Semi-solid AZ91D Magnesium Alloy Research Articles

Corrosion Research and Fractal Characteristics of As-Cast and Semisolid AZ91D Magnesium Alloys

As-cast and semisolid AZ91D magnesium alloy samples were corroded in distilled water, 0.9% NaCl solution, and 3.5% NaCl solution for various times, respectively. With the increase of corrosion time and the concentration of corrosion solution, the corrosion degree of the alloys gradually deepened. The corrosion resistance of semisolid alloy with nearly spherical grain refinement was significantly reduced, resulting in more corrosion pits and a higher corrosion rate. According to the fractal analysis, the corrosion process of semisolid alloy was more uniform under the same conditions. The fractal theory accurately described the corrosion process, further providing a better grasp of the corrosion behavior of AZ91D magnesium alloy.

Effect of heat treatment on microstructure and properties of semi-solid squeeze casting AZ91D

Semi-solid AZ91D magnesium alloy billets were prepared by near-liquidus heat holding. Semi-solid squeeze casting was conducted at 575, 585 and 595 °C, respectively, with 1 mm·s−1 squeeze speed. The semisolid squeeze casting AZ91D samples were heat treated by T4 (solution at 415 °C for 24 h) and T6 (solution at 415 °C for 24 h + 220 °C for 8 h) processes, respectively. The microstructure and mechanical properties of the alloy in different states were investigated by means of OM, SEM and tensile testing machine. The results show that compared to as-cast alloy, the grain size of the semi-solid squeezed AZ91D decreased significantly, and with the increase of semi-solid squeeze temperature, the grain size of AZ91D increased. The grains of the alloy were refined by T4 treatment, and further refined by T6 treatment. T6 treatment greatly improved the tensile strength, elongation, and hardness, but did not significantly improve yield strength. After 575 °C squeeze casting and T6 treatment, the ultimate tensile strength (UTS) reached 285 MPa, the elongation reached 13.36%, and the hardness also reached the maximum (106.8 HV), but the yield strength (YS) was only 180 MPa. During the process of semi-solid squeeze casting and heat treatment, the matrix grain was refined and a large number of precipitated and secondary precipitated phases of Mg17Al12 appeared. Both the average size of matrix grain and secondary precipitated phase decreased, while the volume fraction of secondary precipitated phase increased. All these resulted in high tensile strength, elongation and hardness.

Rheological behavior of semi-solid AZ91D magnesium alloy at steady state

The rheological behavior of semi-solid AZ91D magnesium alloy was investigated in isothermal steady state condition. The effects of stirring temperature and shearing rate on apparent viscosity of semi-solid alloy slurry at steady state were discussed. The results show that the apparent viscosity of semisolid AZ91D alloy increases with increasing solid fraction. It increases slightly before the solid fraction reaches a certain value, about 0.4, and then goes up rapidly after the solid fraction reaches the critical value. However, the apparent viscosity decreases with increasing shearing rate, and the reduction amplitude is higher when the solid fraction is higher. According to the experimental data, an empirical equation that shows the effect of solid fraction and shearing rate on the apparent viscosity of semi-solid AZ91D alloy can be built as ηa=9.7×10−2exp(13.87fs)γ−0.58.

Application of cyclic upsetting-extrusion to semi-solid processing of AZ91D magnesium alloy

The microstructural evolution of AZ91D magnesium alloy prepared by means of the cyclic upsetting-extrusion and partial remelting was investigated. The effects of remelting temperature and holding time on microstructure of semi-solid AZ91D magnesium alloy were studied. Furthermore, tensile properties of thixoextruded AZ91D magnesium alloy components were determined. The results show that the cyclic upsetting-extrusion followed by partial remelting is effective in producing semi-solid AZ91D magnesium alloy for thixoforming. During the partial remelting, with the increase of remelting temperature and holding time, the solid grain size increases and the degree of spheroidization tends to be improved. The tensile mechanical properties of thixoextruded AZ91D magnesium alloy components produced by cyclic upsetting-extrusion and partial remelting are better than those of the same alloy produced by casting.

Numerical Simulation of Thermal-Fluid Coupled Field in Casting Mouth During Rheocasting-Rolling of Semi-Solid Magnesium Alloy

In the initial stage of rheocasting-rolling of semi-solid AZ91D magnesium alloy, the semi-solid slurry was injected into the casting mouth. The whole process was investigated by the VOF model in this paper. The influents of the geometric shapes of casting mouth on temperature field and flow field in casting mouth was analyzed. The numerical simulation results showed that flow field was very chaotic in the casting mouth with rectangular section, air was involved in the slurry, quality of the strip was badly impacted. But flow field was very gentle in the casting mouth with arcuate cross-section. Coupled numerical simulation in steady stage of rheocasting-rolling of semi-solid AZ91D magnesium alloy was done under different inflow temperature, distribution of temperature field,Solid fraction in casting mouth of arcuate cross section was obtained. This provided the basis for optimizing the processing parameters in rheocasting-rolling of semi-solid AZ91D magnesium alloy.

Rheological behaviour of AZ91D magnesium alloy in semisolid state

The rheological characterisation of semisolid AZ91D magnesium alloy has been successfully studied using a Couette rheometer. Different applied regimes by varying the temperature and the shearing in semisolid state revealed the rheological behaviour of the alloy and were confirmed by microstructural evolution. The apparent viscosity was the critical parameter that was studied during the applied regimes. The change in the morphology of the solid phase during the thermomechanical treatment was the main cause of the evolution of the apparent viscosity. The evolution of the morphology of the solid phase, from dendritic to globular, due to the shearing, had a significant impact on the rheological behaviour of the slurry which was correlated to the operating conditions by an analytical model.L’étude de la caractérisation rhéologique de l’alliage de magnésium de type AZ91D à l’état semi-solide a révélé une étroite dépendance entre les conditions opératoires et les changements structuraux. Les régimes thermo-mécaniques choisis ont montré que la vitesse de refroidissement ainsi que le cisaillement imposé avaient un effet direct sur la viscosité apparente de l’alliage. Le passage d’une structure conventionnelle dendritique à une structure globulaire a réduit de manière significative la viscosité, tributaire de l’intensité ainsi que du temps de cisaillement selon un modèle analytique.

Numerical Analysis on Thermal Field in Rheocasting-Rolling of Semi-Solid Magnesium

The process on rheocasting-rolling of semi-solid AZ91D magnesium alloy was investigated by the finite element method from the aspect of fluid mechanics .The mathematical model of the casting-rolling zone was built based on the analysis on the heat transfer mode and boundary conditions of the casting-rolling zone. The numerical simulations were carried out by different casting-rolling parameters to observe the full solidification point and find the best pouring temperature. The numerical simulation results showed that when the semi-solid slurry temperature was 584°C, magnesium strips with good structure and performance could be obtained by rheocasting-rolling. This simulation provided a valid evidence to determine the optimal process parameters.

Study on Filling Capacity of Semi-Solid Magnesium Alloy

The Semi-Solid Processing technology is an innovation manufacturing process. The filling capacity of the semi-solid metal has very important effects on the properties and qualities of the work pieces. Lots of heat factors have effects on the filling capacity of semi-solid metal. The filling capacity of semi-solid AZ91D magnesium alloy has been studied by means of fluidity test. The results indicate that the filling length increases with the increasing of the temperature and the filling capacity of semi-solid magnesium alloy with shearing is better than that of without shearing. The lower temperature, the more obvious the effect of shearing on the filling capacity is. However, if the temperature is very low, the flow of the slurry is poor and the effect of shearing on filling capacity decreases. The microstructures of semi-solid magnesium alloy after shearing is spherical and homogeneous.

Effects of ultrasonic field on microstructures and properties of semi-solid AZ91D magnesium alloy

The microstructure distribution rule of semi-solid AZ91D alloy treated by ultrasonic was researched, and mechanical properies of specimens before and after ultrasonic treatment were investigated further.Semi-solid AZ91D melt specimens were processed by ultrasonic under different powers, and its microstructures and mechanical properties at different sampling points in specimens were obtained. The experimental results show that the microstructure of AZ91D alloy at different sampling points under the same ultrasonic power is different in grain size and shape, and there is also great difference among their microstructures at the same sampling point under different ultrasonic powers. AZ91D alloy treated by ultrasonic can obtain increment in both tensile strength and plasticity. Under same ultrasonic power, mechanical properties of specimen at different sampling points have obvious difference, and regularity for change of mechanical properties everywhere is similar to regularity for change of grain size and shape everywhere.

Numerical Simulation of Semi-Solid AZ91D Magnesium Alloy in Thixoforming Process

In the process of metal plastic forming severe plastic deformations occur to the workpiece. As a result the constitutive relations between material and geometry are nonlinear. One of the most common tools to solve such problems is the finite element method. In thixoforming processes of semi-solid metal many factors affect thixoforming fluidity. Therefore it is important to simplify the problem and predict the metal flow properties. For that reason the thixoforming process of semi-solid AZ91D magnesium alloy was modeled with the theory of rigid viscoplastic finite element method using the commercial finite element software DEFORM-3DTM. The fluid and effective stress-strain fields in the thixoforming process were obtained and the relationships among stress, strain rate and temperature were analyzed.

Effect of asymmetrical deformation on the microstructure evolution of semisolid AZ91D alloy

The effect of asymmetrical deformation on the microstructure evolution of semisolid AZ91D magnesium alloy manufactured by strain-induced melt activation (SIMA) process was investigated. The specimens were compressed at 230 °C, and then were isothermally held at the temperature of 560 °C for different times. At the isothermal holding stage, recrystallization and liquid phase nucleation occurs in the severe deformed region. And the liquid nuclei grow up rapidly to connect together. At different deformed areas liquid forms in different places. With the passage of time, solid fraction decreases, while solid particle size increases in all parts. The variational speed varies from area to area. After isothermal holding for 20 min, the effect of asymmetrical deformation is small, and homogeneous and appropriate microstructure of a semisolid is obtained in the experiment.

Numerical and experimental investigations of semi-solid AZ91D magnesium alloy in thixoforming process

Abstract Many complicated factors affect thixoforming fluidity in thixoforming process of semi-solid metal (SSM). So, it is important to forecast the flow property of SSM. Rigid visco-plastic constitutive model of semi-solid AZ91D Mg alloy was established at different strain rate. The thixoforming process of semi-solid AZ91D Mg alloy was simulated using commercial finite element software DEFORM-3D™. The fluid and effective stress–strain fields in the thixoforming process were obtained and the relationships among stress, strain rate and temperature also were analyzed. The thixoforming experiments were performed at 570 °C with different holding time. Finite element analysis results were compared with experimental ones, and experimental data has good agreements with the simulation results.

Effect of compression ratio on the microstructure evolution of semisolid AZ91D alloy

The effect of compression ratio on the microstructure evolution of semisolid AZ91D magnesium alloy manufactured by strain induced melt activation (SIMA) process was investigated. The specimens were compressed with different ratio at 230 °C, and then were isothermal hold at the temperature of 560 °C for 15 min. During the compression process, as the increasing of compression ratios, the microstructures of samples changed from dendritic to catenarian, and then to fibrous shape. These changes mean the breakdown of the branches of casting microstructure, and the storage of the deformation energy. At the isothermal holding stage, liquid phase nucleation occurs in the severe deformed region. And the liquid nuclei grow up rapidly to connect together. After that isolated solid particles were left, and the semisolid slurry with the globular microstructure was formed. The solid fraction reduced gradually with the increase of compression ratio, and the solid particle size reached a minimum at the critical compression ratio. After the particle size, solid fraction and sphericity were considered, the optimal compression ratio for this paper was 40%.

Effect of Ultrasonic Vibration on the Dispersion of Carbon Nano-Fibre Reinforcement through Semi-Solid AZ91D Magnesium Alloy

Ultrasonic vibration was applied to the surface of AZ91D/CNF slurries in order to homogeneously disperse the reinforcement through the semi-solid matrix, and eliminate the clusters of nano-fibres originated during previous processes. The method utilized to produce the slurry was a modified compocasting process that generates semi-solid magnesium alloy using an inclined cooling plate and mechanical stirring. The CNF reinforcement was added in quantities varying from 1 to 3vol% and different vibration times and amplitudes were applied to the slurries under isothermal conditions. Macro and microstructure combined analysis showed the occurrence of grain refinement in the whole ingot for higher wave amplitudes at shorter times, while for lower amplitudes longer times up to 10min. were required. Vibration times over 10min were not significant to further improve the results and times up to 4min did not seem to provide any result whatsoever. The best results, however, were obtained for 16 and 18μm amplitudes. Discontinuous application of ultrasound seems to produce better results than continuous vibration. The dispersion of reinforcement seems to occur by the smash of CNF clusters due to the movement of the solid fraction present in the Mg alloy slurry, and by cavitation, responsible also for some microporosity present in the microstructures.

Study on Static Shear-Rheologic Characteristics of Semi-Solid AZ91D Magnesium Alloy by Reheating

Static shear-rheology for self-made semi-solid AZ91D magnesium alloy slurry was studied by using the tester and a universal electronic machine. Shear-stress along with cylindrical surface in the sample was produced by using the tester and then shear-rheologic deformation happened. It showed that on the condition of the same loading, the longer the loading time and holding time were, the larger the deforming rate was. When the holding time attained a certain value, small grains acquired enough energy to grow up or amalgamate with prolonging the holding time and coarse grains started to melt from intergranular or grain boundaries, but the critical shear-stress kept a constant. The sample didn’t flow but appeared to instantaneous shear-strains with starting to load. When the loading exceeded the critical value, the sample started to flow and had the phenomena of elastic after-working, elastic before-working and remained deformation. The rheologic characteristics of semi-solid AZ91D magnesium alloy were expressed by the five element mechanical model: H1—(N1/H2)—(N2/S).

Thixotropic Behavior of Semi-Solid Magnesium Alloy

By a high-temperature Couette type viscometer, we studied the thixotropic behavior of the semi-solid AZ91D magnesium alloy slurry. According to different variable conditions, we could measure the change of the shear stress. The results showed that the shear stress of semi-solid AZ91D magnesium alloy slurry increased at starting shearing, and the shear stress fell down at the maintained shear rate shearing. In our experiment data, we found that the semi-solid AZ91D magnesium alloy slurry had the behavior of shear thinning fluid. As the maximum shear rate increased, the measured shear stress increased at a dissimilar level. When the rising time was shorter, the increasing range of shear stress was larger. As the shearing time of the maintained shear rate increased, the falling down degree of the shear stress increased and it can be presented as a function of shearing time, such as: e 0.0159t min 11331 τ = − . At the same shear rate, the area of hysteresis loop was bigger and the thixotropic behavior was more obvious.

Microstructure and mechanical behaviour of semi-solid die-casting AZ91D magnesium alloy

Microstructures and mechanical properties of an AZ91D magnesium alloy prepared with semi-solid die-casting (SSDC) were characterized in as-cast conditions. The SSDC alloy exhibits a unique microstructure featuring primary α-Mg globules uniformly distributed in the matrix of fine secondary α-Mg grains and β-Mg 17Al 12 intermetallic. High ultimate tensile strength and elongation have been achieved before fracture. Observations on the vertical-section microstructure of the fractured sample by scanning electron microscopy (SEM) show that the crack mainly originated from the brittle fracture of the eutectic phase causes the interface decohesion of the ductile Mg phase, making fracture a rather critical event. Before that, the deformation of ductile α-Mg phase in the matrix as well as the “pulling out” of primary α-Mg phase combines to provide the SSDC alloy a certain strain.

半溶融撹はん凝固したＡＺ９１Ｄマグネシウム合金の組織と機械的性質

Changes in microstructure and viscosity with both stirring conditions and a fraction of solid have been investigated in order to obtain fundamental aspects of various poroperties of a semi-solid stir-cast AZ91D magnesium alloy. Mechanical stirring was conducted during cooling of a liquid phase into mushy zone at a given temperature. Tensile test was performed for as-cast and T6-treated specimens. The primary α phase in specimens changed from the rosettelike α phase, whose dendrite arms were bent by stirring, to spheroid during the isothermal stirring. The spheroidization of the primary α phase and the decrease in their size were accelerated with the increase in stirring speed, stirrer width and the fraction of liquid. The stirring torque during the isothermal holding decreased in accordance with the spheroidization of the α phase. 0.2% proof stress of as-cast specimens was higher than the minimum value specified in JIS because of a fine eutectic phase crystallized by rapid cooling. Tensile properties of both as-cast and T6-treated samples had a tendency to decrease with increasing the fraction of solid. Tensile stress increased with the T6 heat treatment and the increment became higher with the fraction of solid being lower during isothermal stirring.