Investigating the effect of cryogenic pre-cooling on the friction stir processing of AZ31B

Abstract

This manuscript aims to investigate the effect of cryogenic in-process cooling via liquid nitrogen on the outcomes of the friction stir processing (FSP) of twin roll cast (TRC) magnesium alloy AZ31B. Friction stir processing was performed on 3mm thick sheets of TRC AZ31B at a wide range of processing parameters. The tool rotational speed was varied between 600 RPM and 2000 RPM while the tool feed rate varied between 75 mm/min and 900 mm/min. Thrust force and torque values were experimentally measured using a 4- component dynamometer. Temperature measurements were monitored during the different tests using Infrared sensors and thermocouples. The microstructure of processed samples was observed using optical microscopy. It was found that thrust force and torque values of the pre-cooled samples were 5% higher than those of the room temperature samples due to the material hardening induced by the cooling effect. Finer and more homogenous microstructure was observed for the precooled samples when compared with samples processed at room temperature. The average grain size of pre-cooled samples was predicted using a relation -previously introduced by the authors- that relate grain size and the Zener-Hollomon parameter for TRC AZ31B. This equation was found to correctly predict grain diameter for in-line cooled FSP AZ31B samples at temperatures lower than room temperature.