• Dour, Gilles
• Broucaret, A.
• Oudin, Alexis
• Michrafy, Abderrahim
• Rezai-Aria, Farhad

Abstract

In the casting industry metal moulds or dies are used more and more. The reason is that they allow a fa st cooling rate o f the solidifying part, hence allowing higher productivity, finer microstructure and higher mechanical properties. In most cases the die is made out o f 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 o f production. A second but nonetheless important effect o f the coa ting is its thermal effect. The coating acts as a thermal barrier and protects the die against thermal shocks. The topic o f the present paper is to assess this function o f 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 tran sferred from the m elt to the die. H eat flu x den sities fro m 0.5 MW/m2 up to 10 MW/m2 have been reported in literature. The intense heat transfer gene rates high temperature heterogeneity into the die. The corresponding dilatation heterogeneity is responsible fo r 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 o f the die suffers the most stress. The stresses can be high enough to cause yielding o f the steel at high temperature. Because the steels in use fo r dies have a high yield stress at high tempera ture 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 fo r residual tensile stresses in cold conditions (i.e. nearly isothermal conditions). This phenomenon is rather classical in most thermal stresses problems [ 1, 2], A t the lifetime scale o f 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 estim ate the thermal stresses, assuming that the stresses remain below or at least close to the yield stress o f 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 fo r the casting factory nor its client. Other materials should be sought as a first priority. From the estimation o f thermal stresses, it would be delicate to foresee the mecha nical behaviour o f the materials o f the die in fatigue condition. Some materials tend to harden (copper alloys, f cc materials) while others tend to soften (heat treated martensite steels [4])[5], Instead o f trying to guess the behaviour, the method that we suggest is to perform a thermo mechanical fatigue test (TMF). This TMF test consists o f applying measured temperature and evaluated strain/stresses history to a mechanical testing sample [6], The most relevant tempe rature and stress history is, o f course, the one corresponding to the moulding surface o f the die. This test will provide information on the materials behaviour and some relevant data about the lifetime o f the die. This paper provides an example o f 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 o f the thermal stresses and the third part shows some results from the TMF testing. Throughout the paper the influence o f the coating nature and o f the die initial temperature is examined.

Topics

• impedance spectroscopy
• microstructure
• surface
• polymer
• experiment
• laser emission spectroscopy
• steel
• fatigue
• copper
• casting
• plasticity
• copper alloy