Hypereutectic Aluminum-Silicon Casting Alloy

In the past, aluminum alloys, due to their light weight, have been used for engine blocks for internal combustion engines. In order to provide the necessary wear resistance for the cylinder bores, it has been customary to chromium plate the cylinder bores, or alternately, to apply cast iron liners to the bores. It is difficult to uniformly plate the bores and as a result plating is an expensive operation. The use of cast iron liners increases the overall cost of the engine block as well as the weight of the engine.

Hypereutectic aluminum silicon alloys containing 17% to 19% by weight of silicon possess good wear resistant properties achieved by the precipitated silicon crystals, which constitute the primary phase. Because of the wear resistant properties, attempts have been made to utilize hypereutectic aluminum-silicon alloys as casting alloys for engine blocks to eliminate the need of plated or lined cylinder bores.

It has been found that as the silicon content in an aluminum-silicon-copper alloy is increased to the range of 17% to 19%, the castability of the ternary alloy is adversely effected. As an example, a common hypereutectic aluminum-silicon-copper alloy containing 16% to 18% silicon, 0.6% to 1.1% iron, 4.0% to 5.0% copper, 0.1% manganese, and 0.45% to 0.65% magnesium and balance aluminum, has good wear resistance, as well as a desirable low fraction solids at the eutectic temperature, thereby providing good fluidity. However, this alloy has a wide solidification temperature range, in the neighborhood of 250.degree., which severely detracts from its castability. Further, the alloy contains a substantial amount of copper which reduces the corrosion resistance of the alloy in salt water environments and thus prevents its use for marine engines.

Another commonly used hypereutectic aluminum silicon alloy has a nominal composition of 19% silicon, 0.6% copper, 1% magnesium and 0.4% manganese with the balance aluminum. Again, this alloy has good wear resistance due to the precipitated silicon crystals, but has relatively poor corrosion resistance when subjected to salt water environments.

The invention is directed to an improved hypereutectic aluminum silicon casting alloy which has use in casting engine blocks for marine engines.

The alloy of the invention contains by weight from 16% to 19% silicon, up to 1.4% iron, 0.4% to 0.7% magnesium, up to 0.3% manganese up to 0.37% copper and the balance aluminum. The copper content is preferably maintained as low as possible, and below 0.37%.

Due to the precipitated silicon crystals, the alloy has excellent wear resistance.

As the copper content is maintained at a minimum, the alloy has greatly improved resistance to salt water corrosion, so that it is particularly useful for casting blocks for marine engines.

By minimizing the copper content, the ternary aluminum-silicon-copper eutectic is avoided and thus, quite unexpectedly, provides a relatively narrow solidification range, below 150.degree. F. and preferably 100.degree. F. These properties provide substantially improved castability over ternary hypereutectic aluminum silicon alloys.

Other objects and advantages will appear in the course of the following description.

The hypereutectic aluminum silicon casting alloy of the invention has the following general composition in weight percent:

Silicon: 16-19%

Magnesium: 0.4-0.7%

Iron: Up to 1.4%

Manganese: Up to 0.3%

Copper: Up to 0.37%

Aluminum: balance.

The magnesium acts to strengthen the alloy, while the iron and manganese tend to harden the alloy, decrease its thermal expansion, increase its machinability, aid in maintaining the mechanical properties of the alloy at elevated temperatures, and increase soldering resistance in die cast applications.

The copper content is maintained below 0.37% and preferably at a minimum. By eliminating any substantial copper concentration, the corrosion resistance of the alloy to salt water environments is greatly improved, making the alloy particularly useful for engine blocks for marine engines and other parts requiring strength, wear resistance and corrosion resistance. The alloy has a weight loss of less than 1% when exposed for 200 hours to a 5% solution of sodium chloride.

The alloy can also contain small amounts, up to 0.2% each, of residual hardening elements such as nickel, chromium, zinc or titanium.

The alloy has excellent wear resistance, and at the stated silicon content, excellent fluidity is achieved.

As the copper content is minimized, the aluminum-silicon-copper eutectic is correspondingly eliminated with the result that the alloy has a relatively narrow solidification range, less than 150.degree. F., and preferably below 100.degree. F.

These properties of good fluidity and a narrow solidification range, provide the alloy with improved castability over known hypereutectic ternary aluminum silicon casting alloys.

In addition, the alloy has a a yield strength of 15,000 to 30,000 psi, an ultimate tensile strength in the range of 20,000 to 35,000 psi, and an elongation of 0% to 2%.

On cooling from solution the silicon precipitates as relatively large crystals. However, in casting cylinder blocks using metal cores a zone is formed bordering each bore that is substantially depleted of silicon crystals due to the rapid dissipation of heat to the metal core. With normal slow cooling this depleted zone generally has a thickness of about 0.02 inch, while under faster cooling conditions the depleted zone can have a thickness up to 0.05 inch. Due to the lack of silicon crystals the depleted zone has reduced wear resistance. It has been the practice in the past to remove the depleted zone by substantial machining, in order to expose the silicon crystals on the surface of the bore.

However, it has been found that when casting engine blocks with the alloy of the invention, the depleted zone can be eliminated by using a dry sand or salt core, which retards the transfer of heat from the molten alloy, and by cooling the casting at a relatively slow rate. with this procedure, the silicon crystals will extend to the surface of the bore and no heavy machining operation is required, thereby substantially reducing the cost of producing the engine block.

The following are specific examples of the alloy of the invention along with the mechanical properties.

Alloy Chemical (weight %):

Silicon: 16.90

Iron: 0.92

Copper: 0.14

Manganese: 0.12

Magnesium: 0.41

Aluminum: 81.51

Solidification Range: 79.degree. F.

Corrosion weight loss (200 hours in 5% NaCl solution): 0.18%

Ultimate tensile strength: 31,157 psi

Yield Strength: 31,157 psi

% elongation: 0.

Alloy Chemistry (weight %):

Silicon: 16.80

Iron: 1.03

Copper: 0.33

Manganese: 0.18

Magnesium: 0.50

Aluminum: 81.16

Solidification Range: 86.degree. F.

Corrosion weight loss (200 hours in 5% NaCl solution): 0.49%

Ultimate tensile strength: 29,164 psi

Yield strength: 29,164 psi

% elongation: 0.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.