Effect of Mn and Cr Addition on Microstructure and Tensile Property of Al-Mg-Si-Cu Aluminum Alloy for Vehicles

Al-Mg-Si-Cu aluminum alloy ingots with separate addition of Mn, Cr and composite addition of Mn and Cr were prepared by smelting, and then subjected to homogenizing annealing at 560 ℃ for 10 h, hot extrusion forming and T6 treatment (holding at 535 ℃ for 75 min and water quenching+aging at 175 ℃ for 10 h), in sequence. The effects of Mn and Cr additions on microstructure and tensile properties of the alloy were studied. The results show that during homogenizing annealing, Mn or Cr displaced some iron atoms in Al-Fe-Si phase and promoted transformation of rod-like β-Al₉Fe₂Si₂ phase to granular α-Al₈Fe₂Si phase, resulting in the formation of α-Al₈(FeMn)₂Si, α-Al₈(FeCr)₂Si or α-Al₈(FeMnCr)₂Si phase in structure. After T6 treatment, compared with the alloy added of Cr, the alloy with addition of Mn had less acicular β" phases in grains and more sub-micron phases near grain boundaries. Compared with those without addition of Mn and Cr, the strength and elongation of the alloy with separate addition of Mn, Cr and composite addition of Mn and Cr were improved; the tensile fracture modes were all ductile. After composite addition of 0.10wt% Mn and 0.15wt% Cr, the tensile properties were the best.