Some observations on flood and dry finish turning of the Ti-6Al-4V aerospace alloy with carbide and PCD tools

Abstract

The Ti-6Al-4V aerospace alloy is a difficult-to-cut material due to its low thermal conductivity and high strength. To reduce the total machining cost and meet environmental standards, dry machining of titanium alloys could present an attractive option to industry. This study seeks to compare and observe flood and dry finish turning of the Ti-6Al-4V using uncoated carbide and polycrystalline diamond (PCD) tools. Cutting with carbide tools was conducted at speeds of 150, 120, and 80 m/min, while PCD was at 150 m/min. Machining with uncoated carbide tools revealed that the tool life in flood cutting was remarkably longer than in dry cutting at 150 and 120 m/min. However, tool wear mechanisms were obviously different as indicated by the scanning electron microscope and the energy-dispersive spectroscopy (SEM/EDS). For the first time, it was proven that at 80 m/min, the wear resistance of the uncoated carbide tool was similar under flood and dry cutting. At the latter cutting speed, the formation of a V2O5 tribofilm, a high-temperature liquid lubricant only created under dry cutting, could assist heat energy dissipation and prevent sticking of the workpiece material onto the cutting edge at such severe cutting conditions. TiC protective and W-O lubricant tribofilms were found at the lowest cutting speed (80 m/min) under flood and dry cutting. Formation of these tribofilms was confirmed by X-ray photoelectron spectroscopy (XPS). The same difference in tool life between flood and dry cutting was found with the PCD cutting tool at 150 m/min. XPS showed the presence of a high amount of TiC tribofilms at the worn cutting edge of this tool material under flood and dry conditions. This might be a cause of a longer tool life obtained during machining of Ti-6Al-4V with PCD cutting tools compared to carbide tools. As an indication of the frictional action at the tool-chip interface, chip compression ratio values did not show remarkable differences in neither flood nor dry cutting using the two tool materials. Moreover, the produced cutting forces were not affected by the application of cutting fluid.