발간논문

Home > KJMM 논문 > 발간논문

Abstract: When a metal is under continuous deformation, the flow stress usually depends upon strain, strain rate and temperature. However, under interrupted deformation at elevated temperatures such as multi-pass rolling or multi-step forging, it also depends upon the recrystallization behavior. In the present study, a coupled model consisting of recrystallization and flow stress models is proposed for prediction of the flow stress of steel in the austenite region. It is verified by comparing predicted values with experimental results and applied to a multi-stand finish rolling process of a hot strip to predict variations of grain size, flow stress and retained strain with respect to the process time. By incorporating with a finite element method, it is also applied to thick plate rolling processes to predict the variations including those in thickness direction.

keyword : Interrupted deformation, Steel, Recrystallization, Flow stress, Finite element method

Multilayer system composed of TiNi shape memory alloy and PZT ferroelectric material is expected to perform the integrated function which is capable for sensing and actuating. To fabricate the integrated function material, it is necessary to design the multilayer film without inter-diffusion at the interface, and to have appropriate crystalline phases for each layer. In this work, TiNi thin films were deposited on sol-gel derived PZT thin films. When Pt barrier layer was deposited on PZT layer, TiNi layer deposited at 350℃ was successfully crystallized. This multilayer film had no other inter-diffusion layer at the interface and indicated the B2(110) phase for TiNi layer and the perovskite phase for PZT layer. This multilayer film has also shown the shape memory effect for TiNi layer and the ferroelectric behaviour for PZT layer.

keyword :

Computer simulation was carried out to investigate the behavior of magnetic field and eddy current property using FLUX2D on an amorphous mold core. Magnetostatics analysis shows that flux density distributes uniformly throughout the amorphous core and its value was about 0.92 T. It was found that when the direction of magnetic path changes drastically within the angle of 90°in an amorphous core, an uniform magnetic flux density was obtained throughtout the core. Magnetodynamics analysis at the frequency of 30 ㎑ revealed that eddy current was concentrated on both sides of edge, but almost cancelled out at the center. The Flux density induced by eddy current shows almost similar behavior. This can be interpreted that the eddy currents formed at each shell on center were cancelled out each other, while those on both sides of edge remain alive. A Weak leakage of fluxes was detected and it might act as EMI noise. Since the EMI noise can cause some errors on operation and increase the temperature of system, shield materials are useful to reduce leakage flux. The skin depth was calculated to be about 0.17 ㎜ from the equation of skin effect, while the penetration was totally achieved at the most outer and its neighboring shell on both sides of edge by simulation. Although the skin depth obtained from equation corresponds to 1/e of its real value, there was a little difference between simulation and calculation. It may be assumed that the relative permeability and electrical conductivity of a amorphous core would drastically reduce after molding.

keyword :

Wetting balance test is known to be the most versatile method for wettability test, because it can provide quantitative information, time dependent wetting behavior, and reproducibility. However, results for wetting balance test are only good for the specimen of which surfaces around their perimeter show uniform wettability. Therefore, this test is not applicable to wettability evaluation of UBM coated Si substrate for flipchip package. In this study, authors focused on wettability evaluation of Cu/Cr coated Si substrate with Sn Pb solder using wetting balance test. With wetting test, we found that non-wetting behavior of non-coated Si plate results in buoyancy force, which reduces overall wetting forces. In case of one-side coated Si plate, force difference between coated and non-coated surface cause tilting of axis of specimen. Therefore, it was confirmed that conventional wettability analysis using wetting forces for wetting balance could not be adaptable for wettability evaluation of coated Si plate. New equation of wetting force for coated Si plate was introduced which considers buyancy effect for non-coated surface and tilting of axis for one-side coated Si plate. With this equation, we can calculate the contact angle between Cu/Cr coated Si surface and Sn-Pb solder, which shows that wettability of Cu/Cr coated Si plate with Sn-Pb solder is superior to that of Cu plate.

keyword :

Leakage current characteristics and ferroelectric properties of lead zirconate titanate(PZT) films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition were investigated. Pt/Ti and RuO₂ were used as bottom electrodes for PZT film. The nucleation of PZT perovskite phase was more difficult on RuO₂ electrodes than on Pt electrodes, and the PZT films grown on RuO₂ tend to have PbO_x second phase. PZT film without PbO_x phase could be grown on RuO₂ by introducing a proper seed layer and by controlling the flow rates of Metal-organic source (particularly Pb(DPM)₂). For the PZT films on RuO₂ with improved microstructure, an excellent leakage current density of 10^(-6)A/㎠ at 100 ㎸/㎝ was obtained from the Pt(top)/PZT/RuO₂ capacitor. Leakage current characteristics and ferroelectric properties of PZT capacitors with four different electrode configurations were investigated. The RuO₂∥ RuO₂ capacitor showed a leakage current density of as high as 10^(-4) A/ ㎠ at 100 ㎸/㎝, which was attributed to the high temperature processes for RuO₂ top electrodes fabrications. The fatigue characteristics of PZT capacitors depend on the electrode material rather than the microstructure of PZT films.

keyword :

Thixotropic AZ91HP Mg alloy was only focused on aspect of semi liquid forming until now. In this study, for the purpose of proposition as economic lightweight hydrogen absorbing materials, the hydrogenation properties of thixotropic AZ91HP Mg alloy were evaluated by automatic Sieverts type pressure-composition-isotherm(PCI) apparatus. By the results of experiments, α-Mg rich phase(like solid phase) was regarded the storage system as an hydrogen absorbing / desorbing and eutectic phase (like liquid phase), Mg_(17)Al_(12)/Mg-Al-Zn, was considered to be the catalytic system as an improving factor of hydrogenation kinetics. Especially, the hydrogenation properties were dependent on the purity and size of α-Mg rich phase. In the case of 863, 868K specimens(4hr crushing under 2 ㎫ hydrogen atmosphere), liquid fraction was about 50-60% and the hydrogen capacity was reached about 4.5 wt% at 623K.

keyword :

The production technology development of surface fine grain steel by CRCT(TMR) is performed in order to improve the strength and toughness of Dowel Bar without additional treatment. It is possible to obtain surface ultra fine grain steel, when warm deformation of martensite formed after on-line quenching is carried out from 650℃ to 780℃ in the finishing rolling step. The characters of surface with ultra fine grain steel is showing the cementite particles inside the ferrite grain and fine ferrite grain of about 1.2 ㎛ in size. It is also confirmed that the Gleeble simulation of this production technology make ultra fine grain structures at similar temperature range from 740℃ to 800℃.

keyword :