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Abstract
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The microstructure and mechanical properties of the conventional and hybrid friction stir welded
joints of a TRIP steel were investigated. For the aim of hybrid welding, oxy acetylene flame assisted
process was conducted. Optical microscopy, x-ray diffraction, and microhardness test were used to
evaluate the microstructure and mechanical properties. The results showed that the stir zone
composed of mainly the martensite and retained austenite. The volume fraction of the retained
austenite of the base metal i.e. 9.84% was decreased to the 5.73% and 5.09, respectively at the stir zone
of the conventional and hybrid joints. Indeed, the retained austenite in the TRIP steel microstructure
was transformed to martensite under the thermal cycle and deformation induced by rotational tool.
However, hybrid operation induced more martensite formation in the microstructure because it
caused higher peak temperature. As a result, the microhardness profiles of the hybrid weldment
showed the higher hardness in the stir zone in compare with that of conventional process.
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