Grain Refinement And Modification Behavior Of Silicon-contained Magnesium Alloys By Al-P Master Alloys

The influence of Al-P master alloy on the primary and eutectic Mg2Si phase in Mg-Al-Si alloys was investigated in this paper by high resolution video microscopy (HRVM), field emission scanning microscope (FESEM) and electron probe micro-analyzer (EPMA). The grain refinement and modification process were optimized by considering various parameters, and the investigation clarified the heterogeneous nucleation mechanism. More study has been done for the phosphide evolution process after adding Mg into Al-P master alloy. The refinement effect of the Al-Mg-P on the Si and Mg2Si in Al-Si and Al-Mg2Si alloys have been also investigated in this work.The reinforced phase Mg2Si in Mg alloys often shows very coarse dendritic shape and Chinese script shape. A significant modification effect on the Mg2Si phase was achieved after adding the Al-P master alloy in Mg-4Al-2Si alloy. The experiment results show that, an optimized modification effect occurs as1.5%Al-P master alloy is added at about740℃for30min. The morphologies of primary Mg2Si particles change from dendritic or equiaxed to polygonal shape and their sizes decrease from about70μm to less than15μm. The eutectic Mg2Si phase was obviously refined and the morphologies change from coarse Chinese script to finer fiber shape. That’s because the phosphide will undergo the following process:dispersion of AlP crystalsâ†'AlP dissolutionâ†'reactionâ†'Mg3P2clustersâ†'small Mg3P2particles. When the temperature cools to the liquidus temperature, the primary Mg2Si begins to precipitate by attaching to the generated small particles of Mg3P2.The addition of Al-3.5P master alloy brought remarkable modification effect to the eutectic Mg2Si in Mg-5Al-1Zn-1Si-0.36Mn (AZS511) alloy. The morphology of eutectic Mg2Si changed from coarse Chinese script to fine granular shape. Mg3P2-coated Al8Mn5acts as the duplex nucleation site of Mg2Si phase in the P-inducing AZS511alloys. In addition, a small amount of P is absorbed on the solid-liquid interface of Mg2Si phase and inhibits the anisotropy of directional properties, which is beneficial to the improvement of Mg2Si morphology. The average size of α-Mg decreased from1000μm equiaxed dendritic structure to300μm.For the ASZ741alloys with high contain Si, beside the Mg2Si size decreased with the increase of Al-3.5P master alloy the grain size of α-Mg decreased obviously. The average size of α-Mg decreased from302μm to106μm,90μm and46μm separately. The refinement mechanism of the grain size is because the tiny granular Mg2Si particle precipitates preferentially, which provides an effective nucleating substrate for (Mg2Si+α-Mg) eutectic cell and contributes the grain refinement of α-Mg.The investigate result shows that AlP in Al-P transferred to Mg3P2by atomic diffusion after adding Mg. It is also found that the phosphide in Al-8Mg-3P alloy is Mg3P2which has a good refinement effect for the Al-Si and Al-Mg2Si alloy. That because after adding Al-8Mg-3P in the alloy, the small particles of AlP will generate in-situ melt which serve as the heterogeneous nucleation sites of Si and Mg2Si, and we can get the refined microstructure at last.