Effects of Mn Content on Microstructure and Hardness of 6061 Aluminum Alloy

Microalloying is an effective way to improve material properties. By means of scanning electron microscope/energy spectrum, X-ray diffraction, metallographic analysis and hardness tests, the effects of microalloying element Mn on as-cast, homogenization, aging microstructure and hardness were investigated in the basic of 6061 aluminum alloy. The results showed that the as-cast 6061 alloy consisted of the primary Al, the irregular strip-shaped Al-8(FeMn)(2)Si phases, fishbone-shaped Al-5(FeMn) Si phases and Mg2Si phases. Through microalloying, the morphology of phases transformed from irregular strip into fishbone obviously as Mn content increased. After homogenization process, Mg2Si phase dissolved into the alloy matrix but iron-rich phase cannot. With the increase of Mn content, the phase transformation of fishbone-shaped Al-5(FeMn)Si to particle-shaped Al-8(FeMn)(2)Si occurred. During solution-aging treatment, higher Mn content could promote the precipitation of stable phase beta(Mg2Si). Mn had a significant effect on the morphology of the crystalline phases which had a direct influence on the hardness of aluminum alloy. It was found that the addition of 0.24 wt% Mn to 6061 aluminum alloy could make the as-cast and aging alloys achieve the maximum hardness value 67 and 105 HBW, respectively.