Papers by Author: Marc Robelet

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: The deformation process of steel in the semi-solid state, or thixoforming is an alternative solution to the classical industrial processes that are the casting or the hot forging. Thixoformed steel parts are of better quality and may substitute advantageously parts realized in cast iron leading to an improvement of the rigidity, for example, but also parts in aluminium alloys obtained by the main deformation processes: for lower volume, thixoformed steel parts, keeping the same functionality, present superior mechanical properties than the parts realized in aluminium alloys. The industrial development of the steel thixoforming must be accompanied by the knowledge and the various parameters control of the process: steel grades definition and elaboration, the homogeneous high-temperature heating of the slug before deformation, conception or re-conception of the part to adapt it to the process and the practice properties, the equipment definitions which are used for the deformation operation at the semi-solid state. The handling system is also important in the thixoforming process; it is going to allow the slug transfer, at the semi-solid state, between the heating zone and the tooling or to evacuate the thixoformed part towards the mean assuring the quality heat treatment of the part. The means choice which are industrially going to ensure the process have a big importance to obtain successful production line and so, a cost reduction by “finished” part competitive with regard to processes used classically today. After the description of the applicable key points to the main parameters and the solutions used to obtain a successful product: high mechanical characteristics, near net shape or net shape geometry, no flash and limited machining operation, ... are described , The investigations finished with an approach comparing the part cost realised by thixoforming process with regard to classical processes.

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: Thixoforming of steel offers the advantages of casting technology in combination with high mechanical strength that can only be achieved by forging. The progress in establishing this technology in industry depends on the success in the development of suitable steel grades. Recent investigations dealt with the development of steel grades that are especially adapted to the thixoforming process. For this, alloys were developed with a lower solidus temperature and a wider process temperature range compared to classic forging steels. In consequence, the inductive heating process is more tolerant to inaccuracies and for a given liquid fraction the process temperature window is easier to handle. It is desired to obtain great degrees of deformation at rather low forming forces as these parameters determine the size of the needed presses. This behaviour is affected by the present liquid fraction in the slug and the heat transfer between work piece and die. It was detected that variations of the forming force have a direct influence on the quality of the thixoformed parts. In order to make the thixoforming technology of steels competitive versus other forming technologies, the parts must show a favourable microstructure and thus, good in-use properties. In this paper various solutions are compared. The main results obtained in the optimization research, namely, the steel grades adapted to semi-solid forming, the resulting process parameters and the mechanical properties of thixoforming parts will be presented for two exemplary steel grades. By producing a real automotive part, thixoforging of steels with regard to the adapted materials and to the ongoing industrial implementation of this process is proved.

Abstract: In recent years several attempts were made to transfer the thixoforming technology of steel parts into industrial applicable processes. This paper gives an overview about the progress of a European consortium that established a fully automated thixoforming process for the series production of automotive steel parts. Due to the multi-faceted nature of this technology, problems concerning the development of suitable steel grades and tool materials as well as the development and application of an inductive heating system, a handling unit and of a complex forming tool had to be solved. Besides the development of adapted steel grades and the inductive heating, the handling of the semi solid billets plays a special role because during the manipulation of the parts from the heating station into the tool a loss of heat is unavoidable. Furthermore, scaling of the parts must be prevented. By means of a fully automated process line existing constraints were reduced and the forming process is kept reproducible. Improved silicon nitride composites have been developed as a tool material, which show good mechanical properties in combination with an acceptable chemical stability at the occurring process temperatures as well as in contact with semi solid steel. Basing on the practical experience a comparison of the thixoforming technology to existing processes and an outlook for the future are given.

Abstract: This paper focuses on the liquid fraction curves of several steels and the correlation between liquid fraction, temperature and heating rate. The work has been performed along two main axes. First, the solid fraction versus temperature has been obtained experimentally by differential scanning calorimetry (DSC), limited to low heating rates. Then, a shift of the liquid fraction curves has been noticed at high industrial heating rates. The quantification of this effect could not be carried out by DSC and required the elaboration of another experimental device.