• Journal of Korea Foundry Society
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• v.34 no.2
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• pp.54-59
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• 2014

In this study, the extruded Al-6.3Zn-2.4Mg alloys were selected among the 7000 series aluminum alloys sensitive to hydrogen environment in order to examine the effects of both the aging conditions and the length of hydrogen charging period on the mechanical properties of the alloy. The specimens were aged for 24hours at $100^{\circ}C$ (under aging (UA)), $120^{\circ}C$ (peak aging (PA)), and $160^{\circ}C$ (over aging (OA)), respectively. Charging tests were performed at RT for 12, 24, 36 hours under potentiostatic conditions (-2000 mV vs (Ag/AgCl)) for 12, 24 and 36 hours in 1M $H_2SO_4$ and 0.1%$NH_4SCN$ solution. The fracture surface was examined by scanning electron microscopy (SEM). X-ray diffraction (XRD) pattern in peak aged sample was obtained before and after hydrogen charging from extruded Al-6.3Zn-2.4Mg alloys. The decreasing rate of tensile strength and elongation is represented in order of over aging < under aging < peak aging, and it is believed that the hydrogen recharge is more sensitive to elongation than tensile strength. The formation of $AlH_3$ in hydrogen charged Al-6.3Zn-2.4Mg alloys has been confirmed by X-ray diffraction studies.

• Journal of the Korean institute of surface engineering
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• v.54 no.4
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• pp.184-193
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• 2021

This study demonstrates formation behavior and morphological changes of PEO (Plasma Electrolytic Oxidation) films on AZ91 Mg alloy as a function of pre-treatment time in 1 M HF solution at 25 ± 1 ℃. The electrochemical behavior and morphological changes of AZ91 Mg alloy in the pre-treatment solution were also investigated with pre-treatment time. The PEO films were formed on the pre-treated AZ91 Mg alloy specimen by the application of anodic current 100 mA/cm² of 300 Hz AC in 0.1 M NaOH + 0.4 M Na₂SiO₃ solution. Vigorous generation of hydrogen bubbles were observed upon immersion in the pre-treatment solution and its generation rate decreased with immersion time. It was also found that 𝛽-Mg₁₇Al₁₂ in AZ91 Mg alloy was dissolved and a protective thin film of MgF₂ was formed on the AZ91 Mg alloy surface during the pre-treatment process in the 1 M HF solution. PEO film did not grow on the AZ91 Mg alloy specimen when the surface was not pre-treated and irregular PEO films with nodular defects were formed for the specimens pre-treated up to 1 min. Uniform PEO films were formed when the AZ91 Mg alloy specimen was pre-treated more than 3 min. The growth rate of PEO films on AZ91 Mg alloy increased significantly with increasing pre-treatment time.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.405-408
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• 2007

Semi-solid Al-Zn-Mg alloys were produced using a cooling plate method in order to investigate the extrudability. Al melt was poured on cooling plate which was adjusted at $60^{\circ}$ with respect to the horizontal plane, and the melt was cooled by water circulation underneath. Obtained Semi-solid feedstock has globular microstructure but also contains considerable amount of gas pore. Due to the pore, tensile elongation of the semi-solid feedstock was very low and it doesn't show yield point phenomenon. Isothermal hot extrusion was carried out using at $400^{\circ}C$ with a ram speed of 1mm/sec and an extrusion ratio of 25:1. The extruded bar show noticeably improved tensile ductility and strength because pore volume fraction decreased from 5% to 0.8% after extrusion. Mechanical properties of the semi-solid extruded bar were compared with that of commercial casting alloy..

• Transactions of Materials Processing
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• v.16 no.5 s.95
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• pp.391-395
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• 2007

Semi-solid Al-Zn-Mg alloys were produced by using a cooling plate method in order to investigate the extrudability. Al melt was poured on cooling plate which was adjusted at $60^{\circ}$ with respect to the horizontal plane, and the melt was cooled by water circulation underneath. Obtained Semi-solid feedstock has globular microstructure but also contains considerable amount of gas pore. Due to the pore, tensile elongation of the semi-solid feedstock was very low and it doesn't show yield point phenomenon. Isothermal hot extrusion was carried out using at $400^{\circ}C$ with a ram speed of 1mm/sec and an extrusion ratio of 25:1. The extruded bar show noticeably improved tensile ductility and strength because pore volume fraction decreased from 5% to 0.8% after extrusion. Mechanical properties of the semi-solid extruded bar were compared with that of commercial casting alloy.

• Transactions of Materials Processing
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• v.25 no.3
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• pp.155-160
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• 2016

Magnesium(Mg) alloys have been evaluated as replacements for steel or aluminum parts in the automobile industry because of the fuel economy they can provide through reducing weight. The application of Mg alloys has been limited due to its low formability at room temperature, which results from a small number of active slip systems. In the current study, an extruded TAZ711(Mg-7Sn-1Al-1Zn) alloy was warm forged into an automotive control arm to evaluate its formability at various forging temperature. Warm forging was conducted at temperatures of 200, 250, 350 and 450℃. Static strength evaluation was performed on the as-forged specimen at 250℃. The results showed good static strength.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.41-41
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• 2009

To evaluate the applicability of dissimilar joining between Mg and Al alloys in automobile manufacturing process, solid state joining processes such as magnetic pulse welding(MPW), friction stir welding(FSW) and friction spot joining(FSJ) were attempted successfully. MPW process has been concentrated mainly on round section tube to tube and tube to bar welds. AZ31 Mg alloy has been successfully welded to pure Al A1070 as well as to Al alloy A3003. While, for friction stir welding of dissimilar sheet joints, AZ31B/A6061 with the thickness of 2mm were used and a square butt joint with a good quality was obtained at the conditions of 0.8mm/sec of travel speed and tool rotation speed of 850rpm. The maximum tensile strength of 179 MPa, which was about 80 % of the Mg base metal tensile strength, has been obtained. Finally, friction spot joining was attempted to make a dissimilar lap joint between AZ31(0.8mm) and A6061(1mm), while the joint exhibited the same level of tensile shear strength as that of similar Mg joint.

• Journal of Korea Foundry Society
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• v.26 no.6
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• pp.272-276
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• 2006

For the first time AZ91 (MgAl9Zn1) and AM60 (MgAl6) Mg alloy foams with homogeneous pore structures were prepared successfully via melting foaming method by using $CaCO_3$ powder as blowing agent. The possible foaming mechanisms and pore structures of these Mg alloy foams were discussed and investigated. The results show that Mg alloy melt can affect $CaCO_3$ decomposition behavior and AZ91 Mg alloy is relative easy to be foamed into metal foam with high porosity and big pore size.

• Korean Journal of Materials Research
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• v.29 no.10
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• pp.609-616
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• 2019

Oxide coatings are formed on die-cast AZ91D Mg alloy through an environmentally friendly plasma electrolytic oxidation(PEO) process using an electrolytic solution of $NaAlO_2$, KOH, and KF. The effects of PEO condition with different duty cycles (10 %, 20 %, and 40 %) and frequencies(500 Hz, 1,000 Hz, and 2,000 Hz) on the crystal phase, composition, microstructure, and micro-hardness properties of the oxide coatings are investigated. The oxide coatings on die-cast AZ91D Mg alloy mainly consist of MgO and $MgAl_2O_4$ phases. The proportion of each crystalline phase depends on various electrical parameters, such as duty cycle and frequency. The surfaces of oxide coatings exhibit as craters of pancake-shaped oxide melting and solidification particles. The pore size and surface roughness of the oxide coating increase considerably with increase in the number of duty cycles, while the densification and thickness of oxide coatings increase progressively. Differences in the growth mechanism may be attributed to differences in oxide growth during PEO treatment that occur because the applied operating voltage is insufficient to reach breakdown voltage at higher frequencies. PEO treatment also results in the oxide coating having strong adhesion properties on the Mg alloy. The micro-hardness at the cross-section of oxide coatings is much higher not only compared to that on the surface but also compared to that of the conventional anodizing oxide coatings. The oxide coatings are found to improve the micro-hardness with the increase in the number of duty cycles, which suggests that various electrical parameters, such as duty cycle and frequency, are among the key factors controlling the structural and physical properties of the oxide coating.

• Journal of Korea Foundry Society
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• v.38 no.1
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• pp.1-8
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• 2018

In this study, we focused on the correlation between the solidification structure, heat treatment and mechanical properties of the A356 alloy according to the conditions of Sr modification. The microstructural evolution of the eutectic Si and ${\alpha}-Al$ phase in the A356 alloy castings depending on the amount of Sr were investigated during solid solution heat treatment using an optical microscope, a scanning electron microscope and an image analyzer. In addition, tensile tests on the heat treated materials examined the relationship between the microstructure and the fracture surface. The as-cast A356 alloys under 40 ppm Sr showed an undermodified microstructure, but that of the added 60-80 ppm Sr had well modified structure of fine fibrous silicon. After solid solution treatment, the microstructure of the undermodified A356 alloy exhibited a partially spheroidized morphology, but the remainder showed the fragmentation of fibrous shaped silicon. The spheroidization of the eutectic silicon in the modified A356 alloys was completed during heat treatment, which was very effective in increasing the elongation. This is supported by the fracture surface in the tensile test.

• Korean Journal of Metals and Materials
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• v.48 no.5
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• pp.436-444
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• 2010

The hot tear susceptibility of Al alloys was investigated by using a constrained-rod mold designed to quantify 8 types of tear tendency. The severity of the crack was scored by 5 grades on a scale of 0 to 4, with 0 being "no crack formed" and 4 being "complete separation by crack". The Hot Tear Susceptibility index (HTS) which consists of crack type scores and position scores, was proposed to compare the hot tear tendency of Al alloys. A356.0 cast alloy and AA6061 wrought Al alloy showed an HTS value of 27.5 and 53 respectively. The effects of Si, Cu, and Mg content on hot tear tendency were also investigated with a constrained-rod mold. The variation of HTS values with alloying elements represents similar behavior in the variation of the solidification range in a pseudo binary phase diagram.