Thermal Simulation of the Continuous Pulse Discharge for Electro-spark Deposition Diamond Wire Saw


 Due to their excellent physical and mechanical properties, third-generation super-hard semiconductor materials (such as SiC, GaN) are widely used in the field of microelectronics. However, due to its ultra-high hardness, the machining is very difficult, which has become the bottleneck of its development. The electro-spark deposition (ESD) process can deposit electrode materials on the substrate under the condition of low heat input to achieve metallurgical bonding between metal materials. And it can improve the wear resistance, corrosion resistance, and repair the size of the workpiece. It has been widely used in the field of surface modification engineering. It can effectively improve the bonding strength of the abrasive grains, and the sawing ability of the wire saw to make the consolidated diamond wire saw by the ESD process. Due to its thin matrix and poor thermal properties, the saw wire is easy to burning or even breaking in the manufacturing process. At present, the selection of pulse interval time in the ESD process is generally determined by the duty factor. However, the pulse interval time selected according to duty factor is difficult to meet the heat dissipation requirements of electro-spark deposition diamond wire saw (ESDDWS). In this paper, two kinds of motion modes of ESDDWS manufacturing are put forward, according to the manufacturing characteristics of ESDDWS. The boundary conditions of the continuous pulse discharge of ESDDWS are established. The thermal simulations of continuous pulse discharge of ESDDWS under two motion modes are analyzed. According to the simulation results, the basis of the value of pulse interval in the ESDDWS process is put forward. The effect of pulse interval time on the mechanical performance of the wire saw is analyzed experimentally. The results show that the discharge interval time selected base on the simulation results can ensure the continuous production of the ESDDWS.

The principle of fabrication of ESDDWS has been studied [23]. It is not difficult to 95 find the particularities of the ESDDWS process. Generally, the surface modification 96 technology requires a certain thickness of the deposited layer to meet the specific 97 performance requirements, so it needs to be repeated deposition in a certain area. In order to simplify the calculation, there are some assumptions made as follows. 114 (1) In the manufacturing process, the arrangement of the core wire and electrodes 115 makes the electrode column face to the surface of the matrix; 116 (2) In the process of manufacturing, the discharge is single-channel discharge and 117 normal spark discharge; 118 (3) The discharge points are at the minimum gap between the core wire and the 119 electrode; 120 (4) The shape of both electrode and core wire is ideal. It is ignoring the change of 121 electrode material consumption and wire surface material accumulation.
Where r and z are the coordinates of the cylindrical work domain; T is temperature; ρ, 127 c, k, and t are mass density, specific heat capacity, thermal conductivity, and time, 128 respectively. 129

Boundary conditions 130
The single discharge surface heat source of the core wire [23] can be given as Eq. 131 (2): 132 2 0 2 2 2 2 ( , ) exp( 4.5 ) cos ( cos sin ) Where θ is the coordinates of the cylindrical work domain; β is the angle between the ( Where h is the convection heat transfer coefficient; k is the thermal conductivity; R is 138 the radius of the plasma channel; m is the number of discharges. Initial temperatures of the core wire are assumed to be uniform at environment temperature, T0 = 25℃. 140

Meshing 141
Generally, the thermal analysis model of ANSYS is a closed model. In single pulse 142 analysis, the heat-affected zone of discharge point is smaller because of the short 143 action time and less heat input. Limited models can already meet the requirements of 144 simulation(shown in Fig.2(a)). However, Due to the long action time and the high 145 total heat input, the limited model can not meet the requirements in the continuous 146 pulse discharge deposition process, which increases the saw wire temperature (shown 147 in Fig.2(b)). Based on the element independence analysis (shown in Fig.3

Simulation results and discussion 170
In this work, the diamond abrasive used W40 Ti coated diamond. The selection of 171 discharge parameters based on the condition, the melting volume of electrode material 172 is the volume of diamond girt's 5, 10, and 15 times. According to the prediction range 173 of the process parameters [23], the discharge parameters determined as follows. The 174 current is 19A, and the pulse duration time is 12μs, 20μs, and 30μs. The thermal 175 simulations of continuous pulse discharge of ESDDWS under two motion modes are 176 analyzed. Under motion mode 1, when the current is 19A, the pulse width is 20μs, the 177 pulse interval is 600μs, and the moving speed is one discharge channel diameter per 178 period. The temperature field of continuous pulse discharge deposition is shown in 179 Fig.5(a). Under motion mode 2, when the current is 19A, the pulse width is 20μs, and 180 the pulse interval time is 8ms. The temperature field of continuous pulse discharge 181 deposition is shown in Fig.5(b).  Fig.6(a) and (c), it can be seen that during a discharge period, whatever mode 1 193 or 2, the core wire's temperature rises rapidly, and then decreases rapidly. At the 194 heating stages, the heating rate can reach 6×10 9 ℃/s. After discharging, the cooling 195 process can be divided into two stages. At the high-temperature stages, the 196 temperature of the core wire decreases rapidly, and the cooling rate is similar to the 197 heating rate. At the low-temperature stages, the cooling rate decreases gradually. temperature superposition can be ignored when the distance between two discharge 209 points is greater than three times the discharge channel radius. Finally, the core wire 210 tends to equilibrium temperature. In motion mode 2, the discharge centers are in the 211 same position. During the increase of discharge times, the core wire's temperature 212 increases continuously due to the superposition of energy. 213 We take the final temperature at the discharge center as the dynamic equilibrium 214 temperature of the core wire. According to the ANSYS simulation results, the 215 relationship between the core wire equilibrium temperature and the pulse interval time 216 in movement model 2 has obtained, as shown in Fig.7. 217 218 Fig.7 The relationship of balance temperature of wire and pulse interval duration 219 time(move model 2) 220

Experiments 221
The manufacturing experiment is carried out on a self-made ESD machine, and the 222 saw wire was deposited from one side. The experimental parameters are described in 223 Table 1. 224 The mechanical properties of the 304-ss wire changed because of the microstructure's 225 transformation. Because it is slender, the change of local structure will affect the 226 overall performance of the core wire. Researchers have reported that the martensitic 227 transformation of 304-ss begins at 300℃ [26]. Therefore, the local equilibrium 228 temperature of the saw wire should maintain at about 300℃ during the ESDDSW 229 process. Therefore, select the pulse interval time should base on the discharge parameters. In the experiment, when the pulse duration time is 12μs, the pulse interval 231 time is set to 1ms, 3ms, 4ms, and 5ms. When the pulse duration time is 20μs, the 232 pulse interval time is set to 4ms, 6ms, and 8ms. When the pulse duration time is 30μs, 233 the pulse interval time is set to 6ms, 8ms, and 10ms. During the experiment, when the 234 pulse duration time is 12μs and the pulse interval time is 1 ms, the saw wire is broke 235 frequently. And the saw wire deforms obviously after deposition. Under other 236 parameters, the saw wire does not break, and the saw wire keeps its original shape. 237 Tensile tests have been carried out on the deposited wire saw, and the 238 load-displacement curve is shown in Fig.8. It can be seen that: when the pulse 239 duration time is 12μs and the pulse interval time is 1 ms, the wire breaking force is 240 10N lower than that of the raw wire. And its ductility is also greatly reduced. Under 241 other parameters, with the decrease of pulse interval time, the broken force of the saw 242 wire does not decrease obviously, but its ductility also decreases gradually. It 243 indicated that the pulse interval would first affect the plasticity of the wire saw. And 244 when it exceeds a certain range, it would affect the tensile strength of the wire saw. 245 Therefore, in order to maintain its mechanical performance, the balance temperature 246 of the wire saw should be around 300℃ during the ESDDWS process. (2) According to the characteristics of the saw wire, a simulation model of continuous 255 pulse discharge deposition is established. 256 (3) According to the two different motion modes of electrode and core wire, the 257 thermal simulations of continuous pulse discharge of ESDDWS were analyzed. 258 (4) The simulation results show that: in movement mode 1, the temperature of 259 adjacent discharge points has the superposition effect. The effect of temperature 260 superposition can be ignored when the distance between two discharge points is 261 greater than three times the discharge channel radius. The wire saw's temperature 262 tends to balance finally. In motion mode 2, the discharge centers are in the same 263 position. During the increase of discharge times, the core wire's temperature increases 264     The relationship of balance temperature of wire and pulse interval duration time(move model 2) Figure 8 Load and displacement curve of tensile test:(a) 12μs; (b) 20μs; (c) 30μs