Young-Ho Seo

The effect of applied pressure on particle-dispersion characteristics and mechanical properties in melt-stirring squeeze-cast SiCp/Al composites

Abstract The fabrication of metal-matrix composites (MMCs) by casting process is a very promising way of manufacturing near net shape components at relatively low cost. One of these casting processes, the squeeze-casting technique, has characteristics such as fine microstructures as a result of rapid cooling, low porosity and good bonding between the particle and the base alloy. This paper therefore examines, the microstructure and mechanical properties of Al-15%SiCp composite fabricated by the melt-stirring and squeeze-casting method. The correlation between the fabrication conditions and the homogeneous dispersion of particles is investigated for SiCp-reinforced Al composites. The manufacturing parameters in homogeneous mixing are the crucible size, the impeller size, the temperature of the molten metal, the stirring time and the stirring speed in the melt-stirring process. The composite slurry containing SiCp is poured into a pre-heated die, and pressure applied in squeeze casting. The state of dispersion of the reinforcement and the fracture surface are observed by the optical microscope and SEM. The primary product of MMCs billet is prepared under squeeze-casting conditions for applied pressures of 70, 100, and 130 MPa. By assessing observed results systematically, correlation is proposed between the microstructure, the particles behavior, the mechanical properties and the processing parameters for an optimum melt-stirring and squeeze-casting process of MMCs.

The upsetting behavior of semi-solid aluminum material fabricated by a mechanical stirring proces

Abstract The fabrication processing of semi-solid material (SSM) of casting alloy has been studied to demonstrate the possibilities of mass production with a controlled solid fraction, the SSM being fabricated for various solid fractions and pre-heating temperatures of the mould. The behavior of the semi-solid globular microstructure has been investigated for various heating and die temperatures for the solid fraction. The influence of the reheating time on the globularization of the SSM microstructure has been determined and the compression behavior of a SSM having a solid fraction of 0.5% has been observed. The relationship between the stress and the strain were also obtained from the compression testing of the semi-solid materials. The rheological behavior of the semi-solid with a globular microstructure was investigated as a function of the compression velocity, under isothermal holding conditions.

The influence of fabrication parameters on the deformation behaviour of the preform of metal-matrix composites during the squeeze-casting processes

Abstract Through a series of experiments with preforms that had a randomly-oriented fiber distribution, the deformation of the preform and the tensile strength of the metal-matrix composites were investigated with variation of processing parameters such as the punch velocity, the amount of binder, the preform temperature, the fiber volume fraction and the applied pressure. Experimental data are presented on the injection of aluminum-alloy melts into preforms composed of δ-alumina short fibers with a silica binder. The preforms were prepared from liquid slurry using the vacuum filtration process. The base material used in this study was A12024, having poor fluidity compared with a casting aluminum alloy. Scanning electron and optical micrographs are also presented for investigating the effects of processing parameters on the microstructures of the metal-matrix composites. Recommendations are put forward concerning the optimization of the microstructure through control of the processing parameters.

A continuous fabrication of Al2O3 short fiber reinforced aluminium composites by direct rolling process

The importance of high strength, wear resistance, thermal stability, and lightweight materials have attracted much interest in the development of the manufacturing process of metal matrix composites (MMCs). Problems in the rolling process are damage to fibers and a reduction of strength by the debonding of interface as well as defects on the surface and internal imperfection in the fabricated composites sheet. Therefore, a roll experiment of semi-solid materials containing short fiber was carried out in this study to fabricate an MMCs sheet. The composites slurry containing Al{sub 2}O{sub 3} short fiber was fabricated with mixing techniques in order to manufacture a composites sheet. Also, the possibility of strip fabrication for direct rolling was investigated for semi-solid materials containing short fiber. The mechanical properties for the composites sheet were also investigated.

Numerical and Experimental Study on Plate Forming Process using Flexible Die

A flexible forming apparatus is composed a number of punches which have spherical pin tip shape instead of conventional solid die. The flexible forming tool consisted of punch array in a matrix form was proposed as an alternative forming method to substitute the conventional line heating method which use heat source to induce residual stress along specified heating lines. In this study, application of the flexible forming process to the small scale curved plate forming was conducted. Numerical simulations for both solid and flexible die forming process were carried out to compare the shape of the products between flexible and conventional die forming process. In addition, spring-back analysis was conducted to figure out the feasibility of the flexible forming process comparing with the die forming process in view of final configuration of the specimens. Moreover, experiment was also carried out to confirm the formability of the process. Consequently, it was confirmed that the flexible die forming method has capability and feasibility to manufacture the curved plates for shipbuilding.

Study on Application of Flexible Die to Sheet Metal Forming Process

Flexible forming process for sheet material using reconfigurable die is introduced based on numerical simulation. In general, this flexible forming process using the reconfigurable die has been utilized for manufacturing of curved thick plates used for hull structures, architectural structures and so on. In this study, numerical simulation of sheet metal forming process is carried out by using flexible dies model instead of conventional matched die set. The numerical simulation and experimental verification for sheet metal forming process using a flexible forming machine that is more suitable for thick plate forming process are carried out to confirm the appropriateness of the simulation process. As an elastic cushion, urethane pads are utilized using hyperelastic material model in the simulation for smoothing the forming surface which is discrete due to characteristics of the flexile die. In the flexible forming process for sheet metal, effect of a blank holder is also investigated according to blank holding methods. Formability in view of occurrence of dimples is compared with regard to the various punch sizes. Consequently, it is confirmed that the flexible forming for sheet material using urethane pad has enough capability and feasibility for manufacturing of smoothly curved surface instead of conventional die forming method.

Development of Stretch Forming Apparatus using Flexible Die

A stretch forming method has been widely used in sheet metal forming process. Especially, this process has been adopted in aircraft and high-speed train industries for skin structure forming having a variety of curvature. Until now, solid dies, which are designed with respect to the specific shapes and manufactured as a single piece, have been usually applied to stretch forming process. Therefore, a great number of solid dies has to be developed according to the shapes of the curved skin structure. Accordingly, a flexible die is proposed in this study. It replaces the conventional solid dies with a set of height adjustable punch array. A usefulness of the flexible die is verified through a formability comparison with the solid die using finite element method considering an elastic recovery and the stretch forming apparatus with the flexible die is developed.

A robust algorithm for roughness laser measurement based on light power regulation against specimen changes

Methods for measuring surface roughness based on light reflectivity have advantages over methods based on light interference or diffraction, especially in in-situ, on-the-machine and inprocess applications. However, measurement inconsistencies caused by changes in the specimen are still a drawback for field applications. In this study, we propose a new feedback-based algorithm to enhance the consistency against changes in the specimen. The algorithm is deduced from simulations based on light reflectance theory with typical modeled surfaces. The proposed method is similar to a digital controller and regulates the power of reflected light. Experiments varying heights and materials, verified the improvements in robustness of the method against measurement disturbances caused by specimen changes.

Tendency Analysis of Shape Error According to Forming Parameter in Flexible Stretch Forming Process Using Finite Element Method

A shape error of the sheet metal product made by a flexible stretch forming process is occurred by a various forming parameters. A die used in the flexible stretch forming is composed of a punch array to obtain the various objective surfaces using only one die. But gaps between the punches induce the shape error and the defect such as a scratch. Forming parameters of the punch size and the elastic pad to prevent the surface defect must be considered in the flexible die design process. In this study, tendency analysis of shape error according to the forming parameters in the flexible stretch process is conducted using a finite element method. Three forming parameters, which are the punch size, the objective curvature radius and the elastic pad thickness, are considered. Finite element modeling using the punch height calculation algorithm and the evaluation method of the shape error, which is a representative value for the formability of formed surface, are proposed. Consequently, the shape error is in proportion to the punch size and is out of proportion to the objective curvature radius and the elastic pad thickness.

Flexible Die Design and Springback Compensation Based on Modified Displacement Adjustment Method

Springback in metal forming process often results in undesirable shape changes in formed parts and leads to deterioration in product quality. Even though springback can be predicted and compensated for through the theories or methodologies established thus far, an increase in manufacturing cost accompanied by a change in die shape is inevitable. In the present paper, it is suggested that the cost accompanied with springback compensation can be minimized while allowing the processing of various three-dimensional curved surfaces by using a flexible die composed of multiple punches. With the die being very flexible, the iterative trial-and-error method can be readily applied to compensate for the springback. Thus, repeated designing and redesigning of solid or matched dies can be avoided, effectively saving considerable time. Only some adjustments of punch height are required. Detailed designs of the flexible die as well as two core algorithms to control the respective punch heights are described in this paper. In addition, a closed-loop system for the springback compensation using the flexible die is proposed. The amount of springback was simulated by a finite element analysis and the modified displacement adjustment (DA) method as the springback compensation model was used in the closed-loop system. This system was applied to a two-dimensional quadratic shape problem, and its robustness was verified by an experiment.