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

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    STUDY ON MICROSTRUCTURE STABILITY OF A Y2O3 DISPERSION STRENGTHENED LOW-ACTIVATION STEEL
    Xue HU, Lixin HUANG, Wei YAN, Wei WANG, Yiyin SHAN, Ke YANG
    Acta Metall Sin, 2015, 51 (6): 641-650.  DOI: 10.11900/0412.1961.2014.00547
    Abstract   HTML   PDF (9996KB) ( 832 )

    Oxide dispersion strengthened (ODS) steels are being developed as a promising structural material for next-generation nuclear energy systems, due to its excellent resistance to both irradiation damage and high-temperature creep. In this work, the mechanical alloying (MA) and hot isostatic pressing (HIP) technologies were used to prepare a ODS low-activation steel, based on the China low activation martensitic (CLAM) steel. SEM, XRD analysis and EPMA were used to examine the particle size, alloying element distribution and lattice distortion of the ball-milled powders. In order to obtain uniform powders, CLAM powders with 0.3%Y2O3 particles should be milled with hard steel balls of 6 mm in diameter for 50 h in Ar protective atmosphere, and the ball-to-powder weight ratio at 10∶1. The microstructure of well-prepared ODS-CLAM steel was stable till 1200 ℃ for 1 h, with grain size of 50~60 mm and martensitic lath width of 200 nm, meanwhile, the Y2O3 particles could still be found in the steel matrix.

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    HOT DEFORMATION AT ELEVATED TEMPERATURE AND RECRYSTALLIZATION BEHAVIOR OF A HIGH MANGANESE AUSTENITIC TWIP STEEL
    Xiaoyun YUAN, Liqing CHEN
    Acta Metall Sin, 2015, 51 (6): 651-658.  DOI: 10.11900/0412.1961.2014.00680
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    Stainless steel is widely used in both industrial production and daily-life due to its anti-corrosion behavior. In view of the shortage in Cr and Ni resources, there has been an increasing interest in developing low-cost stainless steels for several decades. Under the frame of replacing Ni and Cr with Mn and Al, respectively, a recent study indicates that Fe-Mn-Al-C austenitic twinning-induced plasticity (TWIP) steel possesses good comprehensive properties and excellent resistance to oxidation that make it potential in partially replacing conventional austenitic stainless steels. As a viable alternative to low-cost austenitic stainless steel, a new alloy system of high-manganese low-chromium nitrogen-containing TWIP steel was developed in this study. Considering its corrosion resistance, the alloy is not completely free of chromium, yet the Cr content is relatively low. Nitrogen is added, because it is a strong austenite stabilizer that can reduce the tendency to form ferrite and deformation-induced a'- and e-martensites, thereby reducing the amount of nickel required in austenitic stainless steel. Furthermore, nitrogen is beneficial for pitting corrosion resistance. In this study, hot deformation and recrystallization behaviors of this high manganese austenitic TWIP steel were investigated by single-pass compression tests on MMS-300 thermo-mechanical simulator at temperature ranging from 1223 K to 1423 K and strain rate ranging from 0.01 s-1 to 10 s-1. Microstructure evolution during dynamic recrystallization and the correlation of microstructure change to the stress-strain response were further analyzed by using TEM and SEM equipped with EBSD. The results show that the hot deformation behavior of this steel is more sensitive to deformation rate. Dynamic recrystallization occurs during hot deformation when deformation rate is lower than 0.1 s-1, while dynamic recovery takes place at deformation rate higher than 1 s-1. The hot deformation constitutive equation of the high manganese austenitic TWIP steel was established by regression analysis. There is a close correlation between microstructure evolution and stress-strain curve during dynamic recrystallization. With the increase of strain, the grain boundary migration leads to the nucleation of recrystallization. Sub-grain boundary was also formed with increasing the strain. Dislocations climbing or slipping on the adjacent sub-grain boundary lead to the grain boundary merging, and then, new austenitic grains formed.

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    EFFECTS OF FORGING AND HEAT TREATMENTS ON STRESS CORROSION BEHAVIOR OF 316LN STAINLESS STEEL IN HIGH TEMPERATURE CAUSTIC SOLUTION
    Yueling GUO, En-Hou HAN, Jianqiu WANG
    Acta Metall Sin, 2015, 51 (6): 659-667.  DOI: 10.11900/0412.1961.2014.00466
    Abstract   HTML   PDF (13182KB) ( 2353 )

    The reactor coolant piping in the third generation nuclear power plants of AP1000 is manufactured by integrally forging. Therefore, it is of vital importance to investigate the effects of forging and heat treatments on the stress corrosion cracking (SCC) resistance of 316LN stainless steel (316LNSS), which is the candidate material for the reactor coolant piping in AP1000 nuclear power plants. In this work, electron back scattering diffraction (EBSD) and microhardness measurements (HV) were used to characterize the microstructure and residual strain of the as-received 316LNSS, the forged and solution anneal treated 316LNSS and the forged and stress relief treated 316LNSS, respectively. The average grain size of the as-received 316LNSS was the largest, and the forged 316LNSS followed by solution anneal treatment and stress relief treatment showed no obvious differences on grain size. The as-received 316LNSS exhibited the highest residual strain followed by the forged and stress relief treated 316LNSS and then solution anneal treated 316LNSS. Besides, the residual strain in the as-received 316LNSS concentrated on grain boundaries, while the residual strain in the forged and stress relief treated 316LNSS was characterized by a band-like distribution. The U-bend specimens were utilized to investigate the SCC behavior of the 3 kinds of 316LNSS specimens in high temperature caustic solution. After SCC experiments, the crack morphologies of the 3 kinds of 316LNSS specimens were examined by SEM. Then the macro and micro fracture morphologies were examined by OM and SEM, respectively. Grain morphology, residual strain and grain boundary character distribution near the SCC crack tip of the forged and stress relief treated 316LNSS were investigated using EBSD. The results showed that the forged and solution anneal treated 316LNSS exhibited the lowest SCC sensibility, while the as-received the highest, with the most cracks and the highest growth rate. The as-received and the forged and solution anneal treated 316LNSS showed obvious intergranular cracking, while the forged and stress relief treated 316LNSS showed a mixed cracking mode. The larger average grain size and higher residual strain, especially concentrating on the grain boundaries, were considered to be responsible for the highest SCC sensibility of the as-received 316LNSS. Compared with the forged and stress relief treated 316LNSS, the higher content of coincidence site lattice boundary (CSLB) and lower residual strain contributed to the lower SCC sensibility of forged and solution anneal treated 316LNSS. The stress relief treatment failed to eliminate the band-like microstructure effectively, which disadvantaged the SCC resistance.

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    MICROSTRUCTURES AND PROPERTIES OF AZ91D MAGNESIUM ALLOY PRODUCED BY FORCED CONVECTION MIXING RHEO-DIECASTING PROCESS
    Mingfan QI, Yonglin KANG, Bing ZHOU, Guoming ZHU, Huanhuan ZHANG
    Acta Metall Sin, 2015, 51 (6): 668-676.  DOI: 10.11900/0412.1961.2014.00523
    Abstract   HTML   PDF (8320KB) ( 606 )

    Based on the forced convection mixing (FCM) principle, a self-developed FCM semisolid slurry preparation device was successfully developed. Taking AZ91D magnesium alloy tensile parts for example, the rheo-diecasting process that consists of slurry preparation, transportation and forming was achieved by combining with a diecasting machine. Microstructural characteristics of FCM rheo-diecasting parts in different processing parameters were investigated. Mechanical properties of AZ91D alloy parts in different processes were compared. Besides, the formation mechanism and solidification behavior of semisolid slurry were analyzed in FCM rheo-diecasting process. The results show that processing parameters have a great effect on the microstructures of parts, increasing rotation speed or decreasing barrel temperature appropriately is beneficial to optimizing the microstructure. The process not only can produce parts with fine, spherical and uniformly distributed primary a-Mg particles, but also is able to improve mechanical performance of parts significantly. Compared with traditional diecasting, the yield strength remains unchanged, but the ultimate strength and elongation are increased by 12.5% and 80.0%, respectively. Furthermore, compared with parts subjected to T4 and T6 heat treatment, the ultimate strength of the as-cast is the lowest, and the yield strength and elongation are between T4 and T6.

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    QUANTIFICATION STUDY ON DENDRITE FRAGMENTATION IN SOLIDIFICATION PROCESS OF ALLUMINUM ALLOYS
    Cheng BI, Zhipeng GUO, E LIOTTI, Shoumei XIONG, P S GRANT
    Acta Metall Sin, 2015, 51 (6): 677-684.  DOI: 10.11900/0412.1961.2014.00501
    Abstract   HTML   PDF (4038KB) ( 972 )

    Alloy solidification is an important process to control the mechanical properties of engineering products. During solidification, dendrite fragmentation occurs commonly as a key phenomenon to determine the microstructure and to obtain fine grain size. Recently, in situ synchrotron X-radiography technique was developed and applied to observe thermodynamic behaviors such as dendrite growth and fragmentation during solidification. External forces such as mechanical and electromagnetic stirring, and thermal shock were added into the solidification process to investigate their effects on the fragmentation behavior. However, most work conducted in literature focused on qualitative aspects e.g. morphology transition or solute distribution and quantitative investigation such as determining the specific relationship between fragmentation and solidification conditions was rather limited. In this work, the third generation synchrotron X-radiography technique was used to observe the solidification process of an Al-15%Cu (mass fraction) alloy. Experimental conditions including the strength of the pulsed electromagnetic fields, dendrite growth direction and the temperature gradients were varied and the subsequent effect on fragmentation was studied and quantified. A computer program was developed based on Matlab to perform the image processing and measurement. The fragmentation number according to experiments was counted and correlated to the mushy zone depth and local solid fraction. Results showed that a stronger electromagnetic field, growing against gravity and growing at higher velocity would significantly increase the fragmentation number. Furthermore, the fragmentation number followed a Gauss distribution as a function of either mushy zone depth or local solid fraction, and the maximum fragmentation occurred when the solid fraction was about 0.45. In the end, the extent to which caused those statistic results above were analyzed as the necking process due to the velocity field, the cumulative solid due to the gravity field and the liquid flow due to the electromagnetic field.

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    WHISKER MITIGATION FOR Sn-BASED Pb-FREE SOLDERS BY POSS ADDITION
    Yong ZUO, Limin MA, Sihan LIU, Yutian SHU, Fu GUO
    Acta Metall Sin, 2015, 51 (6): 685-692.  DOI: 10.11900/0412.1961.2014.00495
    Abstract   HTML   PDF (13942KB) ( 413 )

    Whisker growth in Pb-free solder joints is one of the most reliability concerns in electronic industry. Several theories and models were developed to elaborate whisker growth, and many attempts were made to find solutions to solve this issue. Micro alloying, such as introducing Cu, Bi, Ag etc. into solders, is considered to be one of effective method to mitigate whisker growth. However, when alloying with these metal elements, the structure of solders will be changed, therefore the reliability of solders needs to be reevaluated. The purpose of this research is to explore the possibility to mitigate whisker growth by reinforce strategy without destroying the structure of solders. In this study, a novel reinforcement, nano-structured cage-type polyhedral oligomeric silsesquioxane (POSS), was employed and expected to mitigate whisker growth. POSS was added into Sn, Sn3.0Ag0.5Cu and Sn58Bi solders respectively. Whisker growth behaviors of these modified solders under high humidity and temperature environment (85 ℃, 85% relative humidity) were analyzed and discussed. The results indicated that, the driving force of whisker growth was compressive stress generated by the volume expanding of tin oxides. The high humidity and temperature condition facilitated the formation of tin oxides and therefore provided continuous driving force for whisker growth. POSS addition could inhibit oxidation process of metal tin effectively, and reduce the amount of tin oxides formation, consequently whisker growth was mitigated. Among Sn, Sn3.0Ag0.5Cu and Sn58Bi solders, Sn solders was the easiest one to grow whiskers, while Sn58Bi was at the lowest risk to grow whis-kers.

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    MICROSTRUCTURE AND OXIDATION BEHAVIOR OF ZrSi2-NbSi2 MULTILAYER COATINGS ON AN Nb-Ti-Si-Cr BASE ULTRAHIGH TEMPERATURE ALLOY
    Xuan LI, Xiping GUO, Yanqiang QIAO
    Acta Metall Sin, 2015, 51 (6): 693-700.  DOI: 10.11900/0412.1961.2014.00498
    Abstract   HTML   PDF (3529KB) ( 988 )

    The rather poor oxidation resistance of Nb-Si base ultrahigh temperature alloys has seriously limited their practical applications at high temperatures. Niobium disilicide coatings, especially those modified by reactive elements (RE) such as Zr and Y, have been shown to possess good anti-oxidation properties at high temperatures due to the formation of a protective RE-containing SiO2 scale. Halide activated pack cementation (HAPC) is one of the most widely used techniques for preparing protective coatings on Nb-Si base ultrahigh temperature alloys, because compact coatings and metallurgical substrate/coating bonds can be obtained with using this technique. However, only a very limited amount of Zr and Y can be diffused into the coatings by a single co-deposition pack cementation process as a result of their large atomic radii and high melting points. To solve this problem, a method such as magnetron sputtering, which can be used for producing overlay coatings with different composition ratios of coating elements, seems to be feasible. In the present study, ZrSi2-NbSi2 multilayer coatings were prepared on an Nb-Ti-Si-Cr base ultrahigh temperature alloy by first magnetron sputtering 2 μm thick Zr-film, and then Si-Y co-deposition at respectively 1150, 1250 and 1350 ℃ by HAPC process. The structures and formation processes, as well as the static oxidation behavior of the coatings were investigated. The results show that the coating prepared at respectively 1150, 1250 and 1350 ℃ had similar structures, consisting of a ZrSi2 outer layer, a (Nb, X)Si2 (X=Ti, Cr, Zr and Hf) middle layer and a (Ti, Nb)5Si4 inner layer. However, the higher co-deposition temperature (1350 ℃) could cause cracks at the interfaces between the constituent layers of the coatings. The formation of the coating was dominated by inward diffusion of Si, accompanied with a certain degree of outward diffusion of Nb, Ti and Cr from the base alloy during the second Si-Y co-deposition process. The oxidation tests demonstrated that the ZrSi2-NbSi2 multilayer coating possessed excellent oxidation resistance. After oxidation, a dense scale consisting of SiO2, TiO2, ZrSiO4 and Cr2O3 formed on the coating, which can protect the base alloy from oxidation at least for 100 h at 1250 ℃ in air.

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    CORROSION MECHANISM DISCUSSION OF X65 STEEL IN NaCl SOLUTION SATURATED WITH SUPERCRITICAL CO2
    Liang WEI, Xiaolu PANG, Kewei GAO
    Acta Metall Sin, 2015, 51 (6): 701-712.  DOI: 10.11900/0412.1961.2014.00491
    Abstract   HTML   PDF (14106KB) ( 574 )

    In recent years, the corrosion problem of steels under supercritical CO2/H2O system in oil/gas production has got more and more attention. The temperature and pressure of some oil wells in China usually exceed 120 ℃ and 100 MPa, where CO2 is in supercritical state. To transportation easier and cost reduction, the oil/gas in pipelines is usually pressured to a high pressure, normally causes CO2 in supercritical state. The supercritical CO2 corrosion environment includes CO2-saturated water and H2O-saturated CO2 phases. Moreover, corrosive ions such as Cl- usually exists in CO2 corrosion environment, however the influence of Cl- on corrosion of carbon steel in supercritical CO2-saturated NaCl solution and NaCl solution-saturated supercritical CO2 are investigated limited. The corrosion behaviors and corrosion rates of X65 carbon steel exposed in supercritical CO2-saturated 3.5%NaCl solution, supercritical CO2-saturated deionized water and NaCl solution-saturated supercritical CO2 systems were investigated. SEM, EDS and XRD were used to analyze the morphology and characteristic of corrosion product scale on the steel surface. The results show that the addition of Cl- in supercritical CO2-satureated water significantly increased the corrosion rate of X65 steel, and modified the FeCO3 grain morphology. The average corrosion rate of X65 steel in NaCl solution-saturated supercritical CO2 was much lower than in supercritical CO2-saturated NaCl solution, but in supercritical CO2 phase X65 steel suffered serious localized corrosion. The corrosion process of X65 steel in supercritical CO2-saturated NaCl solution could be divided into three stages: the first was the active dissolution stage, the surface of X65 steel was corroded inhomogeneous due to the competitive adsorption between Cl- and H2CO3, HCO3-, and Fe3C as well as some lumpish matrix were residued on steel surface; the second was the initiation stage of FeCO3 precipitation, Cl- postponed the precipitation of FeCO3, the FeCO3 scale formed in this period was incomplete, and increased the area of cathodic reaction subsequently the corrosion rate; the last was the protective stage of FeCO3 corrosion scale, the corrosion product scale formed in this period was denser and provided better protectiveness to X65 steel matrix, however Cl- could pass this scale and reach the scale/matrix interface, resulted in the corrosion rate of X65 steel keeping at a higher value than in deionized water environment. The corrosion model of normal pipelines was developed to better understand the corrosion mechanism in supercritical CO2-saturated Cl--containing solution.

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    NUMERICAL ANALYSIS OF FLUID FLOW IN LASER+GMAW HYBRID WELDING
    Guoxiang XU, Weiwei ZHANG, Peng LIU, Baoshuai DU
    Acta Metall Sin, 2015, 51 (6): 713-723.  DOI: 10.11900/0412.1961.2014.00464
    Abstract   HTML   PDF (6362KB) ( 951 )

    Laser+gas metal arc welding(GMAW) hybrid welding fully combines the merits of both laser welding and GMAW, which can achieve high quality, high efficiency and comparatively low-cost welding of thin and thick plate, thus having great application prospect in manufacturing industry. However, compared with single heat source welding, hybrid welding involves more welding parameters and more complicated physical process, leading to difficult process optimization. When mismatching the process parameters, welding defects can still appear in high-speed welding, which affects the reliability of hybrid welding. Therefore, it is necessary to study the physical mechanism in hybrid welding deeply for suppressing welding defects and improving welding stability. In hybrid welding, fluid flow in weld pool has a critical influence on the weld formation. So, modeling and simulating the fluid flow is helpful for understanding the process mechanism completely. To date, however, there is only little study on velocity field in hybrid welding due to its complexity. In this work, with considering the effects of droplet and keyhole on weld pool, a three dimensional transient model is developed to numerically analyze fluid flow in weld pool of laser+GMAW hybrid welding based on FLUENT software. Arc heat input is modeled using an double-ellipsoid heat source; laser heat input is regarded as a hyperbolic curve-rotated heat source with changing peak power density, its distribution parameters being determined based on the simplified model for keyhole geometry and size. Droplet transfer is described as the process of high temperature liquid metal flowing into weld pool from the certain domain above the weld pool. Using the built model, the keyhole behavior, fluid flow and temperature distribution in laser+GMAW hybrid welding under different welding conditions are calculated. The features of velocity field in hybrid welding are analyzed and the effect of laser power on the weld pool dynamic behavior is discussed. The results show that, in the case of 1 m/min, weld bead hump is generated in single GMAW (laser power 0 W); when laser power is 500 W, bead hump disappears in welding, but there is no keyhole emerging in hybrid weld pool and fluid flow pattern is close to that in GMAW. When increasing laser power to 2000 W, keyhole is formed, which makes the fluid flow in weld pool more complicated. The predicted weld geometries and dimensions for varied laser powers are compared with the measured data, which are in good agreement, thereby indicating accuracy and applicability of the established model.

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    MOLECULAR DYNAMICS SIMULATION OF INITIAL RADIATION DAMAGE IN TUNGSTEN
    Man YAO, Wei CUI, Xudong WANG, Haixuan XU, S R PHILLPOT
    Acta Metall Sin, 2015, 51 (6): 724-732.  DOI: 10.11900/0412.1961.2014.00492
    Abstract   HTML   PDF (1847KB) ( 1187 )

    Tungsten is a candidate material for the first wall and divertor in a tokamak fusion reactor, in which it is required to withstand a high neutron irradiation. The defects created in cascade form the primary state of damage and their subsequent evolution gives rise to important changes in their microstructures and engineering properties. However, the evolution and aggregation of radiation-induced defects in atomic level can not be observed by experiments up till now. In this work, molecular dynamics (MD) method was used to explore the microstructural processes and atomic mechanism of the formation and evolution of defects in the initial stage of radiation in bcc-W. The range of primary knock-on atom (PKA) energies is 1.0~25.0 keV, and simulation temperature range from 100 to 900 K. The number and distribution of defects produced by displacement cascades have been studied; the influence of PKA direction and temperature on the number of steady Frenkel pairs has also been researched; defect clusters and the threshold energy have been simulated. The results showed that for morphology distribution of defects induced in the peak time of cascade, the more intensive the defects are, the less the steady Frenkel pairs numbers are, on the contrary, the more decentralized the defects are, the more the steady Frenkel pairs numbers are; the number of steady Frenkel pairs is insensitive to PKA direction, but has a trend to decline with the temperature elevating; the percentage of interstitial clusters is higher than that of the vacancy clusters, while vacancies tend to form larger clusters; the average threshold energy of W is less affected by temperature and has certain anisotropy. The results of this work can provide data for analyzing the behavior of W material under nuclear environment.

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    NUCLEATION MODEL AND DENDRITE GROWTH SIMULATION IN SOLIDIFICATON PROCESS OF Al-7Si-Mg ALLOY
    Rui CHEN, Qingyan XU, Qinfang WU, Huiting GUO, Baicheng LIU
    Acta Metall Sin, 2015, 51 (6): 733-744.  DOI: 10.11900/0412.1961.2014.00560
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    Due to the extensive applications in automotive and aerospace industries of Al-7Si-Mg casting alloys, its understanding of the dendrite microstructural formation is of great importance to control the desirable microstructure and thereby to modify the performance of castings. In this work, through analyzing the measured cooling curves in different cooling conditions of Al-7Si-0.36Mg ternary alloy during sand casting, a theoretical nucleation model correlated maximum nucleation undercooling with the nucleation density is proposed. Besides, a 2D and 3D cellular automaton (CA) model allowing for the quantitatively predicting dendrite growth of ternary alloys is presented. This model introduces a new tracking neighboring rule algorithm to eliminate the effect of mesh dependency on dendrite growth. The thermodynamic and kinetic data needed in the simulations is obtained by coupling with Pandat software package in combination with thermodynamic/kinetic/equilibrium phase diagram calculation databases. This model has also taken account the multi-component diffusion, constitutional undercooling, curvature undercooling, dendrite preferential growth angles as well as the effect of interactions between the alloying elements etc. This model is applied to quantitatively simulate the dendrite growth with various crystallographic orientations of Al-7Si-0.36Mg ternary alloy in 2D and 3D during polycrystalline solidification, and the predicted secondary dendrite arm spacing (SDAS) shows a reasonable agreement with the experimental results. The experimental observed complicated and diverse dendrite microstructure occurring in solidification process can be well reproduced by this 3D-CA model which has considered the effects of various preferred growth orientations, the interactions of adjacent dendrites as well as the influence of S/L interface anisotropies. The simulated results effectively demonstrated the abilities of this model in prediction of dendrite microstructure in ternary alloys.

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    DEVELOPMENT OF AN INVERSE HEAT TRANSFER MODEL BETWEEN MELT AND SHOT SLEEVE AND ITS APPLICATION IN HIGH PRESSURE DIE CASTING PROCESS
    Yongyou CAO, Shoumei XIONG, Zhipeng GUO
    Acta Metall Sin, 2015, 51 (6): 745-752.  DOI: 10.11900/0412.1961.2014.00604
    Abstract   HTML   PDF (3914KB) ( 489 )

    A 2D inverse heat transfer model between molten metal and shot sleeve was based on the nonlinear estimation method. Die casting experiments under both non-shot and shot conditions (via the plunger) were performed using an Al-9%Si-3%Cu alloy. Based on the temperature measurements from thermocouples embedded inside the shot sleeve, the temperature distribution of molten metal and interfacial heat transfer coefficient (IHTC) were successfully determined. Results show that the heat transfer behavior of non-shot condition was different from that in the shot condition, but the IHTC in the middle zone of shot sleeve decreased along the plunger moving direction. Besides, the surface temperature of shot sleeve was higher in both pouring zone and end zone while lower in the middle zone. In accordance to the movement of the plunger, the IHTC profile in the end zone exhibited double peaks.

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    FORMATION AND MODELING OF VERTICAL OUTSIDE WALL OF COMPOENTS INCLINING INWARD IN LASER SOLID FORMING
    Menghua SONG, Xin LIN, Fenggang LIU, Haiou YANG, Weidong HUANG
    Acta Metall Sin, 2015, 51 (6): 753-761.  DOI: 10.11900/0412.1961.2014.00631
    Abstract   HTML   PDF (5571KB) ( 849 )

    The vertical outside wall is prone to incline inward in laser solid forming, which will deteriorate the dimension precision and processing stability. For solving this problem, samples with different amount of deposited layers were prepared and the variation of boundary single-track clad shape and formation of vertical outside wall inclining inward were investigated. Based on the method of constructing cross-section profile of the single-track clad by height of powders accumulated in molten pool, an analytical model was developed to describe the evolution of vertical outside wall during multilayer superimposition. A series of vertical outside walls under different width/height ratios and critical defocus distance were constructed with this model to investigate their influence. Results indicate that the deposited single-track clad will influence the formation of single-track clad under depositing. Due to the arc-like cross-section profile of single-track clad, the molten pool will shrink inward, which leads the outer edge of boundary single-track clad to shrink inward then the vertical outside wall to incline inward. However, this incline will decrease with the increase of deposited height. For the initial single-track clad with fixed width, decreasing critical defocus distance can decrease the inward incline but increase the offset from the preset dimension. The width/height ratio almost has no effect on outside wall.

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    MAGNETOCALORIC EFFECT OF La0.9Ce0.1Fe11.44Si1.56Hy ALLOY AND POWER BONDED BLOCK
    Lijuan MU, Jiaohong HUANG, Cuilan LIU, Juan CHENG, Naikun SUN, Zengqi ZHAO
    Acta Metall Sin, 2015, 51 (6): 762-768.  DOI: 10.11900/0412.1961.2014.00473
    Abstract   HTML   PDF (844KB) ( 573 )

    Recently, La(Fe, Si)13-based magnetic refrigeration materials have been widely explored due to the advantages of giant magnetocaloric effect (MCE), tunable Curie temperature (TC), low cost of raw materials and excluding deleterious elements compared to other room-temperature giant MCE materials such as Gd5(Ge1-xSix)4, MnFeP0.45As0.55 and MnAs based compounds. In this work, in order to shift the TC to around room temperature and maintain the large MCE, the method of absorbing hydrogen was employed. La0.9Ce0.1Fe11.44Si1.56 hydride was prepared by saturated hydrogen absorption and then hydrogen contents and TC of the hydrides were controlled by subsequent dehydrogenation at different temperatures (Td=200~250 ℃ for 3 h). The phase structure and magnetocaloric effect were investigated. The results show that the samples possess the cubic NaZn13-type structure with a small amount of a-Fe as impurity phase. TC exhibits an approximately linear decrease with increasing the dehydrogenation temperature. The isothermal magnetic entropy change (ΔSm) of the hydrides decreases compared with the parent compound, which is mainly attributed to the fact that the field-induced itinerant-electron metamagnetic transition has been weakened upon hydrogen absorption. For the sample desorbed hydrogen at temperatures above 230 ℃, ΔSm is remarkably decreased and favorably the magnetic hysteresis loss has been reduced simultaneously. With further increasing the temperature to 250 ℃, ΔSm curve is broadened, weakening the characteristic of the first-order phase transition. Due to the intrinsic brittleness of hydrides, the preparation of a certain shape is of great importance for practical application. For a magnetic field change of 1.5 T, the maximum adiabatic temperature change (ΔTad) and ΔSm for the bonded block of fully hydrogen absorption La0.9Ce0.1Fe11.44Si1.56 hydride are about 2.7 K and 7.5 J/(kgK), respectively, which are larger than those of La(Fe, Co, Si)13 materials in the same magnetic field change range. In conclusion, the bonded La0.9Ce0.1Fe11.44Si1.56 hydrides with good MCE and different TC have been successfully prepared and will be very useful for the practical application of layered magnetic refrigerants at ambient temperature under low field change in magnetic refrigerators.

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