Nine kinds of rare-earth free Mg-Al-Sn-Mn magnesium alloys were designed by orthogonal method. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and tension tests were carried out to investigate the microstructures and mechanical properties. As-cast Mg-Al-Sn-Mn alloys have an obvious dendritic structure that is composed of α-Mg, Mg17
, and Mg2
Sn phases. After hot extrusion, the cast dendrite structure changed into a recrystallized equiaxed grain. Mg17
dissolved completely into a matrix, and only α-Mg, Mg2
Sn, and a few Al-Mn phases could be observed. The influence of three alloy elements (Al, Sn, and Mn) on grain size, texture intensity, ultimate tensile strength (UTS), tensile yield strength (TYS), and elongation (EL) were studied by extreme difference analysis method. The content of Mn had the greatest influence on grain size. The AT61-0.2Mn and AT73-0.2Mn alloys had the smallest grain, reaching 6.8 μm. The content of Al had the greatest influence on the strength; therefore, the AT73-0.2Mn alloy had the highest UTS, 322 MPa, and TYS, 202 MPa. The content of Sn had the greatest influence on elongation. The AT52-0.4Mn alloy had the highest elongation in theory, but it was not included in the nine designed kinds of alloys yet. AT52-0.2Mn alloy had the highest elongation in the nine alloys (28.4%).
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