Evolution And Mechanisms Of Saw-Tooth Chip Formation In High-Speed Machining

With the advancement of high speed machining technology, high-speed machining is now used to fabricate the components for aerospace, automobile, power generation industries. The chips generated at high cutting speed are very different from that at low or conventional cutting speeds. Typical chip morphology for most of plastic metals at low or conventional cutting speed is smoothly continuous chip, while at high cutting speed the typical chip morphology is saw-tooth chip or serrated chip. The saw-tooth chip is attributed to periodic shear instability within the primary shear plane. The formation of saw-tooth chip causes vibration of cutting system, deteriorates tool wear, and worsens integrity of machined surface. However, the saw-tooth chip is easy to be broken and disposed, which is helpful for the application of full automatic machines. Therefore, Comprehensive understand of the formation mechanisms of the saw-tooth chip is necessary not only for design of machine and cutting tool, but also for improvement of machined surface integrity and cutting efficiency. Though large mounts of researches on saw-tooth chip formation have been carried out, the issues about chip formation, theotetical prediction for chip serration, and influences of material mechanical property change with cutting speed on chip serration are not clear, and further research is required.In this thesis, analytical and experimental studies are conducted with three different metals to investigate saw-tooth chip formation mechanisms at different cutting speeds. The focus is put on the investigation of influences of material mechanical property change (especially the plastic-brittle transition) with increase of deformation rate on saw-tooth chip formation. The main contents of the thesis include:the procedure of saw-tooth chip formation, the influences due to material mechanical property change with deformation rate on the saw-tooth chip formation, dominant mechanisms of chip formation at different cutting speeds, theoretical predication of the critical cutting conditions for saw-tooth chip formation and experimental validation, chip formation phenomena and mechanisms at ultra-high cutting speeds.Firstly, saw-tooth chip formation procedure and initiation and propagation of the primary shear plane are investigated. Four-Point Two-Stage model for saw-tooth chip formation is proposed. The results show that plastic compression occurs at the thinner end of the segment during indentation stage. The localized shear forms along the border of the compressed region extending from the cutting tool tip end to the free surface end of the primary shear plane. This localized shear is regarded as the initiation of the concentrated shear band. There is no plastic shear across the segment occurred during the indentation stage. The concentrated shear band initiates firstly at inner end and then at outer end of the primary shear plane, the two localized shears propagate face to face to form the whole concentrated shear band. The prerequisite for chip serration is that shear instability of primary shear plane happened within the maximum strain required for continuous chip.Secondly, orthogonal cutting experiments with Inconel718, AerMet100, and 7050-T7451are conducted. The continuous chips, continuous saw-tooth chips, and seperated saw-tooth chips are obtained. Metallurgical and mechanical tests are conducted on the chips to investigate material mechanical properties. The results show that, at the Critical Cutting Speed (CCS) when the chip starts to be serrated, the dominant mechanism of chip serration is periodic adiabatic shear (or thermoplastic shear instability) with in primary share plane. However, with the increase of cutting speed, the material plasticity decreases and brittleness increases, leading to the dominant mechanism for chip serration shifting from periodic adiabatic shear to periodic plastic crack.Thirdly, based on Oxley's parallel-sided shear zone model, the deformation of the primary shear zone at CCS is analyzed, based on which formulates determining strain, strain rate, and cutting temperature within the primary shear zone are drawn. Numerical simulation of material flowing over the primary shear zone is carried out. A model, based on thermal-mechanical coupling simulation, predicating the critical cutting conditions for chip serration, is proposed. Theoretical predication for the critical cutting conditions for chip serration for the three experimental materials, and experimental validation of the theoretical predication are conducted. The influences of process parameters and material thermo-physical parameters on chip serration are discussed based on the proposed model. The results show that the strain rate experiences increasing and then decreasing along a second power curve with the workpiece material flowing over the primary shear zone. The heat contributed to the temperature increase of the primary zone decreases with the increase of strain, leading to temperature increase rate decreasing at higher strain value. Strain, strain rate, and temperature within the primary shear zone together determine the development of the flow stress. The chip becomes easier to be serrated at higher uncut thickness and smaller rake angel (larger minus rake angle), and vice versa. The influence degree of rake angle on chip serration is more than that of uncut chip thickness. The influences of material density and heat capacity on chip serration are more than that of thermal conductivity. The influence of thermal conductivity on chip serration is highly affected by the cutting speed, which is obvious at low cutting speed while unobvious at higher cutting speed.Finally, chip formation phenomena and mechanisms at ultra-high cutting speed are studied. The material properties at critical impact velocity and its influences on chip formation, tool wear, cutting forces, and cutting power are analyzed. The results show that the chip becomes very small brittlely broken fragments when the cutting speed approaches the critical impact velocity of the workpiece mateirals. These small fragments may act as the hard grains in grinding leading to the abrasion of cutting tool. Cutting power increases monotonously and the energy required in cutting decreases with the increase of cutting speed. Cutting speed can be classified into conventional cutting speed, high cutting speed, and ultrahigh cutting speed according to chip morphology.This research is supported by National Basic Research Program of China (2009CB724401) for financial supports.