Finite Element Modelling On High-speed Milling Process Of Titanium Alloy

Titanium alloy with low density, high strength, corrosion resistanceand heat-resistance, are widely used in aviation, aerospace and otherimportant fields. However, it is one of most difficult-to-cut materials.Milling is the most widely adopted method in Titanium alloy machiningprocess. High speed milling process is of high efficiency and accuracy, andcan also reduce the effect of the cutting heat on the machined surface. Mostcurrent titanium alloy milling process researches are based on finiteelement (FE) modeling method of ideal equivalent to orthogonal cuttingprocess. There lacks of an accurate FE model to capture the actual millingprocess.Sponsored by the National Natural Science Foundation of ChinaProject under the No.5117533, this paper intends to reveal the mechanismof titanium alloy Ti6Al4V milling process, establish the FE model ofTi6Al4V milling process based on ABAQUS software, and simulate themilling force, milling temperature, chip morphology and residual stressduring the milling process. The model is validated by comparing thepredicted results with the measured data in terms of milling force, chipmorphology and residual stresses.Firstly, a two-dimensional FE model of Ti6Al4V high-speed millingprocess is developed based on ABAQUS software, and the whole modelincludes two sub-models:1) chip formation sub-model: it simulates the milling force, milling temperature and chip morphology byABAQUS/Explicit calculation;2) Surface residual stress sub-model: itsimulates the surface residual stress by importing the machining workpiecein sub-model1into ABAQUS/Standard implicit calculation, and cooling itto room temperature25℃to release thermal stress. This model enable topredict the effect of undeformed chip thickness on milling force, chipmorphology and residual stress. As the undeformed chip thicknessdecreases, chips morphology transfers waveform chip into saw-tooth chipmorphology, the milling force fluctuates and reduces to zero, and thepenetration depths of residual stresses also reduce accordingly.Secondly, the established FE model is applied to simulate the effectsof rotation speeds, feed rates, rake angles and tool nose radii on thehigh-speed milling process. As tool rotation speeds and feed rates increasefrom600rpm,10mm/s to2400rpm,40mm/s, the number of saw-toothincreases significantly, while the maximum milling force decreases, andthe peak milling temperature also increases; as the tool rake angleincreases from5°to15°, the crimp degree of chip will increase, theaverage milling force decreases, and the peak milling temperature alsodecreases; As the tool nose radii increases from5μm to25μm, themaximum tangential milling force and average milling force increaseobviously, but it has a slight effect on chip morphology and millingtemperature distribution.Thirdly, experiments of Ti6Al4V high-speed milling process areconducted by CNC milling machine center to validate the developed modelin terms of milling force, chip morphology and residual stress. Thecomparisons of predicted results and measured data show that the predictmilling forces errors are less than17.0%, the errors in metric parameters of saw-tooth chip are less than17.8%, and predicted surface residual stresserrors are less than21.4%, it indicate the accuracy of this FE model.Finally, a sequential milling FE model is developed based on thevalidated single milling FE model to investigate the effect of sequentialmilling process on residual stress, and predict the surface residual stressdistribution more correctly. In the meantime, a novel approach is proposedto achieve the rapid simulation of high-speed milling process andautomatic post-processing of the calculated results data by Pythonlanguage programming of ABAQUS. This method is further applied toquickly investigate the effect of tool nose radius, tool rake angle andworkpiece preheated temperature on the machined surface residual stress.The analysis indicate that the residual stresses in the shallow layer of themachined surface reduce as the tool nose radii increase, and significantlyreduce as the tool rake angles increase; the residual stresses in the deeplayers of the machined surface reduce as the workpiece preheatedtemperatures increase.