Heat Treatment, Microstructure and Mechanical Properties of the Squeezing Cast Al-7.8Zn-2.0Mg-1.6Cu-0.15Zr-0.18Sc Alloy

The microstructure of squeezing cast Al-7.8Zn-2.0Mg-1.6Cu-0.15Zr-0.18Sc alloy and those after industrial homogenization treatment and multi-stage solution treatments, were studied by differential scanning calorimetry (DSC), optical microscopy (OM), transmission electron microscope (TEM), scanning electron microscopy with energy dispersive X-ray spectroscopy (TEM-EDX). The results shows that, by complex adding the micro-alloying elements Mn, Sc, Zr, etc,the microstructure of squeezing casting ingot are fine, uniform, equiaxed, and with the average sizes of 40 mu m. The melting point of the non-equilibrium eutectic is 468 degrees C for the casting ingot. After industrial homogenization treatment at 450 degrees C for 24 h, the tensile strength of as-cast alloy is 340-350 MPa and the elongation is 3.5%-4.5%. The multi-stage solution treatment at 465 degrees C, 48 h +475 degrees C, 8 h +485 degrees C, 2 h results in the disappearance of the non-equilibrium eutectics at grain boundary, though some phases containing Fe and Cu can not dissolve completely. After 120 degrees C, 24 h aging treatment, the tensile strength is nearly 600 MPa, and the elongation improve to more than 10%.