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The change in reflectivity of Cu-Zn alloys was investigated with the modulation of Zn content. The reflectivity of the prepared alloys were compared with pure copper and divided into wavelength regions corresponding to blue, green, yellow, and red colors. As the amounts of Zn increased, the difference in reflectivity of the yellow region increased, and that of the red region decreased. To determine the color difference, the measured reflectivity of the alloys was transformed into a tristimulus value using the Reflectance-Tristimulus value converting equation. Then the tristmulus value was converted into the CIE L*a*b* color space. The color difference was calculated using the color difference formula of the converted L*a*b* values. When 5 at%Zn was added, the color difference value was 3.224, and it increased with increasing Zn content. As the difference in the reflectivity of the green and yellow regions increased, the b* value of the alloys increased and the a* value of the alloys decreased. In this study, the addition of Zn content to Cu alloy induced changes in the reflectivity of the green and yellow wavelength regions, which influences color differences of α-Cu solid solution alloys. (Received November 1, 2017; Accepted December 27, 2017)

keyword : Cu alloys, color metal, reflectivity, optical property, color difference

The drive towards weight reduction in the automotive industry has led to the use of high strength steel sheets for structural applications. Pressing and stamping are important processes in automotive production and result in additional straining on the vehicle body and chassis sheet material. It is therefore important to have knowledge of the effect of pre-straining on the reliability and mechanical behavior of sheets such as tensile and fatigue strength. This work sought to evaluate the mechanical properties of hot-rolled steel sheets prepared with different levels of pre-straining, which can cause a change in the final thickness of the sheets. Hot-rolled steel sheets, SAPH440 and SPHC with a nominal initial thickness value of 2.4 mm were used to obtain pre-strained sheets with pre-straining levels of 5~20%. Final thickness with pre-strain was measured to verify the correlation between thickness reduction ratio and mechanical properties. Macro and microscopic analysis results showed there was a uniform relationship between the reduction ratio and pre-strain despite the increasing sheet straining levels. The mechanical test results showed that by increasing the pre-straining level up to uniform elongation, the tensile and fatigue strength also increased, because of the work hardening effects with deformation. When pre-straining exceeded the uniform elongation, the mechanical properties decreased drastically when the thickness was modified by the initial value before pre-straining. Based upon the relation between mechanical behaviors and pre-straining, one can precisely predict design reliability and material durability by measuring the thickness change of the sheet. (Received November 14, 2017; Accepted December 11, 2017)

keyword : hot-rolled steel sheet, automobile, pre-strain, tensile strength, fatigue limit

This study investigated the effects of manufacturing conditions, such as patenting temperature and drawing strains, on the tensile fracture behaviors, especially on reduction of area (RA), of hyper-eutectoid steel wire. Steel wires with the chemical compositions of Fe-0.92C-1.3Si-0.6Mn-0.3Cr (wt%), were austenitized at 900 ℃ for 3 min and isothermally transformed at 580 ℃ and 620 ℃ for 3 min. Patented steel wires were cold drawn from 4.90 mm to 1.49 mm in diameter with an 18% reduction per pass. The cup-and-cone fracture surfaces of the tensile specimens consisted of a fibrous zone (crack initiation zone), radial marks (crack propagation zone), and shear lips. Four types of fracture mode were found in the tensile fractured surfaces of the patented specimens: dimple (fibrous zone), shear cracking (fibrous zone), boundary fracture (radial marks), and cleavage-type fracture (radial marks). Increasing transformation temperature contributed to an increase in the area fraction and size of the shear cracking and cleavage-type fracture. The increase in RA with the reduction in transformation temperature was attributed to a reduction in the pearlite block size as well as refinement of the interlamellar spacing. Meanwhile, as drawing strain increased, the area fraction of the fibrous zone in the tensile fractured surface decreased until a drawing strain of 1.79 and then increased, while RA increased up to a strain of 1.79 and decreased. It was found that the area fraction of the fibrous zone in the tensile fractured surface is closely related to the difference in RA in hyper-eutectoid steel wires. (Received November 20, 2017; Accepted January 2, 2018)

keyword : hyper-eutectoid steel, strength, ductility, cold drawing, tensile properties, fracture

Rhenium coatings were fabricated on a graphite substrate at 1100 and 1250 ℃ and a molybdenum substrate at 1150 and 1250 ℃ by chemical vapor deposition (CVD) using ReCl5 precursor, and the effects of deposition temperatures and substrates on microstructure and crystal orientation were investigated. The rhenium coating on graphite infiltrated the pores about hundred micrometers into the graphite, resulting in a large contact area between the coating and the graphite. However, the rhenium coating on molybdenum showed inferior adhesion, introducing voids and cracks in the Mo metal due to additional reactions between the precursor and Mo. As deposition temperature increased, rhenium grains on the graphite increasingly developed a preferred (0002) orientation normal to the coating, while on molybdenum they tended to develop other orientations, such as pyramidal and prism planes. These behaviors were discussed in relation to interfacial microstructure. (Received October 18, 2017; Accepted January 5, 2018)

keyword : coating, vapor deposition, rhenium, microstructure, texture

The optimum acid cleaning conditions for removing CaO formed by the reaction of Ca with Ti powder were investigated in this study. In this experiment, we used Ti powders composed of irregular polygonal Ti particles with average sizes of 125 mm and 250 μm. Ca granules were charged at 50% of the Ti mass into a deoxidation pot with Ti powder, and then the Ti powder with the Ca granules was deoxidized at 800 ℃ in a vacuum atmosphere of 5.0 × 10^-4 torr. It was confirmed that subsequent acid cleaning with 10% HCl for 5 minutes reduced the initial oxygen concentration of the 125 μm-sized Ti powder from 2,000 ppm to 1,105 ppm. In the case of the non-contact deoxidation using Ca vapor at 900 ℃, it was established that the non-contact deoxidation effect was higher than the contact type under the optimum acid cleaning condition of 10% HCl for 5 minutes. XRD and SEM analyses detected some Ca particles, and residual CaO on the Ti powder surface was confirmed without acid cleaning, but the acid cleaning process completely removed the residual CaO. It was also found that the deoxidation and acid cleaning decreased the c/a axial ratio of the Ti powder from 1.5885 to 1.5851. (Received October 23, 2017; Accepted December 29, 2017)

keyword : titanium, powder, deoxidation, acid cleaning, calcium

Material parameters, extracted from mechanical behavior in a uniaxial direction without considering anisotropy in the rolling direction, are commonly used when doing forming analysis. However, when these material parameters are used for the forming analysis of a material which has large anisotropy, inaccurate analysis results can be obtained. In this study, the material parameters of the Yoshida-Uemori model, 0°, 45°, 90° to the rolling direction were extracted. When we used the material parameters extracted from three directions, each according to their rolling directions, the accuracy of the springback prediction was increased. We analyzed the effect of material parameters on the accuracy of the springback prediction, using nine material parameters. As a result, it was found that ‘B-Y’ and ‘m’ affected the accuracy of the springback prediction the most. When we compared this result to other studies, it showed outstanding accuracy for sidewall curl prediction. This shows the importance of considering the anisotropy of steel sheet, to increase accuracy of the springback analysis. (Received June 14, 2017; Accepted January 5, 2018)

keyword : anisotropy, springback, kinematic hardening, Yoshida-Uemori Model, U-bending

In this study, the effects of hot forging and heat treatment (quenching and tempering) of cast Cr alloy steel on the microstructures and mechanical properties were investigated. The hot forging was performed at a compressive ratio 0.5 at 1,250 ℃. The heat treatment process was quenching (860 ℃ for 2 hours and water quenching) and tempering (655 ℃ for 2 hours and air cooling). The microstructures of the hot forged specimen showed bainite, pearlite and ferrite mixed phases with high tensile strength, but showed low fracture toughness. The heat treated specimens after hot forging showed tempered martensite microstructure and high fracture toughness but relatively low yield and tensile strengths. After tensile and fracture toughness tests, the cast and the hot forged specimens both showed cleavage fracture surfaces, which occurred between lamellar structures. However, the heat treated specimen had a ductile fracture surface with dimple shaped fractures. From these results, we could conclude that the high fracture toughness was caused not by the cleavage fracture mode in the pearlite and bainite phases, but delayed fracture due to a ductile fracture mode in the tempered martensite phase. (Received September 11, 2017; Accepted December 27, 2017)

keyword : Cr steel, heat treatment, fracture toughness, microstructure, tempered martensite

To ensure the reliability of high-power light-emitting diode (LED) bulbs, temperature management is very important, including reduction of the junction temperature on the LED chip and enhancing outward thermal dissipation. Simulation can be an effective tool for predicting the temperature distribution over all bulbs, which enables a quantitative analysis. In this study, we investigated a high-power LED bulb consisting of 21 LED packages with a single LED chip for each. We experimentally measured the temperature at the surface of the LED bulb using a thermocouple, and a finite volume method (FVM) simulation was conducted for the same position to confirm the accuracy of the prediction model. In addition, thermal equilibrium models based on heat transfer equations are proposed for quantitative analyses using calculations based on the models. Using this method, we explain the effect of the lens on the surface temperatures of the LED bulb and predict the junction temperature. By comparing the analysis of the heat dissipation characteristics from the calculations to the thermal equilibrium models, we determined the accuracy of the FVM simulation in this study for the variation in thermal conductivity of the heat sink. Based on the reliability of the FVM simulation, we investigated the effects of the surface emissivity of the heat sink and heat convection conditions. (Received November 2, 2017; Accepted December 27, 2017)

keyword : LED(light emitting diodes), LED bulb, heat dissipation, heat transfer, heat sink, FVM simulation

The optimal design of electrical initiation devices for explosive charges and other high energy materials critically depends on the bridge burst or the electrical explosion of the thin-film metallic bridges. It is therefore quite important to select the right material with suitable explosion properties for the metallic bridge film. However, so far, no dedicated studies, experimental or theoretical, have been carried out to quantify those explosion properties for thin-filmed bridges of various metals. This study employed numerical modeling of the joule heating and bridge plasma expansion by intense electrical energy deposition in aluminum and copper bridges to evaluate and compare the explosion properties such as bridge burst time, bridge burst current density, and threshold charging voltage for the two metallic bridge materials. The prediction model of bridge burst also considered a pulsed electrical input from an RLC initiation circuit as well as plasma property calculations of the aluminum and copper plasma vapor for the state equation of the ionized vapor and the plasma electrical conductivity. The numerical predictions were in excellent agreement with the corresponding measured data of bridge bursts in a series of exploding foil initiator firing tests. The copper bridge was found to possess better explosion properties than the aluminum bridge, showing significantly higher bridge burst current density and threshold charging voltage. These findings confirmed that the copper bridge may provide higher safety and reliability in electrical initiation devices. The theoretical model and results in the present study could be useful for selecting bridge materials and designing electrical initiators or other metallic bridge explosion applications. (Received December 4, 2017; Accepted January 10, 2018)

keyword : metallic bridge, bridge burst, aluminum, copper, plasma, electrical initiator

Samples with compositions of 99 w/o MgH₂+1 w/o Zn(BH₄)₂ (designated MgH₂-1Zn(BH₄)₂) and 96 w/o MgH₂+2 w/o Zn(BH₄)₂+2 w/o Ni (designated MgH₂-2Zn(BH₄)₂-2Ni) were prepared by milling in a planetary ball mill in a hydrogen atmosphere (reaction-involving milling). The hydrogen release properties of MgH₂- 1Zn(BH₄)₂ and MgH₂-2Zn(BH₄)₂-2Ni were compared. A larger quantity of additives and the addition of Ni decreased the temperature at which MgH₂ decomposes in the as-milled samples. Activation processes were not required for these two samples. A larger quantity of additives and the addition of Ni by milling in a hydrogen atmosphere increased the quantity of hydrogen absorbed in 60 min (U (60 min)), the initial hydrogen release rate, and the quantity of hydrogen released in 60 min (R (60 min)). MgH₂-2Zn(BH₄)₂-2Ni had an effective hydrogen storage capacity of about 5.5 w/o at the cycle number, CN, of one (CN=1). A ZnH₂ phase, which has not been reported to be formed, was found in the MgH₂-1Zn(BH₄)₂ sample and the Zn(BH₄)₂ and Ni-added MgH₂ sample after hydrogen uptake-release cycling. Mg₂Ni was formed in the Zn(BH₄)₂ and Niadded MgH₂ sample after hydrogen uptake-release cycling. The rapid hydrogen release rate of the Mg₂NiH₄ formed in MgH₂-2Zn(BH₄)₂-2Ni is believed to increase the initial hydrogen release rate of MgH₂-2Zn(BH₄)₂-2Ni. The Mg₂Ni decomposed from Mg₂NiH₄ might facilitate the nucleation of a Mg-H solid solution in the MgH₂-2Zn(BH₄)₂-2Ni sample. There is also a slight possibility that the contraction due to the relatively rapid hydrogen release of the Mg₂NiH₄ provides passages for the hydrogen released from neighboring MgH₂. (Received December 20, 2017; Accepted January 9, 2018)

keyword : hydrogen absorbing materials, mechanical milling, hydrogen, microstructure, Zn(BH₄)₂ and/or Ni-added MgH₂ alloy