Characterization and optimization of abrasive water jet machining parameters of aluminium/silicon carbide composites

Due to the complexity of high temperature and cutting tool wear, most machined components are still facing problems in terms of harder functional fillers that reinforce aluminium matrix composites. Conversely, abrasive water jet machining (AWJM) incredibly useful for the cutting of anisotropic and non-homogeneous metal matrix composites. In this research article, silicon carbide (SiC) particulates were utilized as reinforcement in the AA6026 matrix material (AA6026/SiC) and machined using AWJM under different process parameters namely SiC loading, traverse speed and stand-off distance. Two different compositions of SiC (4, and 8 wt%) were considered to fabricate AA6026 composites using the stir casting. In addition, outputs have been examined, e.g., surface roughness, material removal rate, and kerf angle. An optical microscope, scanning electron microscope, Brinell hardness tester and universal testing machine have been used to characterize the matrix material AA6026 and its composites. Microstructural analysis revealed that the inclusion of SiC particulates in AA6026 affects the very fine grain size of the composite. Furthermore, the 8 wt% composite exhibits the evolution of the Al-Si eutectic phase during solidification. Processing of these composites was performed using the L27 orthogonal geometry, successfully improving the parameters of the abrasive water jet process. The output response shows that reducing the SiC load improves the surface roughness under the key parameters of traverse speed and stand-off distance. However, increasing the SiC loading increases the material removal rate and kerf angle under the key parameters, namely traverse speed, and stand-off distance.


Introduction
Aluminium based alloys, mainly alloys of 6262 and 6026 generally contain minor additions of the elements such as Mn, Cr, Cu.Zn and Fe, in addition to the major elements of Mg, and Si.These minor elements serve the purpose of enhanced machinability when operated at high cutting speeds.As a result, the aluminium (AA) based 6xxx series alloys are of high interest in the engineering fields such as rails, and electricals due to their excellent strength to weight ratio.Other advantages of AA6026 alloys are their machinability, weldability and superior corrosion resistance [1,2].Exploring AA6026-based alloys with ceramic particles is becoming more popular due to these benefits.Moreover, AA-based composites possess superior specific properties in terms of hardness and strength, toughness, lower density, outstanding wear resistance and lower coefficient of thermal expansion (CTE).Further, adding nonmetallic compounds results in enhanced high temperature properties and provides challenging opportunities for designing and developing new products [3][4][5].However, AA is reinforced anisotropically and heterogeneously with ceramic particles, making these composites difficult to machine [6].It causes adverse effects on the workpiece and the cutting tool in terms of wear.Conventional machining processes such as drilling, milling, and turning are largely useful for machining/cutting composites based on equipment availability and experience.
AA-based composites usually contain ceramic reinforcements such as oxides, carbides or borides and are limited to 30 vol. % when applied to structural and load-bearing applications.Although some materials used as reinforcement in composites, such as titanium dioxide, aluminium, and SiC are very hard, conventional machining is well-thought-out for composites because their reinforcement is brittle, and separation of the material is difficult.Machining is achieved by brittle breaking rather than plastic deformation in front of the cutting tool.In addition, the cutting tool material must be appropriately selected to minimize wear due to hard abrasive components as reinforcement in the mixture representing the bar stock.Machining of AA-based composites depends on the inherent properties and relative proportions by volume fraction of the reinforcement and base material and its response to the machining process.In addition, the type of machining process differs on the following factors: (i) the type of machining operation, (ii) the shape and dimensions of the part, (iii) the accuracy and finish requirements improvement, (iv) part numbers, (v) part variety, (vi) availability of suitable machines and cutting tools, (vii) availability of in-house technology, (viii) machining practices current, (ix) production schedule, (x) capital and rationality requirements for new equipment, (xi) environmental and safety considerations, (xii) overall costs and so on.
Many researchers are interested in AA-based composites in automotive, sporting goods, electronic packaging, aerospace, armour, defence, and construction [7,8].It reveals superior mechanical properties, especially at high temperatures which cannot be attained in alloys.In addition, the induction of ceramic-like reinforcement in the AA matrix results in higher strength and stiffness, making machining difficult [9].Owing to its improved mechanical properties, it requires unique machining processes, especially non-traditional machining processes for example electrical discharge machining (EDM), laser beam machining (LBM), electro chemical machining (ECM) process and so on.Electrical discharge machining (EDM) is a non-traditional machining method that involves removing material from a component by a series of repetitive electrical discharges between tools, known as electrodes, and the part being machined while in the presence of a dielectric fluid.Whereas Laser beam machining (LBM) is a nontraditional machining method that basically refers to the process of material removal achieved via interactions between the laser and the target materials.Laser drilling, cutting, grooving, writing, scribing, ablation, welding, cladding, milling, and other operations are examples.Electrochemical machining (ECM) is a metal removal-based manufacturing technique that relies on electrochemical dissolution to remove metal.It is one of the most modern manufacturing procedures for making items with complicated forms from low-machinability metals and alloys.Furthermore, EDM major shortfall is its surface integrity mainly caused by thermal cracking.It is decent to use ECM but then create an oxide layer on the surface due to corrosion which affects the accuracy [10].In the case of LBM process, an efficient contact less machining process but cracks developed in the cutting surface during the machining process limits its wider usage in cutting the hard materials [11].Similarly, the high-energy electron-beam machining (EBM) process is also thermally involved and more expensive [12].Precision machining for AAbased composites was found to be abrasive water jet machining (AWJM) due to reduced amount of thermal strain and negligible residual stress on the machined surface.In addition, AWJM is proficient and cheap machining process which requires less shear force to cut the hard surfaces [13][14][15].
AA reinforced with silicon carbide (AA/SiC) composites exhibit good mechanical properties and superior wear resistance than common metals [5].Hence the underlying mechanism of how the machining behaviour of AA-based composites is affected by various parameters needs to be evaluated.Generally, the test parameters are established with profound knowledge.There are several statistical models that use statistical regression techniques to conduct experiments [16].Design efficient fractional factorials and greatly reduce time.The fractional plane may not contain the best plane point.The Taguchi method is one of the many statistical tools that proved to be an efficient method in various fields, majorly from engineering, agriculture and biotechnology [17].Taguchi's method uses a fractional factorial design with two, three, or more levels [18].Taguchi uses an efficient way of approach to optimize in terms of enhancing productivity.This method has resulted in a limited number of applications in majority industries worldwide [19,20].Therefore, the main purpose of this study is to inquiry the machining of unreinforced aluminium alloy (AA6026/0%SiC), and AA6026 reinforced with 4 wt.% and 8 wt.% of SiC particulates abbreviated AA6026/4%SiC and AA6026/8%SiC composites manufactured by two-step stir casting.Based on Taguchi's design of experiments, the test conditions adopted are traverse speed and stand-off distance.Furthermore, an analysis of variance was conducted to determine the most critical factors influencing the machining parameters.

Experimental details 2.1. Materials
As reinforcement, the matrix material AA6026 is considered with 20-30 μm sized silicon carbide (SiC) particulates.Table 1 summarizes the compositional features of the three sets of AA/SiC composites created by altering the SiC loading by weight percentage.

Abrasive water jet machining setup
Abrasive water jet machining (AWJM) is a process in which a high-speed water jet nozzle ejects abrasive particles after impact to degrade work piece material.Experiments were conducted using AWJM, and an image of AWJM is shown in figure 1. Abrasive particles are introduced to the liquid and pressurized to 400 MPa, driving them out at 900 m s −1 through a diamond/sapphire nozzle to improve the water jet.The aperture diameter of the cutting head is 0.25 mm, and the nozzle diameter is 0.76 mm.The impact angle is 90 degrees.Minimum and maximum abrasive mass flow rates range from 200 g min −1 to 700 g min −1 respectively.The distance between the jet nozzle and the workpiece surface was kept constant at 2 mm.The abrasive particles were garnet, and the particle grain size was 80 mesh (177 μm).Table 2 summarizes the chemical composition and physical characteristics of garnet abrasives.

Fabrication of composites
The stir casting technique is a low-cost, straightforward method for producing particle reinforced metal matrix composites.The two-step casting process is a more complex version of the stir casting method.The matrix material is usually heated to above its melting temperature and then cooled down to a temperature where it maintains in a semi-solid state, and reinforcement material that has been preheated is now added and mixed with the help of a mechanical stirrer, and the slurry is heated to molten temperature and completely mixed.
In this study, matrix material AA6026 and SiC reinforcement were employed in the production of AA6026/ SiC composites to achieve a more homogeneous microstructure than the routine stir casting procedure, the bottom pouring type of stir casting machine is used and illustrated in figure 2 and the schematic of stir casting setup is depicted in figure 3.  The matrix material AA6026 with SiC particulates were produced by a two-step stir casting method as illustrated in figure 2. AA6026 alloy, small pieces of equivalent weight were then fed into the graphite crucible of a coil induction furnace and heated to a temperature of 750 °C.A calculated amount of flux has been added to minimize the oxidation of molten AA6026.To minimize voids, porosity and blistering in the fabricated sample, degassing pellets are placed in molten AA6026.The melt was then permitted to cool.The molten AA6026 temperature was brought down to 600 °C to reach a semi-solid state.The SiC particulates are preheated to 50 °C   to improve wettability and eliminate the oxide layer in the SiC particulates before adding the SiC particulates.The compounded slurry was again heated and kept at a temperature of 750 °C and after adding SiC particulates, the stirring process was continued under optimized parameters.Later the molten metal was drenched into the mild steel mold.The cast billets were cut by wire EDM at 6 mm × 75 mm × 100 mm for abrasive water jet machining.For comparison, the matrix material AA6026 was cast using similar processing conditions.In addition, all samples were cut along the pouring direction to allow for compression, hardness, and metallographic testing.Optical emission spectrometer was performed to find out the chemical composition of AA6026 based composites, which are listed in table 3. Figure 4 shows the typical photographs of the samples prepared for microstructural studies, hardness, and compression strength properties.

Microstructure examination
Microstructural analyses were performed on both AA6026 matrix material and SiC reinforced AA6026 composite samples.The specimen was prepared in three steps: ground, polished, and etched, and then the identical samples were examined under an optical microscope.The specimens were mounted and physically polished on a succession of silicon carbide (SiC) abrasive sheets with increasing grit sizes, as well as with diamond paste and a special polishing cloth.For roughly 30-45 seconds, specimens were chemically etched with Keller's Reagent (190 ml distilled water, 5 ml Nitric acid, 3 ml Hydrochloric acid, and 2 ml Hydrofluoric acid).The samples were then cleaned with running water and alcohol before being fully dried.The samples were examined optically using optical microscope equipment interfaced to a computer.

Mechanical testing
Hardness and compression tests were performed to evaluate the mechanical properties of AA6026 and its composites.

Hardness test
The ASTM E384 standard was used to measure the hardness of AA6026 and SiC reinforced AA6026 composite samples [21][22][23][24][25].The coupons were polished using coarse emery sheets and the final polishing completed using very fine alumina particulates.The hardness of the AA6026/SiC filled AA6026 composites samples were measured using a Vickers hardness tester at 300 g for 30 s.

Compression test
The strain rate is a key factor that can significantly influence the results of a compression test.They affect both the elastic and plastic behavior of the material being tested and can dramatically change the material's response to the applied load.High strain rates can increase the apparent strength of many materials.This is because the internal structures of the material (like dislocations in metals) do not have enough time to rearrange themselves and accommodate the deformation.Conversely, at very low strain rates, the material has more time to rearrange its internal structures to accommodate the strain, which can lead to a lower apparent strength.Furthermore, true stress reduces as actual strain lowers, and the strain hardening component is dependent on the stress and strain.Based on compression testing theory, the compression strength of AA6026 and AA6026/SiC composite samples was evaluated in an INSTRON-3369 with a capacity of 50 kN and a crosshead speed of 0.5 mm min −1 (low strain rate) in the current work.The compression test on the composite was carried out in accordance with ASTM E9-9 guidelines.In each series, three samples were prepared for compression testing, and the mean results were reported.

Surface roughness, material removal rate and kerf angle measurements
As shown in figure 5, the surface roughness (Ra) was measured using a TESARUGOSURF 90 G, which is appropriate for high-precision measurements.The response value was measured at three positions on the machined surface (top, middle, and bottom), and the average response value was used for discussion.
The value of material removal rate (MRR) has been obtained as the quantity of workpiece material removed during unit machining time and can be calculated using equation (1).
where V = velocity of abrasive jet at the point of impact, H = flow strength/hardness of the work material and M g = mass flow rate of abrasive particles.
The kerf was determined using an optical microscope with 100X magnification at three points each alongside the span of the top and bottom cut surfaces, respectively called top kerf width (T) and bottom groove width (B kw ) as illustrated in figure 6.The kerf width has been calculated by taking the average of T kw and B kw .The kerf angle (θ) or the kerf taper is an important quality characteristic of machined parts.
Generally, a tapered slot of cut is generated during through cut operation of the work material where the K w at top is more than that at bottom.The tapered angle is the measure of angle of deviation between Tkw and Bkw, as shown in figure 6, and can be determined using equation (2).
where t is thickness of the work piece.

Taguchi design
The combination of factors and parameters was carefully chosen in the AWJM, so its purpose is to increase the quality of machining.Machining performance has one/more output variables (material removal rate, kerf geometry, and surface quality).Response variables in this research are surface finish, material removal rate, and kerf angle.Each input parameter is evaluated at three levels.Input parameters are SiC loading, traverse speed and stand-off distance.The nominated process parameters were examined on three levels and are summarized in table 4.

Microstructural and phase analysis of AA6026 alloy with varying SiC particulates
Figures 7(a)-(c) depict the material structure of as-cast samples of AA6026 alloy and composite with 4 wt% and 8 wt% SiC particulates, respectively.The uniform distribution of SiC particulates in the AA6026 matrix is not visible in the micrographs.In contrast to the 4 wt% SiC reinforced AA6026 composite, figure 7(c) shows a slight agglomeration of SiC particulates in the 8 wt % reinforced AA6026 composite.Measurements made using the linear intercept method showed that the average grain size of the AA6026 and 4 wt % and 8 wt % SiC/AA6026 composites were 122, 43, and 27 μm, respectively.This shows that the SiC particulates refined the grain structure and served as a nucleation site during the solidification.Figure 8 shows the phase analysis of AA6026 with various SiC particulates.Interestingly, the XRD analysis shows how the intermetallic compound Al 4 C 3 changed when SiC particulates were incorporated during solidification.This stage is typically one that is hard and brittle.
To further validate the influence of SiC particulates on microstructure of AA6026 alloy, SEM analysis was accomplished on the cast coupons.Figure 9 explains the matrix microstructure of AA6026 alloy, and the EDS spectrum measured on AA6026 (point-1) is presented with its respective elemental composition.However,  adding 4 wt% SiC particulates to the AA6026 alloy, the microstructure attests to the spreading of SiC particulates.
The microstructure further reveals that the SiC particulates were mostly segregated in the grain boundaries, as depicted in figure 10.It could be the cause for the grain refinement in the 4 wt% added AA6026 composite as compared to the matrix material.Further, the attendance of SiC particulates is anticipated to strengthen the composite by arresting the dislocations during the plastic deformation.It is noteworthy that as depicted in figure 11, the 8 wt% SiC reinforced AA6026 composites has undergone a sizable morphological change.More intriguingly, the microstructure further reveals that the evolution of Al-Si eutectic phases during solidification is influenced by the concentration of SiC particulates.
The Al-Si eutectic phase is confirmed using the SEM-EDS analysis which is also depicted in figure 11 for AA6026 with 8 wt% SiC particles.The brittle Al-Si eutectic phase formation is triggered by the following reasons.Initially, inclusion of SiC particulates into the matrix material produces the following reaction [26]:  ( ) Further, at high temperatures, it was evidenced that the solid SiC might tend to segregate as Si+C.This reaction leaves Si to react with the molten aluminium and results in the eutectic Al-Si phase.The reason for a more eutectic Al-Si phase is that the tendency of silicon reacts with aluminium is more as compared to the carbon reacts with aluminium.Moreover, carbon solubility in aluminium is limited, which leads to more silicon settling with aluminium.Further, the left out C reacts with aluminium and forms another brittle phase namely, Al 4 C 3 [26].This phase is also confirmed in the phase investigation (figure 8).The microstructure also reveals that certain unreacted SiC particulates can be observed in both the grain boundaries and inside the grains.

Impact of SiC particulates on mechanical properties of AA6026
The cast AA6026/SiC composite samples underwent the Brinell's hardness test as well as the compression test.Table 5 contains a list of the samples' average hardness values.As per the findings in table 5, the samples with a higher SiC loading, AA6026/8%SiC, have a higher hardness value.The improved hardness of the samples of AA6026/8%SiC can be ascribed in large part to the strong interfacial bond between the AA6026 and SiC particulates and grain refinement.It is predicted that the evolution of the Al-Si eutectic phase will improve the alloy's hardness property as the AA6026/8%SiC composite solidifies.However, the outcomes of the compression tests revealed that the AA6026 composite, which contains different SiC particulates, has poor compression strength.The compression test results show the opposite trend from the hardness data obtained.The composites are decreasing as the wt% of SiC particulates increases.According to table 5, compression test results, the matrix material has a cast compression strength of 422 MPa.However, compared to the matrix material AA6026, the compression strength of 4 wt% and 8 wt% is only 271 and 259 MPa, respectively.The reasons listed below are primarily responsible.First, SiC dissolves into intermetallic compounds Al-Si and Al 4 C 3 , which are formed.These two phases are brittle in nature [27].Second, the transfer of load from AA6026 to the SiC particulates is not appropriate because these brittle phases are vulnerable to heavy loading.Consequently, during the uniaxial compression testing, the composites suffered a brittle failure.

Machining of AA6026/SiC composites
The experimental findings and an estimation of S/N ratio for the output variables namely surface roughness, material removal rate, and kerf angle are summarized in table 6.

Effect of Process parameters and levels on surface roughness
In figure 12, it is possible to understand how the process parameters P1-SiC loading (wt%), P2-traverse speed (mm/min), and P3-stand-off distance (mm) affected the surface roughness (Ra).Here, the smaller is better quality characteristics are determined by S/N ratios.Consequently, the parameter settings with levels as F1S3D2 provide the surface roughness's minimum value.
In general, the flushing pressure and particle velocity used during the machining process have an impact on the Ra of the brittle materials machined through AWJM [28].The distribution of abrasive particles in the AWJM is an important factor that decides roughness of the surface of the machined composite.If the abrasive particles distribution is uneven, Ra of the machined components will typically tend to increase [29].With percentage contributions of only 24.37 and 2.06, respectively, the addition of SiC particulates (F) and traverse speed (S) are found to be the least important factors affecting the surface roughness.F-test is described as a type of hypothesis test, that is based on Snedecor f-distribution, under the null hypothesis.The test is performed when it is not known whether the two populations have the same variance.F-test can also be used to check if the data conforms to a regression model, which is acquired through least square analysis.When there is multiple linear regression analysis, it examines the overall validity of the model or  determines whether any of the independent variables is having a linear relationship with the dependent variable.
A number of predictions can be made through, the comparison of the two datasets.In the present work, a statistical analysis is conducted to identify the contribution of each influencing factor to Ra. Different statistical parameters, including degrees of freedom (DF), the sequential sum of squares (SS), percentage contribution (%), the adjusted sum of squares (Adj.SS), the adjusted mean squares (Adj.SS), and Fisher test (F-Test) for each factor are calculated as part of the analysis.The level of confidence used in this study was 95%.Table 7 makes clear that, with a percentage contribution of 50.76, the stand-off distance (D) is the factor most responsible for influencing the Ra.Previous study indicates that the close stand off distance has significant effect on the surface roughness.This study indicates that the Ra is impacted by the increase in stand-off distance to 1.5 mm.Further, the stand-off distance directly affects the extent of the machining operation and the jet kinetic energy.As the stream pressure increases, it tends to produce smooth surface finish.However, with the amount of abrasive particles increases during the machining  process, it results in the reduction of kinetic energy and produces poor surface finish.It is mainly due to the collision of abrasive particles generally results in affecting the flow of the stream.As a result, the amount of abrasive particles involved must be controlled in order to have to the better surface finish [30].

Effect of process parameters and levels on MRR
Figure 13 depicts the impact of process parameters P1-SiC loading (wt%), P2-traverse speed (mm/min), and P3-stand-off distance (mm) on the material removal rate (MRR).In this case, the higher the S/N ratio, the higher the quality features.Table 8 clearly shows that the traversal speed (S) is the extremely important component.In general, the workpiece with high hardness requires harder abrasive particle to facilitate the easy removal of the materials.Hence increase in the abrasive particle hardness normally increases the material removal rate and the increase in depth of cut, eventually the high machining efficiency [31].Figure 13 clearly shows that increase in the traverse speed increases MRR.At room temperature, raise in traversal speed affects the intermolecular forces, which tend to detach more material from the workpiece.Furthermore, as seen in figure 13, higher loading of SiC particulates lowered the MRR.It is mostly initiated by the emergence of brittle phases like eutectic Al-Si and Al 4 C 3 .Because of their great hardness, as listed in table 5, these intermetallic compounds block the machining process.Furthermore, the stand-off distance has no effect on the process variables.The curve represents steady behaviour over various distances.It is primarily because increasing the stand-off distance tends to disrupt the water jet, which has highest impact on the material removal.As a result, the addition of SiC particulates (F) and the stand-off distance (D) were shown the least important factors impacting the MRR.

Impact of process parameters and levels on material kerf angle
Normally, the kerf angle and surface roughness in the abrasive water jet machining can be influenced by the factors such as transverse speed and the pressure [30].The statistical study performed in this study indicated in   9 shows that the SiC loading (F) is the utmost significant parameter, followed by the traversal speed (S), and the stand-off distance (D) is the least significant component influencing the kerf angle (Ka).The SiC particle plays a crucial role in the kerf angle.It is mainly due to the brittle nature of the composite material [32].Due to the higher hardness, the kerf angle tends to have significantly influenced by the SiC particles.As a result, the SiC particles found to be most influential parameter.As expected, the standoff distance has less significant effect on the kerf angle.Since the variation in the standoff distance affects or reduces the velocity which in turn affects the material removal rate, and the kerf angle.Figure 14 depicts the impact of process factors such as P1-SiC loading (wt%), P2-traverse speed (mm/min), and P3-stand-off distance (mm) on the Kerf angle (Ka).The S/N ratios for the smaller and better-quality attributes are determined here.As a result, the parameter settings with levels like F3S1D2 give the smallest value of Ka.

Conclusions
• The matrix material AA6026 with 4 wt% and 8 wt% SiC particulates were created by two-step stir casting.
• Microstructural development demonstrated a homogeneous combination of the reinforced particulates, resulting in grain refinement of the AA6026 composites (43 μm in 4 wt% and 27 μm in 8 wt%) in comparison to the base metal (122 μm).
• In the Brinell hardness test, the hardness of the composites improved significantly.For 4 wt% and 8 wt% SiC particulates, the matrix material AA6026 has hardness of 41 raises to 54 and 71, respectively.The emergence of Al-Si eutectic and Al 4 C 3 brittle phases had a substantial impact on the composite's compression strength.The matrix material AA6026 has a maximum compression strength of 422 MPa, whereas the SiC-containing composites have compression strength of 271 and 259 MPa, respectively.
• The machining operation was conducted using the Taguchi orthogonal array (L 27 ).For various process parameters and found that the stand-off distance (D) is the most important element impacting Ra, the traversal speed (S) is the most important factor for MRR, and the addition of SiC particulates is the most important parameter for the kerf angle.
Influence process parameters on Ka.

Figure 1 .
Figure 1.Equipment setup: (a) Photograph of AWJM, (b) Higher magnified image of the work piece mounted on the table.

Figure 3 .
Figure 3. Schematic of a stir cast to fabricate Al based composites.

Figure 12 .
Figure 12.Influence process parameters on R a.

Table 2 .
Chemical and physical properties of the physical properties of abrasives.

Table 4 .
Process control parameters and levels.

Table 6 .
Test results and S/N ratio of AA6026/SiC composites.

Table 7 .
Analysis of variance for SN ratios of Ra.

Table 8 .
Analysis of variance for SN ratios of MRR.

Table 9 .
Analysis of variance for SN ratios of K a .