Development of a Process-Oriented Tribological Test Rig for the Performance Assessment of Tool Coatings in Turning of Titanium Ti6Al4V

Abstract

The interaction of cutting tool and workpiece in the contact zone is crucial with regard to tool wear. This is of particular relevance for difficult to machine materials such as titanium Ti6Al4V in continuous operations like turning. Due to the low thermal conductivity of titanium, the heat generated during cutting mainly flows into the tool leading to high tool temperatures. In addition, the low Young’s modulus in combination with the high yield strength of Ti6Al4V leads to high normal and shear stresses acting on the tool surfaces and to detrimental ploughing unless sharp cutting edges are used. Previous attempts to increase the tool life in turning of Ti6Al4V by coatings have not been satisfactory. Temperature active coatings are expected to reduce friction and thus tool wear, allowing for more productive cutting conditions or reduced use of coolant. Tribological conditions strongly differ between rake and clearance face in terms of sliding velocity, stress state and temperature, while insights into the contact zone during cutting are difficult to obtain. Thus, for preliminary assessment of the tool performance, tribological pin-on-disc tests are generally used. However, they suffer from the drawback of a closed friction system, as the pin repeatedly engages the same track on the material specimen, thereby affecting its tribological behavior. In order to suitably emulate mechanical and thermal conditions in continuous cutting, an open friction system has been designed, which is characterized by simultaneous adjustment and measurement of acting forces. At the same time, tool surface temperatures are captured by intermittent radiation measurement. Consequently, the open friction test rig enables the separate investigation of the tribological conditions of rake and clearance face. In this paper, the test rig design and first experimental results obtained from tribological investigations are reported. Separation of forces and subsequent evaluation of stresses acting on both tool surfaces is explained.