Measurements of cell and primary dendrite arm spacings in directionally solidified aluminium alloys

Abstract

Cell and primary dendrite arm spacings have been measured in AlCu and AlMgSi alloys directionally solidified at steady state over a very large range of growth conditions. The AlMgSi alloys have been chosen so as to minimize gravitational fluid flow in the liquid. A spacing parameter, λ, has been correlated with the liquid temperature gradient G L , the growth velocity, V and the bulk alloy composition, C ∞. It has been found that for dendrites in the AlMgSi system λ = 272 G −0.55 L V −0.28 C +0.32 ∞ Si where C ∞ Si is the atom fraction of Si in the alloy. It has also been observed that there is a discontinuous change in the spacing relationship, as measured by λ, when the growth morphology changes from cellular to dendritic. Cells were found to form a roughly hexagonal array whereas dendrites adopted a much less regular pattern. This discontinuous change in λ is explained by considering how the nature of an array affects its average nearest neighbour spacing λ ∗.