Effect of pressure on the microstructure refinement, solidification mechanism and fracture behavior of wrought Al-Zn-Mg-Cu alloys prepared by squeeze casting

Abstract

Squeeze casting process can effectively resolve the casting difficulties for wrought aluminum alloys by pressurized solidification. The effects of applied pressure (0.1–125 MPa) on the microstructure refinement, non-equilibrium eutectic phase aggregation and fracture behavior of Al-Zn-Mg-Cu alloys prepared by squeeze casting were investigated through metallography, scanning electron microscopy and energy dispersive spectroscopy. The results showed that the primary α-Al grains transformed from coarse dendrites to refined equiaxed grains with increasing pressure. Considerable primary refinement was achieved at 100 MPa, with a significant decrease by 63% in the average grain size from 273 µm to 101 µm; the non-equilibrium eutectic phases were homogenized and refined, and almost dissolved into the Al matrix after T6 heat treatment. The mechanism of solidification behaviors in pressurized solidification was elucidated. Firstly, the applied pressure increased the thermodynamic driving force to promote primary grain nucleation. Secondly, the solute diffusion and constitutional undercooling at the front of the solid/liquid interface front were inhibited by pressure, and grain growth in a dendritic structure was thus restrained. Thirdly, interdependence theory and thermodynamic calculations were conducted to quantitatively describe the primary grain size changes under pressurized solidification. Lastly, the ternary eutectic reaction L → α-Al + T-Mg (Al, Zn, Cu)2 + Al2Cu and binary eutectic reaction L→ α-Al + Al7Cu2Fe were suppressed by the reduced solute segregation. Furthermore, when the pressure increased from 0.1 MPa to 125 MPa, the ultimate tensile strength was raised from 428 MPa to 538 MPa, almost approaching that of samples by forging process. The refined eutectic phases decreased stress concentration and improved fracture behaviors.