Controlling your aluminum melt.

Molten aluminum has characteristics that can be controlled in the melting process to enhance the properties of the final casting. It combines with oxygen to form aluminum-oxides that can become trapped in the casting, has an affinity for hydrogen gas and is sensitive to chemistry and trace elements.

Metalcasters producing castings with high performance requirements can control the melting process to produce molten metal that has the proper chemistry and hydrogen level and is free of oxides and inclusions. In addition, the melting process may include the addition of trace elements to modify and manipulate the casting microstructure for improved mechanical properties.

Two common melt treatments for aluminum alloys are grain refining and silicon modification. Grain refining provides more, smaller grains during solidification. Silicon modification changes the shape of the silicon crystals in the casting microstructure.

Here Come the Grains

During solidification, aluminum grains grow into dendrites with multiple arms, like the branches of a tree. Dendrites will continue to grow in a long, columnar fashion until growth is impeded by the mold wall or another dendrite.

Grain refining is the process of adding nucleating sites into the melt to initiate grain growth. As more grains start growing, they impinge on each other and limit the size of neighboring grains, resulting in smaller grain structure. This has several benefits during solidification. It reduces hot tearing, provides better feeding and helps disperse potential shrinkage porosity. In addition, the final casting typically shows improved properties, including:

* increased pressure tightness;

* improved mechanical properties;

* better response to heat treatment;

* improved machinability;

* uniform anodize appearance.

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Almost any intermetallic material will initiate grain growth. But, for maximum control and reproducibility, the industry standard is the addition of approximately 0.01-0.08% titanium/ boron in the form of an aluminum master alloy. However, if the aluminum bath is held in the molten state over a period of time, the titanium-diboride can drop to the bottom of the melt, reducing its effectiveness for initiating grain growth. For maximum effectiveness, grain refining should be performed as close to the point of pour as possible.

Grain size is best tested through thermal analysis or by viewing the casting microstructure. Thermal analysis is used to measure the cooling curve to determine the amount of undercooling required to initiate grain growth. Smaller undercooling indicates a smaller grain structure that is less resistant to nucleation.

Although some metalcasting facilities use a spectrometer to test for titanium level in the alloy as an indication of grain size, the presence of titanium does not indicate that active grain refining is present in the melt, nor does it reflect grain size. However, the amount of titanium present in the spectrographic analysis can be used effectively as a quality control tool to verify that the grain refining process is consistent.

Implant Some Silicon

Silicon modification changes the shape of the silicon crystal in the solidifying aluminum alloy. The silicon phase in aluminum-silicon alloys solidifies in the form of large platelets, called acicular silicon. With the addition of a modifying agent, the eutectic silicon solidifies with finer morphology, forming an interconnected network, called fibrous silicon.

The addition of strontium master alloy is the primary method of silicon modification, although antimony is used in some European and Asian countries. Strontium is an effective modifier at very low concentration levels--typically .01-.02%. Aluminum alloys with silicon levels above the eutectic point (hypereutectic alloys) can be modified using phosphorus at an additional level of 0.003-0.006%.

For reliable and reproducible silicon modification, careful control of the strontium addition is critical. It is possible to over-modify (greater than 0.09% strontium or 0.18% sodium), creating an intermetallic phase as inclusions or sludge. Inadequate modification can produce a semi-treated phase called lamellar silicon.

Some research has indicated that both sodium and phosphorus may negate the effect of strontium, making modification difficult and requiring more modifier. The effect of strontium fades as a function of time, temperature, agitation of the melt and the use of aggressive degassing agents, such as chlorine or Freon.

Silicon modification is best measured through thermal analysis of the cooling curve. Full modification will lower the eutectic temperature 6-8F (14.4-13.3C). Among other benefits, silicon modification can have a significant effect on feeding during solidification, reducing shrinkage porosity and hot tearing.