Impact of experimental conditions on material response during forming of steel in semi-solid state

Semi-solid forming is an effective near-net-shape forming process to produce components with complex geometry and in fewer forming steps. It benefits from the complex thixotropic behaviour of semi-solids. However, the consequences of such behaviour on the flow during thixoforming, is still neither completely characterized and nor fully understood, especially for high melting point alloys. The study described in this paper investigates thixoextrusion for C38 low carbon steel material using dies at temperatures much lower than the slug temperature. Four different process parameters were studied: the initial slug temperature, the die temperature, the ram speed and the presence of a ceramic layer at the tool/material interface. The extruded parts were found to have an exact shape and a good surface state only if the temperature was below a certain value. This critical temperature is not an intrinsic material property since its value depends on die temperature and the presence of the Ceraspray©layer. Two kinds of flow were highlighted: a homogeneous flow controlled by the behaviour of the solid skeleton characterized by a positive strain rate sensitivity, and a non homogeneous flow (macro liquid/solid phase separation) dominated by the flow of the free liquid. With decreasing ram speed, heat losses increase so that the overall consistency of the material improves, leading to apparent negative strain rate sensitivity. Finally, some ways to optimise thixoforming are proposed

Thixoforging tools materials: determination of appropriate features and experimental evaluation

Whereas thixoforming of low melting point alloys as aluminium 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 if classical hot-work tool steel could be an appropriate tool material. This evaluation has been realized thanks to experimental trials and to simulations on the finite elements code Forge2009©. The effect of the loadings on the tool’s failure modes are highlighted and compared to the ones observed in classical hot forging. Beyond this, the failure modes of hot-work tool steel, the X38CrMoV5 or H11, is presented

Tooling materials and solutions for thixoforming steel

The aim of this work is to evaluate the thermal and mechanical loadings applied to the tools during steel thixoforming process in order to determine appropriate tool materials and solutions. This evaluation was realized thanks to experimental trials and to the finite elements simulations. The effect of these loadings on the tool’s failure modes are highlighted and compared to the ones observed in classical forming processes. Beyond this, the failure modes of different tool materials and solutions are presented. The tested materials are hotworking tool steels. Other possibilities and tool coating or surface treatments are discussed as well

Caractérisation des propriétés d'emploi des aciers thixoforgés (vers la maîtrise du processus de fabrication)

Le thixoforgeage est un procédé de mise en forme innovant permettant l'élaborationde pièces complexes à l'état semi-solide. Il nécessite moins d'opérations et des efforts plusfaibles que les autres procédés de mise en forme plus classiques. L'objectif de ce travail et decaractériser la microstructure de plusieurs nuances d'acier à chaque étape du procédé afin demieux comprendre l'influence des différents paramètres et mécanismes de déformation, car lamicrostructure à l'état semi-solide, et en particulier la fraction de liquide, est très importante.Plusieurs techniques 2D et 3D (analyses MEB-EDS, CSLM, microtomographie X) ont étéutilisées pour évaluer la fraction de liquide des aciers étudiés (M2, C38LTT et 100Cr6). Lesrésultats obtenus pour l'acier M2 montrent une bonne corrélation entre les observations 2DMEB-EDS et les analyses 3D en microtomographie après trempe, ce qui indique que ces deuxtechniques sont très efficaces pour caractériser l'état semi-solide des aciers fortement alliés. Lemicroscope CSLM est utilisé quant à lui pour observer directement l'état semi-solide à hautetempérature : apparition du liquide, processus de solidification etc. Des essais de thixoforgeageont ensuite été réalisés pour étudier l'influence des différents paramètres du procédé sur lamicrostructure, la géométrie finale des pièces, les écoulements de matière etc. Après avoiranalysé la microstructure des pièces thixoforgées, des mécanismes permettant d'expliquer lesécoulements de matière sont proposés. Qui plus est, la comparaison avec la simulation duprocédé de forgeage à chaud montre que ces écoulements sont très différents, ce qui est dû aucomportement du matériau. Ce dernier est très sensible aux paramètres du procédé : il estdonc nécessaire de bien maîtriser et contrôler toutes les étapes lors de la mise en forme.The thixoforging process is an innovative forming process for the manufacturing ofcomplex parts in the semi-solid state, in fewer forming steps and with a decreased formingforce. The objective of this work is to characterize the microstructure of different steel grades ateach step of the thixoforging process in order to better understand the influence of the processparameters and the mechanisms of deformation, as the microstructure of the material in thesemi-solid state, especially the volume fraction of liquid, is very important. Several 2D and 3Dtechniques (SEM-EDS analyses, CSLM, X-ray microtomography) have been used to evaluatethe liquid fraction of various steel grades (M2, C38LTT and 100Cr6). The results for M2 steelshow a good agreement between 2D SEM - EDS observations and 3D X-ray microtomographyafter quenching, which proves that both techniques are efficient in characterizing high-alloyedsteels in the semi-solid state. The CSLM technique is used to observe the microstructuredirectly at high temperature, with the apparition of liquid and the solidification. Thixoforgingexperiments are finally performed in order to study the influence of the process parameters onthe microstructure: the final part geometry, the material flow etc. After analyzing themicrostructure of the thixoforged parts, some mechanisms of material flow are proposed.Moreover, by comparing the results between the thixoforging experiments and the hot forgingsimulations, it is found that the material flow is very different from that of hot forging process,which results from the material behavior. The latter is very sensitive to the process parameters;an accurate process control is necessary.PARIS-Arts et Métiers (751132303) / SudocSudocFranceF

Experimental investigation of the thixoforging of tubes of low-carbon steel

This article presents the outcomes of using thixo-extrusion (backward and forward) to produce tubes in low-carbon steel, grade SAE1006 (C05). According to the literature data, the semisolid state of this steel, required for thixoforging, is very difficult to obtain and has never been studied for this type of shaping. The experimental tests involve using an inductive heating and a flashless forging process. This work shows the route and possibility to obtain a usable semisolid by inductive heating for thixoforging of this steel grade. It shows by simulation the validation steps before the shaping and the forming constraints. A geometrical analysis of the parts revealed the good dimensional performance of thixoforging processes with high repeatability and a good microstructure. Such thixoforged tubes extend the dimensional limits of the tube wall thickness, as compared to the case of conventional forging in one step and in the framework of a typical industrial installation. These results could be integrated into a knowledge management system of thixoforging to identify new possibility of components

Tribological behaviour of the green anode paste with a steel plate at 150 °C

Abstract In order to accurately predict the mechanical behaviour of paste during forming process, the friction law between the carbon paste and the mould wall is an important parameter to be determined. This paper presents the tribological behaviour of the lubricated paste/steel interface subjected to high stress conditions at the anode forming temperature of 150 °C. A method to characterize the tribological behaviour has been developed and an apparatus was built. The method is based on the comparison of two successive experiments. In the first experiment, the paste is in contact with the friction plate. In the second one, a layer of Teflon is placed under the paste in order to excite another parameter thereby allowing the identification of the friction coefficient between the paste and steel wall. These experiments were performed with a paste under different normal loads. The static and kinetic friction coefficients of the Teflon/steel, steel/steel and paste/steel interfaces have been estimated. The static and kinetic friction coefficients of the Teflon/steel are respectively 0.17 and 0.13. The steel/steel friction coefficients were evaluated twice which gave a static coefficient that varies between 0.22 and 0.30. The kinetic coefficient varies between 0.18 and 0.25. The static and kinetic paste/steel friction coefficients obtained from both experiments are clearly similar. Their values are 0.15 and 0.13 respectively.</jats:p

Preliminary study on the capability of the novel near solidus forming (NSF) technology to manufacture complex steel components

The benefits of the novel Near Solidus Forming (NSF) process has shown previously in its ability to produce steel components with comparable as-forged mechanical properties but with a cost reduction of 10−15%. This study further pushes the NSF technology to produce parts that are conventionally difficult to produce via conventional methods. A 2.7 kg 42CrMo4 steel grade component was manufactured into a complex geometry using only a 400t press. Different manufacturing parameters were evaluated to show their influence on the process and final component. A combination of X-ray fluorescence (XRF), optical microscopy and SEM analysis of the microstructure was also conducted revealing the deformation pattern of the material and shedding some light on how the material evolves during the process. The successful forging of these components shows the capability to produce previously deemed difficult geometries, with much a lower specification forging press, in a single deformation

Preliminary Study on the Capability of the Novel Near Solidus Forming (NSF) Technology to Manufacture Complex Steel Components

The benefits of the novel Near Solidus Forming (NSF) process has shown previously in its ability to produce steel components with comparable as-forged mechanical properties but with a cost reduction of 10–15%. This study further pushes the NSF technology to produce parts that are conventionally diffcult to produce via conventional methods. A 2.7 kg 42CrMo4 steel grade component was manufactured into a complex geometry using only a 400t press. Different manufacturing parameters were evaluated to show their influence on the process and final component. A combination of X-ray fluorescence (XRF), optical microscopy and SEM analysis of the microstructure was also conducted revealing the deformation pattern of the material and shedding some light on how the material evolves during the process. The successful forging of these components shows the capability to produce previously deemed diffcult geometries, with much a lower specification forging press, in a single deformation