Papers by Author: Jose Manuel Prado

Abstract: Low alloy transformation-induced plasticity aided (TRIP) steels have attracted much interest over the last years. TRIP steels were initially developed for automotive applications as they offer an excellent combination of strength and ductility at reasonable costs. These excellent mechanical properties mainly arise from a complex multiphase microstructure of a ferrite matrix and a dispersion of multiphase grains of bainite, martensite and metastable retained austenite. The relevant influence of microstructure on physical and mechanical properties makes metallographic study essential for an appropriate understanding and improvement of the mechanical behavior. An accurate microstructural characterization and quantification of the amount of the different constituents is indispensable to know how the stresses and strains are distributed within the different microstructural constituents. Among the different characterization methods commonly used electron backscatter diffraction (EBSD) appears to be the unique technique able to observe retained austenite grains often no larger than 1 μm. The present work shows the evolution of retained austenite while straining. Microstructural and textural evolution after different strains was examined by optical microscopy OM, EBSD and XRD techniques on TRIP800 steel. EBSD technique appears as a powerful tool for characterizing the complex multiphase steel microstructure and provides an accurate evaluation of the local crystallographic texture. It allows to measure orientation gradients within individual grains of each different phase. The distinction between some phases is observed.

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.

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.

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.

Abstract: Samples of nanostructured and ultrafine grained steels with carbon content ranging from 0.05 to 0.55%wt. have been obtained by a warm consolidation process from mechanically milled powders and subsequent heat treatments. In general, homogeneous grain size distributions were obtained except for the low-carbon steel in which a bimodal grain size distribution was observed when it was heat treated at high temperatures. The stress-strain response has been studied by means of compression tests. Nanostructured materials showed high strength but poor results in terms of ductility. In the low-ultrafine range (mean grain size between 100-500 nm) the three materials showed an increase in the ductility with strain softening. Finally, when the average grain size was close to 1 µm samples showed larger ductility and strain hardening.

Abstract: Semi-solid materials (SSM) in the thixotropic state behave like liquids, i.e they show low or null shear resistance and, at the same time, they behave like solids as do not fall to pieces under applied forces. At present, the potential advantages and industrial applications of these materials are well recognized, in particular for the production of Al-alloy components for the aerospace and automotive sectors. This work is focused on the evaluation and characterization of the thixotropic behaviour of a metal mixture in the semi-solid state obtained by “Compocasting”. The mixture is obtained by mixing spherical solid Cu particles with a liquid eutectic tin-lead alloy. Measurements of the time-dependence of the viscosity of the mixture using an instrumented rheometer showed that, after mechanical stirring, the slurry acquires thixotropic properties. The best conditions to obtain such mixture are presented. Additionally, once the mixture is cooled down, the material is reheated and then tested in a laboratory backward extrusion process. The behaviour of the material is analysed on the basis of the microstructure obtained, and the process parameters considered.

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.

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.

Abstract: In this work the elastic behaviour of metallic powder compacts is studied. Cylindrical specimens with different levels of density have been submitted to uniaxial compression tests with loading and unloading cycles. The analysis of the elastic loadings shows a non linear elasticity which can be mathematically represented by means of a potential law. Results are explained by assuming that the total elastic strain is the contribution of two terms one deriving from the hertzian deformation of the contacts among particles and another that takes into account the linear elastic deformation of the powder skeleton. A simple model based in a one pore unit cell is presented to support the mathematical model.

Abstract: The results of monotonic and cyclic uniaxial compression tests, in which the deviatoric component of the stress is predominant, carried out on green and recrystallized iron compacts with different levels of density are presented and discussed in order to analyse the macro and micromechanisms governing the mechanical behaviour of non-sintered PM materials. The plastic deformation of the particles, especially at the contact areas between neighbouring particles, produces an internal friction responsible for the main features observed in the behaviour of green metallic compacts. These experimental results show important discrepancies with the plasticity models, Cam-Clay and Drucker-Prager Cap, used to simulate the powder compaction stage. Possible causes for these discrepancies are pointed out.