al.
[0071] Then, in operation S12, the Mg master alloy prepared by using the above-described
method is added into the molten Al. In this case, the Mg master alloy used in operation
S12 may be added by 0.0001 parts by weight to 30 parts by weight based on 100 parts
by weight of Al. If the added Mg master alloy is added by less than 0.0001 parts by
weight, effects (hardness, corrosion resistance, weldability, etc.) achieved by adding
the Mg master alloy may be insufficient. Also, if the Mg master alloy is added by
more than 30 parts by weight, intrinsic properties of Al alloy may not appear.
[0072] In this case, the Mg master alloy may be added in the form of an ingot. However,
the Mg master alloy is not limited thereto and may be added in various forms such
as a form of powder and a form of granules. Also, the size of the Mg master alloy
is not limited.
[0073] When the Mg master alloy is added, a Ca-based compound included in the Mg master
alloy is provided together into the molten Al. As described above, the Ca-based compound
provided into the molten Al may include at least one of an Mg-Ca compound, an Al-Ca
compound, and an Mg-Al-Ca compound.
[0074] In this case, a small amount of a protective gas may be additionally provided in
order to prevent the Mg master alloy from being oxidized. The protective gas may typically
use SF
6, SO
2, CO
2, HFC-134a, Novec™612, an inert gas, an equivalent thereof, or a gas mixture thereof,
and may suppress the molten Mg from being oxidized.
[0075] However, in the present invention, the above protective gas is not essentially needed
and may not be provided. That is, if the Mg master alloy including the Ca-based compound
is added as in the present invention, ignition resistance is increased due to an increase
in oxidation resistance of the Mg master alloy and an inclusion of an impurity such
as an oxide in molten metal is remarkably reduced in comparison to a typical case
when Mg not including a Ca-based compound is added. As such, according to the above-described
method, even without using a protective gas, the cleanliness of molten Al may be greatly
improved and thus the quality of molten metal may be significantly improved.
[0076] Then, in operation S13, the molten Al is stirred or held for 1 to 400 minutes. Here,
if the stirring or holding time is less than 1 minute, the Mg master alloy is not
sufficiently mixed into the molten Al. Otherwise, if the stirring or holding time
is more than 400 minutes, the molten Al is stirred or held for a unnecessarily long
time.
[0077] After operation S13 is completed, operation S14 for solidifying the molten Al in
a mold is performed to prepare the Al alloy. In operation S14, the temperature of
the mold may be in a range of room temperature (for example, 25°C) to 400°C. Also,
a master alloy may be separated from the mold after the mold is cooled to room temperature.
However, if the master alloy is completely solidified, the master alloy may be separated
even before the temperature reaches room temperature. The casting method is described
in detail above in relation to the method of preparing an Mg master alloy and thus
is not described here.
[0078] In this case, the prepared Al alloy may be any one selected from 1000-series, 2000-series,
3000-series, 4000-series, 5000-series, 6000-series, 7000-series, and 8000-series wrought
Al, and 100-series, 200-series, 300-series, 400-series, 500-series, and 700-series
casting Al.
[0079] As described above, if an Mg master alloy including a Ca-based compound is added,
the cleanliness of molten Al may be improved and thus mechanical properties of a casted
Al alloy may be remarkably improved. That is, due to an improved cleanliness of molten
metal, an impurity such as an oxide or another inclusion which deteriorates mechanical
properties does not exist in an Al alloy casted by using the molten metal, and bubbles
inside the casted Al alloy are also reduced remarkably. Since the above-casted Al
alloy has a cleaner internal state than a conventional Al alloy, the Al alloy according
to the present invention has mechanical properties, for example, yield strength, tensile
strength, and elongation, superior to the conventional Al ally.
[0080] Accordingly, although an Al alloy having the same content of Mg is prepared, according
to the present invention, the cleanliness of molten metal may be improved and thus
a casted Al alloy may have excellent properties.
[0081] Also, since the loss of Mg added into Al in molten metal is reduced, even when a
smaller amount of Mg is added in comparison to a conventional case, an Al alloy may
be economically prepared to have substantially the same content of Mg as the conventional
case.
[0082] Furthermore, if an Mg master alloy according to the present invention is added into
molten Al, instability of Mg in the molten Al may be remarkably improved and thus
the content of Mg may be more easily increased in comparison to a conventional case.
[0083] Mg may be dissolved by up to 15wt% in Al and thus may increase mechanical properties
of an Al alloy. For example, if Mg is added into a 300-series or 6000-series Al alloy,
strength and elongation of the Al alloy may be improved.
[0084] However, in a conventional case, due to the above-described high oxidizing potential
of Mg, an oxide or another inclusion may be mixed into molten metal and thus the quality
of an Al alloy may deteriorate. Since the above problem becomes more serious if the
content of Mg is increased, even when a protective gas is used, the content of Mg
added into molten Al may not be stably increased.
[0085] On the other hand, according to the present invention, an Mg master alloy may be
stably added into molten Al such that castability may be ensured while increasing
the ratio of Mg by easily increasing the content of Mg in an Al alloy in comparison
to a conventional case. Accordingly, if an Mg master alloy is added into a 300-series
or 6000-series Al alloy according to the present invention, an oxide or another inclusion
may be suppressed from being mixed and strength, elongation, and castability may be
improved. Furthermore, a 500-series or 5000-series Al alloy which is not substantially
used at present may be used.
[0086] For example, an Al alloy according to the present invention may easily increase a
dissolved amount of Mg up to 0.1wt% or more, up to 5wt% or more, up to 6wt% or more,
further up to 10wt% or more, and even further up to 15wt% corresponding to a limit
of solubility.
[0087] The stability of Mg in an Al alloy may act favorably when a waste of the Al alloy
is reused. For example, if the content of Mg is high in the waste used to prepare
the Al alloy, a process of reducing the content of Mg to a required ratio (hereinafter
referred to as a 'demagging process') is performed. If the required ratio of the content
of Mg is low, the difficulty and the cost of the demagging process are increased.
[0088] For example, in a 383 Al alloy, it is technically easy to reduce the content of Mg
to 0.3wt% but it is very difficult to reduce the content of Mg to 0.1wt%. Also, a
chlorine gas (Cl
2) is used to reduce the ratio of Mg; however, the chlorine gas is environmentally
harmful and requires an additional cost.
[0089] However, an Al alloy prepared by using an Mg master alloy including a Ca-based compound,
according to the present invention, may maintain the ratio of Mg to be more than 0.3wt%
and thus has technical, environmental, and cost advantages.
[0090] Also, the method of preparing an Al alloy, according to the present invention, may
further include adding a small amount of iron (Fe) after operation S11 or S12. In
this case, the amount of added Fe may be less than that added in a conventional case.
That is, if an Al alloy is conventionally casted, for example, die-casted, due to
soldering between a die made of an iron-based metal and an Al cast material, the die
may be damaged. In order to solve this problem, 1.0wt% to 1.5wt% of Fe has been conventionally
added into an Al alloy when the Al alloy is die-casted. However, the addition of Fe
may cause another problem of reducing corrosion resistance and elongation of the Al
alloy.
[0091] However, as described above, an Al alloy according to the present invention may include
Mg at a high ratio and the conventional soldering problem of a die may be significantly
improved even though a considerably small ratio of Fe in comparison to a conventional
case is added. Accordingly, the conventional problem of reducing corrosion resistance
and elongation may be solved.
[0092] In this case, the content of Fe added to prepare the Al alloy may be less than or
equal to 1.0wt% (and greater than 0wt%) with respect to the Al alloy, and more particularly,
less than or equal to 0.2wt% (and greater than 0wt%). As such, Fe within the above
composition range may be included in a matrix of the Al alloy.
[0093] Properties of an Al alloy prepared by using the method of preparing the Al ally,
according to the present invention, will now be described in detail.
[0094] An Al alloy prepared by using the method of preparing the Al ally, according to the
present invention, includes an Al matrix and a Ca-based compound existing in the Al
matrix, wherein Mg may be dissolved in the Al matrix.
[0095] In this case, Mg may be dissolved in the Al matrix within a range of 0.1wt% to 15wt%.
Also, Ca may be dissolved in the Al matrix by an amount less than or equal to a limit
of solubility, for example, less than or equal to 500ppm.
[0096] As described above, Ca reduced from a Ca-based additive added into an Mg master alloy
mostly exists in the form of the Ca-based compound and only a part of it is dissolved
in an Mg matrix. If the Mg master alloy is added into molten Al, since Ca dissolved
in the Mg master alloy is diluted, the amount of Ca substantially dissolved in the
matrix of the Al alloy also has a small value less than or equal to the limit of solubility.
[0097] Accordingly, the Al alloy according to the present invention has a microstructure
in which Ca is dissolved in the Al matrix by an amount less than the limit of solubility,
for example, less than 500ppm, and the Ca-based compound is formed separately in the
Al matrix.
[0098] In this case, the Al matrix may have a plurality of domains divided from each other
by boundaries therebetween, and the Ca-based compound may exist at the boundaries
or the inside of the domains.
[0099] The Al matrix may be defined as a metal structure body in which Al is a major component
and another alloying element is dissolved, or another alloying element other than
the Ca-based compound or a compound including the other alloying element is formed
as a separate phase.
[0100] In this case, the plurality of domains divided from each other may typically be a
plurality of grains divided by grain boundaries therebetween and, as an another example,
may be a plurality of phase regions defined by two or more different phase boundaries
therebetween.
[0101] The Al alloy according to the present invention may improve mechanical properties
depending on the Ca-based compound formed in the Mg master alloy. As already described
above, if the Mg master alloy is added into the molten Al, the Ca-based compound included
in the Mg master alloy is also added into the molten Al. All Ca-based compounds are
intermetallic compounds formed due to reaction between Ca and other metal elements
and have higher melting points than Al.
[0102] Accordingly, if a master alloy including the Ca-based compound is added into the
molten Al, the Ca-based compound may be remains without being melted in molten metal.
Moreover, if an Al alloy is prepared by casting the molten metal, the Ca-based compound
may be included in the Al alloy.
[0103] The Ca-based compound may be dispersed and distributed in the Al alloy in the form
of fine particles. The Ca-based compound, as an intermetallic compound, is a high
strength material in comparison to Al which is a matrix. Due to the dispersive distribution
of such a high strength material, the strength of the Al alloy may be increased.
[0104] Meanwhile, the Ca-based compound may provide a site for nucleation during phase transition
of the Al alloy occurs from a liquid phase to a solid phase. That is, the phase transition
from the liquid phase to the solid phase when the Al alloy is solidified will be carried
out in the form of nucleation and growth. In this case, since the Ca-based compound
itself acts as a heterogeneous nucleation site, nucleation for phase transition to
the solid phase is initially occurs on the interface between the Ca-based compound
and the liquid phase. The nucleated solid phase grows around the Ca-based compound.
[0105] If the Ca-based compounds are distributed, solid phases growing at the interfaces
of the different Ca-based compounds meet each other to form boundaries, and these
boundaries may form grain boundaries or phase boundaries. Accordingly, if the Ca-based
compound functions as a nucleation site, the Ca-based compound exists inside grains
or phase regions, and the grains or phase regions become finer in comparison to a
case when the Ca-based compound does not exist.
[0106] Also, the Ca-based compound may be distributed at grain boundaries, i.e., boundaries
between grains, or phase boundaries, i.e., boundaries between phase regions. This
is because such boundaries are further open and have a relatively high energy in comparison
to the inside of the grains or the phase regions, and thus may provide a favorable
site for nucleation and growth of the Ca-based compound.
[0107] If the Ca-based compound is distributed at grain boundaries or phase boundaries of
an Al alloy, since this Ca-based compound acts as an obstacle to the movement of the
grain boundaries or the phase boundaries, the movement of the grain boundaries or
the phase boundaries may be suppressed and thus an average size of the grains or the
phase regions may be reduced.
[0108] Accordingly, the Al alloy according to the present invention may have averagely finer
and smaller grains or phase regions in comparison to an Al alloy not including the
Ca-based compound. These fine and small grains or phase regions due to the Ca-based
compound may improve both strength and elongation of the Al alloy.
[0109] The Al matrix may be any one selected from 1000-series, 2000-series, 3000-series,
4000-series, 5000-series, 6000-series, 7000-series, and 8000-series wrought Al, and
100-series, 200-series, 300-series, 400-series, 500-series, and 700-series casting
Al.
[0110] In the Al alloy according to the present invention, a total amount of Ca may be 0.0001
parts by weight to 10 parts by weight based on 100 parts by weight of Al. The total
amount of Ca is a sum of the amount of Ca dissolved in the Al matrix and the amount
of Ca existing in the Ca-based compound.
[0111] In this case, most of Ca existing in the Al alloy exists in the Ca-based compound
and the amount of Ca dissolved in the Al matrix is small. That is, as described above,
most of Ca reduced from the Ca-based additive added into the Mg master alloy is not
dissolved in the Mg matrix and forms the Ca-based compound. Accordingly, if the Mg
master alloy is added to form Al, since the amount of Ca dissolved in the Mg master
alloy is small, the amount of Ca dissolved in the Al matrix through the Mg master
alloy is also small, for example, less than or equal to 500ppm.
[0112] Meanwhile, the Al matrix may include dissolved Mg by 0.1wt% to 15wt%, by 5wt% to
15wt%, further by 6wt% to 15wt%, and even further by 10wt% to 15wt%. That is, as described
above, if the Mg master alloy prepared by adding the Ca-based additive, according
to the present invention, is used, the amount of Mg added into the molten Al may be
stably increased. Accordingly, the amount of Mg dissolved in the Al matrix may also
be increased.
[0113] The increase in the amount of dissolved Mg may greatly contribute to improvement
of strength of the Al alloy according to solid solution strengthening and heat treatment,
and may achieve superior castability and excellent mechanical properties in comparison
to a conventional commercial alloy.
[0114] Furthermore, an Al alloy according to an embodiment of the present invention has
an improved oxidation resistance in comparison to an Al alloy corresponding to the
Al alloy according to an embodiment and not including the above-described Ca-based
compound. As will be described below, the oxidation resistance of the Al alloy according
to the present invention may be increased if the content of the Ca-based additive
added to prepare the Mg master alloy is high. The improvement in oxidation resistance
is related to an improvement in quality of an Al alloy and/or a distribution of the
Ca-based compound in a matrix.
[0115] Here, an Al alloy corresponding to the Al alloy according to an embodiment may refer
to a typical Al alloy including the same additive elements other than the Ca-based
additive as the Al alloy according to an embodiment, for example, the same elements
according to the standards of the Aluminum Association of America.
[0116] For example, if the Al alloy according to an embodiment is prepared by adding an
Mg master alloy including a Ca-based compound, instead of Mg, into a typical 6061
alloy, an Al alloy corresponding to the Al alloy according to an embodiment may be
the typical 6061 alloy.
[0117] Meanwhile, in a narrow sense, an Al alloy corresponding to the Al alloy according
to an embodiment may refer to an Al alloy including the same-composition additive
elements other than the Ca-based additive as the Al alloy according to an embodiment.
For example, if the Al alloy is a new Al alloy that does not belong to those according
to the standards of the Aluminum Association of America, an Al alloy corresponding
to the Al alloy may refer to an Al ally having substantially the same contents of
additive elements (except for the Ca-based additive) as the new Al alloy. Here, the
same does not mean mathematically the same but means practically the same in consideration
of, for example, an experimental error range.
[0118] Experimental examples will now be provided for better understanding of the present
invention. The experimental examples described below are only for better understanding
of the present invention and the present invention is not limited by the experimental
examples below.
[0119] Table 2 comparatively shows cast properties of an Al alloy prepared by adding an
Mg master alloy including CaO as a Ca-based additive into Al (Experimental Example
1) and an Al alloy prepared by adding pure Mg including no Ca-based additive into
Al (Comparative Example 1).
[0120] Specifically, the Al alloy of Experimental Example 1 is prepared by adding 305g of
the Mg master alloy into 2750g of Al, and the Al alloy of Comparative Example 1 is
prepared by adding 305g of pure Mg into 2750g of Al. The Mg master alloy used in Experimental
Example 1 employed an Mg-Al alloy as a parent material, and a weight ratio of CaO
with respect to the parent material is 0.3.
[Table 2]
|
Experimental Example 1 |
Comparative Example 1 |
Amount of Dross (Impurity Floating on Surface of Molten Metal) |
206g |
510g |
Content of Mg in Al alloy |
4.89% |
2.65% |
Fluidity of Molten Metal |
Good |
Bad |
Hardness (HR Load 60kg, 1/16" Steel Ball) |
92.6 |
92 |
[0121] Referring to Table 2, the amount of an impurity floating on the surface of molten
metal (the amount of dross) is remarkably smaller in the case when the Mg master alloy
is added (Experimental Example 1) than the case when pure Mg is added (Comparative
Example 1). Also, the content of Mg in the Al alloy is larger in the case when the
Mg master alloy is added (Experimental Example 1) than the case when pure Mg is added
(Comparative Example 1). As such, it may be seen that, according to the present invention,
the loss of Mg is remarkably reduced in comparison to a method of adding pure Mg.
[0122] Also, the fluidity of the molten metal and the hardness of the Al alloy are superior
in the case when the Mg master alloy is added (Experimental Example 1) than the case
when pure Mg is added (Comparative Example 1).
[0123] FIGS. 4A and 4B are images showing the states of molten metal according to Experimental
Example 1 and Comparative Example 1. Referring to FIGS. 4A and 4B, the state of the
molten metal is good in Experimental Example 1 (FIG. 4A), but the surface of the molten
metal changes to black due to oxidation of Mg in Comparative Example 1 (FIG. 4B).
[0124] FIGS. 5A and 5B are images comparatively showing the surfaces of cast materials of
the Al alloys according to Experimental Example 1 and Comparative Example 1.
[0125] Referring to 5A and 5B, the surface of the cast material of the Al alloy prepared
by adding the Mg master alloy according to Experimental Example 1 (FIG. 5A) is cleaner
than that of the cast material of the Al alloy prepared by adding pure Mg according
to Comparative Example 1 (FIG. 5B). This is because castability is improved by CaO
added into the Mg master alloy. That is, when the Al alloy prepared by adding pure
Mg (Comparative Example 1) is casted, ignition marks are shown on the surface due
to oxidation of pure Mg during casting. However, when the Al alloy prepared by adding
the Mg master alloy including CaO (Experimental Example 1) is casted, ignition is
suppressed and thus a clean surface may be obtained.
[0126] As such, it may be seen that the quality of the molten metal is remarkably improved
and thus castability is improved in the case when the Mg master alloy is added in
comparison to the case when pure Mg is added.
[0127] FIGS. 6A and 6B are images showing results of energy dispersive spectroscopy (EDS)
analysis on the Al alloys according to Experimental Example 1 and Comparative Example
1 by using a scanning electron microscope (SEM). Referring to FIGS. 6A and 6B, in
the Al alloy prepared by adding pure Mg according to Comparative Example 1 (FIG. 6B),
only Mg and Al are detected. However, in the Al alloy prepared by adding the Mg master
alloy including CaO according to Experimental Example 1 (FIG. 6A), Ca exists. Also,
Mg and Al are detected at the same position and oxygen is hardly detected. As such,
it may be seen that Ca exists as a Ca-based compound by reacting with Mg and/or Al
after being reduced from CaO.
[0128] FIG. 7A is an EPMA image of a microstructure of the Al alloy of Experimental Example
1, and FIGS. 7B through 7E are EPMA images showing component mapping results of Al,
Ca, Mg, and O, respectively.
[0129] Ca and Mg are detected at the same position in an Al matrix as shown in FIGS. 7B
through 7D, and O is not detected as shown in FIG. 7E.
[0130] The above result corresponds to the result of FIG. 6A, and thus it may be seen once
again that Ca exists as a Ca-based compound by reacting with Mg and/or Al after being
reduced from CaO.
[0131] Table 3 comparatively shows mechanical properties of die-cast alloys according to
comparative examples and die-cast alloys according to experimental examples of the
present invention.
[Table 3]
|
Tensile Strength (MPa) |
Yield Strength (MPa) |
Elongation (%) |
Experimental Example 2 |
314 to 366 |
228 to 306 |
6 to 8 |
Comparative Example 2 |
201 to 258 |
184 to 204 |
1.4 to 2.3 |
Experimental Example 3 |
357 to 435 |
270 to 390 |
5 to 17 |
Comparative Example 3 |
300 to 350 |
210 to 300 |
5 to 20 |
[0132] Experimental Example 2 shows a die-cast binary Al-Mg alloy and prepared by adding
10wt% of an Mg master alloy including CaO into Al. Comparative Example 2 shows, as
a commercial Al alloy, a Magsimal-59 alloy including 5.0wt% to 6.0wt% of Mg, which
is a quite high content of Mg in a commercial Al alloy.
[0133] Referring to Table 3, when Experimental Example 2 and Comparative Example 2 including
relatively high contents of Mg are compared, the tensile strength, the yield strength,
and the elongation according to Experimental Example 2 are higher than those according
to Comparative Example 2. In particular, in Experimental Example 2, although 10wt%
of Mg is included in Al, a high tensile strength greater than 360Mpa and a high elongation
of 8% are obtained.
[0134] As described above, if the content of Mg included in molten Al is increased, the
quality of molten metal is reduced due to oxidation of Mg. In an actual case, if 10wt%
of Mg is included in Al, commercialization through die-casting may not be easily acheived.
[0135] However, according to Experimental Example 2, although the content of Mg is increased
to 10wt%, since the molten metal is maintained in a good state, it may be seen that
a bad influence due to addition of Mg is suppressed and an improvement in mechanical
properties due to addition of Mg is acheived.
[0136] FIG. 8 is an image showing a microstructure according to Experimental Example 2.
Referring to FIG. 8, in the alloy according to Experimental Example 2, although the
content of Mg is 10wt%, an impurity such as an oxide or another inclusion is not observed
and very fine grains are obtained. The size of the grains according to Experimental
Example 2 is very small in comparison to a general commercial alloy. These clean and
fine-grain properties are regarded as one factor for achieving excellent mechanical
properties of Experimental Example 2.
[0137] Comparative Example 3 shows a die-cast material prepared by using a 7xx alloy including
Mg by about 2wt% to about 3.5wt%, and Experimental Example 3 shows a die-cast material
the same as that of Comparative Example 3 except that an Mg master alloy including
CaO is added instead of pure Mg as an alloying element.
[0138] In comparison to Comparative Example 3, Experimental Example 3 achieves a significantly
higher tensile strength and yield strength and achieves an equivalent elongation.
As described above, it is regarded that the above result is related to an improvement
in cleanliness of molten metal when an Al alloy according to an experimental example
of the present invention is prepared.
[0139] FIGS. 9A and 9B are images comparatively showing microstructures according to Experimental
Example 3 and Comparative Example 3. Referring to FIGS. 9A and 9B, in comparison to
the alloy according to Comparative Example 3, the alloy according to Experimental
Example 3 has much finer grains. As described above, it is regarded that these fine
grains of the die-cast Al alloys according to the experimental examples are achieved
because growth at grain boundaries is suppressed by the Ca-based compound distributed
at the grain boundaries or because the Ca-based compound functions as a site for nucleation
during solidification. Also, these fine grains are regarded as one factor for achieving
excellent mechanical properties of the Al alloys according to the experimental examples.
[0140] In Table 4, Comparative Example 4 shows a die-cast material of an ALDC12 alloy that
is the most commonly used as a commercial die-cast alloy and its mechanical properties
are as shown in the ASM handbook. Experimental Examples 4-1 and 4-2 show die-cast
materials having the same composition as that of Comparative Example 4 except that
an Mg master alloy including CaO is added instead of pure Mg as an alloying element.
In Experimental Example 4-1, 0.3wt% of the Mg master alloy including 0.5wt% of CaO
is added to Al. In Experimental Example 4-2, 0.5wt% of the Mg master alloy including
1.0wt% of CaO is added to Al.
[Table 4]
|
Tensile Strength (MPa) |
Yield Strength (MPa) |
Elongation (%) |
Comparative Example 4 |
160 |
325 |
4 |
Experimental Example 4-1 |
163 |
344 |
5.83 |
Experimental Example 4-2 |
180 |
370 |
6.19 |
[0141] As shown in Table 4, in comparison to the commercial ALDC12 alloy according to Comparative
Example 4, Experimental Examples 4-1 and 4-2 according to embodiments of the present
invention achieve higher tensile strengths and yield strengths and superior elongations.
In particular, in Experimental Example 5 including a larger content of CaO, the elongation
as well as the strength is superior. As described above, the above result may be related
to an improvement in cleanliness of molten metal when an Al alloy according to an
experimental example of the present invention is prepared.
[0142] FIGS. 10A through 10C are images showing microstructures of the die-cast Al alloy
according to Comparative Example 4 and the die-cast Al alloys according to Experimental
Examples 4-1 and 4-2.
[0143] Referring to FIGS. 10A through 10C, in comparison to Comparative Example 4, Experimental
Examples 4-1 and 4-2 achieve finer grains. As described above, it is regarded that
these fine grains of the die-cast Al alloys according to the experimental examples
are achieved because growth at grain boundaries is suppressed by the Ca-based compound
distributed at the grain boundaries or because the Ca-based compound functions as
a site for nucleation during solidification. Also, these fine grains are regarded
as one factor for achieving excellent mechanical properties of the Al alloys according
to the experimental examples.
[0144] Table 5 shows mechanical properties of Al alloy extruded materials according to experimental
examples and Al alloy extruded materials according to comparative examples. Experimental
Examples 5, 6, and 7 respectively show Al alloy extruded materials prepared by adding
an Mg master alloy including CaO into a 5056 alloy, a 6061 alloy, and a 7075 alloy
which are commercial Al alloys, and Comparative Examples 5, 6, and 7 respectively
show the 5056 alloy, the 6061 alloy, and the 7075 alloy.
[0145] Specimens according to Experimental Examples 5, 6, and 7 are obtained by performing
casting, extruding, and then T6 heat treatment, and Comparative Examples 5, 6, and
7 use data according to the ASM standards (T6 heat treatment data).
[Table 5]
|
Tensile Strength (MPa) |
Yield Strength (MPa) |
Elongation (%) |
Experimental Example 5 (5056) |
424 |
231 |
34.2 |
Comparative Example 5 (5056) |
290 |
152 |
35 |
Experimental Example 6 (6061) |
349 |
329 |
17.8 |
Comparative Example 6 (6061) |
310 |
276 |
12 |
Experimental Example 7 (7075) |
662 |
610 |
13.6 |
Comparative Example 7 (7075) |
572 |
503 |
11 |
[0146] As shown in Table 5, in comparison to the commercial Al alloy extruded materials
not including the Ca-based compound, the Al alloy extruded materials according to
experimental examples of the present invention achieve higher tensile strengths and
yield strengths and superior or equivalent elongations. In particular, with respect
to the 5000-series alloy, in comparison to the commercial Al alloy extruded material
(Comparative Example 2), in the Al alloy extruded material according to the present
invention (Experimental Example 2), the tensile strength may be greatly increased
by about 1.46 times and the elongation may be maintained at an equivalent level. Furthermore,
with respect to the 6000-series alloy and the 7000-series alloy, in comparison to
the commercial Al alloy extruded materials (Comparative Examples 3 and 4), in the
Al alloy extruded materials according to the present invention (Experimental Examples
3 and 4), both of the tensile strength and the elongation may be increased.
[0147] In more detail, with respect to the tensile strength, Experimental Example 2 is about
1.46 times of Comparative Example 2, Experimental Example 3 is about 1.13 times of
Comparative Example 3, and Experimental Example 4 is about 1.16 times of Comparative
Example 4. That is, the tensile strengths according to the experimental examples are
about 1.13 times to about 1.46 times higher than the tensile strengths according to
the comparative examples. Meanwhile, with respect to the elongation, Experimental
Example 2 is about 0.98 times of Comparative Example 2, Experimental Example 3 is
about 1.48 times of Comparative Example 3, and Experimental Example 4 is about 1.24
times of Comparative Example 4.
[0148] In general, if the strength of an alloy is increased, the elongation of the alloy
is relatively reduced. However, the Al alloys according to the experimental examples
of the present invention have ideal properties for increasing the strength and the
elongation. As described above, the above result may be related to an improvement
in cleanliness of molten metal of an Al alloy.
[0149] FIGS. 11 through 13 are images comparatively showing microstructures according to
Experimental Examples 5, 6, and 7 and Comparative Examples 5, 6, and 7 shown in Table
3.
[0150] Referring to FIGS. 11A and 11B, grains according to Experimental Example 5 (FIG.
11A) have an average size of about 25
µm, and grains according to Comparative Example 5 (FIG. 11B) have an average size of
about 60
µm. That is, the grain size according to Experimental Example 5 is merely about 0.42
times of the grain size according to Comparative Example 5. Referring to FIGS. 12A
and 12B, grains according to Experimental Example 6 (FIG. 12A) have an average size
of about 30
µm, and grains according to Comparative Example 6 (FIG. 12B) have an average size of
about 50
µm. That is, the grain size according to Experimental Example 6 is merely about 0.6
times of the grain size according to Comparative Example 6. Referring to FIGS. 13A
and 13B, grains according to Experimental Example 7 (FIG. 13A) have an average size
of about 25
µm, and grains according to Comparative Example 7 (FIG. 13B) have an average size of
about 50
µm. That is, the grain size according to Experimental Example 7 is merely about 0.5
times (half) of the grain size according to Comparative Example 7.
[0151] Accordingly, it is seen that the Al alloy extruded materials according to the experimental
examples of the present invention have much finer grains in comparison to the commercial
Al alloy extruded materials. For example, the grain size of the Al alloy extruded
materials according to the experimental examples of the present invention is in a
range of about 0.42 times to about 0.6 times of the grain size of the commercial Al
alloy extruded materials.
[0152] It is regarded that these fine grains of the Al alloys according to the experimental
examples are achieved because growth at grain boundaries is suppressed by the Ca-based
compound distributed at the grain boundaries or because the Ca-based compound functions
as a site for nucleation during solidification. Also, these fine grains are regarded
as one factor for achieving excellent mechanical properties of the Al alloys according
to the experimental examples.
[0153] Table 6 comparatively shows mechanical properties of Al alloy extruded materials
according to experimental examples and Al alloy extruded materials according to comparative
examples, based on heat treatment conditions. Comparative Examples 6-1, 6-2, and 6-3
respectively show specimens (extruded materials) prepared by casting, extruding, and
heat-treating a commercial 6061 alloy under conditions T1, T5, and T6, and Experimental
Examples 6-1, 6-2, and 6-3 respectively show specimens prepared by casting, extruding,
and heat-treating an Al alloy, which is prepared by adding an Mg master alloy including
CaO into the commercial 6061 alloy, under conditions T1, T5, and T6. In Table 4, an
Al alloy substantially refers to an Al alloy extruded material that is completely
heat-treated after being extruded.
[Table 6]
|
Hardness (HRF) |
Tensile Strength (MPa) |
Yield Strength (MPa) |
Elongation (%) |
Experimental Example 6-1 (T1) |
47.5 |
117 |
215 |
20.9 |
Comparative Example 6-1 (T1) |
43.7 |
108 |
200 |
20.7 |
Experimental Example 6-2 (T5) |
83.4 |
203 |
269 |
16.1 |
Comparative Example 6-2 (T5) |
63.5 |
160 |
234 |
17.8 |
Experimental Example 6-3 (T6) |
92.5 |
385 |
405 |
15.7 |
Comparative Example 6-3 (T6) |
94.1 |
372 |
396 |
14.5 |
[0154] As shown in Table 6, regardless of the heat treatment conditions, in comparison to
the commercial Al alloy extruded materials not including the Ca-based compound (Comparative
Examples 6-1, 6-2, and 6-3), the Al alloy extruded materials according to embodiments
of the present invention (Experimental Examples 6-1, 6-2, and 6-3) achieve higher
tensile strengths and yield strengths and superior or equivalent elongations.
[0155] In general, if the strength of an alloy is increased, the elongation of the alloy
is relatively reduced. However, the Al alloys according to the experimental examples
of the present invention have ideal properties for increasing the strength and the
elongation. As described above, the above result may be related to an improvement
in cleanliness of molten metal of an Al alloy.
[0156] Meanwhile, after heat treatment is performed under conditions T1 and T5, the Al alloy
extruded materials according to embodiments of the present invention (Experimental
Examples 6-1 and 6-2) have higher hardness levels than the commercial Al alloy extruded
materials (Comparative Examples 6-1 and 6-2). However, after heat treatment is performed
under condition T6, the commercial Al alloy extruded material (Comparative Example
6-3) has a higher hardness level than the Al alloy extruded material according to
an embodiment of the present invention (Experimental Example 6-3).
[0157] FIG. 14 is a graph showing oxidation resistance of Al alloys based on the content
of CaO added to prepare an Mg master alloy. In this test, oxidation is performed in
an O
2 atmosphere at about 550°C for about 40 hours. The content of CaO added to prepare
the Mg master alloy varies from 0wt%, to 0.35wt%, to 0.7wt%, and to 1.0wt%, and Al
alloys prepared by using the Mg master alloy are represented as Al-5Mg, Al-5(Mg-5Al-0.35CaO),
Al-5(Mg-5Al-0.7CaO), and Al-5(Mg-5Al-1.0CaO). In these alloys, the contents of additive
elements other than CaO are substantially the same.
[0158] Referring to FIG. 14, under the same condition, in comparison to the comparative
examples not including CaO, in the experimental examples including CaO according to
the present invention, weight gains (%) of the specimens with respect to the increase
in isothermal oxidation time are small. Furthermore, if the content of CaO is increased,
that is, if the content of the Ca-based compound in the Al alloy is increase, the
weight gain of the specimen is small. Considering that the weight of the specimen
is increased as oxidation is processed, if the content of CaO is increased or if the
content of the Ca-based compound in the Al alloy is increased, oxidation resistance
of the Al alloy is increased.
[0159] FIG. 15 is a graph comparatively showing oxidation resistance of Al alloys according
to comparative examples and Al alloys according to embodiments of the present invention,
based on the content of Mg. FIGS. 16A through 16G are images comparatively showing
oxidation resistance of Al alloys according to comparative examples and Al alloys
according to embodiments of the present invention, based on the content of Mg. The
Al alloys according to embodiments of the present invention have the same contents
of additive elements as the Al alloys according to the comparative examples except
that a Ca-based additive is added to prepare an Mg master alloy. The Al alloys according
to embodiments of the present invention are marked as "Eco" in FIGS. 15 and 16.
[0160] Referring to FIGS. 15 and 16, in overall, if the content of Mg is increased, oxidation
resistance is reduced. However, with respect to the same content of Mg, the Al alloys
according to embodiments of the present invention have higher oxidation resistances
than the Al alloys according to the comparative examples. In particular, the Al alloy
including 2.5wt% of Mg according to an experimental example of the present invention
(Eco Al-2.5Mg) has a higher oxidation resistance than pure Al. In this sense, the
Al alloys according to embodiments of the present invention may be referred to as
oxidation-resistant Al alloys in comparison to typical Al alloys.
[0161] Meanwhile, the Al alloys according to embodiments of the present invention have excellent
corrosion resistance. FIG. 17 is a graph showing corrosion resistance of an Al alloy
according to a comparative example and an Al alloy according to an embodiment of the
present invention. FIG. 18 is an image showing corrosion properties of an Al alloy
according to a comparative example. FIG. 19 is an image showing corrosion properties
of an Al alloy according to an embodiment of the present invention.
[0162] As a reaction rate representing corrosion of metal, a corrosion rate may be represented
as a corrosion loss for a unit period of time. In FIG. 16, the corrosion rate is calculated
in units of milimeters/year (mmy). Here, a K-factor is calculated as 8.75×10
4. In this test, a commercial 7075 alloy (AA7075) is used in the comparative example,
and an Al alloy prepared by adding an Mg master alloy including CaO into the commercial
7075 alloy (Eco 7075) is used in the experimental example. In the corrosion test,
a salt spray test is performed by using a 3% NaCl solution, at 25°C and pH 7.0, for
240 hours.
[0163] Referring to FIGS. 17 through 19, although differences exists based on specimens,
a corrosion rate of the Al alloy according to an embodiment of the present invention
(Eco 7075) is lower than or equivalent to the Al alloy according to the comparative
example (AA7075). In this sense, the Al alloy according to an embodiment of the present
invention may be referred to as an oxidation-resistant Al alloy in comparison to a
typical Al alloy.
[0164] Table 7 shows results of a fatigue test of an Al alloy according to an experimental
example of the present invention. The present experimental example uses an Al alloy
having the same composition as a commercial 7075 alloy except that an Mg master alloy
including CaO is added (hereinafter referred to as ECO-7075). The Al alloy according
to the present experimental example has a yield strength of 590.89Mpa (29.92kN). In
the fatigue test, stresses are 40%, 60%, and 80% of the yield strength (590.89MPa),
a stress amplitude is 5kN, and frequencies are 10 Hz and 2Hz.
[Table7]
Stress |
Cycle (N) |
|
10Hz |
20Hz |
80% |
1,021,196 |
2,012,008 |
60% |
1,784,082 |
- |
40% |
- |
- |
[0165] As shown in Table 7, according to the present experimental example, if a cyclic load
is applied under the stress condition of 40% of a tensile strength, fatigue fracture
does not occur. Under the stress condition of 80%, if frequency is 10Hz, fatigue fracture
occurs when the test is performed more than about a million times. If the frequency
is 2Hz, fatigue fracture occurs when the test is performed more than about two million
times. The above result may not be easily achieved by a commercial Al alloy.
[0166] Accordingly, the Al alloy according to the experimental example of the present invention
has superior fatigue properties to a corresponding commercial Al alloy (i.e., a 7075
alloy).
[0167] FIG. 20 is a graph showing mechanical properties of an Al alloy used in a fatigue
test, according to an experimental example of the present invention. Referring to
FIG. 20, the Al alloy according to the experimental example has a yield strength of
590.89Mpa, a tensile strength of 651.9Mpa, and an elongation of 13.6%. The above strength
and elongation are much higher than a typical 7075 alloy. As such, it may be seen
that the Al alloy according to the present experimental example has a high strength
and excellent fatigue properties in comparison to a conventional Al alloy.
[0168] While the present invention has been particularly shown and described with reference
to exemplary embodiments thereof, it will be understood by those of ordinary skill
in the art that various changes in form and details may be made therein without departing
from the spirit and scope of the present invention as defined by the following claims.