Structural characterisation of reaction zone for friction stir welded aluminium–stainless steel joint

In the present investigation, commercially pure Al has been joined with 304 stainless steel (SS) by
friction stir welding. The assembly finds widespread application in the field of cryogenics, nuclear,
structural industries and domestic appliances. Microstructural characterisation was carried out
using scanning and transmission electron microscopes. It has been found that diffusion of Fe, Cr
and Ni is substantial within Al; however, diffusion of Al within 304SS is limited. Owing to
interdiffusion of chemical species across the bondline, discrete islands of Fe3Al intermetallic form
within the reaction zone. The rubbing action of tool over the butting edge of 304SS removed fine
particles from 304SS, which were embedded in the stirring zone of Al matrix. Subsequently,
austenite underwent phase transformation to ferrite due to large strain within this grain. Fracture
path mainly moves through stirring zone of Al alloy under tensile loading; however, in some
places, presence of Fe3Al compound has been also found.