Abstract
This work presents the first practical application of ionic electrolytes for electropolishing of nickel-based superalloys. It contains the results of an experiment-driven optimization of the applied potential and electrolyte temperature during electropolishing of laser powder bed-fused IN625 components containing surfaces oriented to the building platform under angles varying from 0 to 135°. For comparative purposes, the roughness profilometry and confocal microscopy techniques were used to characterize the surface finish topographies and the material removal rates of IN625 components subjected to electropolishing in ionic and acidic (reference) electrolytes. After 4 h of electropolishing in both electrolytes, a roughness of Ra ≤ 6.3 µm (ISO N9 grade number of roughness) was obtained for all the build orientations. To elaborate, both electrolytes manifested identical roughness evolutions with time on the 45°(75% Ra reduction) and 90°-oriented (65% Ra reduction) surfaces. Although the roughness reduction on the 135°-oriented surface in the ionic electrolyte was 17% less than in the acidic electrolyte, the former provided a more uniform roughness profile on the 0°-oriented surface (30% Ra reduction) and 44% higher current efficiency than the acidic electrolyte. This work proves that ionic electrolytes constitute a greener alternative to industrial acidic mixtures for electropolishing of three-dimensional (3D)-printed parts from nickel-based superalloys.
Highlights
Almost all three-dimensional (3D)-printed metal parts require surface finishing to meet performance standards
Chemical polishing is characterized by a low material removal rate and inability to eliminate all laser powder bed fusion (LPBF)-induced surface features
A successful removal of the LPBF surface features using electropolishing (EP) with HClO4–CH3COOH mixed acidic electrolytes has been demonstrated on Ni-based superalloys IN718 [2] and IN625 [3]
Summary
Almost all three-dimensional (3D)-printed metal parts require surface finishing to meet performance standards. Efforts to develop surface finish techniques for parts that are manufactured by metal laser powder bed fusion (LPBF) technology are increasing exponentially. Examples of such techniques include mechanical and vibratory grinding, sand blasting, chemical treatment, chemical-mechanical and electrochemical polishing. Chemical polishing of nickel alloys requires the use of strong and toxic acids, such as HF for example [1]. Chemical polishing is characterized by a low material removal rate and inability to eliminate all LPBF-induced surface features. A successful removal of the LPBF surface features using electropolishing (EP) with HClO4–CH3COOH mixed acidic electrolytes has been demonstrated on Ni-based superalloys IN718 [2] and IN625 [3]. The main drawbacks of conventional electropolishing are the use of highly corrosive liquids, which may cause workplace-related health issues and negative environmental impacts, and extensive gassing, which leads to poor current efficiency [4]
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