Sang-Kon Lee

Effect of heat treatment on the characteristics of tool steel deposited by the directed energy deposition process

The directed energy deposition process has been mainly applied to re-work and the restoration of damaged steel. Differences in material properties between the base and the newly deposited materials are unavoidable, which may affect the mechanical properties and durability of the part. We investigated the effect of heat treatment on the characteristics of tool steel deposited by the DED process. We prepared general tool steel materials of H13 and D2 that were deposited onto heat-treated substrates of H13 and D2, respectively, using a direct metal tooling process. The hardness and microstructure of the deposited steel before and after heat treatment were investigated. The hardness of the deposited H13 steel was higher than that of wrought H13 steel substrate, while that of the deposited D2 was lower than that of wrought D2. The evolution of the microstructures by deposition and heat treatment varied depending on the materials. In particular, the microstructure of the deposited D2 steel after heat treatment consisted of fine carbides in tempered martensite and it is expected that the deposited D2 steel will have isotropic properties and high hardness after heat treatment.

Evaluation of Adhesive Properties Using Cohesive Zone Model : Mode I

Fracture models and criteria of adhesive with two parameters, namely C G and max σ , have been developed to describe the fracture process of adhesive joints. Cohesive zone model(CZM) is a representative two parameter failure criteria approach. In CZM, max σ is a critical, limiting maximum value of the stress in the damage zone ahead of the crack and is assumed to have some physical significance in adhesive failure. Based on CZM and finite element analysis method, the relationship between fracture load and adhesive properties, , as IC G and ( ) max I σ , was investigated in adhesively bonded joint tensile test and T-peel test. The two parameters in tensile mode loading were evaluated by using the relationship. The value of IC G evaluated by proposed method showed close agreement with analytical solution for tapered double cantilever beam(TDCB) test which proposed in an ASTM standard.

The Evaluation of Surface and Adhesive Bonding Properties for Cold Rolled Steel Sheet for Automotive Treated by Ar/O2Atmospheric Pressure Plasma

Cold rolled steel sheet for automotive was treated by Ar/ atmospheric pressure plasma to improve the adhesive bonding strength. Through the contact angle test and calculation of surface free energy for cold rolled steel sheet, the changes of surface properties were investigated before and after plasma treatment. The contact angle was decreased and surface free energy was increased after plasma treatment. And the change of surface roughness and morphology were observed by AFM(Atomic Force Microscope). The surface roughness of steel sheet was slightly changed. Based on Taguchi method, single lap shear test was performed to investigate the effect of experimental parameter such as plasma power, treatment time and flow rate of gas. Results shows that the bonding strength of steel sheet treated in Ar/ atmospheric pressure plasma was improved about 20% compared with untreated sheet.

The effects of heat treatment condition and Si distribution on order-disorder transition in high Si steels

The decomposition and formation of ordered phases in steels with 5 to 6.5 %Si were investigated by a combined analysis of TEM and electrical resistivity measurements in order to determine the optimal heat treatment conditions for removal of ordered phases. TEM diffraction pattern study revealed that B2 ordered phase in 6.5 %Si steel was sufficiently dissolved by heat treatment at 850 °C for 1 h, and then rapidly re-formed during cooling. The critical cooling rate, above which the suppression of B2 phase formation was possible, increased rapidly with Si content higher than 5.6%. DO3 phase appeared only in the case of as-cast samples containing Si content as high as 6.5%. The measurement of electrical resistivity change during the heat treatment showed that the atomic movement was substantially spurred above 800 °C, resulting in an active order-disorder transition. The removal of solidification segregation is necessary to reduce the amount of B2 ordered phase in the as-cast or hot-rolled state, to lower the heat treatment temperature for dissolution of B2 ordered phase, and to shorten the heat treatment time.

Porthole Extrusion Process Design for Magnesium-Alloy Bumper Back Beam by Using FE Analysis and Extrusion Limit Diagram

In recent years, several studies with focus on developing state-of-the-art manufacturing technologies have been conducted to produce light vehicles by employing parts made of light materials such as aluminum and magnesium. Of such materials, magnesium has been found to pose numerous issues, because it cannot be deformed (plastic deformation) easily at low temperatures. Furthermore, oxidation on the surface of manganese occurs at high temperatures. This study analyzes the extrusion process for manufacturing magnesium bumper back beams used in vehicles, using finite element (FE) analysis. The properties of magnesium were determined through a compression test performed at high temperatures. And the temperature at which oxidation occurs at its surface was evaluated via an extrusion test. FE analysis was used to evaluate the extrusion load and temperature during the extrusion process, according to changes in initial material temperature and ram speed. Extrusion limit diagram of the extrusion process was derived based on the results of the FE analysis. Process conditions required to be established during the extrusion process were determined by using the derived extrusion limit diagram. The conditions were further validated by the extrusion test.

Advanced simulation of die wear caused by wire vibrations during wire-drawing process

An advanced simulation that considers the effect of wire vibrations was proposed for predicting accurately wear profiles of a die used in a wire-drawing process. The effect of wire vibrations, the changes in the wear profiles, and the generation of ringing during die approach were investigated by this simulation. Wire vibrations occurring between the die and the drum are governed by a partial differential equation called the wave equation, which is a function of the wire length, tension, density, and initial wire velocity. The wire-drawing process was simulated by the commercial code Abaqus FEA, and the die wear profiles were predicted by Archards wear model. The predicted profiles were compared with measured profiles of a worn drawing die after producing 5 t of AISI 1010 wire; the die was made of tungsten carbide with a Brinell hardness of HB 682. The profiles predicted by considering the effect of wire vibrations are in good agreement with the experimental data, indicating that the advanced simulation can be used to accurately predict the die wear profiles when ringing is observed during die approach.

Process Development of Variable Curvature Extrusion for Automotive Aluminum Bumper

Abstract The effectiveness of vehicle parts made through extrusion is in the limelight because of the advantages of high strength stiffness materials can be produced and the number of processes can be drastically reduced. Therefore, the parts should have sufficient stiffness and be lightweight enough to improve fuel efficiency. However, the application of extruded aluminum requires pre-bending technologies that can manufacture the complex designs profiles demanded by vehicle parts. The aim of this research is that the development of the variable curvature extrusion technology that can produce a variety of curvature. In order to produce a variable curvature, the guide transfer speed and transfer time should be controlled properly. The guide transfer speed and transfer time were examined by the theoretical analysis. A model was developed to simulate the deformation behaviors of extrusion and bending process from the symmetric bumper with range of radii from 1863mm to 2163mm. The theoretical analysis and FE analysis were verified through experimental method. 기호설명

Flash Design for Automatic Transfer System of Bearing Hub in Hot Forging Process

The aim of this study is to design flash geometry of bearing hub to apply the automatic transfer system in hot forging process. The flash geometry is very important in hot forging process because the flash geometry effects on the metal flow, material losses, forging load, die pressure and so on. In this study, the problem of designing the flash geometry is studied with flash thickness and width considering the maximum die pressure to apply an automatic transfer system in hot forging process for bearing hub. The numerical analysis was conducted by means of the commercial S/W DEFORM. On the basis of numerical analysis the flash geometry of hot forging die was redesigned, and experiment was conducted. From the experimental results, it was possible to produce bearing hub with an automatic transfer system without any deterioration of die lifetime.