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Title: The friction stir welding of oxide dispersion strengthened ferritic alloys
Author: Moustoukas, Kiriakos
ISNI:       0000 0004 6422 5292
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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FeCrAl ferritic alloys are highly oxidation and corrosion resistant alloys but have limited creep strength at high temperatures. The incorporation of a stable rare earth oxide such as yttrium oxide in a FeCrAl alloy increases the in service temperature of the alloy up to 1100°C. Oxide Dispersion Strengthening (ODS) is achieved by the Mechanical Alloying (MA) process where the yttrium oxide is added to the alloy powders, and is forced into solution with the matrix powder. MA is a time consuming and expensive process but it results in high number density of fine nanometer size particles that provide excellent creep resistance at high temperatures. One such alloy is PM2000 alloy made by Plansee™ GmbH A more cost effective processing route for the inclusion of oxides in the matrix is the Rapid Solidification Powder (RSP) process where the oxide powder is added to the gas atomised powder from the melt of the alloy to form an ODS alloy but with a much lower number density of larger particles. The alloy still has excellent oxidation and corrosion resistance but limited high temperature stressed applications. Kanthal. APMT is one such alloy that is commercially available. Conventional joining techniques that produce a melt zone cannot be used to join ODS steels as the nanoparticles tend to agglomerate and slag off to the surface weakening the weld. Friction Stir Welding (FSW) is a relatively new solid state joining technique that preserves the nanoparticle distribution in the matrix. In this work 4 different types of welds of ODS steels, using FSW were studied. The welds were of a combination of either MA produced PM2000 and/or RSP produced APMT alloys. The FSW of butt configuration as-rolled PM2000 were studied. Fragments of the polycrystalline cubic boron nitride (PCBN) tool were observed in the thermal mechanically affected zone (TMAZ). On the application of post weld heat treatment (PWHT) these fragments were partially broken down due to a phase change in PCBN and TiN grains were observed around the fragment with the surrounding matrix being rich in Ti. The FSW of as-extruded plates of PM2000 has revealed an area in the TMAZ with an unsuitable grain structure, the grain structure being controlled by the high number of nucleation sites for new grains during recrystallisation. In both types of weld above, coarsening of the nanoparticle distribution was observed in the TMAZ with extensive agglomeration of the particles observed in the top part of the welds. This was attributed mainly to the flow of the material creating more 'encounters' between particles rather than the heat created by the friction of the tool. The FSW of APMT has revealed that the wormhole type voids observed in the TMAZ, originated from a feature on the tool surface, as the voids were formed in a spiral along the welding direction. Finally in the dissimilar metal FSW between APMT and as-extruded PM2000, a distinct region of PM2000 was observed in the APMT half of the weld. Diffusion due to concentration differences were observed by Fe, Mo and Cr as expected. However diffusion of Ti from PM2000 towards APMT had resulted in the formation of Ti(C,N) rich particles after the application of PWHT at the prior boundary between APMT and PM2000. The higher C and N concentration in APMT had resulted in diffusion towards the PM2000 and the capture of Ti present in PM2000 in particles before Ti could diffuse out toward the APMT.
Supervisor: Tatlock, G. J. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral