Chemical-assisted magnetorheological finishing of additively manufactured Inconel 718 alloy

Additively manufactured Inconel 718 is extensively engaged for the developments of various high temperature components owing to its favourable mechanical and tribological properties. In most of the cases, precision surface finishing is required for better functional performance of those components. In the present study, a multi-step finishing strategy is employed that comprises standard magnetorheological finishing (MRF) and chemical-assisted magnetorheological finishing (CAMRF) processes to achieve nano-scale surface finish of Inconel 718 coating fabricated via laser direct energy deposition (LDED) technique. At first, standard MRF is conducted over the ground surface and a surface roughness (Sa) of 210 nm is achieved. The performance of standard MRF process is restricted owing to the considerably higher hardness of Inconel 718 and limited effectiveness of MRF process and that finally leads to the non-uniform polishing of coating. To further improve the surface finish of coated surface, CAMRF is introduced with the employment of two distinct chemical etchants of IN 718, i.e., etchant 1 (i.e., glyceregia reagent) and etchant 2 (i.e., hydrofluoric acid (HF) based). The optimized vol% of both chemical etchants in MR fluids is found to be 8 %. During CAMRF process, the etchant present in CAMR fluid ribbon interacts with the surface peaks and the surface gets oxidized owing to the oxidizing nature of the chemical etchant. As a consequence, the oxides of major constituents of Inconel 718 (i.e., NiO, Cr₂O₃, etc.) are formed (i.e., passivation film). This hypothesis of oxide formation is confirmed by X-ray photoelectron spectroscopy (XPS) analysis. This passivation film makes the surface asperities/peaks weak and less resistant to removal. As a results, those passivated and weak asperities get sheared off easily by the abrasion action of the abrasive particles present in the CAMRF tool. Due to these integrated effects of chemical etching and mechanical abrasion, the effectiveness of CAMRF process is higher than the standard MRF process. The surface roughness (Sa) associated with CAMRF with etchant 1 and etchant 2 is found to be 145 nm and 100 nm, respectively. The improved surface finish in CAMRF with etchant 2 is attributed to the formation stable oxide film owing to the high reactivity of HF. The friction coefficient and specific wear rate of finished coating are reduced by almost 44 % and 49 % than the ground coating, respectively and this indicates a significant improvement in tribological performance of the finished coating.