Modes and Progression of Tool Deterioration and Their Effects on Cutting Force During End Milling of 718Plus Ni-based Superalloy Using Cemented WC-CO Tools
Understanding the detailed progression of cutting tool deterioration and how deterioration affects cutting force (F) during milling of difficult-to-cut Ni-based superalloys is important for the improvement of machinability of the alloys. It also serves to clarify whether and how an F-based method for monitoring tool deterioration is possible. This understanding is however far from sufficient, as is explained in this thesis after a comprehensive review of the literature. The aim of the present research is thus to determine and explain the modes and progression of tool deterioration and how cutting forces may vary due to the various deterioration features of the cutting tool edge.
Experimentally, the study started by using a typical milling condition with both uncoated and coated cemented carbide (WC-Co) tools. Milling was conducted in either dry or wet conditions. After each pass of a selected distance, the tool was examined in detail in the same manner. Thus, tool deterioration could be monitored more closely and failure mechanisms could be identified and explained. Following on the study on determining the modes of tool deterioration, the progress of deterioration and cutting forces during milling were carefully monitored. Through analysing the monitored tool deterioration features and measured force data, how edge wear, chipping and breakage in cutting edge and beyond the edge contribute to the variation of cutting forces could be studied and better understood. Furthermore, experiments have also been conducted using workpiece in a hardened state.
It has been observed that the commonly recognised build-up layer in the initial stage does not significantly affect the tool deterioration process. Instead, from the beginning of milling, cutting forces/stresses could cause small chipping locally in the initially sharp cutting edge. Fracturing locally with cracks propagating outside the cutting edge along the flank face in the subsurface region could also take place and was consistent with the direction of the cutting force. There was an initial period of time during which a number of microcracks had initiated in and near the cutting edge on the rake face side. These cracks soon propagated resulting in extensively fracturing and blunting of the tool. Coating of the tools had provided little protection as in the cutting edge area the coating had broken away soon after milling started. The major tool failure mode was Co binder material having heavily deformed to fracture, separating the WC grains. Loss of strength in binder material at cutting temperatures is also discussed. As would be expected, the general trend of how F increased as the number of pass (Npass) increased agreed with the general trend of increasing flank wear (VB) as Npass increased. However, the F-VBmax plot has shown a rather poor F-VBmax relationship. This was the result of the different modes of tool deterioration affecting VBmax differently, but VBmax did not represent fully the true cutting edge of the deteriorating tool insert. Chipping and breakage of the inserts confined in the cutting area, resulting in the significant blunting of the edge area, causing a high rate of F increase as VBmax increased and completely deteriorated 6 minutes within of milling time. Fracturing along the face of thin pieces effectively increased VBmax without increasing the cutting edge area and without further blunting the edge, thus no increase in F was required. The high rate, meaning high ∆F/∆VBmax, results from the effect of the edge deterioration/blunting on the reducing the effective rake angle and thus increasing F is suggested and discussed. The use of coolant has not been found to affect tool deterioration/life and cutting force. Explanation for this will be given considering the deformation zone for which coolant does not have an effect. An increase in feed rate has reduced the tool life and the mode of deterioration has become more edge chipping/fracturing dominant, leading to a better F-VBmax relationship.
Finally, it has been observed that the rate of tool deterioration is not higher when the hardened workpiece material is used. The modes and progression of deterioration of tools using hardened workpiece were determined to be comparable to those when annealed workpiece was used. Furthermore, the trends of increase in cutting force as milling pass increases have been observed to be similar for both workpiece material conditions. Interrupt milling experiments followed by hardness mapping has indicted that the workpiece hardened state has not affected the deformation area significantly, although increase in hardness in a similar amount in the severe deformed region has been found for both cases. It is suggested that temperature increases in the narrow deformation zone to be similar for both workpiece conditions and at high temperatures hardening mechanisms do not operate, and thus cutting force values do not differ significantly. Furthermore, the modes and rate of tool deterioration on the hardened workpiece was comparable to the annealed workpiece.