• 열처리공학회지
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• 제34권6호
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• pp.287-293
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• 2021

This study was intended to investigate the influence of Al content on hardness and microstructural characteristics of discontinuous precipitates (DPs) formed by isothermal aging in Mg-8.7%Al and Mg-10%Al alloys. In order to obtain large amount of DPs in the microstructure, the alloy specimens were solution-treated at 688K for 24 h followed by water quenching, and then aged at 418K for 48h. The Mg-Al alloy with higher Al content was characterized by higher volume fraction of DPs at the same aging condition, lower interlamellar spacing of the DPs, thinner β phase layer and higher β phase content in the DPs. This is closely related to the higher velocity of discontinuous precipitation process resulting from the higher Al supersaturation in the α-(Mg) matrix. The Mg-10%Al alloy showed higher hardness of the DPs and greater difference in hardness between as-cast state and DPs than the Mg-8.7%Al alloy.

• 한국기계가공학회지
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• 제20권12호
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• pp.16-23
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• 2021

Pin-to-disc wear testing experiments were conducted to investigate the wear characteristics of commercial oil (5W-40) with nano-diamond particles. The upper specimen was a SUJ-2 high-carbon chromium steel ball with a diameter of 4 mm, and the lower specimen was made of the Al-6061 alloy. The applied load was 5 N, and the sliding speed was 0.25 m/s. The wear tests were conducted at a sliding distance of 500 m. The friction coefficients and wear rates of the Al-6061 specimens were tested using commercial oil with different nano-diamond concentrations ranging from 0 to 0.02 wt.%. The addition of nano-diamond particles to commercial oil reduced both the wear rate and coefficient of friction of the Al-6061 alloy. The use of nano-diamond particles as a solid additive in oil lubricants was found to improve the tribological behavior of the Al-6061 alloy. For the Al-6061 alloy, the optimal concentration was found to be 0.005 wt.% in view of the friction coefficient and wear rate. Further investigation is needed to determine the optimal concentration of nano-diamond particles for various loadings, sliding speeds, oil temperatures, and sliding distances.

• Archives of Metallurgy and Materials
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• 제66권3호
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• pp.699-702
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• 2021

Effect of Cu addition on oxide growth of Al-7 mass%Mg alloy at high temperature was investigated. As-cast microstructures of Al-7 mass%Mg and Al-7 mass%Mg-1 mass%Cu alloys showed α-Al dendrites and area of secondary particles. The 1 mass%Cu addition into Al-7 mass%Mg alloy formed Mg₃₂(Al, Cu)₄₉ ternary phase with β-Al₃Mg₂. The total fraction of two Mg-containing phases in Cu-added alloy was higher than the β-Al₃Mg₂ fraction in Cu-free alloy. From measured weight gains depending on time at 500℃ under an air atmosphere, it was shown that all samples exhibited significant weight gains depending on time. Al-7mass%Mg-1mass%Cu alloy showed the relatively increased oxidation rate when compared with Cu-free alloy. All the oxidized cross-sections throughout the entire oxidation time showed coarse and dark areas regarded as oxides grown from the surface to inside, but bigger oxidized areas were formed in the Al-7mass%Mg-1mass%Cu alloy containing higher fraction of Mg-based phases in the as-cast microstructure. As a result of compositional analysis on the oxide clusters, it was found that the oxide clusters contained Mg-based oxides formed through internal oxidation during a long time exposure to oxidizing environments.

• 한국재료학회지
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• 제31권11호
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• pp.642-648
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• 2021

The precipitation effect of Al-6%Si-0.4%Mg-0.9%Cu-(Ti) alloy (in wt.%) after various heat treatments was studied using a laser flash device (LFA) and differential scanning calorimetry (DSC). Solid solution treatment was performed at 535 ℃ for 6 h, followed by water cooling, and samples were artificially aged in air at 180 ℃ and 220 ℃ for 5 h. The titanium-free alloy Al-6%Si-0.4%Mg-0.9%Cu showed higher thermal diffusivity than did the Al-6%Si-0.4%Mg-0.9%Cu-0.2%Ti alloy over the entire temperature range. In the temperature ranges below 200 ℃ and above 300 ℃, the value of thermal diffusivity decreased with increasing temperature. As the sample temperature increased between 200 ℃ and 400 ℃, phase precipitation occurred. From the results of DSC analysis, the temperature dependence of the change in thermal diffusivity in the temperature range between 200 ℃ and 400 ℃ was strongly influenced by the precipitation of θ'-Al₂Cu, β'-Mg₂Si, and Si phases. The most important factor in the temperature dependence of thermal diffusivity was Si precipitation.

• Archives of Metallurgy and Materials
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• 제66권4호
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• pp.1013-1018
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• 2021

In this paper, as a purpose to apply the supersaturated solid-solutionized Al-9Mg alloy to the structural sheet parts of automotive, tensile tests were conducted under the various conditions and a constitutive equation was derived from the tensile test results. Al-9Mg alloy was produced using a special Mg master alloy containing Al₂Ca during the casting process and extruded into the sheet. In order to study the deformation behavior of Al-9Mg alloy in warm temperature forming environments, tensile tests were conducted under the temperature of 373 K-573 K and the strain rate of 0.001/s~0.1/s. In addition, by using the raw data obtained from tensile tests, a constitutive equation of the Al-9Mg alloy was derived for predicting the optimized condition of the hot stamping process. Al-9Mg alloy showed uncommon deformation behavior at the 373 K and 473 K temperature conditions. The calculated curves from the constitutive equation well-matched with the measured curves from the experiments particularly under the low temperature and high strain rate conditions.

• 한국재료학회지
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• 제31권3호
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• pp.156-161
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• 2021

Cr-Si based alloys are not only excellent in corrosion resistance at high temperatures, but also have good wear resistance due to the formation of Cr3Si phase, therefore they are promising as metallic coating materials. Aluminum is often added to Cr-Si alloys to improve the oxidation resistance through which stable alumina surface film is formed. On the other hand, due to the addition of aluminum, various Al-containing phases may be formed and may negatively affect the heat resistance of the Cr-Si-Al alloys, so detailed investigation is required. In this study, two Cr-Si-Al alloys (high-Si & high-Al) were prepared in the form of cast ingots through a vacuum arc melting process and the microstructural changes after high temperature heating process were investigated. In the case of the cast high-Si alloy, a considerable amount of Cr3Si phase was formed, and its hardness was significantly higher than that of the cast high-Al alloy. Also, Al-rich phases (with the high Al/Cr ratio) were not found much compared to the high-Al alloy. Meanwhile, it was observed that the amount of the Al-rich phases reduced by the annealing heat treatment for both alloys. In the case of the high temperature heating at 1,400 ℃, no significant microstructural change was observed in the high Si alloy, but a little more coarse and segregated AlCr phases were found in the high Al alloy compared to the cast state.

• Archives of Metallurgy and Materials
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• 제66권4호
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• pp.1023-1027
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• 2021

This study investigated the effect of flux type and amounts on recovery behavior of aluminum alloy during the melting process of Al can scrap. The heat treatment was conducted to remove the coating layer on the surface of can scrap at 500℃ for 30 min. The molten metal treatment of the scrap was performed at 750℃ in a high-frequency induction furnace with different flux types and amounts. It was observed that the optimum condition for recovery of Al alloy was to add about 3 wt.% flux with a salt and MgCl₂ mixing ratio of 70:30 during melting process. The mechanical properties of recovered Al alloy were about 254.8 MPa, which is similar to that of the virgin Al5083 alloy.

• 열처리공학회지
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• 제34권6호
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• pp.302-308
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• 2021

The aim of this study was to investigate the dependence of the hardness and thermal conductivity on the volume fraction of discontinuous precipitates (DPs) in the Mg-9.3%Al alloy with (α-(Mg)+DPs) dual phase structure. In order to obtain various DPs volume fractions, the alloy was solution-treated at 688 K for 24 h and then aged at 418 K for up to 144 h. The volume fraction of DPs increased from 0% to 63% with an increase in the aging time up to 72 h, over which, continuous precipitation was observed within the α-(Mg) grains. It is noticeable that the hardness and thermal conductivity of the alloy increased linearly with the volume fraction of DPs. The improved hardness and thermal conductivity with respect to volume fraction of DPs are closely associated with the higher hardness of the DPs with fine (α+β) lamellar structure and the lower Al concentration in the α phase layer of the DPs, respectively.

• 한국재료학회지
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• 제31권12호
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• pp.672-676
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• 2021

In this paper, the effect of Ni (0, 0.5 and 1.0 wt%) additions on the microstructure, mechanical properties and electrical conductivity of cast and extruded Al-MM-Sb alloy is studied using field emission scanning electron microscopy, and a universal tensile testing machine. Molten aluminum alloy is maintained at 750 ℃ and then poured into a mold at 200 ℃. Aluminum alloys are hot-extruded into a rod that is 12 mm in diameter with a reduction ratio of 39:1 at 550 ℃. The addition of Ni results in the formation of Al₁₁RE₃, AlSb and Al3Ni intermetallic compounds; the area fraction of these intermetallic compounds increases with increasing Ni contents. As the amount of Ni increases, the average grain sizes of the extruded Al alloy decrease to 1359, 536, and 153 ㎛, and the high-angle grain boundary fractions increase to 8, 20, and 34 %. As the Ni content increases from 0 to 1.0 wt%, the electrical conductivity is not significantly different, with values from 57.4 to 57.1 % IACS.

• 한국주조공학회지
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• 제41권1호
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• pp.3-10
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• 2021

알루미늄 합금과 주철의 복합주조 공정 중에는 주철로부터 철 성분이 용해되어 알루미늄 용탕에 혼입될 수 있으므로 다양한 Fe함유 금속간 화합물이 형성되며, 이로 인해 알루미늄 합금의 인장 특성이 크게 저하 될 수 있다. 반면 불순물로 첨가되는 Fe와 는 달리 Cu의 경우 알루미늄 합금의 기계적 물성을 향상시키기 위해 첨가되는 합금원소이다. 본 연구에서는 Fe와 Cu의 첨가로 인한 알루미늄 합금의 미세조직 및 인장특성의 변화를 조사하였다. 첨가된 Fe 함량이 1% 이상일 경우 조대한 Al₅FeSi 상과 같은 Fe 함유 화합물들이 다량 형성되어 인장 특성이 현저히 감소하는 것으로 나타났다. Cu가 첨가 된 알루미늄 합금의 경우 Al2Cu 상이 추가로 형성되었으며, 인장 강도가 뚜렷하게 향상되는 결과를 보였다.