Papers by Author: Ahmed Rassili

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: Up to now, no technology exists for producing as-cast thixotropic steel at the industrial or pilot level. The existing processes need to reheat a billet that has been previously cast and wrought on separate equipment. The new processing route, we are developing, will allow producing as-cast steel (or high melting point alloy) with thixotropic properties. CRM has developed and patented a continuous casting technology, which by using a specially designed nozzle (HJN nozzle-hollow jet nozzle), allows to combine powder injection and a low superheat. It has been proven by pilot and industrial trials that the combination of iron powder injection and a low superheat leads to achieve a globular solidification structure in the core of a continuously cast steel billet. Several industrial trials on continuous caster have been carried out both with carbon and stainless steel but also on copper in order to modify the solidification structure of these materials. The application of this HJN technology is being studied in order to achieve a globular structure on as-cast steel with the objective to obtain a thixotropic behaviour of the material after an adequate reheating. Casting trials have been performed on CRM continuous casting pilot-line. Specifically designed equipment has been built for use in a steel foundry. The influence of the solidification rate, the superheat, the amount and the composition of the powder has been studied.

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: The development of the thixoforming process made great progress since its beginning. But whereas thixoforming of aluminium and other low melting point alloys is now an industrial reality, thixoforming of high melting point alloys, as steel, is still at the research level. High temperature issues, die wearing and production rate are problems that must be solved and are more and more investigated. This work is a study of the robustness of a complete thixoforming unit dealing with steel. This flexible unit allows parts of various shapes be formed in conditions close to industrial ones. The study focus on the heating strategy, heat exchanges between the slug and the tool, the feeding and the cycle time. In this work, modelling has been used to simulate heating and feeding. In parallel, experiments have been made to obtain the needed parameters, to validate the modelling and to evaluate the capacities of such a line.

Abstract: Inductive re-heating of billets is the state-of-the-art method to enable the desired liquid fraction for thixoforming. In laboratory experiments the temperature might be directly controlled since it is usually possible to apply thermocouples for temperature measurements. Due to technical restrictions temperature measurements especially in the interior of a billet are difficult in industrial heating processes. For this reason, thixoforming billets are heated without temperature monitoring by using proven heating functions. Such functions are usually obtained by trial-and-error or on the basis of FEA simulations. In both cases exhaustive experimental work is usually necessary. Moreover, FEA simulations require thermo-physical material data that is difficult to obtain for industrial relevant heating processes. In this paper an alternative method of experimentally obtaining optimized heating functions by using a fuzzy logic controller is introduced. Under steady-state environmental conditions this method allows a quick and reproducible re-heating. The main advantages of the fuzzy logic based approach are that no specific material data is required, system specific properties such as the efficiency have not be explicitly determined and that the practical implementation could be realized with a minimum of experimental work.

Abstract: Thixoforging is a type of semi-solid metal processing at high solid fraction (0.5

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