Combating Al-Si porosity: the strontium hydrogen myth.

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While strontium aggravates porosity, tests prove it isn't due to increased hydrogen. Maintaining rapid cooling minimizes porosity.

It is common practice for aluminum foundrymen to modify aluminum-silicon (Al-Si) alloys with strontium (Sr) to change the shape of the silicon and enhance the metal's mechanical properties. Unfortunately, Sr also tends to increase porosity in castings, sometimes to the point where the porosity cancels out the Sr's contributions to mechanical properties.

Porosity in Al-Si castings occurs because of the negative pressures generated by solidification contraction, as well as the pressure from the evolution of dissolving hydrogen from the growing solid into the adjacent liquid. It has been thought that Sr introduces more hydrogen to the melt, thus increasing porosity. However, there is no quantitative data to verify that assumption.

Besides hydrogen content, factors such as cooling rate, modification, alloying elements, grain refinement and inclusion content play a role in forming porosity. This article details an extensive study of several of these factors, with an emphasis on the effect of cooling rate on porosity formation.

The research's objective was to clearly establish quantitative correlations between the above-mentioned variables, the pore volume fraction and pore size in Al-Si alloys to comprehensively explain the effect of modification on porosity. In the process, the data proved that while Sr does increase the severity of porosity, it adds no significant amount of hydrogen to the metal, and must therefore affect it in other ways.

Developing a Test

For the research, three Al-Si alloys with different Silicon levels were used. In addition to the standard A356 alloy (7.0 wt% Si), pure silicon and commercial purity aluminum were added to create alloys with 5.7 and 8.1 wt% Si, respectively.

For each experiment, a 22-lb (9-kg) melt was prepared in a silicon carbide crucible using an electric resistance furnace. Melt temperature was held constant at 1346F (730C). The tests were conducted on both Sr-modified and unmodified melts at various hydrogen levels and without grain refiners.

To study the effects of cooling rate, directional solidification of the test castings was necessary. The alloys were poured into preheated molds made from inorganic refractory insulating board coated with iron oxide wash [ILLUSTRATION FOR FIGURE 1 OMITTED]. At the base of each mold was a copper plate sprayed with a jet of cooling water to quickly establish...

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