Microstructure Simulation of Friction Stir Welded UNS S32205 Duplex Stainless Steel

Duplex stainless steels (DSS) combine corrosion and mechanical properties due to a balanced proportion of ferrite and austenite phases, which is only achieved with high composition and heat treatment control. However, during conventional fusion welding, the imposed thermal history impairs this balanced microstructure and the material properties. Friction stir welding (FSW) is a solid state joining process and thus, is not subjected to solidification-associated problems. Although FSW has been mainly studied and applied to low melting temperature alloys, studies have pointed out a number of advantages over conventional fusion welding of DSS. Full penetration joints and their thermal history were obtained and a detailed microstructure characterization was carried on using optical and scanning electron microscopy.

Hot torsion tests were used to reproduce thermo-mechanical history in order to obtain microstructures similar to the ones observed in FSW. A thermo-mechanical simulator Gleeble 3800® was employed to reproduce both deformation and thermal history. Heating and cooling rates and peak temperatures were estimated from FSW joints produced on plates with thermocouples. Deformation and deformation rate cannot be obtained from the original process and were estimated through microstructure comparison. These parameters are essential for reliable implementation of FSW computational simulations. Moreover, physical simulation provides larger volumes of homogeneous microstructure from specific zones of FSW joints, while, in actual joints, microstructure can drastically change from area to area in a few microns. Therefore, mechanical and corrosion properties can be better evaluated through the simulated material, rather than in actual welded joints.

Physical simulation was successfully employed to reproduce microstructure of areas of the FSW joint; grain refinement and balanced microstructure were observed. Microstructure comparisons were based on electron back-scattered diffraction (EBSD), grain size and phase fraction analysis. Austenite, which undergoes partial recrystallization, shows smaller grain sizes located on ferrite grain boundaries.