Semi-solid Magnesium Alloy Research Articles

Study on Semi-Solid Magnesium Alloy Produced by Mechanical Stirring

Semi-solid magnesium alloy slurry was produced by a newly self-developed mechanical stirring machine in order to prepare continuous semi-solid slurry. The changing principle of semi-solid microstructure was studied under different conditions—the changing of temperature, stirring rate, cooling rate and stirring time. It was showed that the lower the temperature of slurry, the higher the stirring rate, the longer the stirring time and the faster the cooling rate, the grain will be smaller and more evenly rounded. When the stirring speed and temperature are the same, better semi-solid spherical grains can be gained by increasing the stirring time. At the same stirring temperature and higher stirring speed, semi-solid spherical particles are smaller, rounder and more evenly; at a low stirring speed, we can't get ideal semi-solid spherical grains even if we increase the stirring time. The ideal technological parameters of semi-solid AZ61 alloy produced with no-dendritic structure were recommended to be 600°C～605°C (stirring end temperature) 、3min～ 5min(stirring time) and 500r/min(stirring rate). Compared with conventional casting alloy, semi-solid alloy mechanical properties were greatly improved. The non-dendrite semi-solid forming mechanism was studied; mechanical stirring semi-solid non-dendritic formation was the result of a variety of factors, the key cause is mechanical shear stress.

Investigation in Rheocasting-Rolling for Semi-Solid Magnesium Alloy Used by Slope

The experiment of the rheocasting-rolling for semi-solid magnesium was carried out on the equipment made by ourselves in our laboratory. Semi-solid AZ91D magnesium slurry was prepared by cooling slope and the evolution of microstructure under different technological parameters such as pouring temperature, casting temperature, standing time, angle of slope and length of slope et al were studied. The results show that the technological parameters have great influence on the microstructure of the semi-solid magnesium slurry. The results show that the optimum parameters of slope method were: pouring temperature 630 degrees；angle of slope 60°；length of slope 0.570m.

Study on Microstructure of Semi-Solid Magnesium Alloy Manufactured by Gas Bubbles Stirring

The semi-solid slurry of AZ31 magnesium alloy was manufactured by gas bubbles stirring. The effects of stirring temperature and stirring time on the microstructure of semi-solid slurry were researched. The results indicate that it is feasible to manufacture the slurry with particle-like primary phases. When the stirring temperature is higher, the morphology of the primary a-Mg presents rosette-like morphology and the size of grain is coarse, and the morphology mainly was α- Mg dendrite at lower stirring temperature. Long stirring time could reduce dendrites and liquid phase, and made the non-dendritic grains well spheroidized. But the effects of longer stirring on shape and size of primary particles are not significant. The optimal process parameters for ideal semi-solid slurry manufactured were stired 3min at 645°C.

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.

Formation of Non-Dendritic Microstructure of Semi-Solid Magnesium Alloy by Introducing Gas Bubbles

A new technique of introducing gas bubbles during isothermal period of AZ31 magnesium to prepare semi-solid slurry has been reported. The effect of resting time after stirring on the microstructure of the slurry is discussed. Results show that the dendrites and liquid phase reduced, the non-dendritic grains grew up and became spheroidized. A fully grain-refined structure can be obtained with 60s resting time.

Coupled Numerical Simulation of Process in Rheocasting-Rolling for Semi-Solid Magnesium Alloy Used By Slope

By the finite element method, the N-S momentum, energy equations and continuum equations, etc. which describe the fluid flow and heat transfer in the slope of the rheocasting-rolling for Semi-solid Magnesium Alloy were computed. The influence of the different slope parameters( pouring temperature, length of slope, lean angle)on the metallographic structure of semi-solid magnesium alloy was analyzed. The simulative results can provide effective data to confirm optimized slope size and technical parameter.

Liquid segregation behavior of semi-solid AZ91 alloy during back extrusion test

The present work is focused on the segregation behavior of semi-solid AZ91 magnesium alloy with Ca content. For this purpose a simple back extrusion test was used and the semi-solid billet was fabricated through a cooling slope. Different shear rates were obtained by variation in ram speed and extrusion ratio. Microstructures of the extruded samples were investigated and liquid segregation was described by Darcy's law. Homogeneity was improved by using higher shear rates, produced either by increasing plunger velocity or extrusion ratio, while at very high shear rates the air was entrapped inside the wall of the extruded sample.

Study on Thixo-Forging of AZ61 Wrought Magnesium Alloy

The semi-solid AZ61 wrought magnesium alloy was fabricated by mechanical stirring method. Thixo-forging set-up were designed and made. The traditional forging and thixo-forging processes of AZ61 magnesium alloy were studied with both computer numerical simulation and experimental methods. The constitutive model of semi-solid AZ61 alloy was established in our prior literature. The differences between traditional forging and thixo-forging processes were analyzed. Results indicated that smaller load and uniform strain & stress were acquired in thixo-extrusion process. Simulation results were good agreement with experimental ones.

Simulation of Temperature Field during Semi-Solid Magnesium Alloy Produced by Casting-Rolling Technology

The temperature fields during semi-solid magnesium alloy produced by casting-rolling technology has been simulated by finite element method on the basis of ANSYS. The temperature fields for different conditions were obtained, which is consistent with the experimental results. Results show that there is a high temperature field in the casting and rolling zone. The temperature fluctuates from the center to edge of the strip near the entry of the casting and rolling zone. but The temperature decreases gradually from the center to edge of the strip near the exit of the casting and rolling zone. There are some remarkable effects of the temperature of the casting and rolling, the velocity of the casting and rolling, the gap of two roll, the cooling of the roll and the diameter of the roll on the temperature field, which are in agreement with the experimental results.

Study on Semi-Solid Magnesium Alloy Fabricated by Ultrasonic Stirring Method

The perfect semi-solid magnesium alloy had been prepared using ultrasonic stirring method. The effect of technological parameters such as melt temperature, ultrasonic power and ultrasonic treatment time on the microstructure of semi-solid AZ61 alloy had been investigated. The results showed that the better refined and well-distributed globular semi-solid magnesium alloy had been acquired with the increasing of output power of ultrasonic stirring when melt temperature and ultrasonic treatment time were at a certain range.

Numerical Simulation of Rheo-Diecasting Mould Filling Process of Semisolid Magnesium Alloys

Base on the coupled three-dimensional model of flow temperature and apparent viscosity on semisolid materials, the mould filling processes of liquid and semisolid magnesium alloys have been analyzed. The simulation results showed that the semisolid mould filling with the characteristic of pseudo-plasticity and Bingham type is steady and the air entrapment is much smaller than that of liquid die casting. Therefore, the semisolid die casting can prevent from those defects produced by the traditional die casting, which offer a new technology for lower interior defects and better quality.

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.

The influence of shearing conditions on the rheology of semi-solid magnesium alloy

The influence of the shearing conditions on the rheology of semi-solid magnesium slurry was investigated by a Couette type viscometer. As the stirring time increased, the apparent viscosity first decreased until reaching a minimum value, and then gradually increased. The phenomena might be affected by the destruction of the dendritic structure (resulting in a decreasing curve) and particle coarsening (resulting in an increasing curve). The apparent viscosity was also influenced by the solid fraction, pre-stirring time, and resting time. When the pre-stirring time was fixed, particle coalescence might lead to an increase in the apparent viscosity as the resting time increased at higher solid fraction. At lower solid fraction, the release of entrapped liquid by Ostwald ripening might cause the apparent viscosity to decrease with increased resting time. When the solid fraction was fixed and the resting time increased, the apparent viscosity might increase due to particle coalescence (for a shorter pre-stirring time) and Ostwald ripening might make the apparent viscosity decrease by releasing entrapped liquid (for a longer pre-stirring time).

Selected Aspects of Semisolid Forming Magnesium Alloys

The conventional and novel routes of injection molding magnesium alloys are described, including thixo- and rheomolding, semisolid extrusion molding, near-liquidus molding, composite molding and alloy generation by mixing thixotropic slurries. For each technique the basic concepts along with their experimental verifications are explained. The examples show the universal nature of the injection molding and its capability of implementing a variety of modern processing methods.

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.

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).

Constitutive Model of Thixotropic Plastic Forming for Semi-Solid AZ61 Magnesium Alloy

Uniaxial compression tests on semi-solid AZ61 alloy fabricated by stain-induced activation (SIMA) process and the conventional casts were carried out using the Gleedle-1500 dynamic material testing machine. The relationships between stress and stain were analyzed. The numerical relationships among processing parameters (strain rate z ε& strain z ε temperature T liquid volume fraction L f ) and stress were studied. The proposed constitutive equation was established for semi-solid AZ61 magnesium alloy using the multiple nonlinear regression method. A scientific basic provided for both numerical simulation of processing process of semi- solid AZ61 alloy and rational choice and control of processing parameters.

Microstructural Evolution of Semi-Solid AZ61 Magnesium Alloy during Reheating Process

The microstructural evolution of AZ61 semi-solid magnesium alloy during semi-solid remelting process was studied in this paper. The semi-solid billet was fabricated by strain-induced melt activation (SIMA) method. The results showed that the initial semi-solid grains melt mainly through coalescence. With the prolongation of isothermal holding time, the grains grew up and spheroidized, in which the mean diameter of grain and liquid volume fraction increased. In the meantime, the higher the holding temperature, the faster the grain grew and spheroidized. The suitable reheating temperature of AZ61 semi-solid magnesium alloy was 592. The samples were susceptible to serious deformation beyond 597.

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.

Microstructure and Aging Behavior of AZ91 Mg Alloy Fabricated by Inclined Cooling Plate

Semi-solid AZ91 magnesium alloys were produced using a cooling plate followed by quenching in the mold. The cast and T6 heat treated microstructures were microscopically characterized and subjected to hardness tests. It was found that the resultant microstructure in water quenched semi-solid specimen were finer than that in conventional permanent mold cast specimen. The hardness tests also showed that the hardness of quenched semi-solid cast specimen was higher than that of permanent mold cast specimen. This was attributed to large amount of precipitates which results from fine precipitation of eutectic phase at intergranular areas during the aging treatment after its complete dissolution.