Counter-rotating electrochemical machining of intensive cylindrical pillar array using an additive manufactured cathode tool
Graphica abstract
Introduction
With the rapid development of modern industry, array structures have been widely employed on metallic surface for achieving superior functional performance. For example, dimple array prepared on the frictional surface can help reduce the friction coefficient and increase the service life [1,2]. Channel array on metallic bipolar plate is essential for the energy conversion efficiency in fuel cells [3], [4], [5], [6], [7]. Hole array is commonly used for air cooling in the aero engine field [8], [9], [10], [11]. Micro pillar array is of great significance to prepare superhydrophobic surfaces [12,13]. In addition, pillar array in millimeter scale is an important cooling structure in combustion chamber flame tube, owing to its superior properties in facilitating the heat transfer [14].
So far, many techniques are developed to fabricate array structures on metallic surface, including mechanical cutting [15,16], electrical discharge machining [17,18], laser machining [19], electrochemical machining (ECM) [20], etc. Among these techniques, ECM, as a non-contact anodic dissolution process, holds the unique advantages such as absence of tool wear, lack of heat affected layers, regardless of the material hardness and toughness [21], [22], [23], and has been an efficient method for the machining of array structures. Mahata et al. fabricated square micro-dimples on stainless steel 304 using through-mask ECM (TMECM), in which an insulated patterned mask is generally prepared to cover the workpiece [24]. Zhu et al. used a thin metal film coated with patterned insulation plate as cathode, and micro hole and dimple arrays of several hundred micrometers diameter were produced [25,26]. Madore et al. employed TMECM to fabricate well-defined three-dimensional topographies in titanium [27]. Wang et al. used TMECM to fabricate micro-grooves with semi-circular profiles on titanium alloy foil [28]. Chen et al. machined micro-dimple arrays on titanium alloy surfaces using a reusable mask [29]. Holstein et al. fabricated microstructures in sizes of 100 μm on tungsten surface by combination of ECM with advanced micro-lithography [30]. Tsai et al. used Taguchi method in TMECM to obtain the best combination of processing parameters, and the uniformity of array holes was remarkably improved [31]. Ming et al. [32] and Chen et al. [33] presented a modified active TMECM with flexible nonconductive porous mask to machine micro dimples on planar and non-planar workpieces. Patel et al. produced texture arrays of micro channels, micro dimples on free-form surfaces with a flexible electrode in TMECM process [34]. Furthermore, Wang et al. used array tube electrodes with vacuum extraction of electrolyte to process hole array [35]. Zhang et al. fabricated variable-section pit arrays using ECM method with a pre-shaped array structured cathode tool [36]. Bo et al. exhibited the machinability for ECM of hole-array on stainless steel 304 with multiple electrodes [37]. Wu et al. used a mask electrolyte jet to fabricate complex superficial concave structures [38,39].
Taking a review of the literatures, TMECM technique has been one of the most popular methods for the production of array structures. Nevertheless, there are still many challenges remaining for TMECM of array structures in sub-millimeter scale. In ECM, to achieve favorable machining accuracy, the machining gap is generally controlled to be a small value through the feed motion of the cathode tool [40]. Smaller machining gap can help achieve higher machining accuracy [41,42]. However, the cathode tool in TMECM is unable to feed. The inter-electrode gap is expanded continuously with the increase of the processing time, which will lead to poor anodic dissolution localization. Furthermore, most of reported literatures focused on the fabrication of concave structures such as dimples, holes and channels, and little work has been done in related to convex pillar structures. For concave structures, the mask patterns employed are cross-linked, which can easy obtain enough bonding force with the substrate owing to the large contact areas. In contrast, the mask patterns for pillar arrays are isolated from each other, which can be easily peeled off under the high-speed electrolyte flow [43]. Moreover, due to the existence of inevitable undercutting in TMECM [44], [45], [46], the contact area will be reduced with increasing machining time, which will further aggravate the peeling process.
Counter-rotating electrochemical machining (CRECM) is a new ECM method, in which the material is dissolved evenly under the synchronous counter-rotating motion of anode workpiece and cathode tool [47,48]. This method has exhibited excellent performance for the machining of convex structures on the outer surface of revolving parts. Complex convex structures were successfully on conical part using a frustum cone-like cathode tool [49]. In particular, a rectangular convex array has been also fabricated on revolving surface by using CRECM method. The feasibility was verified numerically and experimentally [50].
This paper aimed to machine intensive cylindrical pillar array in millimeter scale on revolving surface by using CRECM method. The design and fabrication procedures of the cathode tool with hollow windows were described elaborately. To obtain desired circular pillar structures, non-circular hollow windows with insulated sidewalls were designed. A hybrid additive manufacturing process combining 3D printing and electrodeposition was proposed to machine the cathode tool with multiple non-circular hollow windows. Finally, a 60 × 7 cylindrical pillar array with average dimensions of 4.3 mm diameter and 2.4 mm height was successfully machined on the revolving surface using CRECM with the additive manufactured cathode tool. The sidewall angle of the pillar was measured to be around 9.6°, and the profiles of different pillar structures exhibited favorable uniformity with a maximum roundness error of 0.18 mm.
Section snippets
Principle of CRECM of intensive cylindrical pillar array
Fig. 1 shows the principle of CRECM of intensive cylindrical pillar array. The revolving workpiece is connected to the positive terminal of the power supply, and works as an anode. A small cylindrical cathode tool with multiple hollow windows is connected to the negative terminal of the power supply, and used as a cathode. The anode workpiece and cathode tool keep a small machining gap and rotate oppositely at a constant speed ratio n. The cathode tool feeds simultaneously toward the anode
Design of cathode tool in CRECM of intensive cylindrical pillar array
In CRECM, the diameter of the anode workpiece is decreased continuously with the feed of the cathode tool, while the diameter of the cathode tool remains unchanged. As the angular speed ratio needs to be strictly a constant value, there is a difference in the linear velocity of rotation between anode workpiece and cathode tool. This is quite different from that in the gear hobbing process, in which the workpiece and cutter are pure rolling on the base circle. The shaping process of the pillar
Hybrid additive manufacturing of cathode tool
As shown in Fig. 9, to obtain pillar structure with cylindrical shape in CRECM, the hollow windows on the cathode tool should be designed to be non-standard circles, and their sidewalls need to be electrically insulated to reduce stray current attack. These multiple specific-designed hollow windows bring challenges for conventional methods. On the one hand, the non-circular profile can hardly be machined by conventional drilling or boring methods. Even by milling, the machining process is
CRECM of intensive cylindrical pillar array
Based on the additive manufactured tool electrode in Fig. 13(b), CRECM experiment is conducted to machine intensive cylindrical pillar array on revolving surface. The experimental set-up is shown in Fig. 14. The revolving anode workpiece and tool electrode are fixed on two inversely rotating shafts, and surrounded by an epoxy fixture, in which the electrolyte flows from one side to the other. The anode workpiece is made of stainless steel 304, and the diameter is Φ200 mm. The tool electrode
Conclusions
In this paper, an intensive cylindrical pillar array in millimeter scale on revolving surface were fabricated using CRECM method with an additive manufactured tool electrode. The hybrid additive manufacturing process combining both 3D printing and electrodeposition techniques was proposed. The design and fabrication procedures of the cathode tool were described elaborately. The conclusions can be summarized as follows:
- (1)
The radius of cathode tool and the profile of the hollow windows were
Declaration of Competing Interest
None
Acknowledgement
This research was funded by National Natural Science Foundation of China (51805259), Natural Science Foundation of Jiangsu Province (BK20180431), Postdoctoral Science Foundation of China (No. 2019M661833), Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology and Young Elite Scientists Sponsorship Program by CAST.
Author statement
Dengyong Wang: Methodology, Writing- Original draft preparation. Qianqian Wang: Investigation, Validation. Jun Zhang: Investigation, Validation. Huayong Le: Investigation. Zengwei Zhu: Visualization. Di Zhu: Conceptualization, Supervision.
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