High-speed ball nose end milling of burn-resistant titanium (BuRTi) alloy

Following a brief introduction to aero-engine materials and the development of BuRTi alloy (Ti-25V-15Cr-2Al-0.2C), the paper details a statistically designed machinability experiment involving high-speed ball end milling. Testing utilised 8 mm diameter AlTiN-coated carbide ball nose end mills in a Taguchi L8 fractional factorial design with six factors, each at two levels. Output measures related to tool life/wear, cutting forces, workpiece surface roughness, microstructure and microhardness. Main effect plots, tabulated ANOVA data, percentage contribution ratio (PCR) values together with graphical and SEM data are presented. Use of the lowest material removal rate, high-pressure (70 bar) cutting fluid and a workpiece orientation of 45° resulted in the longest tool life with a machining time of ~60 min; however, surface roughness was poor, and there was smeared/adhered material to a depth of 20 μm. Additionally, carbide fracture/pull-out was observed near the workpiece surfaces whereas microhardness depth profiles from sectioned, mounted and polished samples showed a moderate increase in surface hardness of ~80HK0.25 above the bulk value.