Evaluation of microstructure in friction stir welded austenitic stainless steels

Abstract

In this study, AISI 304 (X5CrNi18-10, material number 1.4301) austenitic stainless steels, 3.0 mm thick, were joined by friction stir welding method. Microstructure characteristic were evaluated. Four main microstructures were observed around the weld joint. These are base materials (BM), heat affected zone (HAZ), stirred zone (SZ) and thermo-mechanically affected zone (TMAZ). The average grain size in the SZ was between 3 and 7 ?m, which is smaller than that in the BM. The average grain size in the HAZ was about 20 ?m, which is half of that in the BM. The microstructure of the transition zone between the base material and the weld zone is different from the microstructures of both of the regions. The dark bands seen in the weld zone can be also found in their microstructure. It was determined that these bands are not ultra fine grains. They are Cr2O3 oxide layers which over the surface of stainless steels may have been ruptured during friction stir welding and may form bands inside the welding bead due to stirring.