Effect of Minor Cr, Mn, Zr, Ti and B on Grain Refinement of As-Cast Al-Zn-Mg-Cu Alloys

The effects of joint addition of minor elements Cr, Mn, Zr, Ti and B on grain refinement of Al-Zn-Mg-Cu alloys were investigated by optical microscopy and scanning electron microscopy (SEM) as well as EDS. Results show that grains can be refined from 256 mu m to 102 mu m via adding (mass fraction) 0.04%Ti, 0.18%Zr and 0.008%B to the cast alloy. Joint addition of 0.20%Cr, 0.20%Mn, 0.04%Ti and 0.008%B may attain better grain refinement, and the average grain size is as fine as 55 gm. It is because atom clusters containing minor Cr and Mn help Al3Ti nucleation to become crystallization substrate. Joint adding of 0.20%Cr, 0.20%Mn, 0.03%Ti, 0.14%Zr and 0.006%B can result in a remarkable refinement with average grain size about 22 mu m. This is because a part of Zr atoms enters the crystal nuclei of Al3Ti based on the atom clusters containing minor elements Cr and Mn to replace the Ti to form new crystal nuclei Al-3(Ti-x, Zr1-x). While minor elements Cr and Mn decrease the surface tension between liquid aluminum and solid Al3Ti, Al3Zr and Al-3(Ti-x, Zr1-x) particles, and impede the nucleation of particles to grow up.