Wide gap braze repairs of nickel superalloy gas turbine components

Abstract

The effect of microstructure and processing parameters on the bend properties of wide gap braze repairs has been investigated for BNi-2 and BNi-5 filler metals. BNi-2 braze alloys developed a brittle eutectic constituent that was the source for crack initiation and propagation. BNi-5 braze alloys developed large pores and lack of fusion to the base metal René 108 that decreased the strength of the joint. Three types of crack behaviors were observed within the two braze alloys. (1) Crack initiation and propagation through the brittle eutectic constituent. (2) Crack initiation and propagation through the brittle eutectic constituent/ matrix interface. The crack would propagate through grain boundaries if the eutectic constituent was dispersed. (3) Crack propagation follows type 1 or type 2, but propagated due to a major defect and coalesced with the defect. Braze alloy chemistry was improved by changing the filler metal-additive powder ratio. For the BNi-2 braze alloys, a mixing ratio of 40 wt.% BNi-2 produced the lowest volume percent of the brittle eutectic constituent. These alloys produced the highest strengths. A ratio of 50 wt.% BNi-2 produced the highest volume percent of the brittle eutectic constituent and the lowest strengths. For the BNi-5 braze alloys, 40 wt.% BNi-5 produced the highest volume percent of voids and therefore exhibited the lowest strengths. Sixty weight percent BNi-5 exhibited the lowest volume percent of voids and therefore exhibited the highest strengths. Processing parameters were improved for the various stages in the brazing cycle by changing the time and temperature. The “brazing hold” was held at 1200°C or 1232°C for 10 minutes. The bend strength and angular deflection increased when brazed at 1232°C for 10 minutes. A brazing temperature of 1232°C improved the microstructure by reducing the amount of detrimental microstructural features. The “diffusion hold” parameters were held at 1100°C and 1121°C for 2 hours and 4 hours. The bend strength and angular defection increased with a “diffusion hold” temperature of 1121°C for both 2 hours and 4 hours. However, at 1100°C, there was an increase in strength and angular deflection with a “diffusion hold” of 2 hours but a decrease in strength when held for 4 hours.