High Aspect Ratio Machining of Carbon Fiber Reinforced Plastics by Electrical Discharge Machining Process

Microcomponents made of carbon fiber reinforced plastics (CFRPs) have been manufactured by various manufacturing techniques for a variety of applications, mainly because of CFRPs high strength and lightweight [1]. Such composite materials are characterized by marked anisotropic, lack of plastic deformation, and structural nonhomogeneity [2,3]. Thus, compared to the machining of metals, precision CFRP machining is much more challenging due to high tool wear, delamination, and uncut fibers [4,5]. Mechanical machining on carbon/epoxy laminates reduces strength and fatigue life of the part [6]. Holes drilled by abrasive water jet method have taper and the delamination. Laser machining causes heat affected zone that degrades the property of the work piece [7]. Another nontraditional method, namely, electrical discharge machining (EDM), is a widely used process in the production of microtools and microstructures on conductive materials [8]. A study found that EDM reduces the delamination of CFRP [9]. EDM has been successfully applied to machine through hole with aspect ratios up to about 0.83 in CFRP [10].

The major challenge in the machining of CFRPs by EDM is that only the carbon fibers are conductive whereas the epoxy resin, which is the material that binds the conductive fibers, is nonconductive, which results in discontinuous energy transfer resulting in poor machining quality. Therefore, it is necessary to ensure the conductivity of the work piece through the entire machining process to maintain the quality of the machined feature. An aspect ratio of 10.9 with a hole of diameter 110 μm and a depth of 1.2 mm in CFRP by EDM was reported in Ref. [11]. But it must be noted that the hole in the work material in this study was drilled only through the carbon fiber layer of the composite as shown in the Fig. 1(a). The hole was made without the tool electrode touching the epoxy layer. This does not represent the real-world application where the drilling is required to pass-through multiple layers of a composite material.

From the above reviewed literature, the reports on the CFRP machining by EDM in general is very limited, and only few articles related to the macrohole drilling of CFRP by EDM have been reported. Literature available on manufacturing of through microholes by EDM is almost absent. Table 1 summarizes the information available on the aspect ratio of holes machined in CFRPs by EDM. The motivation behind this study is to propose a novel method of machining CFRP that eliminates present challenges and adds to the literature on machining CFRP. In this work, the authors test the hypothesis that by increasing the conductivity of CFRP, the EDM hole can be drilled in CFRP as shown in the Fig. 1(b), where the hole is intended to be made through alternate layers of carbon and epoxy layer. The experimental setup used to test this hypothesis is elaborated in the section that follows.

The EDM setup and the process parameters used in this study are shown in Fig. 2 and Table 2. Properties of the CFRP work material are listed in Table 3. The rotation of the tool is achieved by a DC motor whose RPM can be adjusted by a potentiometer. The work piece is held firmly with the help of a suitable fixture inside a tank that contains the dielectric fluid. The feed rate of the tool is maintained by a highly precise stepper motor that is controlled by Arduino. An RLC power supply is used in the experiment. Tektronix current probe TCP2020 is used to monitor the current throughout the experiment. Using the EDM power supply, the voltage and current were controlled to remain constant at predetermined levels. The representative images of current data, revealing whether the machining process is in control or not, is shown in Fig. 3. A LabVIEW interface was used to collect the current values from the current sensor and stored using a data acquisition device.

Carbon fiber reinforced plastics consists of a combination of carbon fiber with other materials to form composites. When impregnated with epoxy and baked, forms CFRP. Carbon fibers are conductive, the nonconductive epoxy layer at the top and bottom of the CFRP makes the entire workpiece nonconductive. Aside from the top and bottom layers, resin is present in between the carbon fibers to bind them. However, after the initial spark, this resin melts due to its low melting point than carbon. This eliminates the main challenge of machining the CFRP. A conducting tape made of copper is stuck around the edge of the CFRP sample to ensure conductivity during the entire machining process through every layer in the CFRP.

Experiments were performed with Ø 300 μm tungsten carbide tool to a depth of 1000 μm to obtain optimum voltage level for the experiment. The feed rate is maintained at constant at the value of 6 μm/s. For each value of voltage ranging 40 V, 70 V, and 100 V, 12 experiments were carried out. Overcut results from these experiments are shown in Fig. 4. It was found that the overcut of the hole machined increased with the increasing voltage. This is due to the reason that spark energy increases as the voltage increases and thus removes more material at higher voltages. At lower voltage, overcut was minimum, but the machined hole was improper which is a disadvantage for high aspect ratio through hole. This effect can be clearly seen in Fig. 5.

When the feed-rate decreased, the overcut increased as shown in Fig. 6. At lower feed rates, the tool travels for a longer time to complete the drilling process and therefore, the side walls of the hole are exposed to a greater number of sparks. This causes larger radial overcut and also irregular shape at lower feed rates. For each value of feed-rate ranging 3 μm/s, 6 μm/s, and 11 μm/s, 12 experiments were carried out and the overcut results are shown in Fig. 6. Figure 7 shows the holes drilled at different feed rates. The first hole which was drilled at 3 μm/s resulted in larger diameter and uneven shape due to more sparks at the side walls. The last image shows the hole drilled with 11 μm/s. Due to increased feed rate, the tool wobbled and tried to drill a hole by traditional drilling instead of creating sparks.

Two experiments were carried out with different polarities of the tool and the workpiece. Voltage and feed-rate were kept constant at 70 V and 6.0 μm/s, respectively, for both the experiments. Figure 8 shows the results. There is evidence of burnt epoxy around the hole caused by surge current when first spark occurs.

It was noted that the tool wear was nearly seven times lesser when the tool was used as an anode and CFRP workpiece as cathode. Similar observation was reported in Ref. [14]. It was reasoned that a layer of carbon film, formed on the surface of anode in EDM, protects the anode from eroding. Since the CFRP is made of carbon fiber, there is a possibility of carbon being deposited on the anode tool, thus forming a protective layer reducing the tool wear when the tool is used as anode. Thus, we can conclude that the tool with positive polarity was able to drill a through hole in the same condition but the tool with negative polarity wore out much earlier before making a through hole.

Aspect ratio is the ratio of the hole depth to the average diameter of the hole. The average diameter of the hole is calculated by the average of the entrance and the exit diameter of the machined hole. It is hypothesized that high aspect ratio can be achieved by maintaining the conductivity of the workpiece (i.e., carbon fiber must be conductive during the EDM machining). In this experiment, it is made sure that every fiber is conducting as explained in preparation of workpiece. A maximum aspect ratio over 11 was achieved in this study by using 70 V open circuit voltage, 6 μm/s feed-rate, and tool diameter of Ø120 μm. The hole had an entrance diameter of 300 μm and an exit diameter of 130 μm on a 2500 μm thick CFRP sample. Figures 9(a) and 9(b) shows the entrance and exit diameter of one of the high aspect ratio holes drilled and represent the several high aspect ratio holes drilled in this study. Figure 10 shows the cross-sectional image of a hole drilled with a Ø300 μm tool. Figure 11 shows the fibers present in the sectional view of the drilled hole.

This section summarizes the results found above. Effect of voltage, feed-rate, and tool-polarity on machining CFRPs was studied. We found that increased voltage removed more material than desired from our work piece and the machined hole had irregularities in its finish, while lower voltages did not achieve sufficient machining. Optimum voltage was found at 70 V. Similar effects were found with changing feed-rate. Optimum feed-rate was found at 6 μm/s. In experimenting with tool-polarity, we observed that when tool was used as anode, the tool showed less wear than when as cathode. With these experiments, optimum machining parameters were deduced.

The major challenge in machining CFRP by EDM is due to the nonconductivity of epoxy material which is used as a binder for manufacturing these CRFPs. Experimental study was conducted to assess the feasibility of high aspect ratio microhole drilling on CFRP by EDM wherein the workpiece was maintained conductive throughout the drilling process by providing a conductive layer around the edges of a carbon fiber during the EDM process. It was found that with the increase of voltage, the overcut of the hole machined increased as expected. Also, as the feed-rate decreased, the overcut increased. Tool wear was found to be significantly less when positive tool polarity was used in the micro-EDM of CFRP. These positive observations were useful for the high aspect ratio machining. An aspect ratio over 11 was achieved on microdrilling of CFRP by EDM using Ø120 μm tungsten carbide tool fed at 6.0 μm/s at 70 V. These machining parameters and the method of conductive CFRP can be used to drill high aspect ratio microholes whenever necessary.

• National Science Foundation (Grant No. CMMI-1454181; Funder ID: 10.13039/100000001).