Thermomechanical fatigue behaviour and mode of failure of laser welded single crystal Ni based superalloy

Abstract

The baseline metallography of single crystal alloy N5 was compared with laser welded material to evaluate the effect of the welding process on cracking behaviour. In addition, baseline thermomechanical fatigue data were compared with the fatigue properties of welded specimens. The solution treated and aged single crystal alloy was found to contain microporosity, Ta rich MC carbides, Cr–Mo rich grain boundary borides, gamma prime and fine secondary gamma prime. In addition, recrystallised grains were present in the weld heat affected zone and provided a path for crack propagation out of the fusion zone. The laser beam welds showed defects mainly associated with the fusion zone, while thermomechanical fatigue analysis of the laser weld data showed inferior values compared to the baseline solution treated and aged material. Metallographic analysis showed that this was due to increased defects in the fusion zone associated with surface oxidation and heat affected zone recrystallisation compared to the non-welded alloy.

Keywords:

• Superalloys
• Metallography
• Welding
• Crack initiation
• Thermomechanical fatigue

Technical assistance from M. Boskwick, J. van Dorp and D. Mardis is much appreciated. Funding for J. Durocher’s research was partly provided by an NSERC Canada, Strategic Project Grant to Dr Richards.