Abstract
In this study, an endeavour have been made to depositing the electrode materials over the surface of the magnesium alloy using electrical discharge machining (EDM) with WC-Cu powder compacted sintered electrode. Various process parameters such as compaction load, discharge current and pulse on time are selected to carry out the experiment in order to attain the maximum material migration rate (MMR) or deposition rate and microhardness (MH). It was concluded that the MMR and MH increased with increase in discharge current and pulse on time at low compacted electrode but it is decreased at lower discharge current and pulse on time. Highest MMR and MH were attained successfully at partial sintered low compaction load electrode. Microstructure evaluation has been carried out on deposited surface using scanning electron microscopy (SEM) and presence of electrode element in the deposited surface was confirmed by energy dispersive spectroscopy (EDS). Defects mechanism such as globules and craters are formed during EDC with high current and pulse on time respectively, which diminishes the surface roughness. It was observed that the compaction load is the influence parameter on the MMR and MH.
1 Introduction
Non-traditional machining (NTM) processes are the electro thermal process, in which there is no physical contact between the tool and work piece. In the NTM process, several forms of energies are used to remove the unwanted material from the work piece surface [1]. Among NTM processes, electrical discharge machining is widely used in the several industrial applications due its precision and quite economic cost [2]. In which very hard materials such as tungsten carbide and titanium carbide can be machined by supplying the series of electric spark between the tool electrode and workpiece in order to attain the good machining characteristics [3]. Nowadays, EDM is introduced for modifying the surface of the workpiece by depositing the electrode materials [4]. This novel approach of EDM is called electrical discharge coating (EDC). For the purpose of surface modification, powder metallurgy electrodes such green compact, sintered and semi sintered electrode have been used to achieve the deposition characteristics such as material deposition rate (MDR), layer thickness (LT) and surface roughness (SR) [5]. In recent few decades, several researches have carried out on the surface modification of different workpiece materials using electrical discharge coating technique with powder metallurgy electrode [6]. Sahu et al. [7] modified the surface AISI 1040 stainless steel using copper-tungsten composite electrode fabricated through powder metallurgy. Compaction load and sintering temperature were selected to control the deposition of electrode materials in the study. Effect of current, duty cycle and pulse on time on the average surface roughness were studied. Prakash et al. [8] fabricated the Ti-Nb electrode using PM route and partially sintered about 800∘C to attain the maximum coating thickness and micro hardness. It was concluded that the coating layer thickness increased with increase in discharge current and pulse duration. Elaiyarasan et al. [9] studied the wear behaviour of magnesium alloy coated with powder metallurgy electrode under different sliding condition. Sahoo and Bhaskar [10] optimized the electro discharge coating process by desirability function technique. Process parameters such as discharge current, duty cycle and sintering were selected to attain the EDC characteristics. Das et al. [11] investigated the surface alloying of TiB2 and SiC on aluminium alloy surface using electrical discharge deposition technique with powder compact electrode. Alloying characteristics such as deposited layer thickness and microhardness were achieved by controlled parameters like duty cycle, gap voltage and flushing pressure. It was revealed that the higher current is the significant parameter to developed uniform deposition over the surface.
Even though several investigations focused on the deposition of electrode materials on the surface, the micro structural effect in the deposited surface is still limited. In this present research, deposition experiment has been conducted on the magnesium alloy surface with different controlled parameters using electro discharge coating. Microhardness of the deposited surface is measured using vicker hardness tester. Microstructure analysis is carried out in the deposited surface using scanning electron microscopy (SEM) and presence of electrode materials in the deposited surface is assessed by energy dispersive spectroscopy (EDS).
2 Materials and methods
2.1 Preparation of electrode and workpiece
In this research, the electrodes have been prepared through powder metallurgy route using tungsten carbide (WC) and copper (Cu) powder. The average particle of the electrode powders is 2-4 microns, purchased from metal powder company, Pune. First the powders were taken in the combination of WC70:Cu30 (wt%) and mixed thoroughly using ball mill for 4 hours. Further the mixed powders are compacted using punch and die setup of 10 mm diameter through hydraulic press for 10 min. Three different loads (150MPa, 175 MPa and 200MPa) are applied to fabricate the electrode and prepared green compact electrode is given in Figure 1. Sequentially, the green compactions are partially sintered and fully sintered are 700∘C and 900∘C respectively using tubular furnace for 30 min in the argon gas atmosphere.
WC/Cu partial and fully sintered electrode
In this investigation, surface of magnesium alloy is modified through electrical discharge deposition technique using prepared powder compact electrode. ZE41A cast magnesium alloy is selected as the substrate material and it chemical combination is given in Table 2. The surface roughness of the base materials (ZE41A) is 2.56 μm. Before experiments, the workpiece is structured in the shape of 20mm × 20mm using surface grinding machine in order to make the smooth surface. After grinding the workpiece is polished using 1200 grid emery sheet to remove the fine sharp edges.
Process parameters and their range in the study
| Parameter | Unit | Levels | ||
|---|---|---|---|---|
| Compaction load (CL) | MPa | 150 | 175 | 200 |
| Discharge current (I) | A | 2 | 3 | 4 |
| Pulse on time (Ton) | μs | 50 | 70 | 90 |
Chemical composition of ZE41A magnesium alloy
| Si | Cu | Zn | Zr | Fe | TRE | Ni | Mn | Al | Mg |
|---|---|---|---|---|---|---|---|---|---|
| 0.003 | 0.002 | 3.80 | 0.60 | 0.004 | 1.18 | 0.002 | 0.003 | 0.006 | Bal |
The experiments with three parameters and three level has been selected using central composite design of response surface methodology (RSM). Twenty experiments are conducted as per the design matrix and input parameter used in this study is given in the Table 1. Different parameters like compaction load, discharge current and pulse on time are selected as the input parameter. Each parameter has the three levels, which has been fixed based on the trial experiments.
To assess the deposition characteristics, material migration rate and microhardness are the responses. MMR is calculated by weight difference between the before and after weight of the deposition, which is measured using SF400D weighing machine with accuracy of 0.001g. Microhardness of the deposited surface is assessed by vicker microhardness tester available at centre for material joining and research (CEMAJOR), Annamalai University, Chidambaram. The deposition is developed over the surface of ZE41A magnesium alloy using ordinary EDM machine with negative polarity condition (electrode as anode and workpiece as cathode) and EDM oil is used as the dielectric medium in this study. The experiments are carried out for the twenty different conditions and rate of deposited materials over the surface is recorded.
3 Results and discussion
3.1 Microstructure of deposition
ZE41A magnesium alloy surface was coated using two types of powder metallurgy electrode such as partially sintered electrode and fully sintered electrode. The deposited surface was analysed using scanning electron microscope to study the microstructure. Microstructure of deposited surface was taken in the magnification of 100 μm. Bulk deposition was observed at EDC experiment conducted in low compaction load (150MPa), high current (4A) and pulse on time (90μs) as depicted in Figure 2(a) and their surface roughness is 5.65 μm. It was observed in the SEM microstructure that the lump deposition was identified at electrode prepared with low compacted partial sintering. In this condition, the mechanical bonding between the powder particles is not so good. Hence the density of the electrode will be low and also sintering temperature is not enough to bond the powder particles.
(a) SEM image of bulk deposition at 150 MPa, 4A, 90μs; (b) deposited layer thickness; (c) EDS peak plot
Hence, during the EDC high quantity of electrode materials melted and deposited over the surface. But in fully sintering condition, the electrode density will be high, hence during the EDC process produced the electrical energy is not sufficient to melt and vaporize the electrode, therefore the deposition rate is minimum. The deposited sample was cross sectioned to study the layer thickness and thickness was measured in the two different places to determine average layer thickness as shown in Figure 2(b). Average layer thickness of the deposited surface was 85.60 μm. Deposition of electrode materials on the base materials were confirmed by the energy dispersive spectroscope (EDS). EDS peak plot revealed that the electrode materials such as copper (Cu), magnesium (Mg), nickel (Ni), iron (Fe), zinc (Zn) and tungsten were presented in the deposited surface. Among the all experiments, the high quantity of electrode materials is deposited at low compaction load partial sintered electrode. The weight percentage of Tungsten and copper in the deposited surface were 0.33 and 0.34, respectively as shown in Figure 2(c).
3.2 Effect of compaction load on MMR and MH
During electrode fabrication, the role of compaction load is important because deposition of electrode materials over the surface of the workpiece is controlled by the compaction load. The green compacted electrodes are sintered in the two conditions, which promotes the migration of electrode materials on the surface. Figure 3 shows the effect of compaction load on material deposition rate and micro hardness. From the effect plot, both partial and fully sintered electrode, MMR and MH decreased with increases in compaction load but the decrement level is high at high compacted sintered electrode because bonding of electrode has high density, hence small quantity of materials eroded from the electrode and deposited over the surface [12]. At low compacted partial sintered electrode, MMR is high, this is owing to density of bonded electrode is not good, therefore high quantity of materials dislodged from the electrode and deposited over the surface. Figure 4 shows the microstructure of the deposited surface at different parameter conditions. Uniform deposition was observed at electrode prepared with high compaction load full sintered electrode as depicted in Figure 4(a-b) and small craters also identified in the surface, which shows the smooth surface. Figure 4(c-d) shows the SEM image of bulk deposition and bigger craters, formed at low compaction load partial sintered electrode,which provides the poorer surface finish [13]. The values of MH are high at low compaction load partially sintered electrode than filly sintered condition because hard deposition is formed in this condition, therefore MH is high.
Effect of compaction load on MMR and MH
SEM microstructure (a, b) uniform deposition at high compaction load fully sintered electrode; (c) lump deposition at low compaction load partial sintered electrode; (d) craters at low compaction load
3.3 Effect of discharge current on MMR and MH
Current is one of the electrical parameter in the EDM which influences the deposition rate of electrode materials. Electrode materials are deposited on the surface by supplying the series of electric spark between the electrode and workpiece. Graphs are plotted on MMR and MH with respect to discharge current as shown in Figure 5. MMR and MH increased with increase in current for both partial and fully sintered electrode. At lower current, small quantity of electric sparks are produced, which is not enough to melt the electrode material, hence small amount of materials deposited over surface. At higher current setting, the migration rate is high because of high density of spark [14]. Figure 6 illustrates the microstructure of the surface deposited with partial and fully sintered electrode. Surface deposited with partial sintered electrode at low current formed the cracks as shown in Figure 6(a) and further increased the current, the size of the craters increased as shown in Figure 6(b), which provides the rough surface. Small size of voids are formed in the surface deposited with fully sintered electrode at higher current as depicted in Figure 6(c),which does not affect the surface finish. Size of the voids increased by increased discharge spark for longer time. Figure 6(d) shows the SEM image of bigger voids, developed at fully sintered electrode. MH increases with increase in current because of large quantity of electrode materials deposited over the surface. While micro-hardness tests the deposited hard material resists the indentation. Hence MH increases at this condition [15]. At lower current setting,MHvalues are low due to presence of low quantity of deposition in the workpiece surface, which allows the indentation in to the deposited surface, hence the MH values is decreased.
Effect of discharge current on MMR and MH
SEM microstructure (a) cracks at partially sintered electrode with low current; (b) crater at high current; (c) small sized voids at fully sintered electrode with higher current; (d) Bigger voids
3.4 Effect of pulse on time on MMR and MH
Pulse on time is the one of the important parameter for melting electrode during EDM, in which the materials deposition rate is directly proportional to the quantity of energy applied during the pulse on time [16]. Figure 7 shows the effect of pulse on time on material migration rate and micro hardness. MMR and MH increased with increases in pulse on time for partial and fully sintered conditioned electrode. At high pulse on time, high quantity of discharge energy is produced which melts electrode material in large quantity and deposited over the surface. Hence, MMR increased. MMR is minimum at lower pulse on time setting because of diameter of the plasma channel produced in the machining zone is small [17]. Therefore produced plasma channel not enough to crumbles the material from the electrode. Surface deposited using high compaction load fully sintered electrode creates the craters and globules which make the rough surface in the deposited zone as depicted in Figure 8, this could be due to the huge amount of discharge energy. On the other hand,MH increased at workpiece deposited with low compaction load partial sintered electrode. In this condition, binding energy of partial sintered electrode is less than fully sintered electrodes, so material freely disintegrated from the electrode and deposited on the workpiece surface [18], hence MH is increased. At lower pulse on time the value of the MH is low because of poor deposition.
Effect of pulse on time on MMR and MH
SEM image (a) craters at fully sintered electrode with low pulse on time; (b) globules at higher pulse on time
4 Conclusion
In this study, ZE41A magnesium alloy was deposited by electrical discharge coating technique with WC-Cu powder metallurgy partial and fully sintered electrode. An effect of compaction load, discharge current and pulse on time on material migration rate and microhardness were studied. It was observed that the MMR and MH increased with increase in current and pulse on time at low compaction loaded partial sintered electrode but it is minimum at full sintered electrode. Microstructure of deposited surface was analysed by SEM, in which different defect mechanisms were identified. Uniform deposition was identified at surface coated with fully sintered electrode. A highest microhardness value was achieved at low compaction loaded partial sintered electrode because of bulk and lump deposition but MH is deceased at high compaction load, low current and pulse on time. Cracks and voids were formed at fully sintered electrode with low current and pulse on time, resulted the poorer surface finish. Surface deposited with partial sintered electrode increases size of the craters and globules, results the irregular surface.
Conflict of Interest
Conflict of Interests: The authors declare no conflict of interest regarding the publication of this paper.
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