Deformation mechanism in wax supported milling of thin-walled structures based on milling forces stability

https://doi.org/10.1016/j.cirpj.2021.01.020Get rights and content

Abstract

At mesoscale, dynamic alternating milling forces and relative stiffness of thin wall are the main factors affecting the deformation of thin-walled structures. To improve the machining accuracy and effectively control the thin wall deformation, this paper investigates the control strategies of dynamic milling forces and deformation mechanism in milling of different thin-walled structures. The instantaneous deformation of thin wall in high temperature casting wax supported milling is analyzed. A deformation control method of thin wall based on the dynamic stability of micro milling forces is proposed in high-speed milling process. Three representative milling experiments on microchannel cold plate with cantilevered boundary, single impeller blade with mixed boundaries, and cantilevered structure shaped like variable curvature wavy line are presented. The milling deformation mechanisms of these three thin-walled structures with different target thickness are further investigated in free milling and casting wax supported milling. Compared with free milling, the milling forces and thickness dimension of thin wall machined under different control strategies are analyzed. By monitoring the milling forces in milling of different micro-walled structures, the small difference and fluctuation of milling forces at different positions, together with the reduced thickness errors verify the effectiveness of the control strategies, using high temperature precision casting wax as auxiliary support in reducing the thin wall deflection. Additionally, the optimal milling control strategy of thin wall is determined.

Introduction

Thin-walled features are widely used in complex aircraft components such as microchannel cold plate, micro impeller, terahertz slow wave and thin-walled metal circular tube [1], [2]. Micro milling can effectively produce three-dimensional complex features with good precision and performance on these micro components. The machining efficiency is relatively high and the machining characteristics such as curved surfaces can be complex, so micro milling plays an important role in machining thin wall (TW) [3]. However, due to strict tolerance requirements, large aspect ratio of TWs have become one of the most typical and challenging machining features.

According to the published reports, some research works focus on the deformation mechanism of TW through theoretical analysis and cutting experiments. Duong and Kim [4] mainly investigated the machining deformation of rectangular micro-straight grooves in microstructures, established a cantilever beam deformation prediction model under decentralized loading, and carried out finite element analysis and experimental verification. Gao et al. [5] carried out a series of micro milling tests on heat-resistant stainless steel (12Cr18Ni9), and explored the influence of different factors on machining quality of mesoscale low-rigidity structures. Their proposed model can predict the deformation of rectangular micro-straight grooves very well. Lauro et al. [6] used special cutting energy to study the effect of grain size in micro milling of high hardness steel. They pointed out that for large and small grain size steels, the special cutting energy is respectively reduced by 70% and 73% by increasing feed rate. Using an inverse evaluation method for solving the heat source intensity iteratively, Peng et al. [7] established a theoretical model for describing the increase in cutting temperature of the workpiece, and carried on a series of micro-milling experiments to study the milling temperatures and deformation. By comparing the simulated temperature and deformation with the experimental results, the simulation model showed considerable reliability. Yi et al. [8] studied the dimensional precision of straight TW at mesoscale and found that for micro straight TWs, 100 μm wall thickness is an inflection point to distinguish the dimension precision between macroscale and mesoscale.

Considering the thermomechanical loads acting on TW, Lazoglu and Mamedov [9] established the prediction model of TW deformation by finite element method, and carried out micro milling tests of titanium alloy TW to verify the model effectiveness. Yi et al. [10] studied the milling deformation mechanisms of thin-walled parts, and established the deformation equation and boundary conditions of TW with mixed boundaries under concentrated milling forces.

The milling deformation of thin-walled parts with titanium alloy is one main factor affecting the machining accuracy, and the prediction and measurement of micro milling forces is especially important for controlling TW deformation. In high-speed end milling, the cutting forces are found to be influenced most considerably by axial depth of cut, and thus the axial depth of cut plays a dominant role in the thin-walled parts deformation [11]. An innovative cutting force modelling concept is presented by Niu et al. [12], which reveals the underlying micro-cutting mechanics and physics in micro-milling using an innovative multi-scale method, i.e. the specific cutting force at the unit length, unit area and unit volume by considering the size effect, cutting fracture energy, material modulus, and cutting heat and temperature partition. Moges et al. [13] presents a methodology to determine cutting forces and surface error of micro milling in the presence of tool deflections. Based on the deformation characteristics of TW, a mathematical model of milling force for ball-end milling tool is established by Cheng et al. [14], and the built model is verified by the combination of three-dimensional finite element benchmark and experiments. The results show that the model provides an effective basis for future research upon controlling the deformation of titanium alloy. Liu et al. [15] studied the milling process of copper TW, established the mathematical model of cutting forces, predicted the TW deformation by finite element method, and carried out the milling experimental validation. Their research shows that not only the deformation and burr of TW become smaller, but also the machining quality is effectively improved, by reducing the depth of cut. In order to exactly predict the cutting forces, Yuan et al. [16] proposed an innovative uncut chip thickness algorithm, which takes into account the combination of the exact trochoidal trajectory of the tool tip and the cutting trajectory of all previously passing teeth, tool run-out, minimum chip thickness and the material’s elastic recovery. According to cutting edge size effect and the minimum chip thickness, Zhang et al. [17] proposed a cutting forces model of micro end milling process in the ploughing-dominant and shearing-dominant regimes, respectively. The experimental results carried out on Al6061 alloy in a wide range of cutting conditions show a good agreement with the proposed instantaneous cutting forces and tool deflection models. Bolar et al. [18] studied the influence of machining parameters in achieving low cutting force and surface roughness, and one important findings was that the cutting tool of 8 mm diameter produces superior surface quality at moderate cutting power consumption when employed with lower values of feed per tooth, axial depth of cut and radial depth of cut.

In mesoscale, the deformation control and machining quality improvement of TW has still been a research hotspot so far. Annoni et al. [19] have made much research on improving the geometric quality of carbon steel C40 thin-walled parts using micro milling. The control of thin-walled parts quality is explored by direct method and indirect cutting force controlled method, and the tolerance requirement of thin-walled parts is satisfied by constricting cutting force in a certain range. Diez et al. [20] studied the stability of deformation compensation in flexible milling, and established an on-line compensation system of workpiece deformation based on milling force, as shown in Fig. 1. Focusing on the machining errors caused by micro milling force during machining thin-walled structure, Ratchev et al. [21] put forward an advanced error prediction and compensation strategy. The theoretical elastic deflection was used to predict the machining errors, and the tool path was optimized by mirror compensation. The experimental results show that the overall error of thin-walled side milling can be obtained with high prediction accuracy, and most of the machining errors can be eliminated by mirror compensation.

Due to its low stiffness and time-varying modal parameters, thin-walled workpiece milling has also been a challenging problem. Llanos et al. [22] used experimental methods to study the milling ability of large aspect ratio of TWs for Al6061-T4 and CuZn36Pb3 alloys, including the influence of milling parameters and tool path on the surface quality, straightness, thickness uniformity and burr of TW. Matsubara et al. [23] investigated the design process of different support systems for suppressing vibrations during milling TW. Because of the low stiffness of thin-walled parts with curved surfaces, cutting forces become an important influencing factor on machining deformation [24]. In addition, compared with macro cutting, high speed micro milling has smaller cutting force, which provides a more effective method for machining titanium alloy micro thin-walled parts with curved surface. Because the stiffness of micro thin-walled curved surface parts has been always changing during the whole process of machining, this leads to more complex deformation under different tool paths, and also affects the quality of machining. In order to reduce the machining deformation, Gao et al. [25] found out reasonable cutting parameters, established a milling force model that directly affected the machining deformation, and then put forward the deformation control strategies under different tool routes. Additionally, they also proposed an effective deformation compensation method based on the modified tool position. Based on the cutting force model, the reasonable cutting parameters are obtained to reduce the machining deformation, and the control strategy of deformation compensation is finally obtained by planning the tool path, the deformation amount is thus reduced by 52.88%. Interestingly, the study of Wan et al. [26] have shown that auxiliary support using low melting point alloy (LMPA) containing Bi, Sn, and Pb can increase the rigidity of TW and effectively reduce surface defects, mainly burrs. However, after cutting, the LMPAs on workpiece surface are difficult to be completely removed, with metal residues such as surface oxidation easily left in the machined parts, and the resulting contamination during melting off the LMPAs make operators get hurt. On this basis, Kou et al. [27] investigated the mechanism of deformation control in micro milling of 80 μm TW, using LMPAs as auxiliary support. Nevertheless, the machining of titanium alloy thin-walled structures with target thickness of < 50 μm is not easy to be arrived at, and some associated problems such as mesoscale deformation [17], [18], [19], [20], [21], [22], [23], [24], [25], [27], chip-tool sticking [11], tool wear [28] and surface defects [22], [26], [29] are encountered in milling of thin-walled structures for titanium alloy.

The above research mainly focuses on the deformation control and compensation in micro milling of TW based on micro milling force control and tool path optimization, which provides a certain guiding role in improving the milling quality and dimensional accuracy of TW. And, there is little research upon the dynamic stability of alternating milling forces by improving self-stiffness of TW and optimizing milling path. Therefore, in this research, a deformation control method based on the dynamic stability of milling forces is proposed in the process of milling different micro thin-walled structures. Additionally, a series of micro milling experiments with different structures are carried out according to different target thickness of TW. Eventually, compared with free milling, the milling force and thickness dimension of TW under different control strategies are analyzed in auxiliarily supported milling.

Section snippets

Control mechanism of auxiliary support

In macroscopic milling of TW, the deformation is controlled by mirror compensation method. The deformation amount is calculated by experiments or finite element simulation, and then the program is written to compensate the deformation amount by tool path, so as to achieve the designed precision of TW. However, at mesoscale, the TW thickness is too small, usually ranging from tens to hundreds of microns. Due to too low stiffness, TW deformation is easy to occur. Under the over-extrusion of the

Experimental design

The deformation mechanism of milling auxiliarily supported by casting wax is investigated. To better compare with free milling in Ref. [10], the wax supported milling experiments used the same milling parameters, shown in Table 2. In all milling experiments of thin-walled structures, the workpiece material is Ti-6Al-4V titanium alloy, and the cutting tool used is KENAMETAL double-edged flat end milling cutter (cemented carbide, made in Germany) from the same batch. The real-time deformation is

Experimental design

Fig. 10 shows the deformation control of HTPCW support in micro milling of TW with mixed boundaries. In order to investigate micro deformation law during micro milling, the milling experiments were carried out in air-cooled dry cutting. After filling with HTPCW, the thickness dimension accuracy of casting wax supported milling is investigated. These small slots were generated after several TWs were machined under different milling.

Analysis of micro milling forces

Fig. 11 shows the forces analysis process in milling of TW with

Experimental design

Based on terahertz zigzag quasi-plate slow-wave structure [30], a cantilevered micro thin-walled structure with variable curvature wavy line is designed. Two micro thin-walled arrays are terahertz zigzag quasi-plate slow-wave structure, while several micro thin-walled arrays make up microchannel liquid cooled plates. The deformation control of the cantilevered TW with variable curvature wavy line was experimentally investigated in casting wax supported milling. Fig. 19(a) shows the geometric

Conclusions

  • (1)

    A deformation control method based on dynamic stability of micro milling force is proposed, which is auxiliarily supported by HTPCW. The milling experiments on the microchannel cold plate with cantilevered boundary, single impeller blade with mixed boundaries, and cantilevered structure shaped like variable curvature wavy line were carried out. The instantaneous deformation in these three representative milling process of TW is analyzed. Compared with the free milling, the average value of

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is financially supported by the Fundamental Research Funds for the Central Universities (Grant No. G2020KY0501), Basic Research Programs of Taicang (Grant No. TC2020JC10), National Natural Science Foundation of China (Grant No. 51605262), Natural Science Foundation of Shandong Province (Grant No. ZR2020QE180), and Youth Innovation Support Program of Shandong Colleges and Universities (Grant No. 2019KJB003).

References (30)

  • G. Tosello et al.

    High Aspect Ratio Micro Tool Manufacturing for Polymer Replication Using Μedm of Silicon, Selective Etching and Electroforming

    Microsystem Technologies: Sensors, Actuators, Systems Integration

    (2008)
  • E. Kuram et al.

    Effects of Tool Paths and Machining Parameters on the Performance in Micro Milling of Ti6al4v Titanium with High-speed Spindle Attachment

    The International Journal of Advanced Manufacturing Technology

    (2016)
  • T.H. Duong et al.

    Deformation Analysis of Rectangular Channel Structures in Micro Pattern Machining

    International Journal of Precision Engineering and Manufacturing-Green Technology

    (2015)
  • S.F. Gao et al.

    Research on Machining Quality Control in Micro Milling of Low-rigidity Characteristics

    The International Journal of Advanced Manufacturing Technology

    (2017)
  • C.H. Lauro et al.

    Specific Cutting Energy Employed to Study the Influence of the Grain Size in the Micro-Milling of the Hardened AISI H13 Steel

    The International Journal of Advanced Manufacturing Technology

    (2015)
  • Cited by (13)

    • Towards high milling accuracy of turbine blades: A review

      2022, Mechanical Systems and Signal Processing
      Citation Excerpt :

      Fei et al. [112] presented a moving fixture to suppress deformation of thin-walled structures. Xiang and Yi [105] analyzed the effect of high temperature precision casting wax (HTPCW) as auxiliary support, and concluded that HTPCW can improve the relative stiffness, reduce elastoplastic deformation, increase material removal and improve machining accuracy. Liu et al. [113] studied the supporting effect of air jet assistance during milling of thin-walled workpiece.

    View all citing articles on Scopus
    View full text