The application of friction stir processing (FSP) on three Ni-base alloys, Alloy 625, Alloy 718, and Hastelloy X utilizing a tungsten-rhenium tool was investigated. A processing window for each alloy was defined in terms of travel speed and tool rotation rate. Process forces and thermal histories were successfully recorded for each alloy. Peak process temperatures were recorded at 1150°C for Alloy 625 and Hastelloy X and 1100°C for Alloy 718 using embedded type K thermocouples. FSP microstructures were investigated using optical and electron microscopy. Significant grain refinement was experienced by each alloy with stir zone grain sizes of 6µm, 5µm, and 4µm for Hastelloy X, Alloy 625, and Alloy 718, respectively, relative to starting grain sizes of 88µm, 26µm, and 44µm. Optical microscopy revealed streaking on the advancing side of the stir zone for each alloy. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis showed these bands to be a result of tool wear. Hardness traverses show that all three alloys experience peak hardness outside the stir zone in the thermomechanically-affected zone. Additionally, it was observed that Alloy 718 and Hastelloy X experienced hardening in the stir zone in comparison to the base metal whereas Alloy 625 underwent softening. Longitudinal sub-size tensile specimens extracted from stir zones showed that Hastelloy X experienced significant strengthening due to FSP relative to the base material whereas Alloy 625 and Alloy 718 experienced virtually no improvement in strength in the FSP region relative to base material.
FSP was further investigated as a means to reduce the susceptibility of these alloys to heat-affected zone (HAZ) liquation cracking. Spot-varestraint testing was used to evaluate the effects of FSP on HAZ liquation in terms of total crack length (TCL) and maximum crack length (MCL). Testing showed a reduction in HAZ liquation susceptibility due to the reduction in HAZ grain size. Optical and electron microscopy evaluation revealed that HAZ liquation cracking resistance is enhanced in the FSP microstructure due to increased grain boundary area resulting in finer, more discontinuous networks of low melting eutectic and second phases.
The modified Gleeble® hot torsion test was utilized to simulate FSP microstructure and investigate shear stress and shear strain encountered by the material. The thermomechanically-affected zone was successfully reproduced for all three alloys, however, stir zone microstructures could not be simulated. Shear stress and shear strain were calculated for each simulation. Examination of the stress-strain curves generated for each simulation showed that a strong correlation between dynamic recrystallization (DRX) and the stress-strain signature. From observation of the curves it can be determined if DRX has occurred.