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Study On Semi-solid Slurry Preparation And Mechanism Of AZ91-Ca Ignition-proof Magnesium Alloy By Gas Bubble Stirring

Posted on:2015-04-18Degree:MasterType:Thesis
Country:ChinaCandidate:J XuFull Text:PDF
GTID:2181330452464198Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
The semi-solid forming method was developed in1970s and is anadvanced metal forming technique. The key to semi-solid forming is thepreparation of semi-solid slurry with non-dendritic microstructure. Untilnow, many methods have been developed to prepare semi-solid slurry,such as mechanical stirring, electromagnetic stirring and so on. But manyof them may have defects such as high cost, complexity, low efficiency,and inhomogeneous or impure slurry. Therefore, novel slurry makingmethod and corresponding equipment is still needed to be developed forthe industrial applications. In addition, our country is rich in magnesiumresources, thus it has great value to find the way to overcome thedeficiency of being easily oxidized during the semi-solid forming.In the study, gas bubble stirring method was used for the preparationof semi-solid slurry. The effect of processing parameters on themicrostructures and the mechanism of gas bubble stirring treatment wasstudied. In addition, magnesium alloy was easily oxidized during themelting or forming process. Ca element was added to improve theoxidation resistance of AZ91alloy. The effect and mechanism of Caaddition on the microstructures was studied.The results show that it is feasible for the application of gas bubblestirring method for the semi-solid slurry preparation and the morphologyof primary α-Mg particles are influenced significantly by the processingparameters. The viable processing parameters for the rotating gas bubblestirring technique is the gas flow rate of about5L/min, the cooling rate of about0.245°C/s and the rotation speed of about150r/min. In thiscondition, the average particle size is71μm, the average shape factor is0.57and the solid particles distribute uniformly in the slurry.Because of the heterogeneous nucleation sites provided by theinserted gas bubbles and the uniform temperature and composition fieldcaused by intensive convection, the gas bubble stirring treatment improvethe nucleation rate. In addition, the convection will accelerate the transferof solute and heat, resulting in the improvement of growth rate. With theincrease of convection intensity, the thickness of the solute diffusionboundary layer will decrease and the effect of the constitutionalundercooling ahead of the liquid-solid interface will weaken, resulting inthe morphology of primary particles changed to rosette-like or globular-like. The morphology of the primary phase is globular-like under theturbulent condition and rosette-like under laminar flow condition.The addition of Ca element will refine the grains, reduce the quantityof β-Mg phase and new Al2Ca phase will form. When the Ca contentreaches0.97%, the Al2Ca phase will form a continuous network aroundthe grains.With the increase of Ca content, the particle density after the gasbubble stirring treatment will first increase and then decrease, while theparticle size has an opposite tendency. When the Ca content is0.51%, theaverage particle size reaches the minimum of76μm, the average shapefactor reaches the peak of0.58and the particle density is7982/cm-2. Inaddition, the intense convection will change the diffusion boundary layergeometry around the growing solid, which will weaken the effect of Ca.
Keywords/Search Tags:Gas bubble stirring, Process parameters, AZ91-Ca, Semi-solid slurry, Semi-solid microstructure
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