Section dimension induced microstructure and mechanical properties variations of sequential solidified sand–cast Mg–10Gd–0.2La–0.1Zn–0.4Zr (wt%) alloy

Abstract

Casting dimension significantly influences the quality and performance of finished products when employing a sequential solidification process. Tapered cylinder Mg–10Gd–0.2La–0.1Zn–0.4Zr (wt%) castings with variable section dimensions were fabricated by sand–cast process, and the role of section dimension on the solidification behavior, microstructures and mechanical properties under different thermal conditions were systematically investigated. Increasing section dimension from Φ23 mm to Φ32 mm, the average cooling rate during solidification decreases from 1.70 °C/s to 0.86 °C/s, and the actual chemical composition of Gd element descends from 10.39 ± 0.11 wt% to 10.05 ± 0.06 wt%. Meanwhile, the primary α–Mg grains are coarsened in size from 29 ± 2 μm to 39 ± 3 μm, and the area ratio of the secondary phase degrades from 1.87 ± 0.19 % to 1.14 ± 0.31 %. With a solution treatment of 500 °C × 8 h, the area ratio of the residual secondary phase rises from 0.55 ± 0.06 % to 0.75 ± 0.12 %, accompanied by the decrease of Gd solubility with a grand value of 0.33 wt%. The decline of Gd solubility is unfavorable for the precipitation of strengthening phases during the ageing treatment and leads to a reduction of ∼13.5% in the area number density of the β′ precipitates. The strength and ductility at ambient temperature simultaneously worsen with increasing section dimension, in which the yield strength, ultimate tensile strength and elongation respectively descend from 230 ± 7 MPa, 364 ± 7 MPa and 8.5 ± 1.1 % to 219 ± 6 MPa, 336 ± 5 MPa and 5.5 ± 0.9 % in the peak–aged condition. The strength debasement is due to the grain coarsening and lower number density of the β’ precipitates, while the reduced ductility is attributed to the coarser grains and higher content of the residual secondary phase with larger size. Casting dimensions should be meticulously designed when developing high–performance Mg alloys by sequential solidification processes.