A comparative study between conventional and hybrid friction stir welding of a TRIP steel

Abstract

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.