The effect of zinc on microstructure and solidification characteristics of Al-Zn-Mg-Cu alloys

In this study, the effect of zinc on microstructure and solidification characteristics of super high strength Al-Zn-Mg-Cu has been investigated. The solidification studies were performed using cooling curve thermal analysis. This method represents quick and accurate results of solidification path of an alloy. The microstructure studies showed increment in the amounts of zinc increases the dendrite arm spacing (DAS), fraction of second phases and eutectic structure and results in a coarse dendrite structure. However, the zinc content did not affect the present phases in this alloying system. Thermal analysis evaluations revealed decrease in nucleation temperature with zinc addition. The formation of Al13Fe4 phase was observed using by cooling curve. The solidification range in the presence of 8wt.% of zinc was 175 °C although the adding of zinc up to 25 wt.% increased it to 190 °C. Cooling curves represented the increase of the fraction of eutectic structure which was in accordance with image analysis results. The addition of zinc resulted in the decrease of the solidified fraction at dendrite coherency point from 0.32 to 0.1 which matched by increment in porosity fraction from 0.09 to 0.32.