Papers by Author: Pierre Cezard

Abstract: 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.

Abstract: 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 %.

Abstract: Since the first research works in the end of 1980s on the semi-solid forming of steel, this process has presented a great interest and a real industrial potential. Several research teams, all over the world, have shown the feasibility of such a process. Working on the parameters which have an influence on the process, they pointed out the "technical locks" which must be overcome to allow industrialization of the process. A first and perhaps most important difficulty is the reliability of the forming tools in an industrial production context. Much work has therefore been devoted to identify ways to increase tools life. A second important point is the possibility to obtain sound microstructure and satisfactory mechanical properties. This paper is a state of the art review on the subject of the thixoforming of steel, restricted to forming of semi-solid reheated steel. Semi-solid forming process carried out after partial solidification are therefore not covered. The reader interested in such processes may refer to the review recently published by Hirt et al. [1]. The present review considers, in turn, the different steps of an hypothetical production line and their particular challenges, from the raw material to the final product.

Abstract: 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.

Abstract: 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.