Research On The Melting, Microstructure, And Mechanical Properties Of AZ31-xNd Mg Alloys

This paper deals with the systematic investigation on the melting, preparation, corrosion resistance, microstructure and mechanical properties of as-cast and as-extrusion AZ31-xNd Mg alloy, aimed at improving the properties of AZ31 Mg alloy.based on the thermodynamics analysis on the chemistry of alloy melt and the evaporation behavior of alloy ingredients, the possible chemical reaction during melting process were defined and the excess of different ingredients in alloy formulation was calculated so as to refrain unfavorable reactions and realize the accurate contral on alloy composition. The improvement in melting device and the optimization in melting parameters in accordance with the melting characteristics of Mg alloy assured the preparation of high-quality AZ31-xNd Mg alloy, which lays the foundation for the subsequent characterizations. The novelty of the new technology is the integration of melting, disposing and casting, which realized the safe, clean and effective melting of Mg alloy through automatic and continuous operation in closed system and reduced air pollution by preventing the use of melting agents and SF6. The closed integration system of melting and casting magnesium alloy features repeated purging of impurity gases in melting chamber with Ar so that the melting and casting processes can be proceeded in Ar atmosphere. The casting mould features the combination of connector principle and the melting characteristics of Mg to give Mg alloy with higher purity for test and measurement.The effect of Nd and its content on the microstructure of AZ31 alloy was investigated. It was shown that Nd exists in as-cast Mg alloy in the form of particle- or needle-like Al2Nd and Mg12Nd intermetallic compounds. With the increase of Nd in Mg alloy, both the amount and the particle size of these compounds increases. During extrusion process, Al2Nd and Mg12Nd intermetallic compounds, which are enriched on grain boundaries or dissolved inα-Mg matrix of as-cast Mg alloy, re-precipitates as fine particles and are uniformly dispersed in the extruded structure.It was found that Nd can significantly refine the grain of AZ31 alloy. In as-cast alloy, the minimum grain size(29μm) occurs at Nd content of 0.6 %, showing a decrease by 57.9% from the original size (68μm) for Nd-free sample. The main refinement mechanism of as-cast Mg alloy by Nd is the increase in composion overcooling at solid-liquid interface that is induced by the re-distribution of solvate during comdensation. In as-extruded alloy, the minimum grain size (10μm) occurs at Nd content of 0.6%, showing a decrease by 61.5% from the original size (26μm) for Nd-free sample. The main refinement mechanism of as-extruded Mg alloy by Nd is the amount, shape and distribution of the second phase particles precipitated during extrusion.The effect of Nd and its content on the mechanical properies of AZ31 alloy was investaigated. It was shown that the addition of Nd can improve the room temperature tensile property of as-cast Mg alloy. The maximum tensile strength (245MPa),yield strength (171MPa) and elongation (9%) occur at Nd content of 0.6%, showing increase by 14.3%, 24.6% and 44.4%, respctively, from their corresponding values for Nd-free sample. The refined grain strengthening and precipation strengthening are the main factors improving the mechanical properties of as-cast Mg alloy.The addition of Nd can improve the room temperature tensile property of as-extruded Mg alloy. The tensile strength (325MPa), yield strength (247MPa) and elongation (18.1%) for the alloy with 0.6% of Nd show increase by 13.6%, 27.5% and 14.9%, respectively, from their corresponding values for Nd-free sample. The second phase strengthening and precipation strengthening are the main factors improving the mechanical properties of as-cast Mg alloy.Similar relationship between microstructure and room temperature tensile for as-cast and as-extruded Mg alloys were observed, that is, tensile strength, yield strength and elongation increase with the decrease in grain size. The Mg alloy with 0.6% of Nd shows the smallest grain size, therefore, the best tensile properties.The addition of Nd can improve the high temperature tensile properties of as-extruded AZ31 alloy. With the increase in temperature, tensile strength and yield strength decrease, while elongation increases. At the same temperature, the tensile strength, yield strength and elongation of Nd-containing alloy are higher than those of Nd-free one. The main strengthening mechanism for improving the high temperature mechanical properties of as-extruded AZ31-xNd alloy is the thermal stability of second phase.The effect of Nd on the corrosion resistance of as-extruded AZ31 was investigated. It was shown that the addition of Nd can significantly improve the corrosion resistance. The alloy with 0.6% of Nd shows the best corrosion resistance. The existence of second phase and the decrease in grain size favor the corrosion resistance. Nd affects the corrosion resistance through the grain refinement and the action of second phase.Because Nd can improve the mechanical and corrosion resistance properties of AZ31 alloy simultaneously, the modification of Mg structure by Nd forms an important research area for the development of Mg alloy.The measurement of mechanical properties indicates that the tensile strength and elongation of AZ31-xNd are higher than those of AZ31 alloy. The tensile strength and elongation of as-cast AZ31-0.6% show increase by 81% and 136%, respectively, from the corresponding values of as-cast AZ31 alloy. For as-extruded alloy, AZ31-0.6%Nd shows the increase by 25% and 20.7% in tensile strength and elongation, respectively, from the corresponding AZ31 alloy.