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Iridium Welding Process Improvement

Stanley Pierce & Paul Moniz Manufacturing Engineering and Technologies

Nuclear and Emerging Technologies for Space

Aerospace Nuclear Science and Technology

February 23-26, 2015

Abstract 5076

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Overview

• Introduction and goals

• Iridium DOP-26 alloy

• Welding concerns and history

• Magnetic arc oscillator

• What really matters

• How to optimize weld

• How to evaluate

• Results to date

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Introduction and goals

• GPHS iridium welding process is being upgraded • Replace obsolete 1990’s automatic

welding equipment

• Investigate a new welding procedure without use of the magnetic arc oscillator

• Collaboration with ORNL and SNLA

• Goals: • Meet or exceed weld characteristics of

legacy process

• Maximize weld ductility

• Improve process control

1990

2015

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Iridium DOP-26 alloy

• Iridium, 0.3% W, 60 ppm Th, 50 ppm Al

• Thorium improves grain boundary (GB) cohesion

• Ir5 Th precipitate retards elevated temp. grain growth

• 200 ppm Th found to be the best for high temp. properties, but … what about welding?

McKamey, et.al, ORNL-6935, 4/98

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• Th > 100 ppm → GB eutectic → cracking & low ductility • Segregation during weld solidification → localized Th > 100 ppm

• Wide weld → more solute volume → higher concentration at GB

• Larger grain size → less GB area → higher concentration at GB

• Larger grain size → easier fracture path

• Weld shape → grain structure & size → solute concentration & fracture path

• Weld heat input→ weld size & shape, grain structure & size

• Cyclic manipulation of solidification: pulsed current & arc oscillation

Welding concerns Hot cracking and low impact ductility

Liu & David, Met. Trans A, Vol 13A, 6/82-1043

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Welding history Cracking in “old process” DOP-26

• 1970’s & 1980’s • “Old process” DOP-26, crude alloy production

methods

• Porosity, grain boundary voids

• Non-homogenous thorium distribution

• Large lot to lot variation

• Poor accuracy in Th determination, 50 ppm could be > 100 ppm

• Unrefined welding procedure

• Poor welding process control

David & Liu, WJ, 5/82, 157-s Franco-Ferreira, et.al, ORNL/TM-2000/84 David & Woodhouse, WJ, 5/87, 129-s

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Welding history Cracking in “old process” DOP-26 (magnetic oscillator to the rescue)

• 1974 • Travel changed from 12 ipm to 30 ipm: “significant modification of the

weld microstructure”

• 2 pole oscillator, “Some further improvement in weld microstructure”

• 1984 • Large batch variation: 0 to 26% crack reject rate

• 4 pole oscillator, the silver bullet?

• Batches at 7% & 26% rejects → 2%

• Grain size reduced 12%

• Also reduced arc length and increased downslope (could this have helped?)

Coffey et.al., WJ 12/74, 566-s Scarbrough & Burgan, DP-MS-83-83 Kanne, WJ 7/83, 17

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Welding history “New process” DOP-26

• 1990 - 1997 Cassini Mission: Cracking eliminated • “New process” DOP-26: more control, more uniform, cleaner

• Equipment upgraded: enhanced process control.

• Refined the weld process

• 4 pole magnetic arc oscillation: was it needed?

• 1997 – 2015: Approximately 450 welds, no cracking

• 2015 - future • Equipment upgrade: enhanced process control

• Refine the weld process

• Additional process monitoring: solidification structure & impact tensile

Franco-Ferreira & George, WJ, 4/96, 69-75

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Magnetic arc oscillator Really?

• Oscillation: 0.005” transverse x 0.010” longitudinal? • Is this a repeated error in the literature?

• How could they measure this, a sheet of paper is 0.005”, this is tiny!

• Could be plausible since arc gap is only 0.035”

• How could this oscillation affect the molten weld pool?

• Arc Oscillator.mp4

Scarbrough & Burgan, DP-MS-83-83

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Magnetic arc oscillator No effect on grain size or morphology

• 1991, 4 pole oscillator • Sigmajig crack test

• “New process” DOP-26

• “14% increase in threshold cracking stress at 55 Hz”

• How does the oscillator reduce crack susceptibility? • “No effect on grain size or morphology”

• “Change in solute segregation”?

• “Change in thermal gradients and stress around weld”?

• “Healing incipient cracks”?

Ohriner & Goodwin, ORNL letter report, 8/91

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What really matters Small weld size, small grain size

Excessive grain size Acceptable grain size ( ≥ 6 grains/thickness)

Ulrich, ORNL, LANL production weld schedule

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What really matters Weld speed, pool shape, solidification structure

• Elliptical: Desirable • Curved growth pattern • Competing grain growth • Interlocking grains • More homogenous solute distribution • Crack resistant

• Transitional: Marginal • Center core of columnar grains • Legacy welds (30 ipm)

• Teardrop: Undesirable • Columnar growth pattern • Uninterrupted growth to weld centerline • Linear weld centerline • Solute concentration along weld centerline • Prone to centerline cracking

Reduced travel speed

Increased travel speed

David & Liu, WJ, 5/82, 157-s

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What really matters Weld speed, pool shape, solidification structure

Liu & David, Met Trans A, Vol 13A, 6/82-1043

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How to optimize the weld Reduce heat input, constrict arc size, alter solidification

• Minimize heat input, weld size, and grain size. • High travel speed, but not too high

(reduce from 30 to 20 ipm)

• Pulsed current • Lower current & varied solidification

• Constrict the arc & increase arc force • Minimize arc gap (0.035” → 0.025”)

• Electrode tip shape (Mound design)

• Argon - 2% hydrogen torch gas

• Ultra-high frequency pulsing (> 10k Hz)

• Minimize O2 & H2O (< 20 ppm)

Jones & Burgan, SME Technical Paper AD75-877, 1975 Fuerschbach & Knorovsky, WJ 9/91, 287-s Lin & Eagar, Met Trans B, Vol 17B, 9/86-601 Eagar, MIT, The Physics of Arc Welding Processes Ohriner, et.al., American Institute of Physics, 1992 AWS C5.5/C5.5M:2003, figure 11, p. 14

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How to evaluate? What is done in production and in prior process re-qualifications?

• Visual 10 – 20X: reject cracks, pores, etc.?

• Weld width gauge: reject > 0.120” (< 0.080” LOP?)

• Ring gauge weld diameter: reject excessive bulge (wall thinning)

• UT: reject indications > 0.0052” • Weld shield fusion confirmed by RT is acceptable

• Metallography of pre-campaign simulant fueled clad • Full penetration, weld shield fusion acceptable (not desirable)

• > 5 grains through thickness

• No single grain > 50% of wall thickness

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How to evaluate? Solidification and grain structure

• Transverse section: • Acceptable grain size

• Top weld surface: • Desirable curved grain structure

• Bottom weld surface: • Less desirable columnar grain

structure

• Top and bottom weld surfaces: • Desirable elliptical ripple pattern

• A more rounded shape may promote a more curved grain structure

(a) Transverse cross section of weld region.

(b) Planar section top view of weld surface. (c) Planar section bottom view of weld surface.

(d) Top weld surface. (e) Bottom weld surface.

Bottom Top

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How to evaluate? Impact tensile ductility test

• The test process • 2” clad blank (disc that is formed

into clad cup)

• Recrystallize to grain size of clad cup, 1350°C / 1hr

• Weld across the middle of blank

• Machine two tensile bars

• Simulate RTG and re-entry exposure 1500°C / 19 hr

• Impact test at 980°C & 200 ft/sec

• Measure % elongation

Friske, ORNL, MET-MatP-SOP-81 Rev.3

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Results to date Metallography, weld on blank

• Developed pulsed welds on clad blanks for impact testing • 100 Hz pulse, amperage adjusted to achieve full penetration

20 ipm Good

30 ipm Like the

legacy welds

40 ipm Undesirable

Travel Speed

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Results to date Impact tensile ductility, lot data & weld data

• No baseline data on legacy oscillated weld

• No cracking in non-oscillated welds

• Good ductility in non-oscillated welds

• Relationship of weld ductility to blank ductility and Th content?

• Note: Old process DOP-26 & 2 pole oscillation

Wide weld = 2.9, 3.7%

Narrow weld = 8.6%

• Lot data: 68 measurements, range = 15 – 33%, average = 23.6, standard deviation = 4.96

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Future

• Establish and execute welding development plan • Try those things listed under “How to optimize the weld”

• Establish weld qualification criteria

• Investigate molybdenum as surrogate metal for process development • Similar grain growth issues, huge cost savings

• Collect impact tensile ductility data • Correlate lot chemistry, lot ductility, and weld ductility

• Satisfy safety basis concerns

• Weld clads • Send to infinity and beyond