José Deodoro Trani Capocchi

Heat transfer coefficient at the metal-mould interface in the unidirectional solidification of Cu-8%Sn alloys

Abstract The heat transfer coefficient at the metal–mould interface of a unidirectional solidification system was calculated by an algorithm that uses the whole domain method for the inverse solution to the heat conduction differential equation with phase change. Experimental curves of temperature as a function of time, collected during solidification of Cu–8%Sn alloys subject to four different conditions, were used as input to the algorithm. Accordingly, the heat transfer coefficient at the metal–mould interface was obtained for those conditions. The estimated heat transfer coefficient values are in good agreement with the ones published in the literature.

Kinetics and catastrophic swelling during reduction of iron ore in carbon bearing pellets

AbstractThe rate of reaction between iron oxides and charcoal, agglomerated together as pellets, has been correlated with the swelling of the pellets and with the morphology of the iron formed. The experimental techniques employed were thermogravimetric analysis, X-ray diffraction analysis, and microscopic examination of partially reduced pellets. The experiments were performed in the temperature range 1223–1423 K. The conditions that favour the growing of iron in the form of whiskers, and hence lead to catastrophic swelling, have been determined.

Polymeric precursor synthesis of alumina containing manganese oxide

Ceramic materials have been successfully prepared by using organic precursors. Synthesized compounds include, for example, tin dioxide, zinc oxide, niobates, ferrites and silicates. Among all the additives that can promote alumina densification, manganese oxide is especially effective in the absence of liquid phase. Addition of 7 mol % of manganese during aluminum oxide preparation by organic precursors synthesis promotes α-Al 2 O 3 crystallization at 900°C. The resulting powder presents a high specific surface area. Manganese oxide has not been detected by X-ray diffraction analysis even when high manganese concentration was used, suggesting that segregation occurs to the interfaces, similarly to the Mn doped-SnO 2 powder prepared by polymeric method.

Dendrite structure control in directionally solidified bronze castings

Tin bronze (Cu-8%Sn) was cast into cylindrical samples using a unidirectional solidification device monitored by thermocouples. Four cylindrical samples were obtained under four different experimental conditions where pouring temperature, heat extraction and addition of inoculant changed. Thermocouple temperature curves did not show any strong effect of inoculant additions, though, at the beginning of solidification, recalescence seemed to decrease with an increase in inoculant efficiency. Macrostructures of samples were examined on longitudinal sections and an increase in the columnar zone length was observed when the pouring temperature and the heat extraction flux were raised simultaneously. Secondary and primary dendrite plates were seen to form from the coalescence of secondary dendrite arms. Dendrite arm spacing was measured and related to the local solidification time and to the cooling rate, showing some agreement with relationships published in the literature.

Study of the aluminothermic reduction of niobium pentoxide through thermal analysis experiments and high energy milling processing

Aluminothermic reduction of niobium pentoxide was studied through thermal analysis techniques such as differential thermal analysis (DTA) and thermogravimetry (TG) as well as through high energy milling processing. Reactants mixtures were composed by powders of Nb2O5 and Al. In the case of DTA-TG experiments, different molar ratios Nb2O5:Al were heated in a dynamic atmosphere of synthetic air under controlled conditions. The high energy milling runs were carried out via SPEX vibratory mill under argon atmosphere and with milling power equal to 7:1 (ratio of mass of balls to mass of mixture) with 10 pct excess of Al over the stoichiometric mass of aluminum necessary. In both kinds of experiments, X ray diffraction was used in order to identify the products of reaction. From DTA-TG experiments, it was possible to determine the experimental value of the enthalpy change (-595.9 kJ.mol-1), which is near to the theoretical one. From the milling experiments, it was possible to verify the possibility of the occurance of aluminothermic reducion of niobium pentoxide via this kind of processing.

Mathematical modelling of microsegregation in eutectic and peritectic binary alloys

Abstract A mathematical model of microsegregation for eutectic and peritectic binary alloys was implemented using a finite volume method to solve the differential equations for mass transport. In this model simple ideas are used to handle phase boundaries and coarsening without the need to employ node jumping schemes or any transformation of variables to fix the domain size. Some model results were compared with available analytical solutions, revealing excellent agreement, which proved the approach useful to solve dissolution and diffusion coupled problems as well as microsegregation ones. Furthermore, good agreement was observed between the model results and measurements of eutectic volume fractions published previously for an Al–Cu alloy. The model was also capable of showing some important features of a typical peritectic transformation. Some instability was observed during model calculations, but it was easily handled by a time step refining technique.

Comparative Study of Mechanical Activation Improved by High Energy Ball Mills in Chromium Oxide Reduction

The processes of high-energy milling and gained importance among the unconventional methods. In this work, we seek to compare the power supply two types of high energy mills (vibratory mill (SPEX) and planetary mill) with the variation of the milling power. The millings were carried out with a mixture of chromium oxide and aluminum metalic. The reduction of chromium oxide does not occur instantaneously, but gradually as the progress of milling with mechanical activation of powders, this mechanical activation occurs leading to the solid state reaction occurs. The results were obtained for thermal analysis of the samples. The energy released varies, exhibiting a maximum mechanical activation for the range of powers milling studied. The correlation between the energy mills can be made by identifying the milling power is reached at which the maximum in each mechanical activation mill and quantifying this activation.

Obtainment of a NiCrNbC Alloy by Mechanical Alloying

The present work investigate the possibility of obtainment by mechanical alloying of Ni superalloys based on the Ni-Cr-Nb-C system strengthened by γ”(Ni3Nb), since γ”(Ni3Nb) as γ’ (Ni3Al) are typical coherent phase strengthening mechanisms in nickel superalloys. In order to evaluate this possibility, a composition with 71,65wt%Ni, 7,90wt%Cr, 20,00wt%Nb and 0,45wt%C was processed in a SPEX mill by 8 hours, consolidated and sintered at different temperatures (1200oC, 1250oC and 1300oC). The powder processed by MA and the sintered products were characterized by x-ray diffraction, SEM and EDS.

Syntesis of Niobium Nitride Using Cryogenic Milling

In the present paper a preliminary study was performed on the influence of mechanical milling on the synthesis of niobium nitrade. The niobium metal powder sample, passing # 635 mesh sieve, was processed by mechanical milling in SPEX mill for 8 hours using a ball-to-powder ratio of 7:1 and a nitrogen atmosphere. The powder was annealed at different temperatures, 900 °C, 1000 °C, 1100 °C and 1200 °C for 1 hour in a hydrogen and argon atmosphere to study their crystallization, which then were formed into blanks for analysis of the compressibility curves. These samples were also subjected to X-ray diffraction and the data were compared between the annealing temperatures. The compressibility curves of niobium samples with and without grinding were also evaluated, showing high compacting capacity. These samples were subjected to X-ray diffraction and X-ray fluorescence. As the formation of nitrides (Nb2N) was observed in SPEX type mill, the interest in studying the synthesis of nitrides came up, using mechanical milling in Attritor type mill. Same previous results of Attritor processing indicate a Nb2N and NbN synthesis after annealing treatments.