3.1 Particle generation of crushed sanitary ware rejects
In general, abrasive grit used for the AWJM application was 80 mesh grit. The aim of the crushing process is to obtain higher yield of 80 grit particles from the crushing method. Hence, the experiment for crushing of electric insulator rejects were done with different jaw distance. After crushing process, the particles were generated with different grit sizes. The grit yield was calculated by standard weighing and sieve analysis method. The obtained results indicate that the larger jaw distance yields maximum amount of coarse grit particles. If the jaw distance is decreased, there is an increase in the grit yield of medium size grit particles. This is shown in Fig. 3 (a). In all the cases, the lowest jaw distance of 1mm produces the required grit size of 80 mesh particles. Smaller jaw distance increases the effective crushing load, creating more finer fraction. Likewise, a higher crushing load was preferred to get a higher yield of medium grit particles. Figure 3 (b) shows the number of pass required to obtain 80 grit particles during crushing of electric insulator rejects. The results show that as the number of crushing pass is increased, the number of particles produced in the 80 grit increases as well. Similar observation was made on ball mill crushing process of ceramic sanitary ware rejects by Cuhadaroglu and Kara [26].
3.2 Friability analysis of recovered electric insulator rejects
Friability analysis gives toughness and life of the abrasives of reprocessed (crushed electric insulator rejects) and standard garnet. Figure 4 depicts the friability results of standard garnet and EIR abrasives. The findings indicate that both abrasives have a similar crushing pattern. In case of EIR abrasive, the crushed down grains are mostly settled in the base pan. However, the garnet abrasive generates a greater number of fines as compared with the EIR abrasives. It means that EIR abrasive has less breakage than the garnet abrasive. Friability percentage of the standard garnet and electric insulator rejects were 29% and 33%. Since both particles have a close friability pattern, the newly generated EIR abrasive particles can be used as an alternative abrasive in AWJM applications.
3.3 Mechanical and geometric dimension of recycled abrasive particle
Table 2
Geometric parameters and mechanical property of abrasive particles.
Parameters | Unit | Garnet | EIR particle |
Density | g/cm3 | 4.03 | 2.64 |
Sphericity | Sp | 0.844 | 0.891 |
Shape factor | Fs | 0.734 | 0.712 |
Elongation ratio | rE | 1.49 | 1.57 |
Hardness | Mohs scale | 6.5–7.5 | 6 |
Table 2 presents the geometric parameter results of studied abrasive particles. The outcome demonstrates that, the elongation ratio of EIR abrasive particle is slightly higher than that of garnet. Likewise, the shape factor of the EIR abrasive particle is lower than the garnet abrasive. It is clear from both cases that the EIR particles are marginally sharper than the garnet abrasive, which is helpful for making indentation on work materials during AWJM process. The density and hardness of garnet were 4.03 g/cm3 and 6.5 mohs scale. Comparatively, the EIR abrasive particle results in lower density and hardness value of 2.64 g/cm3 and 6 Mohs scale. Sphericity of EIR particles was higher when compared with garnet abrasives. This indicates that EIR particles have a larger number of sharp edges as compared to garnet abrasive. Qu et al., [27] made a similar observation on the mineralogical properties of various abrasives such as garnet and silica sand on shale minerals.
3.4 Cutting feature analysis of standard garnet and EIR particles
Table 3
Performance results of EIR abrasive with respect to garnet abrasive.
Performance parameters | Garnet | EIR particle | Efficiency |
MRR (gm/min) | 1.89 | 1.78 | 0.94 |
Cutting width (mm) | 0.95 | 0.84 | 0.88 |
Cutting depth (mm) | 14.22 | 12.25 | 0.86 |
Cutting wear zone depth (mm) | 5.41 | 4.30 | 0.79 |
Kerf angle entry (degree) | 5.21 | 6.74 | 1.29 |
Kerf angle exit (degree) | 4.47 | 5.44 | 1.22 |
Cutting time (sec) | 76.4 | 74.9 | 0.98 |
Table 3 compares the cutting performance of EIR abrasive to that of garnet abrasive particle. Material Removal Rate (MRR) measures the amount of material extracted from the machined surface during the cutting process. The result shows that the higher volume of material (1.89 gm/min) was recorded for garnet abrasive as compared to EIR abrasive. The reason is that garnet abrasive has a higher hardness and a high mass to volume ratio (density), which contributes to a high kinetic energy of the water beam. In present case, the volume of material removal for the EIR abrasive is slightly lower, but it can be matched with the commercially available garnet abrasive. Cosansu and Cogun [20] previously published a report on reusing colemanite powder as a substitute abrasive for garnet abrasive in the AWJM process.
Cutting width is a measure of the actual size of the cutting performed by the water jet coming out of the nozzle. According to Fig. 5, the cutting width of the aluminium sample with EIR abrasive was narrower. When EIR abrasive was used, the top and bottom width of the cut down part were less than 12%. Aydin et al [10] reported a similar impact on cutting width reduction by using reclaimed granite abrasive as an alternate abrasive in the AWJM process. The experimental results indicate that the cutting width of an aluminium sample with EIR abrasive was close to the cutting width obtained with garnet abrasive.
Kerf geometry is an important parameter for determining the angle of the cutting of the cut down section. In the AWJM process, a cutting slot on work material is seen in two sections: top and bottom width. The top width is often more than the bottom width. According to the experimental findings, the kerf angle at entry and exit was greater for the EIR particle as an abrasive. This is because the EIR abrasive loses its cutting characteristic as well as water jet energy as the water jet penetrates in the thickness direction. When machining thicker samples, the standoff distance between the work and the nozzle increases. This increases the likelihood of obtaining a higher taper angle. As can be seen from the results, garnet abrasive was used to achieve the smaller kerf angle than the EIR abrasive. This is attributable to the garnet abrasive's higher hardness and heaviness. Several researchers made similar observations about the lower kerf angle on glass material with different abrasives such as silicon carbide, alumina and garnet [28][22].
The aim of measuring the cutting depth and time is to determine the maximum thickness cut and the time required for machining when deploying new abrasives. Cutting depth is a measurement of an abrasive's ability to penetrate deeper under constant cutting parameters. According to Table 3, the EIR abrasive has a maximum penetration depth of 12.25 in mm and the garnet abrasive has a maximum penetration depth of 14.22 in mm. Hardness and density are two properties that must be considered in order to achieve greater cutting depth. Axinte et al. [21] explained that the hardness of the abrasive determines the cutting wear zone depth and cutting depth when cutting work materials using an abrasive water jet machining method. In both cases, the EIR particle has a lower hardness and density, resulting in a lower cutting depth.
Cutting time for the EIR abrasive was estimated for a 100 mm length of 15 mm thick aluminium sample while keeping the cutting parameters constant. The results reveal that the EIR particle takes 2% longer time than the garnet abrasive. However, the current EIR abrasive will open up a new arena for alternate abrasives that satisfy all of the properties of the garnet abrasive. As a result, this current abrasive could be used to replace garnet AWJM processes.
3.5 Surface feature analysis of AWJM machined aluminium workpieces
Surfaces produced by abrasive machining are divided into two zones: cutting and deformation wear zone. The cutting wear zone has a smooth surface texture, and the deformation zone has craters and valleys. In this study, Fig. 6 shows the surface characteristics such as maximum peak to valley roughness (Rz) and average surface roughness (Ra) were determined along the thickness direction. The experimental results show that, the highest Ra and Rz value of 4.91µm and 26.77µm was observed in the deformation wear zone with the EIW abrasive while machining of aluminium sample. The increased values are due to the abrasive jet's lack of kinetic energy and the water jet's deflection in that deformation region, which results in uneven cutting of multiple sharp edges by EIR abrasive [29]. Alsoufi et al. [30] found a similar effect of increased surface roughness in the deformation zone on Carrara marble. Increased water pressure, on the other hand, increases the kinetic energy of the water beam, resulting in a smoother cutting operation. In all the cases, the surface roughness of the machined surface was fine at the entrance and became gradually rougher at the exit.
The material removal mechanisms for ductile aluminium material were abrasion and erosion in the cutting wear region, as well as ploughing and micro cutting in the deformation zone. SEM photographs were used to reflect the cutting function of the machined aluminium sample's cut part. Figure 7 (a, b) depicts a SEM image of the cutting wear region of an aluminium sample machined with EIR and garnet abrasives. Because of the ductile nature of the aluminium material, garnet and EIR abrasive grain were embedded in the cutting wear region, as seen in Fig. 7 (a, b). By performing elemental analysis on the respective region, the embodied particle is confirmed to be EIR and garnet abrasive. Many researchers observed the embodiment of garnet abrasive particles on machining surface when machining with ductile materials [31–33].
SEM representations of the cutting wear region of an aluminium sample machined with garnet and EIR abrasives are seen in Fig. 8 (a1, a2). In the cutting wear region, micro erosion and abrasion is clearly visible, resulting in the forming of a burr. Figure 8 (a2) shows that a comparatively larger proportion of burr was formed while machining with EIR abrasive, resulting in an increase in the surface roughness of the cutting wear region.
SEM images of the deformation zone of an aluminium sample machined with garnet and EIR abrasives are seen in Fig. 8 (b1, b2). Normally, the jet angle at the entrance is shallow and capable of easily penetrating without deflection. As the sample thickness increases, the jet becomes more diversified, resulting in deflection of the abrasive water jet and irregular machining in the form of craters and valleys in the deformation zone. When compared EIR abrasive to garnet, the EIR abrasive produces marginally higher cutting marks, resultant in a higher surface roughness in this region.
3.6 Recycling capability of EIR particle
Reusability of abrasive is calculated in terms of number cycle by recycling EIR abrasive particles repeatedly in AWJM process. The abrasive grains were filtered from the collector tank after each cycle, and then abrasives were sieved. The grits retained at 80 grit sieves were reused in the next machining experiment run. The response of recycling capabilities was measured in terms of total quantity of grains that could be reused in the next cycle. Figure 9 depicts the relative size of EIR grain size represented in optical images after each cycle of operation. It is known that the abrasive efficiency was decreased with each number of consecutive cycles due to continuous size reduction caused by abrasive particle breakage, as compared to fresh abrasive. The first cycle retention quantity for reusing is estimated to be 76 %, the second cycle to be 35 % reused, and the third cycle to be less than 10% of the abrasives reused. This clearly shows that the recycling potential of EIR particles was good before the second cycle; after that, the abrasive's recycling potential is low. Similarly, Babu and Chetty [23] measured the retention quantity for reusing and the number of recycles for garnet abrasive in AWJM application and found that retention quantity was 31 % in the 4th cycle. This finding leads to a comparison of the performance of EIR recyclability to garnet abrasive, which was found to be less than an order of magnitude.
3.7 Sustainable approach on economic aspects of using EIR particle as alternative abrasive
The use of waste, such as electrical insulator rejects, as one of the primary raw materials for the AWJM application conserves resources. The performance of the EIR abrasive is competitive, and in some cases, it matches the performance of regularly used garnet abrasives. More crushing plants are now commercially available on the market, and they are capable of crushing ceramic waste from its original shape to the appropriate grit size. This will result in cleaner production and improved waste disposal. In terms of cost and environmental considerations, sustainable use of this kind of waste contributes wealth. The cost of electrical insulator waste per kg was approximately ₹27.47 including crushing and sieving cost, while the cost of garnet per kg was approximately ₹52.45. This clearly reveals that garnet consumes almost double the amount of expense as compared to EIR abrasive. It is concluded that the substitution of EIR in the AWJM process would undoubtedly increase the wealth of the abrasive machining industry while also providing greater environmental benefits.