Chang Dong Yim

Effects of Non-Flux Purification on the Microstructure and Mechanical Properties of AZ31+xCa Mg Alloy

The effects of non-flux purification on the mechanical properties, microstructure and fracture pattern of AZ31+xCa magnesium alloys have been studied. The results show that the purification process of ceramic foam filters incorporating gas bubbling process can improve obviously the ultimate tensile strength σb and elongation δ of AZ31 alloy. With 20ppi MgO filter incorporating gas bubbling treatment under Ar flow rate of 2 l/min and temperature of 730°C, the σb and δ of AZ31 can be improved from 144.6MPa and 5.21% to 180.8MPa and 9.06% by 25% and 73.9%, respectively. Addition of 0.13%Ca to magnesium can not only increase the strength but also improve the elongation. With the further increase of more than 0.13% Ca content, the strength and elongation decrease. The experimental results show that under low Ca content, filtration purification process can greatly improve the σb and δ of AZ31+xCa alloy. However, with the increase of Ca content, the filtration effects decrease. The effective sequence of different filters is as follows: 20ppi MgO>20ppi Al2O3>10ppi MgO>10ppi Al2O3.

Microstructural change in gravity cast Mg−Ni alloys with Ni contents

The effects of Ni content on the microstructures of gravity cast Mg−Ni alloys were examined. Different shapes of primary solid particles and microstructures of the eutectic matrix were observed with varying Ni content. The eutectic matrix in the hypoeutectic alloy consisted of rod-like Mg2Ni phase with a small amount of α-Mg phase. The eutectic matrix in the near-eutectic and hypereutectic alloys consisted of eutectic cells oriented in different directions. A transition of lamellae to fiber occurred. The formation of cellular structure and transition of lamellae to fiber resulted from two-phase instability in the coupled region induced by constitutional undercooling.

Oxidation and Corrosion Behavior of Non-Flammable Magnesium Alloys Containing Ca and Y

The combined addition of calcium and yttrium lead to significant increase in non-flammability and tensile properties, compared to conventional Ca-containing magnesium alloys. It means that we can reduce the amount of calcium addition to get certain amount of non-flammability in the alloys containing both calcium and yttrium together. Because the addition of calcium only can increase the ignition temperature but decrease the tensile properties, it should be minimize not to decrease the tensile properties. Corrosion behavior of calcium and yttrium containing alloys were also investigated systematically, and show that they have better corrosion resistance than the any other commercial alloys or calcium containing alloys. Detailed study on the corrosion behavior confirms that there are optimum amount of calcium and yttrium addition to get best corrosion resistance, and corrosion behavior of these alloys are quite different with other alloys.

Electrodeposition of Copper on AZ91 Mg Alloy in Cyanide Solution

Copper electrodeposition on AZ91 Mg alloy was studied in views of preferential deposition on α- or β-phases and how to achieve uniform deposition over the entire surface on α- and β-phases in a cyanide solution. The inhomogeneous microstructure of AZ91 Mg alloy, particularly α- and β-phases, was found to result in non-uniform deposition of zincate layer, preferential deposition of zincate on β-phases, which leads to non-uniform growth of copper layer during the following electrodeposition process. The preferential depositions of zincate can be attributed to higher cathodic polarizations on the β-phases. Pin-hole defects in the copper electrodeposit were observed at the center of large size β-phase particles which is ascribed to gas bubbles formed at the β-phases. The activation of AZ91 Mg alloy in hydrofluoric acid solution was used to obtain uniform growth of zincate layer on both the α- and β-phases. By choosing an optimum activation time, a uniform zincate layer was obtained on the AZ91 Mg alloy surface and thereby uniform growth of copper was obtained in a cyanide copper electroplating solution.

Effect of Zincate Treatment of As-Cast AZ91 Mg Alloy on Electrodeposition of Copper in a Copper Pyrophosphate Bath

In this work, effect of zincate treatment of AZ91 Mg alloy on the following electrodeposition of copper was examined in a non-cyanide bath containing pyrophosphate ions in view of surface morphology and adhesion of the electrodeposited copper layer. Without zincate treatment, the electrodeposited copper layer showed very porous structure and poor adhesion. On the other hand, the copper layer electrodeposited on the zincate-treated surface showed dense structure and good adhesion. The dissolution rate of AZ91 Mg alloy after the zincate treatment appeared to decrease about 40 times in the copper pyrophosphate bath, as compared to that of the surface without zincate treatment. The porous morphology and poor adhesion of a copper layer on the AZ91 Mg alloy surface without zincate treatment were attributed to small number of nucleation sites of copper because of rapid dissolution of the magnesium substrate in the pyrophosphate bath. Based on the experimental results, it is concluded that the zincate treatment to form a conducting and protecting layer on the AZ91 Mg alloy surface is essential for successful electrodeposition of a copper layer on AZ91 Mg alloy with good adhesion and dense structure in the copper pyrophosphate bath.

Microstructure and Mechanical Properties of As‐Extruded Mg‐Sn‐Al‐Zn Alloys

Mg-(9-x)Sn-xAl-lZn (x=1, 2, 3 and 4 wt.%) alloys were subjected to indirect extrusion, and the microstructure and mechanical properties of these as-extruded Mg-Sn-Al-Zn (TAZ) alloys were investigated. The TAZ alloys contain only fine Mg2Sn second-phase particles in the a -Mg matrix, and the amount of precipitates increase with increasing Sn contents. In addition, the TAZ 811 alloy shows finer grain structure than the TAZ 541 alloy due to a larger amount of Mg2Sn particles, which act as nucleation sites for recrystallization and/or prevention of grain growth by the pinning effect. The textures of the alloys show a typical basal pole orientation and an unusual component in which one axis of the HCP unit cell is parallel to the extrusion axis. Tensile yield strength increases with Sn content, while ultimate tensile strength is almost identical due to the increased strain hardenability with Al content. The tension-compression yield asymmetry decreases with reducing Sn content, which is mainly due to the decrease in the amount of particles promoting twin nucleation.

Corrosion Properties of Mg–6Al–0.3Mn–aSn–bZn Alloys

The effect of addition of Sn and/or Zn on corrosion properties of Mg–6Al–0.3Mn alloy was investigated systematically by immersion and salt spray tests. In case of immersion test, average corrosion rate increased by addition of 1 wt% Sn but it decreased by addition of 1 wt% Zn. When the half of the amount of Sn addition was replaced by Zn, the average corrosion rate decreased in comparison with the average corrosion rate of Mg–6Al–0.3Mn–1Sn alloy. In case of salt spray test, the change of average corrosion rate according to composition of the alloy was similar to the result of immersion test. It seemed that the change of corrosion properties according to composition of the alloy was strongly related to the microstructural changes including the sort, fraction and morphology of second phase.

Effects of Microstructural Factors on Corrosion Behavior of As-Extruded Mg-Sn-Zn Alloys in Immersion and Salt Spray Environments

The effects of microstructural factors, including the fraction of second phase particles and solute content in the matrix, on the corrosion behavior of as-extruded Mg-Sn-Zn alloys were evaluated by immersion and salt spray testing. After immersion testing, the average corrosion rate increased with the increase in Sn content and decrease in Sn:Zn ratio. On the other hand, after salt spray testing, the average corrosion rate increased with the increase in Sn content, but it was minimized at the Sn:Zn ratio of 5:2. Hydrogen evolution appeared to be the dominant controlling factor of corrosion during immersion, while the partial protectiveness of the film also contributed to the corrosion during salt spray testing.

Effect of Sn:Zn Ratio on Corrosion Behavior of Mg‐aSn‐bZn Extrusions

The effect of Sn:Zn ratio on corrosion behavior of magnesium alloys containing Sn and Zn was evaluated systematically by potentiodynamic polarization and immersion tests. The hydrogen evolution rate during cathodic polarization and the average corrosion rate measured by immersion test increased with increasing Sn content and Sn:Zn ratio. The changes of fraction of second phase with more positive corrosion potential and solute contents in the matrix phase were responsible for the change of corrosion behavior according to Sn content and Sn:Zn ratio. Mg2Sn particle accelerated the corrosion by formation of micro-galvanic cell, which resulted in faster corrosion rates of Mg-5Sn-xZn alloys than those of Mg-2Sn-xZn alloys. The increase of Zn in the matrix was harmful to the corrosion resistance because Zn accelerated the hydrogen evolution although ZnO was beneficial to the passivity of surface film.

Effect of Ca and Y on Corrosion Behavior of Extruded AZ Series Mg Alloys

It is well known that ignition temperature increases dramatically through combined addition of Ca and Y to AZ series Mg alloys. Therefore, the industrial application of Mg alloys is expected to be expanded gradually. However, studies on corrosion behavior of these alloys have been carried out rarely even though the evaluation and understanding of corrosion behavior are necessary for wide application. In this study, the changes of microstructure and corrosion behavior of AZ series alloys according to the content of Ca and Y were systemically evaluated by immersion and salt spray tests. The corrosion resistance of AZ series alloys was improved dramatically by combined addition of Ca and Y without degradation of mechanical properties. It implies that the AZ series alloys containing optimized amounts of Ca and Y have the excellent combination of ignition resistance, corrosion resistance and mechanical properties.