Heat treatable Al-Zn-Mg-Cu alloy for aerospace and automotive castings

Abstract

A heat treatable aluminum alloy for shaped castings includes from about 3.5-5.5% Zn, from about 1-3% Mg, about 0.05-0.5% Cu, and less than about 1% Si.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser. No. 60/564,813 filed on Apr. 22, 2004, which is fully incorporated herein by reference thereto. It is also closely related to the patent application “A Heat Treatable Al—Zn—Mg Alloy for Shaped Castings” filed concurrently with this application, and which is also incorporated herein by reference thereto.

FIELD OF THE INVENTION

This invention is an aluminum alloy for aerospace and automotive shaped castings, castings comprised of the alloy, and methods of making cast components of the alloy.

BACKGROUND OF THE INVENTION

Cast aluminum parts are used in structural applications in automobile suspensions to reduce weight. The most commonly used group of alloys, Al—Si₇—Mg, has well established strength limits. In order to obtain lighter weight parts, higher strength material is needed with established material properties for design. At present, cast materials made of A356.0, the most commonly used Al—Si₇—Mg alloy, can reliably guarantee ultimate tensile strength of 290 MPa (42,060 psi), and tensile yield strength of 220 MPa (31,908 psi) with elongations of 8% or greater.

A variety of alternate alloys exist and are registered that exhibit higher strength than the Al—Si₇—Mg alloys. However these exhibit problems in castability, corrosion potential or fluidity that are not readily overcome. The alternate alloys are therefore less suitable for use.

Where high strength is required, forged products are often used. These are usually more expensive than cast products. There exists the potential for considerable cost savings if cast products can be used to replace forged products with no loss of strength, elongation, corrosion resistance, fatigue strength, etc. This is true in both automotive and aerospace applications.

Casting alloys exhibiting higher tensile strength and fatigue resistance than the Al—Si₇—Mg material are desirable. Such improvements could be used to reduce weight in new parts or in existing parts which can be redesigned to use the improved material properties to great advantage.

Introduction to the Invention

The alloy of the present invention is an Al—Zn—Mg base alloy for low pressure permanent or semi-permanent mold, squeeze, high pressure die, pressure or gravity casting, lost foam, investment casting, V-mold, or sand mold casting with the following composition ranges (all in weight percent):

• Zn: about 3.5-5.5%,
• Mg: about 1-3%,
• Cu: about 0.05-0.5%,
• Si: less than about 1.0%,
• Fe and other incidental impurities: less than about 0.30%,
• Mn: less than about 0.30%.

Silicon up to about 1.0% may be employed to improve castability. Lower levels of silicon may be employed to increase strength. For some applications, manganese up to about 0.3% may be employed to improve castability. In other alloys, manganese is to be avoided.

The alloy may also contain grain refiners such as titanium diboride, TiB₂ or titanium carbide, TiC and/or anti-recrystallization agents such as zirconium or scandium. If titanium diboride is employed as a grain refiner, the concentration of boron in the alloy may be in a range from 0.0025% to 0.05%. Likewise, if titanium carbide is employed as a grain refiner, the concentration of carbon in the alloy may be in the range from 0.0025% to 0.05%. Typical grain refiners are aluminum alloys containing TiC or TiB₂.

Zirconium, if used to prevent grain growth during solution heat treatment, is generally employed in a range below 0.2%. Scandium may also be used in a range below 0.3%.

In the T6 temper, the alloy demonstrated 50% higher tensile yield strength than is obtainable from A356.0-T6, while maintaining similar elongations. This will allow part designs requiring higher strength than alloys which are readily available today in Al—Si—Mg alloys such as A356.0-T6 or A357.0-T6. Fatigue performance in the T6 temper is increased over the A356.0-T6 material by 30%.

SUMMARY OF THE INVENTION

In one aspect, the present invention is an aluminum alloy including from about 3.5-5.5% Zn, from about 1-3% Mg, about 0.05-0.5% Cu and it contains less than about 1% Si.

In another aspect, the present invention is a heat treatable shaped casting of an aluminum alloy including from about 3.5-5.5% Zn, from about 1-3% Mg, from about 0.05-0.5% Cu, and less than about 1% Si.

In another aspect, the present invention is a method of preparing a heat treatable aluminum alloy shaped casting. The method includes preparing a molten mass of an aluminum alloy including from about 3.5-5.5% Zn, from about 1-3% Mg, from about 0.05-0.5% Cu, and less than about 1% Si. The method further includes casting at least a portion of the molten mass in a mold configured to produce the shaped casting, permitting the molten mass to solidify, and removing the shaped casting from the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a cut surface of a cut sample of prior art A356.0 alloy cast in a shrinkage mold showing the shrinkage cracking tendency of the prior art A356.0 alloy;

FIG. 2 is a photograph, similar to FIG. 1, of a cut surface of a second sample of prior art A356.0 cast in a shrinkage mold showing the shrinkage cracking tendency of the prior art A356.0 alloy;

FIG. 3 is a photograph of a cut surface of a sample of the alloy of the present invention cast in a shrinkage mold showing a lack of shrinkage cracking; and

FIG. 4 is a photograph, similar to FIG. 3, of a cut surface of a second sample of the alloy of the present invention cast in a shrinkage mold showing a lack of shrinkage cracking.

FIG. 5 presents strength and elongation data for directionally solidified samples of the present invention in T6 condition;

FIG. 6 is a photograph of a front knuckle casting of an alloy according to the present invention, showing locations from which tensile test samples were obtained;

FIG. 7 is a plot of strength and elongation data for tensile test samples cut from the casting shown in FIG. 6 after T5 and also after T6 heat treatments;

FIG. 8 is a graph showing the S-N fatigue response (ASTM E 466 testing, R=−1) of the present alloy in T6 condition compared to the response of prior art A356.0-T6.

FIG. 9 is a graph showing staircase fatigue testing of the present alloy in T6 condition compared to the response of prior art A356.0-T6 with the mean fatigue strength for A356.0-T6.

FIG. 10 is a graph showing depth of attack after an intergranular corrosion test of the alloy of the present invention compared to the prior art alloy A356.

FIG. 11 is a photomicrograph of an alloy according to the present invention after an intergranular corrosion test, on the as cast side of the sample.

FIG. 12 is a photomicrograph of an alloy according to the present invention after an intergranular corrosion test, on a machined side of the sample.

FIG. 13 is a photomicrograph of the prior art alloy A356 after an intergranular corrosion test.

FIG. 14 is a graph presenting results of a stress corrosion test on alloys of the present invention, with varied levels of copper.

FIG. 15 is a graph showing the effect of copper and magnesium levels on stress corrosion cracking for alloys of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS AND COMPARISON WITH PRIOR ART ALLOYS

When referring to any numerical range of values herein, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum. A range of about 3.5 to 5.5 wt % zinc, for example, would expressly include all intermediate values of about 3.6, 3.7, 3.8 and 3.9%, all the way up to and including 5.3, 5.35, 5.4, 5.475 and 5.499% Zn. The same applies to each other numerical property and/or elemental range set forth herein.

Table I presents composition data for the alloys which were tested. The first and third lines showing compositions is for directionally solidified castings. The second line is for the composition used in a shaped casting. The shaped casting was the front knuckle shown in FIG. 6.

TABLE I
Alloy Composition
Composition of test samples (Weight %)
	Zn	Mg	Si	Cu	Mn	Fe	Ti	B
DS Casting	4.28	1.99	0.04	0.01	0.00	0.04	0.00	0.0005
Cast	4.2	2	<0.1	0.2	0.05	<0.1	0.06	0.02
Knuckle
DS Casting	4.57	2.03	0.04	0.31	0.04	0.05	0.06	0.02

Table II presents room temperature mechanical properties of the directionally solidified alloys having the compositions shown in the first and third data lines of Table I. The first data line in Table II is for a directionally solidified casting comprised of the alloy of the first data line in Table I after five weeks of natural ageing. The second data line in Table 2 is for the same alloy after T5 heat treatment, and the third data line is for that alloy after T6 heat treatment. The fourth and fifth data lines in Table II are for the alloy in the bottom line of Table 1, which is a high copper alloy. This alloy, also, was subjected to a T6 heat treatment.

TABLE II
DS casting room temperature mechanical properties
Temper	TYS (MPa)	UTS (MPa)	El (%)
F 5 (5 weeks natural ageing)	185	322	18
T5	245	323	14
T6	338	359	7
T6 (High Cu)	382	424	12
Cooling rate: 1° C./sec
T6 (High Cu)	372	412	9
Cooling rate: 0.2° C./sec

The development of mechanical properties of directionally solidified samples of the present invention during heat treatment is presented in FIG. 5. The composition of these samples was presented in the first data row in Table 1. The solution heat treatment was at 1030° F. (554° C.) for 8 hours, which was followed by the cold water quench, and then artificial ageing. Samples were taken out of the oven and subjected to mechanical testing after various amounts of artificial ageing. The properties measured were TYS, UTS and percent elongation. The duration of the artificial ageing was 15 hours. During the first 6 hours, the temperature was 250° F. (121° C.). For the subsequent 9 hours, the temperature was 320° F. (160° C.). Values for the TYS and UTS are referenced to the scale on the left, values of percent elongation are referenced to the scale on the right.

Table III presents data for front knuckle castings as shown in FIG. 6. This is an alloy according to the present invention, and has the composition presented in the second data row in Table 1. The locations of tensile test samples 1, 2 and 3 are indicated in FIG. 6. Tests were performed on one casting subjected to a T5 heat treatment consisting of 160° C. for 6 hours, and one casting subjected to a T6 heat treatment of solution heat treatment at 554° C. for 8 hours followed by a cold water hen by artificial ageing at 121° C. for 6 hours and 160° C. for 6 hours.

TABLE III
CS front knuckle room temperature mechanical properties
Sample				Elongation
Location	Temper	TYS MPa	US MPa	(%)
1	T5	163.4	237.9	6
2		189.6	259.2	5
3		170.3	253.0	10
1	T6	357.8	388.9	6
2		375.8	419.9	11
3		356.5	400.6	13

It is noted that in Table III, extremely high tensile strength values and good elongation are obtained for the alloy in both T5 and T6 tempers. It is noted, again, that the composition was as presented in the second data line in Table I. The data presented in Table III are plotted in FIG. 7.

The graph in FIG. 8 shows the S-N fatigue response of the alloy of the present invention in comparison to the response of the prior art alloy A356.0-T6. This test was ASTM E466, R=−1. It can be seen that after 100,000 cycles, the alloy of the present invention is markedly superior to the prior art alloy. FIG. 9 is a graph showing staircase fatigue testing of the alloy of the present invention in T6 condition compared to the response of the prior art alloy, A356.0-T6 with a calculated mean value for A356.0-T6. The composition of the alloy of the present invention was as presented in the second data row of Table 1.

The samples were solution heat treated at 526° C. or 554 ° C., quenched and artificially aged at 160° C. for 6 hours. As seen earlier, the fatigue response of these samples is appreciably improved when compared to A356.0-T6 material.

The mean fatigue strength of the alloy of the present invention was 109.33 MPa with a standard deviation of 9.02 MPa. The standard deviation of the mean fatigue strength was 3.01 MPa. The calculated mean fatigue strength at 10⁷ cycles of A356.0 T6 is 70 MPa.

Corrosion resistance of the alloy of the present invention was tested using the ASTM G110 corrosion test, which is the “Standard Practice for Evaluating Intergranular Corrosion Resistance of Heat Treatable Aluminum Alloys by Immersion in Sodium Chloride+Hydrogen Peroxide Solution”.

In this test, specimens are immersed in a solution that contains 57 g/L NaCl and 10 mL/L H₂O₂ (30%) for 6-24 hours. The specimens are then cross-sectioned and examined under optical microscope for type (intergranular corrosion or pitting) and depth of corrosion attack.

FIG. 10 is a graph presenting the depth of attack following the ASTM G110 corrosion test after 6 hours and 24 hours for an alloy according to the present invention and for the alloy A356.0.

FIGS. 11 and 12 are photomicrographs of an alloy according to the present invention after 24 hours exposure to the ASTM G110 corrosion test. Very little intergranular corrosion can be seen in these photomicrographs.

FIG. 13 is a photomicrograph of the A356.0 alloy after 24 hours of exposure to the ASTM G110 corrosion test. Considerable intergranular corrosion can be seen in this photomicrograph.

Corrosion tests were also performed employing the ASTM G44 test, which is the “Standard Practice for Exposure of Metals and Alloys by Alternate Immersion in Neutral 3.5% Sodium Chloride Solution”. In this test, stressed specimens are subject to a 1-hour cycle which includes immersion in 3.5% NaCl solution for 10 minutes and then in lab air for 50 minutes. This 1-hour cycle is continuously repeated. During the test, the specimens are regularly inspected for cracks and failures.

Table IV presents the compositions of various alloys according to the present invention, which were employed in ASTM G44 tests.

	TABLE IV
	Alloy Composition
Sample Number	Zn	Mg	Cu	Ti	B	Zr
59	4.23	1.5	0.29	0.029	0.0055	<0.001
86	4.41	1.49	0.23	0.036	0.0038	n/a
110	4.39	1.74	0.28	0.057	0.0129	<0.001
117	4.39	1.74	0.28	0.057	0.0129	<0.001
138	4.19	1.99	0.26	0.073	0.015	<0.001
159	4.31	1.93	0.24	0.105	0.0252	0.1127
A	4.43	2.05		0.06	0.0208
B	4.5	2	0.2	0.06	0.02	n/a
C	4.5	1.2
D	4.5	1.2

Table V presents the test results for the alloy compositions presented in Table IV.

TABLE V
ASTM G44 Test of Alloys with Various Mg and Cu Contents
Run or S		Stress	Percentage		Days to
Number	Temper	Level(MPa)	of TYS	F/N	Failure
59 (Knuckle)	T5	152.37	75%	0/5
86 (Knuckle)	T6	239.25	75%	4/5	17, 17,
					22, 28
110 (Knuckle)	T5	154.44	75%	0/5
117 (Knuckle)	T6	196.50	75%	0/5
138 (Knuckle)	T6	247.52	75%	2/5	45, 61
159 (Knuckle)	T6	270.27	75%	3/5	11, 11, 11
A	T5	186.16	75%	5/5	4, 4, 4, 7, 64
B	T6	182.02	50%	4/4	7, 11, 13, 19
C	T5	135.83	75%	0/5
D	T6	194.43	75%	0/5

FIG. 14 is a graph presenting the results of these tests. It is seen that, for alloys of the present invention, and at these high magnesium levels, increasing copper provides increased resistance to stress corrosion cracking.

FIG. 15 is a graph showing the effect of copper and magnesium levels on stress corrosion cracking for alloys of the present invention. This shows that for alloys according to the present invention which have magnesium in the range from 1.5-2%, it is desirable to include copper in the range from0.25-0.3%.

Table VI and VII present the results of plant trials in which repeated shots were made from a single liquid metal reservoir. One trial was performed on April 4, one was performed on June 4 and one on September 4. On each day, the composition for all the castings made varied very little.

Table VI presents the ranges of the compositions of samples taken on each of the test days. The compositions contained high levels of magnesium and copper, which were expected to provide exceptionally high strength levels.

TABLE VI
MCC plant trial alloy composition ranges
Date	Si	Fe	Mn	Cu	Mg	Zn	Ti	B	Zr
Apr-	0.05-0.09	0.03-0.04	0.03-0.04	0.22-0.27	1.9-2.2	4.1-4.7	0.03-0.06	0.001-0.0007	0.00-0.15
04
Jun-	0.03-0.09	0.04-0.08	0.04-0.05	0.24-0.29	1.5-2.0	4.6-4.9	0.03-0.11	0.008-0.025	0.00-0.12
04
Sep-	0.04-0.05	0.14-0.20	0.03-0.04	0.27-0.32	2.1-2.3	4.4-4.9	0.04-0.07	0.0006-0.0037	0.13-0.14
04

Table VII presents the stress data, ultimate tensile strength, tensile yield strength, and elongation for four different locations in each casting. The column for sample numbers labels the individual castings. The column for location defines individual mechanical test samples cut from the casings.

TABLE VII
Plant trial mechanical properties
		UTS	YTS	Elong.			UTS	YTS	Elong.
Sample	Location	(MPa)	(MPa)	(%)	Sample	Location	(MPa)	(MPa)	(%)
April 2004 tests	June 2004 tests
1-019	1	337	269	9.9	86	1	268	216	9.3
1-019	2	386	349	9.5	86	2	359	321	9.2
1-019	3	375	357	3.1	86	3	319	294	6.7
1-019	4	405	365	10.6	86	4	366	339	6.7
1-024	1	345	301	8.1	117	1	238	193	12.3
1-024	2	384	355	8.2	117	2	305	254	14.6
1-024	3	388	359	6.3	117	3	291	244	8.5
1-024	4	406	370	9.7	117	4	331	286	8.4
2-007	1	371	330	4.7	138	1	264	198	8.9
2-007	2	382	340	8.2	138	2	367	320	8.8
2-007	3	381	352	5.5	138	3	343	308	7.5
2-007	4	419	378	9.25	138	4	375	341	8.9
2-024	1	349	301	8.5	138	1L	373	334	6.6
2-024	2	381	340	9.25	138	1U	356	333	6.4
2-024	3	369	342	3.7	159	1	326	265	9.7
2-024	4	408	370	11.7	159	2	392	352	11.7
3-165	1	331	268	10.1	159	3	368	339	7.2
3-165	4	436	382	8.85	159	4	404	369	9.3
1-014	1	371	335	6.1	159	1L	407	359	9.2
1-014	2	392	352	8.4	159	1U	413	375	7.9
1-014	3	348	318	5.1	166	4	393	354	7.2
1-014	4	392	357	7.5
Sep 2004 Tests
79	1	326	267	12.4	129	1	330.5	270.5	11
79	2	385	338	9	129	2	351.5	296.5	5
79	3	401	356	9.8	129	3	369	316	11
79	4	387	357	5.6	129	4	387.5	352.5	6
53	1	355	303	12.9	151	1	284	269	4
53	2	396	343	10.8	151	2	349	302	10
53	3	396	349	9.5	151	3	380.5	334.5	10
53	4	404	371	5.7	151	4	378	365.5	4
48	1	361	305	11.6	152	1	364.5	311.5	10
48	2	393	342	11.1	152	2	386	349	6
48	3	395	350	8.85	152	3	351	320	3

It is noted that at these high levels of magnesium and copper, excellent strength levels are obtained, with good elongation.

Having described the presently preferred embodiments of the present invention, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.

Claims

1. A heat treatable aluminum alloy for shaped castings, said aluminum alloy comprising, in weight percent, alloying ingredients as follows:

Zn: about 3.5-5.5%;

Mg: about 1-3%;

Cu: about 0.05-0.5%;

Si: less than about 1%.

2. An aluminum alloy according to claim 1 further comprising at least one grain refiner selected from the group consisting of boron, carbon and combinations thereof.

3. An aluminum alloy according to claim 2, wherein said at least one grain refiner includes boron in a range from about 0.0025 to about 0.05%.

4. An aluminum alloy according to claim 2, wherein said at least one grain refiner includes carbon in a range from about 0.0025 to about 0.05%.

5. An aluminum alloy according to claim 1 further comprising at least one anti-recrystallization agent selected from the group consisting of zirconium, scandium and combinations thereof.

6. An aluminum alloy according to claim 5 wherein said at least one anti-recrystallization agent includes zirconium in a range below 0.2%.

7. An aluminum alloy according to claim 5 wherein said at least one anti-recrystallization agent includes scandium in a range below 0.3%.

8. An aluminum alloy according to claim 1 wherein said zinc is at a concentration of about 4.2 to 4.8%.

9. An aluminum alloy according to claim 1 wherein said magnesium is at a concentration of about 1.7 to 2.3%.

10. An aluminum alloy according to claim 8 wherein said copper is at a concentration of about 0.25-0.3%.

11. An aluminum alloy according to claim 10 wherein said copper is at a concentration of about 0.27-0.28%.

12. An aluminum alloy according to claim 1 wherein a concentration of iron in said alloy is less than about 0.3%.

13. An aluminum alloy according to claim 1 wherein a concentration of manganese in said alloy is less than about 0.3%.

14. A shaped casting of an aluminum alloy, wherein said alloy comprises alloying ingredients as follows:

Zn: about 3.5-5.5%;

Mg: about 1-3%;

Cu: about 0.05-0.5%;

Si: less than about 1%.

15. A shaped casting according to claim 14 after T5 heat treatment.

16. A shaped casting according to claim 14 after T6 heat treatment.

17. A shaped casting according to claim 14 wherein said zinc is at a concentration of about 4.2-4.8%.

18. A shaped casting according to claim 14 wherein said magnesium is at a concentration of about 1.8-2.2%.

19. A shaped casting according to claim 14 wherein said copper is at a concentration of about 0.25-0.3%.

20. A shaped casting according to claim 14 wherein said copper is at a concentration of about 0.27-0.28%.

21. A method of making an aluminum alloy shaped casting, said method comprising:

preparing a molten mass of an aluminum alloy, said alloy comprising alloying ingredients as follows:

Zn: about 3.5-5.5%;

Mg: about 1-3%;

Cu: about 0.05-0.5%;

Si: less than about 1%;

casting at least a portion of said molten mass in a mold configured to produce said shaped casting;

permitting said molten mass in said mold to solidify;

removing said shaped casting from said mold.

22. A method according to claim 21 further comprising subjecting said shaped casting to a T5 heat treatment.

23. A method according to claim 21 further comprising subjecting said shaped casting to a T6 heat treatment.

24. A method according to claim 21 wherein said zinc is at a concentration of about 4.2-4.8%.

25. A method according to claim 21 wherein said magnesium is at a concentration of about 1.8-2.2%.

26. A method according to claim 21 wherein said copper is at a concentration of about 0.25-0.3%.

27. A method according to claim 26 wherein said copper is at a concentration of about 0.27-0.28%.