Machinability Characteristics in Drilling of Glass Microballoon /Epoxy Syntactic Foam

Abstract

Polymer composites are steadily substituting the conventional materials in aerospace, marine, automobile and many other engineering applications owing to their unique properties such as lightweight feature combined with high specific strength and superior corrosion resistance. Weight reduction of composite materials is of great interest in aerospace, marine and automobile applications to meet the stringent guidelines of fuel consumption and emissions in the coming years. Structural weight reduction without compromising the desired properties can be achieved by using a unique class of composite called syntactic foams, wherein the matrix is filled with hollow particles called microballoons. Even though the composites are produced to near-net shape, drilling is unavoidable during final stage of production process for the assembly of various structural components using fasteners. Many problems arise during drilling of composites due to non-homogeneous and anisotropic nature of the material. Nearly 60% of the composite parts are rejected during aircraft assembly due to drilling induced damages. The focus of the present study is to achive good quality holes in drilling of glass microballoon/epoxy syntactic foams by selecting appropriate process parameters. In the present investigation, epoxy resin (LAPOX L-12) is used as the matrix resin and borosilicate glass microballoon (GMB) is used as hollow filler without any surface treatment. Syntactic foams are fabricated by dispersing 20, 40 and 60 vol.% GMBs in epoxy matrix using manual stirring method. Nine different types of syntactic foams specimens with 20, 40 and 60 vol.% of GMBs are fabricated using three different densities (varying wall thickness) of GMBs (SID-200Z: 200 kg/m3, SID-270Z: 270 kg/m3 and SID-350Z: 350 kg/m3). All the prepared samples are coded as per EYYY-R convention. Epoxy resin is denoted by ‘E’ and ‘YYY’ represents density of GMBs. Neat epoxy specimens are also fabricated under similar processing conditions for comparison. Extensive micrography of fabricated foams confirms the uniform distribution of GMBs in the epoxy matrix without forming the clusters. Experimental density of all the fabricated syntactic foams is lower than neat epoxy resin. Density of foams decreases with decreasing GMB wall thickness and increasing volume fraction of GMBs. Density reduction in the range of 18-53% is noted as compared to neat epoxy indicating significant weight saving potential of the proposed syntactic foams.Experiments are conducted using vertical computer numerical control machine and TiAlN coated tungsten carbide twist drills of varying diameter based on full factorial design (FFD). Cutting speed (v), feed (f), GMB content (R), GMB wall thickness (w) and drill diameter (D) are taken as input parameters, while thrust force, surface roughness, specific cutting coefficient, cylindricity, exit side circularity error and exit side damage factor are considered as responses for evaluating the quality of drilled hole. Three levels for each input process parameters (v: 25, 75 and 125 m/min; f: 0.04, 0.08 and 0.12 mm/rev; R: 20, 40 and 60 vol.%; w: 0.716, 0.925 and 1.080 µm; D: 8, 12 and 16 mm) are selected to consider the nonlinear effects among the parameters. Experiments are repeated for three times and the average values are used for analysis. Mathematical models based on response surface methodology (RSM) are developed using Minitab 14 software for analyzing the influence of the input parameters on the measured responses. Adequacy of the developed mathematical models is confirmed using analysis of variance. Higher R-squared values indicate that the developed mathematical models can be effectively used as a tool in industrial practices to predict the machinability characteristics of GMB reinforced epoxy foams during drilling. Individual and interaction effect of process parameters on the responses are analyzed using RSM based mathematical models. Individual effects are studied by varying one parameter at a time in the mathematical models while keeping all the remaining process parameters at the intermediate levels. Two parameters are varied at the same time while keeping the other parameters at the intermediate level in the mathematical models to study the interaction effect of process parameters on the chosen responses. Thrust force is found to be increasing with increasing feed and drill diameter, while it decreases with increasing GMB content. Thrust force of all the foams is found to be lower as compared to neat epoxy resin. Thrust force is observed to be decreased by 40-55% as compared to neat epoxy due to the incorporation of GMBs. Drill diameter, feed and GMB content have a significant effect on the thrust force while the effect of cutting speed is found to be insignificant. v125f0.04R60D8 is the optimum condition for minimizing thrust force of E200 and E270 foams while performing machining at v25f0.04R60D8 minimizes the thrust force of E350 syntactic foam. Extensive microscopy is conducted on the drilledspecimens to understand crack initiation and propagation mechanisms. Surface roughness of the drilled hole is measured using Mitutoyo surftest with a cut-off length of 0.8 mm. As compared to neat epoxy, the surface roughness of syntactic foams increases by 14-20 times. However, surface roughness in foams decreases with increasing GMB volume fraction. Surface roughness is strongly governed by drill diameter and cutting speed. Minimum surface roughness for E200 and E270 foams is obtained at v25f0.12R60D16, while v25f0.12R60D12 is found to be optimum for E350 foam. Specific cutting coefficient increases with increasing drill diameter and decreasing feed. Increasing GMB content significantly decreases specific cutting coefficient by 40-55% as compared to neat epoxy specimens. v25f0.12R60D8 is the optimum condition for E350 foam, while machining at v125f0.12R60D8 is found to be beneficial for E200 and E270 foams for minimizing specific cutting coefficient. Coordinate measuring machine is used to measure the cylindricity, exit side circularity and maximum diameter of drilled hole for damage estimations. Cylindricity of the foams increases with increasing the cutting speed, feed and drill diameter. Increasing GMB content decreases the cylindricity by 46-69% as compared to neat epoxy. Drill diameter, feed and GMB content have a significant effect on cylindricity of drilled holes. v25f0.04R60D8 is noted to be the optimum conditions for E200 and E270 foams while v75f0.04R60D8 parametric setting is most suitable for thick-walled (E350) foams to minimize cylindricity. Circularity error increases with increasing cutting speed and drill diameter, while it decreases with increasing feed and GMB content. Increasing the microballoon volume fraction decreases the circularity error of foams by 18-67% as compared to neat epoxy. Circularity error of the holes is highly influenced by drill diameter followed by GMB volume fraction and wall thickness. v25f0.12R60D8 is the optimum condition for minimizing the circularity error of all the type of foams. The damage factor is dependent on the thrust force developed during drilling process. Drill diameter, feed and GMB content have a significant effect on damage factor of the drilled holes. Optimum conditions for minimizing damage factor is observed to same as that of thrust force. A reduction in the damage factor by 26-42% is noted in foams with increasing GMB content as compared to neat epoxy. Optimum conditions based on response surfacemethodology for minimizing all the responses are not same and the trade-off among various process parameters necessitates multi-response optimization. In the present work, grey relation analysis (GRA) is used for finding a specific combination of process parameters for minimizing all the response at the same time to obtain a good quality hole in drilling GMB/Epoxy syntactic foams. According to GRA, v125f0.08R60D8 is the optimal condition for producing a quality hole in E200 foams, whereas v25f0.12R60D8 is found to be optimal for E270 and E350 syntactic foams. Higher GMB content is preferred in the foams from drilling operations perspective, which is also beneficial for weight sensitive applications. Influence of GMB wall thickness on the responses is studied by keeping the GMB content at 60 vol.%, as higher filler content significantly improves the hole quality. Response surface plots for varying wall thickness of GMBs are plotted using the developed mathematical models to study the interaction effects among input process parameters. Increasing microballoon wall thickness from w0.716 to w1.080 increases thrust force, specific cutting coefficient and damage factor by 40%. Surface roughness, cylindricity and circularity error of drilled holes are significantly affected by GMB wall thickness and is found to be decreased by 30, 41 and 56% respectively. Combination of higher particle wall thickness and feed with lower cutting speed and drill diameter (v25f0.12w1.080D8) is the optimum condition for producing a sound hole quality as observed from GRA. Hole quality is highly influenced by drill diameter followed by the interaction between cutting speed and GMB wall thickness. Finally, microscopy is conducted to analyze the shape and size of chips produced during drilling. Cutting tools are inspected using a confocal microscope post drilling operation and micrographs show negligible tool wear due to the superior wear resistance of TiAlN coating. Observations and parameters settings explored in this work offers guidelines for the industrial practitioners to produce quality holes in drilling of GMB reinforced epoxy composites.