Papers by Author: Gérard Bernhart

Abstract: During superplastic forming, dies are subjected to high temperatures and severe environmental conditions. Optimum material grade choice and die design have to take into account all these combined parameters. Microstructure evolution and high temperature mechanical properties are investigated and reported for various Heat Resistant Cast Steels. New die concepts are suggested for energy and cost savings.

Abstract: Superplastic forming of titanium alloy sheets requests long time operating conditions in the range of 900-950°C. Moreover, in a classical press-furnace process environment, die surface temperature drops during sheet unloading and induces high temperature thermo-mechanical fatigue. In order to withstand such extreme conditions in oxidative atmosphere, cast heat resistant nickel chromium steel grades have been developed. The high chromium content (close to 25%) aims to protect against the oxidizing environment, whereas the nickel content is selected with respect to the expected in service loads. The 50% nickel grades are in general used for heating plates, huge casings and cover-plates; whereas 40% nickel grades are selected for inserts and medium size self-standing dies. Cost considerations (Nickel and machining) are also taken into account by the end users for making their choice. An extensive testing program has been performed, in the range of 20 to 950°C, to understand the high temperature fatigue behaviour of these grades and to identify material behaviour models for simulation purposes. This paper presents the major results of these research works and highlights the impact of the nickel content in terms of stress level and life time. Nevertheless, when looking on behaviour, test results show that a unified elasto-visco-plastic cyclic behaviour model is well suited for thermo-mechanical cyclic modelling whatever the grade is. Isothermal identification strategy and out of phase SPF die representative anisothermal fatigue validation are presented.

Abstract: Cyclic behavior and life prediction of two tempered martensitic steels (AISI H11 and L6) are investigated under thermo-mechanical loading conditions. Two non isothermal constitutive models developed in the same framework of the thermodynamics of irreversible processes are introduced. The first one, in relation with the tempering state, considers the fatigue-ageing phenomena whereas the second one is intended to take into account more complex loading paths. This last non unified approach allows to define different strain mechanisms which can be related to microstructural considerations. The strain-stress parameters provided by both approaches can be introduced into a lifetime model which is based on continuum damage mechanics.

Abstract: Today heat resistant cast steels are the nominal solution for Ti-SPF forming die manufacturing. Nevertheless, this materials present some drawbacks related to delivery time and cost. A fibre reinforced refractory castable (FRRC) is proposed as a new solution for prototype SPF die manufacturing. Due to the general brittleness of refractory castables, a short fibre reinforcement has been investigated in order to avoid catastrophic failure during the forming process. General macroscopic behavior of such materials is very complex and presents large evolutions with the testing temperature. The paper addresses the important benefits of the reinforcement for refractory castable in the case of loading on a complex structure. The capability of the material to support several cracks is shown in the case of a technological sample with a complex shape.

Abstract: CROMeP (Research Centre on Tools, Materials and Processes) experience in rapid tooling development is related. Focus is placed on two processes, namely the Direct Metal Laser Sintering/DMLS^Ⓡ and STRATOCONCEPT^Ⓡ currently under consideration for the manufacturing of thermoplastic injection moulds. The principle of the two processing routes as well as the benefits attainable by using those processes will be briefly reviewed. Recent results on the microstructure and the mechanical performances of sample tools built up using both processes are presented.