Amauri Garcia

Determination of transient interfacial heat transfer coefficients in chill mold castings

Abstract The present work focuses on the determination of transient mold–environment and metal–mold heat transfer coefficients during solidification. The method uses the expedient of comparing theoretical and experimental thermal profiles and can be applied both to pure metals and metallic alloys. A solidification model based on the finite difference technique has been used to provide the theoretical results. The experiments were carried out by positioning the thermocouples in both metal and mold. The comparison between experimental and theoretical results is made by an automatic search of the best fitting among theoretical and experimental cooling curves simultaneously in metal and in mold. This has permitted the evaluation of the variation of heat transfer coefficients along the solidification process in unsteady state unidirectional heat flow of Al–Cu and Sn–Pb alloys, as well as the analysis of the effects of the material and the thickness of the mold and melt superheat.

Mathematical modeling and optimization strategies (genetic algorithm and knowledge base) applied to the continuous casting of steel

Abstract The control of quality in continuous casting products cannot be achieved without a knowledge base which incorporates parameters and variables of influence such as: equipment characteristics, steel, each component of the system and operational conditions. This work presents the development of a computational algorithm (software) applied to maximize the quality of steel billets produced by continuous casting. A mathematical model of solidification works integrated with a genetic search algorithm and a knowledge base of operational parameters. The optimization strategy selects a set of cooling conditions (mold and secondary cooling) and metallurgical criteria in order to attain highest product quality, which is related to a homogeneous thermal behavior during solidification. The results of simulations performed using the mathematical model are validated against both experimental and literature results and a good agreement is observed. Using the numerical model linked to a search method and the knowledge base, results can be produced for determining optimum settings of casting conditions, which are conducive to the best strand surface temperature profile and metallurgical length.

Modeling dendritic structure and mechanical properties of Zn-Al alloys as a function of solidification conditions

Abstract In the present article, some important trends have been shown regarding the relationship between mechanical properties, microstructure and solidification variables of Zn–Al alloys castings. Some theoretical dendritic growth models, expressing secondary spacings as function of tip growth rate or local solidification time, have been tested against experimental data obtained during unsteady-state solidification. Based on these dendritic models, on analytical expressions describing the position of solidus and liquidus isotherms in the unidirectional solidification of binary alloys and on experimental results concerning tensile testing of casting samples, expressions have been developed permitting a correlation between ultimate and yield strength, dendrite secondary spacings and solidification processing variables.

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (hi), secondary dendrite arm spacing (λ2), corrosion potential (ECorr), corrosion rate (iCorr), polarization resistance (R1), capacitances values (ZCPE), ultimate tensile strength (UTS, σu), yield strength (YS, σy), and elongation. It is shown that σU decreases with increasing λ2 while the CR increases with increasing λ2, for both alloys experimentally examined. A combined plot of CR and σU as a function of λ2 is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.

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.

Mathematical model for the unidirectional solidification of metals: I. cooled molds

This paper presents a new mathematical model that can be used to predict the solidification rate and the temperature distribution during the unidirectional solidification of metals n molds cooled by fluids such as air and water. The model differs from other analytical methods presented in the literature in the sense that it is more general in application and easier to manipulate, while retaining the advantage of convenience over numerical techniques. The proposed model permits the measurement of the Newtonian heat transfer coefficient at the metal/mold interface. In order to verify the applicability to air cooled molds, the model is compared with experimental results in the literature for the cases of lead, tin and lead-tin eutectic. Finally, the case of water cooled molds is examined, the model being compared with experimental results obtained in this work for lead and aluminum.

Cellular/dendritic transition during unsteady-state unidirectional solidification of Sn–Pb alloys

Abstract Structural parameters such as grain size, dendritic and cellular spacings, segregated products, porosity and other phases are strongly influenced by the thermal behavior of the metal/mold system during solidification, imposing a close correlation between this and the resulting microstructure. Numerous unidirectional solidification studies have been carried out with the objective of characterizing cellular and dendritic spacings and most of the published work has involved solidification in steady-state heat flow conditions. The objective of this article is to determine the thermal solidification parameters affecting the cellular/dendritic transition as well as to compare theoretical models that predict cellular and primary dendritic spacings with experimental results for unsteady-state solidification. Experiments were carried out in a water cooled unidirectional solidification apparatus and dilute alloys of the Sn–Pb system have been used (Sn 1.5 wt.% Pb, Sn 2.5 wt.% Pb and Sn 5 wt.% Pb).

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.

The use of artificial intelligence technique for the optimisation of process parameters used in the continuous casting of steel

Abstract The productivity and quality of a continuous caster depend mainly on process parameters, i.e. casting speed, casting temperature, steel composition and cleanliness of the melt, water flow rates in the different cooling zones, etc. This work presents the development of an algorithm, which incorporates heuristic search techniques for direct application in metallurgical industries, particularly those using continuous casting process for the production of steel billets and slabs. This is done to determine the casting objectives of maximum casting rate as a function of casting constraints. These constraints are evaluated with the aid of a heat transfer and solidification model based on the finite difference technique, which has been developed and integrated with a genetic algorithm. The essential parts of continuous casting equipment, which must be subjected to monitoring, as well as a methodology of mathematical model and physical settlements in each cooling region, are presented. The efficiency of the intelligent system is assured by the optimisation of the continuous casting operation by maximum casting rate and defect-free products. This approach is applied to the real dimension of a steel continuous caster, in real conditions of operation, demonstrating that good results can be attained by using heuristic search, such as: smaller temperature gradients between sprays zones, reduction in water consumption and an increase in casting speed.

Modeling of solidification in twin-roll strip casting

Abstract Twin-roll continuous casting combines solidification and hot rolling into a single operation to produce thin strips that are directly coilable. It offers advantages of low capital investment and low operational cost, and the strips produced have a refined solidification microstructure, which has attracted interest of global metal producers. This is evidenced by numerous pilot-scale casters constructed. The successful development of near-net-shape casting depends critically on an understanding of the fundamental knowledge of heat and fluid flow. Despite sophisticated instrumentation technology, information critical to the understanding of the casting region cannot be measured directly, therefore it is necessary to develop efficient numerical tools to control the process. This paper presents a numerical model for the two-dimensional solidification problem in the twin-roll continuous casting system by using a finite difference technique. The thermal analysis results give valuable insight into the thermal characteristics of solidification and processing for the strip casting. Results of subsequent simulations are compared with data from the literature.