Research On Microstructure And Thermal Stability Property Of AZ91D Magnesium Alloy Containing Rare Earth By Sub-Rapid Solidification

As the lightweight structural materials in the 21 st century, magnesium alloy has broad application prospects in the automotive and electronics fields due to its advantages of high specific strength, specific stiffness and superior damping capacity and good electromagnetic shielding. However, the magnesium alloy produced by conventional casting process exhibits coarse grain, and the continuous network eutectic phase at grain boundary is incline to coarsening under high temperature,which leading undesirable strength and poor creep performance. Consequently, the further application of magnesium alloys is restricted. It is well known that grain refinement can effectively improve the mechanical properties of magnesium alloy. As one of the most valuable elements in magnesium alloys, rare earth can increase the strength via grain refinement and improve the high temperature creep property. With the addition of rare earth, the needle-like phase is formed, which generates a strong fragmentation effects on matrix and thus influence the improvement of mechanical properties. As an important technology for the fabrication of novel high-performance structural materials, sub-rapid solidification technology can not only greatly refine grain, eliminate needle-like phase structure, but also improve mechanical properties,processability and corrosion resistance significantly. Therefore, the combination of rare earth alloying and sub-rapid solidification technology can increase high temperature strength and creep resistance of cast metal, which provide theoretical basis for development of high performance magnesium alloys.In this study, sub-rapidly solidified AZ91 D magnesium alloy containing RE element is fabricated by copper mould spray-casting technique on high vacuum induction melting equipment. Not only the refinement effect of rare earth and sub-rapid solidification on AZ91 D magnesium alloy is researched, but also the effects of different and its content on the primary grain size, solute concentration and phase structures is analyzed. The precipitation of solute-saturation solid solution, the solubility of eutectic phase and grain growth of primary phase were obtained by different heat treatment processes. Based on these studies,the effect of different process parameters on micro-hardness and compression strength of sub-rapid solidification magnesium alloy is studied.After sub-rapid solidification, the average grain size of AZ91 D magnesiumalloys decreases from 400μm to 50μm and the microstructure become to fine equiaxed grain; with the addition of 0.75wt% rare earth(Er, Ce), the average grain size decreases to 50μm, the fine equiaxed grain become to grain. When the content at the same, the refinement effect of Ce is more significant in comparison with that for Er. With Sub-rapid solidified of AZ91 D magnesium alloy containing RE element, the needle-like phase(Al3Er, Al11Ce3) become to grain.The refinement mechanism of rare earth can be attributed to the improvement of the undercooling in front of solid/liquid interface caused by solute concentration during solidification and the formation of Al11Ce3 restrain α-Mg grain growth.Heat treatment results show that the microstructure of sub-rapidly solidified AZ91D-0.75wt%Er alloy occurs obvious grain growth phenomenon with the increase of holding time of isothermal solid solution at 420℃. However, the microstructure of sub-rapidly solidified AZ91D-0.75wt%Ce alloy maintain satisfactory thermal stability behavior of as-fabricated fine-grain. Due to the combined effect of fine grain strengthening, dispersion strengthening and solid solution strengthening, the micro-hardness and compression strength of sub-rapidly solidified alloy increased49.2% and 47.8% respectively. Due to gradual dissolution of β-Mgl7Al12 eutectic phase and grain growth of primary phase, the micro-hardness of sub-rapidly solidified AZ91D-0.75wt%Er alloy decrease with the solution time increase. As the precipitation of β-Mgl7Al12 phase and the Mg-Er phase, the micro-hardness of sub-rapidly solidified AZ91D-0.75wt%Er alloy increase with the rising of holding time of isothermal aging at 200℃.