1. Application of Electro-Spark Deposition for the Joining of Dissimilar Metals
September 14, 2011
Jerry E. Gould
Technology Leader, Resistance and Solid-State Welding
Phone:

2. Topics to be Addressed
Functionality of the electro-spark deposition process
Application for joints between Ni-based superalloys and refractory metals
Observed microstructures
Resulting mechanical properties
Interpretation of effective cooling rates
Implications for dissimilar joint metallurgy

3. Electro-Spark Deposition (ESD) Process
ESD processing characteristics
Repeat fire capacitive discharge power supply
Discharge pulse widths on the order of 35 μs
Peak currents on the order of 100s of amps
ESD torch
Hand held
Rotating electrode
Short circuit sparking
Transfer of small metal volumes (microns in diameter) to substrate

4. Firing Efficiency of Electro-Spark Deposition
Firing rates defined by control system
Individual pulse widths on the order of 10s of micro-seconds
Variable contact pressures affect workpiece resistance and current flow
Sparking “efficiency” quality tool for assessing deposition characteristics
70- 90% sparking efficiency deemed ideal

5. Electro-Spark Deposition (ESD) Process
ESD deposit characteristics
Deposit layers typically microns thick
Deposition rates typically <100 μg/s
Multiple layers to create a deposit
Virtually no heating of the substrate
Application to complex materials joining problems
Small splat sizes to achieve a fine material grain structure
Fast cooling rates (>105-oC/s)
Non equilibrium solidification and suppression of solid-state transformations

6. Thermal Analysis of Various Localized Deposition Processes
Arc and Laser Processes:
Arc and Laser Processes:
ESD:
ESD:

7. Ni-Superalloy to Refractory Metal Joints for Space Nuclear Propulsion
Nuclear-powered space propulsion
Lunar-based power
Gas cooled reactor
Brayton cycle energy conversion
Refractory metals at/near the reactor
Nickel-based superalloys at/near the turbine
Need for dissimilar materials transitions

8. Material Combinations of Interest
Refractory to nickel-based superalloy joints
Refractory metal alloys
Mo-47.5 Re
T-111 (Ta-8%W-2%Hf)
Nickel-based superalloys
Hastelloy X
Mar M247
Melting point suppression associated with eutectics
Solidification cracking
Liquation cracking
Intermetallic formation
Joint ductility
Pre-mature fracture

9. Electro-Spark Deposition Welding Trials
ASAP 50-μF electro-spark deposition system
Welding trials to be done in chamber
Hard glove box
Oxygen levels <1 ppm
Material combinations
Mo-47Re strip to Mar M247 strip
T-111 strip to Mar M247 strip
Mo-47Re strip to Hastelloy X strip
T-111 strip to Hastelloy X strip
All materials 0.5-mm flat stock
Hard Glove Box with ESD Unit used Throughout these Trials
ESD Torch and Manual Manipulation used Throughout these Trials

10. Electro-Spark Deposition Welding Trials – Set-Up
Bonding procedure
Machined joint prep
ESD fill
Joint prep on reverse side
Final fill
Practice taken from EWI experience
Small adaptations made to maintain surface quality
ESD Electrode
Weld groove to be filled with deposited metal
ESD Specimen

11. Electro-Spark Deposition Welding Trials
Welding of ½ tensile specimens
Weld prep
Deposition practice:
Hastelloy X electrode (1.5-mm diameter)
40-μF capacitance
120-V charging voltage
500-Hz firing rate
Three specimens for materials combination
One for metallographic sections
Fill patterns
Internal fill quality
Intermetallic formation
Two samples each for mechanical testing
Joint Preparation and Electrode Orientation for the ESD Welds made in this Study
ESD Weld Current Waveform Taken from a Representative Deposit

12. Wrought Base Metal Microstructures
Microstructure of the Hastelloy X Base Metal Typical of the Material used in this Study
Microstructure of the Mo-47%Re Base Metal Typical of the Material used in this Study

13. MarM 247 Base Metal Microstructures
Fine-Grained MarM 247 Base Metal
Coarse-Grained MarM 247 Base Metal

14. Electro-Spark Deposition Welding – Mo-47%Re to Hastelloy X
Final Geometry of a ESD Joint
Cross Section of a Mo-47%Re to Hastelloy X ESD Weld
Procedure included
Fill of initial groove
Formation of back groove
Filling back groove
Sideplates used to improve joint geometry
Fill nominally >99% dense
Splat size nominally 10-μm thick
Fill showed good adhesion to both components
Processing time: 8 hr/specimen
Microstructural Details of the Hastelloy X Deposit

15. Electro-Spark Deposition Welding – Mo-47%Re to Hastelloy X
Hastelloy X/deposit interface
Good adhesion of deposited material
No degradation of base material noted
Splat size in deposit on the order of the BM grain size
Mo-47%Re/deposit interface
Good deposit adhesion noted
Little or no dillution with base metal
No evidence of second phases
Details of the Hastelloy X/Deposit Interface
Details of the Mo-47%Re/Deposit Interface

16. Electro-Spark Deposition Welding – Mo-47%Re to MarM 247
Joint macrostructure
Fill characteristics similar to that for other ESD welds
Apparent adherence to both the Mo-47%Re and MarM 247 materials
Two-side deposition evident
MarM 247/deposit interface
Good adhesion of deposited material
Apparent reaction zone between the base metal and deposited splats
Interface morphology similar to that seen for magnetic pulse welds
Reaction zone on the order of microns thick
Cross Section of a Mo-47%Re to MarM 247 ESD Weld
Details of the MarM/Deposit Interface

17. Electro-Spark Deposition Welding – T-111 to Hastelloy X
Joint macrostructure
Fill characteristics similar to that for other ESD welds
Apparent adherence to both the T-111and Hastelloy X materials
Two-side deposition again evident
T-111/deposit interface
Good adhesion of deposited material
No apparent reaction zone
No apparent thermal degradation of the base material
Cross Section of a T-111 to Hastelloy X 247 ESD Weld
Details of the T-111/Deposit Interface

18. Electro-Spark Deposition Welding – T-111 to MarM 247
Joint macrostructure
Fill characteristics similar to that for other ESD welds
Apparent adherence to both the T-111and MarM 247 materials
Two-side deposition again evident
Cross Section of a T-111 to MarM 247 ESD Weld

19. Tensile Properties of ESD Welded Ni-Base Superalloy/ Refractory Metal Combinations
Base Metal Properties of the Various Substrates Taken from Standard Reference Texts
Duplicate tensile tests conducted for each material combination
Tensile yield strengths at or above that of the Hastelloy X filler
Elongations dependent on:
Strengths of the substrates relative to the filler
Ductility of the individual substrates
Material
YS (MPa)
UTS (MPa)
MarM 247
828
966
Mo-47%Re
848
1034
T-111
613
679
Hastelloy X
379
767
Duplicate Tensile Test Results from Each Material Combination ESD Welded with a Hastelloy X Filler
Refractory Metal
Superalloy
Yield stress (MPa)
UTS (MPa)
% strain
Mo-47Re
MarM247
813
817
0.6
784
807
0.4
Hastelloy-X
430
586
3.5
466
698
5.5
T-111
611
682
4.7
607
683
8.5
465
618
4.3
460
645
6.4

20. Estimation of Cooling Rates During Electro-Spark Deposition – Stage 1: Heat of Fusion Driven
Two stage analysis
Heat of fusion conduction
Latent heat conduction
One dimensional non-steady-state solution (1)
Conduction driven boundary condition (2)
Heat generation through solidification (2)
Defined thermal gradient at solidification boundary (3)
Integration to define solidification time (4)
Defined transition time between solidification and latent heat driven cooling rates (5)
(1)
(2)
(3)
(4)
(5)

21. Estimation of Cooling Rates During Electro-Spark Deposition – Stage 2: Latent Heat Affects
(6)
(7)
Transition to latent heat driven conduction
Definition of thermal gradient from solidification analysis (6)
Substitution to define final expression (7)
Predictions of cooling rates at solidification terminus
Cooling rates from 107 to 109-oC/s
Cooling rates affected by conductivity of the substrate

22. Metallurgical Implications of ESD welding Dissimilar Material Joints
Effects of high cooling rates on solidification
Equally high solidification rates
Dendritic solidification without segregation
Solidification morphology surface tension rather than compositionally driven
Elimination of drivers for solidification cracking
Potential for a wide range of solid-solution phases
Effects on solid-state phase transformations
High cooling rates suppress second phases
Allows super-saturated phases to exist at room temperature
Potential for complex phase distributions with post-weld heating
Parallels in other pulse weld technologies
Magnetic pulse welding
Percussion welding
Hastelloy X Deposit on a Mo-47%Re Substrate

23. Summary Characteristics of electro-spark deposition processing
Repetitive capacitive discharge process
Deposition volumes on the order of 100 μm2
Adaptable to difficult materials combinations
Electro-spark deposition welding Ni-based superalloys to refractory metals
Adaptable to all material combinations
Fill densities >98%
No evidence of detrimental phase formation
Processing times ~8 hr/sample
Mechanical properties defined by those of the base materials
Thermal analysis of the ESD process
One dimensional analysis
Based on solidification of a single splat
Cooling rates at termination of solidification on the order of 107 to 109-oC/s
Influence of high cooling rates
Solidification without apparent segregation
Suppression of solid-state phase transformation products
Presence of uniform non-equilibrium super saturated phases
Considerable potential for dissimilar material combinations

24. Questions? Jerry E. Gould
Technology Leader, Resistance and Solid-State Welding
Phone: