Tool wear and surface quality in milling of a gamma-TiAl intermetallic

Advanced structural materials for high-temperature applications are often required in aerospace and automotive fields. Gamma titanium aluminides, intermetallic alloys that contain less than 60 wt.% of Ti, around 30–35 wt.% of aluminum, and other alloy elements, can be used as an alternative to more traditional materials for thermally and mechanically stressed components in aerospace and automotive engines, since they show an attractive combination of favorable strength-to-weight ratio, refractoriness, oxidation resistance, high elastic modulus, and strength retention at elevated temperatures, together with good creep resistance properties. Unfortunately such properties, along with high hardness and brittleness at room temperature, surface damage, and short and unpredictable tool life, undermine their machinability, so that gamma-TiAl are regarded as difficult to cut materials. A deeper knowledge of their machinability is therefore still required. In this context the paper presents the results of an experimental campaign aimed at investigating the machinability of a gamma titanium aluminide, of aeronautic interest, fabricated via electron beam melting and then thermally treated. Milling experiments have been conducted with varying cutting speed, feed, and lubrication conditions (dry, wet, and minimum quantity lubrication). The results are presented in terms of correlation between cutting parameters and lubrication condition with tool wear, surface hardness and roughness, and chip morphology. Tool life, surface roughness, and chirp morphology showed dependence on the cutting parameters. Lubrication conditions were observed to heavily affect tool wear, and minimum quantity lubrication was shown to be by far the method that allows to extend tool life.