Eliane Giraud

The strain rate sensitivity and constitutive equations including damage for the superplastic behaviour of 7xxx aluminium alloys

Superplasticity is a characteristic of certain materials, in particular aluminium alloys, whereby very large deformations (up to 1000 %) can be obtained before fracture under certain conditions. Superplastic forming is therefore the process of deforming a flange under these conditions by applying a variable pressure. The final geometry is obtained when the flange takes the form of a die. In order to deform a material superplastically, the temperature of the material should be approximately a half of the absolute melting point of the material and the strain rate (or flow stress) should remain within a certain range. The most important issues concerning the industrial process are the prediction of the final thickness distribution and the computation of the optimal pressure law to maintain superplastic conditions. Finite element simulations make these predictions possible for industrial components. To ensure the precision of the simulations, it is important to have good knowledge of the material behaviour in...

Experimental and Numerical Study of Single Point Incremental Forming for a Spiral Toolpath Strategy

Incremental sheet forming is a flexible forming process based on the sheet progressive localized deformation where a forming tool follows a trajectory predetermined in advance by CAD/CAM programs. Although it is a slow process compared to conventional processes, this relatively new technique is very adequate for prototyping and small series production since it does not require complex tooling (die, punch, etc.). This paper presents a numerical and an experimental study of the Increment Sheet Forming process for a spiral toolpath. A 30 mm depth conical forming part is considered for this purpose. The experimental forming operation has been conducted on a 3-axis milling machine and the experimental data analysis was performed using a Kistler measurement system and a 3D scanner. Moreover, a Finite Element simulation has been realized in order to predict the evolution of the forming forces and the part final profile. The numerical results were in good agreement with the experimental ones. Moreover, the analysis of the scanning data has successfully restituted the part final profile and the surface details.

Cold forming by stretching of aeronautic sheet metal parts

In this article, the development of an industrial prototype for manufacturing aeronautical fuselage panels is investigated. Deep drawing of large components such as aircraft fuselage panels is not an easy task in terms of dimensional accuracy, reliable material behaviour laws and failure criteria. Hot stretching processes ensure large ductility range of some materials. Nevertheless, when using high-performance aluminium alloys with acceptable low-plastic strain at ambient temperature, cold forming might be employed. A special stretching machine of 40-ton (400 kN) capability was instrumented and piloted in that way. Typical operations involved in the forming of parts are carried out with a die on which the sheet metal is successively stretched and drawn in several steps. Currently, the shape of the forming tool is directly determined from CAD models of the final sheet geometry without taking into account springback or residual effects. To increase the dimensional accuracy of the final components, a methodology to define the die shape and to control the process is proposed, taking into account the parameters influencing the forming operations. A feedback loop based on digitalised physical geometry and numerical simulation is carried out in order to ensure that the final shape of the sheet will be accurately obtained.

Creep forming of an Al-Mg-Li alloy for aeronautic application

Creep forming of Al-Mg-Li alloy sheets is studied. An instrumented bulging machine is used to form a double curvature panel at a reduced scale. The deformation of the work-sheet is ensured by a 7475 aluminum alloy lost sheet deformed by a gas pressure applied on its upper surface. A numerical model using the ABAQUS software is developed in order to obtain the pressure law and to ensure the forming conditions during the cycle. This model is validated by comparing experiments and numerical results in terms of deformed shape and thickness evolution.

Numerical and experimental analysis of cold stretching of aluminium sheets using an instrumented bench

Cold stretching is a forming process meanly used in aeronautic industry to obtain deep drawing parts from thin sheets. It’s not very easy to characterize the process using industrial machines, due to production constraints and complexity of their structures. In this study, an instrumented bench is developed to analyse the forming of double curvature panels in 5754H111 Aluminium alloys. A numerical tool using ABAQUS software is developed to predict the behaviour of thin sheets during the stretching process and also to estimate the residual mechanical field in the formed shapes. The bench is calibrated by comparing experiments and numerical results in terms of deformed shape, in-plane strain levels and thickness evolution.

Plasticity Criterion for Hot Forming of Aluminum-Lithium Alloy

Anisotropic behavior at high temperature of an Aluminum-Lithium alloy was studied. Mechanical tests at a temperature of 350°C and a strain rate of 10-2 s-1 were carried out on samples taken at different angles with respect to the rolling direction of the sheet. Two plasticity criteria (HILL48 and HU2005) were identified and implemented in ABAQUS to predict the anisotropic behavior of the alloy for other angles. Results show that: (i) the alloy exhibits an anisotropic behavior at high temperature and some recrystallization occurs during plastic deformation; (ii) the coefficients of anisotropy depend on strain level and (iii) HU2005 criterion allows describing the behavior of the alloy at high temperature.