US20100180989A1

US20100180989A1 - Aluminum alloy

Info

Publication number: US20100180989A1
Application number: US12/656,691
Authority: US
Inventors: Zaki Ahmad; Anwar Ul-Hamid; B.J. Abdul-Aleem
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-06-23
Filing date: 2010-02-12
Publication date: 2010-07-22
Also published as: US20070297936A1

Abstract

The aluminum alloy is an aluminum-magnesium-scandium-zirconium alloy having a long term corrosion resistance combined with high strength as compared to standard AA 5052 alloy, and is suitable for use in marine and salt water environments with a minimum of corrosion. The aluminum alloy contains about 2.2-3.0 wt. % magnesium, about 0.1-0.97 wt. % scandium, and about 0.14-0.9 wt. % zirconium. The alloy may also contain about 0.1-0.4% wt. % iron, 0.001-0.2 wt. % chromium, 0.02-0.94 wt. % titanium, and silicon, copper, zinc and manganese up to about 0.20 wt. %, 0.1 wt. %, 0.1 wt. %, and 0.01 wt. %, respectively, either as additives intentionally added during processing or as impurities, the remainder being aluminum.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an aluminum alloy with magnesium, scandium and zirconium contents in the range of 2.2-3.0 wt. %, 0.1-0.97 wt. % and 0.14-0.9 wt. %, respectively, in the form of extruded products suitable for applications that require a combination of strength and corrosion resistance. More specifically the alloy is suitable for applications requiring service within seawater or in marine or industrial environments. Further the invention relates to a method of manufacturing the alloy.
2. Description of the Related Art
Several efforts have been made in the past to lower the density of Al—Mg (5XXX series) alloys to the level of Al—Zn—Mg—Cu (Al 7075) alloys and elevate the strength to the levels of precipitation hardening alloys. Although Al—Mg alloys with substantially high magnesium content (10 wt. %) approach the strength of precipitation-hardened alloys, such alloys create enormous processing difficulties and exhibit stress corrosion cracking. Since, for practical purposes, the amount of magnesium that can be retained in solid solution at room temperature is around 3 wt. %, any higher amount results in the precipitation of Al—Mg intermetallics at the alloy grain boundaries. This makes the alloy susceptible to corrosion damage. Therefore, the need exists for aluminum alloys with magnesium content lower than 3 wt. % to counter corrosion attack.
Aluminum alloy AA5052 is such an alloy, which has good corrosion resistance to seawater and marine and industrial atmosphere. However, due to relatively lower magnesium content, it exhibits strength of only medium high level. An increase in strength levels of AA5052 without an appreciable decrease in its corrosion resistance will widen its field of applications and allow more flexibility in the use of material dimensions for various components.
Aluminum-magnesium alloys are lower in density compared to 2000 and 7000 series alloys and are weldable by conventional fusion techniques. To increase the strength of aluminum-magnesium alloys, small amounts of scandium are added to the alloy. Scandium combines with aluminum in spherical configuration, producing dispersoids that stabilize the structure, and pushing the strength of aluminum-magnesium-scandium alloys to the level of precipitation-hardened alloys. The Al₃Sc (LI_z) phase forms a fine dispersion of spherical particles, which provides a substantial increase in strength. Scandium reinforced aluminum alloys exhibit a high degree of grain refinement and weld strengthening, a high resistance to hot cracking in welds, and inhibition of recrystallization up to 600° C.
The addition of zirconium reduces the amount of scandium required to strengthen aluminum-magnesium-scandium alloys. Zirconium tends to provide stability to the dispersoids during high temperature operations, thereby maintaining alloy strength. In the absence of zirconium, the Al₃Sc dispersoids tend to grow in size at high temperatures and lose their ability to inhibit recrystallization.
While several aluminum-magnesium alloys with scandium and/or zirconium have been studied, none show the particular composition ranges of the present aluminum alloy. Thus, an aluminum alloy solving the aforementioned problems is desired.

SUMMARY OF THE INVENTION

The aluminum alloy is an aluminum-magnesium-scandium-zirconium alloy having a long term corrosion resistance combined with high strength as compared to standard AA 5052 alloy, and is suitable for use in marine and salt water environments with a minimum of corrosion. The aluminum alloy contains about 2.2-3.0 wt. % magnesium, about 0.1-0.97 wt. % scandium, and 0.14-0.9 wt. % zirconium. The alloy may also contain about 0.1-0.4% wt. % iron, 0.001-0.2 wt. % chromium, 0.02-0.94 wt. % titanium, and silicon, copper, zinc and manganese up to about 0.20 wt. %, 0.1 wt. %, 0.1 wt. %, and 0.01 wt. %, respectively, either as additives intentionally added during processing or as impurities, the remainder being aluminum.
Testing has shown that the addition of scandium provides greater strength than aluminum alloys of comparable magnesium content through the formation of fine, evenly distributed aluminum-scandium dispersoids, while the addition of zirconium prevents dispersoid coarsening at elevated temperatures, thereby providing flexibility in forming products with the alloy. Electron microscopy and polarization studies show reduced corrosion upon exposure to salt water, with mainly surface crystallographic pitting. The reduced corrosion is thought to be due, in part, to the formation of a protective boehmite layer on the surface, probably resulting from the fine, homogenous distribution of precipitates in the microstructure of the alloy. The aluminum alloy may be used in cast or wrought form, but is preferably used for the production of high strength, corrosion resistant extruded aluminum products.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a scanning electromicrograph of an age-hardened aluminum-magnesium-zirconium-scandium alloy (X300) (Alloy 3).

FIG. 1B is a Scanning Electron Microscope/Electron Detector System spectrograph obtained from a precipitate in an aluminum-magnesium-zirconium-scandium alloy (Alloy 3).

FIG. 2 is a transmission electromicrograph showing Al₃Sc precipitates and dislocations at the grain boundaries of an aluminum-magnesium-zirconium-scandium alloy (Alloy 3) according to the present invention.

FIG. 3 is a chart showing the effect of scandium and zirconium addition on the tensile and yield strength of aluminum-magnesium-zirconium-scandium alloys.

FIG. 4 is a graph of the corrosion rate vs. time of various alloys of aluminum with magnesium, scandium and zirconium upon exposure to a 3.5 wt. % solution of NaCl.

FIG. 5 is a chart showing the corrosion rate vs. time of an Al—Mg-0.9% Sc-0.14% Zr alloy upon exposure to a 3.5 wt. % solution of NaCl.

FIG. 6 is a chart showing potentiodynamic polarization curves of various Al—Mg—Sc—Zr alloys in a 3.5 wt % solution of NaCl.

FIG. 7 is an electromicrograph showing crystallographic pitting on the surface of an Al—Mg-0.14% Zr alloy (Alloy 2) upon exposure to a 3.5 wt. % solution of NaCl (X1000).

FIG. 8 is an electromicrograph showing crystallographic pitting on the surface of an Al—Mg-0.15% Sc—Zr alloy (Alloy 3) upon exposure to a 3.5 wt. % solution of NaCl (X1000).

FIG. 9 is an electromicrograph showing mud cracking of a thick non-barrier oxide layer and the formation of pits on the surface of an Al—Mg-0.9% Sc—Zr alloy (Alloy 6) upon exposure to a 3.5 wt. % solution of NaCl (X1000).

FIG. 10 is an electromicrograph showing elliptical oxide growth around a white precipitate in an Al—Mg-0.9% Sc—Zr alloy (Alloy 6) upon exposure to a 3.5 wt. % solution of NaCl.

FIG. 11 is a graph of corrosion rate vs. scandium content in Al—Mg—Sc—Zr alloys.

Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is an aluminum-magnesium-scandium-zirconium alloy having a long term corrosion resistance combined with high strength as compared to standard AA 5052 alloy, and is suitable for use in marine and salt water environments with a minimum of corrosion. The aluminum alloy contains about 2.2-3.0 wt. % magnesium, about 0.1-0.97 wt. % scandium, and 0.14-0.9 wt. % zirconium. The alloy may also contain about 0.1-0.4 wt. % iron, 0.001-0.2 wt. % chromium, 0.02-0.94 wt. % titanium, and silicon, copper, zinc and manganese up to about 0.20 wt. %, 0.1 wt. %, 0.1 wt. %, and 0.01 wt. %, respectively, either as additives intentionally added during processing or as impurities, the remainder being aluminum.
All references to alloy compositions herein are in weight percent unless otherwise indicated. References to any numerical range of values include each and every number and/or fraction between the stated range minimum and maximum.
Specifically the invention includes aluminum-magnesium-scandium-zirconium alloys and the products made therefrom, preferably in the form of an extrusion, whose weight % composition comprises Mg 2.2-3.0, Sc 0.1-0.97, Zr 0.14-0.9, Mn 0.0032-0.01, Cu 0.002-0.1, Zn 0.006-0.1, Si 0.08-0.20, Fe 0.15-0.4, Cr 0.001-0.15, Ti 0.02-0.94, and the balance being aluminum.
By the invention we can provide aluminum-magnesium-scandium-zirconium alloy products in the form of an extrusion with similar long term corrosion resistance and improved strength levels as compared to those of the standard AA5052 alloy. Further, the alloy products in accordance with the invention have been found to show no significant increase in the rate of corrosion upon age-hardening for fifteen (15) days at 290° C.
The alloy products of the present invention have been found to exhibit only crystallographic type of pitting with small pit depths.
Further, the alloy products of the present invention have shown a strong tendency to form protective boehmite films.
The improved properties of the invention are thought to be due to a combination of different elements employed in the alloy development. An increase in strength is achieved by the addition of a combination of scandium and zirconium in the alloys. Scandium plays a role of a strengthener due to its precipitation in the form of fine and evenly distributed dispersoids (˜15 nm), while zirconium prevents dispersoid coarsening at elevated temperatures, thereby providing flexibility in product forming operations and also allowing lower product dimensions to be employed for specific applications.
Higher levels of strength are achieved by scandium containing dispersoids, which pin down alloy grain boundaries and dislocations, thus restricting their movement. This makes it hard for recovery and recrystallization processes to occur within the alloy, which results in an increase in its strength. The use of relatively low magnesium content allows retaining high corrosion resistance of the product.
It is also believed that the relatively fine size of scandium rich dispersoids results in small-sized pits, thereby reducing the possibility of damage to the product by localized corrosion. Formation and prolonged existence of a protective boehmite layer is also thought to be due to the fine microstructure formed by the alloy at its surface. Highly homogeneous boehmite films were formed due to a fine (15-100 nm), homogeneous distribution of precipitates. The protective oxide layer, therefore, remained uniform. Control of precipitate size and distribution can therefore provide some degree of control over corrosion properties.
The amount of other elements are kept to a minimum within a close range; Mn 0.0032-0.01, Cu 0.002-0.1, Zn 0.006-0.1, Si 0.08-0.20, Fe 0.15-0.4, Cr 0.001-0.15 and Ti 0.02-0.94.
Manganese is believed to stabilize the scandium-containing dispersoid, a characteristic similar to zirconium. However, a high manganese content (i.e. >0.9 weight %) can influence rolling operations. Copper higher than 0.2 weight % can deteriorate pitting corrosion resistance. High zinc content can influence casting and rolling processes. The presence of silicon, iron, chromium and titanium is limited in order to avoid formation of coarse primary products. Titanium is preferably kept below 0.03 weight %.

Example

The following is an example to illustrate how to make the aluminum alloy and the properties of the alloy. It is not intended to limit the scope of this invention in any manner.
Table I shows the chemical compositions of six alloys. Alloys 1 and 2 are comparative examples where Alloy 1 is within the AA 5052 range. Alloy 2 is within AA 5052 range but also contains zirconium. Alloys 3 to 6 are all examples of the alloy in accordance with this invention.

TABLE I

Concentration of elements (weight percent)

Alloy
Designations	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Zr	Sc

1	0.087	0.166	0.002	0.003	2.96	0.002	0.0025	0.03	—	—
2	0.11	0.153	0.002	0.002	2.90	0.0014	0.001	0.023	0.14	—
3	0.08	0.16	0.002	0.0032	2.97	0.0014	0.006	0.024	0.14	0.16
4	0.09	0.15	0.002	0.003	2.95	0.0013	0.010	0.024	0.14	0.29
5	0.10	0.16	0.002	0.003	2.96	0.001	0.002	0.021	0.14	0.62
6	0.092	0.160	0.003	0.004	2.87	0.001	0.007	0.028	0.14	0.91

Aluminum alloys were made by melting 4N Aluminum in a 60 kg induction furnace at 2 kHz frequency in a graphite crucible and adding Mg-metal, Si-metal, Zr-master alloy and Sc-master alloy to the crucible. Mixing and degassing was carried out and the melt was analyzed for any possible composition correction. The melt was then cast at a temperature of 695-705° C. into billets with a size of 60 mm×2500 mm. The billets were cut to a length of 100 mm. Homogenizing was carried out at 590° C. for 14 hours in an indirectly heated, electrically-powered annealing furnace. Preheating in an induction furnace to 400° C. was undertaken before extruding to 5 m long strip in a direct 315 MN press. Finally, cutting to desired length was accomplished.
Specimens measuring 15 mm in diameter were used for electrochemical investigations and weight loss studies. The specimens were polished with 320,400 and 600 μm SiC paper using de-mineralized water as lubricant. Final polishing was done with a 6 μm diamond paste. The specimens were washed with de-mineralized water rinsed with acetone and dried 12 hours before use. The rate of corrosion of the specimen was determined in accordance with ASTM G-31-72 Practice. Specimens in triplicate were used. The corroded specimens were treated with a mixture of Cr₂O₃and H₃PO₄at 80° C. to remove corrosion products. All specimens were treated separately in boiling benzene for five minutes and ethanol at 35° C. and 5% acetic acid at 48° C. Tafel extrapolation and polarization resistance techniques were used to determine the corrosion behavior of experimental alloys.
The specimens were immersed in the test solution for two hours prior to commencement of polarization. The polarization was commenced from the corrosion potential (E_cor) in the cathodic direction up to −1300 mV_SCEand to −400 mV_SCFin the anodic direction. The scanning rate was maintained at 1.0 mV/min. The microprocessor, fitted with a potentiostat, examined the data on both the anodic and cathodic sites to find a straight line segment that would yield a Tafel constant. Commercially available software was used to obtain the plots and electrochemical parameters. Corrosion rates are computed by input of βa, βc, E_corrand I_corrvalue in the units of mils per year or millimeter per year. The measurements were made in accordance with ASTM specification G5-87.
The specimen was immersed in seawater for forty-five minutes prior to commencement of polarization. The experiments were performed by applying a controlled potential scan over a small range of potential (±25 mV_SCE) with respect to corrosion potential. A scanning rate of 1.0 mV/min was used. The slope of the potential current function at E_corrwas used with Tafel constants βa, βc in order to determine I_corr(corrosion current) and hence the rate of corrosion.
Tensile tests were carried out in accordance with ASTM procedure. Tensile properties were calculated from the load versus displacement plots. Standard specimen dimensions were used.
The microstructural studies were conducted by a low vacuum scanning electron microscope (LV-SEM). A microanalysis system with a mapping software package for X-ray mapping was used for energy dispersion analysis.
The Al—Sc alloy is slightly hypereutectic and a very small amount of Al₃Sc could be formed prior to solidification of Al phase. Alloys containing scandium addition show rectangular white precipitates, whereas Sc-free alloys exhibit round white precipitates. FIG. 1A shows an SEM image of Alloy 3 containing 0.6 wt % Sc, in which distinct white rectangular precipitates are seen that were found to be enriched in scandium, as evidenced by the EDS spectrum of FIG. 1B. The distribution of Al₃Sc precipitate on grain boundaries is shown in the TEM micrograph of FIG. 2. The Al₃Sc precipitate was identified by EDS analysis.
Dislocation at the grain boundaries can also be observed in FIG. 2. It is difficult to resolve Al₃Sc precipitate because of their small size (˜15-25 nm). The Al₃Sc coherent precipitates appear to pin down the grain boundaries and substantially contribute to the strengthening of the alloy. The dislocation generation is also pre-dominant, as observed in the TEM micrograph.
The mechanical properties of the experimental AlMgScZr alloys are shown in Table II and FIG. 3. The lowest value of 0.2% yield strength and ultimate tensile strength (UTS) are exhibited by Alloy 1. That zirconium is a strengthener is shown by the increase in the yield strength of Alloy 1 (without zirconium and scandium) from 58 MPa to 110 MPa by addition of 0.14 wt % scandium. Scandium addition acts synergistically with zirconium and enhances the yield strength as shown by Alloys 3 and 4. The yield strength of Alloy 3 increases significantly on increasing the scandium content from 0.16% (Alloy 3) to 0.29% (Alloy 4) with a significant increase in UTS. No significant increase in the yield strength is observed on increasing the scandium content from 0.29% to 0.9%.

TABLE II

Mechanical Properties of Al—Mg—Sc—Zr Alloys

	Young's	0.2% Yield		%
Alloy Designation	Modulus	Strength	UTS	Elon-
And Name	(MPa)	(MPa)	(MPa)	gation

Alloy

1	50220	58	195	14.2
Al—Mg—Zr0.0—Sc0.0
Alloy 2	56520	110	210	10.7
Al—Mg—Zr0.14—Sc0.0
Alloy 3	58220	172	265	10.5
Al—Mg—Zr0.14—Sc0.15
Alloy 4	56460	242	322	9.1
Al—Mg—Zr0.14—Sc0.30
Alloy 5	76870	240	325	9.1
Al—Mg—Zr0.14—Sc0.60
Alloy 6	56450	220	320	10.4
Al—Mg—Zr0.14—Sc0.90

On the other hand, the yield strength is slightly lowered by addition of 0.9% scandium compared to addition of 0.6%. From previous works, it was established that the maximum amount of useful Sc addition was 0.6%, although addition up to 1.0 wt % scandium have been made to Al and AlMg alloys. The results obtained by previous investigators conclude that each 0.1% Sc addition causes an average increase in σ_UTSof 50 N/mm²up to 0.4% is principally in agreement with the results shown in Table II. A significant increase in 0.2% σ^yfrom 172 to 242 and O_ursfrom 265 to 322 MPa is shown by increasing Sc content from 0.15 to 0.29%; however, the effect is not as significant on increasing the Sc to 0.62%. No beneficial effect on the increase of σ_yand σ_UTSis observed on addition of 0.9% Sc.
Zirconium plays the role of stabilizer and strengthener. The role of zirconium is clearly observed by an increase in the strength of Alloy 1 (without scandium and zirconium) by adding 0.14% zirconium, which almost doubles σ_yand increases σ_UTS. Addition of 0.14% Zr strengthens the influence of scandium in concentrations from 0 to 0.3%. The particles of Al₃(Sc, Zr_x) formed during crystallization of molten metal are primarily responsible for enhancement of recrystallization temperature and strengthening effect. Zirconium dissolves in the Al₃Sc phase without changing the lattice cast structure. Addition of 0.1% Zr requires a lesser amount of scandium to produce the same strengthening effect. For instance, the required scandium amount is reduced from 0.5% to 0.2% due to an addition of 0.1% zirconium. Scandium does not form compounds with Mg, Zn, or Li. Also, magnesium does not enter the precipitate structure of Al₃Sc. The strengthening effect of Al₃Sc precipitate is in addition to the solution strengthening by magnesium.
The results of weight loss studies of the alloys in 3.5 wt % NaCl solution are described in Tables III and IV and FIGS. 4 and 5. Alloys 3, 4 and 5, containing 0.15,0.3 and 0.6 scandium, respectively, show a decreased loss in weight with increased exposure period. Increased scandium content up to 0.9% does not cause a significant increase in the rate of corrosion. All alloys show a decreased tendency to corrode with time due to their strong tendency for film formation. Alloys containing scandium show low rates of corrosion and no appreciable increase in corrosion rate caused by the addition of 0.16, 0.29, 0.62 and 0.91% scandium, as shown by Table III. Also, the addition of zirconium (0.14%) to Al—Mg alloys results in a slight increase of corrosion rate after 1600 hours (0.2325 vs. 0.342 mpy) and addition of 0.9% Sc slightly lowers the rate of corrosion rate (0.291 vs. 0.268 mpy).

TABLE III

Variation of Corrosion Rate of Al—Mg—Sc—Zr
Alloys with Time in 3.5% NaCl Solution

	Alloy 1	Alloy 2	Alloy 3
	Al—Mg—Zr0.0—Sc0.0	Al—Mg—Zr0.14—Sc0.0	Al—Mg—Zr0.14—Sc0.15
Time	Corrosion Rate	Corrosion Rate	Corrosion Rate

(hours)	mpy	mm/yr	mdd	mpy	mm/yr	mdd	mpy	mm/yr	mdd

200	2.1945	0.056	4.04	2.3995	0.06095	4.41508	1.597	0.0406	2.94
400	1.5305	0.039	2.82	1.9435	0.04936	3.57604	0.815	0.0207	1.50
600	1.0305	0.026	1.90	0.9696	0.02463	1.78406	0.699	0.0177	1.29
800	0.5920	0.015	1.09	0.5975	0.01518	1.0994	0.549	0.0139	1.01
1000	0.8655	0.022	1.59	1.1085	0.02816	2.03964	0.419	0.0106	0.77
1200	0.2755	0.007	0.51	0.5225	0.01327	0.9614	0.333	0.0085	0.61
1400	0.3825	0.010	0.70	0.423	0.01074	0.77832	0.306	0.0078	0.56
1600	0.2325	0.006	0.43	0.342	0.00869	0.62928	0.237	0.0060	0.44

	Alloy 4	Alloy 5	Alloy 6
	Al—Mg—Zr0.14—Sc0.30	Al—Mg—Zr0.14—Sc0.60	Al—Mg—Zr0.14—Sc0.90
Time	Corrosion Rate	Corrosion Rate	Corrosion Rate

(hours)	mpy	mm/yr	mdd	mpy	mm/yr	mdd	mpy	mm/yr	mdd

200	1.663	0.0422	3.06	3.998	0.1015	0.1015	1.5970	0.0406	2.94
400	1.230	0.0312	2.26	2.130	0.0541	0.0541	0.9405	0.0239	1.73
600	0.588	0.149	1.08	0.876	0.0223	0.0223	1.0300	0.0262	1.90
800	0.474	0.0120	0.87	0.790	0.0201	0.0201	0.8930	0.0277	1.64
1000	0.359	0.0091	0.66	0.586	0.0149	0.0149	0.6900	0.0175	1.27
1200	0.322	0.0082	0.59	0.577	0.0146	0.0146	0.8385	0.0213	1.54
1400	0.314	0.0080	0.58	0.532	0.0135	0.0135	0.7385	0.0188	1.36
1600	0.245	0.0062	0.45	0.299	0.0076	0.0076	0.2635	0.0067	0.48

Age hardening increases the rate of corrosion of all alloys, as shown by FIG. 5 and Table IV. Table IV shows the result of electrochemical polarization studies. Typical polarization plots are shown in FIG. 6.

TABLE IV

Summary of Electrochemical Polarization Tests of Al—Mg—Sc—Zr
Alloys in 3.5% NaCl Solution

POLARIZATION RESISTANCE

E(I = 0)

P. Res.

I_Corr

Corrosion Rate

Material	mV	KΩ/cm²	μA/cm²	mpy	mdd

AlMg—Zr0.0—Sc0.0	−903.0	7.46	8.01	3.44	6.33
AlMg—Zr0.14—Sc0.0	−764.0	18.10	2.77	1.19	2.19
AlMg—Zr0.14—Sc0.15	−916.7	8.74	4.79	2.06	3.79
AlMg—Zr0.14—Sc0.30	−835.0	7.20	7.82	3.36	6.18
AlMg—Zr0.14—Sc0.60	−828.0	6.43	8.72	3.74	6.88
AlMg—Zr0.14—Sc0.90	−865.2	5.61	9.36	4.01	7.38

Age-Hardened (4 Weeks)

AlMg—Zr0.0—Sc0.0	−847.5	9.32	6.40	2.75	5.05
AlMg—Zr0.14—Sc0.0	−861.2	4.48	13.20	5.67	10.44
AlMg—Zr0.14—Sc0.9	−894.2	6.38	6.88	2.95	5.43

TAFEL ANALYSIS

E(I = 0)

ATC

CTC

I_Corr

Corrosion Rate

Material	mV	mV/decade	mV/decade	μA/cm²	mpy	mdd

AlMg—Zr0.0—Sc0.0	−784.20	177.40	611.80	6.20	2.66	4.89
AlMg—Zr0.14—Sc0.0	−712.80	157.10	439.20	2.16	0.93	1.71
AlMg—Zr0.14—Sc0.15	−816.90	130.50	369.40	2.71	1.16	2.14
AlMg—Zr0.14—Sc0.30	−807.00	184.30	437.70	6.40	2.75	5.05
AlMg—Zr0.14—Sc0.60	−787.0	176.20	484.80	8.24	3.54	6.51
AlMg—Zr0.14—Sc0.90	−857.10	211.20	282.50	5.61	2.40	4.42

Age-Hardened (4 Weeks)

AlMg—Zr0.0—Sc0.0	−737.80	173.80	653.10	6.71	2.88	5.30
AlMg—Zr0.14—Sc0.0	−759.80	179.90	561.20	11.18	4.80	8.83
AlMg—Zr0.14—Sc0.9	−763.20	120.00	641.80	8.55	3.67	6.75

These results are in confirmation with the results obtained by weight loss technique. The lower corrosion rates obtained by weight loss techniques may be ascribed to the sufficient time that is available for the formation and growth of protective film of boehmite that has been reported to be formed on the alloy surface. Aging increases the rate of corrosion as shown by Table IV; however, the effect is not very pronounced because of the small initial size of the Al₃Sc precipitate (˜15 nm).
The surface morphology shows mainly crystallographic pitting on the Alloy 2 containing Al—Mg-0.14Zr and no scandium, as shown in FIG. 7. FIG. 8 shows crystallographic pitting on the surface of alloy (Al—Mg-0.16Sc—Zr). No evidence of hemispherical pitting was observed.
Alloy 6 containing Al—Mg-0.9Sc—Zr shows mud cracking of the thick, non-barrier oxide layer leading to formation of irregular pits, as shown in FIG. 9. Alloy 6 shows the formation of oxide film in elliptical shapes around the white rectangular precipitate containing mostly scandium element after exposure to 3.5% NaCl for 1600 hour, as shown in FIG. 10. Numerous small circular pits are observed on the elliptical shaped protective oxide growth. Mud cracking is also observed on the surface. The growth of the oxide around the rectangular precipitate is shown more clearly in FIG. 10. It appears to be related to the growth of the oxide layer with an extended period of exposure. The age-hardened samples also show crystallographic pitting. Crystallographic pitting is generally associated with low dissolution rates. Mud cracking is a common phenomenon in these alloys and related to the breakdown of film at a certain thickness. The geometry and morphology of mud cracking in the above alloys is similar.
The maximum pit depth of 1.02 μm was shown by Al-2.5Mg alloy and a minimum of 30 μm by Al—Mg-0.3Sc—Zr (Alloy 4). The maximum depth measured in Alloy 5 (AlMg-0.6Sc—Zr) and Alloy 3 (Al—Mg-0.15Sc—Zr) was 120 μm and 135 μm, respectively, and the average pit depth ranged between 60-80 μm. The maximum pit depth of AlMg-0.3Sc—Zr— was 30 μm. From the above investigation it is established that scandium addition up to 0.6% does not cause any appreciable increase in the rate of corrosion of Al—Mg—Sc—Zr alloys, and pitting is not of significance, as shown by surface morphology of the alloys investigated. Because of the small size of the precipitates, it has an advantage over alloys like 2024, 6061 and 6013, which contain large size precipitates of CuAl₂as Cu—Mg—Al₂, since large precipitates provide active sites for intensive pitting. From the above studies, it is established that scandium up to ˜0.6% can be used as a strengthener without any increased risk of corrosion, and the excellent combination of mechanical properties of Al—Mg—Sc—Zr alloy is further supported by its good resistance to corrosion.
Impingement test was performed to determine the corrosion rates of alloys in aqueous environments. Water was circulated between a PVC tank and a copper reservoir by a recirculating pump. Two nozzles of diameters 3/16″ and ¼″ were used to achieve different impingement velocities. The distance between the tip of the nozzle and the surface of the specimen was kept at 250 mm. Specimens measuring 49 mm×20 mm were used. The specimens were polished with 400, and 600 μm SiC paper. Final polishing was performed with 0.05 micron alumina powder. The specimens were then washed with de-mineralized water and rinsed with acetone and dried for 10 hours. Weight of the specimen was determined before and after the experiment. Experimental results in Table V illustrate the beneficial effect of Sc on the corrosion resistance of alloys under impingement conditions. The same results are plotted in a graph shown in FIG. 11.

TABLE V

Corrosion Rate of Alloys Obtained in Impingement Tests

	Scandium	Flow Rate	Velocity	Corrosion
NOZZLE	wt %	(4 liters)	(m/sec)	Rate (mpy)

⅛″
1	0	95 sec	5.31	271
2	0.3	90 sec	5.61	246
3	0.6	85 sec	5.94	221
3/16″
4	0	68 sec	3.31	125
5	0.3	72 sec	3.11	94
6	0.6	74 sec	3.03	82

It will be appreciated that an improved aluminum alloy with combined strength and corrosion resistance has been disclosed. This aluminum alloy has better combined strength and corrosion resistance compared to conventional Al—Mg alloys. A small amount of zirconium 0.14% allows higher strength of mechanical strength to be achieved by smaller scandium content (0.3% Sc). Microstructural studies show the presence of Al₃Sc precipitate of very small size (˜15 nm). These precipitates pin down the grain boundaries and are responsible for the alloys' strengthening effect.
The alloys also show only crystallographic pits with small pitting depths. They exhibit a strong tendency to form a protective oxide film. By virtue of an outstanding combination of strength, outstanding mechanical properties and a good resistance to corrosion, these alloys could be exploited in marine and salt water environment with a minimum risk of corrosion.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.

Claims

1. An aluminum alloy, comprising:

from about 2.2 to about 3.0 weight percent magnesium;

from about 0.1 to about 0.97 weight percent scandium;

from about 0.14 to about 0.9 weight percent zirconium;

up to about 0.20 weight percent silicon;

up to about 0.1 weight percent copper;

up to about 0.1 weight percent zinc; and

up to about 0.01 weight percent manganese, the balance being aluminum.

2. The aluminum alloy according to claim 1, further comprising from about 0.1 to about 0.4% weight percent iron.

3. The aluminum alloy according to claim 1, further comprising from about 0.001 to about 0.2 weight percent chromium.

4. The aluminum alloy according to claim 1, further comprising from about 0.02 to about 0.94 weight percent titanium.

5. The aluminum alloy according to claim 1, wherein about 0.14 weight percent zirconium is present in the alloy.

6. The aluminum alloy according to claim 5, wherein about 0.15 weight percent scandium is present in the alloy.

7. The aluminum alloy according to claim 5, wherein about 0.3 weight percent scandium is present in the alloy.

8. The aluminum alloy according to claim 5, wherein about 0.6 weight percent scandium is present in the alloy.

9. The aluminum alloy according to claim 5, wherein about 0.9 weight percent scandium is present in the alloy.

10. The aluminum alloy according to claim 5, wherein from about 2.85 to about 3 weight percent magnesium is present in the alloy.

11. The aluminum alloy according to claim 1, wherein the scandium is homogenously distributed throughout the alloy in dispersoids having a diameter of between 15 and 100 nm.

12. The aluminum alloy according to claim 1, wherein the scandium is homogenously distributed throughout the alloy in dispersoids having a diameter of about 15 nm.

13. An extruded aluminum product formed from the alloy according to claim 1.

14. A cast aluminum billet formed from the alloy according to claim 1.

15. An aluminum alloy, consisting essentially of:

from about 2.2 to about 3.0 weight percent magnesium;

from about 0.1 to about 0.97 weight percent scandium;

from about 0.14 to about 0.9 weight percent zirconium;

up to about 0.20 weight percent silicon;

up to about 0.1 weight percent copper;

up to about 0.1 weight percent zinc; and

up to about 0.01 weight percent manganese, the balance being aluminum.

16. The aluminum alloy according to claim 15, wherein about 0.14 weight percent zirconium is present in the alloy.

17. The aluminum alloy according to claim 16, wherein about 0.15 weight percent scandium is present in the alloy.

18. The aluminum alloy according to claim 16, wherein about 0.3 weight percent scandium is present in the alloy.

19. The aluminum alloy according to claim 16, wherein about 0.6 weight percent scandium is present in the alloy.

20. The aluminum alloy according to claim 16, wherein about 0.9 weight percent scandium is present in the alloy.

21. The aluminum alloy according to claim 16, wherein from about 2.85 to about 3.0 weight percent magnesium is present in the alloy.

22. The aluminum alloy according to claim 15, wherein the scandium is homogenously distributed throughout the alloy in dispersoids having a diameter of between 15 and 100 nm.

23. The aluminum alloy according to claim 15, wherein the scandium is homogenously distributed throughout the alloy in dispersoids having a diameter of about 15 nm.

24. An extruded aluminum product formed from the alloy according to claim 15.

25. A cast aluminum billet formed from the alloy according to claim 15.

26. A corrosion resistant and high strength aluminum-magnesium-scandium-zirconium alloy product, the alloy consisting essentially of:

from 2.2 to 3.0 weight percent magnesium;

from 0.1 to 0.97 weight percent scandium;

from 0.1 to 0.9 weight percent zirconium;

from 0.1 to 0.4 weight percent iron;

from 0.001 to 0.2 weight percent chromium;

from 0.02 to 0.94 weight percent titanium;

up to 0.20 weight percent silicon;

up to 0.1 weight percent copper;

up to 0.1 weight percent zinc; and

up to 0.01 weight percent manganese, the balance being aluminum and incidental impurities.

27. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains up to about 0.91 weight percent scandium.

28. The aluminum-magnesium-scandium-zirconium alloy product of claim 27, wherein the alloy contains about 0.16 to 0.9 weight percent scandium.

29. The aluminum-magnesium-scandium-zirconium alloy product of claim 28, wherein the alloy contains about 0.29 to 0.8 weight percent scandium.

30. The aluminum-magnesium-scandium-zirconium alloy product of claim 29, wherein the alloy contains about 0.45 to 0.75 weight percent scandium.

31. The aluminum-magnesium-scandium-zirconium alloy product of claim 30, wherein the alloy contains about 0.62 to 0.70 weight percent scandium.

32. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains about 2.4 to 3.0 weight percent magnesium.

33. The aluminum-magnesium-scandium-zirconium alloy product of claim 32, wherein the alloy contains about 2.7 to 2.97 weight percent magnesium.

34. The aluminum-magnesium-scandium-zirconium alloy product of claim 33, wherein the alloy contains about 2.8 to 2.95 weight percent magnesium.

35. The aluminum-magnesium-scandium-zirconium alloy product of claim 34, wherein the alloy contains about 2.87 to 2.93 weight percent magnesium.

36. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains about 0.14 to 0.8 weight percent zirconium.

37. The aluminum-magnesium-scandium-zirconium alloy product of claim 36, wherein the alloy contains about 0.3 to 0.7 weight percent zirconium.

38. The aluminum-magnesium-scandium-zirconium alloy product of claim 37, wherein the alloy contains about 0.4 to 0.6 weight percent zirconium.

39. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains about 0.12 to 0.35 weight percent iron.

40. The aluminum-magnesium-scandium-zirconium alloy product of claim 39, wherein the alloy contains about 0.15 to 0.30 weight percent iron.

41. The aluminum-magnesium-scandium-zirconium alloy product of claim 40, wherein the alloy contains about 0.16 to 0.30 weight percent iron.

42. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains about 0.05 to 0.15 weight percent chromium.

43. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains about 0.02 to 0.9 weight percent titanium.

44. The aluminum-magnesium-scandium-zirconium alloy product of claim 43, wherein the alloy contains about 0.03 to 0.7 weight percent titanium.

45. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains up to 0.20 weight percent silicon.

46. The aluminum-magnesium-scandium-zirconium alloy product of claim 45, wherein the alloy contains up to 0.08 weight percent silicon.

47. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains up to 0.09 weight percent copper.

48. The aluminum-magnesium-scandium-zirconium alloy product of claim 47, wherein the alloy contains up to 0.002 weight percent copper.

49. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains up to 0.09 weight percent zinc.

50. The aluminum-magnesium-scandium-zirconium alloy product of claim 49, wherein the alloy contains up to 0.006 weight percent zinc.

51. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy contains up to 0.009 weight percent manganese.

52. The aluminum-magnesium-scandium-zirconium alloy product of claim 51, wherein the alloy contains up to 0.0032 weight percent manganese.

53. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the alloy is chill cast in copper molds.

54. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the product is formed from an extrusion of the alloy.

55. The aluminum-magnesium-scandium-zirconium alloy product of claim 54, wherein the extrusion has a yield strength of 242 MPa and an ultimate tensile strength of 322 MPa.

56. The aluminum-magnesium-scandium-zirconium alloy product of claim 55, wherein the corrosion rate of the extrusion in 3.5% NaCl solution is 0.245 mpy.

57. The aluminum-magnesium-scandium-zirconium alloy product of claim 55, wherein the corrosion rate of the product in impinging aqueous water is 94 mpy when scandium is present at about 0.3 weight percent in the alloy.

58. The aluminum-magnesium-scandium-zirconium alloy product of claim 55, wherein the corrosion rate of the product in impinging aqueous water is 82 mpy when scandium is present at about 0.6 weight percent in the alloy.

59. The aluminum-magnesium-scandium-zirconium alloy product of claim 55, wherein the alloy has a protective layer of boehmite on the alloy's surface, whereby the alloy shows decreased corrosion rate with increased exposure period.

60. The aluminum-magnesium-scandium-zirconium alloy product of claim 55, where the scandium is distributed through the alloy in dispersoids having a diameter of about 15 nm.

61. The aluminum-magnesium-scandium-zirconium alloy product of claim 55, wherein the extrusion exhibits only crystallographic pitting with a maximum pit depth of 30 μm.

62. The aluminum-magnesium-scandium-zirconium alloy product of claim 54, wherein the extrusion has an ultimate tensile strength of up to 325 MPa.

63. The aluminum-magnesium-scandium-zirconium alloy product of claim 26, wherein the product comprises a marine product adapted for use in sea water.

64. A method for making an aluminum-magnesium-zirconium-scandium alloy, comprising the steps of:

(a) melting 4N Aluminum in a 60 kg induction furnace at medium frequency (˜2 kHz) in a graphite crucible;

(b) adding Mg-metal, Si-metal, Zr-master alloy and Sc-master alloy to the crucible to form a melt;

(c) mixing and degassing the melt;

(d) analyzing the melt to determine the composition of the melt;

(e) adding material to the melt, when necessary, to correct the composition to: from 2.2 to 3.0 weight percent magnesium, from 0.1 to 0.97 weight percent scandium, from 0.1 to 0.9 weight percent zirconium, from 0.1 to 0.4 weight percent iron, from 0.001 to 0.2 weight percent chromium, from 0.02 to 0.94 weight percent titanium, up to 0.20 weight percent silicon, up to 0.1 weight percent copper, up to 0.1 weight percent zinc, and up to 0.01 weight percent manganese, the balance being aluminum and incidental impurities;

(f) casting the melt at a temperature of 695-705° C. to form billets having a size of about 60 mm×2500 mm;

(g) cutting the billets to a length of about 100mm for extrusion;

(h) homogenizing the cut billets at 590° C. for 14 hours in an indirectly heated electrically powered annealing furnace;

(i) preheating the homogenized billets in an induction furnace to 400° C.;

(j) extruding the preheated billets in a direct 315 MN press to form a flat material; and

(k) cutting the flat material to desired length.