An Investigation on the Effect of Mn on the Castability of Hypoeutectic Al-2Ni-xMn Alloys

The effect of Mn addition (2 and 4 wt%) on the microstructure and castability of hypoeutectic Al-2Ni alloy was investigated. According to the results, Mn promotes the formation of Mn(Ni)-rich intermetallics in the microstructure. In the case of Al-2Ni-2Mn alloy the intermetallic compounds are interdendritic type whilst in Al-2Ni-4Mn alloy in addition to interdendritic intermetallics, the large and primary Mn-rich compounds with platelets, polyhedral and dendritic morphologies are present. The fluidity results show that the addition of 2 and 4 wt% Mn enhances the mushy solidification of Al-2Ni alloy leading to a fluidity reduction of 7 and 30%, respectively. Based on the microstructural observations and thermal analysis results, this reduction can be attributed to the formation of primary Mn-rich compounds in the molten alloy. Manganese addition, also, exerts negative impact on the hot tearing resistance of Al-2Ni alloy. The hot tearing susceptibility index (HTS) of Al-2Ni-4Mn alloy is 5 and 12 times higher when compared to those of Al-2Ni-2Mn and Al-2Ni alloys, respectively. SEM investigation of hot teared microcracks and the presence of free dendrites and primary Mn-rich compounds on the fractured surfaces imply on the critical role of primary compounds on the formation of hot tear microcracks.