F.R. Wan

Grain characteristic and texture evolution in friction stir welds of nanostructured oxide dispersion strengthened ferritic steel

Abstract The present paper investigated the grain and texture characteristics in a nanostructured oxide dispersion strengthened ferritic steel subjected to friction stir welding. The ‘onion rings’ structure obviously exhibited in the macrostructure overview of the welds. The electron backscatter diffraction (EBSD) work revealed that the ‘onion rings’ comprised alternate layers made by coarse and fine grains, while no strong texture was exhibited in the alternate layers of the ‘onion rings’. Image quality maps of EBSD indicated that layers of fine grains were deformed under high strain conditions. Textures within the stir zone and thermomechanically affected zone were weak and exhibited some characteristics of bcc simple shear textures. Results of grain boundary revealed that the mechanical action in welding process promoted the transformation of low angle to high angle boundaries and contributed to the grain refinement.

Effect of trailing heat sink on residual stresses and welding distortion in friction stir welding Al sheets

Abstract This paper investigates a trailing heat sink, which was designed and applied to friction stir welding (FSW) in order to control the residual stresses and welding distortion. Residual stresses, residual plastic strains and welding distortion of 2024-T3 and 5083-H321 Al sheets welded by FSW with and without the trailing heat sink were compared. The optimal placement of the heat sink was discussed. The results revealed that the reductions in peak tensile stresses were 66% for 2024-T3 and 58% for 5083-H321 by application of the trailing heat sink in FSW. In addition, the welding distortion could be reduced drastically by this method. The 5083-H321 sheet with a size of 1000×100×3·5 mm welded by this method was very flat and had almost no distortion. This method achieved in-process control of stresses and welding distortion, without additional complicated work before or after welding operation.

Effect of Deuterium Ion Implantation on Microstructures of Fe-M (M =V, W, Ta) Alloys

To study the effect of tungsten, vanadium and tantalum on the microstructures in CLAM (China Low Activation Martensitic) steel after irradiation respectively, the microstructures of Fe-M (M = V, W, Ta) model alloys were investigated after implanted deuterium ions using an ion accelerator at 773 K. After implanted deuterium ion, TEM (Transmission Electron Microscope) observation and EDX (Energy Dispersive X-ray Spectrom) analysis have been carried out. The result showed that tiny voids were observed in all model alloys after implanted the same dose of deuterium ions. The swelling rate in FeTa alloy was the smallest among the three alloys. Unlike FeW and FeV alloys, there was the segregation in FeTa alloy under a fluence of 5×1017 D+/cm2 at 773 K. A theoretical analysis showed that the void growth in FeTa alloy slowed down due to tantalum segregation near voids. It indicates that tantalum plays an important role in the improved irradiation resistance of CLAM steel.

The Structure and Properties of Low Activation Ferritic / Martensitic Steels

This paper reported the low activation martensitic steels which are being studied to develop the structural materials in fusion reactors. The steels were based on 9Cr1.5WVTa, but the effect of alloy elements was investigated by changing the amounts of alloy elements or adding other elements. The structure and properties of the steels were studied by tensile experiment, X-ray diffraction, SEM, TEM. Also the metallurgical process and heat treatment effect were discussed.

Joint inhomogeneity in dissimilar friction stir welded martensitic and nanostructured ferritic steels

ABSTRACT Dissimilar welding between oxide dispersion strengthened ferritic (ODS) steel and reduced activation martensitic steel would be required for constructing the advanced blanket of progressive fusion reactors. In this study, we achieved dissimilar joints by friction stir welding, and aimed to characterise and ameliorate joint inhomogeneity. Main results reveal that the joint inhomogeneity is generated from discrepant microstructural evolutions within the martensitic and ODS ferritic steels. The ODS steel achieves evolution by the dynamic recrystallisation, while the martensitic steel undergoes phase transformation that drastically hardens the stir zone. By a proper post-weld heat treatment, the joint inhomogeneity can be effectively ameliorated due to carbide reprecipitation and stress relief in the joint.

Microstructures and ion-irradiation behaviour of friction stir welded 12Cr-ODS steel

ABSTRACT 12Cr oxide dispersion strengthened steel was successfully joined by friction stir welding (FSW), and the joints were irradiated with 3u2009MeV Au ions at 500°C. The morphology of grains, dislocations, sizes and distributions of nano-sized oxide particles (NPs) in different zones of the FSW joints were analysed. It was found severe plastic deformation occurred and fine equixed grains were formed in stir zone (SZ). The NPs coarsen obviously in thermal-mechanically affected zone (TMAZ) on advancing side (AS) due to the relative movements of plastic materials with high-velocity gradient. The size distributions of the NPs in base metal (BM), heat affected zone (HAZ) and RS-TMAZ are concentrated in a smaller range than those in SZ and AS-TMAZ. Finer grains and high density of dislocations result in hardness increasing obviously in SZ. Irradiation introduced higher hardening in HAZ because of its less grain boundaries and dislocation density compared with SZ and BM.

Microstructure and Nano-hardness of Pure Copper and ODS Copper Alloy under Au Ions Irradiation at Room Temperature

The microstructure and nano-hardness of the pure copper and oxide dispersion-strengthened (ODS) copper alloy subjected to 1.4xa0MeV Au ions irradiation at room temperature were investigated. After irradiation, dislocation-loops form in both materials, while voids can only be generated in the pure copper. Compared with the irradiated pure copper, larger average diameter and lower number density of irradiation-induced dislocation-loops were detected in the ODS copper alloy, revealing that high-density dislocation and large volume of Al2O3 particles existing in the ODS copper alloy can act as effective sinks for the irradiation-induced defects. It was also detected that irradiation hardening in the ODS copper alloy is lower than that in the pure copper. The microstructure and nano-hardness results reveal that the ODS copper alloy has a better irradiation tolerance than the pure copper. In addition, the average diameter of the Al2O3 particles in the ODS copper alloy decreases after irradiation, because the Al–O chemical bonds are decomposed and the atoms are redistributed in the matrix during the irradiation process. This work reveals that the irradiation tolerance of the copper can be effectively enhanced by adding nano-sized Al2O3 particles into the matrix.