Papers by Author: Víctor H. Jacobo

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Authors: Rafael Schouwenaars, Víctor H. Jacobo, Armando Ortiz
Abstract: In asymmetric roll bonding, two metal sheets with dissimilar properties are subject to a rolling step to create a bimetal strip. The paper reviews earlier work in this field which generally uses the upper-bound or finite element methods. The use of the slab method appears simpler than the former, but allows identifying critical aspects of process and model which were not addressed before. To obtain a complete set of equations, the bonding problem must be analysed not only from the cinematic viewpoint, but also from a mechanical one. After defining mechanical equilibrium, one additional cinematic equation is needed to solve the problem, which is obtained by minimising the plastic work in the zone where both sheets deform plastically but are not yet bonded. The new model has the advantage over earlier approaches in that it not only predicts the final thickness of both sheets but also permits to determine whether the bonding is possible under given process conditions. Keywords: Roll Bonding, Cladding, Modelling, Adhesion, Slab method.
Authors: Rafael Schouwenaars, Víctor H. Jacobo, Sara M. Cerrud, Armando Ortiz
Abstract: Journal bearings are graded systems with a metal-metal composite as the functional layer. Estimation of the microscale stress distributions is used to analyse the interaction between microstructure, material properties and damage mechanisms during wear. The analysis is executed by means of simple plane-strain finite element models mimicking experimentally observed microstructures. It is found that under realistic macrosstress conditions no tensile microstresses are induced in the triboalloy and that plastic flow is inhibited by the graded structure.
Authors: Agustín Bravo, Rafael Schouwenaars, Armando Ortiz, Víctor H. Jacobo
Abstract: Severe plastic deformation processes are used to refine grain structure by means of very high deformations. Equivalent Von-Mises strains in the range of 5.06 to 11.57 were obtained using the accumulative roll-bonding (ARB) process. The effect of the initial microstructure is analyzed. Microstructure, microhardness and tensile strength of the alloy, both starting with previously cold-rolled material and after a 30 minute, 300 °C heat treatment, are compared. The effect of a heat treatment after 5 steps of ARB ( = 8.11) is also presented. Results show that even though both the microhardness and tensile strength go up with increasing deformation, they are not related to each other by a factor of 3. The highest values of these parameters depend more on the initial microstructure than on the total deformation. A better distribution of the second phase and an increase in solid solubility are also observed after ARB.
Authors: Rafael Schouwenaars, Hugo A. Duran, Víctor H. Jacobo, Armando Ortiz
Abstract: Al-Sn alloys for tribological applications are industrially important alloys which have attracted little attention over their history. Being cold rolled directly from thin cast slabs and consisting of two ductile phases, their processing and physical behaviour are distinct from classical Al-Alloys. During cold rolling, the coarse-grained, random texture of the slab is transformed into the classical rolling texture of a fine-grained Al-alloy, with elongated Al-grains delimited by thin Sn-ribbons. During annealing at 300°C, the interior of the Al-grains recrystallises rapidly while the liquid Sn-phase migrates toward Al-grain triple lines to form a reticular structure. A weak texture, dominated by Goss and P is produced. Grain growth within the original cold-rolled grains is fast, but once the recrystallised grain size reaches the length scale of the second-phase distribution, it slows down and both phases coarsen simultaneously, accompanied by a significant texture change.
Authors: Rafael Schouwenaars, J.I. Romero, Víctor H. Jacobo, Armando Ortiz
Abstract: Rapidly solidified industrially stripcast Pb-Sb-Sn triboalloys showed microstructures typical for the eutectic decomposition of metallic glasses. Evidence was found of crystallisation above the glass transition temperature and ageing at room temperature. The observation supports a previous hypothesis by the authors on the wear properties of soft tribological alloys.
Authors: Agustín Bravo, Rafael Schouwenaars, Víctor H. Jacobo, Armando Ortiz
Abstract: Mg-containing bronzes have received little attention in general technological applications due to their relatively complex processing conditions. However, Mg is one of the few metallic elements which may exhibit good tribological compatibility with iron and steel and as such is a possible candidate to replace lead in sliding bearing materials. This work describes the casting of such alloys in the form of thin ingots to produce cold rolled strip, as is done for commercial Al-Sn-based ductile triboalloys. Sound ingots could be produced with simple laboratory equipment, yielding slabs in the compositional range of Cu1Mg1Sn, Cu1Mg5Sn, Cu5Mg1Sn and Cu5Mg5Sn. Cooling curves were monitored by embedded thermocouples. Invariant points could be identified after appropriate filtering of the signal but did not correspond to the ternary equilibrium. Segregation and non-equilibrium phases were confirmed by metallography.
Authors: Rafael Schouwenaars, J.A. Torres, Víctor H. Jacobo, Armando Ortiz
Abstract: A 95% cold-rolled Al-20%Sn-1%Cu alloy was heat-treated in a range of temperatures and times to investigate the evolution of mechanical properties and microstructure. The most interesting combination of properties can be achieved between 300 and 400°C in a process that is simpler than what is used in industry. The physical phenomena which are active during the heat treatment are precipitation, recovery, recrystallisation and change in phase distribution due to surface tension. The former two predominate at low temperatures, while the latter achieve faster kinetics at higher temperatures, where recrystallisation is coupled to the change in morphology of the contiguous Sn-phase.
Authors: Rafael Schouwenaars, Víctor H. Jacobo, Sara M. Cerrud, Armando Ortiz
Authors: Rafael Schouwenaars, Víctor H. Jacobo, Sara M. Cerrud, Armando Ortiz
Abstract: Soft anti-friction alloys are metal-metal composites used in journal bearings as a functionally graded system. The physical processes occurring during severe wear of these products are analysed and placed within the framework of theories on tribolayer formation. A simplified analysis of the thermodynamics of the phenomena permits analysing the anti-friction properties of the alloys in terms of the activity coefficients of their constituents and indicates that during wear, a wear-resistant contact layer is formed which retards further damage.
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