INVESTIGATIONS ON MICROWAVE DRILLING OF CHARACTERISTICALLY DIFFERENT MATERIALS

Abstract

The art and science of hole making is being perfected over the time and has undergone vast changes over the centuries. Today, the need is slowly drifting towards generation of micro holes. Unfortunately, the industry does not have the luxury of processes for this. Laser drilling, although found highly competitive in terms of precision and quality, has limitation of its own in terms of applicability and operational complexity. Microwaves on the other hand, are being increasingly tried in the field of material processing. Results of microwave processing of engineering materials in the area of sintering, polymeric processing, ceramics joining and glass joining have been very encouraging. Joining of metallic materials using microwave energy has given a different dimension to this activity. The microwave-drill invention was reported in 2001, and its applications for various materials have been presented in the literature since then by a few authors. This machining is limited to mostly drilling of ceramics (basalt, mullite, masonry brick, concrete, thermal barrier coatings) and waste animal bones; the results were encouraging but needed further investigation in details. The present study is concerned with microwave drilling of two characteristically different materials - one polymer based low melting point material (PMMA) and the other high strength brittle material. Drilling of these two materials were attempted in a customised low power settings (90 W - 900 W) applicator at 2.45 GHz frequency. Workpieces of depth of 1.2 mm, 2 mm and 5 mm were placed inside the customised applicator at an optimal position determined through simulation of the process. The simulation was carried out using a commercially available soft computing tool (COMSOL version 4.4.0.195). Experimental trials were carried out using nichrome, copper and stainless steel drill bit (concentrator) with diameters ranging from 250 micron to 800 micron. An input power setting was varied from 90 W to 900 W with variable exposure time. Holes on 2 mm and 5 mm thick PMMA and 1.2 mm thick glass could be drilled with a maximum exposure time of 4 s. The stainless steel concentrators were provided better results. A major problem observed associated with drilling was cracking of the workpieces. This is attributed to thermal stresses developed during microwave drilling processes. Surface treatment approach in terms of application of surface precursors on the target drilling area was also attempted, which yielded positive results. Seven different precursors were studied; glycerine and perspex showed best results among the candidates in terms of reduction in chances of cracking. This was attributed to attenuation of v 2015 Abstract vi heat energy at the point of drilling by the precursors, which resulted in reduction in induced thermal stresses. The phenomenon of drilling through material ablation as practiced in the current approach was simulated using the similar input condition as used experimental trials. The use of metallic concentrator (the drill bit) inside the applicator plays an important role in the drilling process. The concentrator being pointed at the drilling end attracts microwaves incident in the applicator. The skin depth of the drill bit at the tip overlaps, which causes a concentration of microwave energy at the tip. This results in breakdown of the dielectric (air) around the tip and a plasma sphere is produced. The heat due to the plasma sphere is sufficient to create localized microwave heating (LMH) and causes the material to melt/ablate in the vicinity of the plasma sphere. On continued feeding of the tool (under gravity or mechanical feeding) and continued creation of plasma, the material gets removed in the proximity of the LMH causing a hole to form. The microwave drilled holes were characterised through different available techniques in terms of HOC (hole overcut), hole accuracy, circularity, taper, aspect ratio and assessment of the heat affected zone (HAZ). Optical microscope, stereoscopic microscope and field emission scanning electron microscope (FESEM) were used to characterise the drilled holes and monitor the tool conditions. A low microwave power setting (180 W) and less exposure time combination was observed to be favourable for obtaining better holes in terms of overcut, accuracy and taperness. Steel drill bit at 180 W also generates minimum residue and minimum HAZ, however, frequent cracking in glass was observed at this power settings level. A customised photoelasticity setup was developed to identify the HAZ. Photoelasticity methodology was used to predict the induced thermal stresses on the target material. Microhardness tester and Energy dispersive spectroscopy (EDS) spectra were used to estimate the surface quality and change in composition of materials before and after microwave drilling. The work contributes to the knowledge in the application of feasibility of metallic concentrator-based microwave drilling process for materials like PMMA and soda lime glass. The processing conditions could be optimised for minimal cracking of the workpiece and minimal defects including tool sticking under the given constraints. The data generated shall be useful for use in the industry as well as future research. The work also generates enough scopes for further investigations in the area including use of different concentrator material, exploring further sets of parametric settings for defect minimizations and adopting the technology for processing of more materials.