| The effect of friction stir processing (FSP) on the mechanical wear behavior was investigated for Ti-1Al-8V-5Fe (Ti-185) and stainless steel (Incoloy RTM A-286) alloys. The Ti-185 and A-286 alloys were tested in different processing conditions, including as rolled (AR), AR+FSP, and AR+FSP+aged. A high frequency reciprocating rig was used to simulate frettingtype wear of these alloys at room temperature. The Vickers micro-hardness and wear rates were calculated and compared for each processing condition. It was determined that along with increasing hardness in the stir zones, FSP resulted in improved wear resistance for both alloys. Specifically, wear rates in the stir zones were reduced to lowest values of 1.6 x 10-5 and 5.8 x 10- 7 mm3/N˙m for the AR+FSP+aged Ti-185 and A-286 alloys, respectively, despite lower hardness for A-286 alloy. Mechanistic studies were conducted to determine the reason behind these improvements in wear resistance and the effect of FSP on the microstructural evolution during wear. For the Ti-185 alloy, x-ray diffraction revealed that there was a phase transformation from beta-Ti (AR+FSP) to alpha-Ti (AR+FSP+aged). This beta-phase decomposition resulted in the harder and stiffer alpha-Ti phase responsible for lowering of wear rate in Ti-185. While x-ray diffraction confirmed the A-286 alloy retains its austenitic structure for all conditions, scanning electron microscopy revealed completely different wear track morphology structures. There was increased coarse abrasion (galling) with the AR+aged A-286 alloy compared to the much finerscale abrasion with the AR+FSP+aged alloy, which was responsible for smaller and less abrasive wear debris, and hence lower wear rate. Furthermore, cross-sectional focused ion beam microscopy studies inside the stir zone of AR+FSP+aged A-286 alloy determined that a) increased micro-hardness was due to FSP-induced microscopic grain refinement, and b) the corresponding wear rate decrease was due to even finer wear-induced grain refinement. With both effects combined, the level of damage and surface fatigue wear was suppressed resulting in lowering of the wear rate. In contrast, the absence of FSP-induced grain refinement in the AR+aged A-286 alloy resulted in lower hardness and increasing wear rate. In addition, micro- Raman spectroscopy inside the stir wear zone determined that the wear debris contained metal oxides of Fe 3O4, Cr2O3, and NiO, but were a consequence and not the cause of low wear. Overall, FSP of titanium and stainless steel alloys resulted in lowering of wear rates suggesting it is a viable surface engineering technique to target and mitigate site-specific wear. |
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