Joining of a Nickel-base SX Alloy by Diffusion Brazing Marcus Klemm, Dieter Schneefeld, Herbert Hanrieder and Karl-Hermann Richter Seattle, September 17, 2014 September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 1
Contents Introduction Objectives Diffusion Brazing Process Material and Test Pieces Results Outlook and Conclusions September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 2
Contents Introduction Objectives Diffusion Brazing Process Material and Test Pieces Results Outlook and Conclusions September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 3
Introduction High-pressure turbine blades Exposed to very high temperatures Subject to high stress Operated in a corrosive environment Made of SX materials Coatings, cooling air holes, etc. Very high value Typical HPT blade damages Abrasion Burn-off Cracks HPT blade damaged by burn-off It is highly desirable to provide a repair process to replace the tip September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 4
Introduction Possible repair process - main process steps Removal of the upper region of the blade Investment casting of a replacement part Manufacture of a brazing preform Diffusion brazing (DB) of the replacement part + Residual blade Replacement part & brazing preform Repaired blade September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 5
Contents Introduction Objectives Diffusion Brazing Process Material and Test Pieces Results Outlook and Conclusions September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 6
Objectives Feasibility study Development of suitable process technology Metallurgical evaluation Strength tests Effect of different orientation on the properties of the joint September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 7
Contents Introduction Objectives Diffusion Brazing Process Material and Test Pieces Results Outlook and Conclusions September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 8
Diffusion Brazing Process Fundamentals Initial condition Surface-fusion of base material Extension of the liquid zone Diffusion zone Patented by Pratt & Whitney in the 1970s Isothermal solidification controlled by diffusion of MPD *) (e.g. boron) Pre-condition for epitaxial growth of dendrites across the fusion area (SX brazing joint) Isothermal solidification Homogenization *) Melting Point Depressant Epitaxial growth September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 9
Diffusion Brazing Process Process technology Fundamental DB trials in a high-vacuum brazing furnace Determination of temperature profile and loading pressure Testing of various brazing filler materials DB of demonstration parts Press Test pieces September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 10 Vacuum brazing chamber Limited temperature load at blade root for planned application Transfer of process technology to inductive DB process featuring identical - temperature profile - loading pressure and - high vacuum
Contents Introduction Objectives Diffusion Brazing Process Material and Test Pieces Results Outlook and Conclusions September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 11
Material and Test Pieces Identical orientation of joining partners Cast plates made of SX material Simple brazing filler made of a Ni-Cr-B alloy Cast plate made of SX material Test pieces for diffusion brazing September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 12
Material and Test Pieces Different orientation of joining partners Measurement of crystallographic axes of the plates by Laue spectroscopy Manufacture of test pieces featuring a well-defined tipping of the primary axis (identical orientation of the secondary axis) Manufacture of test pieces featuring a well-defined twist of the secondary axis (identical orientation of the primary axis) Test pieces featuring well-defined tipping Test pieces featuring well-defined twist September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 13
Contents Introduction Objectives Diffusion Brazing Process Material and Test Pieces Results Outlook and Conclusions September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 14
Results Identical orientation of joining partners Metallurgical evaluation Furnace brazing Stray grains Conventional brazing process Optimized DB process Conventional brazing process generates stray grains Optimized DB process generates an epitaxial brazed joint Depletion of Ɣ -content in the epitaxial brazed joint September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 15
Results Identical orientation of joining partners Metallurgical evaluation Induction brazing Induction heating using a susceptor Depletion of Ɣ -content just as with furnace brazing Metallurgical evaluation shows no difference in structure between furnace brazing and induction brazing September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 16
Results Identical orientation of joining partners EBSD *) analysis Orientation mapping Confirmation of epitaxial brazed joint No grain boundary Misorientation of insular grains less than 2 EBSD image along a diffusion brazed joint *) Electron backscatter diffraction September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 17
Results Identical orientation of joining partners EDX *) analysis EBSD image along a diffusion brazed joint EDX analysis shows distribution of chemical elements Use of a simple Ni-Cr-B braze filler Elements of SX base material diffuse into brazed joint *) Energy-dispersive X-ray spectroscopy September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 18
Elongation Results Identical orientation of joining partners Re-melting of brazed joint Constant Shear Force Non-epitaxial brazed joint Epitaxial brazed joint Fixing Brazing temperature Experimental set-up Temperature Result of re-melting trials of brazed joints Conventional brazed joint has low re-melting temperature Re-melting temperature of an epitaxial brazed joint is much higher as compared with diffusion brazing temperature September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 19
Results Identical orientation of joining partners Creep rupture test Test temperature 980 C Rupture in joining area Lower strength as compared to SX base material due to depletion of Ɣ -content Creep rupture strength within 3σ-scatter band of SX base material possible (depending on DB parameters) September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 20
Results Identical orientation of joining partners Influence of process technology on creep behavior Induction brazing and furnace brazing have a similar time to rupture Specimens for other tests can be produced by simpler furnace brazing. September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 21
Results Different orientation of joining partners EBSD analysis Well-defined tipping Well-defined twist Even minor disorientation of joining partners result in the formation of stray grains Twist and tipping of joining partners feature the same properties September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 22
Results Different orientation of joining partners Creep rupture test Significant reduction in time to rupture if the crystallographic orientation of the joining partner is only slightly different. Failure caused by stray grains in the brazing area. September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 23
Results High-cycle fatigue behavior HCF strength is significantly reduced if the crystallographic orientation of the joining partner is only slightly different. Induction brazing and furnace brazing feature similar HCF strength September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 24
Results Summary of strength measurements Minor misorientation results in a reduction in strength by more than one order of magnitude. The critical angle of misorientation without significant reduction in strength is unknown. Measurement accuracy to determine the crystallographic orientation: only about ±1. September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 25
Contents Introduction Objectives Diffusion Brazing Process Material and Test Pieces Results Outlook and Conclusions September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 26
Outlook Provision of replacement parts Requirement: The crystallographic axes of the replacement part and of the residual blade have to be in line with one another as much as possible. I.Statistical approach to provide the replacement part Casting process produces replacement parts featuring various crystallographic orientations The number, n, of replacement parts which have to be made available depends on the number of blades to be repaired, k, and the probability, p, which indicates that an appropriate replacement part is available, i.e., n=n(k, p). Replacement part If the number of replacement parts is high enough, a suitable replacement part which fits the crystallographic orientation of the residual blade can be found. September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 27
Outlook Provision of replacement parts II. Tailored replacement part *) 1. Take a disk-shaped raw part made of the SX alloy. 2. Measure the crystallographic orientation. 3. Realign the disk-shaped raw part in such a way that its crystallographic orientation is identical to that of the residual blade. 4. Manufacture the replacement part out of the aligned disk-shaped raw part. 5. Join the replacement part with the residual blade. 1 2 3 4 5 *) See Offenlegungsschrift DE 10 2008 058 140 September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 28
Conclusions If the joining partners feature identical crystallographic orientation, the optimized DB process - generates an epitaxial brazed joint - results in a creep rupture strength within the 3σ-scatter band of SX base material Minor misorientation of joining partners results in - the formation of stray grains in the brazing area and - a significant reduction in strength by more than one order of magnitude. Induction brazing and furnace brazing feature similar strength properties September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 29
Questions? Karl-Hermann Richter Dept. TEF MTU Aero Engines GmbH Dachauer Str. 665 Munich, 80995 DEUTSCHLAND September7t1h7I,nt2e0r1n4ational Seminar on Joining Aerospace Materials 30