1. Introduction
Technology industries develop rapidly, and electronic products continue to develop in the direction of compact size with high performance. In addition, to cope with the global trend of energy saving, the demand for energy-saving lighting products has also increased significantly. Sapphire has the advantages of excellent heat conductivity [
1], non-electrical conductivity [
2], wide light transmittance wavelength [
3], high hardness [
4], high melting point [
5], resistance to corrosion [
6], etc., and it can be used for high/low temperature laboratory observation windows [
7], optical lenses requiring high surface scratch resistance (such as cameras’ front lenses, premium watch glass, and optical finger identifier glass), etc. Sapphire can have surface epitaxy with gallium nitride (GaN) capable of emitting light in order to be used as a substrate for light emitting diodes (LEDs) [
8], and its cost is lower than the LED substrates of gallium arsenide (GaAs), silicon carbide (SiC), etc.; therefore, its applicability is increased significantly [
9].
As sapphire has the characteristics of non-electrical conductivity and cannot be cut easily [
10], the present machining methods adopted in the industry, such as abrasive machining [
11], laser machining [
12], ultrasonic machining [
13], and chemical etching techniques [
14], all have certain drawbacks and limitations. The use of laser machining and ultrasonic machining methods to perform micro-machining on glass can cause the problems of surface cracks, degradation of the surface cleanliness, etc. Traditional machine abrasive machining is limited to the cutting of simple structures [
15]. The material removal rate of chemical etching of hydrogen fluoride (HF) is relatively slow and has the issue of environmental pollution [
16]. Accordingly, the method of electrochemical discharge machining (ECDM) is capable of performing effective material removal machining on non-electrically conductive hard and brittle materials. This is a non-traditional machining method [
17,
18] that is extremely suited to performing highly efficient micro-machining on sapphire. However, regarding the ECDM method, during the machining process, for the machining electrode and machined workpiece, the electrolyte circulation, electrolyte concentration maintenance, discharge of machining wastes, etc., can become more difficult as the machining depth increases. These phenomena can cause severe hindrances [
19] of micropore machining with high precision.
In 1998, Gautam et al. [
20] used a gravity fed spindle, and this rotational laboratory device was able to perform ECDM on glass materials. The experiments were conducted using two types of machining methods: rotating spindle and fixed spindle. The experimental results proved that the use of a spindle was able to allow the tool electrode to rotate such that the machining efficiency of ECDM can be effectively improved. In addition, as the discharge generated by the tool electrode rotation was not concentrated at a particular area, the wear of the tool electrode was reduced.
In 2001, Yang et al. [
21] studied the feasibility of using electrolytes of the solvents of KOH, NaOH, H
2SO
4, NaNO
3, NaCl, and NaClO
3 in ECDM. The results indicated that the use of H
2SO
4, NaNO
3, NaCl, and NaClO
3 were able to generate harmful gases during the electrochemical discharge reaction process and the material removal rate was poor, and thus unsuitable for ECDM. After the experimental result comparison, they found that since the electro mobility (7.62 × 10
−8 m
2s
−1V
−1) of K
+ was higher than the electro mobility (5.19 × 10
−8 m
2s
−1V
−1) of Na
+, and KOH solution had a lower viscosity than that of the NaOH solution, it was able to facilitate the electrolyte flow circulation at the machining area such that when the KOH solution was used as the electrolyte, it demonstrated superior results in the machining efficiency and machining quality.
In 2014, Jiang et al. [
22] used finite element analysis to compare the column electrode and conical electrode current density distributions, and found that the conical electrode current distribution concentrated at the tip portion, and the machining energy concentrated at the tip. In addition, the electrolyte circulation effect was found to be better than the one of the column electrodes, and was able to improve the machining efficiency and reduce the problem of side wall discharge. In 2015, Jiang et al. [
23] used the electrolyte concentration, the electrode diameter, and the electrode immersion depth into the electrolyte as the variables to perform simulation through the finite element analysis method, and discharge machining threshold currents of ECDM were obtained. The result proved that when the electrode immersion depth into the electrolyte was shallower, the machining threshold current was greater, and the machining energy was greater.
In 2015, the Master’s thesis [
24] by Sen-Fu Chung at National Central University discussed the feasibility of the use of ECDM to perform machining on sapphire materials. The experimental results indicated that when a machining voltage of 80 V was used with a tungsten carbide tool electrode (with a diameter of 350 μm and a rotational speed of 300 rpm in an electrolyte mixture of phosphoric acid and sulfuric acid, where sulfuric acid accounted for 55.56% of the mixture), in order to perform machining on the sapphire, the maximum machining depth could reach 24.4663 μm. The machining efficiency was higher than the machining efficiency of the wet etching method used by the industry for machining sapphire.
The findings from the literature [
19,
23,
25] demonstrated that the tool electrode rotation provided a higher material machining efficiency of ECDM and aided in the improvement of the hole circularity [
26]. However, the tool rotation resulted in a larger surface contact between the tool tip and the workpiece and the hole size increased from the required hole diameter [
27]. Thus, overcutting and cracking [
28] could be observed at the exit of the hole. In our previous research [
29], we showed that the assist nozzle can prevent the effect of the discharge around the sidewall of the machining tool and reduce stray electrolysis. However, the hole circularity was significantly affected by the assisted nozzle.
In ECDM, if the method of an increased machining voltage and an increased electrolyte concentration is used, it can still achieve the objective of increasing the material removal rate. It can also cause over cutting and adverse heat impacts on the machined material due to the machining energy, resulting in the reduction of the machining quality and machining precision. Accordingly, in this research, an innovative combinational machining assisted method was proposed in order to use a self-developed coaxial-jet nozzle to allow the ECDM to combine with two assisted methods of tool electrode rotation and coaxial-jet at the same time.
4. Conclusions
In this paper, we investigated the machining issues related to the use of electrochemical discharge machining (ECDM) to perform non-electrical conductive hard and brittle material removal machining. To overcome the problems of overcutting, hole expansion, and rough surfaces at the thermally-affected area of hole edges that often occur in ECDM, causing degradation of the machining quality and difficulty in the improvement of precision, we proposed an innovative combinational assisted machining method to overcome the machining problems. In addition, the efficiency and precision of the assisted machining method were studied and tested in order to ensure that this method was able to overcome the machining problems of overcutting and hole expansion. We also confirmed that the method was able to improve the machining quality.
In this study, through the machining experiments conducted on sapphire, a transparent, hard, and brittle material widely used in industrial applications, a self-developed coaxial-jet nozzle was used and ECDM was combined with two types of assisted methods of tool electrode rotation and coaxial-jet, simultaneously. Accordingly, the electrolyte of the machining area was maintained at the low liquid level and the electrolyte could be renewed at the same time, thereby allowing the spark discharge to be concentrated at the contact surface between the front end of the tool electrode and the machined material. In addition, prior to the machining and micro-drilling, an assisted mechanism for energy limitation was further adopted.
The results of this research can be summarized as follows:
We researched and developed an assisted nozzle capable of combining two types of assisted methods of the tool electrode rotation and coaxial-jet at the same time.
We established an experimental system architecture for the online observation of the ECDM experiment.
The combinational machining assisted method proposed in this research indicated a reduction of axial wear by 39.29% and radial wear by 84.09% in comparison to the tool electrode without assisted machining. In addition, we discussed the tungsten carbide tool electrode wear principle during ECDM.
The method of ECDM was used to perform the machining of sapphire with a voltage of 53 V in KOH electrolyte at a concentration of 5 M, and the machining depth reached 193 μm.
In this research, observations were made before and after the experiments, and data were collected for verification. The electrochemical discharge combinational machining assisted method proposed in this research, in comparison to the unassisted machining method, demonstrated an increased machining efficiency and a greater ability in the machining of sapphire material. Thus, the machining precision for transparent hard and brittle materials can be increased in the future, and such methods can be widely applied to the industries of material machining, optical components, semiconductors, batteries, etc., thereby achieving greater quality.