Correlation of Microstructures and Process Variables in FSW HSLA-65 Steel

The present study focuses on developing a relationship between process variables and postweld microstructure in friction stir welded HSLA-65 steel. Fully consolidated welds were produced in HSLA-65 steel using a PCBN convex-scrolled-shoulder-step-spiral (CS4) tool over a wide range of parameters. Microstructures in the nugget center (NC) are governed by lath bainite and with some polygonal/allotriomorphic grain boundary ferrite, which are highly dependent on heat input. Friction stir welded dependent variables are related with FSW independent variables by nonlinear relationship. Heat input is identified as the best parameter index. With increasing heat input, the volume of bainite decreases and the ferrite grain and lath sizes increase. A linear relationship was established between heat input and semiquantitative postweld microstructures.