Influence Of KBF₄ On The Microstructure And Performance Of Mg-6Zn-1Si Alloy

Proper Si element added to the Mg-Zn alloys can signiicantly increase the fluidity and mechanical properties of the molten metal. With the addition of Si in Mg alloys, the intermetallic compound Mg2Si phase, with a highmelting point, low density, high hardness low thermal expansion coefficient, which can effectively improve processing and mechanical properties of the Mg-Zn alloy, is formed. However, the bulk-Chinese-character Mg2Si phase deteriorate the mechanical and deformation properties of materials and impede their applications. So, it is very significative to modify Mg2Si phase in Mg-Zn-Si system magnesium alloy to broaden the scope of application of Mg-Zn system.In this paper, KBF4 is added into the Mg-6Zn-1Si alloy, the effects and mechanism with KBF4 on the microstructures and properties have been investigated using modern analysis and test facilities such as Optical Microscopy (OM), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectrometer (EDS). It offers some experimental basement and theoretical reference for high performance and low cost Mg-6Zn-1 Si alloy.The results show that KBF4 can significantly modifies bulk-Chinese-character Mg2Si phase, it still has significant grain refinement on Mg-6Zn-1Si alloy. When the addition of KBF4 is 1.5wt.%, the morphology of Mg2Si phase completely changed from Chinese-character type into uniform and dispersed granular or blocky, with an average size from the original 100μm reduced to 12μm, and grain size of Mg-6Zn-1Si alloy were also refined from 137μm to 82μm. The optimum temperature of modification is 780℃.The ultimate tensile strength and elongation of Mg-6Zn-1Si alloy were 106MPa and 3.1%. Adding 1.5wt.% KBF4, the ultimate tensile strength up to 171MPa, which has increased 61%, the elongation of Mg-6Zn-1Si alloy was 6.4%, which has increased 106%.Corrosion experimental results showed that the corrosion took place whole sample surface in Mg-6Zn-1Si alloy. While corrosion took place a part of the sample of the alloy with 1.5wt.% KBF4 addition and the degree was lighter than Mg-6Zn-1Si alloy. The corrosion current density of alloy with 1.5wt.% KBF4 addition was 5.756×10-5A/cm2 lower than that of Mg-6Zn-1Si alloy, and the lost weight corrosion rate was 47.7% lower than that of Mg-6Zn-1Si alloy.The mechanism of modification about KBF4 to Mg2Si phase is that, after KBF4 joining the melt, KBF4 and Mg react to separate out B atoms. B elements in the form of active atoms exist, enriched in the solid-liquid interface front, not only improves the nucleation rate, but also reduces the crystal growth rate, while B become adsorbed on the Mg2Si crystal plane boundary and change the surface energy of the Mg2Si crystals by lattice distortion, since the atomic sizes of them are different from those of Mg and Si. This may effectively poison the growth steps and suppress the anisotropic growth of the Mg2Si crystals.