Evolution of externally solidified crystals, porosity and thermal conductivity of high-pressure die-cast hypoeutectic Al-Fe-Ni alloys under varying process parameters

To quantitatively reveal the correlation between processing conditions and thermal properties in die-cast aluminum alloys, this paper investigates effects of process parameters on evolution of the externally solidified crystals (ESCs-α), porosity, and thermal conductivity in a die-cast Al-1.5Fe-1Ni alloy using experimental analysis and model calculations. The microstructure of the die-cast Al-1.5Fe-1Ni alloy comprises primary α-Al, including externally solidified crystals and α-Al solidified in the die cavity, together with (α-Al+Al 9 NiFe) eutectics. Increasing the slow-shot speed from 0.1 m·s -1 to 0.9 m·s -1 reduces the number and area fraction of ESCs-α by 54.1% and 61.6%, respectively, because of the shortened melt residence time in the shot sleeve. This reduction correspondingly decreases the volume fraction and equivalent diameter of shrinkage pores by mitigating the dendritic networks. The fast-shot speed fragments ESCs-α via melt flow, whereas intensification pressure primarily enhances casting density by reducing porosity from 0.4% to 0.09%. A novel thermal conductivity model accounting for each constituent of die-cast alloys is established, which exhibits better accuracy below 5%. The model reveals that thermal conductivity slightly decreases with increasing slow-shot speed because the percentage contribution of low-solute ESCs-α to thermal conductivity decreased from 6% to 3%. Thermal conductivity exhibits the insensitivity to fast-shot speed and intensification pressure. This work clarifies the process-dependent microstructure evolution caused by ESCs-α and porosity, and the quantitative relationship between microstructure and thermal conductivity by a novel model, which reveals the ESCs-α-controlled thermal conductivity variation under different slow-shot speeds. These results provide guidance for the design of high-performance alloys.