Ni-based Mar-M247 superalloy as a friction stir processing tool

摘要

Ni-based superalloy Mar-M247 was evaluated as a potential tool material for friction stir processing (FSP) of X70 steel. The superalloy consists of a gamma-gamma' structure and minor precipitates, such as MC carbide. FSP tests evaluated the tool performance at different rotation and welding speeds and the effect of shielding gas was also analyzed. Prior to FSP tests, hot compression tests showed that the tool material microstructure was stable below 950 degrees C, which affected positively its mechanical strength. It was observed the FSP tool overheating at rotation speed of 400 rpm. In this condition the gamma-gamma' structure was destabilized forming "gamma' free zones" at the tool shoulder. In these zones new gamma-grains were formed by recrystallization. Oxygen diffusion occurred at gamma-grain boundaries embritding the gamma' free zones that were ripped out by abrasion during FSP tests. The tool wear damage was mitigated by controlling friction processing parameters, such as rotation and welding speeds, Z-force values and gas shielding. The FSP made in this work improved surface properties of X70 steel.