Microstructure, Mechanical Properties, and Corrosion Resistance of Ag–Cu Alloys with La2O3 Fabricated by Selective Laser Melting
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Ag and its alloys, when prepared by a selective laser melting (SLM) process, have a low density and poor overall performance due to their high reflectivity when the most commonly used laser (λ = 1060 nm) is used, and they have exorbitant thermal conductivity. These characteristics lead to the insufficient melting of the powders and severely limit the applications of additive manufactured silver alloys. To improve the absorption of the laser, as well as for better mechanical properties and higher resistance to sulfidation, Ag-Cu alloys with different La2O3 contents were prepared in this work using the SLM process, via the mechanical mixing of La2O3 nanoparticles with Ag-Cu alloy powders. A series of analyses and tests were conducted to study the effects of La2O3 in Ag-Cu alloys on their density, microstructure, mechanical properties, and corrosion resistance. The results revealed that the addition of La2O3 particles to Ag-Cu alloy powders improved the laser absorptivity and reduced defects during the SLM process, leading to a significant rise from 7.76 g/cm3 to 9.16 g/cm3 in the density of the Ag-Cu alloys. The phase composition of the Ag-Cu alloys prepared by SLM was Silver-3C. La2O3 addition had no influence on the phase composition, but refined the grains of the Ag-Cu alloys by inhibiting the growth of columnar grains during the SLM process. No remarkable preferred orientation existed in all the samples prepared with or without La2O3. An upwards trend was achieved in the hardness of the Ag-Cu alloy by increasing the contents of La2O3 from 0 to 1.2%, and the average hardness was enhanced significantly, from 0.97 GPa to 2.88 GPa when the alloy contained 1.2% La2O3 due to the reduced pore defects and the refined grains resulting from the effects of the La2O3. EIS and PD tests of the samples in 1% Na2S solution proved that La2O3 addition improved the corrosion resistance of the Ag-Cu alloys practically and efficaciously. The samples containing La2O3 exhibited higher impedance values and lower corrosion current densities.Keywords:
Selective Laser Melting
Sulfidation
Isothermal process
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Abstract By alloy melting, microstructure analysis and micro-hardness test, the effects of Sm addition on the microstructure of Mg-12Gd-2Y-0.5Zr alloy with solution treatment and aging treatment were investigated. The results showed that the microstructure of the tested alloy consisted of α-Mg matrix, Mg 41 Sm 5 , Mg 5 Gdm, Mg 24 Y 5 and β’ phases, and Sm improved the micro-hardness value of α-Mg matrix attributed to its better solid solution strengthening effect in α-Mg. The micro-hardness of the alloy with Sm increases obviously from HV121.4 to HV134.3 when compared with that of Mg-12Gd-2Y-0.5Zr alloy.
Matrix (chemical analysis)
Vickers hardness test
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Cu–10% P–6% Sn alloy rods with columnar crystal structure were synthesized in this study utilizing Cu–10% P–6% Sn alloy as the starting material. Continuous directional solidification was used to produce Cu–10% P–6% Sn (mass percentage) alloy rods with a diameter of 8 mm, and the heat treatment procedure was arried out under the condition of cast alloy. Optical microscopy and field emission scanning microscopy were then used to investigate the microstructure of the cast alloy. The impact of heat treatment on microstructure evolution was also investigated. The microstructure of a Cu–10% P–6% Sn alloy produced by continuous directed solidification had a Cu-rich dendritic structure, and the second phase structure was linear or spherical, according to the findings.
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Selective Laser Melting
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Abstract The effect of Sr addition on the microstructure of a melt-spun Al-12% Si alloy has been investigated using X-ray diffraction and transmission electron microscopy. The Sr addition has no effect on the phase constitution but has a marked effect on the microstructure of the melt-spun alloy. The distribution, morphology and orientation of Si in the melt-spun alloy with the Sr addition are different from those in the alloy without Sr. Under rapid solidification conditions, cooling rates play a dominant role in the microstructure formation of the Al-12% Si alloy.
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The present article describes a possible mechanism behind the microstructure development in aluminum (Al) alloy (AlSi10Mg: Al–10wt%Si–0.3wt%Mg) parts additively manufactured by selective laser melting (SLM) combined with powder bed system (one of the most conventional additive manufacturing processes for metals). It introduces the thermodynamic calculation applied to the studied alloy to understand the microstructure development during the SLM process and then elaborates microstructural features of SLM-fabricated Al alloy parts characterized by electron microscopy and electron back-scattered diffraction analyses. The aforementioned results are utilized to present the formation process of microstructure through the rapid solidification during the SLM process. An attempt was made to quantify the cooling rate and molten pool dimensions using the preliminary two-dimensional finite element model. The analyzed thermal gradient and cooling rates are presented as well.
Selective Laser Melting
Temperature Gradient
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The corrosion behavior of Fe Ce alloy containing approximately 15 wt% cerium has been studied at 600～800℃ in H 2 H 2S atmospheres with a sulfur pressure of 10 3 Pa. The results showed that the alloy was sulfidized more slowly than pure Fe, but more rapidly than pure cerium.The corrosion rate of the alloy generally increased with temperature.The complex scales formed during the sulfidation of the alloy included the outer layer of FeS, the inner layer of FeS and Ce 2S 3 , and the thin region of internal sulfidation of Ce. In the internal sulfidation region, the alloy was not depleted in Ce. Fe 15Ce alloy didn't form a protective Ce 2S 3 scale because of the limited solubility of Ce in the base metal and the presence of intermetallic compounds in the alloy.
Sulfidation
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This chapter presents a brief theoretical consideration of the sulfidation behavior of metals and alloys, and discusses the basic phenomena involved in the processes. It provides an overall picture of the sulfidation process concerning the kinetics of sulfidation of metals, the sulfidation behavior of alloys, and the defect structure of metal sulphides. Two types of reaction kinetics, such as linear and parabolic, have been observed during the sulfidation of metals under isothermal exposure conditions that lead to a formation of sulfide scales. The chapter emphasizes that the physicochemical properties of the sulfide scale formed on a metal entirely govern the sulfidation process. When comparing the available data relating to the defect structures of metal sulfides, on the one hand, and of metal oxides, on the other, it becomes apparent that more fundamental studies have been attempted with oxides than with sulfides. A brief attention is given to the experimental methods used to provide these data.
Sulfidation
Isothermal process
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Quasi in Situ Sequential Sulfidation of CoMo/Al2O3 Studied Using High-Resolution Electron Microscopy
Using a specially developed, quasi in situ vacuum-sample-transfer system, the stepwise sequential sulfidation of CoMo/γ-Al2O3 was studied with transmission electron microscopy without exposing the catalyst to air. In a first step, the sulfidation was performed at 400 °C at either 1 or 10 bar pressure. In a second step, the sulfidation was performed at 600 °C and 1 bar. As the second sulfidation step was performed on the sample on the transmission electron microscopy grid, the same areas of the samples that had been studied for the first sulfidation step could be studied once more, providing detailed data on the changes in the sulfided phase. The slab length and degree of stacking have been studied in detail from the TEM/HREM images. Sulfidation at higher pressure or higher temperature results in a higher average slab size. Detailed study of the same areas of the catalysts before and after the second, more severe, sulfidation step shows that although the dispersion of the sulfided phase decreases, the total amount visible in the TEM images remains unchanged. This indicates that practically all the sulfide present is actually imaged on the micrographs. Additionally, detailed HRTEM study shows increased crystallinity of the slabs after the second sulfidation step. XPS analysis shows that after sulfidation at 400 °C and 1 bar, the degree of sulfidation is 95%, whereas after sulfidation at 600 °C and 1 bar, sulfidation is complete. The catalytic performance for thiophene hydrodesulfurization is much higher after sulfidation at 10 bar and 400 °C as compared to 1 bar and 400 °C, and decreases after the second sulfidation step (1 bar 600 °C). The latter effect is more pronounced for the sample sulfided at 10 bar in the first step, indicating that a distorted yet fully sulfided structure is the most active phase for thiophene HDS.
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The sulfidation rate constants of various Fe-Mo binary alloys in S2 vapor with different sulfidation parameters were collected to summarize the correlation between the sulfidation rate and alloy composition, sulfur pressure and sulfldation temperature. The octivation energies indicate that the sulfidation process of the alloys is exclusively controlled, at a given temperature, by one alloying component i.e. either Mo or Fe. The sulfidation rate declines exponentially with the increasing of Mo content in alloy. And the influence of sulfur pressure is relatively slight.
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Binary alloy
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