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The constant need for the development of new alloys with an improved ratio between weight and strength is the driving force for the progress of the present work, which deals with the production of a new Al-cast alloy with the addition of lithium (Li). 0.8 wt% Li was added to the AlSi7Mg alloy. The solidification behaviour of such a new alloy was the focus of the work. The melt was produced in an inert gas atmosphere in an induction furnace. The solidification behaviour was described by CALPHAD simulations, optical microscopy, scanning electron microscopy (SEM) and simple thermal analysis. In addition, the Vickers hardness (HV) was measured 29 days after casting and compared with the results of the AlSi7Mg base alloy. Solidification proceeds somewhat differently from the base alloy, where the liquidus temperature is raised to 637 °C, where the primary AlLiSi phase forms, followed by αAl dendrites, eutectic βSi and other phases in the form of eutectics, which are known to occur during solidification of the AlSi7Mg alloy. The hardness of the new alloy is increased compared to the base alloy, especially at higher cooling rates, where the hardness gain is up to 36%.