Amélioration du fini de surface par électropolissage des composants en alliage de titane (TI64) et nickel (IN625) fabriqués par FSL

Abstract

This study is about electropolishing of selectively laser melted (SLM) parts made of Ti-6Al-4V and IN625 alloys. Topographical differences between surfaces of different build orientations with respect to the building platform are caused by four major factors, namely: staircase effect, bailing, rippling and partially bonded particles. The influence of these factors can be minimised by tuning up the building parameters, but only to a certain limit, hence the need in post-build surface finishing operations. The mechanical and chemical finishing techniques have severe limitations. On one hand, internal cavities are impossible to finish using conventional mechanical polishing techniques. On the other hand, chemical polishing often requires the use of strong acids in addition to not completely eliminating the SLM-related surface features. Given the preceding, electropolishing (EP) is considered to be one of the best-suited candidates for the surface treatment of SLM parts, and it will be used in the framework of this study. First off all, the applied potential - surface current density relationship was established for both alloys within their respective electrolytes. It was found that Ti-6Al-4V does not present a stable current density plateau, while IN625 does. Afterwards, the electropolishing parameters optimisation has shown that the optimal current density for polishing Ti-6Al-4V is of 240 mA cm-2 and that IN625 polishes well when a constant potential of 9 V is applied instantly, resulting in a current density raging between 26 and 31 mA cm-2. The optimal conditions of electropolishing were then used to simulate the electropolishing of a tube (barrel), composed of 7 staves, each one built at a different orientation: 0, 22.5, 45, 67.5, 90, 112.5 and 135 °. It was shown that, despite a large scatter in the as-built surface finish, ranging from Ra = 4 μm (0°) to Ra = 23 μm (135°) for Ti-6Al-4V and from Ra = 1.8 μm (0°) to Ra = 18.6 μm (135°) for IN625, all the differently-oriented surfaces were polished down to a uniform roughness of Ra=1-3 μm (Ti-6Al-4V) et Ra = 1.5-3.5 μm (IN625). Finally, thickness evolution measurements during finishing of the differently oriented test coupons allowed us to establish the build orientation-dependent allowances for the electropolishing of selectively laser-melted Ti-6Al-4V and IN625 parts.

Date	3 Feb 2017
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Vladimir Brailovski (Supervisor)