FORCED-CONVECTION FLOW CHANNEL DEFECTS IN CASTINGS

Slow filling of castings may lead to remelting of the partially solidified casting thereby forming channels through the structure. The channels are analogous to the channel defects formed by the segregation of solute in the pasty zone leading to the A and V segregates in ingots and large castings. Flow convection channels, however, are characterised externally by a rougher surface finish on the casting and internally by almost zero segregation. The internal structure comprises an edge region of crushed dendrites (leading to the only significant segregation); an intermediate region of refined equiaxed grain structure, coarse dendrite arm spacing, and coarse porosity; and a central region of large grains and large dendrite arm spacing. Some gas pores tend to float to the top of the channel, where they decorate its upper boundary. It is concluded that the channels are not severe defects per se, but are undesirable in castings where good control over quality is required. In particular, because of the delayed solidification, they allow the growth and alignment of porosity in sections which would otherwise freeze rapidly and be sound, or at worst, have fine distributed porosity.