Papers by Author: Ana Isabel Fernández-Calvo

Abstract: Primary AlSi10MnMg alloy is the most widely used alloy for manufacturing of vacuum assisted high pressure die castings (VPDC) with high ductility requirements. In this alloy, die soldering is avoided by a high Mn level (0.5 - 0.6 wt. %) while Fe is kept low (< 0.25 wt. %). Such combination guarantees that the Al-Fe-Mn-Si intermetallic compounds are of the α-iron rich polyhedral or Chinese script type, which is less harmful to the ductility. However, secondary alloys are cheaper and their production requires less energy than the one of primary alloys. The higher amount of Fe, a common impurity in secondary alloys, reduces ductility but also die soldering and thus manufacturing costs. Microadditions based on Mn are known to be very effective in transforming the harmful needle/platelet shaped β-compounds into α-iron compounds with a less harmful morphology. In this work a secondary alloy with 0.60 wt. % Fe and different Mn microadditions has been cast in test parts with different wall thicknesses using VPDC technology. The Mn content of the new alloy has been optimized. Mechanical properties of the optimised alloy have been determined in different heat treatment conditions and been compared to the corresponding AlSi10MnMg primary alloy. Mechanical properties similar to those of the primary alloy have been achieved.

Abstract: Both thermal analysis (TA) and differential thermal analysis (DTA) have been used since long to evaluate latent heat release and solid fraction evolution during solidification of metallic alloys. TA makes use of cooling curves recorded under "natural" cooling while DTA consists in recording the temperature difference between the sample temperature and an inert reference during a controlled cooling, i.e. at imposed constant cooling rate. In both cases, the solid fraction evolution is deduced from a calculation of the latent heat release as estimated by means of a heat transfer model. This paper provides a comparison of such evaluations performed on one Al-Si alloy.

Abstract: The influence of initial grain size on the softening-precipitation interaction in a low niobium microalloyed steel has been investigated. The study has revealed that for the largest initial grain size (1000 μm), the recrystallised fraction remains lower than the softening fraction until relatively long times are reached. In contrast, for the smallest initial grain size (166 μm) both magnitudes are similar. As a result, precipitation interacts with recrystallisation in the case of the finest austenite grain size, whereas for the coarsest one, since recrystallisation is significantly retarded, interaction with recovery process is observed. Apparently, the initial austenite grain size does not affect precipitation kinetics.

Abstract: The interaction between softening and precipitation mechanisms in hot worked Nb microalloyed austenite is analysed with the help of a physically based model. The model is able to calculate the evolution over time of the dislocation density (stress), stored energy and precipitate pinning force, the recrystallized fraction, the average precipitate diameter and precipitate number density, as well as the concentrations of the precipitating Nb over time. It is assumed that nucleation of precipitates occurs heterogeneously at dislocations with recovery producing a continuous decrease in dislocation density. This results in a reduction of the available nucleation sites for precipitation as well as a decrease in the driving force for recrystallization along time. By comparing the model predictions and the experimental results the values of several physical parameters involved in the model are discussed.

Abstract: In this contribution strain induced precipitation of niobium carbides has been analyzed making use of different hot-rolling simulators and combining the advanced precipitation characterization methods of selective chemical extraction and transmission electron microscopy. A laboratory cast Fe-0.1C-0.07Nb alloy has been employed for the study. Thermomechanical simulations were carried out by torsion, plastodilatometry and plane strain compression techniques. The results have shown that, in spite of the different deformation modes a relatively good correlation is obtained between the measurements of the precipitate size and the amount of Nb precipitated in the different experiments.

Abstract: The microstructural evolution during the hot rolling of coarse grain sized austenite has been modeled taking into consideration all the microstructural mechanisms (dynamic, static and metadynamic recrystallization, strain induced precipitation) that could take place during the industrial TSDR production of a Nb microalloyed steel. Based on the results obtained from the model, processing maps have been drawn for a 0.035%Nb microalloyed steel. Optimum processing conditions to exploit all the benefits of Nb microalloying have been defined considering a final gauge thickness range between 1.5 and 12.65 mm.

Abstract: The dynamic recrystallization (DRX) behavior of a coarse grain sized Nb microalloyed austenite (∼ 800 µm), typical of thin slab casting processes, has been studied. Continuous torsion tests were carried out at different, Z, Zener-Hollomon parameter values. It has been observed that as Z increases the curves move to higher values of stress and both the peak, εp, and steady state, εss, occur at larger strains, with an increase in the εss - εp strain difference. Consequently, an increment in Z produces a delay in the beginning and the progression of the DRX process. In the present work, it has been found that the pre-existing grain boundaries are the most favorable nucleation sites for DRX for all Z values, the nucleation mechanisms being related to strain induced migration of high angle grain boundaries. However, in the case of high Z values, intragranular nucleation on defects generated during deformation is also observed. The microstructure analysis denotes also that dynamic recrystallization is a process dominated by repeated nucleation with limited growth.