Using submerged friction stir processing to create nano grained bulk materials

In recent year, considerable efforts have been devoted to fabricate bulk ultrafine and nano grained materials without porosity by imposing severe plastic straining. Friction stir processing (FSP) has a same approach as friction stir welding (FSW), in which a spinning bit is inserted into a confined plate or sheet and the heat generated due to friction increase the sample temperature until it flows plastically, and an automatic axial feed is then implemented to stir a region of material[1,2].The weld or process nugget of FSP sample will display recrystallized microstructures. This would be true for heat treatable alloys. In addition, for heat treatable alloys, the FSP temperatures reached within the nugget and parts of the TMAZ will cause at least partial dissolution of the hardening phases. Some softening within the nugget should be expected in heat treatable alloys that were welded or processed in -T tempers. Some grain coarsening and softening could also take place in the HAZ. The microstructure of the HAZ should transition the base metal into the TMAZ, which, in turn, should transition the HAZ into the nugget. The trends just indicated would tend to give rise to a W-shaped hardness distribution, across the joint profile, with minimum in the HAZ around the HAZ / TMAZ interface. Here, the nugget hardness, depending on the alloy and temper, could range from just above the HAZ minima to values approaching that of the base metal [3].Operating parameters such as processing speeds (V) and tool rotation speeds (ω) have strong effect on microstructure hardness and grain size developments.Submerged friction stir processing (SFSP) has been used by Hofmann and Vecchio to show that the method could be used to creation of ultra-fine-grained bulk samples. The addition of water dramatically reduces the amount of conductive heat flow typically generated by multiple passes of FSP done in air. In addition, the grain size achieved through SFSP can be reduced below200 nm, even in aluminum alloys, where fast recrystallization kinetics generally prevents ultrarefined grain sizes from being obtained