Takuya Tamura

Fluidity and Microstructures Characteristics of AZ 91D by Using New Type Semi-Solid Injection Process

This research has been investigated fluidity and microstructures characteristics of AZ 91 D alloy using new type semi-solid injection machine. To ensure good casting products, uniform temperature distribution was required during heating in the injection cylinder of this machine. So, the injection cylinder was divided with six heating zones. Then temperature distribution in the injection cylinder was precisely controlled. AZ 91 D billets were heated to the desired temperatures in the injection cylinder, and injected into the permanent mold with injection speed about 430 mm/s. Fluidity was measured by using spiral permanent mold with the cavity of 1045 mm in length and 5 mm in thickness. The fluidity test has been done with the fraction solid from 0% - 60%. The fluidity was 905 to 153 mm for fraction solid 0% to 60%, respectively. At the fraction solid from 50% to 60% microstructures are consisted of spherical solid particles and the solid particles surrounded by liquid phase. The shape of solid particles begins to change at the fraction solid of 40%.

On the role of vibration frequency on the solidification of AZ91D magnesium alloys during electromagnetic vibration

In the present paper, we solidified magnesium-based AZ91D alloys in a superconducting magnetic field when an alternating current flowed through the alloy. As the direction of the magnetic field is perpendicular to that of the alternating current, a periodic electromagnetic force is produced to activate an electromagnetic vibration (EMV) on the alloy during solidification. The microstructure formation and microtexture evolution processed by EMV were examined. A significant difference arises in electrical resistivity between a solid and a liquid in the mushy zone of the alloy, making the solid move faster than the liquid and thus generating uncoupled motion, from which melt flow is initiated. The texture evolution obtained by x-ray diffraction and electron backscatter diffraction (EBSD) mapping reveal a strong dependence of melt flow intensity versus vibration frequency. A further analysis reveals that melt flow is rather weak when the vibration frequency is too low and thus the segmentation of growing crystals cannot be thoroughly completed. At medium vibration frequencies, severe fluid flow occurs, which favors fragmentation and thus results in a refined microstructure and a random microtexture. When the vibration frequency is too high, the relative leading distance covered by the mobile solid is rather short and melt flow once again becomes weak. Meanwhile, the static magnetic field makes the crystals orient to their easy magnetization direction and thus yields highly aligned textures. Experimentally, the present systematic observation indicates that the role of melt flow is of substantial importance in revealing the origin of structure formation when the alloy is solidified at various vibration frequencies.

Effect of Volume Fraction Solid and Injection Speed on Mechanical Properties in New Type Semi-Solid Injection Process

We have developed new type semi-solid injection process, that is, runner-less injection process which can obtain high material yield of about 90% for magnesium alloy. In this process, alloy billets are heated to the semi-solid temperature in the injection cylinder and are injected into a permanent mold. In order to investigate the effects of volume fraction solid and injection speed on microstructure and mechanical properties of AZ91D magnesium alloy injected into the permanent mold, semi-solid forming testing machine which has the same system as a runner-less injection machine, has been made on an experimental basis. The magnesium billet precisely controlled at given temperature has been injected into a permanent mold with two kinds (slow and high) of speed and plate-like specimens with each fraction solid have been fabricated. Microstructure has been observed by optical microscopy and X-ray computerized tomography (CT) scanner. Mechanical properties have been measured by tensile test. The effects of volume fraction solid of the alloy slurry and injection speed on mechanical properties have been clarified.

Microstructures and Casting Defects of Magnesium Alloy Made by a New Type of Semisolid Injection Process

We have developed a new type of semisolid injection process that allows magnesium alloys to be formed in high material yields approximating 90%. In this process, generic magnesium billets are heated into their semisolid temperature range in an injection cylinder, without cover gas, and then the material is injected into a mold.

Microstructure formation and grain refinement of Mg-based alloys by electromagnetic vibration technique

Abstract The microstructure formation and grains refinement of two Mg-based alloys, i.e. AZ31 and AZ91D, were reported using an electromagnetic vibration (EMV) technique. These two alloys were solidified at various vibration frequencies and the microstructures were observed. The average size of grains was quantitatively measured as a function of vibration frequencies. Moreover, the grain size distribution was outlined versus number fraction. A novel model was proposed to account for the microstructure formation and grain refinement when considering the significant difference of the electrical resistivity properties of the solid and the liquid during EMV processing in the semisolid state. The remarkable difference originates uncoupled movement between the mobile solid and the sluggish liquid, which can activate melt flow. The microstructure evolution can be well explained when the fluid flow intensity versus vibration frequency is taken into account. Moreover, the influence of the static magnetic field on texture formation is also considered, which plays an important role at higher vibration frequencies.

Effect of Solid Fraction on Microstructure and Casting Faults of AZ91D in New Type Semi-solid Injection Process

We have developed new type semi-solid injection process, that is, runner-less injection process. In order to investigate the effects of solid fraction on microstructure and casting defects of AZ91D in new type semi solid injection process, semi-solid forming testing machine which has the same system as a runner-less injection machine has been made on an experimental basis. Its temperature controlling system has been established to obtain the homogeneous solid-liquid coexisted state in its injection cylinder. AZ91D billets are injected into a permanent mold by this machine in the semi-solid state. A shearing in the part of nozzle of injection cylinder is the most important to reveal thixotropic property of alloy slurry in semi solid forming process by injection machine. So it needs controlling of solid fraction to affect thixotropic property. In order to decrease casting defects and hold homogeneous structure, solid fraction more over 50% is needed. But when the solid fraction increases more than 50%, primary solid particles grow coarser, and then controlling method is required to suppress coarsening. In the case of less than 50% of solid fraction, liquid part preferentially fills inside the permanent mold and alloy slurry continue to fill the mold behind alloy liquid. Then large casting defects form at the boundary of both flows.

Production of Zr-Based Metallic Glass Matrix Composites by Semisolid Process

It was reported that micrometer-sized ductile crystalline phases can improve the ductility of Zr-based bulk metallic glasses. The present authors reported the synthesis of bulk metallic glass composites by a process combining cooling slope casting and suction casting for Zr66.4Nb6.4Cu10.5Ni8.7Al8 alloys. This study aims to investigate basics of Zr-based metallic glass matrix composites with semisolid structure. As a result, it was found that the Zr-based metallic glass matrix composites with very uniform semisolid structure can be produced by heat treatment process at semisolid region. Moreover, it was found that experimental results can be explained by the pseudo-binary phase diagram.

Application of semisolid process to Zr-based metallic glass matrix composites

The effect of the cooling slope on the structure of Zr-based metallic glass matrix composites was investigated by changing the cooling slope. The synthesis of bulk metallic glass composites was made by a process combining cooling slope casting and Cu mold casting for Zr66.4Nb6.4Cu10.5Ni8.7Al8 alloys. The results show that the semisolid slurry which consists of the spheroidal or rosette-type BCC crystals and the liquid phase which forms metallic glass phase can be formed by the cooling slope process in this alloy system. However, the semisolid slurry cannot reach to the mold. It is considered that higher viscosity of the liquid phase which forms metallic glass phase causes this result. Thus, parameters of the cooling slope have to be examined further.

Creation of Fine Structure in Magnesium Alloys by Electromagnetic Vibration Process

We have developed the refinement process of the microstructure of metallic materials by imposition of electromagnetic vibration force during solidification. This process is effective for both wrought magnesium alloys and cast ones. By simultaneous imposition of a static magnetic field of 10 Tesla under an alternative electric current of 60A, the average grain sizes of the AZ31B wrought alloy and the AZ91D cast alloy were obtained about 50 micron in both alloys. The grain size was affected by electric current frequency and had the minimum value at the special electric current frequncy of 500 to 2000 Hz and 900 Hz for wrought alloy and cast alloy, respectively. From experimental results, we suggested the mechanism of refinement of microstructure during solidification by imposition of electromagnetic vibration force. The cavitation phenomenon in liquid phase during electromagnetic vibration was effective to break down th esolid phase. And also the difference of electric conductivity between the solid phase and the liquid one brought vigorous vibration of the solid phase. Then the solid phase was suppressed its growth.

New Application of Electromagnetic Vibration Process for Creation of High Potential Metallic Materials

We have developed the new process for refinement of metallic materials during solidification without addition of refiners or without rapid cooling. This process uses electromagnetic body force based on the vibrations caused by simultaneous imposition of direct magnetic field and alternative electric current on the alloy melt during solidification. The vibrations create cavitation in the melt and it breaks out during growth of it. Then explosive force is released toward the surroundings such as the primary solid particles and they are fractured finely. Finally fractured solid particles solidified as very fine grains. This process was applied to hyper- and hypo-eutectic Al-Si alloys, and AZ91D Mg alloys. Primary silicon crystals in Al-17mass%Si alloy decreased the size remarkably by the optimum electromagnetic vibration condition. Primary aluminum dendrites particles in Al-7mass%Si and primary magnesium dendrite particles in AZ91D are also decreased its size markedly. This process has also been applied to create non-equilibrium state metallic materials such as metallic glasses. We have succeeded to obtain metallic glasses in Mg-Y-Cu alloy system and Fe-Co-Si-B-Nb alloy system. These alloys solidified as fully crystals in this cooling rate without electromagnetic vibrations. These glasses are bulky and are used as structural material.