Jean-Christophe Pierret

Conception of tooling adapted to thixoforging of high solid fraction hot-crack-sensitive aluminium alloys

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.7<ϕs<1), which involves the processing of alloys in the semi-solid state. Tooling has to be adapted to this particular process to benefit shear thinning and thixotropic behaviour of such semi-solid material. Tooling parameters, such as the forming speed and tool temperature, have to be accurately controlled because of their influence on thermal exchanges between material flow and tool. These thermal exchanges influence the high-cracking tendency and the rheology of the semi-solid material during forming, which affects parts properties and therefore their quality. Extrusion tests show how thermal exchanges influence quality of thixoforged parts made of 7075 aluminium alloys at high solid fraction by modifying process parameters like forming speed, tool temperature and tool thermal protector. Thus an optimum in terms of thermal exchanges has to be found between surface quality and mechanical properties of the part. A direct application is the evaluation of surface quality of thixoforged thin wall parts made of 7075 aluminium alloy.

Improvement in Thixoforging of 7075 Aluminium Alloys at High Solid Fraction

Thixoforging is a type of semi-solid metal processing at high solid fraction (0.5<fs<1). 7075 aluminium alloys have been used as a feedstock for thixoforging in order to investigate thixoformability of a high performance aluminium alloy at high solid fraction. Higher solid fraction of 7075 alloy is less sensitive to temperature, avoids metal splash at high speed and allows laminar flow at high speed. Hot tool combined with lubricant tool coating are used to slow down the solidification rate of the high solid fraction metal by decreasing thermal exchanges with the tool. Improved thermal and forming parameters [1-2] will be applied to produce an automotive component by thixoforging and mechanical properties have been measured from tensile samples. High mechanical properties are obtained after T6 thermal treatment.

Thixoextrusion of A357 aluminium alloy

The aim of this work is to attempt the application of A357 Al-Si-Mg cast alloy in the thixoextrusion process, evaluating the different forming parameters effect. Thixoextrusion offers several advantages compared with traditional hot-extrusion such as lower pressure, minor friction forces, higher material fluidity and longer tool life. This type of semi-solid process requires high solid fraction (0.7 0.85) is less sensitive to temperature drops and allows more stable material flow at higher speeds. To know the forming parameters that improve the mechanical properties of thixoextruded parts, some extrusion tests were carried out. The results were used as the main criteria for assessing the thixoforming viability of the A357 alloy.

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.

Stability of steel thixoforming process

Abstract To improve the industrialization of the process, the study of a thixoforming line stability was proposed. The thixoforming line is fully automated to optimize the repeatability of the experiments. Parameters of the heating cycle, the slug temperature, the tool temperature and the forming speed were studied. For each of them, a range of the expected variations in a steady-state process as well as the effects of these variations on the process itself (forming load and parts quality) were given. These variations are shown to be acceptable. Three different tools were used in the experiments. Some mathematical simulations were realized on the finite elements code Forge2008(c) with a semi-solid constitutive law. The capacity of the model to represent the process stability was discussed. The simulation results are in agreement with the experiment results.

X38CrMoV5 hot-work tool steel as tool material for thixoforging of steel: Numerical and experimental evaluation

Abstract Although thixoforming of low melting point alloys as aluminum or magnesium is now an industrial reality, thixoforming of high melting point alloys as steel is still at the research level. High working temperature, die wearing and production rate are problems that must be solved and are under investigation. The aim of this work is to evaluate the thermal and mechanical loadings applied to the tools during the steel thixoforging process in order to determine whether classical hot-work tool steel can be an appropriate tool material. This evaluation has been realized thanks to experimental trials and to simulations on the finite elements code Forge2008©. The effect of the loadings on the tools failure modes are highlighted and compared with the ones observed in classical hot forging. Beyond this, the failure modes of hot-work tool steel, the X38CrMoV5 or H11, were presented.

Characterization of thixoforming process of 100Cr6 steel

The main goal of this work is to analyze the semi-industrial process of steel thixoforming. The process was carried out using industrial equipment. This equipment consists of a heating device, industrial robots and a hydraulic press. The globular microstructure ensuring thixotropic properties was obtained using the SIMA method. It is one of the simplest and cheapest methods which could be easy applied in the case of steel alloys. In this work, the hot forged rods, commercially produced from 100Cr6 steel, were used. The first part of the work concerned the determination of the proper temperature range, for thixoforming of 100Cr6 steel. Next, some heating tests were carried out in order to obtain as uniform temperature distribution as possible. Heating process was executed using inductive heating. Microstructure analysis of heated samples reveals globular particles surrounded by liquid phase. At last, the thixoforming process was carried out using closed-die forming technique. Completely filled die cavity and good microstructure of the part show that applied process parameters were properly selected.

Robustness of a Thixoforming Production Line

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.

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 %.