Investigation of surface roughness post-processing of additively manufactured nickel-based superalloy Hastelloy X using electropolishing

Abstract

A major problem of additively manufactured parts using laser powder bed fusion (LPBF) is the as-built high surface roughness. The poor surface finishing is detrimental to the mechanical and electrochemical properties of the parts. In this work, we present systematic investigations on surface roughness reduction of LPBF produced nickel-based superalloy Hastelloy X (HX) using electropolishing. The electrolyte used in this study is an environmentally safe deep eutectic solvent (DES) comprising a eutectic mixture of choline chloride and ethylene glycol in a 1:2 M ratio, respectively. Two different groups of LPBF HX parts separated for the purpose of electropolishing the external and internal surface were prepared. The results show that both the external and internal surface roughness can be effectively reduced using electropolishing in DES. For the parts with the external surface electropolished for 5 min, a surface roughness reduction down to 1.2 μm (Ra) with a bright and reflective surface appearance was achieved. This is a significant improvement in comparison to the high as-built surface roughness of 10.3 μm (Ra) with a dull and non-reflective surface appearance. For the internal surface electropolishing, the surface quality of a 3 mm diameter internal channel in an angled LPBF HX cylindrical tube was successfully enhanced. The DES electropolishing efficiency on LPBF HX was investigated using weight reduction as a function of the electropolishing time, and the microstructure of the electropolished LPBF HX was studied. A further improvement of the electrochemical properties for the electropolished surface was revealed by anodic polarization tests and electrochemical impedance tests. The electropolished surface was also revealed to be more hydrophobic than the as-built surface by water contact angle tests, which is beneficial to the corrosion resistance. The findings herein demonstrate the surface quality of the LPBF produced HX can be effectively improved using electropolishing, and also showed the feasibility of using DES as a promising alternative electrolyte to the conventional hazardous acid mixtures.