Papers by Author: Oscar A. Ruano

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Authors: Fernando Carreño, M. Pozuelo, José A. Jiménez, Oscar A. Ruano
Abstract: Bend and shear tests were used to characterize the improvement in impact behavior of various ultrahigh carbon steel laminated composites. These tests turned out to deliver much more useful information about the mechanical properties of the laminates than the Charpy impact tests and were especially interesting for characterization of laminates of very high toughness values. The toughness of the various laminates was controlled by the rolling conditions that determined the quality of the bond and the appearance of delamination by the interfaces. The bend test allows determination of yield and maximum stresses, absorbed energy and permits graphical visualization of layer fracture and delaminations as testing proceeds. The shear test allows mechanical characterization of the bond quality between layers, permitting prediction of possible delaminations, and therefore, the mechanical properties of the layered material.
Authors: Ling Jiang, Maria Teresa Pérez-Prado, Oscar A. Ruano, M.E. Kassner
Abstract: The bond strength of ultrafine grained Zr with a grain size of 0.4 µm, fabricated by accumulative roll bonding (ARB), was assessed. The shear strength of the bond was estimated to be about 20% of the shear fracture strength of the as processed metal, a ratio significantly higher than that found in other materials processed by similar methods. The favorable degree of bonding achieved is attributed to the high ductility of Zr as well as to the high reductions used during the ARB process.
Authors: José A. Jiménez, Manuel Carsí, Georg Frommeyer, Oscar A. Ruano
Authors: Jesus Castellanos, Ignacio Rieiro, Manuel Carsí, Julio Muñoz, Oscar A. Ruano
Abstract: The influence of methods for converting torsion data (torque, number of turns and speed rotation) into equivalent variables (true stress, true strain and true strain rate) on the Garofalo equation parameters is studied. Torsion tests for a high strength low alloy steel at temperatures in the range 900 to 1150°C and strain rates in the range 0.5 to 30 s-1 were conducted. The flow behavior of the material was analyzed by means of the Garofalo equation which is especially adequate for the correlation of torsion data in wide ranges of temperatures and strain rates. The Garofalo equation was fitted at different strains: from peak strain to a value of strain where a steady-state is assumed. The parameters of this equation, A, Q, n, , were determined by the RCR method which does not need initial values and provides coefficients indicating the quality of the fitting. Finally, the Garofalo equation parameters were related to creep mechanisms operating in this steel.
Authors: Manuel Carsí, José A. Jiménez, Xabier Gomez-Mitxelena, Oscar A. Ruano
Abstract: In the present work, 1 wt.% of (Ti,Nb)C carbide particles prepared by self-propagating high temperature synthesis (SHS) were introduced into a melt of a conventional P23 steel to obtain a reinforced material with improved creep properties. The as-cast material showed a eutectic type microstucture, indicating partial dissolution of these carbides in the melt. Inside the dendritic regions, a bainitic/martensitic structure similar to that of the unreinforced material was present. A significant refinement of the prior austenitic grain size was revealed in the reinforced material. Brinell hardeness measurements reveal an increase of hardness in the reinforeced material due to the addition of the carbides. High strain rate compression tests were perfomed at temperatures in the range 950 and 1250oC to determine the optimum forming conditions. Stability maps for a wide range of temperatures and strain rates were drawn. The optimum temperature for the reinforced steel is about 77 K higher than for the non-reinforced steel.
Authors: J.A. del Valle, Oscar A. Ruano
Abstract: The effect of annealing treatments on the evolution of the strain rate sensitivity with strain of AZ61 magnesium alloy processed by severe rolling was investigated and related to previous results on normal plastic anisotropy (r-value). The various annealing treatments produce two effects on the microstructure: grain coarsening and slight weakening of the texture. In addition, these treatments produce a noticeable decrease in strain rate sensitivity and an increase of work hardening rate that is related to the decrease of the anisotropy. It is concluded that these effects are related to an enhanced contribution of basal slip as a consequence of the microstructural changes induced by the annealing treatments.
Authors: P. Rey, D. Gesto, J.A. del Valle, D. Verdera, Oscar A. Ruano
Abstract: Friction Stir Processing (FSP) has attracted much interest as a tool for refining grain size and achieving high angle boundary misorientation in magnesium alloys. These characteristics have a great influence in key engineered properties such as strength and ductility, which could be markedly improved by means of this technique. The main objective of this work is to study the microstructural modifications produced when FSP is applied to homogenized cast AZ91 and wrought AZ61 magnesium alloys. Several attempts were made for achieving a homogenous microstructure without defects and enhancing the refinement of the grain size in the stir zone. It was revealed that is of great importance to break the initial microstructure, of coarse grains unfavourably oriented for deformation, in order to facilitate the process, particularly in the case of cast AZ91 alloy. It is highlighted that, after breaking up the initial microstructure, is possible to process the material, in subsequent passes, Furthermore, the use of different backing materials as heat sink and a previous heating treatment of the sample were evaluated. Changing the backing plate can improve more the reduction of the grain size during a second pass. Using a copper plate instead of a steel one can promote a refinement up to 700 nm in AZ91 and 1 μm in AZ61. A coolant agent can be used for inhibiting the grain growth causing a little more reduction of the grain size.
Authors: A. Fernández-Vicente, Manuel Carsí, Félix Peñalba, José A. Jiménez, Oscar A. Ruano
Authors: J.A. del Valle, P. Rey, D. Gesto, D. Verdera, Oscar A. Ruano
Abstract: The effect of friction stir processing (FSP), on the microstructure and mechanical properties of a magnesium alloy AZ61 has been analyzed. This is a widely used wrought magnesium alloy provided in the form of rolled and annealed sheets with a grain size of 45 μm. The FSP was performed with an adequate cooling device in order to increase the heat extraction and reduce the processing temperature. The final microstructure showed a noticeable grain size refinement down to values close to 1.8 μm and an important change in texture. The change in texture favors basal slip during tensile testing leading to an increase of ductility and a decrease in yield stress. The stability of the grain size and the creep behavior at high temperatures were investigated. The optimum conditions for superplastic forming were determined; however, the presence of a large amount of cavities precludes the achievement of high superplastic elongations. Additionally, these results are compared with those obtained by severe hot rolling.
Authors: Ignacio Rieiro, Manuel Carsí, Oscar A. Ruano
Abstract: A precise description of the hot deformation behavior as well as determination of the stability conditions as influenced by temperature and strain rate is fundamental for the simulation of metal forming processes. In this work, a revision of various stability criteria of magnesium alloy WE54 is conducted. The study corresponds to own work and that of Lentz et al. and is based on compression tests at high temperature and high strain rates. Stability and processing maps were obtained using a variety of stability criteria, some based on the efficiency parameter η and others on the strain rate sensitivity parameter, m. This parameter is usually determined by fitting the curves strain rate, ε, versus stress, σ, by means of a potential equation named “power law” or by a polynomial of second or third degree, and calculating the slope of the logarithmic curve at each point using successive derivatives. This procedure is compared with one developed by us where all experimental points are fitted to a single hyperbolic sine equation of Garofalo type and then m and η are calculated for each ε and T using this equation. The maps obtained by one or the other method differ considerably. The predictions of these maps were contrasted with microstructural observations and conclusions on the deformation behavior of the alloy are reached.
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