The Research On The Microstructure And Process Of Thixomolding AZ91D Magnesium Alloy

At present,magnesium alloy is the lightest metal can be used as structuralmaterial. As a green engineering material for the 21st century, magnesiumalloy is getting more and more applications in various fields, and themagnesium alloy research is becoming the focus of the material development.On the other hand, Semi-solid forming (SSF) are being a potential technologyfor metal forming for its many advantages. Thus, the magnesium alloy partsfabricated by SSF technology are got more and more attention.Now Thixomolding, the only SSF method for magnesium alloy on industrialscale, is suited to manufacture products with complicated figure and thin wall,especially to the electronic component shells. Thixomolding is considered asone of the most promising metal forming technology. This new technologywas firstly introduced into China in 2002. For the foreign patents protection,the properties and technology of Thixomolding process have not beenunderstood till now.The 220 Tons Thixomolding machine made by Japan Steel Works (JSW)was used to fabricate the samples in this research. According to therequirement to Thixomolding materials and the characteristic of this formingmethod, AZ91D, the most popular magnesium alloy in the present industrialfield, was selected to fabricate the mobile phone house. The mobile phonehouse has complicated structure and thin wall about 1mm, and this kind ofproduct can adequately show the advantages of Thixomolding. Themicrostructure, forming technology and magnesium alloy chips meltingbehavior of Thixomolding were investigated in this research, and an evaluatesystem for semi-solid microstructure and a melting model of Thixomoldingwere also brought up to analyze the microstructure and explainmicrostructure transition.The solid phase fraction, effective solid phase fraction, fractional parameter,etc. of semi-solid microstructure are needed to analyze in this research. Thecurrent commercial software, such as SISC developed by Zhongke Company,can not meet all analysis demands. For example, the solid phase wasselected by the mouse in solid phase analysis process for most ofcommercial software. This selection is according to similar gray degreedefined before the analysis. If you want to select the whole solid phase, youmust change the pre-defined gray degree consequently. The changepre-defined gray degree process needs more time and patient. The fatalproblems is the selection method can not deal with the effective solid phasefraction which is including liquid phase fraction in the solid phase and somenoise phase have similar gray degree with solid phase. Moreover, thecommercial software can not deal with the area and perimeter of each solidphase particle and calculate the fractional parameter. In order to solve theabove problems, the evaluating rules for semi-solid microstructure werebrought up, and the analysis software was developed on MATLAB 7.0platform in this study. Furthermore, the analysis system can be used not onlyin the Thixomolding magnesium alloy microstructure analysis, but also in theother SSF metal.The microstructure and elements distribution in solid and liquid phase ofThixomolding process were investigated in this research. Thixomoldingmicrostructure has little porosity and impurity and consists of different solidphase forms and fined equal-axial solidified microstructure. The matrix, thefined equal-axial solidified microstructure, consists of α-Mg eutectic solidsolution and inter-metallic β phase surrounding the grain boundary. The solidphase particles distributes in the matrix at random. Two type of solid phaseparticles morphology are found in microstructure. Firstly, some α-Mg solidparticles close to spheroid scatter in the matrix. Secondly, the solid particlecontain some liquid phase which is entrapped the solid phase during stirringof the screw. According to the grain boundary phase analysis of matrix insemi-solid microstructure, the grain boundary phase was mainly composed ofMg, Al, and including a little Zn. The above analysis show the microstructuremorphology of matrix is similar to that fabricate by high pressure die casting(HPDC).The effects of process conditions including barrel temperature, moldtemperature, screw rev and injection velocity on the microstructure andmechanical properties of AZ91D magnesium alloy has been carried out. Theselection rule of main Thixomolding process parameters was put forward.The experimental results can be summarized as follows: 1. the primary solidphase fraction mainly depends on the barrel temperature in Thixomoldingprocessing. When the other process conditions are constant, highertemperature of barrel means more proportion of melting α-Mg and lowerproportion of the residual solid phase in solidification;2. Mold temperaturedecided the cooling rate after the semi-solid slurry entered into the mold.When the other process conditions are constant, lower temperature of moldmeans quicker cooling rate, and the products with finer microstructure andbetter mechanical properties could be acquired. If the mold temperature is toolow, the products would have some defects, such as the crack, the warp, etc.In general, mold temperature should be set higher to thinner products thanthicker ones. 3. Screw rev mainly determines the morphology of the solidphase among the Thixomolding microstructure. The opposite consequencehas been get, that is, when the barrel temperature is lower, faster screw revmeans finer solid phase particulate, and the products with better mechanicalproperties would be gained. When the barrel temperature is higher, the resultis reverse, namely, lower screw rev means finer solid phase particulate andbetter properties. 4. The injection velocity could decide the flowing rate thatthe semi-solid slurry entrance into the mold cavity. The value of injectionvelocity should guarantee that the semi-solid slurry fills the mold in a planarflow and bigger filling density. When the value of injection velocity is too low,the semi-solid slurry could not full the mold cavity completely. On one hand,the products' density might be lower. On another hand, the mechanicalproperties of the products might be worse. If the injection velocity is too high,the melting metal enters the mold in a turbulent way, which could inducemore entrapped air and porosity. Therefore, the injection velocity usually isset at medium value, and should be set higher to thinner products thanthicker ones. By optimizing the process parameters, the conditions ofmanufacturing mobile phone house in this research were as follows:Under different processing parameters, the fracture mechanism ofThixomolding AZ91D magnesium alloy was examined. It was revealed thatthe fracture mechanism of Thixomolding AZ91D alloy exhibits quasi-rupture.Most of the cracks is formed in the porosity or wrap of the microstructure. It isdifficult to distinguish the solid phase particulate and the matrix. The rupturesurface consists of different size and depth holes and different quantity crack.The mechanical properties of Thixomolding sample decrease with surfacemicrostructure is coursing.The processing from the feedstock to semi-solid slurry, especially themelting behavior of Thixomolding slurry, was analyzed. So the structuralcharacteristic of Thixomolding screw was analyzed. A mathematics model forthe melting behavior analysis was built. By calculating, it is proposed that thesize and morphology of the solid phase particulates depended on theviscosity of the semi-solid slurry and rotate rev of Thixomolding screw.Meanwhile, the calculated tendency about the size and morphology of solidphase particulates was consistent with the testing data, which also provedthat the mathematics model founded in this research is correct.The results obtained in this dissertation will play an important guidance rolefor Thixomolding of magnesium alloy in the further industrial application.