Papers by Author: Ignacio Rieiro

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.

Abstract: The improvement of optimization numerical methods for constitutive equations has been the first aim of this research. A subordinate optimization algorithm, based on Newton method, through “ad hoc” assessment, has been developed. Application to hot torsion data of AISI4145 with two grain sizes has been carried out. It is concluded that the Newton method is an excellent algorithm for the optimization of strain dependent constitutive equations. Two models are presented as alternative to the generalized Garofalo model: the normalized stress exponent model (NSE) and the Generalized Sherby model (SG). The NSE model is the most precise to restitute the experimental stress-strain curves.

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.

Abstract: A model for describing the plastic flow has been developed. The model is based on a strain dependent Garofalo equation and predicts the variation with strain of grain size refinement by dynamic recrystallization using non-linear optimization methods. The predictions have been applied to two wrought magnesium alloys, AZ31 and AZ61 and are in good agreement with experimental data.