Abstract:The effects of microalloying elements Ti, Sc, Zr and Er on grain refinement behaviors and hardness properties of wedge-shaped Al-Mg-Mn alloy castings were investigated. The results indicate that alloys containing Sc and Zr can remarkably reduce the grain sizes of Al-Mg-Mn castings. Combination of Sc, Zr and Er can completely eliminate the columnar dendritic grains and further obtain refined grains with nondendritic sub-structure; the whole wedge-shaped cross-section of the casting consequently exhibits more homogeneous cast structures instead of the typical tri-crystal region structures. Large amounts of Al₃Sc-based intermetallic compound particles, such as Al₃(Sc_1−x, Zr_x), Al₃(Sc_1−x, Ti_x), Al₃(Sc_1−x−y, Zr_x, Ti_y) and Al₃(Sc_1−x−y, Zr_x, Er_y) are present in the microalloyed alloys, resulting from their numerously forming in high-temperature melt before solidification. These phases have the same L1₂-type crystal structure to Al₃Sc phase as well as smaller misfits with the primary α(Al) grains, which leads to more efficient epitaxial growth for α(Al) grains on all crystal planes of these composite phases. The experimental alloys have been hardened in different levels and, show the low susceptibilities of hardness change with varying cooling rate. The high hardness of the castings are caused by grain-refined strengthening and solid solution strengthening.

Key words: Al-Mg-Mn alloy; filler metal; wedge-shaped casting; grain-refined strengthening; solid solution strengthening