Department of Engineering Design and Manufacture, The University of Hull
Abstract
A study has been made of the respective performance of cubic boron nitride (CBN) and polycrystalline diamond (PCD) cutting tool materials and compared to various coated and uncoated tungsten carbide grades when cutting titanium alloy workpieces. Two important experimental techniques were employed during the course of this work, firstly a quasi-static contact method was employed to establish the workpiece/tool interfacial temperature above which strongly adherent layers may be formed. This technique revealed that the critical temperatures which marked adhesion and welding, were 740, 820 and 800 °C for coated and uncoated carbides, and 760 and 900 °C for PCD and CBN tools respectively. Furthermore, the technique has been used to study the integrity of the bulk tool material, and/or individual coatings on their substrates, when welded junctions formed between the tool and workpiece are separated. With regard to the latter it was observed that in all cases fracture was initiated in the bulk of the harder tool material rather than in the workpiece or at the welded junction interface. Secondly, a quick-stop technique was used to study chip formation and tool wear when cutting with carbides, CBN and PCD tools under nominally the same conditions.The predominant wear mechanisms for each of the tool materials was found to be based on a diffusion/dissolution process. The wear process is discussed in detail for each of the tool materials and reasons advanced for observed differences in performance when removing material from a titanium alloy workpiece. The wear resistance and quality of the machined surface was found to be superior when cutting with the ultra-hard materials than with the carbide grades and in particular the PCD tool was found to produce exceptionally good surface finish. In the case of coated carbide tool grades rapid removal of the coated layers occurred leaving the substrate vulnerable to reaction with the workpiece material and this is considered to explain the seeming absence of beneficial effects when cutting with these grades
