Effect of Fe, Si and Cooling Rate on the Formation of Fe- and Mn-rich Intermetallics in Al-5Mg-0.8Mn Alloy

The Fe- and Mn-rich intermetallics are of great effect on the mechanical properties of the AA5083 type alloy (Al-5Mg-0.8Mn). The effect of Fe, Si and cooling rate on the formation of the intermetallics were investigated by analyzing the microstructures of the alloys with different Fe and Si contents. The results indicated that increasing the Fe content resulted in the increase of the Al6(Fe,Mn) phase in both amount and size. In the alloys with high Fe content, the platelet-like Al6(Fe,Mn) compounds lined up and formed a band. Increasing the content of Si resulted in the increase of the Mg2Si phase which formed a network structure. Increasing the cooling rate significantly refined the intermetallics. However, increasing the cooling rate did not change the characteristic of the intermetallic compounds in the alloys with low or medium Si contents. For the alloys with high contents of Fe and Si under the condition of near-rapid cooling, the Fe- and Mn-rich intermetallic compound changed to the quaternary Al15(Fe,Mn)3Si phase and displayed a fine fish bone or Chinese script structure. The refinement of the intermetallics would allow higher tolerance of Fe and Si contents in the AA5083 alloy produced, for instance, via the continuous strip casting process.