C. Levaillant

Thermomechanics of the cooling stage in casting processes: Three-dimensional finite element analysis and experimental validation

A thermomechanical three-dimensional (3-D) finite element analysis of solidification is presented. The heat transfer model is based on a multidomain analysis accounting for noncoincident meshes for the cast part and the different mold components. In each subdomain, a preconditioned conjugate gradient solver is used. The mechanical analysis assumes the mold is rigid. A thermoelastic-viscoplastic rheological model is used to compute the constrained shrinkage of the part, resulting in an effective local air gap width computation. At each time increment, a weak coupling of the heat transfer and mechanical analyses is performed. Comparisons of experimental measurements and model predictions are given in the case of a hollow cylindrical aluminum alloy part, showing a good quantitative agreement. An application to an industrial aluminum casting is presented, illustrating the practical interest of thermomechanical computations in solidification analysis.

Determination of precipitate strength in aluminium alloy 6056-T6 from transmission electron microscopy in situ straining data

Abstract The transmission electron microscopy in-situ straining technique is employed to measure the breaking angles of strengthening precipitates in aluminium alloy 6056-T6 as they are sheared by dislocations. The experimental determination of the character of bowed dislocation segments when dislocations are pinned on precipitates allows us to calculate the corresponding line tensions. From this, the maximum forces F m that precipitates can sustain before being sheared by dislocations are deduced. It is suggested that F m may be regarded as a quantitative parameter which includes the effects of the various strengthening mechanisms operative in precipitation-hardened alloys. An attempt is made to relate the maximum force calculated from in-situ straining data to the macroscopic yield strength of the material.

Transmission electron microscopy study of precipitate morphology and precipitate overcoming processes in aluminum alloy 6056 T6

High resolution electron microscopy and in situ straining experiments are performed on aluminum alloy 6056 T6 to charaterize the morphology of precipitates and the processes of precipitate overcoming by the dislocations. Two types of precipitates (needles and laths) are identified. They are found to be sheared by the dislocations and the maximum force precipitates can sustain before being sheared is calculated.

Numerical simulation of failure prediction for ceramic tools: comparison with forging test results

A fracture prediction criterion for brittle materials has been introduced in the POLLUX finite-element code in order to predict the risk-of-rupture of ceramic tools during a forging operation. POLLUX is a software dedicated to the simulation of forging operations, initially developed by INSA (Lyon). The chosen probabilistic fracture model is based on the weakest-link theory and the statistical theory of Weibull. A surface approach or a volume approach can be retained on the basis of the type of critical flaws in the ceramic. Two different criteria are available in order to characterise the stress state, considering the tensile normal stresses and neglecting the compressive stresses. An identification procedure of the critical flaw type is presented for a particular ceramic material. Statistical parameters of ceramic fracture have been determined experimentally using bending tests performed under environmental conditions close to those of forging. A constitutive equation of the workpiece material has been proposed, issued from torsion tests. In order to validate the model in the case of ceramic tools subjected to multi-axial stress states, a particular configuration has been defined to compare the simulation predictions with the experimental results. A forging test has then been developed, in which a billet of superalloy is formed in a ceramic tool up to its fracture at the temperature of 1423 K. The experimental distribution of tool fracture, according to the strain of the billet, is in good agreement with fracture predictions computed by the simulation.

Failure prediction for ceramic dies in the hot-forging process using FEM simulation

A fracture prediction criterion for brittle materials has been introduced in the POLLUX finite-element code, in order to predict the risk of rupture of ceramic tools during a forging operation. The POLLUX code, developed by INSA (Lyon) especially to simulate forging operations, is presented. The fracture model is based on the weakest link theory and Weibull analysis. Two different criteria were chosen in order to characterise the stress state, considering the tensile normal stresses. Comparison between the simulation results and the analytical calculations, in a simple compression case, enables the validation of the numerical model. Applications are presented, in which the design of ceramic forging tools is realized using the failure prediction software. A run-strategy of the program is proposed in order to improve the design of the forging tools.

A new tensile test on notched specimens to assess the forming limit diagram of sheet metals

Abstract The assessment of the Forming Limit Diagram (FLD) of the sheet metals used in automotive stamping requires several testing geometries including stamping tests for example like ERICHSEN or NAKAZIMA tests. These tests involve many parameters for material behaviour and friction conditions. This paper aims at determining the FLD through testing procedures that avoid any friction problem. Moreover it intends to use only in plane stress enabling an easy numerical simulation by the Finite Element Method. This planar configuration is also very convenient for the continuous monitoring of strain field during the tests.