Microstructure Control And Its Effect On Properties Of Casting AlZnSiMg Alloys

The AlZnSiMg casting alloys has combined the mechanical performance of Al-Zn alloys and the casting performance of Al-Si alloys.However,the coarse Si phases still exist in the alloys modified by Sr or Na in the sand-mold casting,thus,the elongation is very poor.In this paper,therefore,the microstructure and mechanical properties of AlZnSiMg alloys were investigated through grain refinement,modification and optimization of alloy composition,which has important industrial application value.The effects of La and B on the microstructure and property of Al-10Zn-8Si-0.35 Mg were investigated.With the combined addition of B and La to the Al-10Zn-8Si-0.35 Mg alloys,the grain size is decreased from about 1200 ?m to 150-170 ?m and Si phases are sufficiently refined.For the Sr-modified alloys,the tensile strength and elongation of the alloy is increased from 227 MPa and 1.3% to 256 MPa and 2.1%,respectively.When it comes to Na-modified alloys,the strength and elongation is increased from 239 MPa and 1.6% to 253 MPa and 1.9%,respectivly.For the Sr-modified alloys,the addition of Ti refined both the dendrites and Si phases,thus,the strength and elongation of the alloy reaches to 249 MPa and 2% respectively.The number of pores in the Na-modified alloys is less than Sr-modified alloys,but the modification effect of Sr is more stable than that of Na and Sr is more suitable for the modification of Al-10Zn-8Si-0.35 Mg alloy for sand casting.Effects of composition on the microstructure and property of Al-Zn-Si-Mg casting alloys were investigated and the results show the less Zn and Si in alloys,the better the modification of Si phases and the elongation of the alloys is and the elongation of Al-9Zn-7.2Si-0.35 Mg and Al-7.5Zn-6Si-0.35 Mg reachs 2.0% and 2.6%,respectively.With the combained addition of La and B,the microstructure of Al-9Zn-7.2Si-0.35 Mg and Al-7.5Zn-6Si-0.35 Mg got remarkably refined and the strength and elongation of Al-9Zn-7.2Si-0.35 Mg alloy reaches 258 MPa and 2.4%,respectively.The Celluar Automation method was carried to investigate the distribution of Si at the front of the liquid/solid interface in the process of solidification.Results show that before the nucleation of eutectic phase,the concentration of Si has reached to the eutectic composition due to the local enrichment at the front of the liquid/solid interface.In that case,the growth of dendrites got severely restricted and only the concentration of Si in the liquid was decreased by the nucleation growth of Si phases,the dendrites can continue to grow.Moreover,the size and quantity of the coarse Si phases are influenced by the cooling condition and the number of dendrites.The decrease of cooling speed enlarges the size of the Si phases and the dendrite refinement can be achieved by increasing the number of dendrite,also the concentration of Si solute at the front of the solidification interface can be reduced.