Effect of Etchants on Superalloy HIP Oxide Scales

Abstract

In the aviation industry producers are met with high standards for material specifications. This has led to the development of new techniques to manufacture parts for airplane engines. One such method is additive manufacturing. Due to the unfavorable microstructure after additive manufacturing the parts have to be post-processed in multiple steps to reach the final structures. These include hot isostatic pressing and an etching process. When manufacturing parts in nickel-based superalloy Inconel 718 oxides can form during hot isostatic pressing. These oxides need to be removed from the part to meet standards. This is done in a two-step etching process, pickling and etching. In this project different pickling solutions are evaluated for their ability to remove the oxide scales formed during hot isostatic pressing of the nickel-based superalloy Inconel 718. Additionally the chemical composition of the oxides and bulk material is determined. The pickling solutions used were HCl, HF mixed with HNO3 and Metex M-629 used with electrolysis. Analysis of the oxides was made using an optical microscope, laser microscope, a scanning electron microscope and energy-dispersive x-ray spectroscopy. The results show that no matter the pickling solution the etching solution removes all of the surface oxides. Among the pickling solutions, HF/HNO3 had the strongest effect on the oxide scale during the pickling step while the HCl solution does very little to the oxides. The bulk material is found to be mostly nickel mixed with chromium and iron. There are two different kinds of oxides found on the surface. The first one is mostly aluminum oxides with a little titanium oxide while the other one is aluminum and titanium oxides with a niobium rich phase.