In this thesis a modified energy efficient T6 heat treatment was performed on AlSi7Mg0,3 and AlSi9Cu3(Fe) castings pilot scale produced, whereas a part of the heat from the casting process was used during solution annealing. Calculations of the possible energy savings showed that the novel energy saving heat treatment can gain up to 260 kJ of thermal energy per kilogram of casting compared to conventional methods. To produce samples of the energy efficient heat treatment the castings were ejected from the squeeze casting machine at 300°C and immediately transferred to the annealing furnace, and subsequently water quenched an artificially aged. The process parameters annealing time, quench temperature, ageing temperature and time were chosen constant for all samples of the same alloy in order to determine the mechanical properties and the microstructures. No significant differences in yield strength, tensile strength, elongation at fracture, micro hardness and present phases could be obtained in samples of conventional and energy efficient heat treatment. Depending on the alloying system, the variation of the annealing time of both T6 treatments (conventional and energy efficient) showed varying influence on the mechanical properties. A longer annealing time on samples of the alloy AlSi7Mg0,3 (relatively high annealing temperature) showed no impact on the mechanical properties, whereas samples of the alloy AlSi9Cu3(Fe) (relatively low annealing temperature) indicate an increase in yield strength and tensile strength. Overall almost identical mechanical properties were obtained for the modified energy efficient heat treatment.