Solidification behaviour of Al-7% Si-0.3% Mg during rotary spray forming

The solidification behaviour of an Al-7% Si-0.3% Mg alloy during rotary spr ay forming was studied. The ability to form a coating was insensitive to th e thermal processing parameters, yielding material exchangess greater than 90%. The level of porosity varied typically between 1.5 and 4.75%. The dend rite arm spacing was evaluated and used to estimate the cooling rates. Typi cal dendrite secondary arm spacings were of the order of 3 mu m, 12 mu m an d 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 experimental ly determined and the partition coefficient calculated indirectly using the Scheil equation. The partition coefficient is increased during rotary spra y forming. This is explained by the presence of trapped vacancies at the so lidification front. The vacancies change the solid's free energy and thus c hange the phase diagram and the partition coefficient. A simplistic analysi s of entrapment and condensation of vacancies and their influence on the pa rtition coefficient is made. (C) 1998 Kluwer Academic Publishers.