Metallographic investigation of hot tear in direct chill cast Al-Zn-Mg billet

The key metallographic features of a hot tear in a direct chill cast Al-Zn-Mg billet were investigated in the present work. The hot tear was found to run through a highly porous region in the centre of the billet. The pores, judged to be of the shrinkage variety, were invariably at grain boundaries and were segregated to the inside of the hot tear. The mushy zone at the centre of the billet was squeezed to force the liquid against the solid shell. The liquid was subsequently sucked back in to feed the last material to solidify. The failure to compensate for the local solidification shrinkage with the intergranular liquid at high solid fractions led to pore nucleation and growth along the solidification front. The hot tear has formed via bridging of the coarser pores. The coalescence of pores is thus claimed to be a key mechanism for the formation of hot tear in this case.