assume that there is two initial microstructure with the same chemical composition (austenitic stainless steel) the first one has an average grain size of 4 micron and the second one's is about 50 micron, and both of these microstructures were friction stir welded with the same welding parameters which generate the same heat and strain.
Is there any different in the final grain structure and grain size after welding? which one become more refined? and is it possible that the first specimen with finer grain size having less potential for recrystallization?
There is difference in the behavior. When the size of grain is the littlest the system has height level of energy.
FSW is characterized by intense deformation and heating which result in fine crystalline structure in the stirring zone. Given that deformation is too high it makes no difference for this metal what was the initial grain size. It could only have some effect on the plunging force since fine crystalline metal has higher hardness. Bbut intense heating makes this difference negligible. However, there is a heat affected zone too where initial grains are not deformed and may only grow ( or experience precipitation) while heated. This may affect the weld strength.
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