Effects Of Sr And High Frequency Electromagnetic Field On AZ91D Alloy Microstructure And Properties

Magnesium alloy has the advantages of light-weight, high strength-to-weight ratio, high stiffness-to-weight ratio, good solidification, high electromagnetism shielding property and re-cyclability and so on, so it has bright promise for magnesium alloy to be widely used in vehicle and spaceflight, computer, communication, and is regarded as the most promising material of the 21 st century. However, for its h.c.p structure, it exhibits bad deformation ability, low efficiency, and bad mechanical properties. Solving these problems is the key to develop more application of magnesium alloy. It is found that grain refinement can increase the intensity and plasticity of materials, so it plays a very important role in the research of magnesium alloys.Structural refinement during solidification of AZ91D alloy in Mg-AI-Zn system has been studied with a focus on the effects of solute elements Sr and high frequency electromagnetic field stirring on microstructures and mechanical properties. The relations of chemical composition, microstructures and properties are discussed to improve its mechanical properties.Four kinds of new AZ91D alloys have been prepared by adding small amount of Sr in this dissertation. Using modem analysis and test facilities such as Optical Microscope (OM), X-ray Diffraction (XRD), Differential Scanning Calorimetric (DSC), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectrometer (EDS) and the test of their tensile strength, specific elongation and fluidity, effects and mechanism of Sr addition on the microstructures and properties have been systemically and deeply investigated. In the meanwhile, by using high frequency electromagnetic field stirring in the solidification of alloy melt, the evolution process and mechanism of initial crystal structure have been investigated, and the effects of high-frequency electromagnetic field on microstructure have been studied.Experimental results showed that the microstructure of AZ91D casting magnesium alloy was made up ofα-Mg phase andβ-Mg₁₇Al₁₂ phase, and the new phases which were different from the primary were not found in the AZ91D alloys which 0.2%Sr and 0.5%Sr were added to. But the new A14Sr phase was gained by adding 0.8Sr% to the alloy. The alloy grains were effectively refined by the addition of small amount of Sr. And the degreeof refinement was deeper and the grain size was decreased from 250μm to 120μm with the increase of the content of Sr. It is found that the Sr in the alloys almost existed in the grain boundary and the Mn, Zn and A1 also existed in the grain boundary by the analysis of surface scan. Sr exists in the alloys with two forms: one dissolves in the alloys which influence the configuration of Mg₁₇Al₁₂ phase, as the surface activation element to refine the grain, the other is one element of new compound which is Al₄Sr phase. Al₄Sr phase is a high temperature steady phase which can increase the alloy eutectic temperature. The result is that the grains were refined.The mechanical properties of alloys were increased gradually because of the refined grains. The tensile strength of as-cast alloy with 0.8wt%Sr addition was 23.6% higher than AZ91D alloy, from 129MPa to 160MPa. The rates of elongation of alloys with 0.5%Sr and 0.8%Sr addition were 1.95%, 1.3%, and the one without Sr was 1.1%. With an h.c.p structure, the failure of AZ91D alloy is usually brittle through cleavage fracture. The fracture of AZ91D with Sr additions obvioused features the combination of quasi-cleavage and local gliding fracture, and the ductility of AZ91D became better.The addition of Sr made small influence to the solidus temperature, but could obviously decrease the liquidus temperature and reduce the interval of crystalloid temperature. When the liquidus temperature of AZ91D alloy which is 596℃reduced into 566℃after adding 0.5Sr, the solidus temperature increased from 435℃to 437℃, the interval of crystalloid temperature decreased for 32℃. The length of AZ91D fluidity sample was 33mm. But it became to 40mm after the 0.5wt%Sr was added. Then the fluidity was increased 21% before the 0.5wt%Sr addition.Because of the composition super-cooling,α-Mg phase of AZ91D magnesium alloy presents thick and obvious dendritic crystal. At the crystal boundary, the eutectic presents continuous, reticular and thick structure. After the high frequency electromagnetic stirring, the grains became smaller and near spherical, and the size of grains was reduced from 180μm to 120μm. Si and Mg can make a reaction into Mg₂Si in the quartz pipe used as a heating container. Much closer to the wall of the quartz pipe, much more Mg₂Si will be produced. By thermodynamic calculation, we know that the reaction 2Mg(1)＋SiO₂(s) Mg2Si(s)＋O₂(g) can be spontaneously achieved. The effects of electromagnetic field to alloys in the solidification is breaking up the dendritic crystal and increasing the solubility of the alloy elements in the magnesium.