| The mechanical properties of aluminum alloy castings are affected by porosity which is caused by a combination of dissolved gases and shrinkage. The objective of this research is to study the effects of solidification under pressure on microstructure and mechanical properties of two aluminum alloys, A356-T6 and A206-T6.;Wedge-shaped aluminum castings were produced with chemically bonded sand molds. Castings were poured at 704ºC (1300ºF) or 760ºC (1400ºF) and solidified under 1 atmosphere or 10 atmospheres of applied pressure. Density, x-ray radiography, metallographic inspection, and tensile testing were performed on samples from various section sizes of the castings. For aluminum alloy A206-T6, one additional casting was poured under 1 atmosphere pressure and hot isostatically pressed (HIPed).;For the A356-T6 aluminum alloy, solidification under pressure increased ultimate tensile strength of the casting by 4 to 9% and elongation to failure increased by 28 to 36%. The total volume of porosity was reduced by 70-72%. In section thicknesses of one inch or less, the casting cooling rate was increased, and the solidification time decreased by as much as 62%. The combination of solidification under pressure and low pouring temperature produced the greatest improvement in mechanical properties.;For A206-T6 aluminum alloy, solidification under pressure increased ultimate tensile strength of the casting by 17 to 19% and elongation to failure increased by 112 to 120%. The total volume of porosity was reduced by 78-84%. The A206-T6 alloy solidified under 10 atmospheres of applied pressure had similar properties to those of the HIPed casting. The combination of solidification under pressure and low pouring temperature produced the greatest improvement in mechanical properties. The casting cooling rate and the solidification time of wedge-shaped castings were not affected by the application of pressure in this alloy. |
