The solidification behaviour of an Al-7% Si-0.3% Mg alloy during rotary spray forming was studied. The ability to form a coating was insensitive to the thermal processing parameters, yielding material exchangess greater than 90%. The level of porosity varied typically between 1.5 and 4.75%. The dendrite arm spacing was evaluated and used to estimate the cooling rates. Typical dendrite secondary arm spacings were of the order of 3 mu m, 12 mu m and 25 mu m, corresponding to cooling rates of 4630 K s(-1), 72 K s(-1) and 8 K s(-1), respectively. The fraction primary precipitation was experimentally determined and the partition coefficient calculated indirectly using the Scheil equation. The partition coefficient is increased during rotary spray forming. This is explained by the presence of trapped vacancies at the solidification front. The vacancies change the solid＇s free energy and thus change the phase diagram and the partition coefficient. A simplistic analysis of entrapment and condensation of vacancies and their influence on the partition coefficient is made.