Friction Stir Welding of Steel: Heat Input, Microstructure, and Mechanical Property Co-relation

Friction stir welding was performed to join carbon steel plates at tool rotational rate of 800-1400 rpm.
Microstructure and microhardness of welded specimens were evaluated across weld centerline. Torque base
index, peak temperature, cooling rate, strain, strain rate, volumetric material flow rate, and width of
extruded zone at weld nugget were calculated. Peak temperature at weld nugget was 1300-1360 K. At this
temperature, ferrite transformed to austenite during welding. Austenite was decomposed in to ferrite and
bainite at cooling rate of 4-7.5 K/s. The presence of bainite was endorsed by increment in microhardness
with respect to base material. Ferrite grain size at weld nugget was finer in comparison to as-received alloy.
With the increment in tool rotational rate strain, strain rate, total heat input, and peak temperature at weld
nugget were increased. High temperature at weld nugget promoted increment in ferrite grain size and
reduction in area fraction of bainite. Heat-affected zone also experienced phase transformation and
exhibited enhancement in ferrite grain size in comparison to base alloy at all welding parameters with
marginal drop in microhardness. Maximum joint strength was obtained at the tool rotational rate of
1000 rpm. Increment in tool rational rate reduced the joint efficiency owing to increment in ferrite grain
size and reduction in pearlite area fraction at heat-affected zone.