Microstructure Evolution of Dissimilar AISI 316L and AISI 444 Stainless Steel Joints by Friction Stir Welding

The present study investigated the main microstructural characteristics of dissimilar welding between AISI 316L and AISI 444 stainless steel. In the welds, 25 to 50 kN axial forces were applied, varying the rotation speed between 350 and 450 rpm and keeping the welding speed constant at 1 mm/s. Three types of joint configurations were used: The first type was the butt joint, and the other two types were overlapped joints. Under all conditions, relevant microstructural changes were not observed between base metal (BM) and heat-affected zone (HAZ). In the thermomechanically affected zone (TMAZ) and stir zone (SZ) of the butt joint, the increase in the axial force and the rotational speed shows austenitic and ferritic grains that are poorly refined at the top and more deformed grains with intense refining at the bottom. In the overlap joints, the positioning of AISI 316L steel, with low SFE and easier recrystallisation at the top of the joint, provided a more negligible difference in deformed and recrystallised grains between the upper and lower TMAZ and SZ, where AISI 444 steel is positioned. In none of the welded conditions is it possible to observe areas of excessive grain growth and formation of undesirable phases such as chi, sigma and Laves despite these phases being predicted in the phase diagram of these steels in the temperature range reached in these dissimilar FSW welds. Therefore, it is possible to produce dissimilar joints between AISI 444 and AISI 316L steels by the FSW process with excellent grain refinement, absence of defects and undesirable phases and high cohesion between the two materials.