On experimental optimization of friction stir welding of aluminum 6061: understanding processing-microstructure-property relations

Abstract

A comprehensive investigation into processing-microstructure-property relations for optimization of ultimate tensile strength (UTS) of friction stir welded aluminum 6061 plates is presented. A customized experimental setup has been employed to measure temperature at multiple points of welded plates as well as the friction stir welding (FSW) tool axial force, transverse force, torque, and temperature under various combinations of process parameters. After performing a set of FSW tests based on a full factorial design, X-ray and ultrasonic tests were employed to detect process-induced failure in test samples. Using design of experiments, the main effects and percentage contributions of the process parameters on the maximum UTS were then identified. During the latter analysis, a new methodology is proposed to cope with the effect of “variable” axial force, as it is often uncontrollable during FSW tests. Samples with the highest and lowest UTS were selected and examined in more detail by comparing their fracture surfaces using scanning electron microscopy (SEM), as well as their grain size distributions using electron back scattered diffraction (EBSD) and microhardness experiments. Finally, through the observed microstructure, temperature distribution, and welding force, it could be explained why the UTS and microhardness are found to notably vary between sample sof different RPM and weld speeds.