Process Damping Effect in Milling Process of Titanium Alloy
Key words: milling; flutter; titanium alloy; process damping; plowing effect
Titanium alloy has been widely used in the aerospace manufacturing industry. It has excellent comprehensive properties such as high specific strength, low density, high heat resistance and low temperature resistance. It can be used to manufacture aircraft parts, which not only can extend the life of the aircraft, but also reduce the weight. , reducing fuel consumption, thereby greatly improving its flight performance.
However, titanium alloy is also a typical difficult-to-machine material, which has poor thermal conductivity, high chemical activity, severe work hardening, short tool life, and due to large unit cutting force and high chattering during processing, flutter is left to the workpiece. The underlying slanting vibration pattern often needs manual honing to remove, affecting the processing efficiency, seriously causing the workpiece to be scrapped, or even destroying the tool. The flutter problem of titanium alloy processing is a major bottleneck restricting the quality and efficiency of aviation manufacturing.
The method of controlling flutter can generally be attributed to increasing system damping. The damping of the cutting system can be divided into machine structure damping and damping generated by the tool flank and the workpiece surface interfering with each other, also known as process damping. The modeling and calibration of process damping is a research hotspot in international academic circles in recent years. The famous Canadian scholar Altintas has listed it as an unsolved research difficulty in cutting flutter.
Tlusty and Sission first discovered process damping in machining, and as the cutting speed decreases, the stability limit of turning can be significantly improved. Sission also concluded that cutting speed, tool relief angle and cutting edge radius are key factors affecting process damping . Later, many scholars studied the process damping, pointing out that the force formed by the interference between the flank face and the surface vibration of the workpiece is the source of process damping. The process damping coefficient in dynamic cutting forces is identified by a series of orthogonal tests controlled by a fast servo system that causes the tool to oscillate at the desired frequency and amplitude, but the test system is complex and requires a lot of work. Budak and Tunc and others overcome the weaknesses of experimental modeling and have done more detailed work on process damping modeling and coefficient calibration. The stability limit prediction analytic method of orthogonal turning and the flutter experiment are combined, and the process depth damping coefficient is directly calibrated by using the limit depth of cut obtained by the two. Based on this, combined with energy analysis, the intrusion coefficient is obtained, then the intrusion area and cutting force are calculated, and the stability analysis model of turning is established. The literature  systematically analyzes the influence of cutting parameters and tool geometry parameters on process damping. Ahmadi and Ismail et al. based on the small amplitude assumption, the process damping is equivalent to linear viscous damping, and the semi-discrete method is used to calculate the milling stability limit.
At present, the international research on process damping mainly focuses on turning. For the process damping of milling, there is still no perfect dynamic analysis model. There are many degrees of freedom, the force analysis requires coordinate transformation, and the time-varying coefficient exists in the cutting force equation. The description of the intrusion area and process resistance is much more difficult than turning. However, there are no scholars in China who have conducted in-depth research on process damping. In the current literature, the cutting stability analysis adopts a more traditional linear model. Without considering the process damping, the model will produce large errors in the low-speed region. For titanium alloy processing, in order to ensure tool life, the cutting speed is generally low. If a conventional linear model is also used, the predicted limit cutting depth is far below the actual limit depth, which will inevitably affect the machining efficiency.
In view of this problem, this paper establishes a milling dynamics model considering process damping, and uses the implicit fourth-order Runge-Kutta method to calculate the intrusion area and interference resistance of the tool flank and workpiece vibration corrugation during machining of typical titanium alloy materials. Draw a stability limit graph. Finally, the experimental results show that the nonlinear model built in this paper can accurately predict the stability limit of the low-speed zone and provide a reference for the selection of processing parameters of titanium alloy.
The flutter problem in the milling of titanium alloy materials is a major bottleneck restricting the efficiency of aerospace manufacturing. To ensure tool life, titanium alloy materials are cut at a relatively low speed. At this time, if the stability limit is very low according to the conventional linear model, selecting the depth of cut according to the line model will be very disadvantageous for the processing efficiency. In this paper, a nonlinear milling dynamics model considering process damping is established to calculate the intrusion area formed by the plowing effect and the process resistance. The critical depth of cut is calculated by the time domain simulation method. The experimental results show that the nonlinear calculation model proposed in this paper can accurately predict the stability limit of low-speed region during titanium alloy processing. This provides a necessary reference for the selection of parameters at normal operating speeds for titanium alloys.
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