Wear development of self-lubricating CrAlVN coatings during turning Ti6Al4V

Abstract

Interactions between cutting tool and workpiece material are crucial to tool wear at the cutting edge. In particular difficult to machine materials like the titanium alloy Ti6Al4V require tailored solutions to avoid extensive tool wear. Temperature-active, self-lubricating chromium-aluminium-vanadium-nitride (CrAlVN) coatings deposited by physical vapor deposition (PVD) have shown the potential to reduce tool wear during Ti6Al4V turning. These coatings form lubricating oxide phases at temperatures between 700 °C ≤ ϑ ≤ 800 °C which might be suitable to reduce thermal and mechanical loads during cutting. However, the initial wear development as well as the formation of lubricating oxide phases during Ti6Al4V turning while using cooling lubricant have not been adequately investigated. Consequentially, it is unclear whether the lubricating oxide phases form during the turning operation with flood cooling. Nevertheless, this is of interest, because the use of cooling lubricant is state of the art in turning of Ti6Al4V in order to reach an increased tool life. That is why, the initial tool wear within the first two minutes of turning with flood cooling was investigated in this study. For this purpose, two different CrAlVN coatings with varying V/Al ratios were deposited on cemented carbide inserts by a hybrid direct current magnetron sputtering (dcMS) / high power pulsed magnetron sputtering (HPPMS) process using an industrial coating unit. Subsequently, cutting tests with a depth of cut ap = 1.2 mm, feed rate f = 0.12 mm, setting angle κr = 95° and cutting velocity vc = 80 m/min were conducted at defined cutting intervals of tc = 5 s, 10 s, 20 s, 40 s, 80 s and 120 s using a computer numerical controlled (CNC) lathe machine. The initial tool wear development, prevailing wear mechanisms as well as the formed oxide phases during the intervals were analyzed. Uncoated cemented carbide inserts were used as a reference. The results highlight that the formation of lubricating vanadium oxides is possible in turning process with cooling lubricant. Furthermore, it seems that adhesion processes are the main causes of tool failure. The results offer the possibility to adapt the CrAlVN coatings for Ti6Al4V turning.

Keywords: Tool coatings; PVD; self-lubrication; turning; Ti6Al4V