September 14, 2011

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 Email: jgould@ewi.org Phone: 614.688.5121

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

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. ESD Electrode ESD Specimen Weld groove to be filled with deposited metal 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

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 Coarse-Grained MarM 247 Base Metal Fine-Grained MarM 247 Base Metal

14. Electro-Spark Deposition Welding – Mo-47%Re to Hastelloy X • 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 Cross Section of a Mo-47%Re to Hastelloy X ESD Weld Final Geometry of a ESD Joint 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 • 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 Base Metal Properties of the Various Substrates Taken from Standard Reference Texts Duplicate Tensile Test Results from Each Material Combination ESD Welded with a Hastelloy X Filler

20. 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) Estimation of Cooling Rates During Electro-Spark Deposition – Stage 1: Heat of Fusion Driven (1) (2) (3) (4) (5)

21. Estimation of Cooling Rates During Electro-Spark Deposition – Stage 2: Latent Heat Affects • 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 (6) (7)

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 e-mail: jgould@ewi.org Phone: 614.688.5121