SHEAR LOCALISATION AND ITS CONSEQUENCE ON TOOL WEAR IN HIGH SPEED MACHINING

摘要

(1) The flow stress decrease accompanying major microstructural softening events cause shear localisation in metal cutting. Dynamic recrystallisation and phase transformation are identified as major microstructural softening events occurring in the matrix that cause shear localisation. (2) A quantitative model to predict the onset of shear localised chip morphology is developed, which is rooted in Oxley's model for shear angle rotation to reach equilibrium shear angle. The occurrence of microstructural softening events before reaching the equilibrium shear angle is quantitatively analysed using the concept of critical strain for dynamic recrystallisation or phase transformation temperature. The model predictions on shear localised chip morphology due to dynamic recrystallisation are validated with the available flow stress data on 4340 steel. (3) The effect of lowering the phase transformation temperature in Fe-Ni-C model alloys by increasing the Ni content is shown to decrease the critical speed for the onset of shear localised chip morphology. This is consistent with the model predictions. (4) The consequence of shear localisation in the secondary shear zone is to cause chemical crater wear. The consequence of shear localisation in the primary shear zone is to cause chemical wear localised at the cutting edge of the tool. (5) By lubricating the tool-chip interface through glassy oxide inclusions engineered into the steel, the tribology of seizure can be prevented. By preventing the shear localisation in the secondary shear zone, it is shown that shear localisation in the primary shear zone can be suppressed. Thus, inclusion engineering offers a viable strategy to prevent chemical tool wear caused by shear localisation in the secondary and primary shear zones. This is the basis for the design and development of self-lubricating steel.