| The microstructural features of MA956 friction welds were examined using optical microscopy, STEM and EDS. The results of this investigation have confirmed that friction welding has a major influence on the microstructural features of MA953. The friction weld zone is considered as three distinct microstructural region, namely, region R I containing recrystallized grains, region R II where the grains were reoriented by the upsetting pressure and unaffected base material(region R III). The friction welding operation substantially alters the particle chemistry, dimensions and shape in the joint region. The number of Al-rich or Ti-rich particles is substantially decreased and large irregularly-shape particles are formed. It is postulated that the clustering of Y₂O₃ dispersoids, Al₂O₃ and Ti(C, N) particles is promoted by high strain rate superplastic behavior in the region immediately adjacent to the contact surface. The shape of elongated particles observed in friction welded joints, the variation in chemical composition across particles and the substantial increase in particle dimensions at the periphery of welded samples are all consistent with the proposed particle agglomeration mechanism. |
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