Session: Student Poster Competition

Submission Number: 187174

Evaluating the Metallurgical Effects of Rejuvenation Repair of Service Run Single Crystal Turbine Blades 

Full Solution Rejuvenation (FSR) has the capability to reverse the effects of alloy ageing, restoring microstructure and mechanical properties to the like-new condition, thus extending the service life of turbine blades. Extending service life is particularly critical for single crystal blades due to their high manufacturing cost and lack of availability. FSR has been successfully applied to conventionally cast and directionally solidified alloys, however, its application to single crystal turbine blades remains a key area of interest. One major concern with applying FSR to single crystal blades is recrystallization. Prior work looked at developing the heat treatment schedule and the resulting microstructure and mechanical properties related to FSR of Rene N5 single crystal alloy. The aim of this study was to examine the metallurgical effects of FSR on service run single crystal blades which are comprised of a variety of materials (base alloy, coating(s), braze). Trials were conducted on life expired, service run Rene N5 alloy aeroderivative (LM2500+ and LM6000) first stage blades. Each of the blades had the external coating removed, followed by the application of full solution rejuvenation heat treatment, including the primary and secondary ageing heat treatments. Non-destructive (thermal and chemical macro etching) and destructive metallurgical analysis was conducted on the airfoil and root surfaces through both optical microscopy and scanning electron microscopy (SEM). The blades were examined for the extent of recrystallization, surface defect formation and base alloy microstructure. Results from these trials revealed potential challenges associated with the application of FSR to single crystal turbine blades. During service, inter-diffusion layers formed at the interface between the base alloy and coating. These layers have different microstructure and chemical composition from both the coating and base alloy. As a result, these layers were not removed during chemical stripping of the coating, which preferentially attacks the beta aluminide phase. The remnant inter-diffusion layer was found to result in void formation, likely as a result of melting, at the airfoil surface during subsequent solution heat treatment. The melted remnant inter-diffusion layer likely led to additional challenges in non-destructive testing to inspect for recrystallized grains. Overall, no significant recrystallization was identified on the airfoil of the blades by both destructive and non-destructive examination. An expected, uniform layer of recrystallization was observed on the peened root surfaces. Additional potential challenges that were identified included re-melt of brazed details (root cover plates and tip caps) as well as the removal and replacement of internal coatings. It was confirmed that the applied FSR heat treatments restored the microstructure in the airfoil of the service run blades from an over-aged condition to the like new condition. The study demonstrated that there are additional metallurgical considerations beyond recrystallization and bulk microstructural recovery that need to be considered when performing FSR repair of single crystal blades. Specifically, the presence of coating inter-diffusion zones, removal and replacement of internal coatings, and braze details can complicate the repair to a greater extent than typically applies to conventionally cast and directionally solidified alloys.

Presenting Author: Kristina Siiman Liburdi Engineering Limited

Presenting Author Biography: Kristina Siiman is a fourth-year student in Materials and Biomedical Engineering at McMaster University. She is currently completing a co-op term at Liburdi Engineering Limited in a Materials Engineering Co-op role, where her work focuses on turbine blade failure analysis and developmental efforts aimed at extending blade service life. Her research interests include innovative solutions targeted at advancing the field of materials engineering, specifically related to coating applications.

Authors:

Kristina Siiman Liburdi Engineering Limited