Papers by Author: Jose Manuel Prado

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Authors: V.G. García, Jose María Cabrera, Jose Manuel Prado
Abstract: Modelling hot flow stress during grain refinement operations of fcc metals has largely included the use of an Avrami type equation to describe the decrease in stress due to Dynamic Recrystallization (DRX). However when refining large-grained copper, the processing temperatures and strain rates often produce a multi peak behaviour, which is not predictable by an Avrami equation alone. If an initial grain size, D0, is greater than the stable dynamically recrystallized grain size, Drex, which is a function of the Zener-Hollomon Parameter, Z, then the material will tend to refine. However if the current the Zener-Hollomon value, given by current temperature and strain rate conditions, is lower than a critical value, Zc, which depends on D0, then a multi peak stress behaviour is expected while refining. The latter Relative-Grain-Size model (i.e. the D0-Zc and Drex-Z relationships plotted on the same log-log graph) is a practical model that allows determination of whether a material will grain coarsen or refine and whether the dynamic recrystallization behaviour will be monotonic or with multi peaks. The present authors devised a dynamic recrystallization algorithm to measure the stress due to the diminishing initial grain volume and to measure the correction stress due to recrystallizing grains. Analysis on the hot (600°C-950°C) compression data of a 99.9% pure copper inductively lead to the use of an Avrami type equation to describe the stress contribution produced by the deformation of the remaining initial grain volume and a damped cosine equation to describe the stress contribution of the synchronized volume of new grains. This work discusses the experimental evidence and analytical findings that inductively support the mathematical description of the stress-strain curve given by a Damped Cosine Avrami Model for discontinuous DRX.
Authors: Céline Bertrand, Jose María Cabrera, A. Herrero, P. Mateos, Jose Manuel Prado
Authors: Tara Chandra, Jose María Cabrera, Jose Manuel Prado
Authors: Josep Antonio Benito, Robert Tejedor, Rodriguez Rodríguez-Baracaldo, Jose María Cabrera, Jose Manuel Prado
Abstract: This paper reviews the ductility of nanostructured and ultrafine iron obtained using a variety of methods. Mechanical milling of powder and subsequent hot consolidation, one of the most popular methods offer high mechanical strength but poor ductility. Improvements made in the consolidation processes and the introduction of final heat treatments, in addition to new approaches such as spark plasma sintering and high pressure torsion, have increased the total plastic strain of nanostructured iron. The development of bimodal structures enables the existence of strain hardening and more uniform deformation. The paper also includes a steel study, which finds that the hardness of milled powder and the role of carbon atoms inside ferrite grains make it more difficult to improve the ductility of nanostructured samples.
Authors: David Gutiérrez, A. Lara, Daniel Casellas, Jose Manuel Prado
Abstract: The Forming Limit Diagrams (FLD) are widely used in the formability analysis of sheet metal to determine the maximum strain, which gives the Forming Limit Curve (FLC). It is well known that these curves depend on the strain path during forming and hence on the test method used to calculate them. In this paper, different stretching tests such as the Nakajima and the Marciniak tests were performed, with different sample geometries to obtain points in different areas of the FLD. An optical analysis system was used, which allows following the strain path during the test. The increasing use of advanced high-strength steels (AHSS) has created an interest in determining the mechanical properties of these materials. In this work, FLCs for a TRIP steel were determined using Nakajima and Marciniak tests, which revealed different strain paths depending on the type of test. Determination of the FLCs was carried out following the mathematical calculations indicated in the ISO 12004 standard and was also compared with an alternative mathematical method, which showed different FLCs. Finally, the tests were verified by comparing the strain paths of the Nakajima and Marciniak tests with a well-known mild steel.
Authors: V.G. García, Jose María Cabrera, Jose Manuel Prado
Abstract: Previous research works assert that the observed increase in hot flow stress of commercially pure copper is attributed to the interactions between solute atoms and dislocations, specifically by interstitial oxygen. This work shows TEM images of the formation of Cu2O precipitates after warm working temperatures that in part help explain the increase of stress during hot compression of 99.9% pure copper. Three commercially pure large-grained coppers with 26, 46 and 62ppm of oxygen were tested at different temperatures (600°C-950°C) and strain rates (0.3s-1- 0.001s-1). At temperatures below 850°C, the stress differences between coppers, tested at same the strain rate, became increasingly higher. A correlation between stress increase and oxygen content was found. Precipitation of nanometric Cu2O did not show any difference in dynamically recrystallized grain size; however hardness tests showed that the final properties were modified. This work discusses the effect precipitation of Cu2O has on the hot flow curve and the final microstructure of hot formed 99.9% pure copper with different oxygen levels.
Authors: Jose María Cabrera, J.A. Cubero, A. Subirats, Jose Manuel Prado
Authors: Gonzalo Varela-Castro, Jose María Cabrera, Jose Manuel Prado
Abstract: The microstructural control of rolled products is based on managing the austenite phase transformations during and after hot deformation to attain the desired microstructure after the cooling step. Therefore, it is very important an appropriate description of the kinetics of the hardening and softening phenomena taking place during the deformation at high temperatures, namely, dynamic recovery (DRV) and recrystallization (DRX). This investigation examines the effect of manganese contents on the hot flow behaviour of plain carbon steels. For this purpose, uniaxial hot compression tests were carried out in carbon steels in an extensive range of temperatures, from 1123 to 1373 K and strain rates, from 510-4 to 110-1 s-1. This work is focused in determining the physically-based constitutive equations that govern the plastic behaviour of plain carbon steels. Experimental results were compared with the predictions of the model and an excellent agreement over a broad range of temperatures and strain rates was obtained.
Authors: A. Al Omar, A. Chenaoui, Rachid Dkiouak, Jose María Cabrera, Jose Manuel Prado
Abstract: The main aim of the present investigation was to study the flow behaviour of two medium carbon microalloyed steels under hot forming conditions, and to analyse its microstructural evolution. The dependence of recrystallized grain size (Drec) on the Zener-Hollomon parameter Z shows a bimodal behaviour with transition from single to cyclic dynamic recrystallization. Also we observed that the variation of Drec normalized by Burgers vector (b) with sss normalized by shear modulus ( µ) shows the same bimodal behaviour cited above. The Derby’s universal equation cited in literature for recrystallized grain sizes was not followed; it seems that the presence of fine precipitated particles has a clear effect on this disagreement.
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