Papers by Author: Christophe Desrayaud

Abstract: This paper is devoted to the numerical computation of a steady-state thermo-fluid modeling related to the Friction Stir Welding Process in a two-dimensional cylindrical geometry. It analyzes the efficiency of the implementation on parallel architectures of two finite-difference schemes on a structured grid. The first one applies the Newton-Raphson method to compute a numerical solution of this non-linear elliptic type equation, and uses an iterative sparse solver. The second one is based on a time-marching approach converging to the steady state solution thanks to a time-explicit computation. Their respective performance is presented and discussed. Some numerical simulation results are presented to validate the proposed approach.

Abstract: Subtransus multiaxial hot forging of α+β Ti-6Al-4V (TA6V) titanium alloy with a β-transformed microstructure aims at obtaining an equiaxed microstructure through α phase globularization. The activation of mechanisms involved in microstructural evolution, such as globularization, depends on parameters such as time, temperature, strain and strain rate. It is also sensitive to the crystallographic orientation of α-lamellae. As a result, multiaxial processing of titanium alloys leads to significant microstructural gradients depending on thermomechanical conditions and initial microstructure. In this study, we focused on the effect of complex thermomechanical paths on microstructural evolutions. Thanks to the MaxStrain Gleeble device, we were able to reproduce such thermomechanical treatments to β-transformed TA6V samples. Stress strain fields obtained with finite element modelling of the MaxStrain test were compared to experimental microstructure gradients. This experimental method offers the opportunity to get closer to industrial open die forging conditions.

Abstract: A simple three dimensional thermomechanical model for FSW is used in the present paper. It is developed from the model proposed by Heurtier [1] improved by Jacquin [10] to account for the eulerian cooling flux due to the tool motion during welding. The velocity fields used to describe the bulk flow around the tool are introduced in the particular derivative of the thermal equilibrium equation. The complete thermomechanical history of the material during the process can then be accessed by temperature and strain rate contours.

Abstract: Thanks to their oxide layer, aluminium alloys are remarkable for their ability to resist corrosion. However, in welding, this protective layer acts as a barrier which must be broken in order to succeed in the thermomechanical joining of aluminium. The chosen alloy (6082-T6 or AlSi1MgMn) has been subjected to various deformation path. The first of them consists in the channel-die (plane strain) compression of two cuboids, one above the other. Considering the configuration of the test, the surface size between the two samples rises, so that the fragmentation of the oxide layer creates welding bonds. However, the friction effects in the channel lead to a heterogeneous deformation, so that the contact surface undergoes different behaviors: a microscopic study then shows that the welds appear in areas with significant shear. Channel-die and uniaxial compressions of beveled samples confirm that more significantly than the global deformation, the shear strain is the most active phenomenon for achieving an effective thermomechanical joining. Another approach is the cumulative deformation as a result of a cyclic load: a tube is cut through its section and undergoes both a compression and cyclic torsion load. The contact surface between the two semi-tubes is under a shear behavior and the combination between plastic deformation and local heating leads to a fragmentation of the oxide layer: all this factors allow the thermomechanical joining of aluminium alloys.

Abstract: The thermo-mechanical simulation of Friction Stir Welding focuses the interest of the welding scientific and technical community. However, literature reporting material flow modeling is rather poor. The present work is based on the model developed by Heurtier [2004] and aims at improving this thermo-fluid simulation developed by means of fluid mechanics numerical and analytical velocity fields combined together. These various velocity fields are investigated separately and especially according to the power dissipated during the flow. Boundary conditions are considered through a new approach based on the kinematic analysis of the thread of the pin. An equilibrium is established between the vertical motion of the bulk material dragged in the depth of the metal sheet, and its partial circulation around the pin. The analyses of the obtained velocity fields enable the understanding of the welded zone asymmetry and highlights the bulk material mixing between the welded coupons in the depth of the sheet. A regression is performed on the relative sliding velocity of the aluminium according to the surface of the tool: shoulder and pin. Two dimension flow lines in the depth of the metal sheet are then obtained and successfully compared with the results obtained by Colegrove (2004) [1].

Abstract: The aim of this work is to study the flow instabilities occurring during hot forging of titanium alloy blades. In this view, the viscoplastic deformation behaviour of Ti-6Al-4V alloy is investigated by means of torsion tests under isothermal hot working conditions at temperatures ranging from 800 to 1020 °C and strain rates of 0.01, 0.1 and 1s−1. The thermomechanical processing is performed up to a true strain of 10. The flow stress data are analysed in terms of strain rate and temperature sensitivities. A constitutive equation that relates not only the dependence of the flow stress on strain, strain rate and temperature, but also for the fraction of each phase α and β is proposed. Two mechanical models are compared : the uniform strain rate model (Taylor) and the uniform plastic energy model (IsoW). The usual strain rate sensitivity and activation energy values of Ti-6Al-4V alloy are obtained by fitting the experimental data. Furthermore, specific values of strain rate sensitivities and activation energies are calculated for the α and β phases providing thus a constitutive law based on the physics of the α / β phase diagram. The flow stress is then related to strain by an empirical equation taking into account the flow softening observed after a true strain of 0.5 and the steady state flow reached after a true strain of 4. Comparison of the calculated and measured flow stresses shows that the constitutive equation predicts the experimental results with a reasonable accuracy. The above constitutive equation is then used for simulating forging processes by the finite element method. The calculations exhibit the localisation of deformation produced by shearing effects in the form of the classical X shape.

Abstract: The present work deals with the influence of niobium in solid solution on the dynamic recrystallization of pure nickel. High-purity nickel and two model nickel-niobium alloys were deformed to large strains via torsion at temperatures between 800 and 1000°C. Niobium additions considerably increased the flow stress, while they lowered the strain-rate sensitivity and increased the apparent activation energy. EBSD of the steady-state microstructures revealed strong grain refinement. Substructure development was favored, whereas thermal twinning was reduced by niobium. More generally, discontinuous recrystallization kinetics were considerably decreased.

Abstract: Thermomechanical processing involving severe plastic deformation (SPD) is a popular approach to ultrafine grain formation in bulk samples. In the present study, two grades of highpurity α-iron were deformed within the ferritic domain in cold and warm torsion to large strains (>> 1). Examination of the deformed samples using orientation imaging microscopy revealed a highly fragmented, lamellar structure aligned almost parallel to the direction of shear. Between 37 and 54 % of boundaries detected are high angle ones (HAB). Some of these HAB are associated with the original grain boundaries. However, a good number are believed to originate from dislocation accumulation processes, during which the misorientation angle across certain strain-induced low angle boundaries (LAB) rises with increasing strain. The resultant microstructure is composed of ultrafine crystallites on the order of 1 – 2 1m. In fact, localised regions of equiaxed grains on the micron scale were observed within samples deformed between room temperature and 300 °C. Nonetheless, other areas remain relatively unfragmented despite persistent straining until failure. At higher temperatures, the microstructure is more homogeneous, but the average grain size is coarsened. Optimal grain refinement thus appears to be a compromise between several competing factors: large strains at relatively low temperatures for high dislocation density, higher temperatures to enable sufficient dynamic recovery, and low grain boundary mobility that is aided by low temperatures and/or pinning by solute atoms or second phase particles. Furthermore, the development of a torsion texture composed of a single ideal orientation at large strains is unfavourable towards the generation of HAB.

Abstract: Multiaxial compression (MAC) is a severe plastic deformation (SPD) method that allows sequential uniaxial compression of prismatic samples to relatively large cumulative strains. The technique involves a change in loading direction (x to y to z to x…) between successive compression passes. A high-purity α-iron containing 60 mass ppm C was thus strained using passes of ε ∼ 0.4 at room temperature (0.16 Tm) and 450 °C (0.40 Tm) to total ε ranging from 1.4 to 2.9. Both optical and electron microscopy were used to characterise the deformed microstructures. Fragmentation of the initial grain structure occurs mainly in the form of a dense, homogeneous network of low angle boundaries (LAB) delimiting subgrains of about 1 3m. The original grains are easily distinguishable and maintain a relatively equiaxed appearance even at larger strains. At room temperature, high angle boundaries (HAB) are observed within some of the initial grains, and not necessarily close to the grain boundaries. These HAB may be open or closed, and tend to align themselves at approximately 45° to the orthogonal axes, suggesting the presence of microshear bands and thus a heterogeneous deformation. Such bands of localised strain criss-cross as a result of different slip systems being activated from one pass to another. When the temperature is increased to 450 °C, grain boundary migration becomes significant owing to the lack of impurities that could otherwise provide a pinning effect. The resultant subgrain structure is coarsened to about 4 3m. Besides, the enhancement of recovery at higher temperatures also appears to discourage the generation of HAB by dislocation accumulation processes.