Aluminium metal matrix composites (AMMCs) are finding application in aerospace, automobile and civil industries. The machining capability for fabricating components in AMMCs is challenging due to their intricate metal structure, an example of which is the presence of reinforcement, that reduces the cutting efficiency of the tool. The wirecut electrical discharge machining (WEDM) is used in the manufacturing industries to fabricate intricate shapes and components in difficult-to-machine materials. WEDM is a well-known unconventional machining system for manufacturing complicated shapes in difficult-to-cut materials. WEDM is mainly used in the tool room for developing tool and dies. The advantages of WEDM are non-contact type, good geometrical accuracy and efficiency. In recent years, the emergence of new sustainable materials that can offer a good range of performance has upheld the research interest in the development and application of metal matrix composites (MMCs). An overview of the literature is presented in Table 1.
A brief overview of the literature
1 | Pulse on time, pulse off time, wire feed rate, current, voltage, thermal conductivity, co-efficient of thermal expansion, density and wire tension [1] | Al 2124 SiCp MMCs | In the WEDM of AlSiCp MMC, DA and ANN-based predicted models for surface roughness and MRR were developed. | With an increase in pulse duration and thermal conductivity, the rate of material removal and surface roughness both increase significantly. |
2 | Pulse on time, pulse off time and wire feed [2] | HEA-reinforced aluminium metal–metal composite | The best parameter combination for a better surface finish, a faster MRR and a smaller KW is found using the Taguchi method and an L18 OA. | The pulse ON time has a significant influence on surface roughness (76.70%), KW (41.96%) and MRR (35.37%), and increasing the pulse ON time enhances the response variables. |
3 | Pulse on time, pulse off time, wire feed, wire tension, current and voltage [3] | AA6061-TiB2 | Studied the effect of reinforcement and wire material on the surface roughness in MMCs. |
The results of the experiments show that the percentage of particle reinforcement was the most important factor in surface quality (62.04%) and machinability (34.2%). The machinability and surface quality of the TiB2 (5 wt.%) reinforced composite are excellent. Zinc-coated brass wire outperforms plain brass wire. |
4 | Pulse on time, pulse off time, wire feed and wire tension [4] | SiCp/Al composite | Prepared casted, coated, annealed and plastic processed wire for WEDM of MMCs. | The use of zinc coating on the wire resulted in increased MRR by 16.67%, reduced surface roughness by 21.18% and reduced wire breakage by 16.67% under the same discharge parameters when compared to brass wire electrode. |
5 | Short pulse time, wire feed rate, pulse width, spark gap, servo control mean reference voltage and time between pulses [5] | Al/ZrO2 (p)-metal matrix composite | Surface veracity aspects such as surface defects and recast layer thickness are investigated. | The result finding shows lower value of pulse on/off time, and frequency of pulse plays an important role in surface veracity. |
6 | Gap voltage, wire feed, pulse on time and pulse off time [6] | Al-Si12/B4C/fly ash | In WEDM of Al-Si12/B4C/fly ash composites, the effects of control parameters on MRR and surface roughness were examined using the Taguchi and ANOVA methods. | MRR increases as the pulse on time and reinforcement increase. Optimal machining conditions resulted in a maximum MRR of 38.01 mm3/min and a minimum surface roughness of 3.24 m. |
7 | Voltage, peak current, wire tension and dielectric pressure [7] | AMMCs with 6% and 8% weight fraction of Al2O3 | AMMC with weight fraction of Al2O3 is machined through WEDM | Based on the TOPSIS approach, the optimal MR and Ra process parameters were ascertained as 1.5 mm/min and 3.648 m, respectively. According to ANOVA, the peak current has a significant influence on MR and Ra. |
8 | Pulse on time and pulse off time, gap voltage, peak current and wire feed [8] | Aluminium-based composite materials (AA 7075) with (Al2O3) particles | The effect of wirecut EDM process parameters on MRR and surface roughness of Ni-P-coated and un-coated alumina-reinforced composite materials was investigated. | By combining grey relation analysis with principal component analysis, an ideal set of process parameters was observed. |
9 | Pulse on time and pulse off time, gap voltage, reinforcement and wire feed [9] | LM5/ZrO2 AMMCs | By using the Taguchi technique, the study sought to determine the optimal wire-EDM machining parameters for achieving maximum MRR, minimum SR and minimum kerf width KW. | The main statistical factors influencing MRR are the gap voltage (29.92%) and pulse on time (64.84%). |
10 | Pulse on time and pulse off time, gap voltage, percentage of reinforcement and wire feed [10] | Aluminium (LM25) rein-forced with fly ash and boron carbide (B4C) hybrid composites | WEDM experiments were planned and carried out using the Taguchi methodology's L27 OA approach, and the corresponding MRR and surface roughness were measured. | The grasshopper optimisation algorithm performed better than the others in terms of maximising volume removal rate and minimising surface roughness values, according to the results. |
11 | Doping percentage, reinforcement percentage, pulse on time and pulse off time, and wire feed [11] | Magnesium MMC | Investigation in WEDM has been carried out to oversee the effect of process variables on the machining performance parameters such as MRR and Ra of magnesium composite. | The results of the experiment show that increasing the duration of pulse ON and wire feed rate in WEDM increases the MRR. Surface roughness increases noticeably as pulse ON increases. |
12 | Cutting speed, feed and depth of cut [12] | Aluminium (AA6061) and alumina powder sized <1 mm with 99.9% purity | The study investigated the effects of varying alumina amounts ranging from 1 wt.% to 5 wt.% added to recycled aluminium chip using hot press forging. Ultimate tensile strength and elongation to failure were the primary responses studied. | The addition of 2 wt.% alumina to the recycled aluminium alloy produced high-quality and consistent results. |
13 | Current, pulse on time, wire speed, voltage and pulse off time [13] | SiCp reinforced Al6061 composite | The effect of parameters such as current, pulse on time, wire speed, voltage and pulse off time on wire-EDM machining of 4–8 wt.% SiCp/Al6061 alloy was investigated. | MRR was significantly influenced by current, pulse on time, pulse off time, wire speed and voltage. The MRR increased as the current, pulse on time, wire speed and voltage increased, but it decreased as the pulse off time and wire speed exceeded 700 rpm. |
14 | Stirring temperature, stirring speed, stirring time, preheat temperature of reinforced particles, preheat temperature of permanent die and squeeze pressure [14] | AlSi7Mg + alumina; scrap aluminium alloy + alumina; AlSi7Mg + SAC; scrap aluminium alloy + SAC | In the present study, stir-squeeze casting was successfully used to create AMCs using a novel method. The viability of using SAC from oil refineries as reinforcement material and SAAWs as the matrix material was examined. | According to the micrograph analysis, the scrap aluminium alloy alumina composite had the most uniform distribution of reinforcements and the lowest porosity among the four composites. |
15 | Current, pulse on time, wire feed rate, pulse off time, ultimate tensile strength and micro hardness [15] | AZ61 magnesium alloy with boron carbide and silicon carbide as an reinforcement with varying percentage levels | The fabricated magnesium MMC is machined through WEDM for MRR and surface roughness. | The highest MRR of 0.212 mm3/s was obtained at pulse on time of 115 μs and pulse off time of 50 μs, and the minimum values of surface roughness were obtained as 1.003942 μm. |
16 | Alumina weight percentage, amplitude percentage and pulse time [16] | SAAWs | Using a L9 OA and the Taguchi method, an experimental study was carried out. Multi-objective optimisation based on ratio analysis technique was used for optimisation. | The findings showed that compared to other composites, SAAWs reinforced with 1 weight percent of nanosized alumina particles and 5.5 weight percent of micro sized alumina particles had lower porosity and metal loss (wear), higher hardness, tensile strength, and compressive strength. |
17 | Cutting speed, surface topography, surface roughness, recast layer formation, residual stresses and microstructural and metallurgical alterations [17] | Inconel 706 | To determine the feasibility of machining these components, research was carried out on Inconel 706 superalloy using the WEDM process. | Despite the fact that zinc-coated wire improves productivity, hard brass wire was noticed to be advantageous in terms of improved surface quality of machined parts. |
18 | Pulse off time, pulse on time, gap voltage and peak current [18] | [Difficult-to-cut materials] | The study concentrated on the impacts of various optimisation techniques, such as single and multi-objective techniques, on difficult-to-cut materials. | Reviewed the recent and early research articles on the WEDM process to cut hard conductive materials along with single response and multi response optimisation. |
19 | Pulse off time, pulse on time, gap voltage and peak current [19] | A286 superalloy | Optimised the WEDM performances by particle swarm optimisation. | The best MRR and surface roughness, respectively, were 19.90 mm2/min and 3.49 m. |
20 | Pulse off time, pulse on time, gap voltage and peak current [20] | Hard-to-cut materials | Six algorithms, namely MOALO, NSMFO, MODA, MOGWO, MOGOA and NSWOA, are used in the Pareto optimisation of a WEDM process. | The results reveal that MOGWO, MOGOA and MODA can identify the optimum solutions in 47%, 28% and 20% of the situations, respectively. |
AMMCs, aluminium metal matrix composites; ANN, artificial neural network; ANOVA, analysis of variance; DA, dimensional analysis; KW, kerf width; MMC, metal matrix composite; MOALO, multi-objective ant lion optimisation; MODA, multi-objective dragonfly algorithm; MOGOA, multi-objective grasshopper optimisation algorithm; MOGWO, multi-objective grey wolf optimiser; MRR, material removal rate; NSMFO, non-dominated sorting moth flame optimisation; NSWOA, non-dominated sorting whale optimisation algorithm; OA, orthogonal array; SAAWs, scrap aluminium alloy wheels; SAC, spent alumina catalyst
In this research, the machinability of scrap aluminium alloy-based composites is machined using WEDM, and this study paves way for reusability and sustainable product development. In general, the alloy wheel already consists of silicon and magnesium, and addition of Al2O3 greatly improves the mechanical properties of the material. AMMCs is finding applications in various industries such as aerospace, marine and automobile, primarily in the body structure. The machining of AMMCs becomes an essential area of research due to the above applications. In this research, the performance of zinc-coated wire on AMMC was analysed through experimental performance by varying one parameter at a time and characterisation using scanning electron microscopy (SEM) and energy-dispersive x-ray spectrometry (EDS). Moreover, the L18 orthogonal array (OA) experimental design is considered to find the optimal combination of factors using the CRiteria Importance Through Intercriteria Correlation (CRITIC) and simple additive weighting (SAW) methods, and the significant factor affecting the WEDM process is ascertained using the ANOVA method.
Electronic Computer Numerical Control (CNC) WEDM is used for making slots on the fabricated MMCs. A zinc-coated brass wire electrode of Φ 0.25 mm is used for the cutting. The zinc coating improves the flushability and instant cooling ability of the wire [15]. The workpiece is formed from the scrap aluminium alloy wheels (SAAWs) of vehicles as a matrix and 5% alumina as reinforcement.
The AMMCs are fabricated through stir-squeeze casting technique. The SAAWs of necessary size are melted in an electric furnace on a graphite crucible and heated to a temperature of 900°C. To enhance the wettability among the matrix and the reinforcements, magnesium of 1wt.% is added to the melt. A two-blade stirrer is used for 5 min to mix the molten metal to ensure uniform mixing of reinforcement in the Al matrix. The molten mixture is poured into a permanent mould of dimensions 50 mm × 50 mm × 250 mm and cooled and solidified at room temperature. The workpiece consists of recycled aluminium alloy reinforced with silicon and magnesium, and the chemical composition of the workpiece is confirmed using energy-dispersive X-ray spectroscopy (EDS) analysis, as shown in Figure 1 and in Table 2. Deionised water is used as the dielectric medium. The material is removed by a sequence of discrete discharges between the wire electrode and the workpiece in the presence of dielectric fluid, which creates a path for each discharge as the fluid becomes ionised in the gap. The area where discharge takes place is heated to a tremendously high temperature, so that the surface is melted and removed [16,17,18]. The removed particles are flushed away by the flowing dielectric fluids.
EDX analysis of AMMC. AMMC, aluminium metal matrix composite; EDX, energy-dispersive X-ray
Chemical compositions of composite materials
C K | 4.49 | 9.08 | 21.36 |
O K | 16.53 | 25.11 | 9.43 |
Na K | 1.86 | 1.96 | 11.36 |
Mg K | 5.01 | 5.01 | 6.53 |
Al K | 31.72 | 28.58 | 4.71 |
Si K | 29.47 | 25.5 | 6.15 |
S K | 0.07 | 0.05 | 29.63 |
Cr K | 1.44 | 0.67 | 12.35 |
Fe K | 6.53 | 2.84 | 5.11 |
Ni K | 2.9 | 1.2 | 10.1 |
The experimental variables, namely voltage (V
Control variables and their levels
Voltage | V | V | 30 | 40 | 50 | 60 | 70 |
Wire feed rate | F |
mm/min | 3 | 4 | 5 | 6 | 7 |
Current | I |
A | 10 | 15 | 20 | 25 | 30 |
Pulse on time | ON |
μs | 100 | 105 | 110 | 115 | 120 |
Pulse off time | OFF |
μs | 50 | 55 | 60 | 65 | 70 |
Performance measure of wire breakage
1 | 30 | 7 | 30 | 120 | 70 | 304 |
2 | 40 | 7 | 30 | 120 | 70 | 60 |
3 | 50 | 7 | 30 | 120 | 70 | 32 |
4 | 60 | 7 | 30 | 120 | 70 | 27 |
5 | 70 | 7 | 30 | 120 | 70 | 22 |
6 | 70 | 3 | 30 | 120 | 70 | - |
7 | 70 | 4 | 30 | 120 | 70 | 28 |
8 | 70 | 5 | 30 | 120 | 70 | 21 |
9 | 70 | 6 | 30 | 120 | 70 | 19 |
10 | 70 | 7 | 30 | 120 | 70 | 15 |
11 | 70 | 7 | 10 | 120 | 70 | 727 |
12 | 70 | 7 | 15 | 120 | 70 | 32 |
13 | 70 | 7 | 20 | 120 | 70 | 26 |
14 | 70 | 7 | 25 | 120 | 70 | 23 |
15 | 70 | 7 | 30 | 120 | 70 | 21 |
16 | 70 | 7 | 30 | 100 | 70 | 1114 |
17 | 70 | 7 | 30 | 105 | 70 | 847 |
18 | 70 | 7 | 30 | 110 | 70 | 30 |
19 | 70 | 7 | 30 | 115 | 70 | 25 |
20 | 70 | 7 | 30 | 120 | 70 | 19 |
21 | 70 | 7 | 30 | 120 | 50 | 120 |
22 | 70 | 7 | 30 | 120 | 55 | 90 |
23 | 70 | 7 | 30 | 120 | 60 | 8 |
24 | 70 | 7 | 30 | 120 | 65 | 7 |
25 | 70 | 7 | 30 | 120 | 70 | 6 |
Overview of the experimental work and methodology. ANOVA, analysis of variance; CRITIC, CRiteria Importance Through Intercriteria Correlation; MR, machining rate; SAW, simple additive weighting
Table 5 presents the levels and parameters used for the performance optimisation of WEDM process. Machining rate (MR) and surface roughness (R
Parameters and their levels chosen for optimisation
Voltage | V | V | 30 | 50 | 70 |
Wire feed rate | F |
mm/min | 3 | 5 | 7 |
Current | I |
A | 10 | 20 | 30 |
Pulse on time | ON |
μs | 100 | 110 | 120 |
Pulse off time | OFF |
μs | 50 | 60 | 70 |
L18 OA
1 | 30 | 3 | 10 | 100 | 50 | 1.02 | 3.600 |
2 | 30 | 5 | 20 | 110 | 60 | 1.02 | 3.795 |
3 | 30 | 7 | 30 | 120 | 70 | 1.52 | 3.748 |
4 | 50 | 3 | 10 | 110 | 60 | 0.9 | 3.218 |
5 | 50 | 5 | 20 | 120 | 70 | 1.25 | 3.789 |
6 | 50 | 7 | 30 | 100 | 50 | 1.24 | 3.780 |
7 | 70 | 3 | 20 | 100 | 70 | 1.04 | 3.392 |
8 | 70 | 5 | 30 | 110 | 50 | 0.85 | 3.392 |
9 | 70 | 7 | 10 | 120 | 60 | 1.06 | 3.722 |
10 | 30 | 3 | 30 | 120 | 60 | 0.85 | 3.570 |
11 | 30 | 5 | 10 | 100 | 70 | 1.28 | 3.575 |
12 | 30 | 7 | 20 | 110 | 50 | 1.35 | 3.405 |
13 | 50 | 3 | 20 | 120 | 50 | 0.82 | 3.532 |
14 | 50 | 5 | 30 | 100 | 60 | 0.92 | 3.420 |
15 | 50 | 7 | 10 | 110 | 70 | 1.23 | 3.228 |
16 | 70 | 3 | 30 | 110 | 70 | 0.76 | 3.729 |
17 | 70 | 5 | 10 | 120 | 50 | 0.88 | 3.686 |
18 | 70 | 7 | 20 | 100 | 60 | 1.06 | 3.370 |
OA, orthogonal array
The CRITIC weighting method deals with the interdependence between the criteria. The CRITIC method is more appropriate for weighing up the weights of both conventional and modern performance measures, and it comprises all the information in the assessment criteria. Moreover, the SAW method is used to compute the index score.
The weights of the criteria play an essential role in deciding the actual degree of a criterion's control. In describing the output performance of WEDM, the indicator with the maximum weight is considered as the most significant indicator, and against this conceptual background, this research used the CRITIC weighting method to establish the weights of the MR and surface roughness by using the following steps:
Finally, the following expression in Eq. (7) below represents the objective weight for indicator
To optimise the MR and surface roughness, this paper proposes to use the SAW method, as given in Eq. (8):
The greatest value
Figure 3 shows the effect of voltage on time for the first incidence of wire breakage while machining the slot in MMC. It is evident from the graph that at the parameter level of 30 V, 7 mm/min, 30 A, 120 μs (ON
Effect of voltage on the first incidence of wire breakage at 7 mm/min, 30 A, 120 μs (ON
Figures 5A and 5B show the SEM picture of wire before and after machining, and it is evident that no significant changes are observable, except for a small quantity of erosion and re-solidified surfaces. The condition of the wire confirms efficient machining for the parametric combination of 30 V, 7 mm/min, 30 A, 120 μs (ON
The primary parameter that significantly influences the WEDM process is the wire feed rate. The effect of wire feed rate on the machinability of a slot in AMMC is shown in Figure 6. As can be clearly observed from the graph, we have confirmation that usage of the discussed experimental combination of input parameters—i.e., 30 V, 3 mm/min, 30 A, 120 μs (ON
Wire behaviour effect between wire feed and first incidence of wire breakage
Cross-sectional view of the eroded and broken wire:
In WEDM, zinc-coated wire electrode occupies a considerable percentage of the machining cost. Therefore, to attain stable machining without wire breakage, it is required to set a low wire feed rate and a higher level of electrical parameters.
Figure 8A shows the SEM picture of the machined surface at the parametric combination of 70 V, 3 mm/min, 30 A, 120 μs (ON
Figure 9 demonstrates that if we consider the applicable wire breakage as the one corresponding to the lowest value of peak current (30 A), 727 s is obtained as the time taken to machine the slot up to a depth of 9.8 mm. Increase in peak current increases the rate of heat energy. Melting and vaporisation of the wire occur at a rapid rate during the continuous increment of peak current, thereby leading to wire breakage during machining. The peak current governs the maximum amount of amperage for machining the workpiece. Roughing operations are possible with the flow of high current, but they may lead to the creation of cavities. Continuous increment of peak current improves MR but reduces the surface roughness. In AMMC, the continuous increment of peak current leads to increasing frequency of wire breakage.
Effect of peak current on the first occurrence of wire breakage
Figure 10 indicates the effect of pulse on time on the wire breakage during the machining of MMCs. The two slots were successfully completed without wire breakage at 1,115 s and 847 s. Further increase in the pulse on time results in wire breakage. It is evident from the graph that no wire breakage has occurred at lower pulse on time; and with increase in the pulse on time, the frequency of breakage increases. It is due to the fact that increase in pulse on time increases the discharge rate. The high frequency discharge increases the erosion and breaking of the wire. Another important factor for the cause of wire breakage is short circuit. Short circuits take place due to longer pulse on time, and the material removed from the work-piece creates the conductive bridge, resulting in unwanted sparking. Hence, an increased frequency of sparking contributes towards frequent breaking of wires.
Wire behaviour effect for various pulse on time
On comparing the other parameter, the pulse off time (OFF
Effect of pulse off time on the wire behaviour (70 V, 7 mm/min, 310 A and 120 μs [ON
SEM graph of the machined slot at an experimental condition of 70 V, 7 mm/min, 310 A, 120 μs (ON
As per Eqs (1)–(8), the weights for the criteria (MR and R
Standard deviation, criterion value and weighted value
0.2779 | 0.3200 | 0.4549 | |
0.3329 | 0.3833 | 0.5450 |
MR, machining rate
Normalised decision matrix for the CRITIC and SAW methods
0.3421 | 0.3379 | 0.2230 | 0.2385 | 0.7925 | 10 |
0.3421 | 0 | 0.2230 | 0.2514 | 0.7675 | 14 |
1 | 0.0814 | 0.3323 | 0.2483 | 0.9229 | 1 |
0.1842 | 1 | 0.1967 | 0.2131 | 0.8144 | 9 |
0.6447 | 0.0104 | 0.2733 | 0.2510 | 0.8370 | 6 |
0.6315 | 0.026 | 0.2711 | 0.2504 | 0.8351 | 7 |
0.3684 | 0.6984 | 0.2273 | 0.2247 | 0.8284 | 8 |
0.1184 | 0.6984 | 0.1858 | 0.2247 | 0.7715 | 13 |
0.3947 | 0.1265 | 0.2317 | 0.2465 | 0.7885 | 11 |
0.1184 | 0.3899 | 0.1858 | 0.2365 | 0.7457 | 15 |
0.6842 | 0.3812 | 0.2798 | 0.2368 | 0.8737 | 4 |
0.7763 | 0.6759 | 0.2951 | 0.2255 | 0.9192 | 2 |
0.0789 | 0.4558 | 0.1792 | 0.234 | 0.7420 | 16 |
0.2105 | 0.6499 | 0.2011 | 0.2265 | 0.7882 | 12 |
0.6184 | 0.9826 | 0.2689 | 0.2138 | 0.9115 | 3 |
0 | 0.1143 | 0.1661 | 0.2470 | 0.6978 | 18 |
0.1578 | 0.1889 | 0.1924 | 0.2442 | 0.7392 | 17 |
0.3947 | 0.7365 | 0.2317 | 0.2232 | 0.8377 | 5 |
CRITIC, CRiteria Importance Through Intercriteria Correlation; SAW, simple additive weighting
ANOVA results for MR and R
Voltage (V) | 2 | 0.04409 | 0.022046 | 0.43 | 0.669 | 7.03 |
Wire feed rate | 2 | 0.03265 | 0.016323 | 0.32 | 0.739 | 5.21 |
Current | 2 | 0.03130 | 0.015649 | 0.30 | 0.748 | 4.99 |
0.14463 | 0.072317 | 1.40 | 0.308 | 23.06 | ||
0.01268 | 0.006342 | 0.12 | 0.886 | 2.02 | ||
Error | 7 | 0.36188 | 0.051698 | 57.69 | ||
Total | 17 | 0.62724 | 100 | |||
Voltage (V) | 2 | 0.16103 | 0.080517 | 9.28 | 0.011 | 21.23 |
Wire feed rate | 2 | 0.36270 | 0.181350 | 20.90 | 0.001 | 47.82 |
Current | 2 | 0.01343 | 0.006717 | 0.77 | 0.497 | 1.77 |
0.01710 | 0.008550 | 0.99 | 0.420 | 2.25 | ||
0.14343 | 0.071717 | 8.26 | 0.014 | 18.91 | ||
Error | 10 | 0.06075 | 0.008679 | 8.01 | ||
Total | 17 | 0.75845 | 100.00 |
ANOVA, analysis of variance; MR, machining rate
For the first time, the WEDM of a new kind of AMMC produced using scrap aluminium alloy wheels reinforced with 5% alumina in a stir-squeeze casting setup is investigated. The experimental study was performed by varying one parameter at a time and five control parameters were used, namely voltage (V), wire feed (F
However, with increase in the voltage, wire breakage occurs, the earliest being 22 s at 70 V. The completion of slot machining without breakage of wire is made possible using the parameter combination of 70 V, 3 mm/min, 30 A, 120 μs (ON