Investigation to Hole Surface Microstructure Evolution in Drilling of Aerospace Alloys: Ti-5553

Ti-5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe) is a newly developed near beta titanium (Ti) alloy with excellent fatigue performance and corrosion resistance. Hence, it is of significant importance in several high-performance aerospace applications such as landing gear components and helicopter rotors. The machinability of Ti-5553 is low owing to its high strength at elevated temperature, low thermal conductivity and high chemical reactivity. Although there is a profound knowledge about the machinability of alpha + beta Ti alloys (typically Ti-6Al-4V), there is a lack of understanding regarding the surface microstructure evolution during machining of Ti-5553. This paper presents experimental investigations on the microstructure evaluation in the hole surface produced from drilling of Ti-5553. A series of high-speed drilling tests were conducted to evaluate the influence of cutting conditions on the hole surface microstructure alternation in relation to the cutting temperature. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) technique were used to characterize the hole surface microstructure evolution. The precipitation of new alpha phase from beta matrix in the hole surface was observed in dry drilling; however, this phenomenon was not detected in wet drilling with a coolant supplied.