Structure-Property Correlation of Al-Mg-Si Alloys Micro-alloyed with Sn

Microstructure and mechanical properties of the cast and wrought Al-Mg-Si alloy micro-alloyed with 0.04 wt% and 0.08 wt% Sn were investigated at various processing conditions, viz. as-cast, solutionised, homogenised, hot rolled and peak-aged. Additionally, fracture surfaces of the alloys were also examined in the as-cast, peak-aged cast and peak-aged rolled conditions. It was observed that 0.04 wt% Sn favours the formation of a lamella-like eutectic Al + Mg2Si, whereas 0.08 wt% Sn favours the formation of plate/rod-like Mg2Si. The formation of the former deteriorated the hardness and tensile strength, but improves the ductility, whereas the formation of the later was found to contribute to the hardness, tensile strength as well as ductility of the Al-Mg-Si alloy. Homogenisation treatment had reduced the hardness of all the alloys to the lowest value due to the dissolution and spheroidisation of the intermetallic phases. However, hot rolling had significantly increased the hardness, tensile strength and ductility of the alloys. Thus, the increase in hardness, tensile strength and ductility were more prominent in the peak-aged rolled alloys relative to the peak-aged cast alloys. At the studied processing stages, the hardness and tensile strength of Al-Mg-Si alloy were found to decrease with the addition of 0.04 wt% Sn and increase with the addition of 0.08 wt% Sn. However, the ductility increased with Sn addition, and the increment was more prominent when micro-alloyed with 0.04 wt% Sn. Improvement in ductility in 0.04 wt% Sn alloy could be attributed to the formation of script-like Al(Fe, Mn)Si phase, whereas in 0.08 wt% Sn alloy could be attributed to the formation of ductile Mg-2(Si, Sn) phase. Furthermore, the fractography study had revealed that the fracture in the alloys was mainly developed by the cracking of Al(Fe, Mn)Si intermetallic particles. The fractures mode in the as-cast and peak-aged cast alloys was mainly by transgranular brittle with a cleavage fracture surface, whereas in the peak-aged rolled alloys was mainly by transgranular ductile with a dimpled fracture surface.