Investigation of the effect of cryogenic treatment applied on cutting tools on drillability ti-6Al-4V alloy

In this study, for the drilling of Ti-6Al-4V titanium alloy, the effects of cutting parameters, cutting conditions and of the processes (applied to the cutting tools) on the cutting forces, momentum, surface roughness, hole diameter, deviation from circularity and cylindricity, tool wear and tool life were investigated. As the cutting tool, uncoated and TiAlN/TiN coated M42 HSS and tungsten carbide drills were used. On both of the two cutting tool materials, part of the uncoated tools was cryogenically treated at -145 oC for 24h and the other part was tempered at 200 oC for 2h right after the criyogenic treatment. Hence, four different tools (uncoated, TiAlN/TiN coated, criyogenically treated, and tempered) from every cutting tool material were tested. In the drilling tests, for HSS and tungsten carbide tools , the cutting parameters of four different cutting speed and three different feed rate were used. At the CNC vertical machining center longitudinal holes of 15 mm depth were drilled with 5mm diameter drills under dry and wet cutting conditions. Additionally, SEM analyses were made in order to determine the changes in the microstructure caused by criyogenic treatment and tempering with respect to the uncoated tools.Microstructure examinations showed that criyogenical and tempering treatments reduced the carbide sizes and provided homogeneous carbide distribution and converted the retained austenite to martensite after the conventional heat treatment. Furthermore, it was observed that the intensity of secondary carbides increased with tempering which was made after the criyogenical treatment. Whereas in the tungsten carbide tools, criyogenical and tempering treatments increased the intensity of eta carbides. These changes in the microstructure played a significant role in the increase of hardness and wear resistance of cutting tools.At the end of the drilling tests, it was observed that the use of cooling liquid had a positive effect on the test outputs except cutting forces and momentum. Especially, significant increases were obtained in tool life for both of the cutting tool materials. For both of HSS and tungsten carbide tools the best results from the aspect of hole quality and tool life were obtained with TiAlN/TİN coated tools. It was observed that criyogenical and tempering treatment had significant effects in increasing the machining performance of HSS and tungsten carbide tools. In wet cutting conditions criyogenically treated and tempered HSS and tungsten carbide tools provided 87% and 83% life increase with respect to the untreated tools. When taking into consideration the effects on the machining outputs, the effects of criyogenical treatment on HSS tools were much more than the effects on carbide tools.