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 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) Manual Metal Arch (MMA) Metal Inert Gas (MIG) Submerged Arch Welding (SAW)
Small Melt Pool Welding Processes (HAZ mm) 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) • 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) → 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  - 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  - 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)   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  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) 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 • Diffusion Bonding is the joining of two metallic surfaces by the diffusion of atoms under pressure and temperature over time. • 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. • Rotary Friction Welding is a solid state joint that uses rotational energy under an axial load to form a join. • 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 Pulsed Plasma Diffusion Bonding Friction Stir Welding Rotary Friction Weld
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] • 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, 2007, vol. 38, p. 1663-1665] 950°C 1200°C TEM interface • 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] Stress strain curves [Inkson and Threadgill, Materials Science and Engineering A258 (1998), 313-318]
Part 2 - Solid State Joining Techniques- Friction Stir Welding Plan View Rotational direction of tool Transverse direction of tool Advancing side Retreating side • 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 (5x1023m-3) 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.