Processing, microstructure, and material property relationships following friction stir welding of oxide dispersion strengthened steels

Abstract

A comprehensive set of processing, microstructure, and material property relationships is presented for friction stir welded oxide dispersion strengthened MA956 steel. Eight rotational and traverse speed combinations were used to produce friction stir welds on MA956 plates using a polycrystalline cubic boron nitride tool. Weld parameters with high thermal input produced defect-free, full penetration welds. Microstructural analysis showed a significant increase in grain size, a persistent body centered cubic torsional texture in the stir zone, a sharp transition in grain size from the thermo-mechanically affected zone into the stir zone, and an asymmetric reduction in hardness across the weld, all of which were sensitive to weld parameters. Oxide particles were significantly coarsened by friction stir welding resulting in a complete loss of particle strengthening. Base metal MA956 mechanical properties were determined up to 600C and the effect of friction stir welding on these properties was directly correlated to the evolved microstructure. Grain refinement is a dominant strengthening mechanism in the base metal and for all friction stir welding conditions, as the welding process removed essentially all dislocation and dispersion strengthening contributions. Friction stir welded MA956 retains a majority of its high temperature strength, making the alloy and joining method a suitable candidate for a structural material in advanced nuclear reactor designs.