Papers by Keyword: AZ91D

Abstract: The aim of the present work was to study the influence of beta phase precipitates content and distribution on the corrosion behavior of the AZ91D magnesium alloy, using samples subjected to solution annealing and aging. The morphology of the beta phase was observed using optical microscopy and scanning electron microscopy. The volume fraction of each phase was also determined. Potentiodynamic polarization tests and immersion tests were carried out, indicating an increase of the corrosion resistance with the volume fraction of the beta phase and its distribution on the grain boundaries of the alpha-phase. Solution annealed samples were more susceptible to corrosion.

Abstract: An enthalpy equilibrium electromagnetic-stirring process was introduced to semisolid metal processing for rheoforming route. AZ91D was die-casted under normally liquid state of Mg alloy, semisolid state of Mg alloy stirred using traditional EM-stirrer and an enthalpy equilibrium EM-Stirrer, respectively. Further step, effect of pouring temperature on microstructures and tensile properties of castings was researched in the introduced process. Experimental results indicated that samples of rheo-diecasted in the introduced process showed globular primary solid particle microstructure and better comprehensive mechanical properties, typical microstructures consisting of globular primary α-Mg grains and pseudoeutectic organization was observed in the castings with low solid fraction, while in higher solid fraction, the microstructure comprised of agglomerating primary solid particles and Mg₁₇Al₁₂ of divorced eutectic. With decreasing pouring temperature, the density of primary solid particles increased greatly, the size of primary solid particle evolved to coarse.

Abstract: The presence of Hollow particles instead of gas porosity provides a closed cell structure called Syntactic foams. Syntactic foams have gained significant attention in recent years due to their low density, moisture absorption and thermal expansion coefficient compared to other cellular materials, such as open and closed cell structured foams. In terms of mechanical behavior, it is generally more insightful to compare metal matrix syntactic foams with metal foams and metal matrix composites. In comparison with metal foams, they have high compressive yield strength and more homogenous mechanical properties but usually higher densities and lower plasticity. In comparison with metal matrix composites, they have lower strength but offer compressibility, which is not existence in metal matrix composites. Syntactic foams have been extensively studied for aluminum based metal matricesand polymer matrices. Importance in magnesium foams is increasing in recent periods due to their very low density. Only a few studies are available on magnesium matrix syntactic foams processed through powder metallurgy techniques. This review presents an overview of hollow particle filled magnesium matrix (AZ91D/microballons) syntactic foams using powder metallurgy methods.

Abstract: Magnesium alloys are very active and readily ignite during heating and melting. In this study, the ignition-proof property of AZ91D with Nd and Dy addition was discussed, and the effects of these ignition-proof elements on the microstructure and microhardness of AZ91D investigated. The results show that the ignition-proof performance of magnesium alloy is improved obviously. The ignition resistance is attributed to the compact oxide films, which consist of MgO, Al₂O₃, Nd₂O₃, Dy₂O₃. When Dy content increased beyond 0.5%, the change tendency of ignition point of the alloy follows the shape of “v” curve, and the ignition point increased with Nd addition. The optimum composition of the newly developed magnesium alloy is AZ91D-3Nd-0.5Dy with the ignition point about 60K higher than AZ91D. And also AZ91D-3Nd-0.5Dy has fine microstructure and higher microhardness.

Abstract: In order to develop ignition-proof magnesium alloys, the effect of alloying element Ce on the ignition-proof properties and surface tension of AZ91D-2.5Ca alloy were investigated. The results show that the addition of Ce can significantly raise the ignition temperature and change the structure of surface oxide film as well as the surface tension. When the Ce content increases to 1.2%, the ignition point reaches 1371K. Upon the addition of Ce, the oxide film of AZ91D-2.5Ca melt changes to fine and close structure from the porous structure before the Ce addition. The composite oxide film mainly composed of MgO, Al₂O₃, CaO and Ce₂O₃. The surface tension of AZ91D-2.5Ca decrease with the increasing Ce content, as the active-element Ce enriched at the melt surface.

Abstract: The influence of ceramic shell mould thickness and permeability on investment casting of AZ91D alloy using in-situ melting technique was investigated. AZ91D granules together with melting flux were charged into two different moulds having different thicknesses and four various permeabilities; then were heated at 650°C in order to be melted. Visual inspection and scanning electron microscopes were used to characterise the surface quality of cast samples. Thermal analysis was employed to further analyse the effect of mould thickness on cooling and solidification behaviour of molten metal. The findings of this research showed that thinner mould provided higher solidification rate, which is believed to favour in-situ melting enhancement. It enabled melting of the granules at the investigated temperature resulted in suppressing mould-metal reaction and producing cast samples with good surface quality. The results also showed that the permeability of shell mould was ineffective in suppressing mould-metal reaction.

Abstract: Cast magnesium AZ91D, used for cases, covers and housings, for consumer electronics faces increased requirements on aesthetically pleasing surfaces. The casting conditions have strong effect on as-cast surface roughness for thin-walled castings. This is currently not well understood. In the current study surface roughness was measured parallel and perpendicular to the filling direction and on both sides on straight flat thin speci¬men. The parameters studied were First and Second phase injection speed, Cooling time, and Melt temperature. The Fix and Moving side die tempera¬tures were varied, with the fix side was kept hotter. A D-Optimal experimental design was used resulting in 31 different settings. The re¬sults showed that roughness was decreased by mini¬mizing the temperature difference between the two die halves, increasing cooling time and first phase injection speed. Increasing the second phase injection speed increased roughness.

Abstract: Flexibility of the Council for Scientific and Industrial Research’s Rheocasting System (CSIR-RCS) and its rheo-high pressure die casting (R-HPDC) technology is again demonstrated, as with aluminium alloys, by processing and shape casting of three different magnesium alloys (AM50A, AM60B, AZ91D) in a first attempt. All as-cast microstructures are characterised more by rosette shaped globules of the primary-(Mg) phase together with Mg17Al12 as evidence of non-equilibrium cooling rates. Surface liquid segregation is observed in the as-cast microstructure for all three alloys. Minor alloy additions of Mn, in composition specifications, results in the formation of Al8Mn5 intermetallic phase particles dispersed throughout the microstructure. All alloys were homogenised at 415 °C for 16 hours for the T4 condition. The Mg17Al12 phase dissolves with homogenisation while the Al8Mn5 intermetallic phase does not dissolve. The resulting tensile properties of all three alloys in the as-cast and T4 conditions are reported.

Abstract: In this paper, chip recycling technology combined with SIMA method which is called CR-SIMA method was adopted to prepare semi-solid billets. AZ91D magnesium alloy was refined by Er and its microstructural evolution was investigated during semi-solid isothermal treatment. The results show that Er can improve the feature of cast structure and decrease the grain size. Moreover, the γ-Mg₁₇Al₁₂ phase is well refined and disperses in the α-Mg matrix. A semisolid microstructure with small and spheroidal primary particles can be obtained after partially remelting. With increasing heating temperature, the dissolution of eutectic Mg₁₇Al₁₂ phase first took place, resulting in the primary dendritic grains coarsening into interconnected non-dendritic grains. With heating continuously, the residual interdendritic γ-Mg₁₇Al₁₂ at the edges of the primary grains melted in succession and the primary grains separated into small polygon grains. During the semi-solid isothermal treatment, the amount of liquid increased until the solid-liquid system reached its equilibrium state. At the same time, owing to the decreasing of interfacial energy, the grains gradually spheroidized and began to grow with a further increasing of the holding time.

Abstract: Electroless nickel plating coatings on AZ91D alloy by phosphate pretreatment were prepared. The coating resembles a cauliflower and is compact. It consists of amorphous Ni-P. Potentiodynamic polarization curves of the Ni-P coatings in 3.5% NaCl show that the electroless Ni-P coating improves the corrosion resistance of AZ91D alloy and the highest corrosion potential reached to -0.513V vs. SCE. The results of the orthogonal experiment of process parameters indicates that temperature 80°C, time 120min and pH 7.5 can be considered as the optimum process parameters for electroless Ni–P plating on AZ91D alloy and pH value in the bath has more effect on corrosion potential than temperature and time.