MICROWAVE DRILLING OF BOROSILICATE GLASS

Abstract

Microwave drilling is an electromagnetic energy-based machining process in which microwaves at an frequency of 2.45 GHz is concentrated into a narrow region using a thin metallic concentrator. A high electric field region gets developed around the concentrator tip, which ionizes the dielectric media around it to form plasma. The heat of the plasma ablates/melts the target material in the vicinity of the concentrator tip. However, limited control over the heat during the discharge results in thermal damage in the target material; especially, in the materials with poor thermal conductivity. Therefore, materials like glass experience thermal damage as well as frequent cracking. Consequently, a new approach to minimize defects like cracking, heat affected zone (HAZ), overcut, and taper during microwave drilling of borosilicate glass at 2.45 GHz was developed; performance of the process was evaluated in terms of tool wear rate and material removal rate. Simulation and experimental studies were carried out to understand the effect of process parameters such as input power, dielectric medium and dielectric flow rate on heat-affected zone, diametrical overcut, and thermal stresses developed in the workpiece during microwave drilling. Sub-millimeter holes were produced in the workpieces using a graphite tool in air and transformer oil in static (immersion depth = 45 mm) and dynamic conditions (flow rate: 15.7, 78.6, 141.4, and 204.3 cm3s-1). Results indicated that a decrease in input power enhances the HAZ while drilling in air and static dielectric; whereas, HAZ decreases (approximately, 44 % and 24 %) in dynamic dielectric than air and static dielectric, respectively, due to better heat dissipation and flushing of debris. Machining time was minimum while drilling with static dielectric; however, it increased with the increase in dielectric flow rate and a decrease in input power. On the other hand, overcut increased at higher input powers and lower dielectric flow rates due to enhanced ablation and heat accumulation in the machining zone. Higher thermal stresses generated in borosilicate glass while drilling in air and static dielectric; whereas, flowing dielectric produced lower thermal stresses. The study determined an optimum combination of flow rate (204.3 cm3s-1) and input power (70 W) for minimum HAZ, overcut, and thermal stresses during microwave drilling. A study on the distribution of input microwave energy during microwave drilling indicated that ~30 % of input power is utilized in removing material from the workpiece, while the remaining energy(~70%) gets reflected and absorbed by the dielectric and the concentrator. The mechanism of microwave drilling in the presence of a dielectric medium was explained.Results revealed that parameters like dielectric constant of dielectrics, electric conductivity of the concentrator materials, the concentrator shape affect the plasma shape and intensity, whereas immersion depth affects the confinement of plasma into a narrow zone. It was found that higher immersion depth reduced defects like crack, thermal damage due to low-temperature gradient on the workpiece surface, and better heat dissipation from the surface of the workpiece. Further, thermo-physical properties like viscosity and thermal diffusivity of dielectric were also found to have significant effect on HAZ around the hole. The concentrator with a conical tip performed better compared to the concentrator with a cylindrical tip due to the concentration of thermal energy at the tip. Further, it was observed that the thermal damage, overcut, and amount of material removed decreased with the increase in immersion depth and feed rate. An immersion depth of 45 mm and feed rate of 1.2 mm s-1 minimized the defect significantly. A machining gap (~0.3 mm) between the concentrator tip and the workpiece was essential as it facilitated flushing of the glass residue from the machining zone. Effective flushing also reduced thermal damage and roundness error. However, overcut and amount of material removed got increased with an increase in the machining gap due to increased plasma zone area and interaction time between the plasma and the workpiece. A preheating time of 10 s helped in reducing thermal crack and glass residue deposition around the hole. The best result was obtained with a graphite concentrator in the transformer oil dielectric at an immersion depth of 45 mm, machining gap of 0.3 mm, feed rate of 1.2 mm s-1 and preheating time of 10 s. The dielectric in dynamic mode was used to drill hole at relatively lower power (70-350 W). Correlations were drawn between the operating parameters, material removal rate (MRR), and the tool wear based on the computational as well as experimental results. A mathematical model for MRR in terms of power, dielectric flow velocity, and a decrease in concentrator length was developed. It was found that the tungsten carbide (WC) concentrator performed better than graphite and stainless steel concentrators. Further, the effect of wear of the WC concentrator tip on the dimensional accuracy of the holes was studied. An increase in input power showed a positive influence on the material removal rate while enhancing the tendency for thermal cracking. Higher dielectric flow velocity of 2.6 m s-1 helped in minimizing thermal cracks and removed the bulge around the hole. The wear of the concentrator tip also reduced the MRR.