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摘要
Al-Si casting alloys had been extensively used in automobiles, aerospace, communication equipment and other fields owing to their high specific strength and specific stiffness, good casting behavior, high corrosion resistance, and near 100% recyclability. Fe was one of the most common impurity elements in secondary aluminum, which could easily form Fe-rich phase with high hardness but brittleness, significantly deteriorating the plasticity of recycled alloys. But for Al-Si casting alloy, an appropriate amount of Fe could reduce the adhesion between the casting and mold, which was beneficial to improve the precision and life of the mold. While the excessive Fe might easily form coarse needle-like or polygonal iron-rich phase, which were significantly deteriorated the surface quality, processing performance and mechanical properties of casting. The purpose of this paper was to optimize the Fe content, to achieve a balance between the Fe content and the structure and properties of recycled aluminum. The raw materials mainly included commercial ADC12 alloy and Al-20Fe master alloy. Five ADC12 ingots with different Fe contents (0.828%, 0.858%, 0.894%, 0.942% and 0.973%) were prepared by adding a certain amount of Al-20Fe master alloy. The raw materials were melted in a graphite crucible with an electric resistance furnace and poured into a pre-treated (250 ℃) steel mold with size of 30 mm×200 mm×150 mm. Optical emission spectrometer (OES) was used to analyze the chemical composition along the height of the ingot. The metallographic samples were cut from the same position (near the ingot bottom) by electro-discharging machine and prepared by mechanical grinding, polishing and etching. Optical and scanning electron microscope (OM and SEM) equipped with energy-dispersive spectrometry (EDS) were used for microstructure observation and chemical composition test. The characteristics of the Fe-rich phase included area fraction, average area and roundness were calculated by Image Pro-Plus 6.0 software based on backscattered images. The effect of Fe content on the mechanical properties of the alloy was studied using tensile tests, and the morphology of the fracture section was observed by SEM, which was used to analyze the influence of the iron-rich phase morphology on the elongation of the ingot. From the systematic analysis, the main results could be classified as follows: (1) With the Fe content increased to 0.942%, the area fraction of α-Al dendrites and primary silicon gradually decreased, while the secondary dendrite spacing of α-Al and size of primary silicon gradually increased. When the Fe content reached 0.973%, the area fraction of α-Al dendrites increased, but the dendrite spacing decreased sharply and the primary silicon basically disappeared, which improved the uniformity of the structure. (2) The morphological evolution sequence of the iron-rich phase with the increase of Fe content was: compact Chinese-script, Chinese-script+Fe-rich dendritic, Chinese-script + polygonal, needle-like+Chinese-script. (3) The area fraction and average area of??the iron-rich phase increased firstly and then decreased with the increase of Fe content. When the Fe content reached 0.942%, the area fraction and average area of the iron-rich phase reached a maximum value, as well as the roundness, which was due to the formation of a coarse polygonal iron-rich phase. (4) When the Fe content was not higher than 0.942%, the elongation slightly reduced with on significant tensile strength loss. But the elongation of the alloy decreased significantly when the Fe content continuously increased to 0.973%. During the stretching process, the initiation and propagation of cracks changed from mainly eutectic silicon to needle-like iron-rich phases, which was the main reason for the plasticity reduction of the alloys. (5) Based on the analysis of microstructure, mechanical properties and fracture cross-sectional morphology of the alloy with various Fe content, the upper limit content of Fe in ADC12 cast alloy under gravity casting was close to 0.942%, and Fe content range of 0.8%~0.942% was acceptable in secondary ADC12 alloy. By studying the influence of Fe content on the microstructure and properties of ADC12 aluminum alloy, the allowable range of Fe content in ADC12 aluminum alloy under gravity casting was determined, which was beneficial to improve the utilization efficiency and value of renewable resources. ? Editorial Office of Chinese Journal of Rare Metals. All right reserved.
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