Investigations into morphology and surface integrity of micro hole during femtosecond laser drilling of titanium alloy

Abstract To increase an aircraft engine efficiency, thousands of cooling micro-holes should be drilled on the gas turbine blade. Because of the superior thermal and mechanical properties of titanium alloy, it is challenging to produce micro-holes with high dimensional and form accuracy using conventional methods. So, in this work, an attempt has been made to produce micro-holes on Ti-6Al-4V using a femtosecond laser. A detailed experiment is performed using full factorial design to comprehend the combined effect of laser process parameters and laser scanning strategies on micro-hole characteristics like hole circularity at entry, exit, taper hole, surface finish and microstructure. A combination of higher laser fluence and a lower pulse repetition rate improves the entry and exit hole circularity. On the contrary, the taper angle is lowered by increasing laser fluence and pulse repetition rate. The zig-zag scanning strategy reduces the hole taper, while the concentric circle scanning strategy improves the circularity of the hole at the entry and exit. It is found that the optimum process parameters for improving micro hole geometry in Ti-6Al-4V include a laser fluence of 1.90 J/cm2, a pulse repetition rate of 20 kHz, and a concentric circle scanning strategy. The surface finish of the micro-hole deteriorates with increasing laser fluence and repetition rate, while concentric circle scanning yields lower surface roughness with fewer surface imperfections. Furthermore, the hole wall microstructure evolution exhibits deep craters at higher laser fluence and undulating grooves at higher repetition rates.