Research on manufacturing and application of polycrystalline diamond (PCD) tools

Titanium alloys have been wide applied in industry, but they are difficult to machine due to the severe cutting conditions including high temperature at tool/chip interface and highly abrasive interaction at tool/workpiece interface. Polycrystalline diamond (PCD) has been gradually used for machining titanium alloy because of its high hardness and excellent thermal-conductivity. The in-depth understanding of wear behaviour and mechanisms of PCD tools which are made of different PCD materials and refined with different grinding methods is critical for the wide application of PCD tools in industry.

It is known that grain size and volume fraction of diamond in the PCD may have influences on the properties of PCD tools, which, in turn, will affect the wear mechanism of PCD tools. However, it is far from clear how these factors, or structure of different PCD materials, will affect the development of tool wear, and to what extend the affection will be. Through a series of cutting experiments, cutting forces, cutting temperatures, morphological characteristics of wear areas on tool surface, and the geometric parameters of chips were analysed to investigate the wear mechanisms, cutting performance, as well as the effects of material structure. It was found that adhesive-abrasive process and chemical diffusion were the main mechanism of wear of abrasively ground PCD tools. However, the wear processes of the three tools were different due to the difference in material structures. PCD tools made of uniformly sized diamond grains wear in a steady “spalling process”. In contrast, PCD tools made of mix-size diamond grains suffered from large-scale fracture at the tool tip. The shapes of chips and the related geometric parameters reflect different wear processes. Chip shapes changed from spiral to strip with the growing of crater wear, segment chips were generated because of the change of tool geometry caused by the fracture of the tool tip.

Since the wear of PCD materials is a complicated process, it is insufficient to limit the research to experimental analysis only, theoretical studies are necessary for the in depth understanding of the wear mechanism of PCD tools. In this study, a new theoretical model was developed by considering both abrasive and adhesive wear in order to investigate the wear processes and wear mechanisms of different PCD tools. The width of flank wear (VB) and depth of crater wear were calculated by solving the differential equation formulated to describe the rate of flank wear, the rate of crater wear and their relationship with cutting parameters and the properties of tool and workpiece material respectively. The data collected from experimental study was utilized to validate the analytical models. Calculation outcomes matched experimental results when tools made of uniformly sized diamond grains were used. Obvious deviation was found when the tool made of mix-size diamond grains was used due to the occurance of large-scale fracture of tool tip in the cutting processes.

It is known that different grinding methods may result in different kinds of quality of tools. In this research, the quality and performance of PCD tools machined different grinding methods by were analysed. Comparisons between the ground tools (machined by abrasive grinding) and tools machined by electrical discharge grinding (EDG) are conducted, fundamental theories are investigated. After the grinding processes, sharpness of tool edges, roughness of machined surface and residual stress, were measured and investigated. Wear resistance was investigated by a series of cutting tests. By analyzing the experimental results, it was found that the wear and wear mechanism of eroded tools and ground tools were different. In ground PCD tools, compressive residual stress combined with external compressive stress led to the fracture of PCD structure around the tool tip. In contrast, the forming-removing cycle of build-up layer caused a steady adhesive-abrasive wear process, which was the main wear mechanism of eroded PCD tools. Also, residual stress and graphitization are the main factors that accelerated the wear of eroded tools. The stress at cobalt/diamond interface made the structure unstable and reduced the wear resistance of PCD tools. PCD inserts with bigger residual stress have larger worn area on the flank face after machining. The tool with less graphitization showed less worn in the machining of Titanium alloy.