Field of Invention

[0001] The present invention relates to an aluminum based alloy and more particularly to an aluminum based alloy suitable for use in die casting and for use in other articles of manufacture for automotive vehicles.

Description of Prior Art

[0002] The manufacturing industry, and particularly the automotive industry, has increasingly been replacing ferrous materials with light weight materials such as aluminum. The demand for substitute light weight materials has led to the development of aluminum alloys capable of forming structures that will withstand stresses typically reserved for structures formed from ferrous metals. In addition to enhanced strength, (including both high yield strength and high elongation values) an aluminum alloy should be die-
castable, corrosion resistant, and readily machinable.

[0003] While many of the known aluminum alloys exhibit acceptable corrosion resistance for moderately harsh environments, the known aluminum alloys, and in particular the known aluminum die casting alloys, are not sufficiently resistant to corrosion for certain highly corrosive environments. For example, aluminum castings that are used in highly corrosive exterior automotive applications in which the castings are routinely exposed to temperature extremes, water, snow, ice and humidity, as well as corrosion inducing materials such as salt, and dirt and road grime that can retain moisture and salt, eventually tend to exhibit significant corrosion.

[0004] Improving one property of an aluminum alloy without degrading another property has often proved elusive in the known art. ADC 5 is a typically used aluminum die cast alloy having satisfactory corrosion resistance. ADC 5 however has poor castability since the melt has poor fluidity since it is susceptible to solidification as a result of being cooled by the surface of the mold. ADC 12, though widely used in the prior art for aluminum die cast automobile parts due to its superior castability has very low corrosion resistance. Other commonly used aluminum die cast alloys include LM 6 and ENAB 43100, both of which have low yield strength and is hence unable to meet various requirements of the automobile industry.

[0005] Further, beryllium is a constituent in numerous prior art aluminum die cast alloys containing magnesium. One such alloy has been described in US Patent No. 6649126 titled 'Aluminum alloy for high pressure die-casting'. Beryllium is added to prevent loss of magnesium in the alloy by oxidation. However, there have been occurrences of berylliosis, a chronic irreversible lung disease, among workers engaged in the production of beryllium containing alloys. By breathing air contaminated with small particles of beryllium, a person is subjected to the risk of contracting berylliosis.

[0006] It would therefore be highly desirable to have an aluminum die last alloy that is not toxic, maintains superior strength and castability and has high corrosion resistance, the above-mentioned being achieved without degrading other inherent properties of the alloy.

Summary of Invention

[0007] The present invention has been conceived in view of the foregoing drawbacks in the prior art and it is the primary objective of the present invention to provide an aluminum die cast alloy that has superior castability and high corrosion resistance. It is another objective of the present invention to provide an aluminum die cast alloy that has substantially enhanced strength. It is yet another objective of the present invention to provide an aluminum die cast alloy that is ideal for use in automotive applications. It is still another objective of the present invention to provide an aluminum die cast alloy that is essentially free from toxic ingredients.

[0008] The aluminum die cast alloy as per the present invention consists of silicon 10 to 14% by weight, magnesium 0.8 to 2% by weight, iron 0.45 to 0.9% by weight, manganese 0.3 to 1% by weight, copper less than 0.1% by weight, zinc less than 0.4% by weight and the remainder aluminum. The low proportion of Copper and the enhanced proportion of manganese results in present invention achieving superior corrosion resistance with respect to the prior art. Further, the yield strength and the ultimate tensile strength of the alloy is enhanced due to the elevated magnesium content. The present invention also has improved fluidity and machinability in comparison to prior art alloys. Due to the above-mentioned enhanced properties, the aluminum die cast alloy as per the present invention is ideal to be used in chassis, engine covers, transmission covers and engine components of two-wheeled vehicles and three-wheeled vehicles.

[0009] These and other features, aspects, and advantages of the present invention will be better understood with reference to the following description, the appended claims and the accompanying figures. This summary is not intended to limit the scope of the claimed subject matter.

Brief Description of Figures

[0010] The above and other features, aspects, and advantages of the present invention are further illustrated by the accompanying figures. A brief description of the figures is as follows:

Fig. 1 is a bar graph comparing the corrosion resistance of the present invention with that of some of the prior art alloys.

Fig. 2 is a bar graph comparing the ultimate tensile strength, yield strength and elongation percentage of the present invention with that of some of the prior art alloys.

Fig. 3 is a bar graph comparing the machinability and fluidity of the present invention with that of some of the prior art alloys.

Detailed Description of Invention

[0011] The present invention is hereinafter described with reference to the accompanying figures.

[0012] The aluminum die cast alloy as per the present invention comprises silicon 10 to 14% by weight, magnesium 0.8 to 2% by weight, manganese 0.3 to 1% by weight, copper less than 0.1% by weight, zinc less than 0.4% by weight, and the remainder is composed of aluminum and incidental impurities.

The above table tabulates the composition of some of the known aluminum die cast alloys and the composition of the aluminum die cast alloy as per the present invention.

[0013] A preferred embodiment of the present invention comprises silicon 10 to 12% by weight, magnesium 0.8 to 1.2% by weight, iron 0.5 to 0.7% by weight, manganese 0.6 to 0.9% by weight, copper less than 0.05% by weight and the remainder aluminum. The composition of elements in the proportion described above enables the alloy as per the present invention to achieve high corrosion resistance, superior yield strength and excellent castability and machinability.

[0014] Fig. 1 is a bar graph that compares the corrosion resistance of the present invention with that of some of the prior art aluminum die cast alloys namely ADC 6, ADC 12, LM 6 and ENAB 43100. The bar graph has been plotted using results obtained from salt spray test (see ASTM B117) which is the commonly used method for evaluating corrosion resistance. The test was conducted by placing test pieces fabricated from each alloy in a tank sprayed with neutral salt water. Each test piece was taken out of the tank when the test equipment detects a visible layer of white rust was formed on it. The withstanding time of each test piece is the parameter for determining corrosion resistance. The test piece, which is placed for the maximum time in the tank, has the highest corrosion resistance. As shown in Fig. 1, the alloy as per the present invention shows remarkable corrosion resistance. This result is chiefly because of the enhanced manganese content and the reduced copper content in the present invention. In an aluminum die cast alloy, manganese is added to improve mechanical strength and to enhance
corrosion resistance. However manganese content above 1% by weight reduces the fluidity of the alloy. Manganese content is therefore kept in the range 0.3 to 1% by weight to achieve optimum results. Further, addition of copper in the alloy is to improve mechanical strength. Higher copper content
reduces thermal conductivity and corrosion resistance and therefore makes the alloy not suitable for automotive applications. Hence the present invention has a reduced copper content of less than 0.1% by weight.

[0015] Fig. 2 is a bar graph that compares the yield strength, Ultimate Tensile Strength (UTS) and elongation of the present invention with that of the prior art alloys named above. The results have been obtained from tension tests conducted as per ASTM E8 and are tabulated below:

It is evident from Fig. 2 that the present invention has superior yield strength and UTS. The yield strength of the aluminum die cast alloy as per the present invention is 230 Mpa and the UTS is 270 Mpa. The yield strength and the UTS was calculated when the present invention was in a cast state without
thermal treatment. The improved strength of the present invention is primarily due to the enhanced composition of magnesium. Magnesium content in the present invention has been fixed in the range 0.8 to 2% by weight. This composition of magnesium ensures the superior mechanical strength and
castability of the present invention. Zinc also enhances mechanical strength and improves corrosion resistance and castability of the alloy. However zinc content greater than 0.5% by weight may result in casting cracks. Therefore the present invention has zinc content less than 0.4% by weight.

[0016] Fig. 3 is a bar graph comparing the fluidity and machinability of the present invention with that of the above mentioned prior art alloys. As shown, the alloy as per the present invention has excellent fluidity and machinability when compared to the prior art alloys considered. Fluidity has been calculated using the flow length ratio value. Flow length ratio is defined as the flow length in the direction in which the aluminum alloy melts and advances until it solidifies, when poured into a castings mold having a pre-determined cross sectional shape. The improved fluidity and machinability is mainly due to the composition of silicon and iron. Silicon and iron is added predominantly to increase castability of the aluminum alloy. It also enhances the mechanical strength of the alloy. However raising the weight percentage of silicon above 14% causes loss in mechanical strength, ductility and machinability, which is undesirable for automotive components. Iron content above 1% by weight reduces the corrosion resistance and toughness of the alloy. Therefore in the present invention, silicon content is limited to the range 10 to 14% by weight and iron content is limited to the range 0.45 to 0.9% by weight.

[0017] The aluminum die cast alloy as per the present invention is made by firstly adding aluminum-magnesium and aluminum-manganese master alloys to base aluminum alloy. The base aluminum alloy comprises of silicon 9-12% by weight, magnesium not greater than 0.1% by weight, iron not greater than 0.5% by weight, manganese not greater than 0.5% by weight, zinc not greater than 0.1% by weight, copper not greater than 0.1% by weight and the remainder aluminum. The aluminum-magnesium and aluminum-manganese master alloys respectively comprises magnesium 10% by weight and manganese 10% by weight and the remainder aluminum. The quantity of master alloys that is to be added is decided based on the melt loss studies for each element. Casting of the aluminum die cast alloy is done in a melting furnace with nitrogen gas flushing. During melting of the aluminum die cast alloy ingots, flux is added to ensure the composition of the alloy. The aluminum die cast alloy is then modified by addition of modifiers such as calcium, phosphorus and titanium.

[0018] The present invention due to its superior properties is therefore ideal for use in automotive applications. Health hazards are avoided since use of toxic elements such as beryllium is avoided.

[0019] The foregoing description is a specific embodiment of the present invention and has been described for the purpose of illustration only. Persons skilled in the art may practice numerous alterations and modifications of the present invention without departing from its spirit and scope. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.

We Claim:
1. An aluminum based alloy for automotive applications comprising:
silicon 10 to 14% by weight;

magnesium 0.8 to 2% by weight; iron 0.45 to 0.9% by weight; manganese 0.3 to 1 % by weight; copper less than 0.1% by weight; zinc less than 0.4% by weight; and the remainder aluminum.

2. The aluminum based alloy for automotive applications as claimed in claim 1 comprising:

silicon 10 to 12% by weight; magnesium 0.8 to 1.2% by weight; iron 0.5 to 0.7% by weight; manganese 0.6 to 0.9% by weight; and copper less than 0.05% by weight.

3. The aluminum based alloy for automotive applications as claimed in claim 1 wherein the said aluminum based alloy in a cast state without thermal treatment has a thermal strength of atleast 270Mpa.

4. The aluminum based alloy for automotive applications as claimed in claim 1 wherein the said aluminum based alloy in a cast state without thermal treatment has a yield strength of atleast 230Mpa.

5. A method of making an aluminum based alloy for use in automotive
applications comprising the steps of:

adding aluminum-magnesium and aluminum-manganese master alloys to base aluminum alloy;

casting the said aluminum based alloy in melting furnace with nitrogen gas flushing;
adding flux during melting of aluminum based alloy ingots; and

modifying the said aluminum based alloy by addition of modifiers.

6. The aluminum based alloy for automotive applications as herein above described in the specification and the accompanying figures.