Microporosity Formation in Investment Castings of Nickel - Base Superalloys : Metallurgical Effects, Thermal Modelling And Foundry Assessment

Abstract

Although there is a general agreement between foundries on the main trends for controlling microporosity in production parts, the lack of a systematic method for assessing the tendency of super-alloys to form microporosity made necessary a broad research programme carried out by three partners: ENSMP, LTPCM and SNECMA. The aim of this programme is to predict theoretically and to assess experimentally the intrinsic tendency for nickel base superalloys to form microporosity in castings.

The microporosity content has been estimated by careful radiographic examination of longitudinal sections of cylindrical castings cast under standard conditions. Two alloys have been chosen as examples: the incidence of microporosity has been found to increase with increasing carbon content for IN 100 whereas it decreases for René 77.

The study of the heat transfer in the mould and casting consists on one hand in data acquisition during casting experiments on real mould equipped with numerous thermocouples, on the other hand in a computer calculation of heat transfer performed with a finite elements method. The release of the latent heat within the freezing range has been taken into account.

The effect of the chemistry of the alloys on their solidification behavior and the release of the latent heat during cooling has been experimentally studied to afford the proper data for the computer calculations.

Finally, the pressure drop, at any time of the solidification, at; any point of the casting has been calculated. The experimental assessment of the microporosity for the different alloys is tentatively discussed in terms of pressure drop in the mushy zone as calculated above.