Enhancement of tool life by cryogenic and preheated workpiece methods in the milling of Ti-6Al-4V

Abstract

The goal of this study is to observe the tendency of various machining condition in milling as well as the depth of cut, feed per tooth and cutting speed, and to increase the tool life and material removal rate in macro-milling of titanium. In this study, the FE model describes a milling process using the Johnson-Cook plasticity model. The simulation shows that comparing the increasing rate of tangential force followed by increasing feed and depth of cut, there was more increase of tangential force with increasing depth of cut. Thus increasing the feed rate is the preferable method of increasing the material removal rate in Ti-6Al-4V machining. Cryogenic machining has been proved useful for machining hard metals and super alloys such as titanium and titanium alloys. There have been numerous experimental studies of turning procedures involving Ti-6Al-4V under cryogenic machining conditions showing improvement in the tool life. Milling is an essential machining process, however, literature showed very few work in cryogenic assisted milling work. The present work is concerned with cryogenic-assisted milling of Ti6Al4V. Liquid nitrogen (LN2) was supplied to the back side of the tool to help reduce the amount of workpiece precooling. However, the cutting forces were increased due to the cooling effect of the LN2. Workpiece preheating was used to increase the workpiece temperature. Three cutting speeds and three machining environments (dry, cryogenic, and cryogenic plus preheated) were considered in the analysis of tool wear, material removal rate, cutting forces, tool wear morphology, and chip morphology. Soft and hard tool coatings were applied in the present study. It was observed that the tool life could be increased by 50 to 90% with a soft coating (Si), and by up to 50 to 55% with a hard coating (CrTiAlN). The tool wear morphology showed that rubbing and chipping were the primary tool wear mechanisms. It is expected that the present work will be useful for improving the tool life and reducing the cost of hard metal products. It may also be useful for improving the quality of the finished products.