Study of thin-wall machining considering the effects of cutting-induced deformation and CMQL machining
Creators: name: Wu, Ge
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401406
subject: Machining
classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)

description: Thin-wall structural components such as turbines and impellers are widely used in modern industries due to their higher structural efficiency, lightweight characteristics and excellent functional properties. However, chatter vibration and flexible deformation often occur in the machining of the thin-wall components due to the low rigidity, weak damping and time-varying dynamic characteristics of the milling system, which lead to poor surface quality, high noise level, and accelerated tool wear. Study on the machining of thin-wall components is of significant importance to improve the machinability and productivity of these complex parts, especially in high-speed machining (HSM) of thin-wall parts that are made of difficult-to-cut materials such as titanium alloys.

Under chatter-free conditions, the deformation errors of the tool and workpiece are shown to be the dominant source of surface form errors. To eliminate the deformation errors, optimized cutting parameters are normally selected after carrying out a large number of experiments, which is time-consuming and error-prone. Therefore, it is necessary to develop a prediction model to analyse the process response and consequently improve the machining quality and efficiency. In this study, a theoretical model was developed to analyse the thermal-mechanical coupling and part deformation in milling of Ti-6Al-4V thin-wall component. Meanwhile, cutting heat sources and the development of tool flank wear was considered in the modelling process to improve the prediction accuracy.
 
The cutting loads were modelled analytically and calculated using an efficient iterative algorithm and the deformation of the thin-wall part was simulated through a finite element model. Then, a series of thin-wall milling experiments were conducted under various cutting conditions to validate the predicted results. Both the cutting forces and thin-wall deformation values were recorded to validate the predicted results calculated by the hybrid model, and good agreements have been achieved between the measured results and simulated outcomes. The predicted cutting forces in the radial, feed and axial directions are within errors of 14%, 10% and 5%, respectively, concerning the experimental values. Meanwhile, the maximum predicted deformation errors at the initial, middle and end portions of the workpiece are less than 20%. Overall, the proposed model demonstrates acceptable prediction accuracy.

In the metal cutting process, the application of cutting fluid is the most direct and effective method for improving the machining performance. Advanced cooling/lubrication solutions such as minimum quantity lubrication (MQL) and cryogenic minimum quantity lubrication (CMQL) were proposed and employed in green manufacturing to avoid the environmental and health issues that occurred in conventional flood cooling machining. However, the mechanisms and effects of CMQL on the thin-wall milling process have not been explored yet. To further improve the machinability of this complex machining process, this study investigated the performance of CMQL in machining of titanium thin-wall components. Comparative experiments were carried out under different cutting environments, including flood cooling, MQL and CMQL machining. Then, the tool wear state, cutting force, chip morphology, surface quality and workpiece deformation value were compared and analysed. Experimental results show that CMQL machining can significantly lower the cutting loads and deformation values of thin-wall parts, especially at higher spindle speed. Meanwhile, the wear resistance of the cutting tool and surface quality is enhanced in comparison with flood cooling and MQL machining. Overall, the experimental outcomes demonstrate the feasibility of CMQL in improving the machinability of thin-wall components made from difficult-to-cut material. This can be attributed to the superior lubrication and cooling functions of CMQL machining. The cryogenic cooling action and induced thermal effect are especially beneficial to the control of the flexible deformation.
 
Moreover, corresponding finite element models were established to simulate the thin-wall machining process based on the finite element solver Abaqus/Explicit. In these models, the Lagrangian formulation with an explicit solution scheme was employed to simulate the interactions between the cutting tool and workpiece. The Johnson-Cook material constitutive model was employed to define the flow stress behaviour of the workpiece material at high strain, strain rate and temperature. The Johnson-Cook damage law was adopted to realize chip separation, and the Coulomb friction model was used to determine the frictional contact criterion of the tool-chip interface. Comparative simulations were carried out under different cutting conditions, including flood cooling, MQL and CMQL machining, and all the parameters involved in the models were modified according to the actual cutting conditions. The FE models have demonstrated their capacity and reliability in modelling the metal cutting process. The simulation results showed good agreement with the experimental results and also provided some necessary information that cannot be obtained from the cutting tests.