Chung Gil Kang

Transient thermal analysis of solidification in a centrifugal casting for composite materials containing particle segregation

One-dimensional heat-transfer analysis during centrifugal casting of aluminum alloy and copper base metal matrix composites containing Al2O3, SiCp, and graphite particles has been studied. The model of the particle segregation is calculated by varying the volume fraction during centrifugal casting, and a finite difference technique has been adopted. The results indicate that the thickness of the region in which dispersed particles are segregated due to the centrifugal force is strongly influenced by the speed of rotation of the mold, the solidification time, and the density difference between the base alloy and the reinforcement. In the case where the base alloy density is larger than that of the particles, the thickness of the particle-rich region near the inner periphery decreases with an increase in speed, thereby increasing the volume fraction of dispersion. The solidification time of the casting is also dependent upon the speed of rotation of the mold, and it decreases with an increase in speed. This study also indicates that the presence of particles increases the solidification time of the casting.

Two-dimensional model for twin-roll continuous casting

A numerical algorithm for the two-dimensional solidification problem in the twin-roll continuous casting system is presented in this paper. Attention is focused on the elucidation of heat transfer and flow characteristics in both the liquid and the solid phases. The present mathematical model can be applied to general full Navier-Stokes and energy equations, thereby covering the wide range of twin-roll casting conditions. The boundary fixing method (BFM) is adopted to handle the moving boundary, and the resultant transformed governing equations for the solid and liquid regions are solved separately by using a usual explicit-type finite difference method. In this paper, a general numerical methodology is presented, and the quantitative relationships between the important control parameters in continuous casting of twin-roll type (such as the roll speed, the roll gap, the initial temperature of molten materials, the material properties, the solidification profile, and the endpoint of solidification) are clarified in detail. The present numerical results have been compared with experimental results obtained separately to check the validity of the proposed method.

The effect of test specimen size and strain-rate on liquid segregation in deformation behavior of mushy state material

Abstract For optimization of net shape forging process with semi-solid materials (SSMs), it is important to predict the deformation behavior of material. In this study, compression experiments have been performed to investigate the deformation behavior of SSM with variation in processing parameters such as test specimen size and strain-rate. Stress–strain curves and microstructures of SSM were investigated. The characteristics of flow between solid and liquid phase considering liquid segregation is examined through the above experiments. Moreover, porosities were measured to evaluate the effects of experiment parameters on liquid segregation quantitatively.

Determination of Die Design Rules for Semi-Solid Die Casting Process and Its Experimental Investigation

Die design rule for semi-solid die casting (SSDC) with A356 electromagnetic stirring (EMS) aluminum alloy, was proposed. The die design rule included inspection of machine, part requirements, parting line determination, sleeve, plunger, gating system, overflow, air vent, ejector pin, and heating line design. The specification of gating system, overflow, air vent, plunger tip, and sleeve suitable for respective part were regulated. Two steps die system of lower-positioned gate and three steps die system of center-positioned gate were manufactured for 4 automobile suspension parts, based on the die design rule. For the sound filling pattern and solidification behavior, injection speeds of 4 parts were summarized to the interval (from V1 to V4). As a result of observing the microstructure of 4 parts after T6 heat treatment, primary Al-α phase was globularized and fine Si particles were distributed around the grain boundary. The mechanical properties of 4 parts with T6 heat treatment were investigated and showed ultimate tensile strength (UTS) of 330 MPa, yield strength (YS) of 250 MPa, and elongation of 7.5% as average.

Effect of forming conditions on mechanical properties of rheoformed thin plates with microchannels using electromagnetic stirring

Rheoforming is a near net-shape manufacturing technology for fabricating components from light alloys in their semisolid states with improved mechanical properties. In this work, a feasibility study on the fabrication of Silafont 36 aluminum thin plates via rheoforming was conducted. The thin plates were fabricated under different experimental conditions, such as different solid fractions and punch pressures. Electromagnetic stirring was used to prepare a semisolid slurry of Silafont 36 aluminum alloy. Subsequently, the slurry was transferred to die sleeve and injected into the die cavity of the thin plate. The thin plates were successfully fabricated under the optimal conditions of 50% solid fraction and a rheoforming pressure of 130 MPa.

Development of rheology vacuum low-pressure die casting with electromagnetic stirrer and vacuum system

In this study, a rheology vacuum low-pressure die caster is developed by installing a vacuum system in the die and an electromagnetic stirrer in the rasing pipe of a low-pressure die casting system. The cavity dimensions are 130 mm in the filling direction and 110 mm in the vertical direction, with a thickness of 1 mm. The fluidity of the material inside the die cavity is improved by maintaining the vacuum state inside the die. The electromagnetic stirrer is attached to the outside of the rasing pipe, which connects the furnace to the lower die. The application of the electromagnetic stirrer enables the achievement of microstructure with fine and globular primary α-Al particles by disrupting the dendrite structure. A mild filling simulation is conducted using a commercial casting analysis program prior to an experiment with the rheology vacuum low-pressure die caster. The rheological behaviour of the material inside the die is observed. A rheological thin plate is fabricated by applying a melt temperature of 615 °C, a vacuum of 60 Torr, and a gas pressure of 15 bar. There is little eutectic structure, with primary aluminium grains of 30 µm or below that are mostly distributed due to the effects of stirring. The tensile strength and elongation are 120 MPa and 15%, respectively. The hardness is 61–66 HV.

Processing of Low-Carbon Cast Steels for Offshore Structural Applications

Cast steels with carbon contents of approximately 0.05%, 0.1%, and 0.2% (low-carbon steels) were processed. The investigated steels were first cast. Fully lath martensite was obtained after austenitization and water quenching of the cast steels. The mechanical properties of ∼0.05% carbon steel did not significantly change after tempering at 500°C, 540°C, and 580°C. The strength of ∼0.1% carbon content steel decreased after tempering for 2 h to 3 h at 580°C, and then stabilized at longer tempering times of 6 h. The ductility remained almost constant through the processes. The Charpy impact energy increased when the tempering time was increased from 2 h to 4 h, but decreased remarkably after tempering for 6 h. By increasing the tempering temperature from 450°C to 550°C, the ductility of the ∼0.2% carbon content steel increased, followed by a drop at 600°C. The strength of this steel was the highest at 450°C, but decreased and stabilized at tempering temperatures of 500°C to 600°C. The Charpy impact energy increased monotonically and reached its highest value at 600°C. Finally, the applicability of the investigated cast steels for offshore structures was assessed in detail.

Metallic Bipolar Plate Fabrication Process of Fuel Cell by Rubber Pad Forming and its Performance Evaluation

Recently, the demand for energy is growing at a very high rate all over the world. The fossil fuels eventually lead to the foreseeable depletion of limited fossil energy resources. Hydrogen is considered a promising candidate to remedy the depletion of fossil fuels. The bipolar plate is the second most important component of a proton exchange membrance (PEM) fuel cell stack after the membrance electrode assembly (MEA). Its primary roles are to supply reactant gases to the fuel cell electrodes and provide electrical connection between adjacent cells in the stack while removing product water from the cell and transferring away the heat of reaction. Historically, machined graphite had been chosen as a good compromise between all of these requirements, but alternatives are emerging. New materials are light metals. In this study, rubber pad forming process was employed as the manufacturing method for metallic bipolar plates. The rubber pad and the sheet metal plate were pressed together by the punch, and the repulsive force of the deformed rubber is loaded at the plate, and can contribute to improving formability. And then, its surface was coated with TiN. After coating process, the performance characteristics of single stack in the condition of PEMFC using the metal bipolar plate have been investigated.

The effect of different forming parameters on the depth of bipolar-plate channels in static- and dynamic-load stamping

A titanium alloy is selected as an ideal material for a fuel-cell bipolar plate because it has a low density and, like aluminium, exhibits excellent corrosion resistance due to an insulating oxide film. Microchannelled bipolar plates are fabricated using a 25-ton material-testing machine by varying the parameters of the static and dynamic loads (type of load wave, punch load, number of cycles) and the die curvature in order to evaluate the formability. The bipolar plate formed by dynamic-load stamping (square wave of 0.5 Hz, 32 load cycles, punch load of 90 kN) with die curvature of 0.3 mm resulted in 13.0% deeper channels compared to static-load stamping of 90 kN. Moreover, in the case of 0.1 mm die curvature, the bipolar plate formed by square-wave dynamic-load stamping had 22.7% deeper channels, compared to static-load stamping. Dynamic-load stamping with die curvatures of 0.1 and 0.3 mm shows a channel depth of 0.251 and 0.353 mm, respectively. Compared to die curvature of 0.1 mm, 0.3 mm die curvature resulted in channels that are deeper by 28.9%.

Thin-Plate Forming by Thixo- and Rheoforging

Thin plates with a thickness of 1.2 mm are fabricated using two processes, thixoforging and rheoforging, which are semisolid forming techniques. The die design, formability, microstructure, and mechanical properties of the fabricated thin plates are analysed. A fan-shaped gate is designed by analysing the filling behaviour using semisolid material, and uniform filling behaviour of material is obtained by arranging nine overflows in product area. semisolid metal is prepared through a semisolid process in which reheating, a thixoprocess, and cooling with stirring, a rheoprocess, are applied. The semisolid material is injected into a forging die and is formed into thin plate at a punch speed of 300 mm/s and under a pressure of 100 MPa. Since semisolid material with a solid fraction below 45% has mainly small primary α-Al particles, the formability of the thin plate is improved. The formed thin plate also has good mechanical properties since the small and globular grains are evenly distributed. The thin plate formed from semisolid material with a solid fraction above 50% has poor mechanical properties owing to the large quantity of coarse primary α-Al particles. A rheoforged thin plate exhibits poorer mechanical properties than a thixoforged thin plate, but rheoforging produces a more precise thin plate.