Equiaxed solidification of Al-Si alloys

An experimental programme has been undertaken to determine which of the gra in formation mechanisms of equiaxed crystals are dominant in the solidifica tion of Al-Si foundry alloys. Small ingots were cast from alloys of varying silicon concentration with and without gauze barriers, using different typ es of mould materials and different mould preheats. The results show that t wo mechanisms of grain nucleation are operating. The first is a wall mechan ism where crystals are nucleated either on or near the mould wall owing to thermal undercooling. The second is a constitutional supercooling mechanism where nucleants are activated in the constitutionally undercooled zone ahe ad of the advancing interface. As a consequence, the grain size decreases w ith increasing silicon content. However a transition in the growth mode occ urs once a critical degree of constitutional undercooling is exceeded. This change in growth is accompanied by an increase in grain size. The transiti on point can be shifted with respect to solute content by changing the cast ing conditions, and a mechanism is proposed to explain this effect. MST/410 9