Effect Of External Field And Modification Treatment On Microstructure And Properties Of Mg₂Si/AZ91 Composites

Magnesium alloy has been widely used in industry field, with its specific strength, high specific rigidity, low density and so on. At the same time, its good casting property can not only reduce the cost of the product, but also bring a lot of path for the research of the magnesium alloy casting modification. because of the single materials’ performance limitations, people often prepared new metal composite materials to meet the requirements of parts by the way of physical or chemical methods, which according to the different functional requirements required by design or working conditions. In recent years, researchers have found that the introduction of external field treatment in the alloy smelting process or add a trace of rare-earth element can significantly improve the morphology of the matrix, reduce the size of the alloy, improve the performance of the alloy. in this paper in-situ Mg₂Si/AZ91 composite materials were prepared by direct melt reaction method, The effects of ultrasonic wave, pulsed magnetic field, rare earth Er and Gd on the microstructure and mechanical properties of in-situ Mg₂Si/AZ91 composites And analysis of the mechanism were studied, and the results show that:1) With the ultrasonic treatment, The in-situ Mg₂Si/AZ91 composites matrix grain refined obviously, the morphology of primary Mg₂ Si phases was changing from chunk to fine particle, eutectic β-Mg₁₇Al₁₂ phase was Less precipitated. Compared with the ultrasonic treatment time and power, the effect of ultrasonic treatment temperature on the tensile strength and elongation of the composite is greater. The tensile strength and elongation of the composite were the highest when the ultrasonic treatment temperature was 690℃, ultrasonic power was 700 W and ultrasonic treatment was 65 s.The tensile strength increased from 167 MPa to 209 MPa, and the elongation increased by 1.8% to 3.3% after ultrasonic treatment, each were promoted by 25% and 89%.2) With the increased content of rare-earth element Er,the grain size of the Mg₂Si/AZ91 composite showed increased and then decreased, the refining effect was most obvious when Er content was 0.5wt.%, the average size of primary Mg₂ Si phase decreased from more than 50 microns below 20 microns, its shape changed from largepolyhedron into small pieces, and distributed more evenly. At the same time the tensile strength and elongation reached 190 MPa and 2.7%, Compared with the composite without introducted Er increased by 13% and 50% respectively.3) With the increased content of rare-earth element Gd, the grain size of the Mg₂Si/AZ91 composite showed increased and then decreased, the refining effect was most obvious when Gd content was 0.9 wt %, the average size of primary Mg₂ Si phase decreased from more than 50 microns below 25 microns. Morphology of β-Mg₁₇Al₁₂ is changed from coarse and continuous bone to short rod shape. The tensile strength and elongation reached 187 MPa and 2.6%, Compared with the composite without introducted Gd increased by 11% and 44% respectively.4)On the effects of pulsed magnetic fields, the in-situ Mg₂Si/AZ91 composite of the matrix grains are refined obviously, the morphology of primary Mg₂ Si phases is changing from chunk to fine particle, When the preheating temperature of the mold is250-400℃, the pulse voltage is 0-200 V, the pulse frequency is in the 0-8.0Hz, the grain size of the composite material always show decreases first and then increases.