Pierre Cezard

Steel Thixoforging: Heat Exchange Impact on the Mechanical and Metallurgical Features of Thixoforged Samples

Steel thixoforging process combines the advantages of performing parts having highly complex shapes and good mechanical properties. These advantages result from the shear thinning flow behaviour of semi-solids due to microstructure changes during the material deformation. Many parameters such as steel grade, raw speed, slug temperature, tool temperature influence the steel thixoforging process. In this work, we are interested in analysing the influence of thermal conditions as well as ram speed on the flow behaviour and the microstructure of the final parts. To do so, thixo-extrusion tests on a high speed hydraulic press were performed on C38 steels using different ram speeds and different temperatures for the tools. The temperature is measured in the die close to the tool-material interface during the whole process. Correlations between the temperature measurements, the flow profile and the microstructure are discussed. Using the Finite Element code Forge2005, these thixo-extrusion tests are simulated. Using an inverse method, some parameters used in the thermal exchange modelling are identified by successive comparisons between calculated and measured temperatures within the die.

Thixoforming of Steel: New Tools Conception to Analyse Thermal Exchanges and Strain Rate Effects

Through different papers, authors shown that the influence of thermal exchanges was a first order parameter on the semi‐solid steel behaviour, and certainly for every semi‐solid metallic materials. These thermal exchanges hide other parameters effect like, for example, the strain rate influence. This paper tries to determine the influence of these two parameters by using a new extrusion device on a hydraulic press. This new tools conception annihilated the influence of the decrease of the punch speed before stopping and permitted to have a constant speed during the experiment. This work also deals with the homogeneous flow during thixoforming of steel and shows the importance to couple initial temperature of the slug with punch speed. This paper presents different conditions which permitted to have a homogeneous flow by keeping a low load.

Thixoforming of Steel: Experiments on Thermal Effects

This paper presents experimental results on steel thixoforming. The influence of thermal exchanges with tools and environment on the semi-solid response is analysed.

Finite element simulations of thixoforging by using a micro‐macro constitutive equation and comparison with experiments

This paper presents different finite element simulations of thixoforging realised on Forge2© software. Compression and axial extrusion tests are particularly studied. The constitutive equation used for these simulations is based on a micro‐macro approach accounting for the semi‐solid microstructural evolution. The constitutive equation parameters are identified thanks to successive comparisons of the simulated load‐displacement responses with experimental ones for compression tests on steel semi‐solid at different temperatures. The relevance of the modelling is then analysed through extrusion tests. It is shown that the constitutive equation developed by the authors provides simulated results closer to the experimental ones than the classical Norton‐Hoff law. It demonstrates the capacity of prediction of such a modelling. Moreover, these simulations give a lot of information concerning the role of parameters such as solid fraction, initial solid skeleton degree of agglomeration, thermal exchanges, or stra...

Influence of the Pre and Post Treatment Operations on the Properties of the Thixoformed Steel Parts

The pre and post treatment of the thixoformed parts in terms of heating conditions and controlled cooling has a great influence on the final microstructure and the properties of the parts and their homogenization. The paper presents recent investigations on metallographic analysis and hardness measurements on as thixoformed parts and post treated ones. For these tests, different steel grades are investigated and the liquid fraction of the slug before deformation is evaluated to 30 %.

Thixoforming of Steel - Influence of thermal parameters

This paper presents experimental results on steel thixoforming. The influence of thermal effects on the semi-solid response is analysed through the influence of thermal exchanges with tools and environment. Several rheological experiments such as compression, extrusion or radial filling test were developed to understand the semi-solid steel behaviour and determine the parameters that have a major influence on thixoforming. Actually, in our experiments, the temperature of the slug and consequently the solid fraction were found first order parameters while; the morphology of the solid phase plays a minor role.

Simulation of thixoforming: comparison of different constitutive equations

The behaviour of the semi-solid material is complex to simulate and many approaches are proposed in the literature to represent it. In this work, different constitutive equations are studied: the classical Northon-Hoff law, Joly and Merhabian’s one [1] and an innovative micromechanical model using homogenisation technique [2]. These constitutive equations have been implemented in the FE code Forge2® to simulate simple forming tests, such as compression and direct extrusion. The load-displacement curves using the three constitutive equations are compared in order to estimate their capabilities.