Issue 27

A. Brotzu et alii, Frattura ed Integrità Strutturale, 27 (2014) 66-73; DOI: 10.3221/IGF-ESIS.27.08 70 Considering the high affinity between the alloying elements and oxygen, initially casting and pouring were performed in a centrifugal furnace after 8 vacuum-argon washing cycles. Afterwards, in order to improve casting soundness, castings have been manufactured by means of vacuum induction melting. By adopting this preventing measure we obtained castings that did not contain gas defects. A further critical aspect that comes to light in the presented investigation is the presence of shrinking cavities that seem to favour crack initiation and propagation. Shrinkage should not be a problem when manufacturing by centrifugal casting, since the material is constantly forced to instantly fill any vacancies that may occur during solidification and then it is important to evaluate if gas porosities and blowholes favour the formation of shrinkage cavities by avoiding locally compensation of liquid and solidification contraction. Figs. 2c and 4b show shrinkage cavities found on the fracture surfaces of Alloy “C” and Alloy “B” respectively. By pouring under vacuum in a preheated mould and by performing a slow cooling in a furnace gas defects are eliminated. Despite that SEM analyses showed that shrinkage cavities are still present as shown in Fig. 5. A considerable reduction of shrinkage cavities could only be achieved by means of a close control of process temperatures. Although melting under vacuum allows to get rid of gas porosities, as already stressed in some papers [16-18], it causes aluminium loss that requires during the charge material preparation a careful calculation of aluminium excess. Usually, the melting conditions of TiAl lie in the free evaporation area, thus the evaporation loss must be taken into account for the composition control. Experiments reported in literature [18] highlight that there exists a critical pressure and an impeding pressure for the evaporation losses of Al in a TiAl melt during the melting process under vacuum. When the vacuum pressure is less than the critical pressure, the evaporation of components takes on a state of free evaporation. (c) (b) (a) Figure 4 : SEM micrographs of the Alloy “B” specimen showing a crack (a) , a shrinkage cavity on the fracture surface (b) and the transgranular fracture (c) . Figure 5 : Shrinkage cavities found in the Alloy “P”.