Effects of forge axis force and backing plate thermal diffusivity on FSW of AA6056

Using three sets of tool rotation rate and welding speed, series of friction stir welds were made at various forge axis forces on different backing plates with widely varying thermal diffusivity on 4.2 mm thick AA6056. Temperature during the process was measured at the probe core using a thermocouple. Because of the use of different backing plates and forge forces it was possible to obtain a relatively wide range of peak temperature in the nugget. This provided a unique opportunity to investigate changes in joint microstructure and property (1) over a wide range of peak temperature while keeping welding and rotational speed constant (2) at similar peak temperature with different welding and rotational speeds. The results show that for the studied gauge thickness, metallurgically significant temperature variations can be achieved without changing the rotation and welding speed while producing defect free welds. The backing plate conductivity and forge force both independently affect the weld process parameters such as nugget temperature and tool torque significantly. A significant increase in the tool torque and hence power is observed with the increase in the forge force alone.