Milling of gamma titanium aluminides

Abstract

Gamma Titanium Aluminide (gamma – TiAl) alloys are intermetallics with approximately half the density of nickel based superalloys. Their low density, high specific strength, and good structural integrity at high temperatures make them potential candidates for aeroengine components. However, characteristics such as poor ductility at room temperature, high hardness, and substantial surface damage during machining have earned these alloys the reputation of “difficult to cut”. Therefore, the present study aims to assess the machinability and surface integrity of second generation Ti-48Al-2Cr-2Nb gamma – TiAl alloys through end bull milling. First, a tribological characterisation of the tool coating (TiAlSiN) under dry, flood and Minimum Quantity Lubrication (MQL) was carried out. The effectiveness of tool coatings in reducing tribological properties was evaluated by conducting experiments with uncoated pins. The tribological characteristics of the gamma – TiAl alloys were subsequently benchmarked against one of the most used titanium alloys, Ti-6Al-4V. The effect of contact parameters such as contact pressure and sliding velocity on the evolution of tribological phenomena was determined. Secondly, a comprehensive sensitivity analysis of the milling input parameters to the output parameters during the bull end milling of Ti-48Al-2Cr-2Nb was conducted. The influence of microstructures (Near lamellar and Duplex) was analysed together with three types of lubrications (dry, flood and MQL), two bull end mill tools, and milling process parameter (cutting speed). The assessment of output parameters of cutting forces, tool wear, and surface integrity in the form of surface roughness, sub-surface microstructural alterations, and subsurface microhardness demonstrated the relevance of all the input parameters in affecting the output ones. As a result, a process window with long tool life and high surface integrity was established in accordance with industrial requirements.