Showing papers in "Acta Metallurgica Sinica (english Letters) in 2006"

EFFECT OF TOOL WEAR ON MICROSTRUCTURE, MECHANICAL PROPERTIES AND ACOUSTIC EMISSION OF FRICTION STIR WELDED 6061 Al ALLOY

Abstract: Tool condition is one of the main concerns in friction stir welding (FSW), because the geometrical condition of the tool pin including size and shape is strongly connected to the microstructure and mechanical performance of the weld. Tool wear occurs during FSW, especially for welding metal matrix composites with large amounts of abrasive particles, and high melting point materials, which significantly expedite tool wear and deteriorate the mechanical performance of welds. Tools with different pin-wear levels are used to weld 6061 Al alloy, while acoustic emission (AE) sensing, metallographic sectioning, and tensile testing are employed to evaluate the weld quality in various tool wear conditions. Structural characterization shows that the tool wear interferes with the weld quality and accounts for the formation of voids in the nugget zone. Tensile test analysis of samples verifies that both the ultimate tensile strength and the yield strength are adversely affected by the formation of voids in the nugget due to the tool wear. The failure location during tensile test clearly depends on the state of the tool wear, which led to the analysis of the relationships between the structure of the nugget and tool wear. AE signatures recorded during welding reveal that the AE hits concentrate on the higher amplitudes with increasing tool wear. The results show that the AE sensing provides a potentially effective method for the on-line monitoring of tool wear.

Effect of nanocrystalline and twin boundaries on corrosion behavior of 316l stainless steel using smat

Abstract: By means of surface mechanical attrition treatment (SMAT), the grain size with a diameter of about 60nm formed at about 20μm depth and numerous mechanical twins at about 50μm depth from the treated surface were synthesized in 316L stainless steel because of the different distributions of strain and strain rate along depth orientation. For instance the maximum strain rate reached 103–104s−1 on the top surface. The relationship between the microstructure and the corrosion property was studied in 0.05M H2SO4+ 0.25M Na2SO4 aqueous solution, and the results show an extreme improvement of corrosion resistance owing to the appearance of twin boundaries and the obvious reduction in corrosion resistance attributed to the presence of nanocrystaline boundaries.

Study on temperature field induced in high frequency induction heating

Abstract: A mathematical model was established for the temperature field developed during high frequency induction heating (HFIH) by Maxwell's equations. It required solving the coupled equations of the electromagnetic and temperature fields. The numerical simulation was performed using FEMLAB. The comparison of the calculations using the proposed model with experimental results showed a very good correlation. The effects of the heating parameters in high frequency induction such as the distance between the plate and the coil, the applied current, the frequency, and the turns of the coil on the temperature profiles developed in the plate were also discussed using the established model.

Mechanism and kinetics of transformation of alumina inclusions by calcium treatment

Abstract: The basic mechanism and kinetics of the transformation process of alumina inclusions in steel was reported when calcium introduced into the steel by wire feeding or powder injection. To clarify the mechanisms model, experiments were performed by studying reactions between Al 2 O 3 and CaO in a laboratory furnace and by performing calcium treatments in an 8kg induction furnace for Al deoxidised melt. The phases formed during the reaction between Al 2 O 3 and CaO were examined by SEM-EDS (scanning electron microscope-energy dispersive spectrometer), and the reaction sequence of Al 2 O 3 ⇒CA 6 ⇒CA 2 ⇒CA⇒CA x(l) was discussed in term of the experimental observations. The kinetics of the reaction of calcium with alumina inclusions were simulated by immersing alumina plates in a Ca treated steel melt in the induction furnace. Results were compared with observations of real inclusion transformation. A kinetic model was proposed based on the results.

Comparison on Refinement of Iron Powder by Ball Milling Assisted by Different External Fields

Abstract: The cryogenic milling and milling in conjunction with dielectric barrier discharge plasma (DBDP) have been separately set up. The combined effect of low temperature and plasma on ball milling has been investigated by examining the refinement of particle size and grain size of iron powder using scanning electron microscopy, X-ray diffraction, and small angle X-ray scattering. It was found that the mean size of iron particles could reach 104nm only after 10 hours of ball milling in conjunction with DBDP, whereas a minimum average grain size of 8.4nm was obtained by cryomilling at −20°C; however, it is difficult to refine the particle size and grain size under the same milling condition in the absence of DBDP and cryogenic temperature.

Phosphorus Removal Using Steel Slag

Abstract: Steel slag is a byproduct produced in large amounts in the steel-making process. It is an important resource that can be effectively utilized. An experiment was described in which steel slag was tested as an adsorbent for the removal of phosphorus from waste water. Phosphorus removal depended on the amount of steel slag added, the pH value, the contact time, and the initial concentration. Under laboratory conditions when the added slag was 7.5g/L, the contact time 2h, and the pH value was equivalent to 6.5, over 99% of the phosphorus was removed; the experimental data on steel slag adsorption of phosphorus in the water fitted the Freundlich isotherm model. Steel slag was found to be very effective in adsorbing phosphorus.

Experimental investigation and thermodynamic calculation on phase precipitation of inconel 740

Abstract: A novel nickel-based superalloy INCONEL 740 was under development for application in ultra-supercritical superheater tubers above 750°C. The precipitation behaviors of η phase and γ′ particles of the alloy were investigated through experimental study and phase computation. Experimental results showed that η phase formed a Widmanstatten pattern structure following long-term exposure at elevated temperatures and that the coarsening of γ′ particle follows a cube rate law: r−3 ∝ t. Thermodynamic calculation results showed that Al and Ti had an important effect on the precipitation behavior of γ′ and η phases. Two suggested novel modified alloys, wherein the Al and Ti contents were modified, were designed and melted for the experimental study. The preliminary results indicated that the modified alloys exhibited higher structural stability following long-term exposure at 750°C till 5000h.

Rheocasting of a356 alloy by low superheat pouring with a shearing field

Abstract: A novel semi-solid slurry-making process was developed, which was a controlled nucleation and growth technique using a specially designed rotational barrel. Experimental study was undertaken to investigate the effects of pouring temperature and rotation speed of barrel on the microstructure of A356 alloy. Localized rapid cooling, combined with vigorous mixing during the initial stage of solidification enhanced wall nucleation and nuclei survival. High nuclei density combined with a much gradual cooling afterwards led to the formation of the near-ideal semi-solid shiny under a large processing window for the pouring temperature. Primary phase presented in mean equivalent diameter of 50–70 μm and shape factor of 0.812–0.847, and featured zero-entrapped eutectic.

In situ observation of growth behavior and morphology of delta-ferrite as function of solidification rate in an AISI304 stainless steel

Abstract: It was presented the in situ observation of growth behavior and morphology of delta-ferrite as a function of solidification rate in an AISI304 stainless steel. The specimens have been solidified and observed using confocal scanning laser microscopy (CSLM). The δ-phase always appears like cells on the sample surface when critical supercooling occurs, during which the L→δ transformation starts. The solid-liquid (S-L) interface is found to be finger shaped and has no faceted shape. γ phase appears among δ grains due to partitioning of Ni into the melt during solidification, when solidification rate is higher. The mergence of observed δ cells is possible for the steel sample cooled at 7.5°C/min. The formation of dendrites can be observed on the free surface of the steel sample cooled at 150°C/min. The size of solidified delta grains decreases from 120 to 20–80μm, and the volume fraction of solidified austenite increases with increase in solidification rate from 7.5 to 150°C/min. The relation between the tip radius of δ cell and its growth rate is deduced, and the results agree with the experimental values.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Mg-Li ALLOY WITH Ca ADDITION

Abstract: Mg-9wt%Li-2wt%Zn alloy was prepared and studied in this article. The addition of Ca to the alloy from 0.1 wt% to 1wt% can refine the α-Mg and the best effect of refinement occurs when Ca content is 0.4wt%—0.5wt%. The billets can be rolled to thin sheets at room temperature, from which it is obvious that the addition of Ca improves alloys' formality, while it decreases with the excess addition of Ca. The results also show that the ultimate tensile strength (UTS) and yield strength (YS) of the alloy with 1wt% Ca may rise by 28% and 25%, respectively, however, the elongation decreases. It is clarified that the adsorption of proper Ca on the grain boundaries refines α-Mg and improves the tensile properties, but the presence of excess Ca and stable Ca2Mg6Zn3 phase worsens the elongation.

Experimental research and numerical simulation of mold temperature field in continuous casting of steel

Abstract: Mold is the heart of the continuous casting machine. Heat transfer and solidification in a watercooled mold are the most important factors during the continuous casting of steel. For studying the temperature distribution of a mold wall, a simulated apparatus of mold was designed and experiments were performed by it. The measured results indicated that the mold wall temperature approaches the temperature of cooling-water. An equivalent thermal-conductivity coefficient was proposed and deduced on the basis of the conclusion of the experiments. This coefficient was applied to solve the heat transfer between the melt and cooling water, and to characterize the heat transfer capacity of the mold. By this equivalent thermal-conductivity coefficient, it is very easy and convenient to numerically simulate the solidification process of continuous casting. And the calculation results are in agreement with the experiments. The effects of casting speed and water flow rate on the mold temperature field were also discussed.

Effects of different functional group-containing organics on morphology-controlled synthesis of silver nanoparticles at room temperature

Abstract: Silver nanoparticles with average particles sizes ranging from 2 to 131nm were manipulatively synthesized starting from silver nitrate using different functional group-containing organic modifiers at room temperature. The effects of the organic modifiers on the morphology of the resulting silver nanoparticles were strongly dependent on the intrinsic properties of the functional groups and the reducibility of the reductant. Numerous ether bonds ( O ) present in polyethylene glycol and Tween-80 were beneficial to the formation of silver nanoparticles with particle sizes of several nanometers in a narrow size distribution in both weak and strong reducing environments. Cetyltrimethylammonium bromide induced the formation of nanosized silver triangle plates in a weak reducing environment. The crystal growth of the silver nanoparticles with particle sizes of more than 10nm was postulated through an adhesion process of small-sized particles followed by a subsequent coalescence process under the present reaction conditions.

STUDY ON THE TEXTURE OF A FRICTION STIR WELDED Mg-Al-Ca ALLOY

Abstract: Macro-texture of an Mg-Al-Ca alloy prepared by friction stir welding (FSW) was investigated through pole figure measurement and X-ray diffraction (XRD) pattern analysis. It was found that at the top and bottom surfaces of friction stir zone (FSZ), (0002) basal planes of magnesium tend to be arranged parallel to the plate surface. In the cross section of FSZ, no obvious texture had evolved and (0002) basal planes showed a random distribution.

Modeling of microstructural evolution in microalloyed steel during hot forging process

Abstract: The microstructural evolution of microalloyed steel during hot forging process was investigated using physical simulation experiments. The dynamic recrystallized fraction was described by modifying Avrami's equation, the parameters of which were determined by single hit compression tests. Double hit compression tests were performed to model the equation describing the static recrystallized fraction, and the obtained predicted values were in good agreement with the measured values. Austenitic grain growth was modeled as: D i n c 5 = D 0 5 + 1.6 × 10 32 t ⋅ exp ( - 716870 RT ) using isothermal tests. Furthermore, an equation describing the dynamic recrystallized grain size was given as D dyn = 3771·Z −0.2 . The models of microstructural evolution could be applied to the numerical simulation of hot forging.

Microstructure and mechanical property development in the simulated heat affected zone of v treated hsla steels

Abstract: The simulated heat affected zone (HAZ) of the high strength low alloy (HSLA) steels containing 0%, 0.047%, 0.097% and 0.151% vanadium, respectively, were studied with Gleeble-2000 thermomechanical simulator to determine the influence of vanadium addition on the mechanical properties of the HAZ. The HAZ simulation involved reheating the samples to 1350°C, and then cooling to ambient temperature at a cooling rate of 5°C/s ranging from 800 to 500°C(Δt 8/5 =60s). The mechanical properties including tensile strength and −20°C impact toughness were conducted. The microstructures of the base steel and the simulated HAZs were investigated using optical microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Based on the systematic examination, the present work confirmed that about 0.05% vanadium addition to low carbon low alloy steels resulted in expected balance of strength and toughness of the HAZ. And more than 0.10% levels addition led to detrimental toughness of the HAZ. SEM study showed that the simulated 0.097% and 0.151%V HAZs consisted of more coarse ferrite plates with greater and more M-A constituents along austenite grain and ferrite plate boundaries. The impact fracture surfaces of the simulated 0.097% and 0.151% V HAZs showed typically brittle mode with predominant cleavages. The size of the facet in the fracture surface increased with increasing vanadium level from 0.097% to 0.151%. As a result, the simulated 0.151%V HAZ has the lowest impact toughness of the four specimens.

Improving the surface property of tc4 alloy by laser nitriding and its mechanism

Abstract: The mixing technology of laser and heated nitrogen was applied to improve the surface hardness of titanium alloy (TC4). The samples were nitrided with laser power density of 6.5×10 5 W·cm −2 , the scanning speed various from 100 to 500mm·min −1 . The nitrogen gas was pre-heated to 300°C to accelerate the nitriding process. Some interested samples were tested with XRD method (X-ray diffraction) to analyze the composition of nitrides, and the surface hardness of HV was measured. The results show that TiN and Ti 2 N were formed on the surface of Ti alloy with proper nitriding parameters, but TiN is the main composition. The surface hardness increased by three times, which is from the original value of 269 to 794kg·mm −2 . The mechanism of the mixing technology is considered mainly of the activation of nitrogen by laser power and the pre-heated process which accelerated the nitriding process. The nitridation process can be considered as six steps given in detail. The result by analyzing the mechanism of improving the surface property of TiAl alloy shows the improvement of surface property due to two factors: the first reason is the result of laser annealing, and the second one is the formation of TiN.

Damping behavior of ultrafine-grained pure aluminum l2 and the damping mechanism

Abstract: Ultrafine-grained pure aluminum L2 with a mean grain size of 1.0μm was produced by equal channel angular pressing (ECAP) and annealing at 150°C for 2h. Damping behavior of the alloy was measured using a dynamic mechanical thermal analyzer. The alloy had an excellent damping capacity Q −1 with the ambient value being 9.8×10 −3 at 1.0Hz when the strain amplitude was 2.0×10 −5 . The damping behavior of the alloy showed a non-linear damping variation tendency, that is, with an increase in temperature and a decrease of frequency, the damping capacity of the alloy increased. The damping capacity increased with the strain amplitude when the strain amplitude was less than 4.6×10 −5 . When the strain amplitude was higher than 4.6×10 −5 , the damping capacity became a constant and independent of strain amplitude. The high damping capacity was attributed to dislocation unpinning and a drag of dislocation on pinning points.

A neural network for weld penetration control in gas tungsten arc welding

Abstract: Realizing of weld penetration control in gas tungsten arc welding requires establishment of a model describing the relationship between the front-side geometrical parameters of weld pool and the back-side weld width with sufficient accuracy A neural network model is developed to attain this aim Welding experiments are conducted to obtain the training data set (including 973 groups of geometrical parameters of the weld pool and back-side weld width) and the verifying data set (108 groups) Two data sets are used for training and verifying the neural network, respectively The testing results show that the model has sufficient accuracy and can meet the requirements of weld penetration control

Effect of Si on the microstructure and mechanical properties of the Al-4.5%Cu alloys

Abstract: The effect of Si on the microstructure and mechanical properties of binary Al-4.5%Cu alloy has been investigated. The results show that the addition of Si does not have any effect on alloy strength; however, it decreases the ductility of the as-cast Al-4.5%Cu alloy. It is obvious that after T4 heat treatment, the microstructure and mechanical properties of Al-4.5%Cu-Si alloys are improved, especially for the alloy with an Si addition of 2%.

Investigation of laser beam welding process of az61 magnesium-based alloy

Abstract: Laser welding process of AZ61 magnesium alloys is investigated using a special CO 2 laser experimental system. The effect of processing parameters including laser power, welding speed, and protection gas flow at the top and bottom is researched. The results show that an ideal weld bead can be formed by choosing the processing parameters properly. An optimized parameter range is obtained by a large number of experiments. Among them, laser power and welding speed are the two main parameters that determine the weld width and dimensions. The protect gas flow rate has a slight effect on the weld width, but it directly effects the surface color of the weld. The test results for typical welds indicate that the microhardness and tensile strength of the weld zone are better than that of the base metal. A fine-grained weld region has been observed and no obvious heat-affected zone is found. The weld zone mainly consists of small α-Mg phase, (α + Al 12 Mg 17 ). and other eutectic phases. The small grains and the eutectic phases in the joint are believed to play an important role in the increase of the strength of welds for AZ61 magnesium alloys.

PREPARATION AND CORROSION RESISTANCE OF NiP/TiO2 COMPOSITE FILM ON CARBON STEEL IN SULFURIC ACID SOLUTION

Abstract: A NiP/TiO 2 composite film on carbon steel was prepared by electroless plating and sol-gel composite process. An artificial neural network was applied to optimize the prepared condition of the composite film. Corrosion behavior of the NiP/TiO 2 composite film was investigated by polarization resistance measurement, anode polarization, ESEM (environmental scanning electron microscopy) and EIS (electrochemical impedance spectroscopy) measurements. Results showed that the NiP/TiO 2 composite film has a good corrosion resistance in 0.5mol/L H 2 SO 4 solution. The element valence of the composite film was characterized by XPS (X-ray photoelectron spectroscopy) spectrum, and an anticorrosion mechanism of the composite film was discussed.

In situ processing of Al2O3 whiskers reinforced Ti-Al intermetallic composites

Abstract: In situ Al2O3 whiskers reinforced Ti-Al intermetallic composites were fabricated at ∼1200°C by reaction sintering of cold-consolidated fillets consisting mainly of Ti, Al, and different additives. The phases and microstructures of the sintered composites were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The process of synthesis was investigated using differential thermal analysis (DTA). The effects of processing parameters and additives on the microstructures of the composites and the development of whisker were examined. It is found that the morphology of the whisker is strongly influenced by the additives, the exothermal reaction process, and the processing parameters.

Microstructure and mechanical properties of novel 718 superalloy

Abstract: Recently, a novel 718 superalloy with remarkable structural stability at 680°C has been designed and fabricated by CISRI (Central Iron and Steel Research Institute) etc. Phase identification of novel 718 alloy under the above-mentioned heat-treatment condition was performed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Results show that the novel 718 alloy has outstanding structural stability at 680°C. The novel 718 alloy possess excellent structural stability and good mechanical properties, which is attributed to γ-phase strengthening and also to the specific sandwich structure of the γ′ + γ″ strengthening phase.

Analysis of three-dimensional upsetting process by the rigid-plastic reproducing kernel particle method

Abstract: A meshless approach, called the rigid-plastic reproducing kernel particle method (RKPM), is presented for three-dimensional (3D) bulk metal forming simulation. The approach is a combination of RKPM with the flow theory of 3D rigid-plastic mechanics. For the treatments of essential boundary conditions and incompressibility constraint, the boundary singular kernel method and the modified penalty method are utilized, respectively. The arc-tangential friction model is employed to treat the contact conditions. The compression of rectangular blocks, a typical 3D upsetting operation, is analyzed for different friction conditions and the numerical results are compared with those obtained using commercial rigid-plastic FEM (finite element method) software Deform3D. As results show, when handling 3D plastic deformations, the proposed approach eliminates the need of expensive meshing and remeshing procedures which are unavoidable in conventional FEM and can provide results that are in good agreement with finite element predictions.

Coupled simulation of 3d electro-magneto-flow field in hall-heroult cells using finite element method

Abstract: Two full 3D steady mathematical models are developed by finite element method (FEM) to calculate coupled physics fields: the electro-magnetic model is built and solved first and so is the fluid motion model with the acquired electromagnetic force as source body forces in Navier-Stokes equations. Effects caused by the ferromagnetic shell, busbar system around, and open boundary problem as well as inside induced current were considered in terms of the magnetic field. Furthermore, a new modeling method is found to set up solid models and then mesh them entirely with so-called structuralized grids, namely hex-mesh. Examples of 75kA prebaked cell with two kinds of busbar arrangements are presented. Results agree with those disclosed in the literature and confirm that the coupled simulation is valid. It is also concluded that the usage of these models facilitates the consistent analysis of the electric field to magnetic field and then flow motion to the greater extent, local distributions of current density and magnetic flux density are very much dependent on the cell structure, the steel shell is a shield to reduce the magnetic field and flow pattern is two dimensional in the main body of the metal pad.

Effect of chloride on the atmospheric corrosion of simulated artifact iron in no3− bearing pollutant environment

Abstract: The effect of chloride in nitrogen-bearing pollutant on the atmospheric corrosion of cast iron was investigated by using periodic wet-dry test, electrochemical experiment and surface tension test. Scanning electron microscopy (SEM) with energy disperse atomic X-ray (EDAX) was used to identify the corrosion processes and products. The results of the weight loss measurement showed that the whole corrosion kinetics can be approximately described by: ΔW=AtB. With the addition of NaCl, B increases. The result presented that Cl− accelerated the corrosion rate obviously during the whole corrosion process. The initial corrosion process was investigated from the viewpoint of surface tension. At the initial corrosion period, the corrosion rate was proportion to the adsorption of anions contained the solutions. And as corrosion went on, the penetration effect of anions and different characteristics of the corrosion products began to dominant the corrosion process, which led to the accelerated effect.

EFFECTS OF Zn CONCENTRATION ON WETTABILITY OF Sn-Zn ALLOY ON Cu AND ON THE INTERFACIAL MICROSTRUCTURE BETWEEN Sn-Zn ALLOY AND Cu

Abstract: The potential of using a hypoeutectic, instead of eutectic, Sn-Zn alloy as a lead-free solder has been discussed. The nonequilibrium melting behaviors of a series of Sn-Zn alloys were examined by differential thermal analysis. It was found that at a heating rate of 5°/min, Sn-6.5Zn exhibited no melting range. Dipping and spreading tests were carried out to characterize the wettability of Sn-Zn alloys on Cu. Both tests exhibited that Sn-6.5Zn has significantly better wettability on Cu than Sn-9Zn. The reaction layers formed during the spreading tests were examined. When the Zn concentration fell between 2.5wt%-9wt%, two reaction layers were formed at the interface, a thick and flat Cu 6 Zn 8 adjacent to Cu and a thin and irregular Cu-Zn-Sn layer adjacent to the alloy. Only a Cu 6 Sn 5 layer was formed when the Zn concentration decreased to 0.5wt%. The total thickness of the reaction layer(s) between the alloy and Cu was found to increase linearly with the Zn concentration.

Experimental research and numerical simulation of laser shock forming of ta2 titanium sheet

Abstract: Laser shock forming (LSF) was a new technique realized by applying an impulsive pressure generated by laser-induced shock wave on the surface of metal sheet. LSF of metal sheet was investigated with experiments and numerical simulation. The basic theories were introduced; the surface quality and deformation of the processed titanium sample (TA2) were examined; ABAQUS was used to simulate LSF and the central displacement of the shocked region was measured and compared with the simulation. Overlapped LSF treatment was experimentally carried out to produce groove and simulation. The results showed that the surface quality and the microstructure with single laser pulse had no remarkable change, and ablation was observed on the surface of the sample with overlapped pulses. The deformation observed in the numerical simulation agreeed with that observed in the experimental measurement quite well.

Numerical investigation of straight-line laser forming under the temperature gradient mechanism

Abstract: Laser forming is a new flexible and dieless forming technique. To achieve the high accuracy forming, the temperature gradient mechanism (TGM) is studied. In the analysis of TGM, the plate bends about x-axis and about y-axis as well. To understand the deformation trend, the numerical simulation of deformation of plate is conducted by choosing different laser powers, laser spot diameters, scanning speeds, lengths, widths and thicknesses. From the results of simulation, it can be seen that the laser spot diameter, the scanning speed, laser power and thickness of plate play dominant roles in the laser forming process. However, the bending angles αx and αy show different trends with the variation of parameters. In addition, in comparison with above four parameters, the effect of length and width of plate on the bending angle may be neglected, but their effects are significant for the bending radius R.