U.S. patent application number 12/127608 was filed with the patent office on 2008-12-04 for aluminum alloy formulations for reduced hot tear susceptibility.
This patent application is currently assigned to Alcan International Limited. Invention is credited to Neivi Andrade, Joseph Langlais, Alain Lemieux.
Application Number | 20080299001 12/127608 |
Document ID | / |
Family ID | 40074529 |
Filed Date | 2008-12-04 |
United States Patent
Application |
20080299001 |
Kind Code |
A1 |
Langlais; Joseph ; et
al. |
December 4, 2008 |
ALUMINUM ALLOY FORMULATIONS FOR REDUCED HOT TEAR SUSCEPTIBILITY
Abstract
The present invention relates to modified alloy compositions for
reduced hot tear susceptibility, the aluminum alloy comprising from
0.01 to 0.025% by weight of Sr; and TiB2, measured by its boron
content, from 0.001 to 0.005% by weight of B. The invention also
relates to a method of preventing or eliminating hot tears in an
aluminum alloy comprising the step of combining with aluminum: from
0.01 to 0.025% by weight of Sr; and TiB2, measured by its boron
content, from 0.001 to 0.005% by weight of B.
Inventors: |
Langlais; Joseph;
(Jonquiere, CA) ; Lemieux; Alain; (Alma, CA)
; Andrade; Neivi; (Jonquiere, CA) |
Correspondence
Address: |
BANNER & WITCOFF, LTD.
TEN SOUTH WACKER DRIVE, SUITE 3000
CHICAGO
IL
60606
US
|
Assignee: |
Alcan International Limited
Montreal
CA
|
Family ID: |
40074529 |
Appl. No.: |
12/127608 |
Filed: |
May 27, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60941207 |
May 31, 2007 |
|
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|
Current U.S.
Class: |
420/532 ;
148/549; 420/529; 420/535; 420/542; 420/546; 420/548; 420/552 |
Current CPC
Class: |
B22D 17/007 20130101;
C22C 21/00 20130101; C22C 21/14 20130101; C22C 21/16 20130101; C22C
21/06 20130101; C22C 1/06 20130101; C22C 21/08 20130101; C22C 1/03
20130101; C22C 21/12 20130101; B22D 21/007 20130101; C22C 1/026
20130101 |
Class at
Publication: |
420/532 ;
420/552; 420/529; 420/548; 420/542; 420/546; 420/535; 148/549 |
International
Class: |
C22C 21/00 20060101
C22C021/00; C22F 1/04 20060101 C22F001/04 |
Claims
1. An aluminum alloy comprising: i) From 0.010 to 0.025% by weight
Sr; and ii) TiB.sub.2, measured by its boron content, from 0.001 to
0.005% by weight B.
2. The aluminum alloy according to claim 1, further comprising: i)
0.16% or less by weight excess Ti over the amount bound
stoichiometrically with the B in TiB.sub.2.
3. The aluminum alloy according to claim 1, selected from the group
consisting of alloys comprising primarily aluminum and copper;
alloys comprising primarily aluminum and manganese; alloys
comprising primarily aluminum and silicon; alloys comprising
primarily aluminum and magnesium; alloys comprising primarily
aluminum, magnesium and silicon; and alloys comprising primarily
aluminum, magnesium, zinc and copper.
4. The aluminum alloy according to claim 1, comprising aluminum,
magnesium, and silicon.
5. The aluminum alloy according to claim 4, further comprising
0.010 to 0.020% by weight Sr, and TiB.sub.2, measured by its boron
content, from 0.002 to 0.004% by weight B.
6. The aluminum alloy according to claim 1, for shape casting.
7. The aluminum alloy according to claim 1, for die casting.
8. The aluminum alloy according to claim 1, wherein the alloy is an
alloy cast prepared using a semi-solid casting process.
9. The aluminum alloy according to claim 1, with a composition in
weight percent of 0.6 to 0.8 Si, up to 0.12 Fe, 0.15 to 0.40 Cu,
0.8 to 1.2 Mg, 0.04 to 0.10 Cr, 0.006 to 0.025 Sr, 0.005 to 0.025%
TiB.sub.2, measured by its boron content, from 0.001 to 0.005% by
weight B, 0.16% or less of excess Ti over the amount bound
stoichiometrically with the B in TiB.sub.2, incidental impurities
each less than 0.05 and total less than 0.15, and balance with
Al.
10. The aluminum alloy according to claim 9, for casting by
semi-solid processes.
11. The aluminum alloy according to claim 1, with a composition in
weight percent of 0.6 to 0.8 Si, up to 0.12 Fe, 0.15 to 0.40 Cu,
0.8 to 1.2 Mg, 0.04 to 0.10 Cr, 0.006 to 0.025 Sr, 0.005 to 0.025%
TiB.sub.2, measured by its boron content, from 0.001 to 0.005% by
weight B, 0.16% or less of excess Ti over the amount bound
stoichiometrically with the B in TiB.sub.2, up to 0.45% by weight
Mn, provided that the total Mn+Fe is between 0.55 to 0.65% by
weight, incidental impurities each less than 0.05 and total less
than 0.15, and balance Al.
12. The aluminum alloy according to claim 11, for processing in the
fully molten state.
13. A method of preventing or eliminating hot tears in an aluminum
alloy comprising the step of combining with aluminum: i) from 0.010
to 0.025% by weight Sr; and ii) TiB.sub.2, measured by its boron
content, from 0.001 to 0.005% by weight B.
14. The method according to claim 13, further comprising the step
of combining iii) 0.16% or less by weight excess Ti over the amount
bound stoichiometrically with the B in TiB.sub.2.
15. A shape cast part, cast from an alloy as defined in claim
1.
16. The shape cast part according to claim 15, being a die
cast.
17. The shape cast part according to claim 15, cast from the alloy
in a semi-solid state.
18. A method of providing an aluminum alloy comprising combining
with aluminum: i) from 0.010 to 0.025% by weight Sr; and ii)
TiB.sub.2, measured by its boron content, from 0.001 to 0.005% by
weight B.
19. The method according to claim 18, further comprising combining
iii) 0.16% or less by weight excess Ti over the amount bound
stoichiometrically with the B in TiB.sub.2.
20. The method according to claim 18, wherein the alloy comprises
primarily aluminum, magnesium, and silicon.
21. The method according to claim 18, comprising casting the
alloy.
22. The method according to claim 18, comprising die casting the
alloy.
23. The method according to claim 18, comprising casting the alloy
in a semi-solid state.
24. The method according to claim 18, wherein the alloy is cooled
from its liquidus state to its semi-solid state prior to
casting.
25. The method according to claim 18, wherein the alloy in its
molten state may be maintained at temperature for a period of time
sufficient to produce a semi-solid structure in the alloy.
26. The method according to claim 25, wherein the period of time is
from 1 second to about 2 minutes.
27. The method according to claim 23, when the alloy is in its
semi-solid state, said semi-solid state being globular solid phase
dispersed in a liquid phase, prior to casting, at least some, but
not all of the liquid phase is removed.
28. The method for processing an aluminum alloy as defined in claim
1, having a liquidus temperature and a solidus temperature, the
method comprising the steps of providing the alloy having a
semi-solid range between the liquidus temperature and the solidus
temperature of the alloy; heating the alloy to an alloy initial
elevated temperature above the liquidus temperature to fully melt
the alloy; reducing the temperature of the alloy from the initial
metallic alloy elevated temperature to a semi-solid temperature of
less than the liquidus temperature and more than the solidus
temperature; maintaining the alloy at the semi-solid temperature
for a sufficient time to produce a semi-solid structure in the
alloy of a globular solid phase dispersed in a liquid phase,
wherein the semi-solid structure has less than about 50 weight
percent solid phase; removing at least some, but not all, of the
liquid phase present in the semi-solid structure of the metallic
alloy to form a solid-enriched semi-solid structure of the alloy,
wherein the step of removing includes the step of removing liquid
phase until the solid-enriched semi-solid structure has from about
35 to about 55 weight percent solid phase; and forming the alloy
having the solid-enriched semi-solid structure into a shape.
Description
FIELD OF INVENTION
[0001] The invention relates to modified alloy compositions for
reduced hot tear susceptibility.
BACKGROUND OF THE INVENTION
[0002] Hot tears occur during casting wherein brittle
interdendritic fractures initiate during the solidification
process. Alloys which are generally considered to be prone to hot
tearing have relatively long freezing/solidification ranges,
defined as the temperature difference between the liquidus and
solidus temperatures. In addition, during the final stages of
freezing, these alloys have very little eutectic liquid remaining,
and the limited amounts of this eutectic liquid must pass through
narrow spaces left between the solidified grains. This poor feeding
in the final stages of solidification is a significant contributor
to the phenomenon of hot tearing.
[0003] A method of reducing hot tearing is disclosed in
WO2005/056846. WO2005/056846 is silent to the combination of
strontium and titanium diboride of the present invention and
primarily addresses the problem of hot tearing by casting a mix of
a pure aluminum at a first specified temperature and a second
aluminum alloy mixed with copper, zinc, or magnesium at a second
specified temperature. Temperature control of the two alloys is a
central aspect of the method disclosed in WO2005/056846.
[0004] U.S. Pat. No. 4,681,152 is directed to twin roll casting of
an 5xxx alloy. The composition can contain up to 0.05% Sr and uses
a grain refiner comprising aluminum wire containing about 5% by
weight titanium and 0.2% by weight boron. The boron content can be
as much as 1% by weight. Sufficient grain refining alloy is added
to bring the titanium content up to about 0.02% by weight. U.S.
Pat. No. 4,681,152 is not directed to reducing hot tearing. The
alloys of U.S. Pat. No. 4,681,152 are cast by strip casting.
[0005] U.S. Pat. No. 5,453,244 is directed to a bearing alloy
including Al--Zn base (7xxx). The alloy is broadly described as
containing 0.05 to 0.5% Sr and Ti+B in the range 0.03 to 0.5%. U.S.
Pat. No. 5,453,244 is not directed to reducing hot tearing.
[0006] U.S. Pat. No. 5,211,910 mentions Sr from a list consisting
of Zn, Ge, Sn, Cd, In, Be, Sr, Sc, Y, and Ca to be present in about
0.5 to about 4 weight percent total and Ti and/or TiB.sub.2 from a
list comprising Zr, Cr, Mn, Ti, Hf, V, Nb, B and TiB.sub.2 in the
range 0.01 to 2% as constituents of 2xxx alloy. However, the
specific combination of additives of the present invention are not
disclosed, nor in the range of the present invention. U.S. Pat. No.
5,211,910 is not directed to reducing hot tearing.
[0007] EP0432184 mentions Sr from a list consisting of Zn, Ge, Sn,
Cd, In, Be, Sr, Sc, Y, and Ca to be present in about 0.01 to 1.5
and Ti and/or TiB.sub.2 from a list comprising Zr, Cr, Mn, Ti, Hf,
V, Nb, B and TiB.sub.2 in the range 0.01 to 1.5%. However, the
specific combination of additives of the present invention are not
disclosed, nor in the range of the present invention. EP0432184 is
not directed to reducing hot tearing.
[0008] WO 96/10099 discloses a broad range of possible alloys, and
includes grain refiners (including Ti and TiB.sub.2) and modifiers
(including Sr). The principal alloying element is Sc. Alloys are
useful for shape casting and are said to give properties comparable
to wrought alloys.
[0009] U.S. Pat. No. 6,562,165 describes an Al--Si alloy suitable
for semi-solid processing containing Ti 0.005 to 0.5% and Sr 0.005
to 0.030, with spheroidized structure. U.S. Pat. No. 6,562,165
mentions that excessive Ti addition can lead to large, detrimental
TiB.sub.2 crystals and is silent to TiB.sub.2 levels. The additives
of U.S. Pat. No. 6,562,165 are not intended to reduce hot
tearing.
SUMMARY OF THE INVENTION
[0010] The inventors have found that aluminum-based alloys
comprising as additives a narrowly specified range of both
strontium and titanium diboride have surprisingly low incidences of
hot tearing, thereby allowing die casting of these alloys.
[0011] The present invention is directed to a modified alloy
composition applicable to aluminum alloys to control hot tearing by
the selective use of additives, thereby providing for these alloys
to be subject to die casting, wherein the alloys of the invention
have a strength and ductility properties absent in conventional
aluminum alloys. These properties allow for shape casting of either
above the liquidus of the alloy or in the semi-solid region of the
alloy.
[0012] Without being bound to a particular theory, it is thought
that the strontium and titanium diboride additives work in a
synergistic manner on the alloy, wherein the strontium promotes the
formation of spheroidal grains in the alpha grains and the titanium
diboride initiates the formation of new grains. When used in
combination in the amounts specified, these alloying components
allow the liquid aluminum based alloy to flow until the final
solidification, thereby preventing or significantly reducing the
incidence of hot tearing.
[0013] A first aspect of the invention is directed to an aluminum
alloy comprising i) from 0.010 to 0.025% by weight Sr; and ii)
TiB.sub.2, measured by its boron content, from 0.001 to 0.005% by
weight B. Preferably, the aluminum alloy further comprises iii)
0.16% or less of excess Ti over the amount bound stoichiometrically
with the B in TiB.sub.2. It has been found that a number of alloys
normally susceptible to hot tearing are highly suitable to the use
of the additives of the invention in their specified ranges.
[0014] A related aspect of the invention relates to a method of
preventing or eliminating hot tears in an aluminum alloy comprising
the step of combining with aluminum i) from 0.010 to 0.025% by
weight Sr; and ii) TiB.sub.2, measured by its boron content, from
0.001 to 0.005% by weight B.
[0015] A further object of the invention is to provide a shape cast
part, cast from an alloy defined by the present invention. Notably,
the shape cast may be a die cast, which is difficult with hot
tearing susceptible aluminum alloys. Further advantages are
provided by the present invention in that the cast from the alloy
may be in a semi-solid state.
[0016] The invention may be alternatively defined as providing a
method of providing an aluminum alloy comprising combining with
aluminum i) from 0.010 to 0.025% by weight Sr and ii) TiB.sub.2,
measured by its boron content, from 0.001 to 0.005% by weight
B.
[0017] A particularly interesting aspect of the invention relates
to a method for processing an aluminum alloy said aluminum alloy
having i) from 0.010 to 0.025% by weight Sr and ii) TiB.sub.2,
measured by its boron content, from 0.001 to 0.005% by weight B,
and said alloy having a liquidus temperature and a solidus
temperature, the method comprising the steps of providing the alloy
having a semi-solid range between the liquidus temperature and the
solidus temperature of the alloy; heating the alloy to an alloy
initial elevated temperature above the liquidus temperature to
fully melt the alloy; reducing the temperature of the alloy from
the initial metallic alloy elevated temperature to a semi-solid
temperature of less than the liquidus temperature and more than the
solidus temperature; maintaining the alloy at the semi-solid
temperature for a sufficient time to produce a semi-solid structure
in the alloy of a globular solid phase dispersed in a liquid phase.
Optionally, one may remove some, but not all, of the liquid phase
present in the semi-solid structure of the metallic alloy to form a
solid-enriched semi-solid structure of the alloy, wherein the
optional step of removing includes the step of removing liquid
phase; and forming the alloy having the solid-enriched semi-solid
structure into a shape.
DESCRIPTION OF THE INVENTION
[0018] The invention relates to aluminum based alloys. The aluminum
based alloys may be selected from the group consisting of alloys
comprising primarily aluminum and copper such as of the 2xxx and
2xx type; alloys comprising primarily aluminum and manganese such
as of the 3xxx type; alloys comprising primarily aluminum and
silicon such as of the 4xxx type; alloys comprising primarily
aluminum and magnesium such as of the 5xxx and 5xx type; alloys
comprising primarily aluminum, magnesium and silicon such as of the
6xxx type; and alloys comprising primarily aluminum and zinc such
as of the 7xxx type. The term "primarily" in the context of the
alloys of the present invention is intended to mean that these
elements provide the highest weight content in the alloy, with
aluminum being the highest contributor to the weight content.
[0019] As stated, the aluminum alloys of the present invention,
comprise the unique combination of strontium and titanium diboride,
namely i) from 0.010 to 0.025% by weight Sr and ii) TiB.sub.2,
measured by its boron content, from 0.001 to 0.005% by weight B.
Although TiB.sub.2 is a known grain refiner, in this specific
combination of this specified grain refiner with strontium, the
liquid alloy left in the solidifying alloy is not flow
restricted.
[0020] Titanium, zirconium and their borides and carbides are all
known grain refiners. Surprisingly, titanium diboride when used in
combination with strontium, a crystal modifier, gave surprising
improvements to the properties of the aluminum alloy preventing or
eliminating the incidence of hot tearing.
[0021] In a preferred embodiment of the invention, the aluminum
alloy further comprises 0.16% or less by weight excess Ti and more
preferably 0.12% or less by weight excess Ti. By excess Ti, we mean
the amount of Ti over that which forms TiB.sub.2. Although the Ti
can be introduced by a number of manners known to the skilled
person, it is typically introduced either by the addition of
metallic titanium or through the use of a "grain refiner" rod,
which is an aluminum rod or wire containing specified levels of Ti
and B with a stoichiometry designed to generate TiB.sub.2 with an
excess of Ti, as known by the skilled person.
[0022] Suitably, the aluminum alloy of the invention may comprise
additives in addition to strontium and titanium diboride, and
optionally titanium, for a wide range of purposes such as Mg, Cu
and Zn for strength, Mn and Fe for strength and reduction of die
soldering in die casting, and Ca, Na and Sb for grain
modification.
[0023] As can be seen from the Examples, in alloys where the
content of TiB.sub.2 (measured by its boron content) is less than
0.001% by weight, there is no effect on grain nucleation.
Conversely, in alloys where the content of TiB.sub.2 (measured by
its boron content) is more than 0.005% by weight, there is no
measurable benefit in terms of hot tearing but levels of TiB.sub.2
(measured by its boron content) which exceed 0.005% by weight are
expected to have a negative effect on some semi-solid processes.
Excessive titanium diboride has a negative effect on the degree of
control in some embodiments of the semi-solid process and also can
reduce the strength of the cast part. Accordingly, the invention is
directed to an alloy comprising TiB.sub.2 (measured by its boron
content) from 0.001 to 0.005% by weight B, preferably from 0.002 to
0.004% by weight TiB.sub.2 (measured by its boron content).
[0024] As can also be seen from the Examples, in alloys where the
content of Sr is less than 0.010%, there is insufficient
spheroidizing or globularizing effect (too many acicular or needle
shaped grains remain). Above 0.025%, the Sr has a negative effect
on the final cast strength. Accordingly, the invention relates to
an aluminum alloy comprising from 0.010 to 0.025% by weight Sr,
preferably 0.010% to 0.020% by weight Sr.
[0025] As can be further seen from the Examples, the use of excess
titanium (other than in the form of titanium diboride) is not
advantageous in terms of the controlling the hot tearing effect as
it contributes to the formation of excessively large and elongated
grains. Excess Ti has a negative effect on hot tearing and, if in
excess of 0.16%, results in the formation of Al--Ti intermetallics
which are acicular and contribute to hot tearing as well as
increasing the brittleness of the cast product. The alloys of the
present invention have preferably 0.16% or less by weight excess
Ti, more preferably 0.12% or less by weight excess Ti.
[0026] The additives of the invention are highly suited to an
aluminum alloy comprising primarily aluminum, magnesium, and
silicon. Accordingly, an interesting embodiment of the invention
relates to an aluminum alloy comprising primarily aluminum,
magnesium, and silicon and further comprising 0.010 to 0.025% by
weight Sr, and TiB.sub.2 (measured by its boron content) from 0.001
to 0.005% by weight B. A further interesting embodiment of the
invention relates to an aluminum alloy comprising primarily
aluminum, magnesium, and silicon and 0.010 to 0.025% by weight Sr,
TiB.sub.2 (measured by its boron content) from 0.001 to 0.005% by
weight B and preferably 0.16% or less by excess Ti over the amount
bound stoichiometrically with the B in TiB.sub.2.
[0027] Furthermore, the additives of the invention are considered
to be highly suited to an aluminum alloy comprising primarily
aluminum and copper. Accordingly, an interesting embodiment of the
invention relates to an aluminum alloy comprising primarily
aluminum and copper and further comprising 0.010 to 0.025% by
weight Sr, and TiB.sub.2 (measured by its boron content) from 0.001
to 0.005% by weight B. In a further interesting embodiment of the
invention relates to an aluminum alloy comprising primarily
aluminum, and copper and 0.010 to 0.025% by weight Sr, TiB.sub.2
(measured by its boron content) from 0.001 to 0.005% by weight B
and preferably 0.16% or less by excess Ti over the amount bound
stoichiometrically with the B in TiB.sub.2.
[0028] In a further suitable embodiment, the additives of the
invention are highly suited to an aluminum alloy comprising
primarily aluminum and magnesium. Accordingly, an interesting
embodiment of the invention relates to an aluminum alloy comprising
primarily aluminum and magnesium and further comprising 0.010 to
0.025% by weight Sr, and TiB.sub.2 (measured by its boron content)
from 0.001 to 0.005% by weight B. In a further interesting
embodiment of the invention relates to an aluminum alloy comprising
primarily aluminum and magnesium and 0.010 to 0.025% by weight Sr,
TiB.sub.2 (measured by its boron content) from 0.001 to 0.005% by
weight B and preferably 0.16% or less by weight excess Ti over the
amount bound stoichiometrically with the B in TiB.sub.2.
[0029] In this application, alloys are referred to by the
International Alloy Designations of the Aluminum Association. A
2xxx alloy therefore refers to a wrought alloy that is principally
aluminum with Cu as the main alloying element (where the Cu may be
present, for example, up to about 7 percent by weight), a 2xx alloy
therefore refers to a foundry alloy that is principally aluminum
with Cu as the main alloying element (where the Cu may be present,
for example, up to about 9 percent by weight). An example of an
aluminum Cu alloy would be the 206 alloy, which has a composition
in weight percent of Si less than 0.1, Fe less than 0.15, Cu 4.2 to
5.0, Mn 0.20 to 0.50, Mg 0.15 to 0.35, Ni less than 0.05, Zn less
than 0.10, Sn less than 0.05, balance Al with incidental impurities
each less than 0.05 and total less than 0.15, plus 0.010 to 0.025%
by weight Sr, TiB.sub.2 (measured by its boron content) from 0.001
to 0.005% by weight B, and preferably 0.16% or less by weight
excess Ti over the amount bound stoichiometrically with the B in
TiB.sub.2. Another example of an aluminum Cu alloy would be the
2024 alloy, which has a composition in weight percent of Si less
than 0.5, Fe less than 0.5, Cu 3.8 to 4.9, Mn 0.30 to 0.9, Mg 1.2
to 1.8, Cr less than 0.10, Zn less than 0.25, balance Al with
incidental impurities each less than 0.05 and total less than 0.15,
plus 0.010 to 0.025% by weight Sr, TiB.sub.2 (measured by its boron
content) from 0.001 to 0.005% by weight B and preferably 0.16% or
less by excess Ti over the amount bound stoichiometrically with the
B in TiB.sub.2.
[0030] A 3xxx alloy refers to a wrought alloy that is principally
aluminum with Mn as the main alloying element up to about 1.5
percent by weight. A 4xxx alloy refers to a wrought alloy that is
principally aluminum with Si as the main alloying element up to
about 14 percent by weight.
[0031] A 5xxx alloy refers to a wrought alloy that is principally
aluminum with Mg as the main alloying element up to about 6 percent
by weight. A 5xx alloy refers to a foundry alloy that is
principally aluminum with Mg as the main alloying element up to
about 11 percent by weight. An example of an aluminum Mn alloy
would be the 5182 alloy, which has a composition in weight percent
of Si less than 0.2, Fe less than 0.35, Cu less than 0.15, Mn 0.20
to 0.50, Mg 4.0 to 5.0, Cr less than 0.1, Zn less than 0.25,
balance Al with incidental impurities each less than 0.05 and total
less than 0.15, plus 0.010 to 0.025% by weight Sr, TiB.sub.2
(measured by its boron content) from 0.001 to 0.005% by weight B
and preferably 0.16% or less by excess Ti over the amount bound
stoichiometrically with the B in TiB.sub.2.
[0032] A 6xxx alloy refers to a wrought alloy that is principally
aluminum with Mg and Si as the main alloying elements with Mg
present up to about 1.6 percent by weight and Si present up to
about 1.7 percent by weight and where magnesium silicide forms
during solidification. An example of an aluminum Mg--Si alloys
would be 6061 alloy, which has a composition in weight percent of
Si 0.40 to 0.80, Fe less than 0.7, Cu 0.15 to 0.40, Mn less than
015, Mg 0.8 to 1.2, Cr 0.04 to 0.35, Zn less than 0.25, balance Al
with incidental impurities each less than 0.05 and total less than
0.15, plus 0.010 to 0.025% by weight Sr, 0.005 to 0.025% by weight
TiB.sub.2 (measured by its boron content) from 0.001 to 0.005% by
weight B and preferably 0.16% or less by excess Ti over the amount
bound stoichiometrically with the B in TiB.sub.2.
[0033] A 7xxx alloy refers to a wrought alloy that is principally
aluminum with Zn as the main alloying element, typically present up
to about 9 percent by weight.
[0034] The preceding alloys would have, in addition to the elements
named, TiB.sub.2, Sr, and optionally Ti in the ranges stated above
to give reduced hot tearing. These alloys may also contain
additional alloying elements including Si, Fe, Cu, Mn, Mg, Cr, Ni,
Zn, and V.
[0035] The additives of the present invention, namely 0.010 to
0.025% by weight Sr, and TiB.sub.2 (measured by its boron content)
from 0.001 to 0.005% by weight B, are also considered to be highly
suitable for 2xxx and 2xx aluminum-copper alloys. 2xxx and 2xx
Al--Cu alloys are known to be crack prone. The Sr and TiB.sub.2
additives, in their stated amounts allow for die casting of 2xxx
and 2xx alloys and thereby allowing for complex part shapes and
designs. By controlling hot tearing in aluminum-copper alloys (2xxx
and 2xx alloys), it has been possible to die cast 2xxx and 2xx
alloys, which is normally difficult using conventional alloys. The
alloys of the invention retain strength and ductility properties
similar to the unmodified alloys even though die cast. In an
embodiment of the invention, the alloy is cast, such as shape cast
or die cast.
[0036] Similarly, alloys comprising primarily aluminum and
magnesium such as of the 5xxx and 5xx type are known to be crack
prone. The use of 0.010 to 0.025% by weight Sr, and TiB.sub.2
(measured by its boron content) from 0.001 to 0.005% by weight B in
aluminum and magnesium such as of the 5xxx and 5xx type allow for
die casting of 5xxx and 5xx alloys and thereby allowing for complex
part shapes and designs. By controlling hot tearing in alloys
comprising primarily aluminum and magnesium such as of the 5xxx and
5xx type, it has been possible to die cast 5xxx and 5xx alloys,
which is normally difficult using conventional alloys. The alloys
of the invention retain strength and ductility properties similar
to the unmodified alloys even though die cast. In an embodiment of
the invention, the alloy is cast, such as shape cast or die
cast.
[0037] The alloy formulations of the invention are suitable for use
in any number of shape casting processes including, but not limited
to, sand casting, permanent mold casting, and die casting. Example
processes would include gravity permanent mold, low pressure
permanent mold, and vacuum permanent mold. Most remarkably, they
are also suitable for high pressure diecasting processes, including
both conventional high pressure diecasting and high integrity
diecasting processes such as high-vacuum diecasting, semi-solid
forming, and squeeze casting.
[0038] Hot tearing is not, of course, a phenomenon unique to the
shape casting of near-net shape parts but is also a limitation
frequently encountered when casting billets, blooms, or T-Ingot
cross sections via either semi-continuous or continuous casting
processes (e.g. direct-chill casting or horizontal continuous
casting). The formulations of the invention are, of course,
applicable to the reduction in hot tear susceptibility when casting
these types of products as well.
[0039] Remarkably, by controlling hot tearing in Al--Mg--Si alloys
(6xxx alloys), it has been possible to die cast 6xxx alloys, which
is normally difficult using conventional alloys. The alloys of the
invention retain the strength and ductility properties of the
unmodified alloys even when die cast and provide properties
normally absent in conventional aluminum shape casting alloys. In
an embodiment of the invention, the alloy is cast, such as shape
cast or die cast.
[0040] It has been found that the casting can be done at either
above the liquidus of the alloy or in the semi-solid region.
Surprisingly for aluminum alloys, the present invention is
directed, in one embodiment, to casting in the semi-solid region
since it has been found that not only is hot tearing resistance
observed, this gives further improvements in die filling and
general suitability for casting. The semi-solid structure may have
a globular solid phase under certain processing conditions and this
is particularly favourable for casting.
[0041] The casting of the alloy of the invention into a useful
shape starting from a temperature above the liquidus (where the
alloy is fully molten) can be done by any technique known to those
skilled in the art.
[0042] Most of the alloys of the invention can also be cast in
semi-solid form, at a temperature where the alloy is partly solid
and partly liquid (between the "solidus" and "liquidus"
temperatures). Various techniques can be employed.
[0043] A particularly preferred 6xxx alloy for casting by
semi-solid or fully molten processes has a composition in weight
percent of Si 0.6 to 0.8, Fe up to 0.12, Cu 0.15 to 0.40, Mg 0.8 to
1.2, Cr 0.04 to 0.10, Sr 0.006 to 0.025, 0.005 to 0.025% by weight
TiB.sub.2 (measured by its boron content) from 0.001 to 0.005% by
weight B and preferably 0.16% or less by excess Ti over the amount
bound stoichiometrically with the B in TiB.sub.2. For processing by
semi-solid processing the particularly preferred 6xxx alloy balance
Al with incidental impurities each less than 0.05 and total less
than 0.15, whereas for processing in the fully molten state the
particularly preferred 6xxx alloy will have additionally up to
0.45% by weight Mn, provided that the total Mn+Fe is between 0.55
to 0.65% by weight, balance Al with incidental impurities each less
than 0.05 and total less than 0.15. Semi-solid processing can
beneficially use lower levels of Fe and Mn since the process is
less susceptible to die-sticking. For processing above the
liquidus, control of the total Fe+Mn is advantageous to reduce die
sticking.
[0044] In some embodiments, a solid rod or ingot that may have been
specially cast to have a fine globular structure in the solid phase
is reheated to a temperature between the solidus and liquidus
temperatures and then transferred to a shape casting mould.
[0045] In other embodiments, a fully liquid alloy is cooled to a
temperature between the liquidus and solidus temperature to create
a semi-solid slurry which is then cast. Generally the process is
controlled to ensure that the solid fraction has a globular rather
than dendritic structure. This may be accomplished by rapid
cooling, optionally with the addition of solid nuclei, or by
vigorous agitation (e.g. electromagnetic stirring) during cooling
to a predefined temperature. The semi-solid mixture may be held at
this temperature (for a few seconds to several minutes) to allow
the solid particles to grow into globular structures. The
semi-solid slurry having a globular structure is generally
thixotropic which enhances its mould filling capabilities.
[0046] The vessel in which the slurry is formed may be heated
and/or cooled by external means to ensure that the correct
predefined temperature is maintained. In a particularly preferred
embodiment, the temperature and weight of the fully liquid alloy is
adjusted so that when it is added to a vessel of known mass, heat
capacity and temperature, the alloy attains the desired predefined
temperature in the semi-solid region and stabilizes there for a
period of time to permit globularization of the structure.
[0047] In some preferred embodiments, some of the unsolidified
liquid alloy may be removed (e.g. by draining through a filter or
orifice) during or after the period at the predefined temperature.
This achieves a further improvement in the structure of the
semi-solid alloy and permits easier removal of the semi-solid
material to the casting machine.
[0048] The semi-solid slurry after preparation may be cast by known
shape casting techniques. Die casting is a particularly preferred
technique.
[0049] The present invention provides aluminum alloys with reduced
incidences of hot tearing. Without being bound to a particular
theory, the combination of modifiers and grain refiner addition, in
the stated amounts, controls both the primary alpha and secondary
phases of the alloy. The high degree of control of the grain size
and morphology provided by the present invention is achieved using
a narrow range of titanium diboride, preferably well distributed
within the melt. A high control of the primary alpha phase allows
for the eutectic liquid to move more freely (compared to in the
absence of titanium diboride) within the solidifying network. In
addition, the precipitation of the secondary phase, comprising the
intermetallics (such as Mg.sub.2Si in 6xxx alloys) is considered to
affect the flow of the eutectic liquid between the globular
structures and prevent feeding of any incipient hot tears.
[0050] As stated, the grain refiners and modifiers control the size
and morphology of both the secondary and primary phases. As the
primary alpha phase is generally the last phase to form during
solidification, the present invention in particular controls the
size and shape of the alpha phase to ensure that the eutectic
liquid is free to move in the solidifying network. The secondary
phase between the alpha grains affects the flow of eutectic fluid
and therefore refinement and modification of the secondary phase is
also important to ensure adequate alloy feeding and prevention of
incipient hot tears.
[0051] When grain refinement is not very efficient, hot tears are
likely to be initiated and easier to propagate. On the other hand,
if the alloy is over refined, the hot tear will be difficult to
initiate, but once it occurs, it propagates more easily.
Consequently, when refinement is done properly, the start up of the
hot tears is more difficult and they propagate with difficulty.
EXAMPLES
Example 1
[0052] Samples of two base alloys of the Al--Mg--Si type, two of
the Al--Cu and one of the Al--Mg type were prepared as follows
(compositions in weight percent)
TABLE-US-00001 Base Base Base Alloy Alloy B Alloy C Base Alloy D
Base Alloy E Element A (Al--Mg--Si) (Al--Mg--Si) (Al--Cu) (Al--Cu)
(Al--Mg) Si 0.7% 1.2% 0.5% 0.05% 0.20% Fe 0.12% 0.12% 0.5% 0.07%
0.35% Cu 0.25% 0.25% 4.5% 4.8% 0.15% Mn <0.01% <0.01% 0.6%
0.40% 0.35% Mg 1.10% 1.10% 1.5% 0.28% 4.5% Cr 0.07% 0.07% 0.07%
0.10% Zn <0.002% <0.002% 0.25%
[0053] To these base alloys, varying amounts of Sr, TiB.sub.2 and
excess Ti were added. The Ti and Sr were added to the casting
furnace in the form of aluminium master alloys. The TiB.sub.2 was
added as an Al-5Ti-1B grain refiner rod in the casting ladle
immediately before casting to prevent settling of the TiB.sub.2.
This also increased the amount of Ti slightly because of the excess
Ti in the grain refiner rod and this amount was added to the Ti in
the stated alloy compositions. These alloys were cast from the
fully liquid state into a Constrained Rod Casting (CRC) mould. The
hot tearing sensitivity index was measured in each case using the
following method.
[0054] Cracks on the CRC cast bars were inspected visually. Five
categories of hot tear severity are described below and an index
number Cj for that crack assigned:
[0055] No crack: A casting that appears to be crack free
(Cj=0);
[0056] Hairline crack: A hairline crack that extends over half the
circumference of the bar (Cj=1);
[0057] Light crack: A hairline crack that extends over the entire
circumference of the bar (Cj=2);
[0058] Severe crack: A crack that extends over the entire
circumference of the bar (Cj=3); and
[0059] Complete crack: A complete or almost separation of the bar
(Cj=4).
[0060] Two different CRC moulds were used with cast bars (A to D)
of different length and these were assigned length parameters as
shown in the following tables:
TABLE-US-00002 (a) Mould (b) Mould CRC-1 CRC-2 A (2.0) 4 B (3.5) 3
B (6.5) 3 C (5.0) 2 C (8.5) 2 D (6.5) 1 D (10.5) 1
[0061] The value of HTS for a sample is then given by:
HTS = i = A D ( C i .times. L i ) ##EQU00001##
[0062] Where "C" is the assigned numerical value for the severity
of the crack in the bars, "L" is the assigned numerical value
corresponding to the length of the bar, and "i" represents the bars
A, B, C, and D. The HTS value for the specific alloy compositions
was the average value of the five castings that were cast using the
CRC mould.
[0063] The following results were obtained (all additions are in
weight percent):
TABLE-US-00003 Hot Equiv- Ti Total B Added Tearing alent Alloy Base
(excess) Ti (wt Sr Sensitivity TiB.sub.2 Number Alloy (wt %) (wt %)
%) (wt %) Index (wt %) A1 A 0 0.000 0 0 14.50 0.000 A2 A 0.002
0.005 0.001 0.002 10.50 0.003 A3 A 0.052 0.055 0.001 0.005 12.80
0.003 A4 A 0.152 0.155 0.001 0.020 17.00 0.003 A5 A 0.202 0.205
0.001 0 16.80 0.003 A6 A 0.004 0.010 0.002 0.005 14.80 0.006 A7 A
0.154 0.160 0.002 0 17.80 0.006 A8 A 0.204 0.210 0.002 0.002 16.20
0.006 A9 A 0.105 0.115 0.003 0 11.40 0.010 A10 A 0.155 0.165 0.003
0.002 17.20 0.010 A11 A 0.205 0.215 0.003 0.005 14.50 0.010 A12 A
0.107 0.120 0.004 0.002 15.80 0.013 A13 A 0.157 0.170 0.004 0.005
12.80 0.013 A14 A 0.207 0.220 0.004 0.010 18.00 0.013 A15 A 0.102
0.105 0.001 0.010 11.60 0.003 A16 A 0.054 0.060 0.002 0.010 8.00
0.006 A17 A 0.104 0.110 0.002 0.020 11.80 0.006 A18 A 0.055 0.065
0.003 0.020 10.40 0.010 A19 A 0.007 0.020 0.004 0.020 8.80 0.013 B1
B 0 0.000 0.000 0 17.40 0.000 B2 B 0.004 0.010 0.002 0.005 9.00
0.006 B3 B 0.154 0.160 0.002 0.010 6.00 0.006 B4 B 0.105 0.115
0.003 0 15.60 0.010 B5 B 0.155 0.165 0.003 0.005 12.20 0.010 B6 B
0.157 0.170 0.004 0 9.20 0.013 B7 B 0.054 0.060 0.002 0.010 4.40
0.006 B8 B 0.104 0.110 0.002 0.020 3.20 0.006 B9 B 0.005 0.015
0.003 0.010 1.40 0.010 B10 B 0.055 0.065 0.003 0.020 4.00 0.010 B11
B 0.007 0.020 0.004 0.020 3.80 0.013 B12 B 0.057 0.070 0.004 0.010
4.00 0.013 C1 C 0 0.000 0.000 0 16.40 0.000 C2 C 0.052 0.055 0.001
0.005 17.20 0.003 C3 C 0.152 0.155 0.001 0.005 15.60 0.003 C4 C
0.152 0.155 0.001 0.020 15.75 0.003 C5 C 0.202 0.205 0.001 0.010
18.00 0.003 C6 C 0.004 0.010 0.002 0.005 16.20 0.006 C7 C 0.154
0.160 0.002 0 19.80 0.006 C8 C 0.204 0.210 0.002 0.002 20.60 0.006
C9 C 0.105 0.115 0.003 0 23.20 0.010 C10 C 0.102 0.105 0.001 0.010
12.80 0.003 C11 C 0.054 0.060 0.002 0.010 13.20 0.006 C12 C 0.104
0.110 0.002 0.020 16.20 0.006 C13 C 0.005 0.015 0.003 0.010 12.00
0.010 C14 C 0.055 0.065 0.003 0.020 8.80 0.010 C15 C 0.105 0.115
0.003 0.010 15.40 0.010 C16 C 0.007 0.020 0.004 0.010 15.20 0.013
C17 C 0.007 0.020 0.004 0.020 10.40 0.013 C18 C 0.057 0.070 0.004
0.010 11.80 0.013 C19 C 0.057 0.070 0.004 0.020 6.90 0.013 D1 D 0
0.000 0.000 0 16.50 0.000 D2 D 0.052 0.055 0.001 0.005 21.33 0.003
D3 D 0.152 0.155 0.001 0.005 18.50 0.003 D4 D 0.152 0.155 0.001
0.020 12.50 0.003 D5 D 0.202 0.205 0.001 0.010 12.25 0.003 D6 D
0.004 0.010 0.002 0.005 11.60 0.006 D7 D 0.204 0.210 0.002 0.002
12.25 0.006 D8 D 0.207 0.220 0.004 0.010 14.25 0.013 D9 D 0.102
0.105 0.001 0.010 9.00 0.003 D10 D 0.054 0.060 0.002 0.010 8.40
0.006 D11 D 0.104 0.110 0.002 0.020 8.00 0.006 D12 D 0.005 0.015
0.003 0.010 7.00 0.010 D13 D 0.055 0.065 0.003 0.020 10.60 0.010
D14 D 0.105 0.115 0.003 0.010 9.00 0.010 D15 D 0.007 0.020 0.004
0.010 7.50 0.013 D16 D 0.007 0.020 0.004 0.020 8.00 0.013 D17 D
0.057 0.070 0.004 0.010 8.20 0.013 D18 0.057 0.070 0.004 0.020 9.00
0.013 E1 E 0 0.000 0.000 0 9.40 0.000 E2 E 0.052 0.055 0.001 0.005
3.20 0.003 E3 E 0.152 0.155 0.001 0.020 6.40 0.003 E4 E 0.202 0.205
0.001 0.010 4.40 0.003 E5 E 0.004 0.010 0.002 0.005 2.40 0.006 E6 E
0.204 0.210 0.002 0.002 5.00 0.006 E7 E 0.105 0.115 0.003 0 5.00
0.010 E8 E 0.207 0.220 0.004 0.010 3.20 0.013 E10 E 0.102 0.105
0.001 0.010 3.40 0.003 E11 E 0.054 0.060 0.002 0.010 1.40 0.006 E12
E 0.104 0.110 0.002 0.020 2.60 0.006 E13 E 0.005 0.015 0.003 0.010
1.20 0.010 E14 E 0.055 0.065 0.003 0.020 0.80 0.010 E15 E 0.007
0.020 0.004 0.010 0.80 0.013 E16 E 0.007 0.020 0.004 0.020 1.00
0.013 E17 E 0.057 0.070 0.004 0.010 0.40 0.013 E18 E 0.057 0.070
0.004 0.020 0.60 0.013
[0064] In the above table, the boron is present as TiB.sub.2. Total
Ti is the total amount of Ti from all sources and Ti (excess) is
the amount of Ti that is not bound up in TiB.sub.2.
[0065] Alloys A15 to A19, B7 to B12, C10 to C19, D9 to D18 and E10
to E18 represent alloys within the inventive range of additives,
whereas the remaining alloys are outside the range. Alloys A1, B1,
C1, D1 and E1 represent the base alloys with no additive
elements.
[0066] The Hot Tearing Susceptibility Index for the alloys within
the inventive range is less than those outside the range. The
change is sufficient that in actual castings the presence of cracks
due to hot tearing in the inventive alloys is substantially
reduced.
Example 2
[0067] Die cast parts in the form of U-shaped sections (simulated
suspension arm) were prepared from Base Alloys A and B and several
modifications having the preferred compositions of this
application.
[0068] Samples were produced using both a liquid alloy die casting
process and the preferred semi-solid process described above, in
which a mass of alloy above the liquidus was cooled rapidly to a
temperature in the semi-solid region, the temperature determined by
the relative masses and temperatures of the alloy and holding
crucible, holding the mass of metal for a period of time then
draining a portion of the remaining liquid from the crucible before
casting. The amount of liquid alloy drained from the semi-solid
mass prior to casting was from 15 to 18%. The cast parts were heat
treated by two different processes.
[0069] Treatment I: Heat Treatment: 3 h@530.degree. C.+Quench+18
h@160.degree. C.
[0070] Treatment II: Heat Treatment: 3 h@530.degree. C.+Quench+8
h@170.degree. C.
[0071] Liquid die penetrant analysis was used to determine a hot
tearing index for the cast parts. The straight side sections of the
U shape was examined and a hot tearing index determined for these
locations. Tensile tests were run on samples cut from the same
locations. The hot tearing index is a semi-quantitative index which
assigned a score for each defect found in the locations examined. A
score of 0 means no defect, a score of 1 means a point defect (no
propagation), a scope of 2 means propagation length less than or
equal to the width of the side section, and a score of 3 means
propagation length greater than the with of the side section. The
Index was the sum of these scores for four locations along the
straight side sections of the U shape.
TABLE-US-00004 Hot Tearing UTS (MPa) YS (MPa) % El Alloy
Preparation Index Treatment I Treatment II Treatment I Treatment II
Treatment I Treatment II A1 Liquid 11.33 301 318 260 272 6.7 12.5
Semi- 11.00 307 308 262 267 10.6 11.7 solid A19 Liquid 8.71 311 322
267 286 9.5 7.6 Semi- 4.77 298 299 250 257 12.0 13.1 Solid A16
Liquid 9.13 314 316 265 286 14.0 8.4 Semi- 2.00 304 295 253 248
12.7 11.2 solid B9 Liquid 6.00 328 334 289 294 5.2 8.7 Semi- 7.00
332 331 295 297 7.7 10.3 solid
[0072] The results indicate a lower hot tearing susceptibility for
the inventive alloys A19, A16 and B9 compared to the base alloy A1.
Generally then parts cast using semi-solid processing had better
hot tearing performance that the liquid cast alloys and semi-solid
processing resulted in tensile properties less sensitive to the
heat treatment than those produced by liquid processing.
Example 3
[0073] Die cast parts were prepared from the inventive alloy A16.
Samples were produced using the preferred semi-solid process
described above, in which a mass of alloy above the liquidus was
cooled rapidly to a temperature in the semi-solid region, the
temperature determined by the relative masses and temperatures of
the alloy and holding crucible, holding the mass of metal for a
period of time without draining any remaining liquid from the
crucible before casting. The cast parts were heat treated by the
following process.
[0074] Treatment III: Heat Treatment: 8 h@540.degree. C.+Quench+6
h@170.degree. C.
[0075] Tensile tests were done and results are seen in the
following table.
TABLE-US-00005 UTS (MPa) YS (MPa) % El Hot Tearing Treatment
Treatment Treatment Alloy Preparation Index III III III A16 Semi-
2.00 339 296 15.4 solid
[0076] The results indicate a lower hot tearing susceptibility for
the inventive alloy A16 compared to the base alloy A1. Moreover,
results confirm that this inventive alloy A16 has elasticity limits
and mechanical strengths in orders of 5 to 10% above the typical
values of the 6061 wrought alloy. Also, from fatigue test
performed, the inventive alloy A16 in semi-solid has a fatigue life
length similar to the 6061 wrought alloy.
* * * * *