• Journal of the Korean Society for Heat Treatment
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• v.33 no.5
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• pp.219-225
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• 2020

The objective of this study was to investigate the tensile properties and thermal conductivities of Mg9.3%Al alloy in as-cast state and heat-treated state consisting of fully discontinuous precipitates (DPs), respectively. The fully DPs microstructure was obtained by solution treatment at 405℃ for 24 h, followed by furnace cooling to RT. The as-cast alloy showed a partially divorced eutectic β(Mg₁₇Al₁₂) phase particles formed along the α-(Mg) cell boundaries. The DPs had various apparent (α+β) interlamellar spacings, which is related to different transformation temperatures during the furnace cooling. The DPs microstructure exhibited better tensile strength than the as-cast one, resulting from the higher value of elongation in response to its more homogeneous microstructure. It is noticeable that the DPs microstructure had 12.4% higher thermal conductivity in average than the as-cast one between RT and 200℃. The XRD analyses revealed that the lower Al concentration in the α-(Mg) matrix may well be responsible for the better thermal conductivity of the DPs microstructure.

• Journal of the Korean Society for Heat Treatment
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• v.21 no.2
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• pp.69-78
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• 2008

Differences in hardness and microstructure between surface and area at 0.3 mm below the surface after quenching and tempering of ductile cast iron for rear planet carrier of automotive transmission have been investigated. Microstructure of ductile cast iron consisted of ferrite, pearlite, and nodular graphite. The amount of pearlite increased with going down to the half-thickness area. It was found that Cr and Mo segregated to the pearlite and the pearlite transformed to the harder martensite during quenching. The martensite was more resistant to the decomposition to ferrite and cementite during tempering because of segregation of Cr and Mo, resulting in the harder tempered martensite. Consequently, the hardness of the surface with less amount of pearlite, corresponding to the harder martensite in the quenched and tempered microstructure, was lower than that of the area at 0.3 mm below the surface.

• Journal of Korea Foundry Society
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• v.30 no.1
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• pp.46-51
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• 2010

Recently, automotive industry is attempting to replace steels for automotive parts with light-weight alloys such as aluminum alloy, because of the growing environmental regulations governing exhaust gas and the engine effectiveness of a vehicle. The low cycle fatigue (LCF) and high cycle fatigue (HCF) properties as well as the microstructure and tensile property were investigated on the low pressure cast A356 aluminum alloy wheel, which was followed by T6 heat treatment. The cast microstructure of the alloy influenced significantly on the low cycle and high cycle fatigue behaviors. The rim part of cast aluminum alloy wheel showed higher low cycle and high cycle fatigue strength compared with the spoke part, which should be caused by higher cooling rate of rim part. The spoke part of the wheel showed coarser dendrite arm spacing (DAS) and wide eutectic zone in the microstructure, which resulted in the partial brittle fracture and lower fatigue life time.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.493-498
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• 2002

Friction stir welding (FSW) is a relatively new solid-state joining process which can homogenize the heterogeneous microstructure by intensely plastic deformation arising from the rotation of the welding tool. The present study applied the FSW to an A356 aluminum (AI) alloy with the as-cast heterogeneous microstructure in the T6 temper condition, and examined an effect of microstructure on mechanical properties in the weld. The base material consisted of Al matrix with a high density of strengthening precipitates, large eutectic silicon and a lot of porosities. The FSW led to fragment of the eutectic silicon, extinction of the porosities and dissolution of the strengthening precipitates in the Al alloy. The dissolution of strengthening precipitates reduced the hardness of the weld around the weld center and the transverse ultimate tensile strength of the weld. Longitudinal tensile specimen containing only the stir zone showed the roughly same strength as the base material and a much larger elongation. Moreover, Charpy impact tests indicated that the stir zone had remarkably the higher absorbed energy than the base material. The higher mechanical properties of the stir zone were attributed to a homogenization of the as-cast heterogeneous microstructure by FSW.

• Journal of Korea Foundry Society
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• v.32 no.2
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• pp.81-85
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• 2012

The effects of section size and melt holding time on the mechanical properties and microstructure of high silicon ductile cast iron were investigated. The strength, elongation and hardness of the test specimen with smaller cross-section were higher than those with larger one. The nodule count and volume fraction of pearlite of the former were higher than those of the latter. The mechanical properties decreased with increased melt holding time before pouring. Nodularity and nodule count decreased and the volume fraction of pearlite rather slightly increased with it.

• Corrosion Science and Technology
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• v.2 no.1
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• pp.30-35
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• 2003

In this study, sand cast AZ91E and zirconium grain refined MEZ are representative of two typical groups of magnesium alloys: those containing aluminium and those containing no aluminium but with zirconium as a grain refiner. The corrosion performance of these two alloys was evaluated and compared in 5%wt NaCI solution through measurements of weight loss and polarisation curves and examination of microstructure. Corrosion damage of AZ91E was deeper and more localised than that of MEZ, while MEZ had a lower rate of cathodic hydrogen evolution and a higher rate of anodic dissolution than AZ91E. These differences in behaviour can be related to the differences in microstructure and chemical composition between the two alloys.

• Journal of the Korean Society for Nondestructive Testing
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• v.32 no.2
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• pp.155-161
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• 2012

It was attempted to evaluate the microstructure and quality of various types of cast iron by ultrasonic velocity measurement. Three types of commercial gray cast iron and ductile cast iron were used for this investigation, respectively. One type of the ductile cast iron was heat-treated as a function of annealing time to produce different microstructure. Ultrasonic velocity measurement, microstructural analysis (pearlite area fraction, graphite length and nodularity), and hardness measurement were performed to find empirical correlations among these parameters. Ultrasonic velocity of ductile cast iron was markedly faster than that of gray cast iron. Ultrasonic velocity decreased with the decrease of fraction of pearlite structure. As a quality monitoring parameter of cast iron, potential of ultrasonic velocity was suggested.

• Journal of Powder Materials
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• v.17 no.6
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• pp.470-476
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• 2010

In this study, Cu-10Sn and Cu-10Sn-2Ni-0.2Si alloys have been manufactured by spray casting in order to achieve a fine scale microstructure and high tensile strength, and investigated in terms of microstructural evolution, aging characteristics and tensile properties. Spray cast alloys had a much lower microhardness than continuous cast billet because of an improved homogenization and an extended Sn solid solubility. Spray cast Cu-Sn-Ni-Si alloy was characterized by an equiaxed grain microstructure with a small-sized (Ni, Si)-rich precipitates. Cold rolling of Cu-Sn-Ni-Si alloy increased a tensile strength to 1220 MPa, but subsequent ageing treatment reduced a ultimate tensile strength to 780 MPa with an elongation of 18%.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.414-419
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• 2009

Good control of thermal energy helps to increase characteristics and eliminate defects of large cast-forged part, such as large sized forged shell. Thermal energy control is a important factor. We have studied about forging process and after heat treatment process by FEM simulation. There are three ways of process. Changes of temperature and microstructure for forged shell were predicted according to temperature declination in large cast-forged product. So we will be able to choose the proper time from heat treatment conditions by FEM simulation.

• Journal of Korea Foundry Society
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• v.36 no.6
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• pp.181-186
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• 2016

The typical microstructure of Al-5%Mg-2%Zn cast alloy mainly consists of an aluminum matrix with a small amount of AlMgZn 2nd phase. The secondary dendrite arm spacing and the grain size of the cast alloy tend to be inversely proportional to the section thickness of casting; however, the tensile properties cannot be said to be clearly related to the cast microstructure. After T6 heat treatment, the tensile strength of the alloy was enhanced significantly. TEM analysis results show that very fine AlMgZn precipitates were formed after the heat treatment. The corrosion resistance, measured according to the corrosion potential, was found to increase slightly after the conducting of heat treatment.