Discussion 2: Methods to Join ODS steels
Kiriakos Moustoukas
Thomas Boegelein
Karl Dawson
The Problem
•
Conventional joining
techniques create a
melt pool which has a
detrimental effect on
the nanoparticles in
ODS steels.
•
Agglomeration and
slagging off of
nanoparticles in the
melt pool deplete weld
area of nanoparticles
that improve high
temperature creep
resistance.
Slagging off of yttrium aluminium oxide seen in a
laser melt deposition LMD build.
Discussion Structure
Part 1
• Discuss established welding techniques and ways that can
minimise damage to nanoparticles by altering the weld
parameters.
Part 2
• Discuss welding techniques that avoid a melt pool altogether such
as solid state welding
Part 3
• Consider new welding techniques not currently used for ODS
steels
Part 1 – conventional (fusion) techniques with a melt pool
Weld
cleanliness
Tungsten inert gas (TIG)
HAZ width Automation
(mm)
Plate
thickness
(mm)
Good
2-3
Medium
All (multipass)
Poor
5-6
None
All (multipass
Submerged arc welding (SAW)
Poor
7-10
Very high
All (multipass
Metal inert gas (MIG)
Acceptable
3-4
Medium
All (multipass
Laser
Very good
< 𝟎. 𝟓
Very high
Up to 30
Electron Beam
Very good
< 𝟎. 𝟓
Low
Up to 250
Process
Wright, Ian G., et al. Summary of Prior Work on Joining of Oxide Dispersion-Strengthened Alloys. No.
ORNL/TM-2009/138. Oak Ridge National Laboratory (ORNL), 2009.
Smallest melt pool process are Laser and Electron beam techniques.
Large Melt Pool Welding Processes
Tungsten Inert Gas (TIG)
Submerged Arch Welding (SAW)
Manual Metal Arch (MMA)
Metal Inert Gas (MIG)
Small Melt Pool Welding Processes (HAZ < 𝟓mm)
Laser Welding
Electron Beam Welding
Part 1 – conventional (fusion) techniques with a melt pool
Question 1:
What are the main problems that can be expected when welding
ODS steels?
Part 1 – conventional (fusion) techniques with a melt pool
Question 1:
What are the main problems that can be expected when welding
ODS steels?
• Large volume of material molten (melt pool)(e.g. TIG, filler: base
metal)[5]
• Agglomerations of ODS particles
• Slagging off of (low density) ODS particles
• Loss of grain orientation (e.g. hardness↓)
• Grain coarsening (e.g. Tensile strength↓)
• Cracking due to thermal stresses
• Release of entrapped/absorbed gases especially with Al containing
ODS alloys(→porosity)[6]
→ Low strength of the joints
Part 1 – conventional (fusion) techniques with a melt pool
Question 2:
How could welding involving melting be improved sufficiently
to minimise ODS particle agglomeration?
Part 1 – conventional (fusion) techniques with a melt pool
Question 2:
How could welding involving melting be improved sufficiently
to minimise ODS particle agglomeration?
• Careful application of the conventional techniques [6]
- Preheating
- Minimum inter-pass temperatures
- Post-weld heat treatment to avoid cracking
→ Minimising agglomeration of ODS particles
→ Still not perfect
Part 1 – conventional (fusion) techniques with a melt pool
Question 2:
How could welding involving melting be improved sufficiently
to minimise ODS particle agglomeration?
• Careful application of the conventional techniques [6]
- Preheating
- Minimum inter-pass temperatures
- Post-weld heat treatment to avoid cracking
→ Minimising agglomeration of ODS particles
→ Still not perfect
•
Using laser or e-beam welding
Conduction-based laser welding
(Optional)
[7]
[8]
Welding with keyhole formation
•
•
•
•
•
•
•
•
(effect starts at a high energy density)
Laser beam can be finely focussed
Low heat input (small meltpools, small HAZ)
Flexibility: `Razorblades and ship panels are currently laser welded´
Constant stirring of the meltpool due to a strong thermal gradient
High welding speeds
Process monitoring
´Hybrid´ processes are possible
Easy to automate (robots)
E-Beam welding
[10]
Shapes of the welded zone
• Requires a vacuum (chamber can be evacuated within seconds size
limitations)
• Very high energy density -> high welding speeds, deep penetration
• Deep, narrow welds up to 40:1 ratio (laser: 10:1)[11]
• Excellent weld quality
• Process monitoring
• Easy to automate
Joining ODS Alloys:- Literature
•
Fusion welding techniques are unsuitable as the melt-solidification process results in
excessive coarsening and particle agglomerations.
•
TIG - MA754 (Ni-20Cr ODS) - Severe coarsening and agglomeration of yttrium oxides.
Evidence of yttria slagging off during the welding process. [Molian et al., J. Mat. Sci (1992)]
• Laser - Effect of laser welding on oxide distributions was less detrimental than TIG but
particle coarsening and agglomeration still observed. Lemmen estimates up to 24% of
yttria is lost during welding. [H. J. K. Lemmen et al., Journal of Materials Science 2007, vol. 42, pp. 5286-5295]
•
E-beam welded 9Cr ODS Eurofer - Lindau reports “huge coarsening” of oxide particles
in the fusion zone. PWHT weld alloy contained yttrium rich oxides as large as 400nm.
[Lindau et al., Journal of Nuclear Materials, (2011)]
Elevated temperature Tensile Strength
Eurofer ODS
E-beam welded Eurofer
Non-ODS Eurofer
[Lindau et al., Journal of Nuclear Materials, (2011)]
Part 2 - Solid State Joining Techniques
1.
Diffusion Bonding is the joining of two metallic surfaces by
the diffusion of atoms under pressure and temperature over
time.
2. Pulse Plasma Assisted Diffusion Bonding is the joining of two
metallic surfaces by hot pressing and a pulsed direct electric
current through pins that apply pressure to the sample.
3
Rotary Friction Welding is a solid state joint that uses rotational
energy under an axial load to form a join.
4
Friction Stir Welding is a solid state join that uses friction to
plasticise and a stir bit to join the parts together.
Part 2 - Solid State Joining Techniques
Diffusion Bonding
Rotary Friction Weld
Pulsed Plasma Diffusion Bonding
Friction Stir Welding
Part 2 - Solid State Joining Techniques
• Diffusion Bonding
• Pulse plasma assisted diffusion bonding
• Rotary friction welding
• Friction Stir Welding
Question 3:
What is the main advantage of solid state joining techniques?
Part 2 - Solid State Joining Techniques
Question 3:
What is the main advantage of solid state joining techniques?
• Lack of melt pool avoids the agglomeration of nanoparticles but some
coarsening observed after post weld heat treatments
Part 2 - Solid State Joining Techniques - Literature
• Diffusion bonding – 15CrYWT, 950°C -1200°C,
25MPa,
under vacuum of 5x10-4Pa
[S. Noh et al., Acta Materialia, 59 (2011) 3196–3204]
950°C
• Pulse plasma assisted diffusion bonding
micro plasma discharge ablates Al2O3 scale on
PM2000 alloy – recrystallisation across bond line
– 72% of parent strength (incrementally loaded
creep tests at 1000°C) [G. J. Tatlock et al., Met Mat Trans A,
1200°C
TEM interface
Stress strain curves
2007, vol. 38, p. 1663-1665]
• Rotary friction welding - FeAl40 intermetallic ODS alloy. Approximately 90% of
parent strength but coarsening and agglomeration of oxide particles may remain
an issue.
[Inkson and Threadgill, Materials Science and Engineering A258 (1998), 313-318]
[Inkson and Threadgill, Materials Science and Engineering A258 (1998),
313-318]
Part 2 - Solid State Joining Techniques- Friction Stir Welding
Plan View
Rotationa
l direction
of tool
Transverse
direction of
tool
Advancing side
side
Retreating
• FSW of aluminium is an established
technique.
•
FSW of steels now made possible due to the
development of a tool made from
Polycrystalline Cubic Boron Nitride (PCBN).
•
Tool design constantly being improved to
extend tool life.
Application of FSW to joining ODS alloys
• Studies show FSW can be used successfully
to join ODS materials.
•
APT shows high number densities (5x1023m3) of 2-3nm Y-Ti oxides are retained in FSW
MA957; 7% drop in hardness. [A. Etienne et al.,
Materials Science and Technology 2011, vol. 27, pp. 724-728]
•
Fully consolidated, defect free welds
produced in ¼ inch thick Kanthal APMT –
recrystallised grains elongated in the weld
direction [G. Grant and S. Weil, Pacific Northwest National
Laboratory, Fe-Based ODS Alloys: UC San Diego, La Jolla, CA Nov
17th –18th 2010]
• PM2000 friction stir welded at TWI and
analysed at the University of Liverpool [C. L.
Chen, G. J. Tatlock and A. R. Jones, Journal of Alloys and
Compounds 2010, vol. 504, Supplement 1, pp. S460-S466]
Recrystallised APMT FSW
Part 3 – Welding Techniques not currently used for ODS Steels
Any suggestions?
Conclusions
Conventional fusion techniques
• Large melt pool processes unsuitable for ODS joining
• Some success with laser welding and e beam welding but nanoparticle
agglomeration still an issue.
Solid State Joining Techniques
• The lack of a melt pool is a major advantage
• Mainly positive results with further research needed to improve weld
performance.