Characteristics of cutting force and surface finish in diamond turning of polycrystalline copper achieved by friction stir processing (FSP)
Introduction
Polycrystalline copper components fabricated by single point diamond turning have been extensively applied in space science, clean energy, and electronics industry due to its high electrical, thermal conductivity, etc (Lu et al., 2021). Cutting force and surface finish are two critical characteristic parameters for diamond turned components (Zhao et al., 2019). Cutting force affects diamond tool wear and spindle vibration; surface finish has immediate impact on the service performance of copper components (Zhang et al., 2019). Therefore, it is quite necessary to accurately predict cutting force and obtain good surface finish for diamond turned copper components.
To establish an accurate cutting force model, Arcona et al. (1998) measured the shear angle from micrographs of chip cross sections and developed a cutting force model considering chip formation and friction between a diamond tool and cutting chip. Lee et al. (2002) proposed the micro-cutting force model based on microplasticity considering the crystallographic orientation of the work material. They further applied this model to analyse cutting force variation in diamond turning. Wang et al. (2004) presented a cutting force model considering the cutting forces acting on the rake face, cutting edge, and flank face. Zhou et al. (2009) developed a dynamic cutting force model in three cutting directions and analysed their impacts on surface topography generation in diamond turning. Jamshidi et al. (2020) studied the high negative rake angles in material removal mechanism and established a cutting force suitable well for the individual grit interaction process. Venkatachalam et al. (2015) presented a cutting force model considering the microstructure effect of work material in ultraprecision machining. Brinksmeier et al. (2017) performed systematic investigations on cutting forces in high-speed diamond machining. They developed the cutting forces model with the 3D finite element method (FEM), and further validated its accuracy based on cutting experiments. To predict cutting forces in fast/slow tool servo diamond turning of micro-structured surfaces, Zhu et al. (2019) developed the cutting force model based on the dynamic slip line theory. Rahman et al. (2020) took the material grain size and chip thickness into account and developed the flow stress constitutive model, which was further applied to analyse surface generation mechanism for different relative tool sharpnesses. Lu et al. (2020) analysed the friction property of tool-chip interface based on shear-slip theory and developed a prediction model of cutting force for oblique elliptical vibration cutting (Lin et al., 2016). In the interest of cutting forces in ultraprecision machining of microgrooves, Zhao et al. (2019) developed a cutting force model considering the tool geometry, work material spring back, and heat generation in the cutting process.
As depicted in the above state-of-the-art review, great efforts have been made on the cutting force model in diamond turning. However, some issues still have not been satisfactorily solved. First, grain size of work material has a great influence on flow stress, which further affects cutting forces in diamond turning process. However, in the existing cutting force model, its influence has not been comprehensively studied. Second, due to the small cutting edge radius of diamond tool (10∼100 nm), a severe stress concentration was observed on cutting edge (Zong et al., 2007), which had not been fully considered. There are three cutting force components, i.e., principal cutting force, thrust force, and feed force, in the diamond turning process. Current studies mainly focus on the first two force components. A reliable model for feed force is also essential. Furthermore, residual stress in the work material can also affect cutting force, and its influence has not satisfactorily been considered into the cutting force model.
Meanwhile, for diamond turned polycrystalline components, small steps with step height of a few nanometres appeared on the grain boundary has been reported (Brinksmeier et al., 2017), which obviously degrades surface finish (Ding et al., 2012). To improve surface finish of diamond turned components, developing fine-grained work material has been demonstrated to be effective in practical application (He et al., 2019). Friction stir processing (FSP) technology, which derives from friction stir welding (FSW) in principle, has been widely applied in polycrystalline copper as a grain modification technique in the past decades (Wang et al., 2017). It has been demonstrated that FSP can achieve fine-grained work material with grain sizes at several microns (Wang et al., 2021). However, the mechanism in relation to surface finish improvement of with fine-grained copper achieved by FSP has not been comprehensively revealed.
Therefore, an accurate theoretical model was designed to predict cutting forces in diamond turning of polycrystalline copper processed by FSP technology. The influence of Hall-Petch effect, thermal effect, viscous property of copper as well as the residual stress is successively considered. X-Ray Diffraction (XRD) and Electron Back Scattered Diffraction (EBSD) technology were designed to analyse the variation of residual stress and grain size for copper material achieved by FSP at different processing parameters, respectively. Cutting forces on the rake face, cutting edge area, and flank face are modelled and calculated, respectively. Cutting experiments were performed to validate the accuracy of the proposed model and investigate cutting force variations at different parameters. Furthermore, the underlying mechanism of the relationship between surface roughness and fine-grained copper material was demonstrated according to the material characterization results.
Section snippets
Flow stress model
In this work, the flow stress of a polycrystalline cooper is composed of four parts, i.e., Hall-Petch stress σhp, thermal stress σth, viscous stress σvs, and residual stress σre. Therefore, the flow stress of σf is calculated as (Gao and Zhang, 2012):
Hall-Petch effect is extensively observed in polycrystalline cooper, which demonstrates the variation of mechanical parameters (e.g., flow stress, yield stress, and hardness) with grain size. In this work, the Hall-Petch stress is
Cutting edge of diamond tool
In this work, three waviness-controlled diamond tools (natural, Suporhard Co. Ltd.) were employed in cutting experiments. Cutting edge radii of three diamond tools were evaluated using atomic force microscope (Nanosurf Nanite B), and five positions were chosen as sampling sites for each diamond tool. Measurement results of 3D and 2D cutting edge profiles of diamond tool 1 were depicted in Fig. 4. Average value of the cutting edge radii from five different sites was configured as cutting edge
Efficiency of the newly developed model
In this work, 3-axis cutting force components, i.e., principal cutting force, thrust force, and feed force, were measured from radius r = 8 mm to r = 36 mm in cutting process, which corresponds to the FSP area. For further verification of cutting forces on rake face, cutting edge area, and flank face, we recommend the photoelasticimetry method, which has been demonstrated to be successful in visual observation of cutting forces at the desired positions. Corresponding process has been
Conclusions
In this work, a cutting force prediction model was developed considering the Hall-Petch effect, residual stress, etc. Friction stir processing (FSP) was applied to achieve copper materials with different grain sizes. The influence of FSP treatment on surface finish is also discussed. According to the above theoretical and experimental results, some important conclusions are drawn as:
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The stress distribution on rake face, cutting edge area, and flank face is modelled, which can be further applied
Data availability
All data is within the manuscript and figure.
CRediT authorship contribution statement
C.L. He: Conceptualization, Methodology, Data curation, Investigation, Funding acquisition, Project administration, Writing - original draft, Writing - review & editing. J.G. Zhang: Formal analysis, Data curation, Resources, Validation. C.Z. Ren: Supervision, Methodology, Writing - review & editing. S.Q. Wang: Supervision, Methodology, Funding acquisition, Investigation. Z.M. Cao: Writing - review & editing, Formal analysis, Software.
Declaration of Competing 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
The authors would like to thank National Natural Science Foundation of China (Nos. 52175430 and 52105478), China National Postdoctoral Program for Innovative Talents (BX20200234) and Research start-up funds of Tianjin University of Technology (No. 10101/01002008) for the support of this work. We would like to thank Zhang Jinfeng for her professional operation in material analysis and Dr. Huang Tifang for his operation in copper processing.
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He and Zhang contributed equally to this work.