Managed by Triad National Security, LLC for the U.S. Department of Energy’s NNSA

Capacitive Discharge Resistance Welding

of 14YWT and Other Alloys

LA-UR-#

LA-UR-20-XXXXX

UNCLASSIFIED

C.R. Lear1, B.P. Eftink1, L. Lindamood2, T.E. Steckley1,

M.M. Schneider1, J. Gould2, T.J. Lienert3, S.A. Maloy1

1 Los Alamos National Laboratory, Los Alamos, NM 875452 EWI, Columbus, OH 432213 T.J. Lienert Consulting, LLC, Los Alamos, NM 87544

Radiation damage and induced microstructural change

present an on-going materials challenge

02/26/2020 | 2Los Alamos National Laboratory

J.O. Stiegler, 1979

Averback & Ghaly

Dislocation

Loops

PrecipitatesCavities

• Materials development for high T, high ϕ Gen IV and

fusion reactors

• Vetting current materials for Gen II/III reactor life

extension (dashed box)

S.J. Zinkle, 2009

Nanostructured materials trap defects and gasses,

limiting evolution and minimizing deterioration

Los Alamos National Laboratory

G.R. Odette, 2008

• Nanostructured ferritic alloys (NFA) are

formed by mechanical alloying for fine grains

and high densities of nanoscale features

• Resistance to creep and coarsening at

high temperature

• Voids/bubbles kept small and off grain

boundaries via abundant nucleation

• Oxide dispersion strengthened (ODS) alloys

are promising NFAs, as Y, Al, and Ti based

nanoparticles are fully incoherent and very

stable with irradiation

• With proper chemistry, NFA ODS materials

have also shown improved high temperature

oxidation resistance vs. conventional claddings

02/26/2020 | 3

Los Alamos National Laboratory

• Conventional fusion techniques degrade

microstructure via heat input and local melting

• Growth of nanoscale grains

• Coarsening, agglomeration, or redistribution

of ODS particles in molten regions

• Has driven development of solid state

welding (SSW) techniques

• Friction stir welding (FSW) is one of several SSW

techniques in use today

• Friction heating softens material at the joint

without melting, allowing for physical mixing

• Has been used for 20 years for steels, with

some success on NFA’s

• Leaves a hole when the tool is withdrawn and

is not an option for small or complex parts

NFA-ODS microstructures and dispersoids require

advanced welding techniques

02/26/2020 | 4

Z. Feng & W. Ren, 2006

Los Alamos National Laboratory

NFA-ODS microstructures and dispersoids require

advanced welding techniques

• Pressure resistance welding (PRW) is flexible

alternative SSW for cladding

• Resistive heating softens the point of

contact between parts, while mechanical

pressure forces material to flow and bond

• Commonly uses a “projection”, or reduced

contact, to focus heating and deformation.

This collapses, driving aside surface oxides

and minimizing affected material

• Capacitive discharge resistance welding (CDRW)

is a modification of PRW

• A capacitor bank to guarantees fast

discharge of high current

• Rapid thermal cycles, 2-5x faster than most

PRW, limit microstructural evolution and

excess heating

02/26/2020 | 5

Plug

Tube

Plug

Tube

Plug

Tube

J. Gan, 2018

60º Chamfer

Cladding tubes were capped using CDRW,

for minimum heat input

Los Alamos National Laboratory 02/26/2020 | 6

Cross-section View

Cap

Tube

~5 mm

45º

?

Flow at T > 1000 ºC and ሶ𝜀 ≈ 103 s-1 poorly

understood for these materials.

Los Alamos National Laboratory

Backflow

Into Tube

Resolidification

& Coarsening

Incomplete

Sealing

Tube

B126Y

Cap

Kanthal

Rapid screening using optical microscopy;

detailed characterization using electron microscopy

Dynamic

Recrystallization

at Weld Line

MA956

ODS Particles

Tube

B126Y

Cap

Kanthal

02/26/2020 | 7

(left) Grain orientations

mapped using EBSD

(right) Dispersoids imaged

using HAADF STEM

14YWT – 14YWT

(ODS – ODS)

Focus on two groups of welds

Los Alamos National Laboratory

B126Y – MA956

(FeCrAl – ODS)

Typical CDRW Structure?

Parameter Influence?

Radiation Stability?

02/26/2020 | 8

Los Alamos National Laboratory

Cap

MA956

Tube

B126Y

CI > 0.1

Microstructural effects of CDRW were extremely localized

02/26/2020 | 9

Los Alamos National Laboratory

Base

Material

Mechanically

Affected

~100 μm

Recrystallized

~50 μm

CI > 0.1

Microstructural effects of CDRW were extremely localized

Cap

MA956

Tube

B126Y

02/26/2020 | 10

Los Alamos National Laboratory

Base

Material

Mechanically

Affected

~100 μm

Recrystallized

~50 μm

Note the difference between

“hard” and “soft” components.

CI > 0.1

Microstructural effects of CDRW were extremely localized

Cap

MA956

Tube

B126Y

02/26/2020 | 11

Weld microstructure was not sensitive to parameter choice

Los Alamos National Laboratory 02/26/2020 | 12

• Increased heat input lead to recrystallized

and mechanically affected zones that were

slightly thicker and thinner, respectively, …

• Increased force lead to slightly smaller

recrystallized grains, …

• … but all these trends were statistically

insignificant here. This suggest a broad

flexibility for selecting CDRW parameters.

Los Alamos National Laboratory 02/26/2020 | 13

• Increased heat input lead to recrystallized

and mechanically affected zones that were

slightly thicker and thinner, respectively, …

• Increased force lead to slightly smaller

recrystallized grains, …

• … but all these trends were statistically

insignificant here. This suggest a broad

flexibility for selecting CDRW parameters.

Tube Cap

Molten material expelled

through fixture gaps

There are practical, non-microstructural

reasons to optimize CDRW conditions.

Expulsion of material is much more

common at high heat input!

Weld microstructure was not sensitive to parameter choice

Accelerated irradiation testing was carried out

using protons and heavy-ions

Los Alamos National Laboratory

IBL

@ UW

MIBL

@ UM

https://mibl.engin.umich.edu/

https://ibl.ep.wisc.edu/

Interest IonTemperature

(ºC)

Dose

(dpa)

MS

Swelling

3.7 MeV

Fe2+

35010

100

45010

100

Defects

RIS

1.2 MeV

H+ 3601

5

02/26/2020 | 14

Proton irradiation (5 dpa, 360 ºC) did not change

ODS particles near welds

Los Alamos National Laboratory

HAADF of unirradiated and irradiated regions

of same sample, < 10 μm from weld

5 dpa H+0 dpa

02/26/2020 | 15

Proton irradiation (5 dpa, 360 ºC) did not change

ODS particles near welds

Los Alamos National Laboratory

HAADF of unirradiated and irradiated regions

of same sample, < 10 μm from weld

5 dpa H+0 dpa

02/26/2020 | 16

14YWT – 14YWT

(ODS – ODS)

Impact of Strength?

Dispersoid Evolution?

Focus on two groups of welds

Los Alamos National Laboratory

B126Y – MA956

(FeCrAl – ODS)

02/26/2020 | 17

CDRW parameters were adapted for higher strength 14YWT

Los Alamos National Laboratory

Although at higher F and I than previous

materials, Trial 2 proves that CDRW is still

suitable for high strength alloys like 14YWT!

02/26/2020 | 18

• Simplification: assumed minimal effects from

grain size, dislocation density, and resistivity.

• Scaled known, low- ሶ𝜀 flow stresses using best

published estimates (e.g., McQueen, Richards

and Sheppard).

• Extrapolated trend with temperature to ~1100 °C,

running trials for highest and lowest strength.

Tube

14YWT

Cap

14YWT

From Trial 2

Remaining

GapContinuous

Seal

Trial 1

Trial 2

ሶ𝜀 ≈ 103 s−1

Small-scale microstructure unchanged by CDRW

Los Alamos National Laboratory 02/26/2020 | 19

Pre-existing

Large Grains

“Under-quenched”

Material

Warping

of Tube

“Texture”

Pre-existing

Large Grains

Tube

14YWT

Cap

14YWT

Dispersoids remained uniformly distributed after CDRW

Los Alamos National Laboratory 02/26/2020 | 20

Weld ~5 μm~5 μm~10 μm

The smallest dispersoids remain after CDRW

Los Alamos National Laboratory 02/26/2020 | 21

~10 μm from Weld

The smallest dispersoids remain after CDRW

Los Alamos National Laboratory 02/26/2020 | 22

~10 μm from Weld

Future Work

Los Alamos National Laboratory 02/26/2020 | 23

Interest IonTemperature

(ºC)

Dose

(dpa)

Swelling5.0 MeV

Fe2+ 450 600

Defects1.2 MeV

H+ 300 5

D.B. Williams & C.B. Carter, 2009

Improved Filtered Imaging

Irradiation Effects

Mechanical Testing

P. Hosemann, 2018

https://teamafw.com

(left) Petal testing for

qualitative “ductility”

of CDRW joints

(right) Nanomechanical

testing of individual

weld zones

Conclusions

Los Alamos National Laboratory 02/26/2020 | 24

• CDRW was used to join similar and differing ferritic alloys, with a focus on ODS weldability. Joint quality varied, but solid

bonds were produced for even considerably stronger alloys like 14YWT.

• CDRW joints showed considerably thinner – or absent – weld zones than more conventional techniques, suggesting that

weld performance (e.g., strength, resistance to degradation) will be favorable. The extent of these zones was minimally

effected by parameter choice, though material expulsion at high heat input could create practical issues in industry.

• CDRW of MA956 and 14YWT components did not redistribute dispersed particles inhomogeneously in areas nearest the

joint, and the smallest particles in 14YWT (~2 nm diameter) were preserved. Future work in this line will focus on improved

microscopy of these dispersoids.

• CDRW joined MA956 showed no significant changes in dispersed particles after 5 dpa proton irradiation, but analysis of

heavy-ion irradiated samples is still ongoing. Future work in this line will focus on irradiation of newly made 14YWT welds

to understand hardening and high-dose swelling effects.

• Work will begin soon on mechanical testing of old and new CDRW joints, with nanomechanical testing used to probe

individual weld zones. Petal testing will provide a further qualitative benchmark of weld “ductility”.

This work was supported by the U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho

Operations Office Contract DE-AC07- 051D14517 as part of a Nuclear Science User Facilities experiment.