Papers by Author: Kyu Hwan Oh

Abstract: A study has been made of the evolution of the microstructures and textures in three kinds of low-carbon steel sheets (MAFE, BH and IF) having well developed <111>//ND texture that were rolled by low reductions and annealed at 780 °C in Ar atmosphere. The steel sheets developed different microstructures and textures, even though their initial textures and thermomechanical treatments were similar. MAFE steel showed an unusual behavior that grains with high Taylor factors survived and grew very rapidly. This unusual behavior and the differences in microstructure and texture have been discussed.

Abstract: A two-step rolling-annealing process has been developed to increase the <111>//ND (γ fiber) component in the recrystallization texture of a copper-bearing bake hardening steel. The two step process comprises the first rolling by a low reduction in thickness and subsequent annealing at 780°C, followed by the second rolling by a high reduction and subsequent annealing at 780°C. The first rolling process aims at seeding the γ fiber oriented grains, so that they can grow at the expense of differently oriented grains developed in the second rolling process. In this way the density of γ fiber component in the recrystallization texture of the bake hardening steel much increases compared with that in the conventional one-step rolling-annealing process.

Abstract: Bending fatigue behavior of eutectic Sn-3.5Ag solder bump bonded on FR4-PCB was characterized by experimental and finite element method (FEM). To investigate an effect of stress state on bump failure, which had not been weighed in conventional Coffin-Manson model of Nf=K ⋅εp -1~-2, ‘fatigue frequency variable’ and ‘bump viscoplasticity’ were included in analysis procedure. As experimental results, with increasing fatigue cycles from 3,000 to 10,000, bond strength decreased from 98.9% to 76.5%, and from 97.5% to 67.1% at the fatigue frequencies of 2.5Hz and 5.0Hz, respectively. Stress state could be critical components to determine fatigue life, which should be combined in Coffin-Manson criteria. FEM calculation showed that higher bending frequency led to higher normal stress development at the solder and IMC interface, but smaller plastic strain in bump. However, bending fatigue experiment revealed discrepant results from that of Coffin-Manson criteria. Higher bending frequency, which was predicted to give rise to smaller εp at solder, showed dramatic bond deterioration of solder bump on UBM (under bump metallurgy). This was confirmed experimentally through SEM (scanning electron microscopy) observation as cracks were found at the solder bump and UBM interfacial IMC, Ni3Sn4, in case of the higher bending frequency.

Abstract: The development of deformation texture and microstructure was examined for four different initial textures. IF steel sheets with a majority of α-, ε-, and γ-fiber and near random texture were prepared and cold rolled. The specimens exhibited characteristic behaviors in rolling texture evolution and deformation-induced misorientation development, according to their initial textures, especially at small strain levels. Due to the orientation dependence of intra-granular misorientation accumulation, the different texture evolutions affected the induced misorientation distribution. A larger fraction of γ-fiber orientations was related to more prominent misorientation development, while the initial texture stability simultaneously affected the misorientation development. The unstable, initial ε-fiber texture showed a stronger tendency of misorientation accumulation than the stable α-fiber during the subsequent cold rolling.

Abstract: Texture and microstructure of gold sheet were investigated during deformation and subsequent annealing. The Brass, S and Copper (β-fiber) orientations are closely connected together in the deformed microstructure. Recrystallization texture also was examined during isothermal annealing at 500°C with reduction in area. Initial rolling textures with rotated cube and β-fiber resulted in the cube and recrystallized β-fiber orientations after annealing. A two dimensional Monte Carlo (MC) method was used to simulate primary recrystallization in gold sheet. A function of boundary misorientation was introduced to consider anisotropic properties of grain boundary energy and mobility. Stored energy associated with orientations in the deformed grains was evaluated by reconstructing of data measured using electron back-scattered diffraction (EBSD). The nucleation at an initial stage of recrystallization was found at the high angle grain boundaries (HAGBs) and grain interiors. The main texture components obtained by the simulation were similar to those obtained experimentally except cube component.

Abstract: Friction stir welding (FSW) process of aluminum alloys was investigated using a two-dimensional Eulerian formulation coupling viscoplastic flow and heat transfer and strain hardening. The thermal equation for the temperature was modified to stabilize temperature distribution using a Petrov-Galerkin method. The evolution equation for strength was calculated using a streamline integration method. Predicted strength was compared with experiments. Based on crystal plasticity, texture evolution was predicted during FSW of AA6061.

Abstract: A novel process for butt welding of thin metal sheets was developed and named as surface friction welding (SFW), which utilizes friction heat and severe plastic deformation like friction stir welding (FSW). The joining mechanism of the SFW is based on not stirring by the pin tool but surface friction between tool shoulder and joining metals. The developed method was successfully applied to butt welding of various metal sheets thinner than 1.5 mm thick. This paper deals with the principle of SFW, the difference between FSW and SFW, the effect of welding parameters, and the microstructure and mechanical properties of welded sheets.

Abstract: Flow behavior of the surface and center layers of solution-treated, peak-aged, or reversion-treated 2090 Al-Li alloy specimens has been reviewed and discussed in terms of microstructures and textures.

Abstract: Copper and gold bonding wires were characterized and compared using electron backscatter diffraction (EBSD). During drawing, <111> and <100> fiber textures are the main components in the wires and shear components are mainly located under the surface. Grain average misorientation (GAM) and scalar orientation spread (SOS) of the <100> component in copper and gold bonding wires are lower than those of the <111> or other orientations. The bonding wires experience three stages of microstructural changes during annealing. The first stage is subgrain growth to keep elongated grain shapes overall and to be varied in aspect ratio with annealing time. The grain sizes of the <111> and <100> components increase during annealing. The volume fraction of the <100> component increases whereas that of the <111> decreases. The second stage is recrystallization, during which equiaxed grains appear and coexist with elongated ones. The third stage is grain growth which eliminates the elongated grains and enlarges equiaxed grains. The <111> and <100> grains compete with each other and the <111> grains grow faster than the <100> grains during the third stage.

Abstract: Cold drawn gold wires are widely applied in electronic packaging process to interconnect micro-electronic components. They basically provides a conducting path for electronic signal transfer, and experience thermo-mechanical loads in use. The mechanical stability of drawn gold wires is a matter of practical concern in the reliable functioning of electronic devices. It is known that mechanical properties of materials are deeply related to the microstructure. With appropriate control of deformation and heat processes, the mechanical properties of final products, such as tensile strength and elongation can be improved. Severe plastic deformation by torsion usually contributes to grain refinement and increment of strength. In this study, microstructure variations with torsion strain followed by drawing and heat treatment were investigated. Analyses by focused ion beam (FIB) and electron backscattered diffraction (EBSD) were carried out to characterize the effect of deformation and heat treatment on the drawn gold wires. Pattern quality of EBSD measurements was used as a quantitative measure for plastic deformation.