Optimization Techniques with FORTRAN

Effect of Iron-Containing Intermetallic Particles on the Corrosion Behaviour of Aluminium

Microdefects formation during the twin-roll casting of Al–Mg–Mn aluminium alloys

In this work, similar butt joints of AA6061-T6 alloy prepared by underwater friction stir welding (UWFSW) and friction stir vibration welding (FSVW) processes were examined. The characteristics of joints were compared with the joints obtained by conventional friction stir welding (CFSW). The different kinds of microstructural modifications that occurred during CFSW, FSVW, and UWFSW processes were analyzed. The results are employed to analyze the different behaviors in strength, ductility, weldability, and hardness of the joints in different processes at different traverse speeds, rotational speeds, and vibration frequency. It was found that mechanical vibration decreases the grain size in the weld zone and hinders the coalescence and regrowth of the precipitates during FSVW. On the other hand, water significantly decreased the joint temperature during UWFSW and led to a refined microstructure in the stir zone, which substantially improved the mechanical properties of the welded joint. The synergetic effect of refined grains and lower dissolution of β″ precipitates (higher evolution of β″ to β′ and β-Mg2Si) due to fast cooling rate (intensified local deformation) led to higher hardness in UWFSW (FSVW) joint compared to CFSW joint. The small size with the uniform distribution of dimples indicated the combination of strength and ductility in FSVW-ed and UWFSW-ed joints. The simple and highly effective welding procedure, FSVW, can be readily scaled up for industrial welding applications.

Comparison of the Weldability of AA6061-T6 Joint under Different Friction Stir Welding Conditions

Computer simulation of steel quenching process using a multi-phase transformation model

In this work, we present a theoretical scheme that allows us to follow the kinetics of the precipitation of carbides in the Fe-C system. Our scheme is based on reaction equations, which take into account all possible ways of atomic exchange between the different competing phases. Through the use of nonlinear regression methods, we have fitted the results from our model to those obtained from thermoelectric power measurements for the Fe-C system in the temperature range between 20 °C and 450 °C. By deconvolution of optimally fitted theoretical kinetics, we have deduced ranges for coexistence of the different carbides that formed and their activation energies. An adequate agreement between our results and those reported in the literature is obtained.

Multiphase precipitation of carbides in Fe-C systems: Part I. Model based upon simple kinetic reactions

We have used thermoelectric power (TEP) techniques to study the precipitation process in samples of cold-rolled commercial 3003 Al alloys, in which we guarantee the initial microstructure by diverse thermal treatments. Kinetics are obtained, isothermally, at several aging temperatures, proving that the manganese impurity controls the precipitation process. The Nordheim-Gorter rule is used to determine the effect of impurities on the diffusional TEP, permitting us to know the amount of Mn in solid solution for the diverse work conditions. The time-temperature-transformation (TTT) curves, obtained from the isothermal kinetics plots, indicate the existence of different precipitate phases which depend upon the thermal treatment applied to the sample. For homogenized samples, the observed behavior was associated to an equilibrium phase with a nose toward 540 °C and with an activation energy of (41±2) Kcal/mol. For strained samples, we observe a metastable phase with an activation energy of (19±2) Kcal/mol and also a stable phase with an activation energy similar to the one obtained for the homogenized samples. For as-cast and as-cast-heated samples, we predict the existence of two precipitated phases, the activation energy of the precipitated phase at low temperature being (31±2) Kcal/mol. The energy of the second phase seems to be the same for all microstructural conditions. The results obtained for the stable phase agree with the results reported in the literature.

Isothermal precipitation of commercial 3003 Al alloys studied by thermoelectric power

It is well known that when one follows, isothermally, the process of separation of phases of certain binary alloys using electrical-resistivity measurements, an anomalous behavior is observed. The theoretical explanation of this anomaly has been controversial. In this work, we discuss two aspects of that theory that have not received enough attention in the literature. The first is the real effect that an exponential damping term produces on the resistivity anomaly when the mean free path is not a free parameter but rather depends on the wave vector. This leads to an integral equation of the Volterra-type, the solution of which, by the iterative method of Newmann, exhibits rapid convergence when the time constant of the damping factor is associated with the internal mean free path of a Guinier-Preston zone. The second aspect concerns a possible reconciliation of the ideas of Rossiter and Hillel concerning a semiphenomenological model that reproduces well the clustering process. This model takes into account the effect of scattering by zones, separately, through the microstructure and through the boundaries, with a weight function that determines the centers by which the electron is scattered. The results obtained when this model is applied to the binary Al-Zn alloy are completely satisfactory.

Resistivity anomaly during the process of separation of phases of a binary alloy.

Multiphase precipitation kinetics for a 3003 aluminum alloy, pointed out in our previous work[1] and obtained under different microstructural conditions by thermoelectric power measurements, is now modeled under the light of the theory of reaction rates. Our multiphase model encompasses the precipitation kernels of Johnson-Mehl-Avrami (JMA) and of Fujita-Damask (FD), which adequately reproduce the experimental kinetics. The JMA kernel generates almost constant values for the exponent n. These values are less than one in temperature ranges where the log of the rate constant k grows linearly with the inverse of the temperature. The FD kernel provides an idea of the average critical size of the phase which is formed. Our multiphase results are compared with those obtained from the traditional monophase considerations rendering evident important differences between both models. The total kinetics is deconvoluted, allowing us to characterize the different metastable and stable phases formed according to the different microstructural conditions. The activation energy for kinetics in “as-cast” samples behaves differently from that obtained from the microstructures in homogenized and homogenized-strained samples. The activation energy for each of these cases is reported.

Analysis of thermoelectric power measurements in the study of precipitation kinetics in 3003 Al alloy

The precipitation of carbides in the Fe-C system aged at constant temperature is simulated considering two transformation kernels of complex reactions, involving different impingement and rate-time factors. The metastable and stable phase are identified by deconvolution of the experimental kinetics. The activation energy of carbides is found to depend, slightly, on the transformation kernel in which it is used.

Multiphase precipitation of carbides in Fe-C system: Part II. Model based on kinetics of complex reactions

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