The evolution of mechanical properties of steel AISI 316 during direct drive friction welding depends essentially on the heat flow. The generation of this flow depends on two parameters, rotation speed and friction pressure. In the first instance, the optimisation of welding conditions is made to obtain a microstructure closest to that of the base metal. This optimisation is made for a single parameter; the temperature must remain below the phase transformation line, 926 °C for AISI 316 /1, 2/. Results show that the microstructure of the welded joint is built in the last three steps of the process. During the first step of friction phase, the application of friction pressure causes cold deformation creating a mechanical deformation zone (MDZ_1). At the second step of friction phase, MDZ_1 leaves place for softening zone, because of heat diffusion, with a remaining friction pressure. The thermomechanically affected zone (TMAZ) is the final step of the friction phase. The forging pressure applied in the third step defines the microstructure of the welded joint; the TMAZ is replaced by the second mechanical deformation zone (MDZ_2). Additionally, forging pressure application creates inside the new MDZ_2 a hard eccentric core around the weld centre, precisely along the rotation axis. The microhardness of MDZ_2 is higher than TMAZ and less than MDZ_1. Post welding analyses were investigated by microhardness measurements, SEM, and tensile tests. They illustrate the important role of heat flux on the joint microstructure through the welding process.
Keywords: austenitic stainless steel, heat flow, microstructure, welding stages