Friction Stir Welding of Stainless Steel to Al Alloy: Effect of Thermal Condition on Weld Nugget Microstructure

Joining of dissimilar materials is always a global challenge. Sometimes it is unavoidable to
execute multifarious activities by a single component. In the present investigation, 6061 aluminum
alloy and 304 stainless steel were joined by friction stir welding (FSW) at different tool
rotational rates. Welded joints were characterized in optical and scanning electron microscopes.
Reaction products in the stirring zone (SZ) were confirmed through X-ray diffraction. Joint
strength was evaluated by tensile testing. It was found that the increment in average heat input
and temperature at the weld nugget (WN) facilitated iron enrichment near the interface.
Enhancement in the concentration of iron shifted the nature of intermetallics from the Fe2Al5 to
Fe-rich end of the Fe-Al binary phase diagram. The peak microhardness and ultimate tensile
strength were found to be maxima at the intermediate tool rotational rate, where Fe3Al and
FeAl2 appeared along with Fe2Al5.