Zin-Hyoung Lee

Time-varying heat transfer coefficients between tube-shaped casting and metal mold

Abstract An experimental and numerical study were made on the time-varying heat transfer coefficient h(t) between a tube-shaped casting and metal molds. One dimensional treatment was adopted in analyzing the heat flows between the casting and the inner and the outer mold. The sequential function specification method was employed to solve the nonlinear inverse heat conduction problem. In order to investigate the different behavior of h(t) for different alloys, casting experiments were carried out with three Al-base alloys and pure Al having different types of solidification behavior. It was found that the temperature change of the outer mold showed a normal heating and cooling curve. However, that of the inner mold was unusual especially for the alloys with a wide solidification range, i.e. the temperature increases first rapidly, then halts for a while and then increases again showing finally a regular heating and cooing curve. The resulting heat transfer coefficient at the interface to the inner mold hi(t) decreases temporarily and then increases, while the one at the interface to the outer mold ho(t) decreases monotonously to a quasi steady state. The abnormal heat transfer phenomenon at the inner interface for the alloys with a wide solidification range was concluded to be caused by a slight movement of the semi-solid inner wall at the inside of the tube-shaped casting due to the solidification contraction of the casting freezing in a mushy type.

The effect of heat treatments and Si contents on B2 ordering reaction in high-silicon steels

The silicon content was increased up to 6.5% (by weight, unless specified otherwise) to reduce the power loss of the silicon steels. These steels were prepared by the conventional casting method or by spray forming and were investigated with the aid of light optical microscopy (LOM) and transmission electron microscopy (TEM). The difference in the casting method did not result in any difference in suppressing the B2 ordering. The D03 phase was observed only in the as-cast 6.5%Si steel. It was almost impossible to suppress the B2 ordered phase keeping the silicon level as high as 6.5% even after the heat treatment at 1000°C for 24 h or after hot rolling. It was necessary to change the Si level and control the cooling rate to suppress the ordering reaction, especially, in cooling after heat treatment. The silicon level of 5.87% was observed to be a critical value in suppressing the B2 ordering reaction.

The effect of grain refining and oxide inclusion on the fluidity of Al–4.5Cu–0.6Mn and A356 alloys

Abstract The effect of grain refinement and oxide inclusion on the fluidity of Al alloy was investigated with a test casting with eight thin flow channels. Pouring in air increased the amount of oxide in the A356 melt. The fluidity compared between normal A356 melt and contaminated melt. The amount of oxide was evaluated qualitatively by ultrasonic treatment. The flow length varied linearly with the pouring temperature. By adding Ti and Al–5Ti–B, fluidity increased. The grain size decreased by adding grain refiner. The fluidity depended on the degree of grain refining. It was noticed that pouring in air increased the amount of oxides in the melt by ultrasonic treatment. The fluidity of contaminated melt was decreased comparing to the normal one especially in lower temperature.

The effect of nitrogen and heat treatment on the microstructure and tensile properties of 25Cr–7Ni–1.5Mo–3W–xN duplex stainless steel castings

Abstract The effects of nitrogen content and solution treatment temperature on the microstructure and tensile properties of 25Cr–7Ni–1.5Mo–3W–xN Duplex stainless steel (DSS) castings were investigated. An increase of nitrogen decreases the ferrite volume fraction and also can cause gas defects and change the solidification mode from ferrite single phase to ferrite–austenite mode. An increase of heat treating temperature increases the ferrite volume fraction, which decreases the tensile strength and the elongation but increases the yield strength almost linearly.

The effect of casting method and heat treating condition on cold workability of high-Si electrical steel

Abstract The cold workability of Fe–(4.8–6.5)%Si was investigated by varying heat treatment conditions. The effects of casting methods such as ingot casting and spray forming on the cold workability were compared. Amount of B2 ordered phase, Si content, working temperature and Si-oxide were shown to have an effect on the cold workability of high-Si steel.

Computer-aided cooling curve analysis of A356 aluminum alloy

Computer aided cooling curve analysis (CA-CCA) is very useful in the foundry industry for easy and fast evaluation of a variety of properties. Typical applications include the prediction of the temperatures and amounts of different phases appearing during solidification and monitoring of the quality of melt in terms of Si- modification, grain refinement, inoculation, and graphite spheoridization. The use of cooling curve analysis can be extended to many other areas of solidification also, assuming the calculated values are reasonably accurate. The calculation of zero curve, which is vital in cooling curve analysis, offers many problems however. In this study, an attempt was made to investigate the problems of zero curve calculation and a new method is suggested to minimize calculation errors. An in-house developed computer program was used for a complete analysis of aluminum alloy A356 to determine the latent heat and solid fraction values.

Three dimensional finite element analysis with phase change by temperature recovery method

Abstract Finite element analyses for three dimensional problems in solidification have been made. For handling the release of latent heat of solidification, the temperature recovery method was applied. In analyses, the accuracy of the computed results depended upon the element size and the time increment used in simulations. An interface heat transfer coefficient between the mold and the casting was determined numerically as a function of time and used in calculations. The predicted temperatures were in good agreement with the available solutions in the literature.

Lattice matching of D023 and D022 phases in Al-6at.%(Ti,V,Zr) systems

Abstract Composite quaternary alloys with 21–22 vol.% of Al3M (M = Ti,V,Zr) were made by vacuum arc melting. Two inlermetallic phases were found in alloys: one is V rich D022-Al3(Ti,V,Zr) and the other is Zr rich D023-Al3(Ti,V,Zr). Measured lattice constants of precipitate phases were strongly dependent on the composition of transition elements in precipitates and generally obeyed Vegards rule. The lattice misfits between Al3M phases and the matrix did not change much in alloys while the lattice misfit between D022 and D023 Al3M phases was found to decrease with an increase in Ti content implying that the interface between both phases became more smooth, hard to distinguish and separate. A geometrical model was made for the lattice matching between two, D022 and D023 Al3M phases. Titanium has been found to act as a retarding element to separation into D023 and D022 phases and the pre-existing phase may have been L12 or D023 phase.

Coarsening behavior of L12 precipitates in melt-spun AlTiVZr alloys

Aging studies of two melt-spun Al-2at.% (Ti,V,Zr) alloys showed that the would-be metastable L12 Al3 (Ti,V,Zr) precipitates did not transform to stable D023 ones, and the average radius is 5–7 nm and the interparticle spacing is 20–40 nm at 698 K up to 400 h. The coarsening rate of spherical Al3(ti0.2V0.4Zr0.4) precipitates was observed to be five times as fast as that of Al3(Ti0.1V0.4Zr0.5) precipitates. The coarsening behavior in both alloys obeyed the Lifshitz-Slyozov-Wagner (LSW) prediction well. Owing to the low coarsening rate and the high thermal stability of the precipitated phase, AlTiVZr systems show promise as bases for high-temperature high-strength Al alloys.

The anodic behaviour of hot-galvanized zinc layer in alkaline solution

Abstract The anodic behaviour of a hot-galvanized zinc layer characterized by three spangle sectors has been studied in comparison with pure bulk zinc in KOH solution using potentiodynamic polarization and impedance methods. The hot-galvanized zinc layer showed a higher anodic dissolution current than the pure bulk zinc over all potential ranges, due largely to the alloying elements, Al and Pb. Also, the pure bulk zinc gave no notable anodic current peak in the passivation potential range, whereas the spangle specimen gave a marked anodic current peak in the potential range −0.75 to −0.65 V(SCE) due to hydrogen adsorbed/absorbed on spangles. From the variation of impedance spectra with exposure time at an applied anodic potential of −1.0 V(SCE), it is suggested that the passivation process on the pure bulk zinc readily attains a steady state, whereas it is disturbed on the spangle specimen by the adsorbed/absorbed hydrogen, thus proceeding in an unstable manner.