3.1 Kerf top width and Bottom results
Figure 4 shows microscopic observations of AISI 304L with a thickness of 4, 8 and 12 mm at traverse speeds of 90,120, and 150 mm/min, kerf geometries such as kerf top width, and kerf bottom width have been measured. Each trial combination was repeated in 2 runs and calculated for averaged results. In referring to the images in Figure 4, it can be seen that the aspects of the cut have irregular shapes, whereas material thickness increases cut quality has deteriorated at the bottom cut. It can be seen within the figure that increasing traverse speed generates a wider kerf top width than kerf bottom width.
Kerf geometric inaccuracies imparted to machined samples are more prominent with higher material thickness. The initial collision of the abrasive particle towards the workpiece generates forces that are greater than the crushing load, causing particles to get fractured and reduced during the cutting process. Denser abrasive particles move towards the target material and decrease forces, causing a narrowing of the kerf at the bottom part . Regardless of whether cut geometry was in arcs or a straight profile, a lowering of the kerf at the exit cut dimension and irregularities of shape were observed.
Figure 5 shows the correlation of varied traverse speed and material thickness of 4, 8, and 12mm of AISI304L in comparison with contour cutting consisting of (a) external arcs, (b) internal arcs, and (c) straight line.
The results of AWJM cutting of curvature and straight line profiles demonstrate the same trend of a wider kerf top width compared to the kerf bottom width. AWJM cutting operation takes place through an erosion process where abrasives are suspended in a high velocity of water jet stream, resulting in an increasing acceleration of the abrasive particles, where their kinetic energy impingement and the collisions of these abrasive particles transpire the material removal towards the target material . The kinetic energy of the abrasive particles is radically high at first impact, where it moderately decreases during the machining application process . The experimental data presented in Figure 5 demonstrates that the narrowing of top and bottom kerf widths depends directly on the decreasing amount of abrasive particles during the machining process. A similar explanation was study by Jeykrishnan et al. , where a larger kerf top width than bottom width formed by AWJM in an Inconel 625 alloy. In this study, a lower rate of traverse speed at 90 mm/min amounted to lower variation in kerf widths as compared to a higher rate of 120-150mm/min. A lower gap in the kerf width geometry indicates better performance in AWJM cutting operations. The explanation for this is that a low traverse speed rate carries a vast number of abrasive particles that can impinge the target workpiece ; whereas a faster or higher traverse speed reduces the number of abrasive particles that execute cutting operations or machining motions .
3.2 Kerf Taper Angle results and analysis
Figure 6 shows the Kerf taper angles obtained in AWJM profile cutting of AISI304L, where the experiment ranged from 0.825° to 1.550° for 4 mm, 1.092° to 1.575° for 8 mm, and 1.235° to 1.660° for 12 mm material thickness with traverse speed levels of 90,120, and 150 mm/min.
Figure 6 shows that gradual machining with a low level of traverse speed of 90 mm/min has achieved the smallest kerf taper angle value of 0.825º for 4 mm, 1.092° for 8 mm and 1.235° for 12 mm material thickness. For materials such as stainless steel, a disparity in taper cut is due to deformation-induced from ductile material during machining operations . The formation of kerf taper inherent in AWJM cutting application is due to the changing conditions at the interface. Kerf tapering has been observed at the entrance and exit of the jet, initiated by the low energy abrasive particles suspended at the exterior of the coherent jet . Initially, these abrasive particles have high kinetic energy and gradually decrease along with the cutting operation; thus, as material thickness increases, the kinetic energy continuously reduces, causing a higher tapering angle . It has been noted in the findings of Wang et al.  that kerf taper correlates with traverse speed and the thickness of a material. In this research, the values of KTA were visibly higher at 8 and 12 mm thickness than 4 mm AISI 304L, as shown in Figure 6.
The results indicated that kerf geometry inaccuracies within the machined AISI 304L were recognised at a higher or increasing traverse speed. With the feature of abrasive particles, a lower traverse speed increased the influence of cohesion on metal material to create kerf taper angles.
3.3 Material removal rate results and analysis
In the review of the obtained data, Figure 7 presents a graphical analysis of the behaviour of material removal rate towards different traverse speed and material thickness in AWJM profile cutting of AISI304L.
A similar trend of increasing the level of input parameters results in increasing values for output parameters has been observed for both curvature (i.e. arcs and straight line profiles) and different thicknesses of materials. In this study, the lowest value of KTA of 0.825º for arcs profile and 0.916º for straight profile were achieved at the lowest level of traverse speed at 90 mm/min rate. The maximum value of MRR of 769.50 mmᵌ/min was obtained from machining of curvature profile and 751.50 mmᵌ/min achieved when cutting straight line profiles at a higher value of traverse speed at 150 mm/min rate. The process of material removal for AWJM in ductile material, such as steel, takes place through erosion caused by impinging abrasive particles from the waterjet stream. Hence, higher kinetic energy generates a higher erosion rates and leads to higher material removal rate. With a higher level of traverse speed, the machining rate is increased, resulting in more material being removed from the workpiece. In turn, the material removal rate is noted to be mainly influenced by traverse speed, similar findings with previous studies .
In this experiment, the amount of material removed increased by approximately 60 - 80% as the value of material thickness increased from 4 mm to 12 mm. It has been observed that a higher material thickness obtained a higher value of MRR 346.50 mmᵌ/min for 4 mm, 612.00 mmᵌ/min for 8 mm, and 769.50 mmᵌ/min for 12 mm material thickness of AISI304L material. The study also showed that traverse speed is an essential factor in obtaining a higher material removal rate, demonstrating a direct proportional trend to MRR.
3.4 Statistical analysis
Analysis of variance for kerf taper angle and Material removal rate
Analysis of variance (ANOVA) was performed to validate the kerf angle and material removal rate from the varied thickness of twelve AISI 304L profile and is given in Figure 8.
The ANOVA results in Figure 8 denote that the percentage contribution of material thickness on the kerf taper angle ranges from 69-91% with 5-18% for traverse speed. The kerf tapering results show the proportion of kerf top width to kerf bottom width. The variation between the top and bottom geometries denotes a higher kerf tapering. Kerf top or entry width is relatively higher than the exit width because the kinetic energy of abrasive particles is primarily at a high level and consistently decreases during the machining process . An increase in material thickness denotes prolonged cutting operations, which continuously decreases the kinetic energy of abrasive particles, producing a higher taper angle.
Figure 8 also shows the material removal rate obtained under variable conditions. The percentage contribution of material thickness on material removal rate ranges from 62-69%, with 27-36% for traverse speed. According to this statistical analysis, material thickness directly influences the measured output parameter in this case. In AWJM cutting, machining is fundamentally executed by the cohering action produced through impact by a number of abrasive particles towards a workpiece . As a result, material removal rate and thickness are directly proportional, where it is possible to achieve higher MRR even when machining samples with increasing thickness.