The squeeze casting process, in which pressure is applied to a solidifying metal, has found a wide application in the production of high integrity castings. Two variants of the process have been developed, which vary in the method of pressure application. The pressure can be applied to the casting either directly or indirectly [Chadwick and Yue, 1989]. The effects of the direct squeeze casting process on the structure and properties of cast alloys have been investigated in depth. However, a similar study has been missing for the indirect process, and that is the topic of this dissertation. The effects of the indirect squeeze casting process on solidification, structure and mechanical properties of Al-Si alloys were investigated. The castings were produced on a commercial Ube 250 tonne indirect squeeze casting machine. Binary Al-7%Si and Al- 12%Si alloys, both in unmodified and Sr-modified form, were cast and analysed in the project. To obtain meaningful mechanical properties, magnesium was added to a commercial Al-12%Si eutectic alloy which was tensile tested, and the results were compared to an Al-7%Si-0.7%Mg alloy. A limited amount of experimental work was performed on the direct squeeze casting process to obtain data on porosity formation and macrosegregation. The same alloys as used in the indirect squeeze casting were also gravity cast to obtain cooling curves and microstructures under atmospheric pressure. The work was complemented by solidification modelling, which described the thermal distribution in the die and distribution of solid and liquid at different stages of solidification. In the indirect process, the melt was first poured into a shot-sleeve, where a solid layer developed upon contact with the sleeve surface. Broken pieces of the solid layer were carried to the interior of some castings during filling. The progress of solidification in the indirect squeeze casting was dictated by the thermal gradient imposed on the die, which was controlled by die heating and cooling arrangements. In the die configuration used in the present work, the gate solidification is initially faster than that of the die cavity, and this has a significant effect on the feeding of the castings. Inferences were drawn from the solidification model to assist the understanding of the feeding processes. The liquidus temperatures were increased by ~ 5°C by the application of 100 MPa pressure, and this shift in the liquidus temperature can be predicted from a Clausius­ Clapeyron equation. The effect of pressure on the eutectic temperatures was not as clear. 5 to 9°C increase was detected in the Al-12%Si alloys, but a corresponding increase in the transformation temperature was not measured in the Al-7%Si alloys. The addition of Sr did not affect the eutectic temperatures during pressurised solidification. The eutectic point was shifted to a higher Si composition. The indirectly squeeze cast Al-7%Si alloys suffered from severe macrosegregation. Five different mechanisms were found to be responsible for the segregation formation, and they depended upon location in the casting and section thickness. The Al-l 2%Si castings contained some segregated areas, but the segregation was not as severe as that found in the Al-7%Si alloys. The applied pressure and addition of the eutectic modifier had very little effect on the extent of segregation. In contrast to the indirect squeeze castings, the direct squeeze castings were free of any macrosegregation. The microstructure of the segregated areas was also different from the microstructure of the unsegregated regions. In the unmodified castings, the eutectic Si particles in the segregated areas were chill modified to a fibrous morphology, and fine a-phase dendrites and primary Si particles were also present. The fine a dendrites were also present in the segregated areas of the Sr modified castings, but a change to the eutectic Si was not detected. The Si level has an effect on feeding of castings and development of porosity, due to the presence of different fractions of primary phase and eutectic. The porosity was more difficult to eliminate in the castings from Al-7%Si alloys than Al-12%Si alloys. However, the situation was reversed in the feeders (biscuits), where the porosity was more easily eliminated in the Al-7%Si alloys. The Sr did not have a clear effect on the amount of porosity present in the castings, but it increased the number of pores. This was especially noticeable in the direct squeeze castings. The segregation affected the mechanical properties, especially elongation to fracture, which was lower in the segregated areas. The eutectic alloys were not as affected by segregation as the Al-7%Si alloys. The level of applied pressure did not affect the mechanical properties except in the influence of porosity. The heat-treated eutectic Al- 12%Si-0.6%Mg CC401 alloy was not affected by the segregation, and its yield and ultimate tensile strengths were higher than those previously measured in the Al-7%- 0.6%Mg AA603 alloy, but its elongation to fracture was lower. The lack of segregation and better feeding characteristics make this alloy suitable for the indirect squeeze casting process.