• 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.

• Journal of the Korean Society for Heat Treatment
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• v.15 no.6
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• pp.260-268
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• 2002

The effects of calcium and silicon on microstructure and aging characteristics of AZ91 magnesium alloy during T5 treatment was investigated. The addition of 0.88% calcium or 0.25% silicon to AZ91 alloy made dendrite cell smaller. Especially, silicon is more effectively acted as refinement of the dendrite cell than calcium. It is due to that $Mg_2Si$ precipitated at the dendrite cell boundary or in the matrix during T5 treatment of Si added AZ91 alloy retarded the growth of the secondary phase. In the mean while, without inducing the precipitates containing calcium, calcium was segregated mainly around secondary phase such as $Mg_{17}Al_{12}$ and partially dissolved in ternary eutectic (Mg-Al-Ca) structure. In the AZ91 alloy containing both silicon and calcium, more finely distributed $Mg_2Si$ in matrix homogeneously and much finer microstructure were obtained than those containing silicon or calcium. Hence, An AZ91 containing both silicon and calcium was more effective to retarding the growth of the secondary phase than the other AZ91 alloy such as AZ91 alloy containing silicon or AZ91 alloy containing calcium.

• Journal of the Korean institute of surface engineering
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• v.49 no.5
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• pp.401-407
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• 2016

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.

• Journal of the Korean institute of surface engineering
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• v.55 no.1
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• pp.24-31
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• 2022

This study investigated the effect of pre-treatment on the PEO film formation of AZ91 Mg alloy. The pre-treatment was conducted for 10 min at room temperature in 0.5 M oxalic acid (C₂H₂O₄) solution containing various ammonium fluoride (NH₄F) concentrations. The pre-treated AZ91 Mg specimens were anodized at 100 mA/cm² of 300 Hz AC for 2 min in 0.1 M NaOH + 0.4 M Na₂SiO₃ solution. When AZ91 Mg alloy was pretreated in 0.5 M oxalic acid with NH₄F concentration less than 0.3 M, continuous dissolution of the AZ91 Mg alloy occurred together with the formation of black smuts and arc initiation time for PEO film formation was very late. It was noticed that corrosion rate of the AZ91 Mg alloy became faster if small amount of NH₄F concentration, 0.1 M, is added. The fast corrosion is attributable to fast formation of porous fluoride together with porous oxides in the reaction products. On the other hand, when AZ91 Mg alloy was pretreated in 0.5 M oxalic acid with sufficient NH₄F more than 0.3 M, a thin and dense protective film was formed on the AZ91 Mg alloy surface which resulted in faster initiation of arcs and formation of PEO film.

• Journal of Korea Foundry Society
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• v.37 no.1
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• pp.14-20
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• 2017

The microstructure, electrical conductivity and hardness variation of an AZ91-5wt%Sn Mg alloy sample during a solid-solution and aging heat treatment were instigated by optical microscopy, scanning electron microscopy, X-ray diffraction and by Rockwell hardness techniques in this study. The XRD result shows that the main phases in the as-casted alloy are ${\alpha}$-Mg, $Mg_{17}Al_{12}$ and $Mg_2Sn$. From the SEM images of the AZ91-5wt%Sn Mg alloy after the solution treatment, the $Mg_{17}Al_{12}$ phases in the alloy were found to have dissolved into the matrix with an increase in the holding time during the solution treatment, but $Mg_2Sn$ phases were clearly observable. The highest peak hardness of the AZ91-5wt%Sn Mg alloy is 82HRE at an aging temperature of $200^{\circ}C$.

• Transactions of Materials Processing
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• v.32 no.4
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• pp.173-179
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• 2023

In this study, the age-hardening behavior and tensile properties of a cast AZ91-0.3Ca-0.2Y (SEN9) alloy are investigated and compared with those of a commercial AZ91 alloy. Even after homogenization heat treatment, the SEN9 alloy contains numerous undissolved secondary phases, Al₈Mn₄Y, Al₂Y, and Al₂Ca, which results in a higher hardness value than the homogenized AZ91 alloy. Under aging condition at 200 ℃, both the AZ91 and SEN9 alloys exhibit the same peak-aging time of 8 h, but the peak hardness of the latter (86.8 Hv) is higher than that of the former (83.9 Hv). The precipitation behavior of Mg₁₇Al₁₂ phase during aging significantly differs in the two alloys. In the AZ91 alloy, the area fraction of Mg₁₇Al₁₂ discontinuous precipitates (DPs) increases up to ~50% as the aging time increases. In contrast, in the SEN9 alloy, the formation and growth of DPs during aging are substantially suppressed by the Ca- or Y-containing particles, which leads to the formation of only a small amount of DPs with an area fraction of ~4% after peak aging. Moreover, the size and interparticle spacing of Mg₁₇Al₁₂ precipitates of the peak-aged SEN9 alloy are smaller than those of the peak-aged AZ91 alloy. The homogenized AZ91 alloy exhibits a higher tensile strength than the homogenized SEN9 alloy due to the finer grains of the former. However, the peak-aged SEN9 alloy has a higher tensile elongation than the peak-aged AZ91 alloy due to the smaller amount of brittle DPs in the former.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2016.11a
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• pp.124.1-124.1
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• 2016

In this work, uniform thickness and good adhesion of electrodeposited copper layer were achieved on AZ91 Mg alloy in alkaline noncyanide copper solution containing pyrophosphate ion by employing appropriate zincate pretreatment. Without zincate pretreatment, the electrodeposited copper layer on AZ91 Mg alloy was porous and showed poor adhesion which was explained by small number of nucleation sites of copper due to rapid dissolution of the magnesium substrate in the pyrophosphate solution. The zincate pretreatment was found as one of the most important steps that can form a conducting layer to cover AZ91 surface which decreased the dissolution rate of AZ91 Mg alloy about 40 times in the copper pyrophosphate solution. Electrodeposited copper layer on AZ91 Mg alloy after an appropriate zincate pretreatment showed good adhesion and uniform thickness with bright surface appearance, independent of the deposition time but the surface roughness of the electrodeposited copper layer increased with increasing Cu deposition time.

• Journal of the Korean Society for Heat Treatment
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• v.24 no.4
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• pp.193-198
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• 2011

Effect of CaO addition on age hardening response has been studied by using optical microscopy, scanning electron microscopy and differential thermal analysis in AZ91 and CaO-containing ECO-AZ91 alloys. After solution treatment, the ${\beta}$($Mg_{17}Al_{12}$) phase formed during solidification mostly disappeared in the microstructure in the AZ91 alloy, whereas numerous ${\beta}$ precipitates containing Ca were still observed in the ECO-AZ91 alloy due to its enhanced thermal stability. The ECO-AZ91 alloy showed the delayed peak aging time and higher peak hardness compared with those of the AZ91 alloy. The activation energies for ${\beta}$ precipitation calculated by means of Kissinger method increased from 71.4 to 85.6 kJ/mole by the addition of CaO, which implies that CaO plays a role in reducing ${\beta}$ precipitation rate in the AZ91 alloy.

• Journal of the Korean Society for Heat Treatment
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• v.28 no.5
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• pp.229-238
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• 2015

The effects of Sn addition and solution treatment on corrosion behavior were studied in AZ91 magnesium casting alloy. The addition of 5%Sn contributed to the introduction of $Mg_2Sn$ phase, to the reduction in dendritic cell size and to the increase in the amount of secondary phases. After the solution treatment, trace amount of $Al_8Mn_5$ particles were observed in the ${\alpha}$-(Mg) matrix for the AZ91 alloy, while $Mg_2Sn$ phase with high thermal stability was additionally found in the AZ91-5%Sn alloy. Before the solution treatment, the AZ91-5%Sn alloy had better corrosion resistance than the Sn-free alloy, which is caused by the enhanced barrier effect of the (${\beta}+Mg_2Sn$) phases formed more continuously along the dendritic cell boundaries. It is interesting to note that after the solution treatment, the corrosion rate of both alloys became increased, but the Sn-added alloy showed higher corrosion rate than the Sn-free alloy. The microstructural examination on the corroded surfaces revealed that the remaining $Mg_2Sn$ particles in the solution-treated AZ91-5%Sn alloy play a role in accelerating corrosion by galvanic coupling with the ${\alpha}$-(Mg) matrix.

• Journal of Korea Foundry Society
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• v.28 no.1
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• pp.20-24
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• 2008

The characteristics, such as roughness, thickness, microhardness and corrosion resistance, of plasma electrolytic oxide coatings on AZ91D alloy were investigated under the processing condition of various coating times. The coatings on AZ91D alloy consisted of MgO, $MgAl_{2}O_{4}$ and $Mg_{2}SiO_{4}$ oxides. The surface roughness and thickness of coatings became larger with increasing the coating time. The microhardness in cross section of coatings was much higher than not only that in surface but that in the conventional anodic oxide coatings, which increased progressively as the coating time increased. After being immersed in 3.5%NaCl solution and methyl alcohol, the corrosion resistance of AZ91D alloy was markedly improved by plasma electrolytic oxidation coating treatment, and the AZ91D alloy coated for 50min revealed excellent corrosion resistance.