Friction stir welding (FSW)can generate large residual stresses during solid state joining of oxide dispersion strengthened steels.In this work, a plate of MA956 steel was friction stir welded at three conditions: 500 rpm/25 millimeters per minute (mmpm), 400 rpm/50 mmpm and 400 rpm/100 mmpm. The residual stresses across these welds were measured using both x-ray and neutron diffraction techniques.The distribution and magnitude of the residual stresses agreed well between the two techniques. Longitudinal residual stresses up to eighty percent of the yield strength were observed for the 400 rpm/100mmpm condition. The surface residual stresses were somewhat larger on the root side of the weld than on the crown side.Increases in the relative heat input during FSW decreased the measured residual stresses in the stir zone and the thermomechanically affected zone (TMAZ). Increasing the traverse rate while holding the rotational speed fixed increased the residual stress levels. The fatigue strength of the material is predicted to decrease by at least twenty percent with cracking most likely in the TMAZ.
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I. Introduction.Despite the promise of oxide dispersion strengthened (ODS) steels and alloys for use in high temperature nuclear applications, joining these materials through traditional, fusion-based welding is highly problematic. The microstructural attributes that give these materials their strength can be severely degraded by fusion welding techniques. Processes like gas tungsten arc welding and gas metal arc welding result in transport and agglomeration of the finely dispersed oxides to the surface of the weld pool, as well as significant alterations to the grain sizes upon solidification.[1]The result isa significant reduction in yield strength across the weld and degraded creep properties in the large heat affected zone(HAZ) outside the weld pool. Other processes like electron beam welding and laser beam welding give similar results, coarsening and agglomerationof the oxide particles that give ODS steels their strength [2,3].Friction stir welding (FSW) shows significant promise as a solid-state joining technique for ODS steels. FSW involves the insertion of a hard, rotating tool into the surface of the pieces to be joined. The tool rotation heats the material to approximately eighty-to-ninety percent of the melting point of the alloy. [4,5] The tool is then translated across the specimen, thus joining the material. This process does not melt the base metal, and preserves many of the mechanical and microstructural properties that make ODS steels attractive. Several authors have successfully demonstrated friction stir welding on ODS steels. [6-11] While defect-free welds can be achieved in ODS steels by FSW, some evolution in the stir zone (SZ) microstructure has been observed; including an increase in ferrite grain size, an increase in oxide particle size, and a resultant reduction in hardness and yield strength for the stir zone material. [6,7,11,12] Although FSW can significantly reduce the distortion that ofte...
The oxide dispersion strengthened steel MA956 was friction stir welded using eight different rotational speed/translational speed combinations using a polycrystalline cubic boron nitride tool. Weld parameter conditions with high thermal input produced defect-free, full penetration welds. Electron backscatter diffraction showed a significant increase in grain size in the stir zone, a body centered cubic torsional texture in the stir zone, and a sharp transition in grain size across the thermo-mechanically affected zone. Micro-indentation results showed an asymmetric reduction in hardness across the transverse section of the weld that was sensitive to the heat input. This change in hardness is explained by the increase in grain size and may be described using a Hall-Petch type relationship.
A comprehensive set of processing-microstructure relationships is presented for friction stir welded oxide dispersion strengthened MA956 steel. Eight rotational speed/traverse speed combinations were used to produce friction stir welds on MA956 plates using a polycrystalline cubic boron nitride tool. Weld conditions with high thermal input produced defect-free, fullpenetration welds. Electron backscatter diffraction results showed a significant increase in grain size, a persistent body centered cubic torsional texture in the stir zone, and a sharp transition in grain size across the thermo-mechanically affected zone sensitive to weld parameters. Microindentation showed an asymmetric reduction in hardness across a transverse section of the weld. This gradient in hardness was greatly increased with higher heat inputs. The decrease in hardness after welding correlates directly with the increase in grain size and may be explained with a Hall-Petch type relationship.
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