The hot cracking susceptibility of the 7000 series Al-Zn-Mg high strength aluminum alloy has been studied. The cracking behaviour of the specimens were evaluated by TIG-A-MA-JIG Varestraint test process under various augmented strain condition at room temperature. It has been observed experimently that copper and chromium weakened the hot cracking resistivity of the high strength aluminum alloy weld metal deleteriously and they increase the total crack length(TCL). The effect of manganese on the hot cracking behaviour is found to be beneficial to decrease the hot cracking susceptibility markedly as the manganese content of the weld metal increases from 0.68 to 0.72 % and this enhancement by manganese element is understood to be attributed to the reduction of the mush zone size during the solidification process of the weld metal. In addition, the solidification behaviour of the tested alloys are investigated and it is found that as the solidification temperature range(ΔT) becomes narrow, the solidified structure is found to be more dendritic in its feature which is believed to have higher hot cracking resistance. The fracture mode of the hot cracks under the optimum amount of Mn are dendritic type protuberance for the alloy with a low Zr concentration and without Cr, flat type protuberance for the alloy with a high Zr and low Cr concentration, and dendritic columnar type protuberance for the alloy with a high Zr and Cr concentration, respectively.
