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

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    HEAT-AFFECTED ZONE MICROSTRUCTURE EVOLU- TION AND ITS EFFECTS ON MECHANICAL PROPERTIES FOR LASER CLADDING FV520B STAINLESS STEEL
    Binshi XU,Jinxiang FANG,Shiyun DONG,Xiaoting LIU,Shixing YAN,Chaoqun SONG,Dan XIA
    Acta Metall Sin, 2016, 52 (1): 1-9.  DOI: 10.11900/0412.1961.2015.00489
    Abstract   HTML   PDF (11830KB) ( 363 )

    FV520B steel is a martensitic stainless steel developed by Firth-Vickers, with good corrosion resistance and weldability, high strength and toughness. It has been widely used in heavy load and corrosion-resistant components such as compressor impeller, valves, fasteners and pump shafts, which are easy to be damaged because of severe service-environments. The production cycle of those expensive components are long. If these components can be repaired and remanufactured, the accessional value of the products can be reserved. At the same time, it can save time, resources and funds, and reduce environmental pollutions. Laser cladding is an attractive green reconstruction technology, which is widely used for the remanufacturing of faulty metal parts. However, the heat-affected zone (HAZ) of remanufactured parts will experience cycles of heating and cooling during the cladding operation, its properties will change and may be extremely different than that of the unaffected area of the base material. Hence, the study of HAZ of FV520B steel is essential. The laser cladding on FV520B stainless steel was conducted to investigate the evolutions of microstructure and mechanical property of HAZ. The microstructure of the HAZ was characterized by means of OM and SEM, and hardness distribution was measured. Thermo-simulation was carried out to analyze the continuous cooling transformation (CCT) diagram, which provides useful instructions to investigate the microstructure evolution of HAZ. Simulated HAZ specimens and its mechanical properties were obtained by Gleeble 3500 thermal/mechanical simulator and MTS 810 material testing system. The results indicate that, HAZ can be divided into four zones: semi-melton zone, precipitation dissolved zone, completely austenization zone and partially austenization zone. The microstructures of the HAZ are martensite, the grain grows and second phase particles dissolve in the areas near the fusion zone. Meanwhile, its martensite transformation start temperature lower, and hardness higher than that of the unaffected area of the base material. The maximum temperature of thermal cycle dominates the evolution of microstructure and property of HAZ. With the decrease of the maximum temperature, the solid-state transformation temperature, elongation and impact energy increase, and the hardness decrease. Thermal cycle have a little influence to the tensile strength of HAZ under the processing parameters in this study. It can be speculated that the reduction in impact toughness and elongation of the HAZ can be controlled by decreasing the scanning speed of cladding.

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    MgO SECONDARY ELECTRON EMISSION FILM PREPARED BY RADIO-FREQUENCY REACTIVE SPUTERRING
    Bin WANG,Liangyin XIONG,Shi LIU
    Acta Metall Sin, 2016, 52 (1): 10-16.  DOI: 10.11900/0412.1961.2015.00189
    Abstract   HTML   PDF (3587KB) ( 238 )

    High, stable and durable secondary electron emission is an essential property for the application of dynodes of electron multipliers and photomultiplier tubes. The MgO film have been widely used as dynode materials for the applications owing to its good secondary electron emission properties. In this work, MgO and CoO doped MgO films, as secondary electron emission films, were prepared by radio-frequency reactive sputtering deposition on the stainless steel substrate, and also another MgO film at the surface of activated AgMg alloy was prepared. The effect of preparation processes on the secondary electron emission properties of the films was focused. It was found that the film thickness significantly affected the resistance to electron beam bombardment. With the increase of film thickness, the resistance to electron beam bombardment was significantly enhanced. Radio-frequency reactive sputtering deposition could control the film thickness by varying deposition time. The surface quality of MgO film is quite sensitive to the oxygen partial pressure of the deposition atmosphere. Higher oxygen partial pressure caused higher surface roughness, which was harmful to the secondary electron emission. After doping with CoO, the surface of MgO films were much flatter and smoother, resulting in the improvement of the secondary electron emission coefficient. The CoO doping also reduced of the sensitivity of film surface quality to the oxygen partial pressure. The secondary electron emission coefficient of CoO doped MgO film sharply decreased after heated at 550 ℃ for 1 h due to the surface quality degrading and the thermal decomposition induced loss of oxygen. Elevating the substrate temperature or oxygen partial pressure during deposition accounted for the presence of metallic Mg in film and the degrading of surface quality, which finally lead to lower secondary electron emission coefficient.

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    EMBRITTLEMENT OF σ PHASE IN STAINLESS STEEL FOR PRIMARY COOLANT PIPES OF NUCLEAR POWER PLANT
    Yongqiang WANG,Bin YANG,Na LI,Suhua LIN,Li SUN
    Acta Metall Sin, 2016, 52 (1): 17-24.  DOI: 10.11900/0412.1961.2015.00180
    Abstract   HTML   PDF (9278KB) ( 280 )

    Cast austenite stainless steel (CASS) possesses excellent mechanical properties, good workability and high resistance to localized corrosion in chloride environments due to the dual phase microstructure in which the island a-ferrite phase distributes in the g-austenite matrix. So they are widely used in the primary coolant pipes of nuclear power plants. However, undesirable s phase can precipitate in these steels when they are welded or heat treated and it severely decreases the toughness of stainless steels. Although some works have been done to investigate the effect of s phase on mechanical properties of CASS, the mechanism of embrittlement was still lacking. In this work, the effect of s phase on toughness of Z3CN20.09M CASS was investigated, and the embrittlement mechanism of s phase in CASS was discussed by using in situ tensile test, microhardness technology and fracture analysis. It was found that the impact energy of specimens aged at 750 ℃ decreased severely due to the presence of s phases. The (s+g2) structure formed by the eutectoid decomposition of a phase is very hard and its hardness is much higher than that of austenite. This makes the deformation between (s+g2) structure and austenite incoordinate in aged specimens. The precipitation of s phase brought more s/g2 and a/s/g2 high energy non-coherent boundaries. These boundaries hindered dislocation movements and brought stress concentrations. So cracks initiated at the s/g2 or a/s/g2 boundaries preferentially and propagated rapidly when the aged specimen bearded impact stress. The much potential cracking sites (s/g2 and a/s/g2 boundaries) in the (s+g2) structure is the main reason of embrittlement of aged Z3CN20.09M CASS with low toughness.

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    EFFECT OF FINAL ANNEALING ATMOSPHERE ON SECONDARY RECRYSTALLIZATION BEHAVIOR IN THIN GAUGE MEDIUM TEMPERATURE GRAIN ORIENTED SILICON STEEL
    Gongtao LIU,Ping YANG,Weimin MAO
    Acta Metall Sin, 2016, 52 (1): 25-32.  DOI: 10.11900/0412.1961.2015.00200
    Abstract   HTML   PDF (5298KB) ( 319 )

    The development trend of grain oriented silicon steel is reducing the slab reheating temperature and the thickness of final product. Medium temperature slab reheating grain oriented silicon steel bearing copper was characterized by omitting hot band annealing and larger range of secondary cold rolling reduction which was suitable for the preparation of thin gauge product. But less research were reported about thin gauge grain oriented silicon steel produced by medium temperature reheat technique. It was well known that the sharpness of secondary recrystallization Goss texture is deteriorated by the preparation of 0.18 mm thin gauge grain oriented silicon steel, poor secondary recrystallization and deviated Goss grains occurs by the influence of Goss seeds decreasing and inhibitor decrease. So the key point of producing thin gauge grain oriented silicon steel was controlling the precipitations ageing behavior. In order to improve the sharpness of Goss texture and the magnetic flux density after secondary recrystallization, secondary recrystallization behavior was controlled by annealing atmosphere in this work. The microstructure and texture of interrupted annealing specimens were measured by EBSD system. The results show that the magnetic flux density of 0.18 mm gauge specimen was 1.95 T after final annealing in 90%N2 atmosphere. Due to the coarsening behavior of inhibitors was more strongly influenced by atmosphere in thin gauge silicon steel, the primary recrystallization grain size was smaller and secondary recrystallization duration was longer by improving volume fraction of N2 during final annealing. In this condition, deviated Goss grains were inhibited while Goss grains have enough time for abnormal growth. As a result, sharp Goss texture and stable secondary recrystallization were guaranteed and high magnetic flux density of thin gauge final product was obtained.

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    RESEARCH ON RIDGING OF 17%Cr ULTRA PURE FERRITIC STAINLESS STEEL AFTER ENLONGATED ALONG VARIOUS DIRECTIONS
    Zhi FANG,Jingyuan LI,Yulai CHEN,Laizhu JIANG,Wei DU
    Acta Metall Sin, 2016, 52 (1): 33-40.  DOI: 10.11900/0412.1961.2015.00257
    Abstract   HTML   PDF (9316KB) ( 438 )

    Improved mechanical and chemistry properties of ferritic stainless steel (FSS), such as stamping formability and corrosion resistance, have been attained by decreasing the contents of C and N. Therefore, the ultra pure ferritic stainless steel with low content of C and N is a good candidate to replace the conventional Cr-Ni austenitic stainless steel for specific applications to save the higher price of Ni. As compared to conventional austenitic stainless steel, however, the ferritic stainless steel is susceptible to develop narrow ridges on the sheet surface during forming operations. The ridges, which can extend over the whole sheet length and have a depth of 20~50 μm, destroy the smooth appearance and surface shine of the product and thereby reduce the quality of the formed work pieces. This is one of the most serious problems of ferritic stainless steel sheets. Hence, the improvement for resistance of ridging is desired for further wide applications of ferritic stainless steel. In this work, laser scanning confocal microscopy, XRD and EBSD were used to observe the corelation between surface ridging and the evolution of grain orientation of 17%Cr ultra pure ferritic stainless steel after elongated along three different directions. Furthermore, the mechanism of tensile ridging of ferritic stainless steel was discussed. The results show that the ridging direction always parallels to the original rolling direction when the 17%Cr ultra pure ferritic stainless steel is enlongated along 0° (rolling direction, RD), 45° and 90° (transverse direction, TD) with the rolling directions, respectively. However, the height of ridging gradually decreases with the increase of the angle betweeen the rolling direction. Meanwhile, tensile texture of <110>//TA (tensile axis) gradually forms after enlongated along three different directions. The most important phenomenon is that the crystal plane almost does not rotate when enlongated along TD, while {112} and {221} orientations form when enlongated along RD. Thus it can be deduced that there is no relationship between ridging and <110>//TA orientation in 17%Cr ultra pure ferritic stainless steel. Moreover, the rotation of crystal direction in rolling plane has little effect on the ridging. However, the formation of ridging can be attributed to the rotation of crystal plane in rolling plane with cluster distribution.

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    MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Fe-C-Cu POWDER-FORGED CONNECTING ROD
    Linna BAI,Fuping LIU,Sui WANG,Feng JIANG,Jun SUN,Liangbin CHEN,WANG,Fengyuan
    Acta Metall Sin, 2016, 52 (1): 41-50.  DOI: 10.11900/0412.1961.2015.00486
    Abstract   HTML   PDF (5844KB) ( 337 )

    Powder-forged (P/F) connecting rods have been widely used due to their advantages of high strength, less machining, light weight, and consistency etc.. Currently, P/F connecting rods were only supplied by GKN in Britain and Metaldyne in US in commercial quantities. In this work, the microstructure and mechanical properties of the P/F Fe-C-Cu automobile engine connecting rods (H16) were designed and manufactured domestically, and the factors affecting the fatigue performance were systematically analyzed. The Measured results indicate that the density of the connecting rod is greater than 7.80 g/cm3. Microstructure observation showed that there were no oxide penetrations or network near or at rod surface and the surface decarburization layer is thinner than 70 mm. Anisotropy at different locations inside the P/F connecting rod was revealed. Furthermore, the bending of connecting rods was found to affect the fatigue performance significantly. The microstructure and the surface shot-peening condition had certain influence on the sites of fatigue crack initiation. Most importantly, the fatigue strength of H16 P/F connecting rod was found to be superior to that of the wrought steel-forged connecting rod (C70), and similar to that of P/F connecting rods designed and manufactured by entities.

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    EFFECT OF GRAIN SIZE AND TAYLOR FACTOR ON THE TRANSVERSE MECHANICAL PROPERTIES OF 7050 ALUMINIUM ALLOY EXTRUSION PROFILE AFTER OVER-AGING
    Wei GU,Jingyuan LI,Yide WANG
    Acta Metall Sin, 2016, 52 (1): 51-59.  DOI: 10.11900/0412.1961.2015.00163
    Abstract   HTML   PDF (5490KB) ( 465 )

    Generally, it is believed that inside the material the smaller grain size is, the higher yield strength is. In addition to this effect, grain refinement method also ensures that the toughness of the material is not reduced. However, it is found that the relationship between the grain size distribution and mechanical properties is contradiction with this law after the properties have been studied in the transverse direction of a large cross-section 7050 aluminum alloy profile. That is, the impact energy and yield strength in the center with a large grain size is higher than those at the edge with the smaller grain size in the thickest section of the profile. Besides that, during the establishment of the yield strength model in over-aging 7050 aluminum alloy, there are two models for the grain boundary strengthening which are Nes model and Hall-Petch model, so the choice from these model is found to affect the final results of the yield strength model. In order to study and understand the reasons for this phenomenon, the difference of mechanical properties distribution in the cross-section of 7050 aluminum extrusion profile has been investigated by impact test and tensile test at normal temperature, meanwhile, the microstructures have been analyzed by OM, EBSD and TEM. The results show that lots of the harder deformation textures, i.e., copper texture in the core of the profile lead to higher yield strength in the core with grain size of 12 mm than that in the edge with grain size of 6 mm. The Taylor factor could be calculated after the solution strengthening by alloying elements, grain boundary strengthening by the sub-grain and the yield stress of the alloy, at last, it reaches to 3.925 in the core, while that is just 2.257 in the edge. Compared with Nes model, the Hall-Petch model is much preferable to the calculation of grain boundary strengthening in yield stress of 7050 aluminum alloys after solid solution treatment. It is established that there is a linear relationship between impact energy and grain size of three over-aging specimens.

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    INVESTIGATION ON MECHANICAL AND STRESS CORROSION CRACKING PROPERTIES OF WEAKNESS ZONE IN FRICTION STIR WELDED 2219-T8 Al ALLOY
    Ju KANG,Jichao LI,Zhicao FENG,Guisheng ZOU,Guoqing WANG,Aiping WU
    Acta Metall Sin, 2016, 52 (1): 60-70.  DOI: 10.11900/0412.1961.2015.00201
    Abstract   HTML   PDF (8969KB) ( 244 )

    Al alloy 2219 (AA2219) is widely used in the aerospace industry, and friction stir welding (FSW) is an ideal method to join it. The ultimate tensile strength of an FSW AA2219-T8 joint can be as high as 344 MPa which is significantly higher than that welded by other methods such as gas tungsten arc welding. However, the thermo-mechanically affected zone (TMAZ) in the FSW joints of AA2219-T6/T8 is a weakness zone of mechanical property and is susceptible to stress corrosion cracking (SCC), but the reasons are not been well understood. In this work, the mechanical and electrochemical properties of different zones in AA2219-T8 joints obtained by the FSW method were studied. The welding thermal cycles during welding were measured using an array of type K thermocouples. During the tensile process of the joints, digital image correlation (DIC) technique and high speed video technique were employed to investigate the deformational behavior and fracture pathway of the TMAZ, respectively. A microcell method was used to study the micro-electrochemical characteristics of the joints with and without stress. The results showed that the minimum strength located at a position where the weighted strengthening effects of both thermal cycles and stir action were the weakest. The DIC results revealed that the deformation concentrated mainly in the TMAZ during the tensile tests. However, due to the different restraints from the nugget zone (NZ) led to a large strain in the root side than that in the crown side. This made the root side susceptible to cracks initiation. In situ tensile testing indicated that cracks occurred only in the TMAZ at 190 MPa, indicating that the protective surface films in the TMAZ were more prone to crack than those in other zones of the joint. This led the TMAZ to be the weakest zone to pitting corrosion in an aggressive environment. Once pits generate in the TMAZ, the local stress will concentrate near the tip of the pitting, resulting in failure.

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    SLIP SYSTEM DETERMINATION OF DISLOCATIONS IN a-Ti DURING IN SITU TEM TENSILE DEFORMATION
    Jing SHI,Zhenxi GUO,Manling SUI
    Acta Metall Sin, 2016, 52 (1): 71-77.  DOI: 10.11900/0412.1961.2015.00268
    Abstract   HTML   PDF (4483KB) ( 344 )

    Titanium and its alloys have been widely used in automotive industry and aerospace field due to their high mechanical strength and low density. It has been known that a-Ti has an hcp crystal structure and silp in hcp structure is limited because of only 3 independent slip systems. Therefore, twinning is active in hcp structure and the deformation behavior of hcp metals is very complex by the presence of both dislocation slip and twinning. In sub-micron sized a-Ti sample, deformation twins are difficult to produce and the deformation mechanism is mainly dislocation slip. However, it is hard to identify the activated dislocation slip system in a-Ti, as a few avaliable slip planes is corresponding to one slip direction. Usually there are two ways to identify the activated slip systems. One is to deduce the slip plane and the slip direction based on the loading direction and the crystal orientation. But this method is not accurate because of many possible groups of slip planes and slip directions in hcp structure. The other one is judging the Burgers vector of the dislocation under certain diffraction vectors based on Bragg's law by using TEM. It takes time and can only determine the slip direction of dislocation. Therefore, it is important to find an effective method to identify the active slip system more simply and accurately during deformation process. In this work, a nanometer sized tensile sample of a-Ti single crystal was fabricated by using focused ion beam (FIB) technique. In situ tensile test was carried out along [2110] of a-Ti sample by using a homemade bimetal stretching device in TEM. It has been found that three types of the dislocations, one prismatic dislocation and two pyramidal dislocations, were activated in order with strain increasing during tensile process.The Burgers vectors of dislocations were determined by two-beam diffraction contrast imaging in TEM. For hcp structure, one Burgers vector may have the characteristics of a variety of slip planes. By EBSD technique, the crystalline orientation and the loading direction in TEM were indexed accurately and Schmid factors for all the possible slip systems were calculated corresponding to each Burgers vector. Then, the activated slip systems during in situ TEM tensile process are determined by Burgers vector and Schmid factor. This work offers an effective method to identify the activated slip system during tensile process and get more understanding about the plastic deformation mechanism of a-Ti and hcp metals.

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    SECOND PHASE PARTICLES AND THEIR CORROSION BEHAVIOR OF Zr-0.72Sn-0.32Fe-0.15Cr-0.97Nb ALLOY
    Zhen WANG,Bangxin ZHOU,Boyang WANG,Jiao HUANG,Meiyi YAO,Jinlong ZHANG
    Acta Metall Sin, 2016, 52 (1): 78-84.  DOI: 10.11900/0412.1961.2015.00260
    Abstract   HTML   PDF (6000KB) ( 166 )

    Zr-0.72Sn-0.32Fe-0.15Cr alloy, which has much better corrosion resistance than that of Zr-4 alloy, was alloying by adding 1%Nb (mass fraction). In order to understand the effect of Nb on the corrosion resistance of Zr-0.72Sn-0.32Fe-0.15Cr alloy, the second phase particles (SPPs) and their oxidation behavior in this alloy were investigated using TEM, SEM and EDS techniques. Thin foil specimens for TEM observation were prepared for Zr-0.72Sn-0.32Fe-0.15Cr-0.97Nb alloy after recrystallization annealing. The corrosion tests for these thin foil specimens were conducted in an autoclave at 300 ℃, 8 MPa in deionized water for short time exposure. The results showed that a thin oxide layer in several hundred nanometers mainly consisted of the monoclinic ZrO2 formed on the surface, and SPPs embedded in the thin foil specimens at different corrosion levels were observed after corrosion test. The sizes of SPPs were mainly distributed between 30~150 nm and the maximum size was 230 nm. The size and crystal structure of SPPs have a relationship with Nb/Zr ratio (atomic ratio) of their composition. When Nb/Zr ratio was about zero, the size of SPPs was over 150 nm. When Nb/Zr ratio was in the range of 0.10~0.50, the sizes of SPPs were between 60~150 nm. When Nb/Zr ratio were in the range of 0.50~0.75, the sizes of SPPs were between 30~60 nm. When Nb/Zr ratio was over 0.75, the size of SPPs was smaller than 30 nm. With the increase of Nb/Zr ratio of their composition, three kinds crystal structure of Nb-containing SPPs, fcc (lattice constant a=0.701 nm), hcp (lattice constants a=0.508 nm, c=0.832 nm) and bcc (a=0.325 nm) structures were detected. SPPs without Nb containing have fcc structure (a=0.817 nm) while Fe/Cr ratio was over 5.00 and hcp structure (a=0.492 nm, c=0.788 nm) while Fe/Cr ratio was less than 3.00. The oxidation behavior of SPPs also had a relationship with the Nb/Zr ratio. The SPPs were easy to be oxidized to amorphous when Nb/Zr ratio was over 0.50. However, the SPPs with Nb/Zr ratio less than 0.50 were difficult to be oxidized.

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    EFFECT OF THE INTERMEDIATE HEAT TREATMENT PROCESSES ON THE OXIDATION CHARACTERIS- TICS OF Zr-1Nb-0.2Y ALLOY IN 420 ℃ AIR
    Changji LI,Liangyin XIONG,Shi LIU
    Acta Metall Sin, 2016, 52 (1): 85-92.  DOI: 10.11900/0412.1961.2015.00184
    Abstract   HTML   PDF (4254KB) ( 179 )

    Zr-based alloys have been used as cladding tubes in nuclear reactors for several decades due to their superior mechanical properties, good corrosion resistance and low neutron absorption cross-section. Zr alloys consist of hcp-structured a-Zr matrix and dispersed precipitate particles. These precipitate particles play a key role in improving the service performance of the alloy. In general, the manufacturing of Zr-based alloy tubes or sheets involves a series of deformation and annealing processes, which lead to a modification of the precipitate particles in size and distribution and an improvement of comprehensive properties of the alloys. In this work, the effect of intermediate heat treatment processes on precipitate particles and air oxidation characteristics of Zr-1Nb-0.2Y (mass fraction, %) alloy was studied. With increase of rolling and annealing times, the oxidation resistance of Zr-1Nb-0.2Y alloy was improved. The final product from manufacturing route II with intermediate annealing process of 640 ℃, 3 h+570 ℃, 3 h was proved to be most resistant to oxidation in 420 ℃ air. TEM images and EDS results showed that relevant parameters such as precipitate particle volume fraction, precipitate particle mean diameter, Nb+Y content (including mean content and total content) in precipitate particles were modified by intermediate annealing processes, which essentially influenced the oxidation characteristics of Zr-1Nb-0.2Y alloy. The smaller the mean size of precipitate particle and the higher the Nb+Y content in the precipitate particle are, the better the resistance to air oxidation of Zr-1Nb-0.2Y alloy.

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    ARC BEHAVIOR AND JOINTS PERFORMANCE OF CMT WELDING PROCESS IN HYPERBARIC ATMOSPHERE
    Jiqiang HUANG,Long XUE,Junfen HUANG,Yong ZOU,Huli NIU,Deyu TANG
    Acta Metall Sin, 2016, 52 (1): 93-99.  DOI: 10.11900/0412.1961.2015.00204
    Abstract   HTML   PDF (6426KB) ( 381 )

    Underwater hyperbaric dry welding method is one of the key technology for emergency repair of underwater pipeline leakage. Since the ambient pressure grows with water depth for application of the underwater dry hyperbaric welding method, the normal GMAW welding process tends to be unstable with the increase of the ambient pressure, which leads to the decline in the quality of welding. The cold metal transfer (CMT) welding method adopts a push-pull wire feeding mode and it has adaptive ability to control droplet transfer. In order to improve the welding quality under the hyperbaric environment, the experiments using the CMT welding method were conducted in atmospheric pressure (0.1 MPa) and 0.5 MPa environmental pressures respectively with a test system simulating the underwater hyperbaric environment. API X65 pipes were used as the base metal for welding experiments. A high-speed video camera was used to monitor the behavior of the welding arc. The welding processes at both ambient pressures were found to be stable. However, compared with the atmospheric environment, the CMT welding arc contracted at the ambient pressure of 0.5 MPa, and the droplet transfer frequency was reduced a little. Mechanical performance tests and microstructure analysis of the welds were carried out after welding. While welding in the hyperbaric environment, the upper bainite structure emerged in the microstructure of the seam and the heat-affected zone (HAZ) because of the enhanced environmental cooling effect. The tensile properties of the welds were not changed significantly. Although the low temperature impact toughness decreased, the test data were higher than the relevant limitations of standard. The experimental results show that the stability of the welding process is improved by applying the CMT welding method in the hyperbaric environment. It was verified that the CMT welding method can meet the requirements of underwater hyperbaric welding.

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    INFLUENCE OF ALLOY ELEMENT Cu ON KINETIC MECHANISMS OF DEUTERIUM ABSORPTION IN ZIRCONIUM
    Yun YANG,Xiping SONG
    Acta Metall Sin, 2016, 52 (1): 100-104.  DOI: 10.11900/0412.1961.2015.00256
    Abstract   HTML   PDF (3491KB) ( 182 )

    With increasing demand for energy, nuclear fusion has attracted more and more attention. In fusion process, the most promising of fusion reactions is the fusion of deuterium and tritium. Thus, deuterium absorption has become a key issue. At present, the extensively used materials for storage and supply of deuterium are uranium beds. However, upon hydrogenation, uranium is easily disintegrated into fine powder, which causes many undesirable problems. It has been found that zirconium alloys can take as much deuterium atoms as that of uranium alloys but with a lower density and price, thus becoming a candidate material for deuterium carrier. However, zirconium alloys usually occur to crack after deuterium absorption, which badly restricts their application as a deuterium carrier. In order to minimize the cracking, Cu is chosen as an alloying element, expecting to minimize the cracking. In this work, the kinetic mechanisms of deuterium absorption in Zr-xCu (x=0, 5%, 10%, mass fraction) alloys were investigated based on experiments and kinetic function calculations. The results show that with the increase of Cu content, the microstructure transforms from the primary single a-Zr phase of the pure Zr to the a-Zr and Zr2Cu duplex phases of the Zr-5%Cu and Zr-10%Cu alloys. Correspondingly, the equilibrium time of deuterium absorption increases significantly from 20 min for the pure Zr to 80 min for the Zr-5%Cu alloy and to 130 min for the Zr-10%Cu alloy. After deuterium absorption, the phase of pure Zr is e deuteride while the phases of Zr-5%Cu and Zr-10%Cu are e deuteride, Zr2Cu and Zr7Cu10. The kinetic mechanisms of deuterium absorption in these alloys are found to be controlled by a 2-dimensional diffusion mechanism in the pure Zr, and by a chemical reaction mechanism in the Zr-5%Cu and Zr-10%Cu alloys. The addition of Cu changes the kinetic mechanisms of the Zr-xCu alloys, resulting in slowing down deuterium absorption rate. It is attributed that during deuterium absorption of Zr-Cu alloys, Zr2Cu also absorbs deuterium and forms intermediate phase, such as Zr2CuHx. Then the intermediate phase will discompose into Zr7Cu10 and ε deuteride.

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    SYNTHESIS OF Sn3.5Ag0.5Cu NANOPARTICLE SOLDERS AND SOLDERING MECHANISM
    Zhi JIANG,Yanhong TIAN,Su DING
    Acta Metall Sin, 2016, 52 (1): 105-112.  DOI: 10.11900/0412.1961.2015.00263
    Abstract   HTML   PDF (7176KB) ( 399 )

    Solder has been long playing an important role in the assembly and interconnection of integrated circuit (IC) components on substrates, i.e., ceramic or organic printed circuit boards. The main function of solder is to provide electrical, thermal, and mechanical connections in electronic assemblies. Lead, a major component in Sn/Pb solder, has long been recognized as a health threat to human beings, which is the main reason for the requirement of environmental-friendly lead-free solder. A variety of lead-free solder alloys have been investigated as potential replacements for Sn/Pb solders, but there is still no perfect alternative. Three alloy families, Sn-Ag-Cu, Sn-Ag and Sn-Cu, seem to be of particular interest. However, concerns with this alloy family, including higher soldering temperature, poorer wettability due to their higher surface tension, and their compatibility with existing soldering technology and materials, have impeded their steps in completely replacing Sn/Pb solder. As the melting point can be dramatically decreased when the size of the particles is reduced to nanometer size, especially under 20 nm, and nanosolders have much better wettability at the same time. Furthermore, after heated and cooled, nanomaterials become bulk materials, which make them have the ability to endure a higher function temperature. Thus it is of great significance to conduct in-depth investigation on the synthesis of nanosolders and their soldering performance. In this work, Sn3.5Ag0.5Cu nanoparticles as a promising alternative of Sn/Pb solder was developed. The morphology, atomic structure, phase composition, and element composition of nanoparticles were characterized by SEM, TEM, XRD, and EDS, respectively. Size change of Sn3.5Ag0.5Cu nanoparticles under different sintering temperatures and sintering times was discussed. Microstructure of Cu/nanosolder/Cu sandwich structure under different soldering peak temperatures and soldering times was investigated. Shear strength and failure mode of the Cu/nanosolder/Cu sandwich structure under different pressure were also discussed. The results showed that the average diameter of nanoparticles was less than 10 nm with an agglomeration growth tendency. When sintering temperature was relatively low, the neck size increased steadily as temperature and time increased. In contrast, when sintering temperature was relatively high, the agglomeration mainly happened in the initial process and neck size changed little as the time increased. Thickness of intermetallics of Cu/nanosolder/Cu sandwich structure increased with the soldering temperature increased while the size and quantity of voids decreased. Shear strength of bonded sample increased with the increasing pressure, and got the maximum 14.2 MPa when the pressure reached 10 N.

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    THERMODYNAMIC ANALYSIS OF THE FORMATION OF Fe-Al-Zn INTERMETALLIC COMPOUNDS IN Al/GALVANIZED STEEL INTERFACE
    Manjiao CHEN,Jiankang HUANG,Cuicui HE,Yu SHI,Ding FAN
    Acta Metall Sin, 2016, 52 (1): 113-119.  DOI: 10.11900/0412.1961.2015.00203
    Abstract   HTML   PDF (3312KB) ( 257 )

    As for the intermetallic compound of Al and galvanized steel welding interface has greatly affect on welding joint, the researchers study the formation mechanism of intermetallic compound at the Al/galvanized steel interface. In order to research on Al/galvanized steel welding joint interface area, Gibbs free energy calculation model of Fe-Al-Zn intermetallic compounds formation was established based on the lattice model. Using the Gibbs free energy calculation model the formation of Fe2Al5Znx intermetallic phase was calculated and analyzed. At the Al/galvanized steel welding interface area, the calculation result was confirmed by experiments. Fe-Al-Zn ternary compound phases were formed at the welding joint interface of Al/galvanized steel. The Fe-Al-Zn compound phases, Fe2Al5Zn0.4, was the most stable. The calculation results agreed well with experiment results. It is showed that the calculation model was reasonable and the method was appropriate and feasible. The calculation model could reflect the Fe-Al-Zn intermetallic compound formation at Al/galvanized steel welding joint correctly. Fe2Al5Zn0.4 phase formation process could be experienced three stages based on the study of element distribution analysis at the interface center. It is also called the galvanized layer was dissolved in liquid Al, the Zn element was diffused, the Fe2Al5Zn0.4 was generated based on the reaction of Zn element and intermetallic compound Fe2Al5.

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    CRYSTAL PLASTICITY FINITE ELEMENT SIMULA- TION OF SLIP AND DEFORMATION IN ULTRA- THIN COPPER STRIP ROLLING
    Shoudong CHEN,Xianghua LIU,Lizhong LIU,Meng SONG
    Acta Metall Sin, 2016, 52 (1): 120-128.  DOI: 10.11900/0412.1961.2015.00264
    Abstract   HTML   PDF (4487KB) ( 308 )

    When the part size is scaled down to micro-scale, the material consists of only a few grains and the material properties and deformation behaviors are quite different from the conventional ones in macro-scale. In micro-scaled plastic deformation process such as ultra-thin strip rolling, material thickness effect is difficult to reveal and investigate using conventional material models. The distributions of the stress, strain and active slip systems, and the slip and deformation behavior in rolled pure ultra-thin copper strip with the same reduction were simulated by the crystal plasticity finite element method (CPFEM) and Voronoi polycrystalline model with respect to specimen dimension, grain size, grain orientation and its distribution to evaluate quantitatively the influence of grain orientation and structure on inhomogeneous deformation behavior of ultra-thin strip rolling on a mesoscale. A polycrystalline aggregate model is generated and a crystal plasticity based an implicit finite element model is developed for each grain and the specimen as a whole. The crystal plasticity model itself is rate dependent and accounts for local dissipative hardening effects and the original orientation of each grain was generated based on the orientation distribution function (ODF). Voronoi tessellation has been applied to describe the polycrystalline aggregation. The accuracy of the developed CPFEM model is verified by the fact that the simulated stress-strain curves agree well with the experimental results. The deformation behaviors, including inhomogeneous material flow, and slip system activity with the increase of thickness size for the constant size of grain, are studied. It is revealed that when the ultra-thin strips are composed of only a few grains through thickness direction, the grains with different size, shapes and orientations are unevenly distributed in the ultra-thin strip and each grain plays a significant role in micro-scale plastic deformation, slip system activity and leads to inhomogeneous deformation. The simulation result reveals that the deformation behavior in the polycrystalline aggregates is inhomogeneous not only in intracrystalline but also in intergranule regions by simulation of deformation behavior of pure ultra-thin copper strip rolling with 40% rolling reduction. This can be attributed to the different initial grain orientations and structures, the misorientation of neighboring grains, and the different properties of active slip systems and lattice rotation. Activation often initially occurs at free surface and near the boundary adjacent to grains, and then the slip develops in interior grain. The results from the proposed modeling methodologies provide a basic for understanding and further exploring of micro-scaled plastic deformation behavior in ultra-thin strip rolling process.

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