ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M2C eu. . .
Acta Metall, 2014 Vol. 50 (7): 769-776    DOI: 10.3724/SP.J.1037.2013.00621
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Dual-Cluster Characteristic and Composition Optimization of Finemet Soft Magnetic Nanocrystalline Alloys PDF (6752KB)
Yao-Xiang GENG Jianbing QIANG Chuang DONG
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      07 April 2017, Volume 53 Issue 4 Previous Issue   
    Orginal Article
    Flatness Defect Evolution of Cold-Rolled High Strength Steel Strip During Quenching Process
    Qingdong ZHANG,Xiao LIN,Qiang CAO,Xingfu LU,Boyang ZHANG,Shushan HU
    Acta Metall. 2017, 53 (4): 385-396.   DOI: 10.11900/0412.1961.2016.00261
    Abstract   HTML   PDF (2658KB)

    Quenching is a key process in cold-rolled high strength steel manufacturing for the improvement of the material strength and plasticity. The quenching, however, may bring initial flatness defects of the steel strips, which causes problems for subsequent production process. It is thus necessary to study the flatness defects evolution during the quenching process. Using the secondary development of ABAQUS subroutine UMAT, this work establishes a temperature-microstructure-stress coupling finite element modeling (FEM) model to simulate the quenching process of the high strength steel with initial buckling defects. Thermal simulation experiments are further conducted to verify the present FEM model. Then, the elastic-plastic deformation behavior of the steel plates and its effects on flatness buckling during the quenching process is investigated using the FEM model. As a consequence, the buckling defect evolution mechanism in heat treatment process is obtained for the cold-rolled high strength steel. The flatness change or the forming of new flatness defect is mainly caused by the longitudinal extension arising from temperature gradient and the sequential phase transformation different in width and transverse directions. Change rates of the wave height, width, and length are used to describe the flatness change degree, quantifying the influence of the tension and initial transverse temperature difference on flatness change. The simulation shows that the tension has a positive correlation with the improvement of initial bucking defects. The initial edge waves become more severe after quenching along with the appearance of the new quarter waves, when the initial temperature of strip center is higher than that of the edge. On the contrary, the initial central waves become more serious when the initial temperature of strip center is lower. Meanwhile, joint impact of the tension and the initial transverse temperature difference on wave height is revealed for the application of industrial practice. Furthermore, quenching experiments of the high strength steel plates with initial single edge wave buckling defects are carried out using the experiment system in lab. Different sides of the plates quench into the water tank to reproduce the sequence of the phase change. The simulation and experiments produce consistent results qualitatively. This work makes connections between technological parameters and flatness change during quenching process, which can provide support to industrial heat treatment of high strength steel.

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    Study on Evolution Mechanism of bcc Phase During Solution Treatment in 6%Si High Silicon Austenitic Stainless Steel
    Sihan CHEN,Tian LIANG,Long ZHANG,Yingche MA,Zhengjun LIU,Kui LIU
    Acta Metall. 2017, 53 (4): 397-405.   DOI: 10.11900/0412.1961.2016.00294
    Abstract   HTML   PDF (4495KB)

    After decades of development, high silicon austenitic stainless steels are widely concerned about due to their excellent corrosion resistance and good mechanical properties. Till now, 4%Si high silicon stainless steel has been widely used, but it is not doing well under the condition of high temperature and strong oxidizing medium. 6%Si high silicon austenitic stainless steels can resist in the strong oxidizing medium when the temperature is up to 100 ℃. But the increasing of Si may lead to the increasing of precipitation such as bcc phase, which may cause hot cracks during heat processing. As a result, obtaining a temperature range which is without precipitation is essential. The bcc phase evolution mechanism of 6%Si as-cast high silicon austenitic stainless steel under different solid solution treatment temperature was investigated by means of OM, SEM, XRD and TEM in this work. In order to study the precipitation and re-dissolution of bcc phase, the distribution of alloy elements, morphology and crystal structure of the bcc phase were analyzed under different solution treatments. Moreover, the heat-treated schedules were made based on the experimental results. The results indicated that the solid solution treatment temperatures had a great influence on the microstructure of 6%Si high silicon austenitic stainless steel. The precipitates existed in the as-cast structure were mainly bcc phase with a lattice constant of 0.8747 nm, rich in Mo, Si and Ni elements, and distributed in grain interior and grain boundary. The bcc phase redissolved during the solution when the temperature was between 1050~1200 ℃ for 2 h. The contents of Mo, Si and Ni increased with the rising solution temperature. Furthermore, the bcc phase re-precipitated when the test specimen was heat treated at 1250 ℃ for 2 h. The re-precipitated phase has the same composition with that in the as-cast structure. Thus the optimal solid solution treatment temperature of 6%Si high silicon austenitic stainless is 1100~1200 ℃ for 2 h.

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    Effect of Dimple Depth on Friction Properties of Aluminum Alloy Under Different Lubrication Conditions
    Mingming MA,Feng LIAN,Luping ZANG,Qiukuan XIANG,Huichen ZHANG
    Acta Metall. 2017, 53 (4): 406-414.   DOI: 10.11900/0412.1961.2016.00257
    Abstract   HTML   PDF (2760KB)

    Notable properties of aluminum alloy such as high strength-to-weight ratio, easy to be recycled and good welding properties lead to a wide range of applications in marine industry. However, in addition to many advantages, there are also a lot obvious shortcomings in tribological properties. Especially, the passive state film of aluminum alloy could be destroyed by the Cl- in seawater and harsh marine environment, which can erode into the defects and then aggravate the friction behavior, and limit the use of aluminum alloy in the field of marine engineering. In recent years, the super-hydrophobic surfaces are gaining a wide application prospects in the field of marine engineering due to their properties of drag reduction, anti-adhesion and anti-corrosion abilities. In order to improve the tribological properties of aluminum alloy, the amphiphobic aluminum alloy surface is constructed through building dimple of cone frustum texture with depths of 15 and 30 μm on the surface of 5083 warship aluminum alloy by laser processing and changing the surface wettability by coating the nano-SiO2 powders and low surface energy modification. And the tribological performance was examined by high speed reciprocating friction test machine (HSR-2M) in the water/seawater/oil lubrication respectively. The test results show that the surface with dimple depth of 30 μm has stronger amphiphobic performance and tribological performance than that of 15 μm. Compared with the simple texture surface, the amphiphobic surface with both texture and chemical composition can improve the tribological performance significantly. The friction coefficient and the wear loss of amphiphobic surface are minimal in oil. The friction coefficient of amphiphobic surface in seawater is smaller than that in water while the wear loss of the former is bigger. The simulation results showed that the carrying capacity of the lubricating film increases first and then decreases as the increment of the dimple depth. The carrying capacity of the lubricating film is the biggest when the depth of cone frustum was 75 μm. It can be concluded that the amphiphobic surface can significantly improve the tribological properties of aluminum alloy in different lubrications.

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    Microstructure and Mechanical Properties of TC16 Titanium Alloy by Room Temperature Roller Die Drawing
    Zhiqiang ZHANG,Limin DONG,Shaoxuan GUAN,Rui YANG
    Acta Metall. 2017, 53 (4): 415-422.   DOI: 10.11900/0412.1961.2016.00424
    Abstract   HTML   PDF (5376KB)

    Grain refinement is a challenging topic to improve mechanical properties of metallic materials, especially for titanium alloys which show great potential in aerospace and medical implants areas due to the low density and good corrosion resistance. However, severe plastic deformation (SPD) technologies which have been commonly used in laboratory in smaller scale are difficult to be realized in industrial. Considerable researches are therefore paying attention to the development of new technologies for improvement of grain refinement at relatively lower strains. In this work, the dual phase TC16 titanium alloy showing excellent room temperature ductility was investigated with emphasis on the feasibility of producing ultrafine grains by roller die drawing at room temperature. The techniques of XRD, SEM, TEM, Vickers hardness test and tensile test were employed to analyze the phase constitutes, microstructure evolutions and preliminary mechanical properties of the alloy deformed at different conditions. Results reveal that TC16 titanium alloy mainly consists of α and β phases after roller die drawing at room temperature, and a small quantity of stress-induced α" martensite can be additionally identified inside β grains. The grain sizes of α phase and β phase decrease with strain increasing, which result to enhanced tensile strength and Vickers hardness. Indeed, the fibrous morphology of both α phase and β phase with 0.3 μm in thickness and a high value of 365 HV in Vickers hardness were revealed at the applied true strain of 2.14. Ultra-fine grains evidenced by a near-ring SAED spots were therefore achieved in the present case.

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    Effect of Ru on the Solidification Microstructure of a Ni-Based Single Crystal Superalloy with High Cr Content
    Likui NING,Jian TONG,Enze LIU,Zheng TAN,Huisi JI,Zhi ZHENG
    Acta Metall. 2017, 53 (4): 423-432.   DOI: 10.11900/0412.1961.2016.00291
    Abstract   HTML   PDF (10721KB)

    Ni-based single crystal superalloys have been widely used in manufacturing the critical components of aero-engines, such as turbine blades and vanes. Improvements in phase stability on the addition of Ru are well known in the field of Ni-based superalloy development. Cr is beneficial to hot co-rrosion resistance of Ni-based superalloys. Generally, superalloys which used under easy corrosion conditions should contain high levels of Cr. Early researches about the influence of Ru on solidification microstructures in Ni-based single crystal alloys are mostly focused on low-Cr systerms (<6%). Since Cr has complex interactions with Ru, it is meanful to study the effects of Ru on solidification microstructures in high-Cr (>10%) Ni-based single crystal superalloy systems. The materials used in this work are Ni-based single crystal superalloy with high Cr content. Three superalloys by changing Ru addition (0, 1.5%, 3%, mass fraction) were designed. By observing the as-cast structure, the effect of Ru on the elements distribution and the precipitation characters of different phases in these alloys were studied. It is found that as the Ru content increases, the primary and secondary dendrite arm spacings decrease gradually; the volume fraction of (γ+γ′) eutectic increases firstly and then decreases; the γ′ size is reduced progressively. The addition of 3%Ru leads to the formation of β-NiAl phase, which contain a certain amount of Cr, Co and Ru except the basic elements Ni and Al. The typical "reverse partitioning" of other alloying elements is exhibited with the addition of Ru, while the formation of β-NiAl phase can reduce the "reverse partitioning" of other alloying elements. The addition of Ru could enhance the segregation of positive segregation elements Ta, Al and negative segregation element Re while reduce the segregation of positive segregation elements Mo and Cr.

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    Effect of Processing Conditions on Microstructure and Property of Multiphase V-Ti-Ni Alloys for Hydrogen Purifying
    Peng JIANG,Tongxin YUAN,Yandong YU
    Acta Metall. 2017, 53 (4): 433-439.   DOI: 10.11900/0412.1961.2016.00370
    Abstract   HTML   PDF (3235KB)

    The method of separation and purification of hydrogen from a mixed gas based on the permeation of hydrogen through a dense metallic membrane appears as an attractive mean of producing high purity hydrogen at a large scale. V-based alloy membranes with bcc structure are of great interest for hydrogen separation applications due to their low cost and high permeability. The hydrogen flux of the membrane is proportional to its hydrogen permeability and inversely proportional to its thickness. Therefore, V-based alloys should be fabricated in the form of large and thin membranes with the least possible thickness. Rolling process is presently regarded as the most promising route to a large scale fabrication of hydrogen permeable metal membranes. The refractory nature of the most prospective bcc alloys, and their potentially complex compositions, restrict the fabrication techniques which can be applied to form the alloy into a thin foil. In this work, thin sheets of V55Ti30Ni15 alloy were produced by a thermo-mechanical treatment consisting in successive heat treatment, rolling and annealing treatment, and the effect of microstructures resulting from different processing conditions on hydrogen permeability, have been investi gated for the multiphase V55Ti30Ni15 alloy. Precipitation of NiTi particles from V-matrix of V55Ti30Ni15 alloy during heat treatment, reduces the volume fraction of V-matrix contributing mainly to hydrogen permeation, which results in the decreasing of hydrogen permeability. The microstructure of the alloy after heat treatment evolved into a fibrous/lamellar microstructure during hot-rolling deformation, and a significant reduction in hydrogen permeability accompanied this deformation. Subsequent annealing decreased the dislocation density and increased hydrogen permeability. Dislocations have a great impact on hydrogen permeability due to their ability to trap diffusing hydrogen.

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    Fabrication of Novel Bimodal Titanium Alloy with High-Strength and Large-Ductility by Semi-Solid Sintering
    Limei KANG,Chao YANG,Yuanyuan LI
    Acta Metall. 2017, 53 (4): 440-446.   DOI: 10.11900/0412.1961.2016.00491
    Abstract   HTML   PDF (5247KB)

    According to Hall-Petch relationship, high strength of nano-grain and ultrafine-grain meta-llic materials are always accompanied by the cost of ductility because of the lack of work hardening induced by rare or absent dislocation or slip band. And various strategies including semi-solid processing accompanied by rapid solidification, recrystallization induced by plastic deformation and heat treatment, consolidation of blended powders with different grain sizes, and so on, have been developed to fabricate so-called bimodal/multimodal microstructures in the pursuit of high strength and no sacrificing ductility. As one of the most significant types of phase transformation in metallography, eutectic reaction was frequently utilized to tailor phase constitution and its microstructure due to high strength resulted from resultant lamellar eutectic structure. Generally, eutectic structure is more common in solidification and even traditional semi-solid processing for low melting point alloys (such as aluminum and magnesium alloys). In this work, a fundamentally novel approach of semi-solid sintering stemmed from the formation of liquid phase induced by eutectic transformation is introduced. Through regulation of the phase composition of eutectic transformation (or eutectic liquid content), novel bimodal Ti52.1Fe21.7Co8.2Nb12.2Al5.8 alloy with high-strength and large-ductility was successfully fabricated by semi-solid sintering of amorphous alloy powder with multi-phase eutectic system. The fabricated bimodal microstructure consists of fine nearly equiaxed fcc Ti2(Co, Fe) embedded into ultrafine lamellar eutectic matrix containing bcc β-Ti and bcc Ti(Fe, Co) lamellae, which is different from bimodal microstructures reported so far. The fabricated bimodal alloy exhibits ultra-high yield strength of 2050 MPa and large plastic strain of 19.7%, superior to those of bimodal titanium alloys reported so far. The method is conducive to process high-performance new structural metallic alloys in high melting point alloy systems.

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    Corrosion Behavior of Second Phase Alloys of β-(Nb, Zr) in Deionized Water at 360 ℃
    Bing CHEN,Changyuan GAO,Jiao HUANG,Yajing MAO,Meiyi YAO,Jinlong ZHANG,Bangxin ZHOU,Qiang LI
    Acta Metall. 2017, 53 (4): 447-454.   DOI: 10.11900/0412.1961.2016.00389
    Abstract   HTML   PDF (7407KB)

    Zirconium alloys are widely used as fuel cladding materials in water-cooled nuclear power reactors due to their low thermal neutron cross-section, good mechanical properties and corrosion resistance. The waterside corrosion is one of main factors that influence the service life of zirconium alloys. The corrosion of zirconium alloys occurs at the oxide/metal (O/M) interface, so the characteristics of the oxide film do have an impact on the oxidation process of zirconium alloys, which are affected by the oxidation behavior of second phase particles (SPPs) in zirconium alloys. E110 (Zr-1Nb, mass fraction, %) and M5 (Zr-1Nb-0.16O) alloys are Zr-Nb series alloys used in commercial presently. The major SPPs in Zr-Nb series alloys are bcc β-Nb. β-Nb SPPs in zirconium alloys are very fine, and it is inconvenient to analyze their oxidation processes due to the effect of α-Zr matrix. Therefore, based on the composition and crystal structure of β-Nb, two kinds of β-(Nb, Zr) SPPs alloys, 90Nb-10Zr and 50Nb-50Zr alloys were prepared by vacuum non-consumable arc smelting, and were corroded in autoclave with deionized water at 360 ℃ and 18.6 MPa for 11 d. XRD was used to analyze the crystal structure and phase composition of β-(Nb, Zr) specimens before and after oxidation. SEM and TEM equipped with EDS were used to investigate the microstructures of the external surface and the cross-section of oxide layers. Results show that the 90Nb-10Zr and 50Nb-50Zr alloys are oxidized into amorphous and crystalline oxides. The crystalline oxide formed on 90Nb-10Zr alloy is monoclinic Nb2O5, but the crystalline oxide formed on 50Nb-50Zr alloy is tetragonal (Zr, Nb)O2.

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    Life Prediction for Stress Corrosion Behavior of 316L Stainless Steel Elbow of Nuclear Power Plant
    Shu GUO,En-Hou HAN,Haitao WANG,Zhiming ZHANG,Jianqiu WANG
    Acta Metall. 2017, 53 (4): 455-464.   DOI: 10.11900/0412.1961.2016.00462
    Abstract   HTML   PDF (3893KB)

    Stress corrosion cracking (SCC) is one of the main ageing mechanism in light water reactor (LWR). 316L austenitic stainless steel was adopted in nuclear industry for its relatively high corrosion resistance. The SCC of austenitic stainless steel may occur as it is subjected to both the tensile stress and the caustic medium, with regard to maintaining the structural integrity of components in nuclear power plant, an accurate prediction and efficient assessment of the component lifetime is significant and necessary. The stress corrosion crack propagation behavior of the 316L stainless steel elbow of nuclear power plant was investigated through a numerical simulation method. Firstly a finite element (FE) model was created for the stainless steel thick-walled elbow (the outer diameter is 355.6 mm, the inner diameter is 275.6 mm), with a semi-elliptical shaped 3D defect introduced at the internal surface of the elbow as the geometry of the crack, which was consistent with a practical crack, the crack opening displacement (δi) was determined by the calculations through the Dugdale model; subsequently, according to the FE calculation results, establish the fitting formula of the stress intensity factor (K) varying with the crack depth (a) and additional stress (P), and the fitting formula of the stress corrosion crack propagation rate (da/dt) for elbows under two types of cold work deformation was deduced through the combination with the experimental data, the crack propagation time was then calculated using a iterative method for cracks which evolved from different initial crack depth values to certain crack depth values. The calculation results provided effective reference criterion for the nuclear power plant safety assesment. This investigation demonstrated that, when the cold deformation extent of the elbow part is relatively small ( with the hardness of 230~245 HV) and it is under the ideal condition (no initial additional stress), it takes around 57 a for the stress corrosion crack to penetrate the elbow, when the initial additional stress was elevated to 200 MPa, the elbow failure time was shrinked to 1/5 (no stress release), 2/7 (half-stress release) and 3/7 (total stress release) of the former; keep the same initial additional stress (200 MPa) and increase the cold work deformation extent (the hardness was increased from 230~245 HV to 275~300 HV), the elbow failure time was shortened to 2/5 (no stress release), 3/8 (half-stress release) and 1/3 (total stress release) for the elbow part with higher cold deformation extent compared to the part with lower cold deformation extent, thus it was observed that both the decrease of the extent of stress relaxation and the increase of the extent of cold work deformation contributed to the reduction of the residual life of the nuclear power plant 316L stainless steel elbow.

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    Effects of Cu on Microstructure and Mechanical Properties of AlN/TiN-Cu Nanocomposite Multilayers
    Jin LIU,Yuanxia LAO,Yuan WANG
    Acta Metall. 2017, 53 (4): 465-471.   DOI: 10.11900/0412.1961.2016.00259
    Abstract   HTML   PDF (6864KB)

    The nanocomposite multilayers, composed by typical nitride ceramic (AlN and TiN), have been developed for variety of application for its excellent properties such as structure stability and high hardness as well as low friction coefficient. By adding an appropriate amount of soft metal, the mechanical performance of the film can be significantly improved including intensity, tenacity and friction coefficient, but microstructure and hardness will be greatly influenced. In this work, AlN/TiN-Cu nanocomposite multilayers combining AlN with composite layer formed by adding soft phase metal Cu into hard TiN phase were prepared by multi-arc ion plating equipment. The microstructure and phase composition of the films were characterized by FESEM, HRTEM and XRD respectively. The hardness and the bond strength of the films were detected by Vickers hardness test and scratch method. The effects of Cu on microstructure and mechanical properties of AlN/TiN-Cu nanocomposite multilayers were investigated. The results show that the microstructure of the films was affected by the doping of Cu. The average grain size of the films reduced with the increase of Cu content. The hardness of films increased after the dropping of Cu. However, the critical loads of the films with different types have different changing trends. The critical load of the nanocomposite monolayers increased while that of the nanocomposite multilayers decreased.

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    Microstructure and Properties of TiC/Co-Based Alloyby Laser Cladding on the Surface of NodularGraphite Cast Iron
    Wenhui TONG,Zilong ZHAO,Xinyuan ZHANG,Jie WANG,Xuming GUO,Xinhua DUAN,Yu LIU
    Acta Metall. 2017, 53 (4): 472-478.   DOI: 10.11900/0412.1961.2016.00288
    Abstract   HTML   PDF (5514KB)

    Ductile cast iron, with excellent comprehensive mechanical properties, is widely used manufacturing traction wheel, crankshaft, cylinder liner, etc.. However, in the harsh environment, it often leads to failure due to the serious surface wear. At present, the repair methods for the damaged parts are mainly thermal spraying, deposit welding and other methods, but the properties and application effect of the repaired parts need to be improved. In order to significantly improve the surface properties of ductile cast iron, the 30%TiC/Co-based alloy cladding layer prepared by laser cladding is put forward on the surface of ductile cast iron in this work. The microstructure, composition, phase, hardness of the laser cladding layer are investigated and analyzed by OM, SEM, EDS, XRD, TEM and MHV2000 digital microhardness tester. The results show that the cladding layer can be integrated metallurgically with the nodular graphite cast iron matrix. The cladding layer consists of a surface layer of dendritic crystals and an internal cellular crystal. The primary phase of TiC from the melt is precipitated in situ during the solidification after laser heating. The amount of the primary TiC is gradually increased from the inner layer to the surface layer. Meanwhile, the undissolved TiC is dispersively distributed among the dendrites. The laser cladding layer is mainly composed of γ-Co, TiC, CoCx and a small amount of Cr7C3. The hardness maximum of the cladding layer is 1278.8 HV0.2, up to 5 times more than the hardness of the nodular graphite cast iron matrix.

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    Reactive Wetting of TC4 Titanium Alloy by Molten 6061 Al and 4043 Al Alloys
    Peng JIN,Ran SUI,Fuxiang LI,Weiyuan YU,Qiaoli LIN
    Acta Metall. 2017, 53 (4): 479-486.   DOI: 10.11900/0412.1961.2016.00289
    Abstract   HTML   PDF (5282KB)

    In order to improve the inoxidizability of TC4 alloy at high temperatures, hot dip aluminizing process is an efficient and economical way for industrial application. In this process, the wetting of TC4 alloy by molten Al alloy is the main factor which determined the coating quality. In this work, wetting of TC4 alloys by two industrial grade Al alloys (i.e., 6061 Al and 4043 Al alloys) were studied by using the modified sessile drop method at 600~700 ℃ under high vacuum. The results show that Al/Ti system is a typical reactive wetting, and the spreading dynamics can be described by reaction product control model, further the whole wetting behavior can be divided into two stages: the first stage for the nonlinear spreading and the second stage for the linear spreading. The small amount of alloying element Si in the Al alloys can cause significantly segregation at liquid/solid interface and formation of the Si-rich phase (Ti7Al5Si12). Ti7Al5Si12 decomposition is responsible for the nonlinear spreading, and Ti7Al5Si12 decomposition and Al3Ti formation are together responsible for the linear spreading. The formation of precursor film accompanies with the good final wettability.

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    Preparation and Activation Mechanism of Pd Colloid with High Concentration and Performance
    Shuoshuo QU,Qingsheng ZHU,Yadong GONG,Yuying YANG,Caifu LI,Shian GAO
    Acta Metall. 2017, 53 (4): 487-493.   DOI: 10.11900/0412.1961.2016.00388
    Abstract   HTML   PDF (5670KB)

    The non-conductive substrate is often metallized through electroless plating method. Prior to the electroless plating, the substrate surfaces need to be firstly Pd activation pre-treated. The traditional "two-step" activation process, i.e., sensitization-activation, has been gradually obsoleted because of poor controllability and uniformity. A "one-step" activation process using Pd colloid has been widely used in industry, especially for the microvia metallization treatment in printed circuit board (PCB) fabrication. The bottleneck problem of this technology is the preparation of the Pd colloid solution with high concentration and excellent catalytic activity. The aim of this work is to develop a preparation method of the Pd colloid with high concentration and high quality. Pd colloid was prepared by a continuous reduction reaction with minor content. By mean of this process, the Pd concentration of the prepared colloid can exceed 2%. The morphology, microstructure and composition of the Pd colloid were characterized by SEM, TEM, XRD and XPS, respectively. The activate ability of the Pd colloid was examined by electroless Cu and electrochemical test. It was found that the average diameter of the Pd particles was less than 4 nm. Even if the concentration of Pd was less than 25 mg/L, this Pd colloid still had good activation ability for electro less Cu. The result demonstrated that the shell structure of the Pd micelle played a key role for the activation ability. The shell of Pd micelle was consisted of Sn2+, Sn4+ and Cl-, and generally formed two structures, [PdSn2]Cl6 and [PdSn3]Cl8. For the structure of [PdSn3]Cl8, the failure of the hydrolysis could lead to the loss of activation. The preparation method in this work can effectively avoid the occurrence of [PdSn3]Cl8, which greatly improved the activation ability of the Pd colloid.

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    Research on the Development of Mathematical Model of the Whole Process of Electroslag Remelting and the Process Simulation
    Qing LI,Zixing WANG,Shuyuan XIE
    Acta Metall. 2017, 53 (4): 494-504.   DOI: 10.11900/0412.1961.2016.00386
    Abstract   HTML   PDF (5216KB)

    Electroslag remelting (ESR) is duly an important process for the production of high quality special steels and superalloys. Conventional ESR research has long been known as trial and error approach, which is excessively expensive and time-consuming, due to the complex process mechanism involving interactions of multiple physical fields, simultaneous phase transformations and chemical reactions. As the alternative way of study, ESR numerical simulation has been profoundly developed. Till now, systematically formulated model could demonstrate so many aspects of the process including electromagnetic field, fluid flow, heat and mass transfer, electrode melting, ingot solidification, slag/metal interface phenomenon, solidification structure parameters, ingot elements distribution, etc. There is a trend of multi-scale combined simulation, trying to bridge the gap between macro- and micro-scopes, thus could realize the control of solidified structure. Numerical modeling and simulation of ESR process have been widely accepted for its superiority of low cost, high speed, flexible adaptability and systematic results. Through combination of simulation and experiment, the ESR R&D process can be significantly promoted. Further, with the newly developed control technology supported by theoretical models, high precision and perfect quality control are expected to achieve. In this work, a mathematical model and the calculating code for the simulation of practical ESR process were developed based on multi-physics coupling calculation. The model considers many features of the process including the heat of the dropping liquid metal from the electrode, the naturally formed melt pool and the growing of the ingot, the cooling shrinkage of the solidified ingot away from the mould boundary, the changed slag skull thickness along the ingot growing direction, the matching between melt rate and input melting parameters, the specific boundary conditions, etc. The model covers physics of electromagnetic field, fluid flow, heat transfer, and melting and solidification during the remelting process, giving the characteristic information about distributions of temperature and liquid phase volume fraction, shape and size of melt pool and mushy zone, etc. highly concerned with the process control. Using history of temperature distributions and evolution, the model can compute various solidification parameters closely related to the ingot quality. The model realizes predictions for the unknown ESR process with steady state mode calculation and also analysis in transient mode of the whole ESR process from the melting start point of electrode to the end of cooling stage of ingot within the mould. Electromagnetic fields and steady and transient process simulations were carried out and discussed here for the practical IN718 alloy ESR process. The simulated melt pool profile and its depth size approximate to the experimental result of the ingot dissection analysis, and the predicted secondary dendrite spacing distribution coincides with the pictures of dendrite structure analysis fairly well. The model could be applied to the process analysis and optimization, and provide important technical support for the R&D of new product and technology.

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    Experimental and Finite Element Simulation of Milling Process for γ-TiAl Intermetallics
    Li ZHOU,Chao CUI,Qing JIA,Yingshi MA
    Acta Metall. 2017, 53 (4): 505-512.   DOI: 10.11900/0412.1961.2016.00256
    Abstract   HTML   PDF (8772KB)

    γ-TiAl intermetallics are attractive candidates for applications in aircraft turbine engines due to their low density and good mechanical properties at high temperature. However, the low room temperature ductility makes the machinability of these materials poorer compared to the conventional alloys. In this work, a meso-model of γ-TiAl intermetallic was developed using ABAQUS finite element software. The surface morphology and edge fracture mechanism of different material models were analyzed, and the effects of cutting parameters on the surface roughness and size of edge fracture were investigated. The results indicate that the cracks and pits occur between the lamellar and lamellar with different material properties. At the same time, due to the low ductility of γ-TiAl intermetallic, the negative shear angle begins to form at the exit of workpiece, then the edge fracture is formed. In addition, for both surface roughness and size of edge fracture, the experimental data are slightly higher than the simulated data obtained by the hexagonal lamellar model, and smaller than those obtained by the rectangular lamellar model. With the increasing of cutting depth, the surface roughness and the size of edge fracture increase gradually, on the contrary, the cutting speed has a small effect on them. Therefore, in order to obtain a fine surface quality during machining of γ-TiAl intermetallic, the cutting speed can be adopted as higher as possible, but not the cutting depth.

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