Noé Cheung

The columnar to equiaxed transition during solidification of Sn–Pb alloys

Abstract In the present article, some important trends are shown regarding the influence of solidification thermal parameters on the columnar to equiaxed transition (CET) during the unsteady state solidification of Sn–Pb alloys. A comparison of the results obtained in the present work with results from the literature concerning similar experiments, but undertaken under conditions of lower heat transfer efficiency at the metal–mold interface, has shown that a realistic CET criterion should be based on a critical cooling rate at the dendrite tips of about 0.014 K/s, which depends only on the alloy system. The effects of melt superheat, solute concentration and metal–mold heat transfer coefficient on the CET position are also investigated.

Microstructural and hardness investigation of an aluminum-copper alloy processed by laser surface melting

Laser material processing has been widely applied in industrial processes due to the unique precision and very localized thermal action furnished by the lasers high energy density and power controllability. The scanned laser beam can be used to induce melting of a thin layer on metal surface when operated at higher intensity than that used for hardening. With the inherent rapid heating and cooling rates to which this surface layer is submitted, this process provides an opportunity to produce different microstructures from that of the bulk metal leading to useful properties. The major microstructural changes commonly observed in aluminum alloys are the extension of the solid solubility and the refinement of the microstructure. The objective of the current work was to analyze the microstructural and hardness variations throughout samples of an aluminum-copper alloy (Al-15 wt.% Cu) submitted to a laser surface remelting treatment. The analysis procedure consisted of scanning electron microscopy (SEM) characterization and microhardness tests in the resolidified and unmelted substrate regions.

Application of a Solidification Mathematical Model and a Genetic Algorithm in the Optimization of Strand Thermal Profile Along the Continuous Casting of Steel

ABSTRACT This work presents an optimization method based on a genetic algorithm applied to continuous casting process. A simple genetic algorithm was developed, which works linked to a mathematical model permitting the determination of optimum values for the water flow rates in the secondary cooling zones. First, experimental data (industrial) were compared with simulated results obtained by the solidification mathematical model, to determine the metal/cooling heat transfer coefficients along the machine by the inverse heat conduction problem method. The industrial data concerning surface strand temperature were obtained by using infrared pyrometers along a continuous caster machine during casting of both SAE 1007 and 1025 steels. In a second step, these results were used by a numerical code based on a genetic algorithm for determining optimum settings of water flow rates in the different sprays zones, which are conducive to the best quality of the solidified strand. The simulations were carried out by analyzing the solidification process during continuous casting to attain metallurgical restrictions concerning the reheating of strand surface temperature and metallurgical length.

On array models theoretical predictions versus measurements for the growth of cells and dendrites in the transient solidification of binary alloys

Most of the theoretical studies of the growth of cells/dendrites in the literature are based on the assumption that it is a steady-state phenomenon. The analysis of cells/dendritic structures in the unsteady-state regime is very important, since it encompasses the majority of industrial solidification processes. The aim of the present investigation was to validate the predictions furnished by the cellular and primary dendritic growth models in the literature for unsteady-state conditions against a large spectrum of experimental data, which includes those for a variety of Al alloys (Al–Cu, Al–Si, Al–Fe, Al–Bi, Al–Ni, Al–Sn) and low thermal diffusivity alloys, such as Sn–Pb and Pb–Sb. The predictions furnished by the Hunt–Lu model do not match the cellular experimental scatter for any examined alloy system. However, this model matches well with the primary dendritic growth of Al alloys, with the exception of Al–Sn alloys, for which the Hunt–Thomas approach has to be applied. The primary dendritic predictions of Bouchard–Kirkaldys model, performed with the originally suggested a 1 calibration factors are, in most cases, located above the experimental points. Experimental growth laws relating cellular and dendritic spacings with the tip growth rate and the cooling rate, respectively, are established.

Investigation of nonmetallic inclusions in continuously cast carbon steel by dissolution of the ferritic matrix

One of the main steel plants problems has been the occurrence of inclusions throughout the process of steel making. In this sense, it is very important to detect nonmetallic inclusions in the steel, to determine their origin and to control the formation of such inclusions, in order to generate a final product of high quality. The aim of this work is to present a characterization method for nonmetallic inclusions which uses the expedient of dissolving the ferritic matrix in hydrochloric acid (HCl). Scanning electron microscopy connected to an energy-dispersive spectrometer (EDS) system is used to obtain the morphology, size and chemical composition of such inclusions. This analysis allows a better understanding about the nature of the inclusions, their incidence and distribution along the process of steel manufacturing, providing subsidies to formulate corrective actions that minimize the occurrence of nonmetallic inclusions.

An Effective Inverse Heat Transfer Procedure Based on Evolutionary Algorithms to Determine Cooling Conditions of a Steel Continuous Casting Machine

The inverse modeling of heat transfer is a useful tool in analyzing contact heat transfer at the ingot surfaces during the continuous casting process. The determination of the boundary conditions involves an experimental work consisting in the evaluation of the thermal history, generally at the casting surface, experimentally provided by infrared pyrometers. Additionally, numerical simulations, based on the solution of the 2D transient heat conduction equation, are performed in order to be inversely solved in response to the measured thermal data furnished by the sensor. Due to computational time consumption during simulations in searching cooling conditions, this work proposes an interaction between natural inspired algorithms, called evolutionary algorithms, and the numerical model in order to speed up the searching process. The present work aims to compare three algorithms, namely genetic algorithm, improved stochastic ranking evolutionary strategy, and evolutionary strategy with Cauchy distribution. The latter develops a metaheuristic version of an evolutionary strategy workflow, using a Cauchy random number function to generate each individual, instead of the usual uniform distribution function available in almost all programming languages. The surface temperature, solid shell, and molten pool profiles from the determined cooling conditions are analyzed in terms of casting quality.

Steady and unsteady state peritectic solidification

Abstract In the present study, Pb–Bi as a model of alloys from peritectic systems was directionally solidified under transient heat flow conditions, which is the class of heat flow encompassing the majority of industrial solidification processes. Some experiments were also carried out in a vertical tube furnace under cooling rates closer to equilibrium during solidification in order to broaden the range of solidification parameters. Thermal parameters such as the tip growth rate (VL) and the cooling rate were experimentally determined and correlated with the primary (λ1) and secondary (λ2) dendrite arm spacings by experimental growth laws. It is shown that the proposed growth laws are able to encompass also the growth of dendritic branches during steady state growth from the melt.

The variation of the metal/mold heat transfer coefficient along the cross section of cylindrical shaped castings

During solidification, the mathematical analysis of heat flow depends on the transient heat transfer coefficient at the metal/mold interface. The analysis of heat transfer behavior along a cylindrical section is necessary for a better control of solidification in conventional foundry and continuous casting processes. For this purpose, a water-cooled experimental apparatus was developed, and experiments were carried out with Sn–Pb alloys with different melt superheats. The heat transfer coefficients were determined by a theoretical–experimental fit of thermal profiles (IHCP). The results have shown a variation in heat flow conditions along the metal/mold interface provoked by the action of solidification thermal contraction connected with the gravitational effect. In macrostructural terms, this effect was evident with an asymmetric structure due to the variation of metal/mold thermal contact along the cylinder cross section. Experimental equations correlating heat transfer coefficients as a power function of time along the cross section of cylindrical horizontal castings of Sn–Pb alloys are proposed.

Mathematical modeling and experimental analysis of the hardened zone in laser treatment of a 1045 AISI steel

The aim of this work is to develop a mathematical model to predict the depth of laser treated zone in the LTH process. The Fourier equation of heat conduction is solved by using the Finite Difference Method in cylindrical coordinates in order to study the temperature distribution produced in a workpiece and hence to obtain the depth to which hardening occurs. The theoretical simulations are compared with results produced experimentally by a CO2 laser operating in continuous wave, showing good agreement.

Investigation of the chemical composition of nonmetallic inclusions utilizing ternary phase diagrams

Nonmetallic inclusions are normally present in steel products, as they originate from reactions that occur during the steelmaking process. Inclusions, which consist of stable nonmetallic phases depending on their chemical composition, can strongly affect the final quality of the metallurgical product. In this sense, the influence of inclusions in the steel is determined by their chemical composition. The aim of this work is to present the evolution of the chemical composition of nonmetallic inclusions utilizing ternary phase diagrams by means of the analysis of steel samples collected during the steel (aluminum-killed steel, SAE 1015) manufacturing process. A computational program was developed to furnish the positions of the inclusions on appropriate ternary phase diagrams by using as input data the chemical composition of the inclusions, determined with a scanning electron microscope (SEM) connected to an energy-dispersive spectrometer (EDS).