Effect Of Grain Refiner Involving Nitrogen On AZ31 Alloy Microstructure And Properties

Grain size is one of the most important factors determining the quality of cast magnesium alloys products because it significantly affects the mechanical properties. The addition of grain refiner to melt is the most maneuverable methods among all the melt grain refining methods. Currently aluminium-free magnesium alloys can be readily grain refined by Zr. But there isn't any stable and effective refiners which can be adopted in the most popular Mg-Al alloys.In this thesis, effect of AlN in-situed by Mg₃N₂ addition on the microstructure and properties of AZ31 alloy was investigated with optical microscopy (OM), X-ray diffraction (XRD), scanning electric microscopy (SEM) with energy dispersive spectrometer (EDS), corrosion potential test, tensile test, etc. It offers some experimental basement and theoretical reference for developing a novel refiner with N element to used in Mg-Al alloys.It was found that in-situ synthesis of AlN in AZ31 alloy by Mg₃N₂ addition with different time and quantity presents grain refining efficiency on AZ31. When lwt% Mg₃N₂ are added to AZ31 alloy and held for 30min at 720℃, the mean grain size of alloy is 49% lower than that of virgin alloys; the tensile strength and specific elongation as cast improve 14% and 42% respectively. No further refinement of grain size was achieved with holding time for 50min or 90min. In addition, after Mg₃N₂ was added to AZ31 alloys, the number and size of β-Mg₁₇Al₁₂ phase of alloys also changed. As holding time for 90min, the number of β-Mg₁₇Al₁₂ got a remarkable decrease and changed into distributting dispersedly, while the size was smaller.Fabricate process of master alloy including AlN and effect of master alloy on the microstructure and properties of AZ31 were investigated. Result show it is feasible that Al-AlN master alloy is fabricated with AlN in-situed with Mg₃N₂ at the vacuum medium frequency induction melting furnace. The Al-AlN master alloy prepared with this kind of process had grain refinement efficiency in AZ31 alloy. When the addition of the master alloy was up to 2%, the average grain size of the alloy was decreased with 31%, while the microstructure was homogeneous and the mechanical properties were...