Microhardness measurements were conducted at room tem perature conditions in two directions, along the axial direction 
and the interface, over+-4.5 mm. The measurements taken 
along the axial direction provide information about the mate rial characteristics of the welded joint, while those taken at 
the interface illustrate the softening and hardening along that 
interface.This can be explained by the 
larger grain size structure in the peripheral region compared 
to the centerline [2].The 
regional speed and temperature at the peripheral region are 
higher than those at the central region [39].Therefore, the friction between the 
two surfaces at the interface through the FW process, under 
the efect of pressure, encourages bulky plastic deformation 
that leads to fne grain and is the frst reason for the micro hardness amplifcation at the weld centerline.Figure 5a shows the microhardness profle for the 
axial direction, with elevation values related to AISI 316 at 
the weld centerline.