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SEMISOLID MICROSTRUCTURE OF AZ91D MAGNESIUM ALLOY REFINED BY MgCO3 T. Chen, X. Jiang, H. Huang, Y. Ma, Y. Li and Y. Hao


Key Laboratory of Gansu Advanced Nonferrous Metal Materials, Lanzhou University of Technology, Lanzhou, China Copyright © 2012 American Foundry Society


Abstract


The microstructural evolution of an AZ91D alloy refined using 1% MgCO3


is investigated during partial remelting


at 580C (1076F). Simultaneously, the effect of the initial as- cast microstructure on the semisolid microstructure and the microstructure uniformity of semisolid rods are discussed. The results indicate that the microstructural evolution includes four stages: • the initial coarsening • structure separation • spheroidization • final coarsening


The finer the as-cast microstructure, the smaller and more Introduction


Thixoforming is a relatively new metal forming technol- ogy and has many of the advantages of traditional casting and forging methods. Thixoforming’s main advantage is to significantly decrease or even eliminate porosity because of two reasons: one is the relatively small solidification shrink- age due to the existence of a specific amount of solid phase prior to solidification, and the other is the good feeding ability for solidification shrinkage due to the spheroidal morpholo- gies of the solid phase.1,2


This is especially attractive for al-


loys with large solidification intervals. It is well known that magnesium alloys have high specific strength and thus have a large application potential in automobiles, aerospace vehicles, sporting goods, portable tools and electronic products that re- quire more lightweight designs.3


Among the numerous mag-


nesium alloys, AZ91D is the most common. But, AZ91D’s mechanical properties are relatively lower when compared to other light metals and aluminium alloys, and can not match the requirements in many applications.4


thixoforming is considered as a promising way to improve mechanical properties of AZ91D alloy.


The advantage of thixoforming is the resultant semisolid microstructure containing small and spheroidal primary par- ticles uniformly suspended in the liquid phase.1,2


There are


many methods which have been developed to yield semi- solid ingots with such a microstructure. Comparatively, this


International Journal of Metalcasting/Winter 2012


sons is that the solidification interval of AZ91D alloy is quite large and porosity is easily formed during casting.5


One of the main rea- Therefore,


spheroidal the primary particles are in the semisolid microstructure. For alloys with a grain size of 50-100µm, the resulting primary particle size is slightly larger than the initial grain size. But for the alloys having larger size grains (>150µm), the result is just the opposite. The main reason which leads to this difference is the different evolution mechanisms determined by the as-cast microstructure. To obtain a semisolid ingot with uniform microstructure, an ingot with uniform as-cast microstructure must be used.


Keywords: AZ91D magnesium alloy, grain refinement, partial remelting, microstructural evolution, semisolid microstructure, as-cast microstructure


grain refining method does not require special equipment or procedures and only a small quantity of grain-refiner is added during traditional casting followed by partial remelt- ing. So the grain refining method is quite simple and should have large potential in future practice.1,2


The resulting semi-


solid microstructure is closely related to the initial as-cast microstructure and the microstructural evolution during par- tial remelting. However, most of the existing investigations on the microstructural evolution have been focused on the pre-deformed magnesium alloys.6-11


Only a few papers have states.12-18


involved as-cast alloys and they have emphasized the mor- phology change and coarsening behavior of the primary par- ticles or Mg2


Si particles in Si-containing alloys in semisolid But the microstructural evolution prior to liquid


formation has an important effect on the final semisolid mi- crostructure.19


Existing literature has rarely discussed the ef-


fect of the initial as-cast microstructure on the semisolid mi- crostructure or the relationship between these two kinds of microstructures. In addition, the specimen dimensions used in the existing investigations are always less than 20 mm in diameter.6-18


The dimensions of the starting ingots used in


practice are always much larger. How does the microstruc- tural uniformity of the semisolid ingots vary with the larger dimensions? The investigations into this aspect are also very scarce for magnesium alloys.


Therefore, in this study, the microstructural evolution, espe- cially that prior to the liquid formation, of AZ91D alloy re- fined by MgCO3


has been investigated during partial remelt- ing. In addition, the semisolid microstructures of AZ91D 43


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