• Journal of Korea Foundry Society
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• v.36 no.2
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• pp.60-66
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• 2016

In this study, comparison of corrosion properties of the Mg-1.5Ge-1Zn (GZ21) alloy and Mg-9Al-1Zn (AZ91) alloy were investigated. The studied alloys were fabricated by permanent mold casting method. And the potentiodynamic test, hydrogen evolution test, immersion test and A.C Impedance test were carried out in a 3.5% NaCl solution with pH7.2 at room temperature to measure the corrosion properties. The microstructure of GZ21 alloy was composed of ${\alpha}-Mg$ and $Mg_2Ge$ phases and AZ91 alloy was composed of ${\alpha}-Mg$ and $Mg_{17}Al_{12}$ phases. From the test results, the corrosion property was improved by adding Ge. It seemed that the corrosion mechanism was changed from galvanic corrosion (AZ91) to filiform corrosion (GZ21).

• Journal of the Korean institute of surface engineering
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• v.49 no.3
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• pp.238-244
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• 2016

Copper electrodeposition on AZ91 Mg alloy was studied in views of preferential deposition on ${\alpha}$- or ${\beta}$- phases and how to achieve uniform deposition over the entire surface on ${\alpha}$- and ${\beta}$-phases in a cyanide solution. The inhomogeneous microstructure of AZ91 Mg alloy, particularly ${\alpha}$- and ${\beta}$-phases, was found to result in non-uniform deposition of zincate layer, preferential deposition of zincate on ${\beta}$-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 ${\beta}$-phases. Pin-hole defects in the copper electrodeposit were observed at the center of large size ${\beta}$-phase particles which is ascribed to gas bubbles formed at the ${\beta}$-phases. The activation of AZ91 Mg alloy in hydrofluoric acid solution was used to obtain uniform growth of zincate layer on both the ${\alpha}$- and ${\beta}$-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.

• Journal of the Korean Society for Heat Treatment
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• v.28 no.4
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• pp.190-199
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• 2015

The study is intended to investigate the effect of solution treatment on microstructure and corrosion behavior of AZ91(Mg-9%Al-1%Zn-0.3%Mn)-2%Ca casting alloy. In as-cast state, the AZ91-2%Ca alloy consisted of intermetallic ${\beta}(Mg_{17}Al_{12})$, $Al_8Mn_5$ and $Al_2Ca$ phases in ${\alpha}-(Mg)$ matrix. After the solution treatment, Al within the ${\alpha}-(Mg)$ matrix was distributed more homogeneously, along with the slight decrease in the total amount of intermetallic compounds. The corrosion resistance of the AZ91-2%Ca alloy was improved after the solution treatment. The microstructural examinations for the solution-treated samples revealed that the better corrosion resistance may well be related to the incorporation of more oxides and hydroxides such as $Al_2O_3$, $Al(OH)_3$, CaO and $Ca(OH)_2$ into the surface corrosion product without dissolution of the intermetallic phases along the grain boundaries.

• Corrosion Science and Technology
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• v.5 no.6
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• pp.218-221
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• 2006

Influence of microstructure on the corrosion property of Mg-Al-Zn alloy was investigated using potentiodynamic polarization experiments, galvanic coupling experiments, and scanning electron microscopy in sodium chloride solutions. Pitting was the most common form of attack in chloride solution, and filiform corrosion was also occurred in AZ91D-T4 alloy. On the contrary, filiform attack in the bulk matrix was predominant corrosion form in AZ91D-T6 alloy, and the number and size of pit were decreased than those of AZ91D-T4 alloy. Galvanic coupling effect between $Mg_{17}Al_{12}$ and matrix was existed, but the propagation of galvanic corrosion was localized only near the $Mg_{17}Al_{12}$ phase in AZ91D-6T alloy. The corrosion resistance of Mg-Al matrix increased with decreasing Al content in the matrix. And, it could be regarded that Al content in the matrix is decreased by precipitation of $Mg_{17}Al_{12}$ during the aging treatment and it decreases the anodic reaction rate of the matrix and galvanic effect in AZ91D-T6 alloy. It could be considered that the composition and microstructure of surface protective layer would be varied by precipitation of $Mg_{17}Al_{12}$ and subsequent decreasing of Al content in the matrix. And it would contribute the corrosion resistance of AZ91D-T6 aging alloy.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.13 no.11
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• pp.4963-4967
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• 2012

In this study, 18-inch wheels, magnesium alloy AZ91D was developed and we compared overseas go on sale magnesium wheels and same specifications of the aluminum wheels mechanical properties. Prototype 18-inch magnesium wheels by a low-pressure casting method to achieve the same specifications of aluminum wheels and reduced 26% of the weight, the new edition of magnesium wheels compared to the same level of elongation, tensile strength, hardness. Casting and heat treatment process to improve future need to improve the yield strength is expected.

• Journal of Welding and Joining
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• v.22 no.5
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• pp.53-57
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• 2004

Friction stir weldability of AZ31B-H24, AZ61A-F and AZ91C-F Mg alloys were studied using microstructural observation and mechanical tests. The microstructure of stir zone(SZ) was coarse in AZ31B-H24 alloy whereas it was very fine both in AZ61A-F and AZ91C-F alloys. The hardness of SZ was remarkably increased by very fine recrystallized grains both in AZ61A-F and AZ91C-F alloys. On the other hand, the hardness of SZ was decreased in AZ31B-H24 due to the coarse microstructure. In SZ, AZ91C-F alloy showed very high hardness values because of dispersion hardening of $Mg_{17}$Al$_{12}$($\beta$ phase) and Al solid solution hardening. Because of more $Mg_{ 17}Al_{12}($\beta$ phase)$ intermetallic compounds, Mg alloy with high Al content showed poor mechanical properties.s.

• Korean Journal of Metals and Materials
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• v.50 no.9
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• pp.619-627
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• 2012

One of the barriers limiting wide applications of magnesium alloys to various industries is their poor corrosion resistance. The corrosion properties of AZ91 magnesium alloy, which is the most popular magnesium casting alloy, are affected by microstructural and environmental factors. The corrosion properties of AZ91 magnesium alloy are affected by the corrosion properties of ${\alpha}-Mg$ and ${\beta}$ phases, the volume fraction and distribution of ${\beta}$ phase and area ratio of ${\alpha}-Mg/{\beta}$ phases. The corrosion properties of AZ91 magnesium alloy under various environments also change according to the passivity of films and types of corrosion products formed on its surface. The corrosion resistance of the magnesium alloys can be improved by microstructural control through the addition of alloying elements and optimization of the production process.

• Journal of the Korean Society for Heat Treatment
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• v.27 no.4
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• pp.185-190
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• 2014

The influences of small amount of Sn addition on microstructure and creep resistance of AZ91-0.4%Ca alloy have been investigated. The microstructure of the AZ91-0.4%Ca alloy was characterized by ${\alpha}$-(Mg) dendrite cells surrounded by eutectic ${\beta}(Mg_{17}Al_{12})$ and $Al_2Ca$ phases. The 0.5%Sn addition resulted in the formation of rod-shaped CaMgSn particles with the extinction of $Al_2Ca$. The Sn-containing alloy exhibited better creep resistance below $175^{\circ}C$, but the tendency was reversed above $200^{\circ}C$. The reason was discussed in relation to the change in thermal stability of ${\beta}$ phase in response to the Sn addition.

• Korean Journal of Materials Research
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• v.28 no.10
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• pp.544-550
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• 2018

The passivation of AZ91D Mg alloys by plasma anodization requires deliberate choice of process parameters due to the presence of large amounts of structural defects. We study the dependence of pore formation, surface roughness and corrosion resistance on voltage by comparing the direct current (DC) mode and the pulse wave (pulse) mode in which anodization is performed. In the DC plasma anodization mode, the thickness of the electrolytic oxide film of the AZ91D alloy is uneven. In the pulse mode, the thickness is relatively uniform and the formed thin film has a three-layer structure. The pulse mode creates less roughness, uniform thickness and improved corrosion resistance. Thus, the change of power mode from DC to pulse at 150 V decreases the surface roughness (Ra) from $0.9{\mu}m$ to $0.1{\mu}m$ and increases the corrosion resistance in rating number (RN) from 5 to 9.5. Our study shows that an optimal oxide film can be obtained with a pulse voltage of 150 V, which produces an excellent coating on the AZ91D casting alloy.

• Journal of Korea Foundry Society
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• v.29 no.5
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• pp.233-237
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• 2009

The effect of electrolyte on mechanical and corrosion properties of AZ91 magnesium alloy by plasma electrolytic oxidation (PEO) method was investigated. The coating layers formed in the silicate and the aluminate electrolytes showed porous structures. The small pores were randomly distributed on the coatings formed in aluminate electrolyte while the coatings formed in silicate electrolyte showed much bigger pores. In the aluminate electrolyte, the coatings were composed of Mg, MgO and $MgAl_2O_4$, whereas Mg, MgO, $MgAl_2O_4$ and $Mg_2SiO_4$ were identified in the coatings formed in silicate electrolyte. The hardness of coatings in the silicate electrolyte was higher than that of coating grown in the aluminate electrolyte. The AZ91 alloy coated in the silicate electrolyte had higher tensile strength and elongation than that coated in the aluminate electrolyte. In addition, the coatings formed in the silicate electrolyte showed much better corrosion resistance compared to the coatings formed in the aluminate electrolyte.