Microstructural and Statistical Investigation on Dissimilar Friction Stir Spot Welding of Austenitic and Ferritic Stainless-Steels

Abstract

The increasing use of austenitic and ferritic stainless steel sheets, especially in the power plant industry, highlights the importance of research into the mechanical and metallurgical properties of these joints. Friction stir spot welding of these sheets was investigated using full-factorial experimental design. Considering the three main parameters of rotational speed, plunge depth and dwell time, each of which had three levels, led to 27 tests. The highest strength (6400 N) was related to sample number 8 with parameters 900 rpm, 2.7 mm and 20 s and this sample was selected for microstructural studies. The metallurgical properties of this sample were investigated and the microstructures of heat affected zone, thermos-mechanically affected zone and stir zone were extracted. Thermomechanical changes during the process and especially dynamic recrystallization in the stir zone due to heat application and severe plastic deformation by the process reduce the grain size, which in turn increases the bond strength. Using analysis of variance, it was found the direct effect of rotational speed and plunge depth on the lap shear strength is significant. The interaction effect of rotational speed and plunge depth and also, rotational speed and dwell time on the response were significant.