GB2073773A

GB2073773A - Aluminium Alloys Containing Antimony

Info

Publication number: GB2073773A
Application number: GB8012061A
Authority: GB
Original assignee: Nippon Light Metal Co Ltd
Current assignee: Nippon Light Metal Co Ltd
Priority date: 1980-04-11
Filing date: 1980-04-11
Publication date: 1981-10-21

Abstract

Aluminium alloys containing, apart from impurities, 6% to 13% by weight and preferably 7% to 11% by weight of silicon; 3% to 6% by weight and preferably 3.5% to 5.5% by weight of copper; 0.2% to 1% by weight and preferably 0.25% to 0.6% by weight of magnesium; and 0.03% to 1% by weight and preferably 0.10% to 0.60% by weight of antimony; and if required, further containing: one or more of a trace to 1.5% by weight of manganese; a trace to 1.0% by weight of chromium; a trace to 2.5% by weight of nickel; and a trace to 0.3% by weight of titanium; the balance being aluminium. These alloys are superior in castability, heat resistance, and toughness.

Description

SPECIFICATION Improved Aluminium Alloys This invention relates to improved aluminium alloys and more particularly, heat treatable high strength aluminum alloys having superior castability, toughness and heat resistance.

Aluminum materials are now used widely in various fields of industry, including the manufacture of vehicles and machinery, it is strongly desired to develop aluminum alloys having a high degree of toughness and heat resistance.

High strength aluminum alloys are known containing aluminum, copper and magnesium for casting use, for example, ALCOA X149, and ESCO KO-1 alloys developed in the United States.

Although these are very tough alloys exhibiting a tensile strength of at least 40 kg/mm2 and an elongation of about 5 to 10% when subjected to T6 treatment, they have an extremely limited scope of use, since they are liable to hot tearing or misrun during casting, and their heat resistance is low.

High strength aluminum wrought alloys, such as Al-Cu-Mg, Al-Cu-Mg-Zn and Al-Zn-Mg alloys are also known.

Although after precipitation heat treatment alloys having a tensile strength well over 40 kg/mm2 can be obtained they lack sufficient heat resistance, and are very liable to stress corrosion cracking.

Moreover, Al-Si-Cu-Mg alloys are known having a relatively high strength and heat resistance (for example the Aluminum Association's standard alloys AA 332 for casting, AA 4032 for wrought product etc). Copper increases the strength of these alloys, but if they contain 3% or more of copper, there is a sharp reduction in toughness. This, therefore, limits the maximum copper content. The alloys which are in practical use have a maximum strength only in the vicinity of 35 kg/mm2 after subjected to T6 treatment. They cannot possibly be called high strength alloys.

With the aim of developing high strength aluminum alloys having superior castability, toughness and heat resistance, it has been found that if Al-Si-Cu-Mg alloys contain more than about 3% of copper, the addition of antimony markedly improves the strength of the alloy, and its heat resistance, particularly thermal impact resistance, since the hardening of such alloy is promoted during aging.

According to this present invention we propose an aluminium alloy containing, apart from impurities, 6% to 13%, and preferably 7% to 1 1% of silicon; 3% to 6% and preferably 3.5% to 5.5% of copper; 0.2% to 1% and preferably 0.25% to 0.6% of magnesium; 0.03% to 1%, and preferably 0.10% to 0.60% of antimony; the balance being aluminum.

This aluminum alloy is well suited to casting by various methods, can be worked (i.e. can be subject to plastic deformátion) by, for example, forging or rolling and lends itself to mass production.

Further the alloy can be produced more clearly than comparable conventional alloys not least because only a very short time is required for precipitation heat treatment.

Other features of the invention are set forth in the appendant claims and in the following description of preferred embodiments of the invention. Reference is made to the accompanying drawings of which: Figure 1 is a graph showing changes in the mechanical properties of alloys according to this invention as a function of the amount of copper contained therein; Figure 2 is a graph showing changes in the hardness of the alloys in relation to the length of artificial aging time; and Figure 3 is a graph comparing the thermal fatigue resistance of the alloys.

All percentages are given as a percentage of the total weight of the alloy.

Silicon is an element which is essential to improve the strength and castability of the alloy. Less than 6% of silicon is insufficient to improve the castability of the alloy. On the other hand, more than 1 3% is undesirable, since this sharply reduces the toughness and thermal fatigue resistance of the alloy. Preferably, the alloy contains 7% to 1 1% of silicon.

Copper improves the strength of the alloy when the alloy is artifically aged. Less than 3% of copper inhibits the effect of antimony in the promotion of aging as hereinafter described, while the presence of copper in excess of 6% is undesirable in practice, since its residue increases in the mother phase of the Al-Cu intermetallic compound which is still to form a solid solution, thereby reducing the toughness and fatigue strength of the alloy, and making it more likely to crack upon casting. Preferably, the alloy contains 3.5% to 5.5% of copper.

Magnesium strengthens the alloy by forming precipitates, such as Mg2Si, when the alloy is artificially aged. Less than 0.2% of magnesium is, however, insufficient for the formation of such precipitates, while magnesium in excess of 1% seriously affects the elongation and impact resistance of the alloy. The alloy preferably contains 0.25% to 0.6% of magnesium.

Antimony is very effective in improving the strength of the alloy, as it promotes the aging of the alloy by copper when the alloy contains more than 3% of copper (see Example 1, Figure 1 and Example 2, Figure 2) Antimony effectively improves the thermal impact resistance of the alloy, too. Less than 0.03% of antimony fails to produce satisfactory results, while amounts in excess of 1% do not make any appreciable change in the results obtained. Preferably, the alloy contains 0.10% to 0.60% of antimony.

In order to improve the heat resistance of the alloy, a trace to 1.5% of Mn, a trace to 1.0% of Cr and a trace to 2.5% of Ni may be added. In the past, such additions have been used to increase the strength of this kind of alloy at a high temperature.

It is also acceptable during the casting of the alloy of this invention to add a trace to 0.3% of titanium, a trace to 0.05% of boron, or other elements in order to refine the grain of the alloy. The addition of titanium is particularly useful, since it serves to improve any shrinkage of the casting which is formed from the alloy. It is also possible to add a trace to 0.05% of beryllium in order to prevent any oxidation that may take place upon addition of magnesium.

About 1.5% of less of Fe, which may mix into the alloy as an impurity is permissible, since it does not seriously inhibit the effect of this invention.

Preferred embodiments of alloys according to the invention are set forth in the following examples. Various properties of the alloy are described with reference to individual examples but it will be understood that all the alloys of this invention possess these properties.

Example 1 Figure 1 shows the results of the tests conducted to determine the influences which the addition of antimony might have on the mechanical properties of the alloys subjected to aging treatment. The tests were conducted on Al-Si-Cu-Mg alloys containing 9% of silicon and 0.5 of magnesium, of which some further contained 0.1 5% of antimony, while the rest did not contain any antimony, by varying the amount of copper contained therein. In the graph, the ordinate represents the tensile strength and elongation of the alloys, while the abscissa represents the amount of copper. The test samples were prepared by forging and were solution heat treated at 5000C for 10 hours, quenched in water, and artificially aged at 2000C for one hour.

As shown in Figure 1 , the alloys containing no antimony showed an increasing tensile strength with an increase in the copper content while the elongation sharply dropped on the alloys containing more than 3% of copper. The alloys containing antimony and a relatively small amount of copper showed a markedly improved elongation as compared with those containing no antimony, but their strength was relatively low.-Alloys containing antimony and more than 3% of copper exhibited improved elongation and tensile strength as compared with those containing no antimony, and in particular exhibited increasing strength with an increase in the amount of copper over 3%.

It is clear, therefore that antimony promotes the aging of the alloys containing more than 3% of copper to thereby improve their strength and elongation.

Example 2 Figure 2 shows the results of the tests conducted to ascertain the effect which antimony would have in the promotion of aging of the alloys according to this invention. The tests were conducted on samples of Al-Si-Cu-Mg alloys containing 10% of silicon, 4% of copper and 0.4% of magnesium (prepared by forging), of which some contained 0.2% of antimony, while the rest did not contain any antimony. The samples were solution heat treated at 5200C for 10 hours, quenched in water, and aged artificially at a temperature of 1 800 C. The hardness of each sample was measured from time to time during the aging treatment. In the graph, the ordinate represents the Brinell hardness (HB) of the alloys, while the abscissa indicates the lapse of time.

The test results (Figure 2) show that the addition of antimony to the alloys containing more than 3% of copper promotes aging as compared with those which do not contain any antimony. In other words, the addition of antimony does not only contribute to improving the strength of the alloys, but also makes it possible to obtain high strength alloys by a very short period of heat treatment. It will, for instance, be noted from Figure 2 that the strength which the comparative samples not containing any antimony acquired after as long as 8 to 12 hours of aging could be obtained in only about one hour according to this invention. This yields a great economy of thermal energy.

Example3 Attention is now directed to the results of the tests conducted to compare the mechanical properties of alloys according to this invention, and conventional Al-Si-Cu-Mg alloys.

Table 1 shows the chemical composition of each of the alloys and Table 2 lists the mechanical properties exhibited by those alloys. In the tables, samples Nos. 1 to 7 refer to the alloys according to the invention, and among them, sample No. 7 was prepared by forging. Samples Nos. 8 and 9 are typical of the conventional Al-Si-Cu-Mg alloys.

Sample No. 7 was prepared by soaking a 100 mm dia., 300 mm long cylindrical ingot at 4800C for two hours, and upsetting it with a forging ratio of 1:9 at a temperature of 4200 C. A fine forged produced with no cracks was obtained.

Table 1 Chemical Composition (wt.%) Sample Si Cu Mg Sb Ti Mn Ni Cr 1 7.2 3.8 0.35 0.10 0.1 - - - This invnetion 2 9.1 4.2 0.45 0.15 0.1 - - - " 3 11.3 4.5 0.40 0.15 0.1 - - - " 4 9.1 4.2 0.44 0.30 0.1 0.7 - - " 5 9.1 4.8 0.45 0.45 0.1 - 1.0 - " 6 9.0 4.1 0.48 0.30 0.1 - - 0.7 " 7 9.3 5.0 0.48 0.15 0.1 - - - " (forged) 8 12.1 1.0 1.10 - 0.1 - 1.8 - AC8A 9 9.6 2.9 1.21 - 0.1 - 0.9 - AC8B Note: The balance is Al and impure matter.

Table 2 Conditions for heat treatment Tensile Yield strength strength Elongation Hardness Solution heat Quenching Aging Sample (kg/mm) (kg/mm) (%) (HB) treatment 1 46.6 40.3 3.1 148 500 C, 10 hours Water quench 170 C, 10 hours 2 49.0 45.9 2.3 150 500 C, 10 hours Water quench 160 C, 24 hours 3 44.8 38.5 3.9 146 500 C, 10 hours Water quench 200 C, 1 hour 4 44.2 39.3 3.0 146 500 C, 10 hours Water quench 200 C, 1 hour 5 43.9 39.5 2.5 148 500 C, 10 hours Water quench 200 C, 1 hour 6 44.1 39.4 2.7 147 500 C, 10 hours Water quench 200 C, 1 hour 7 47.3 39.2 5.8 147 500 C, 1 hours Water quench 180 C, 5 hours 8 35.5 33.8 0.8 122 500 C, 10 hours Water quench 170 C, 10 hours 9 37.8 35.4 0.6 128 500 C, 10 hours Water quench 170 C, 10 hours As shown in Table 2, samples Nos. 1 to 7 of the alloys according to this invention had a tensile strength of about 45 kg/mm2 or above, and an elongation greater than 2%, while the samples of the known alloys showed a tensile strength only in the vicinity of 35 kg/mm2 and an elongation of less than 1%. As is obvious from the table, the alloys of this invention showed a high degree of toughness, and particularly, samples Nos. 3 to 5 showed considerable strength despite their aging over a very short length of time (i.e., one hour at 2000).

Example4 Figure 3 shows the results of the thermal fatigue tests conducted to demonstrate the superior heat resistance of the alloys according to this invention. The tests were conducted on samples prepared by forging of the alloy of this invention containing 9.5% of Si,4.2% of Cu, 0.5% of Mg and 0.15% of Sb, and comparative samples of the same composition, but not containing any antimony, followed by solution heat treatment at 5000C for 10 hours, quenching in water, and aging at 1 700C for 10 hours.

Each sample was in the shape of a disc having a diameter of 100 mm and a thickness of 3 mm, and formed with a central hole having a diameter of 5 mm. The centre of the sample was rapidly heated using a gas burner and after the temperature of the entire sample reached 3500 C, it was quenched in water at about 200C. This cycle of rapid heating and cooling was repeated until the thermal stress created by the internal constraint of the sample caused a crack to appear at the edge of the centre hole. The number of the cycles repeated before the crack appeared, and the rate of growth of the crack were recorded to compare the thermal fatigue resistance of the alloys.

As shown in Figure 3, the alloy of this invention started to crack only after considerably more rapid heating and quenching cycles than in the case of the comparative alloy, and moreover, the crack grew at a far slower rate in the alloy of this invention.

It is clear from the foregoing description that the aluminum alloys according to this invention are tougher and more heat resistant than conventional alloys and, hence, are suitable for making machine parts, particularly pistons, pulleys and bearings. The alloys of this invention also enable considerable savings of thermal energy, as a result of the drastic reduction in the time required for aging.

Claims

1. An aluminum alloy containing, apart from impurities 6% to 13% by weight of silicon; 3% to 6% by weight of copper; 0.2% to 1% by weight of magnesium; and 0.3% to 1% by weight of antimony; the balance being aluminum.

2. An alloy according to claim 1 , further including at least one of a trace to 1.5% by weight of manganese; a trace 1.0% by weight of chromium; a trace to 2.5% by weight of nickel; and a trace to 0.3% by weight of titanium.

3. An alloy according to claim 1 or claim 2, containing 7% to 11% by weight of silicon.

4. An alloy according to claim 1 or claim 2, containing 3.5% to 5.5% by weight of copper.

5. An alloy according to claim 1 or claim 2, containing 0.25% to 0.6% by weight of magnesium.

6. An alloy according to claim 1 or claim 2, containing 0.10% to 0.60% by weight of antimony.

7. An alloy according to claim 1 or claim 2, wherein a maximum of 1.5% by weight of iron ispresent as an impurity.

8. An alloy according to claim 1 or claim 2, containing not more than 0.3% by weight of titanium.

9. An alloy according to claim 1 or claim 2, and containing a trace to 0.05% by weight of boron.

10. An alloy according to claim 1 or claim 2, and containing a trace to 0.05% by weight of beryllium.

11. An aluminum alloy containing, apart from impurities 7% to 11% by weight of silicon; 3.5% to 5.5% by weight of copper; 0.25% to 0.6% by weight of magnesium; and 0.10% to 0.60% by weight of antimony; the balance being aluminum.

12. An aluminum alloy containing, apart from impurities, 6% to 13% by weight of silicon; 3% to 6% by weight of copper; 0.2% to 1% by weight of magnesium; 0.03% to 1% by weight of antimony; and at least one of a trace to 1.5% by weight of manganese, a trace to 1.0% by weight of chromium, a trace to 2.5% by weight of nickel and a trace to 0.3% by weight of titanium; the balance being aluminum.

1 3. An aluminum alloy according to claim 1 and substantially as hereinbefore described with particular reference to the accompanying drawings and/or in the examples.