17-10-2016, 11:36 AM
1459358456-AnInvestigationofAbrasiveWaterJetMachiningonGraphite.docx (Size: 230.64 KB / Downloads: 6)
Abstract
In the present research work, the effect of abrasive water jet (AWJ) machining parameters such as jet operating pressure, feed rate, standoff distance (SOD), and concentration of abrasive on kerf width produced on graphite filled glass fiber reinforced epoxy composite is investigated. Experiments were conducted based on Taguchi’s L27 orthogonal arrays and the process parameters were optimized to obtain small kerf. The main as well as interaction effects of the process parameters were analyzed using the analysis of variance (ANOVA) and regression models were developed to predict kerf width. The results show that the operating pressure, the SOD, and the feed rate are found to be significantly affecting the top kerf width and their contribution to kerf width is 24.72%, 12.38%, and 52.16%, respectively. Further, morphological study is made using scanning electron microscope (SEM) on the samples that were machined at optimized process parameters. It was observed that AWJ machined surfaces were free from delamination at optimized process parameters.
1. Introduction
Fiber reinforced polymer composite is used in product manufacturing due to its distinct advantages such as lower weight, higher strength and stiffness, ability to mold into complex shapes, better corrosion resistance, and damping properties. In recent days, nanofillers such as graphite particles are impregnated with glass fiber reinforced polymer (GFRP) to enhance specific properties. Shivamurthy et al. [1], found that mechanical properties of glass/epoxy composite, namely, Young’s modulus, tensile strength, flexural strength, impact strength, and wear resistance, show improvement with addition of graphite flakes. Such composites are highly suitable for manufacturing of bearing liners, gears, seals, cams, wheels, brakes, rollers, clutches, bushings, and so forth. Studies on electrical properties of graphite filled composites by Goyal and Kadam [2] and Bhattacharya et al. [3] revealed that these composites are also suitable to shield electromagnetic interference in electronic devices.
In AWJ machining, the kerf profile produced depends on jet energy, jet exposure time on the workpiece, jet orientation, and material properties. Axinte et al. [26] developed a geometrical model to predict jet footprint (kerf) in maskless controlled milling applications. AWJ milling experiments were conducted on silicon carbide ceramic material at 90° jet impingement angle at various jet feed rates to validate the model. Kong et al.. These models were found to be powerful tools to develop advanced jet path strategies on complex geometries using CAD/CAM by considering various process parameters including jet exposure time and orientation. From the literature review, it is noticed that generally addition of graphite improves mechanical, tribological, and electrical properties of GFRP composite material due to which the composite finds a wide range of industrial applications. AWJ machining is the suitable method for processing of such composite materials. The literature review establishes the need to investigate the effect of operating parameters on the machinability characteristics of graphite laced GFRP. Hence, this paper attempts to explore the effect of process parameters, namely, jet operating pressure, SOD, feed rate, and abrasive concentration on kerf width, while machining of graphite filled GFRP composite and their optimization using Taguchi method. In addition, morphological study is carried out using SEM on the cut surface machined at optimum and few selected process settings.
2. Materials and Methods
2.1. Materials Used
Properties of polymer composites are enhanced by impregnating the additives. In the present research, graphite of 200 m size in the particulate form was used as filler in glass fiber reinforced epoxy laminate. Bisphenol-A based epoxy resin was used as matrix; and bidirectional glass fabric of 200 g•m−2 was used as reinforcement to prepare the composite. Addition of graphite to the glass epoxy laminate improves wear resistance with low coefficient of friction as well as toughness. Since graphite also acts as solid lubricant, the machinability of the laminate improves. Machining is carried out by using garnet abrasive of the size 80 mesh.
2.2. Fabrication of Composite Specimen
Graphite filled GFRP laminates were prepared using hand lay-up process as shown in Figure 1. The material compositions for composite were graphite 3% by weight, glass fibers 50% by weight, and epoxy 47% by weight. Bidirectional glass fabric was cut into square shape of 250 mm × 250 mm. Fabric was impregnated with the graphite filled epoxy and layered one above the other. The layered green laminate was transferred to hot compression molding press and compressed with uniform pressure. Curing was done for 2 hours in compression mode at 140°C. Postcuring was done at 180°C for 8 hours in free hanging condition in the furnace. The final thickness of laminate was found to be 3 ± 0.1 mm.
Design of experiment (DOE) is a powerful statistical tool to optimize the machining process parameters economically. Many researchers have used DOE to optimize the AWJ process parameters for machining various materials [23, 33]. In the present work, experiments were designed using Taguchi’s fractional factorial orthogonal array. The AWJ machining parameters and levels of the experiments were chosen based on the literature review followed by trial experiments. Table 1 shows the machining parameters that were chosen to study the performance of AWJ machining of graphite filled GFRP composite.
The main effect of operating pressure (), feed rate (), abrasive concentration (), and SOD () as well as interaction effect of , , and on the response parameter (kerf width) is considered for designing the experiment. The degree of freedom for the present experimental condition is 21 (which means the minimum number of experiments that must be conducted to estimate the effect is 21). The nearest suitable fractional orthogonal array for four factors of each three levels is L27 (43). This design is used for experiments conducted. The maximum material removal from the workpiece occurs at jet impact angle of 90° [18] and the garnet is the best suitable abrasive for AWJ machining polymer composite [13]. In the present investigation, irregular and angular shaped garnet abrasive particles were uniformly dispersed with water in the mixing chamber and jet was expelled through the nozzle at 90° for machining. Combinations of machining parameters used in the present study are shown in Table 1.
Table 1: Machining parameters.
Machining parameter/levels 1 2 3
Operating pressure (MPa) 90 120 150
Feed rate (mm/min) 75 100 125
Abrasive concentration (wt.%) 6 10 14
Standoff distance (mm) 1 3 5
2.5. Experimental Setup
A three-axis CNC AWJ machine (supplied by Dardi Water Cutter Co. Ltd., make: DWJ1525-FA, maximum operating pressure: 400 MPa, repeatability accuracy: ±0.025 mm) was used for experimentation. The experimental setup used for machining is shown in Figure 2. The garnet abrasive water jet was directed at 90° on to the surface of graphite glass epoxy composite through a carbide nozzle of 0.8 mm focus tube diameter. A length of 20 mm cut was made on workpiece in each trial.
3. Results and Discussion
3.1. Mechanical Properties of Target Material
The tensile properties of graphite filled GFRP laminates are determined using universal testing machine (model: Instron 3366). The tensile tests are carried out on three test specimens (size as per ASTM D-638 of standard Type 5) and their average results are tabulated in Table 2. The surface hardness of the composite is determined using Vickers hardness test as per the standard loading procedure. A load of 300 grams is applied on the test specimen using 136° diamond indenter for a duration of 20 seconds and the Vickers hardness number is measured using computerized microhardness tester (make: Matsuzawa-MMTX7A).
3.2. Characterization of Garnet Abrasive Particles
The morphology, particle size analysis, and chemical composition of the garnet abrasive were investigated using SEM (Zeiss EVO 18 special edition) before mixing into water jet. The garnet abrasives particles were coated with thin layer of conducting material (gold) by sputtering method to prevent charging of a specimen with an electron beam in SEM. Figure 3 shows the SEM image of garnet abrasive. It is observed that the abrasives have irregular sharp edges. The particle size of the garnet abrasive was investigated as per ASTM D 422. It was found that the range of particle size was 80–240 m and average particle size was around 174 m. The mineral composition of garnet abrasive as per elemental mapping method by weight percent is shown in Table 3.
3.5. Optimization and Regression Modeling of AWJM Process Parameters
Process parameters were optimized to produce smallest kerf width on the workpiece. Using the experimental results shown in Table 4, the mean value of the top kerf width is calculated at each level of process parameters as shown in Table 6. It is seen that smallest kerf width is obtained at an operating pressure of 90 MPa, feed rate of 125 mm/min, SOD of 1 mm, and 6% abrasive concentration. At these optimum settings of process parameters, the predicted top kerf width is 0.756 ± 0.068 mm and bottom kerf width is 0.731 ± 0.068 mm.
Morphology
Morphological study has been carried out using SEM on the specimens that were machined at the optimum process settings which generate small kerf width. It is seen from Figure 7(a) that the top and bottom edges of the kerf are free from delamination. It is also observed from Figure 7(b) that there are inconsistent and nonuniform pits at few locations on top kerf due to collision between abrasive particles which richochiates from the edges. Further, SEM analysis is carried out to understand the microlevel of destruction in the composite. Figures 7© and 7(d) show the morphology of cut surface when fibers are oriented at 90° and are parallel to jet traverse, respectivly. It is seen that the fibers are cut accurately and no abrasive embeddment is found accoss the machined surface.
4. Conclusions
The following conclusions have been drawn from the present research work.(i)The kerf width increased with an increase in operating pressure (contribution: TKW, 24.72% and BKW, 19.68%) and standoff distance (contribution: TKW, 52.16% and BKW, 44.92%), but it decreased with increase in feed rate (contribution: TKW, 12.38% and BKW, 15.31%). Abrasive concentration has marginal effect on the kerf width.(ii)The interaction effects of the process parameters are found to be insignificant. Hence, process parameters can be independently varied to get required kerf width.(iii)Process parameters are optimized to get small kerf width and regression models are developed to predict the top and bottom kerf width with values of 96.86% and 91.52% for TKW and BKW, respectively.(iv)The morphological study revealed that there is no delamination of the surface machined at optimum process settings chosen to obtain small kerf as compared to that of the surface machined at nonoptimum process parameters.