Rôle du poteyage et de la température initiale du moule sur les sollicitations thermomécaniques des moules permanents de fonderie

Effect of coating and initial die temperature on thermal stresses into die casting dies

École des Mines d'Albi-Carmaux, Albi

Résumé

Cet article traite de la fatigue thermique subie par les outillages de mise en forme. Pour appréhender le problème d’endommagement des outillages, il est nécessaire de bien connaître les conditions de transfert de chaleur et d’évaluer les contraintes thermo-mécaniques subies par l’outillage. La prévision de la durée de vie et l’évolution de la plasticité cyclique qui précède la fissuration est difficile car certains matériaux ont tendance à s’écrouir alors que d’autres ont tendance à s’adoucir. Nous proposons le recours à des expériences de fatigue thermo-mécanique (TMF) pour appréhender cette prévision. Par un exemple emblématique, nous montrons comment il est possible d’aborder le problème. Le cas considéré est la fonderie gravité en coquille d’acier. Nous mettons en évidence l’influence de paramètres process sur le transfert de chaleur et sur les contraintes thermo-mécaniques subies par le moule pendant un cycle de coulée. Les paramètres retenus sont la température initiale du moule et la nature du poteyage. Des essais de fatigue thermo-mécanique ont été menés et les résultats sur 1000 cycles sont exposés.

Abstract

In the casting industry metal moulds or dies are used more and more. The reason is that they allow a fast cooling rate of the solidifying part, hence allowing higher productivity, finer microstructure and higher mechanical properties. In most cases the die is made out of steel and reacts with the liquid cast metal. The usual solution is to cover the moulding surface with a coating or spray. Depending on the casting technology, the coating is sprayed every cycle or every 8 to 10 hours of production. A second but nonetheless important effect of the coating is its thermal effect. The coating acts as a thermal barrier and protects the die against thermal shocks. The topic of the present paper is to assess this function of the coating.

During a casting cycle, the coated die and the molten metal are briefly in contact during the very first moments and then an air gap may form and separate them apart. During the first stage, an intense heat is transferred from the melt to the die. Heat flux densities from 0.5 MW/m² up to 10 MW/m² have been reported in literature. The intense heat transfer generates high temperature heterogeneity into the die. The corresponding dilatation heterogeneity is responsible for internal stresses into the die, so called thermal stresses. They are usually compressive stresses on the hot surface. It will be shown in this paper that the moulding surface of the die suffers the most stress. The stresses can be high enough to cause yielding of the steel at high temperature. Because the steels in use for dies have a high yield stress at high temperature the plastic deformation remains small. However it is a cyclic plasticity because the same phenomenon occurs at every casting cycle. We believe that this plasticity in warm conditions is responsible for residual tensile stresses in cold conditions (i.e. nearly isothermal conditions). This phenomenon is rather classical in most thermal stresses problems [1, 2], At the life-time scale of the die, the moulding surface is cyclically stressed in traction at low temperatures and compression at high temperatures leading to a fatal cracking.

In a first approach we suggest measuring temperatures within the die during a casting cycle. From this measurement it is possible to estimate the thermal stresses, assuming that the stresses remain below or at least close to the yield stress of the die materials. This assumption is usually fair. Indeed, if the plastic deformation were large during every cycle, the die would never last much longer than a few thousands cycles. If this ever occurred, it would not be a great advantage for the casting factory nor its client. Other materials should be sought as a first priority. From the estimation of thermal stresses, it would be delicate to foresee the mechanical behaviour of the materials of the die in fatigue condition. Some materials tend to harden (copper alloys, fcc materials) while others tend to soften (heat treated martensite steels [4])[5], Instead of trying to guess the behaviour, the method that we suggest is to perform a thermomechanical fatigue test (TMF). This TMF test consists of applying measured temperature and evaluated strain/stresses history to a mechanical testing sample [6], The most relevant temperature and stress history is, of course, the one corresponding to the moulding surface of the die. This test will provide information on the materials behaviour and some relevant data about the lifetime of the die.

This paper provides an example of this method. The thermal data was obtained from a gravity casting experiment [3] that is described in the first part. The second part deals with the evaluation of the thermal stresses and the third part shows some results from the TMF testing. Throughout the paper the influence of the coating nature and of the die initial temperature is examined.