Titanium alloys are increasingly becoming the material of choice for a wide range of applications. They have found their way into aerospace, biomedical and automotive industries due to their superior properties such as good strength-to-weight ratio, high corrosion resistance and bio-compatibility. However, machining of titanium alloys has proven difficult over the years owing to several inherent properties of these materials. Titanium has a very strong chemical affinity and, therefore, has a tendency to weld or stick to the cutting tool during machining, which results in chipping and premature tool failure. Their low thermal conductivity causes high temperatures to be generated at the tool/workpiece interface, affecting tool life adversely. Additionally, their high strength at elevated temperature and low modulus of elasticity further impairs their machinability. Researchers have yet to find an effective strategy to overcome these challenges and improve the tool life during the machining of titanium alloys. The present research aims to find suitable tool coatings that will increase tool life during the machining Ti6Al4V, a prominent titanium alloy.
The alloy Ti6Al4V make up 60% of all titanium production [1]. Thus, the present research is focused on this alloy. Standard industrial practice for machining titanium alloys still revolves around the use of uncoated cemented carbide tools, although recently TiAlN coatings are being recommended by tool manufacturers. However, both uncoated and TiAlN coated cemented carbide tools have yet to achieve the desired tool life andproductivity. Hence, a variety of coatings have been investigated in this research study and are compared with uncoated and TiAlN coated carbide tools to find a more effective tool coating that will improve tool life during the machining of Ti6Al4V.
The coatings tested were chosen with the expectation that beneficial tribofilms could be generated on a tool’s surface due to the coatings’ self-adaptability characteristics under the cutting conditions experienced during machining. If the coating is properly engineered, these tribofilms have the potential to improve tool life significantly [2]. Experimental studies were carried out and the results showed that TiB2, CrN/WN and CrN/ZrN coated cemented carbide tools outperformed standard uncoated and TiAlN coated tools during Ti6Al4V machining. Detailed analysis of the best coatings revealed the beneficial tribofilms that each coating generated and how they affected tool wear.
Effect of deposition parameters on TiB2 (best coating) and chip breaker profile of the tools were also studied. Results showed that tools with positive chip breaker profile gave a better tool life and that low hardness TiB2 coating obtained through varying deposition parameters was more suitable for machining Ti6Al4V alloy compared to harder variants of TiB2 coating. End milling tests with TiB2 coated tools revealed that the coating was not as effective at increasing tool life as it was in turning.