Development and Removal of Alpha-Case Layer From Heat Treated Titanium Alloys

The components of the aero engines such as fan blades are generally manufactured from Titanium alloy forgings. At the elevated temperatures, the affinity of Titanium towards oxygen is very high, which results in formation of oxide layer on surface known as alpha-case layer. This alpha-case is both hard and brittle in nature which results in localized micro failure during its application. This gives rise to a fatigue crack initiation zone and compromises the integrity of the component, causing it to fail. To investigate this, Titanium α-β (Ti 64), α (Sn) and β (Mo) alloys were heat treated at 1010°C for 30min, 60min, 90min and 120min followed by air cooling. Formation of alpha-case layer in Ti-6Al-4V, Ti-Sn and Ti-Mo increased from 120.5μm to 391.1μm, 128.77μm to 443.23μm, 105.75μm to 262.46μm at 30mins and 120mins respectively. Chemical treatment, cathodic de-oxygenation, surface coating and laser ablation methods are generally used to remove the alpha case. In the current study, acid pickling is used to remove the alpha case layer, as this process is simple and also easily applicable to any complex shape of the material. In this method, samples were dipped in the solution of HF (5%) and HNO₃ (35%) at 80 °C for fixed time at fixed intervals to find the rate of alpha case removal. Micro indentation was carried out to obtain hardness profile from surface to bulk of heat treated specimen. The quantification of alpha case oxide layer from surface to bulk was done by EDS.