• 열처리공학회지
• /
• 제28권4호
• /
• pp.190-199
• /
• 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.

• 열처리공학회지
• /
• 제31권5호
• /
• pp.231-236
• /
• 2018

The purpose of this study was to investigate the microstructural characteristics and hardness distribution of AZ91 magnesium alloy furnace-cooled to room temperature after solution treatment, and to compare the results with those of as-cast condition. The as-cast alloy showed a partially divorced eutectic ${\beta}(Mg_{17}Al_{12})$ phase and discontinuous precipitates (DPs) with a lamellar morphology, while only DPs were observed in the furnace-cooled alloy. The DPs in the furnace-cooled AZ91 alloy had various apparent interlamellar spacings, which would be ascribed to the different transformation temperatures during the furnace cooling. The average hardness for the furnace-cooled alloy is similar to that for the as-cast alloy. It is interesting to note that the hardness values of the furnace-cooled alloy were distributed over a narrower range than those of the as-cast alloy. This is likely to be caused by the relatively more homogeneous microstructure of the furnace-cooled alloy in comparison with the ascast one.

• 한국주조공학회지
• /
• 제27권3호
• /
• pp.131-134
• /
• 2007

[ $SF_6$ ] gas that is used as a protective cover gas for Mg and Mg alloys during melting and casting has extremely high greenhouse effect. CaO added Mg alloys could maintain their original mechanical properties and original abilities such as fluidity and hot tearing susceptibility. The ignition temperature increased with increasing CaO addition under ambient atmosphere and nitrogen atmosphere. The minimum amount of $SF_6$ gas decreased by 0.13 wt%CaO added AZ91D Mg alloy in the sealed condition.

• 한국생산제조학회지
• /
• 제21권4호
• /
• pp.593-600
• /
• 2012

Thixomolding of Mg-alloy is a semi-solid injection molding process utilizing thixotropic phenomenon. Using this process, higher strength, thinner wall section and tighter tolerance without porosity are obtained. It has been applied for production of near-net-shape magnesium component. To design optimal thixomolding process of Mg-alloy part, molding conditions such as slurry temperature, mold temperature and injection time should be determined properly. Selection of these parameters has been dependent upon engineers' experience and intuitiveness. In this paper, to improve mechanical properties of the thixomolded product, optimal selection of process variables such as injection velocity, barrel temperature and die temperature in the process has been studied through microstructural analysis and Taguchi method. Performance of the process is verified through experiments.

• 한국재료학회지
• /
• 제29권10호
• /
• pp.609-616
• /
• 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.

• 소성∙가공
• /
• 제8권3호
• /
• pp.283-283
• /
• 1999

In the present study, the effect of microporosity on the tensile properties of as-cast AZ91D magnesium alloy was investigated through experimental observation and numerical prediction. The test specimens were fabricated by die-casting and gravity-casting. For gravity-casting, the inoculation and use of various metallic moulds were applied to obtain a wide range of microporosity. The deficiency of the interdendritic feeding of the liquid phase acted as d dominant mechanism on the formation of the micropores in the Mg-Al-alloys, rather than the evolution of hydrogen gas. Although tensile strength and elongation has a nonlinear and very intensive dependence upon microporosity, the yield strength appeared to have a linear relationship with microporosity. However, it was possible to quantitatively estimate the linear contribution of microporosity on the individual tensile property far a range of microporosity, which was below about B %. The numerical prediction suggests that the effect of microporosity on fractured strength and elongation decreased as the strain hardening exponent increased. Furthermore. the shape and distribution of micropores may play a more dominant role than local plastic deformation on the tensile behavior of AZ9lD alloy.

• 한국재료학회지
• /
• 제9권7호
• /
• pp.715-723
• /
• 1999

마그네슘은 20여년간 자동차 산업에서 휠소재로 사용되어 왔다. 마그네슘 휠은 무게가 알루미늄 휠보다 25% 가벼워서 주행성이 우수하다. 이 연구의 목적은 사형주조 및 영구금형주조 공정에 의한 AZ91D 합금제 췰을 개발하는 것이다 보호개스$(SF_6+CO_2)$를 사용하는 비플럭스 용해기술을 적용하여 용탕의 산화와 불순물의 유입을 배제하였다 마그네슘 용탕은 가압식 펌프시스템을 사용하여 가열된 파이프를 통하여 모울드에 자동으로 공급된다. 열처리 및 인고트의 조성에 따른 AZ91B 합금제 휠의 기계적 특성을 조사하였다.

• 열처리공학회지
• /
• 제31권5호
• /
• pp.220-230
• /
• 2018

The objective of this study is to investigate the effect of solution treatment on the microstructure and corrosion behavior of cast AZ91-4%RE magnesium alloy. In the as-cast state, microstructure of the AZ91-4%RE alloy was characterized by intermetallic ${\beta}(Mg_{17}Al_{12})$, $Al_{11}RE_3$ and $Al_2RE$ phase particles distributed in ${\alpha}-(Mg)$ matrix. After solution treatment, the ${\beta}$ particles with low melting point dissolved into the matrix, but Al-RE phases still remained due to their high thermal stabilities. It was found from the immersion and potentiodynamic polarization tests that corrosion rate of the AZ91-4%RE alloy increased after the solution treatment. On the contrary, EIS tests and EDS compositional analyses on the surface corrosion products indicated that the stability of the corrosion product was improved after the solution treatment. Examinations on the corroded microstructures for the ascast and solution-treated samples revealed that dissolution of the ${\beta}$ particles which play a beneficial role in suppressing corrosion propagation, would be responsible for the deterioration of corrosion resistance after the solution treatment. This result implies that the microstructural features such as amount, size and distribution of secondary phases that determine corrosion mechanism, are more influential on the corrosion rate in comparison with the stability of surface corrosion product.

• 한국주조공학회지
• /
• 제26권6호
• /
• pp.272-276
• /
• 2006

[ $CaCO_3$ ]를 발포제로 사용하여 균일한 기공구조를 가지는 AZ91과 AM60 마그네슘 합금의 발포 금속을 주조법을 통하석 제조하였다. 발포 금속의 제조가 가능한 이유와 발포 마그네슘 합금의 기공구조가 연구를 통하여 논의되었다. 마그네슘 합금의 용탕은 $CaCO_3$의 분해 거동에 영향을 미친다. 제조된 AZ91 마그네슘 합금의 발포 금속은 높은 기공률과 큰 기공의 크기를 가졌으며, 발포 금속들 중에서 상대적으로 쉽게 발포되는 것으로 판단된다.

• 대한용접접합학회:학술대회논문집
• /
• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
• /
• pp.516-521
• /
• 2002

A study was carried out to grow an understanding of the microstructural development of friction stir welds on an AZ91D magnesium alloy, and to evaluate the mechanical properties of the welds. AZ91D plates with the thickness of 4mm were used, and the microstructural development of the weld zone was investigated using optical and scanning electron microscopes. Square butt welding joint with good quality was obtained at the conditions of under 187mm/min of travel speed with 1100 to 1250 rpm of tool rotation speed. The microstructure within the weld region consisted of fine equiaxed grains with no evidence of the original dendritic structure. The hardness tests showed slightly increased harness in the weld region, and the minimum hardness measured is in that of the parent material. Tensile strength of the weld zone was remarkably improved due to very fine recrystallized structure. XRD pattern of weld zone revealed the removal of $\beta$ intermetallic compounds, $Mg_{17}$Al$_{12}$, which had been distributed in the base metal.l.