Microstructures variation of spray formed Si-30%Al alloy during densification process

Microstructure variation of spray-formed Si-30%Al alloy during densification process by hot pressing was studied. The results indicate that the microstructure of as-deposited preforms is fine and homogenous. The primary silicon phases distributing in aluminium matrix evenly are fine and irregular. Aluminium matrix is divided into two groups： supersaturated α-Al phase or α-Al phase and Al-Si pseudo-eutectic phase or Al-Si eutectic phase. During hot pressing, the primary silicon and the aluminium matrix realign as follows： the primary silicon fractures at a given compressive stress, the particles congregates in microzone with increasing stress, and the aluminium matrix flows and connects in harness. Al-Si pseudo-eutectic phase turns into Al-Si eutectic phase due to the diffusion of atoms during densification process.     

Modeling of gas phase diffusion transport during chemical vapor infiltration process

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｃｈｅｍｉｃａｌｖａｐｏｒｉｎｆｉｌｔｒａｔｉｏｎ (ＣＶＩ)ｍｅｔｈｏｄｉｓｏｎｅｏｆｔｈｅｍｏｓｔｐｒａｃｔｉｃａｌａｎｄ ｐｒｏｍｉｓｉｎｇ ｐｒｏｃｅｓｓｆｏｒｆａｂｒｉｃａｔｉｏｎｏｆｃｅｒａｍｉｃ/ｃａｒｂｏｎｍａｔ     

Characterization of oxide structures formed on nickel-chromium alloy during low pressure oxidation at 500–600°C

Low pressure oxidation studies of Ni-18%Cr alloy were carried out at temperatures of 500–600°C for very brief periods. Detailed XPS, AES, SEM, and TEM studies identified four stages in the initial oxidation. These are: (1) formation of a mixed nickel-chromium oxide overlayer; (2) growth of submicron-sized oxide nodules; (3) development of dark hole-like patches on the surface; and (4) growth of second generation oxide nodules. Both types of nodules consist primarily of a nickel structure depleted in oxygen. Their formation appears to result from a very rapid outward movement of nickel from localized defects in the metal. The dark patches result from the presence of a chromium oxide-rich underlayer, which appears to form by a lateral migration of chromium from adjacent oxide/metal interface regions and from grain boundaries.     

Evolution of surface chemistry and morphology of oxide scale formed during initial stage oxidation of modified 9Cr–1Mo steel

Initial stage oxidation characteristics of the modified 9Cr–1Mo steel in ambient air at 650 °C have been investigated, for exposure times ranging from 5 to 500 h. Oxygen flux from the gas phase causes high initial oxidation rate, but the growth kinetics do not follow parabolic law. In “as-received” condition, binary oxides of Fe and Cr were found as native oxides. Upon oxidation, segregation of Mn resulted in the formation of MnCr₂O₄ along with FeCr₂O₄ and binary oxides of Fe, Cr and Mn. Thus, the initial oxide scale constitutes multiple oxides with delineated interface, unlikely to have a layered structure.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Correlation between oxidation resistance and crystallinity of Ti–Al as a barrier layer for high-density memories

The Ti–Al intermetallic material system has been investigated for application as a conducting diffusion barrier in a three-dimensional stacked capacitor–transistor geometry. La–Sr–Co–O (LSCO)/Pb–Zr–Ti–Nb–O/La–Sr–Co–O ferroelectric capacitors were fabricated on Ti–Al/polycrystalline-Si/Si substrates. The electrical and ferroelectric properties are found to correlate strongly with the crystallinity of the Ti–Al layer. The crystalline Ti–Al layer shows a distinct chemical reaction with the bottom LSCO electrode thus preventing ohmic electrical contact between the ferroelectric capacitor and transistor. In contrast, the amorphous Ti–Al layer does not react and forms an ohmic contact to LSCO. For crystalline Ti–Al, X-ray photoelectron spectroscopy (XPS) shows the formation of Al₂O₃ induced by the segregation of Al to the LSCO/Ti–Al interface. For amorphous Ti–Al, XPS reveals that no Al₂O₃ layer is formed. In addition, Rutherford backscattering analysis shows almost no difference in the Ti-peak spectrum before and after deposition of LSCO.     

The Asymmetric Laplace Law for distribution of aluminum contact resistance in spot welding

This paper investigates the distribution of contact resistance of the aluminum alloy in the squeeze stage. A new method of measuring path＇ s resistance is proposed firstly. Contact resistances are calculated accurately by the systems of 5 linear equations and solutions show that three contact resistances are different. The probability density functions of contact resistance in workpiece/workpiece（ W/W） and upper electrode/workpiece（ E/W） show that the curve shape has steeper peak and heavier tail than that of the normal distribution. Non-parameter hypothesis test is performed and the result shows that R2 , R4 reject the normal distribution using chi-square and kolmogoroo statistic D method. Therefore, the Asymmetric Laplace distribution is fitted to empirical distributions and is applied to quantify the influence of random contact resistance. The result illustrates that AL distribution is very close to contact resistance of W/W and upper E/W and normal distribution has some deviation. The paper is helpful to research the initial nugget conditions, weldability and the transient multi-coupling field.     

Possibilities for improving the corrosion resistance of Fe–40Al intermetallic strip by prior oxide protection

Iron aluminides do not generally show good aqueous corrosion resistance. The present study attempts to improve their corrosion resistance by prior surface oxidation. While good corrosion resistance can indeed be obtained, it is found to be difficult to ensure that the oxide layer is continuous and defect free.     

Three-dimensional combined finite-discrete element approach for simulation of single layer powder Compaction process

The application of a combined finite-discrete element modeling approach to simulate the three-dimensional microscopic compaction behavior of single-layer metal powder system was described. The process was treated as a static problem, with kinematical component being neglected. Due to ill condition, Cholesky‘s method failed to solve the system equations, while conjugate gradient method was tried and yielded good results. Deformation of the particles was examined and compared with the results of physical modeling experiments. In both cases, the inner particles were deformed from sphere to polygonal column, with the edges turning from arc to straight line. The edge number of a particle was equal to the number of particles surrounding it. And the experiments show that the ductile metal particles can be densified only by their plastic deformation without the occurrence of rearrangement phenomenon.     

Experimental determination of the tool–chip thermal contact conductance in machining process

Tool–chip contact is still a challenging issue that affects the accuracy in numerical analysis of machining processes. The tool–chip contact phenomenon can be considered from two points of view: mechanical and thermal contacts. Although, there is extensive published literature which addresses the friction modeling of the tool–chip interface, the thermal aspects of the tool–chip contact have not been investigated adequately. In this paper, an experimental procedure is adopted to determine the average thermal contact conductance (TCC) in the tool–chip contact area in the machining operation. The tool temperature and the heat flux in tool–chip contact area were determined by inverse thermal solution. Infra-red thermography was also used to measure the average chip temperature near the tool–chip interface. To investigate the effects of the work piece material properties on the tool–chip TCC, AISI 1045, AISI 304 and Titanium materials were considered in the machining experiments. Effects of the cutting parameters such as cutting velocity and feed rate on TCC were also investigated. Evaluating the tool–chip thermal contact conductance for the tested materials shows that TCC is directly proportional to the thermal conductivity and inversely proportional to the mechanical strength of the work piece. The thermal contact conductance presented in this paper can be used in the future numerical and analytical modeling of the machining process to achieve more accurate simulations of the temperature distribution in the cutting zone and better understanding of the tool–chip contact phenomena.     

Enhancement of oxidation resistance of the supercooled liquid in Cu–Zr-based metallic glass by forming an amorphous oxide layer with high thermal stability

In the present study, we propose that the formation of amorphous oxide layers with an enhanced thermal stability can significantly improve the oxidation resistance of metallic glass in the supercooled liquid (SCL) state. Addition of a small amount of Be (2 at.%) in Cu₄₆Zr₄₆Al₈ widens the SCL region and simultaneously improves the oxidation resistance of the SCL. For Cu₄₆Zr₄₆Al₈, the initial amorphous oxide completely transforms into the crystalline oxide far below the glass transition temperature during continuous heating, while for Cu₄₅Zr₄₅Al₈Be₂, the amorphous oxide remains after heating up to the SCL region, thus significantly improving the oxidation resistance of the SCL.     

Modeling for thermal contact resistance of frictional interface under high temperature and high pressure

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｐｈｅｎｏｍｅｎｏｎｔｈａｔｔｈｅｉｎｔｅｒｆａｃｅｏｆｔｗｏｓｐｅｃｉ ｍｅｎｓｃｏｎｔａｃｔｕｎｄｅｒｔｈｅｅｆｆｅｃｔｏｆｍｅｃｈａｎｉｃｌｏａｄｉｓｓｅｅｎｅｖｅｒｙｗｈｅｒｅ[1] ,ｗｈｉｌｅｗｈａｔｉｓｔｈｅａｃｔ     

Nanoscale characterization of the transfer layer formed during dry sliding of Cu–15 wt.% Ni–8 wt.% Sn bronze alloy

The microstructure of the transfer layer, and the underlying severely plastically deformed layer (SPDL), formed during the dry sliding of a spinodally hardened Cu–15 wt.% Ni–8 wt.% Sn bronze against a stainless steel, is characterized at the nanoscale by conventional and analytical transmission electron microscopy, including energy-dispersive spectroscopy and electron energy loss spectroscopy. The SPDL consists of a Cu–Ni–Sn solid solution with elongated nanograins, due to extensive dislocation glide and twinning. In contrast, the transfer layer, 2–3 μm thick, is an equiaxed nanocomposite comprised of a Cu-rich metallic phase with a (Fe,Cr)₂O₃-based oxide precipitates, and forms as a result of the mechanical mixing and compaction of wear debris. The bronze in this layer has undergone dealloying, indicative of the importance of thermal effects. The dispersion of oxide in the transfer layer suggests a different type of forced mixing, possibly turbulent mixing. The transfer layer is observed to improve significantly the wear resistance of the bronze.     

Corrosive-wear resistance of stainless steels for the impeller of slurry pump used in zinc hydrometallurgy process

1. IntroductionA number of pumps have been widely used in thefollowing fields: water power, electric power, smelting,coal mine, oil field, chemical plant, papermaking and soon. The wet parts of pump (mainly consists of impellerand pump shell etc.) can easily be damaged due to theerosion and corrosion of the slurry, namely the combinedcorrosion wear (C-W) or erosion-corrosion (E-C) [1-3]. Ithas been reported that C-W is responsible for more than5 % of the total wear encountered in industrial …     

Fluidity of Al–Si semisolid slurries during rheocasting by a novel process

Lower fluidity of semisolid metallic slurries considerably limits their mold filling ability in rheocasting processes. In this paper, rheo-centrifuged casting (RCC) process was introduced and its performance was evaluated in terms of casting fluidity. Slurries were continuously cooled and stirred until reaching the desired fraction liquids up on which they were poured into a rotating sand mold. It was found that fluidity of Al–7.1 wt% Si semisolid slurry increased approximately linearly with the square of the initial fraction liquid. Casting fluidity also showed a relationship in the form of exp(nr) with section thickness of the channel (r). The exponent n appeared to be a function of the external force (centrifugal force) and was independent of fraction liquid. Casting fluidity was also found to increase with slurry stirring speed due to lower flow resistance resulted from smaller and more rounded primary particles formed. Some degree of liquid segregation was observed along the length of the mold channels which was more severe for smaller casting cross sections. In summary, RCC process was proved as a viable method for improving the casting fluidity of the alloy.     

Prediction of nugget development during resistance spot welding using coupled thermal–electrical–mechanical model

Abstract

An axisymmetric finite element model employing coupled thermal–electrical–mechanical analysis of resistance spot welding is presented. The welding parameters considered include: heat generation at the faying surface and the workpiece–electrode surface; Joule heating at the workpiece and the electrode; and the thermal contact conductance between the electrode and the workpiece. The latent heat of phase change due to melting is accounted for. The effect of friction coefficient on the workpiece interface is also studied. The computed results agree well with the experimental data. Heat generation at the faying surface in the initial stages of welding dominates the nugget development, and Joule heating at long times governs the weld nugget growth. A parametric study is carried out for the nugget growth with specific consideration of resistance spot welding of Al alloys. Process control and modelling of resistance spot welding of Al alloys is more difficult than that for steel because of their high electrical and thermal conductivity and low melting point.     

Influence of process parameters for coating of nickel–phosphorous on carbon fibers

The nickel–phosphorous (Ni–P) coating on carbon fiber was studied, using sodium hypophosphite as a reducing agent in alkaline medium. The effects of process parameters such as time, stabilizer concentrations, pH of the plating bath and plating bath's temperature on the electroless Ni–P coating efficiency were investigated. Structural study using X-ray diffraction (XRD) indicates that nickel deposition rate increases with increasing coating time and temperature. The nickel (Ni) recovery efficiency decreases with an increase of stabilizer concentration. From scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), it has been confirmed that the coating thickness and nickel content increase with an increase of coating time and temperature. The bath temperature of 25 °C, pH of 9, and stabilizer concentration of 25 g/L is good to get a good and uniform coating of Ni on carbon fiber. Thermal stability was studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). From TGA study it is evident that the nickel coating increases thermal stability of the nickel-coated carbon fiber. I–V (current vs voltage) measurement shows Ni-coated fiber is more conducting in nature.     

Electrical-thermal interaction simulation for resistance spot welding nugget process of mild steel and stainless steel

A three-dimensional finite difference electrical-thermal model for resistance spot welding nugget process of mild steel and stainless steel is introduced. A simulation method of the interaction of electrical and thermal factors is presented. Meanwhile, calculation method of contact resistance and treatment method of heater structure is provided. The influence of the temperature dependent material properties and various cooling boundary conditions on welding process was also taken intoaccount in the model. A method for improving the mild steel and stainless steel joint was analyzed in numerical simulation process. Experimental verification shows that the model prediction agrees well with the practice. The model provides a usefultheoretic tool for the analysis of the process of resistance spot welding of mild steel and stainless steel.     

3D FE analysis on the product defects of composites formed by liquid–solid extrusion process

The liquid–solid extrusion process for forming composite products was simulated by the three-dimensional thermo-mechanical finite element method. The mechanism for forming defects was analyzed based on the distribution of temperature and flowage of the work piece. The surface annular cracks occurring at the initial stage was caused by the high deformation temperature together with the axial tensile stress which was caused by uneven axial flow rate. However, the low deformation temperature in the terminal stage led to high resistance of deformation and interfacial strength, resulting in the fracture of fibres during severe deformation. The deformation condition in the middle stage was feasible to obtain composite products that were defect free. The simulation was verified through the comparison of the deforming force between the calculated and the measured one in laboratory conditions. The results indicate that the forming quality depends on the thermal and deformation state of the work piece, which can be controlled by the selection and adjustment of the process parameters.