Low Cycle Fatigue Properties of LPM Wide-Gap Repairs in Inconel

Abstract

Strain controlled, continuous cycling and 2 minute compressive dwell LCF tests were performed on simulated LPMTM wide-gap repair joints in the nickel-base superalloy Inconel 738 at 850°C over a total strain range (A+) of 0.4 to 0.9 percent, 0.33 Hz, A = 03, R= -1. The test specimens were uncoated hybrid bars fabricated with a gauge section consisting of a cast inner core and an equal cross-sectional area overlayer of Inconel 738 LPMTM repair material. For samples with near-surface porosity of 0.25 mm and under, the fatigue lives of the LPMTM samples were about equal to those of cast Inconel 738 when AET < 0.5%, and approximately one-half to three-fourths for strains above this level. A two minute compressive hold reduced the fatigue lives of both the hybrid samples and cast Inconel 738 by a factor of 5 to 10 as compared to the continuous cycling tests, due to the increased creep, oxidation and mean-stress effects. In all cases, the dominant fatigue cracks initiated at near-surface pores. At strains below AE~ = 0.5 % crack propagation was transgranular, but shifted to an intergranular mode at higher strains. For specimens with near-surface porosity of approximately 0.5 mm, the fatigue lives were much shorter and initiation of the dominant cracks took place at these features. The results of these and other mechanical property tests indicate that, using the appropriate non-destructive inspection procedures and defect size restrictions, the LPMTM repair limits can potentially be extended into the more fatiguesensitive areas of components such as airfoil leading edges.