• PDF / 8,249,526 Bytes
• 13 Pages / 593.972 x 792 pts Page_size
• 19 Downloads / 218 Views

DOWNLOAD

REPORT

---

VER the last decades, the increase in operating efficiency of gas turbines has been mainly achieved by continuous improvement of high-temperature materials and the respective investment casting technology for the fabrication of hollow, thin-walled turbine blades.[1] A columnar grain structure achieved by directional solidification (DS) shows numerous beneficial effects due to the low modulus parallel to the main stress axis and elimination of transverse grain boundaries.[2] The application of DS casting to very complex-cored blades is prevented for certain important alloys like IN792[3,4] or CMSX-4[5] because they suffer from bad castability, i.e., crack formation at grain boundaries during the last stages of solidification. Even though hot tearing is a well-known problem that has been studied extensively in the field of casting and welding,[6,7] details of the process are still not well understood. It is generally believed that hot tears are initiated at the last stages of solidification by stresses perpendicular to grain boundaries induced by thermal contraction or solidification shrinkage. When the interdendritic liquid is in a ‘‘film stage,’’ stresses cannot be accommodated by the J. GRODZKI, formerly Research Associate with the WTM Institute, Department of Materials Science, University of ErlangenNuremberg, Martensstr. 5, 91058 Erlangen, Germany, is now Development Engineer with Siemens AG, Huttenstr. 12, 10553 Berlin, Germany. Contact e-mail: [email protected] N. HARTMANN, Post Graduate Student, and R. RETTIG, Research Associate, are with the WTM Institute, Department of Materials Science, University of Erlangen-Nuremberg. E. AFFELDT, Industrial Advisor and Contracting Body, is with MTU Aero Engines AG, 80995 Munich, Germany. R.F. SINGER, Professor and Director, is with the WTM Institute, Department of Materials Science, University of ErlangenNuremberg. Manuscript submitted September 11, 2015. Article published online March 29, 2016 2914—VOLUME 47A, JUNE 2016

deformation of the dendritic structure, if the dendrite arms have insufficient bridges (links between their fragments of solid). In addition, liquid feeding is impeded because the permeability of the dendritic structure is very low.[8,9] Pellini and co-workers stated that the hot tearing susceptibility depends on the localized strain rate and time the alloy is within this critical temperature range.[8] Flemings concluded that a small freezing range results in a good castability.[11] Clyne and Davies[12] proposed a refined cracking susceptibility coefficient (CSC) to predict the effect of alloy composition on hot tearing. They concluded that cracks are easily ‘‘healed’’ by mass and liquid feeding in the solidification stage characterized by liquid volume fractions between 0.1 and 0.6, whereas the ‘‘vulnerable‘‘ film stage is defined by fractions between 0.1 and 0.01. The CSC corresponds to the ratio of the relative times an alloy spends in these two stages. This model was successfully used to predict hot tearing of binary model alloy systems and steels.[13] According to ou