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Wide Gap Braze Repairs of Nickel Superalloy Gas Turbine Components

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Wide Gap Braze Repairs of Nickel Superalloy Gas Turbine Components WIDE GAP BRAZE REPAIRS OF NICKEL SUPERALLOY GAS TURBINE COMPONENTS by Cheryl Hawk A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Metallurgical and Materials Engineering). Golden, Colorado Date: ______________________ Signed: ______________________________ Cheryl Hawk Signed: ______________________________ Dr. Stephen Liu Thesis Advisor Golden, Colorado Date: ______________________ Signed: ______________________________ Dr. Ivar Reimanis Professor and Head Department of Metallurgical and Materials Engineering ii 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. iii TABLE OF CONTENTS ABSTRACT ................................................................................................................................... iii TABLE OF CONTENT ................................................................................................................. iv LIST OF FIGURES ...................................................................................................................... vii LIST OF TABLES ....................................................................................................................... xiii CHAPTER 1 INTRODUCTION .....................................................................................................1 CHAPTER 2 BACKGROUND .......................................................................................................3 2.1 Superalloys ............................................................................................................................3 2.1.1 Development of Nickel-base Superalloys .................................................................4 2.1.2 Microstructure ...........................................................................................................6 2.1.2.1 Gamma Phase.............................................................................................7 2.1.2.2 Gamma Prime ............................................................................................8 2.1.2.3 Carbides and Borides ...............................................................................11 2.1.2.4 Topological Closed-Packed Phases .........................................................11 2.1.2.5 René 108 ..................................................................................................13 2.2 Repair Methods ...................................................................................................................14 2.2.1 Welding Repairs......................................................................................................15 2.2.2 Brazing Repairs .......................................................................................................15 2.2.3 Transient Liquid Phase Bonding .............................................................................17 2.2.3.1 Transient Liquid Phase Bonding Process ................................................17 2.2.3.2 Transient Liquid Phase Bonding Microstructure .....................................19 2.2.3.3 Effect of Processing Parameters ..............................................................22 2.3 Wide Gap Brazing...............................................................................................................23 2.3.1 Activated Diffusion Healing ...................................................................................24 2.3.2 Activated Diffusion Healing Process ......................................................................24 2.3.3 Activated Diffusion Healing Microstructure ..........................................................25 2.3.4 Effect of Processing Parameters .............................................................................26 2.4 Microstructural Characterization ........................................................................................27 2.4.1 Superalloy MAR-M 247 .........................................................................................28 2.4.2 BNi-2 Filler Metal...................................................................................................29 iv 2.4.3 BNi-5 Filler Metal...................................................................................................31 2.5 Mechanical Testing .............................................................................................................32 2.5.1 Four-Point Bend Testing .........................................................................................32 2.5.2 Elevated Temperature Testing ................................................................................33 2.5.3 Digital Image Correlation (DIC) .............................................................................34 CHAPTER 3 EXPERIMENTAL PROCEDURES ........................................................................35 3.1 Powder Analysis .................................................................................................................35 3.2 Braze Alloy Creation ..........................................................................................................36 3.2.1 Braze Alloy Creation ..............................................................................................36 3.2.2 Mixing the Braze Alloys .........................................................................................37 3.3 Machining ...........................................................................................................................38 3.3.1 Spreadability Plates .................................................................................................39 3.3.2 Braze Plates .............................................................................................................40 3.3.3 Four-Point Bend Coupons ......................................................................................40 3.4 Surface Preparation .............................................................................................................41 3.5 Braze Paste Application ......................................................................................................41 3.6 Brazing Cycles ....................................................................................................................42 3.6.1 Spreadability Cycles ...............................................................................................42 3.6.2 Brazing Hold Experiments ......................................................................................43 3.6.3 Diffusion Hold Experiments ...................................................................................44 3.7 Metallography .....................................................................................................................46 3.8 Microstructural Characterization ........................................................................................47 3.8.1 Light Optical Microscopy .......................................................................................47 3.8.2 Volume Fraction .....................................................................................................48 3.8.3 Scanning Electron Microscopy ...............................................................................48
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