Abstract
The investigation was carried out by making use of the design of experiments method in order to achieve its objective, which was to study wear analysis in relation to a cryogenically treated nozzle that was utilized in plasma arc machining. Kerf width and surface roughness are two output characteristics that are key variables in deciding the quality of the cut and the efficiency of the operation. Both of these metrics are outputted by the process. While machining S235 steel, an investigation into the impact that nozzle treatment has on various quality metrics is currently under way. The examination is carried out with the arc voltage, the cutting speed, and the gas pressure, all serving as important components. A cryogenic treatment of the nozzle material using liquid nitrogen at a temperature of −194°C has been attempted in an effort to increase the life of the nozzle. Machining is performed using two different nozzle conditions, such as cryogenically treated and cryogenically untreated, with regard to the input parameter combinations that have been selected. To have a better understanding of the wear behavior of nozzles, an image from a scanning electron microscope is studied. Because of the treatment, the production of wear tracks in the direction that gas flow takes has been drastically decreased. This, in turn, has increased the cutting efficiency by decreasing the amount of arc current that was necessary. In addition, a grey relational analysis is carried out in order to find the best possible machining settings in both conditions. The parameters that were optimized for a nozzle that had been cryogenically treated were 6 bar of gas pressure, 120 amperes of arc current, and 1800 of cutting speed per minute. The use of cryogenic treatment resulted in a reduction of surface roughness by 0.4670 µm and a narrowing of the kerf width by 0.96 mm. It is clear from the SEM pictures of untreated and cryogenically treated nozzles that thermal distortion and wear in the nozzle tip area are minimized to a greater extent in the treated nozzle. This is evidenced by the fact that the treated nozzle has a more uniform appearance.
1. Introduction
Numerous machining processes have been developed, and research is still going on in the field of developing modern techniques for machining tough materials. One of the modern techniques is plasma arc machining (PAM), using which precise components can be cut out of sheet metals. Plasma arc machining has been widely used to machine complex part profiles from materials that are tough enough to cut with traditional machining processes. It is one of the tangible methods of cutting hard metals. Among the steel varieties used in manufacturing industries, S235 material was used in manufacturing of parts for power transmission towers, equipment, offshore structures, oil and gas platforms, automobile bridges, and other structures. The machining of S235 material is a tedious process by any conventional methods.
The process utilizes a high-temperature arc for melting metals at specified locations in a precise manner, resulting in a very lesser heat-affected zone. It is widely used in various industries such as automobiles, locomotives, pressure vessels, chemical machinery, nuclear industry, general machinery, engineering machinery, steel structures, and ships. Various factors are involved in the accuracy of components developed through PAM and efficiency of the process. Among them, gas pressure, arc current, and cutting speed were considered in this experimentation. Several attempts have been made to improve efficiency via optimization techniques [1–4].
Ananthakumar et al. [5] optimized the process parameter value for a better response using the TOPSIS approach. The parameters such as stand-off distance and gas pressure are identified as the most influencing parameters. Bhowmick et al. [6] tried machining AISI 304 material in PAM following full factorial design. Based on the results, linear regression models are developed for predicting MRR (material removal rate) and SR (surface roughness). The models developed have an accuracy of more than 90%. Nas and Kara et al. [7] discussed cryogenic treatment that involves cooling workpieces to temperatures below 190°C (310°F). This is carried out to get rid of residual stresses and make steel and other metal alloys less likely to wear out. Cryogenic treatment has been done on copper electrodes in the electrical discharge machining process . The analysis revealed that the cryogenic treatment would reduce electrode wear and improve MRR. Davis stated that [8] cryogenic treatment reduced adhering particles, micropores, and cracks on the surface of the sample. Davis and Singh stated that [9] powder or abrasive-mixed-microelectric discharge machining (A-M-EDM) is gaining popularity for performing precise machining and simultaneously modifying micromanufactured surface tables for clinical applications. Cicek stated that [10] the cryo-treated end mill achieved the highest surface microhardness during cryo-lubri-coolant milling. However, due to the poorest surface quality in terms of the maximum number of machining-induced cracks, the hybrid-lubri-coolant-milled surface was preferred over the cryo and wet-lubri-coolant-milled surfaces. In addition, FESEM and EDS (energy-dispersive X-ray spectroscopy) analyses confirmed that the oxide layer generated by the cryo-treated end mill during hybrid-lubri-coolant milling was the narrowest (12.16 mm) and had the most uniform passivation layer. Kara stated that [11] cryogenic treatment significantly increased the service life of tools. Fine carbides form on uncoated end mills due to the formation of fine carbides as well as their uniform distribution. During arid conditions, prolonging of tool life was attained until this increase reached 126.1% under damp conditions, whereas it was only 25.0% under dry conditions. Therefore, it is essential to maintain a minimum cutting temperature to maximize the efficacy of cryogenic treatment on tool wear. Kara stated that [12] experiments were conducted with AISI 5140 steel that was cryogenically treated, and it was treated in three ways: CHT, DCTT-15, and DCTT-30. Utilizing the L18 orthogonal array Taguchi optimization method, experimental investigations indicate that 30 hours (DCTT-30) is the optimal holding time for Ra.
Mouda and Siddhi Jailani stated that [13] the DCTT-36 sample produced the greatest results for surface roughness and tool wear based on the results of the experiments. In a comparison of cutting tools, the coated ceramic tool (AB2010) produced the greatest results for surface roughness and tool wear. DCT-36 had the greatest macroscopic and microscopic hardness values. The DCTT-36 sample yielded the finest microstructural results, with homogeneous and thinner secondary carbide formations [14]. Deep cryogenic treatment at varying holding durations was used to improve weld mechanical characteristics. The investigation first detected welding area heat damage. A thermal camera, thermocouple, and solder measured welding zone temperatures. These welding temperatures affected surface topography and mechanical characteristics. Tempering heat treatment after 6, 12, 18, and 24 h deep cryogenic treatment was examined. Tempering 7xxx series aluminum alloys following cryogenic treatment decreases nugget zone hardness but enhances heat-unaffected zone hardness. Deep cryogenic treatment at varying holding durations improved weld area hardness, tensile strength, and % elongation.
Lazarevic and Lazarevic [15] worked on understanding the intensity of the heat impact of a plasma jet on surface layers of stainless steel. The heat-affected zone is investigated by measurement of its phase content and transformations during the material cooling process. Pawar [16] studied the quality properties of 316L stainless steel while machining with a plasma-cutting process. The quality of the cut was measured by determining the width and taper of the cut. Arc voltage is found to be the main influencing parameter on Kerf width (Kf) when cutting speed is accompanied by cutting strain.
Klimpel [17] presented a report on the effect of lasers and plasma arc cutting parameters on steel grades and thicknesses of cuts. Three ways are examined to determine the association of parameters with correlation studies. Naik and Maity [18] optimized machining parameters while cutting 304L stainless steel using the PAM process using the analysis of the means approach. The optimized parameter setting for a leaner dimension with lesser machining time is achieved with 70 A current, 150 V arc voltage, 4 mm stand of distance, and 3000 mm/min cutting speed. Abdulnasser and Bhuvanesh [19] optimized the cutting parameters of PAM with 3 mm and 6 mm thick sheets with objective functions as MRR and SR. The contribution of significant parameters such as current and cutting speed in machining aluminium alloy 1100 is reported. Chamarthi et al. [20] tried to understand the cause of uneven surface formation during PAM cutting of Hardox-400 material. Experiments are conducted with plasma gas, cutting speed, and arc voltage as operating parameters. Results are analyzed using the ANOVA technique, and the optimized setting is reported to be 70 L/Hr plasma flow, 125 V, and 2100 mm/min. Ozek et al. [21] tried to develop models for predicting SR while machining AISI 4140 steel in PAM. The prediction is made with the help of fuzzy logic modelling with an accuracy of 98%. Çelik [22] studied the impact of process parameters in PAM of S235 materials with various thicknesses of 4, 6, and 8 mm. Results revealed that arc current and voltage played a major role in deciding the SR of machined components.
Adalarasan et al. [23] used the grey–Taguchi technique for optimizing the process parameters in the machining of SS304L. The grey-RSM approach is targeted to improve the surface characteristics of the machined workpiece. Dhinakaran et al. [24] utilized the Gaussian heat model approach to study the plasma arc machining of Ti-6Al-4V alloy. The numerical analysis is carried out in COMSOL. The analysis depicted that the parabolic Gaussian heat source (PGHS) model is very appropriate in predicting the heat distribution that happened during experimentation. Maity and Bagal [25] tried to optimize machining parameters during the cutting of AISI 316 stainless steel using the response surface methodology (RSM). RSM has proven to be efficient in determining the effect of parameters over the response values. Suresh Manoj and Gandhi [26] utilized the Taguchi approach for designing the experimental runs in an attempt to improve the mechanical properties of the machined workpiece. It is also stated that the nanocoating of tools improves life to a great extent by increasing the heat dissipation rate. The bond energy required for removing metal from the tool gets increased. Bejaxhin and Paulraj [27] studied vibrations developed during machining and proposed a newly developed copper-coiled vibration monitoring microphone system. It is used in conjunction with the cross-platform tool Audacity 2.1.0 to record vibrations during machining as sound signals, so that surface roughness gets monitored.
From the literature, it is found that the wear study on a cryogenically treated nozzle is yet to be studied in detail in terms of PAM. As a result, using the aforementioned finding as an objective, S235 steel is chosen for the study in nozzle wear of the PAM process in cryogenic treatment. The novel cryogenic treatment of the nozzle improves cutting efficiency while lowering machine costs. Process parameters that affect surface finish and kerf width quality during machining with PAM are considered in the specimen for analysis. As per Taguchi’s design of experiments, L27 trial runs were conducted for both the treated and untreated nozzle. Consequently, grey relation analysis is used to optimize the model and verify the findings.
2. Materials and Methods
The present work focuses on the plasma arc machining (PAM) of S235 steel by using different input parameter combinations in order to understand their effect on the chosen responses. The experimentation is carried out using a CNC plasma arc cutting machine of the model PM1000A1313H. The machine is capable of cutting different materials ranging from normal mild steel to tough materials such as titanium and tungsten. The machine has a CNC-cutting table with a cutting area of 1220 × 1829 mm. The maximum thickness of the material that could be cut in a single pass is 25 mm. The necessary operating specifications required for the machine are provided in Table 1, and the experimental setup and the methodology used are shown in Figure 1. Figure 2 depicts the chosen profile of the cut for the experimentation with a plate thickness of 6 mm.
The work material chosen for machining is steel S235 for its applications in areas requiring a high amount of thermal resistance. The chemical composition and mechanical properties of the same are provided in Tables 2 and 3. The primary focus of this work is to study the effects that occurred on the nozzle used in PAM operation to cut through the chosen material. The two types of nozzles are utilized in this work, i.e., cryogenic treated (seen in Figure 3) and untreated nozzle. The nozzle through which plasma will come out of the machining head is treated with cryogenic conditions in order to observe the increase in performance and life cycle. The cryogenic treatment process is carried out using liquid nitrogen at a temperature of −194°C, as shown in Figure 3. The nozzle is placed in the cooling medium for 24 hours of the time period and allowed to get hardened. Nitrogen atoms get to penetrate into the structure at a very low level and occupy wherever the crystal lattice has a vacancy. Owing to the interstitial occupancy, the strength of the nozzle material gets improved by restricting the movement of dislocations, and the setup is shown in Figure 3.
The experimentation is followed using a full-factorial model having three levels for three input parameters chosen, resulting in 27 trial runs. The parameters chosen are cutting speed, arc current, and gas pressure. The details of the trial runs are presented in Table 4. Complete experimental runs are repeated for both types of nozzles. The average SR and kerf response characteristics were evaluated in the present study. For measuring surface roughness, the arithmetic average roughness, i.e., the average deviation of the surface profile from the mean line within the evaluation length, was considered and expressed in micrometres. At three separate locations, SR was measured using an SJ-210-Mitutoyo surf test equipped with a 2 µm stylus tip diamond indenter. The median of the three values was then chosen. The workpiece samples of steel S235 with the following composition (described in Tables 2 and 3) were obtained from add Corp Steel Solutions in Chennai, India. The 130 A nozzle has been chosen. Based on the trial and error procedure, Table 4 is determined.
3. Results and Discussion
The experimentation is conducted as mentioned, following full-factorial design using the high precision Pro-Arc CNC plasma arc machining unit PM1000A1313H. The primary results observed on the machined workpiece are surface roughness (SR) and kerf width (Kf). The measured results for both cryogenic treated and untreated nozzle machining are provided in Table 5. The machined-out workpiece can be seen in Figures 4(a) and 4(b) for cryogenic treated and untreated nozzle machining, respectively.
(a)
(b)
The machined workpiece is carefully handled immediately after the cutting operation. The surface area is properly covered and stored after sufficient cooling has occurred. The proper handling of machined parts ensured for the accuracy of the SR measured. The comparison between the SR of cryogenically treated and cryogenically untreated nozzle machining is presented in Figure 5(a). Kf for the same is provided in Figure 5(b). From Figure 5(a), it shall be evident that SR reduces with a simultaneous increase of arc current and cutting speed. Cutting speed showed an inverse relationship with SR. The least value of SR is observed for the cryogenically treated nozzle with a value of 0.4670 µm, and the same for the untreated nozzle was about 0.6856 µm. It could be clearly observed that there lies a 31.88% decrement in SR due to the treatment. Initially, the value of SR is slightly higher for the treated nozzle than that of the untreated nozzle at all gas pressures and cutting speeds of 1000 mm/min. A cutting speed of 1000 mm/min shall be maintained as the threshold value for PAM of S235 steel. Only beyond that value, SR started to decrease with the aid of the treated nozzle. While travelling at high speed, the energy density of the operation gets reduced. Due to this, the heat developed on the surface has reduced and resulted in a smooth surface.
(a)
(b)
Similarly, from Figure 5(b), Kf is compared for the workpiece machined using both types of nozzles. It could be seen that the Kf value increased with an increase in arc current. The arc current and Kf values are directly proportional to each other (as seen in Figure 5(b)). It could be observed that Kf increased with arc current for every value of gas pressure and cutting speed. However, the maximum values of 1.7022 mm and 1.7781 mm are obtained for treated and untreated nozzle machining, respectively, at a low gas pressure of 4 bar. A minimum value of Kf is achieved at 6 bar pressure and 1000 mm/min cutting speed for machining carried out with both nozzles.
Table 5 shows a 3D plot which is shown in Figure 6. Surface roughness was analyzed using the plot. Figures 6(a) and 6(b) show that surface irregularity was minimal at the nozzle minimum pressure (4 bar), maximum current (130 A), and maximum speed (1800 mm/min). In general, it can be observed that interactions between various machining parameters were much simpler than the basic rule of thumb and that their combined effect on the process response was ambiguous. Figures 6(c) and 6(d) show that surface irregularity was minimal at medium pressure (5 bar), maximum current (130 A), and maximum speed (1800 mm/min). The inference from Figures 6(e) and 6(f) is that surface roughness was lowest at the cryogenically treated nozzle with maximum pressure (6 bar), medium current (120 A), and maximum speed (1800 mm/min).
(a)
(b)
(c)
(d)
(e)
(f)
3.1. SEM Analysis
It is possible to create photographs of a sample by scanning its surface with a focused beam of electrons, as is carried out by using a scanning electron microscope. Through their interactions with atoms, electrons in a sample generate a wide range of signals that can be decoded to reveal information about the surface’s topography and composition.
The specimen for SEM analysis was cut into a 20 mm diameter nozzle. The cryogenically treated nozzle is examined with a scanning electron microscope before and after machining has been carried out (shown in Figures 4 and 7). The grain orientation has been slightly modified due to the treatment carried out on the nozzle part (seen in Figure 4). It can be clearly observed from Figure 7 that wear tracks are higher on the untreated nozzle surface after machining operations have been carried out. During continuous operation of the machine, the nozzle’s inner surface area has been eroded more in the untreated condition. The cryogenic treatment has improved the bonding strength of surface atoms with that of core atoms, and hence, heat energy from the plasma arc has a lesser effect on the erosion of nozzle material during machining. Moreover, the oxide passive layer formed during the cryogenic treatment enhances the wear resistance of the nozzle [9]. Figures 7(a), 7(c), and 7(e) show the SEM images of the untreated nozzle after machining and reveal the removal of more material from the nozzle layer. On the other hand, the cryogenically treated nozzle shows a smaller number of wear tracks in Figures 7(b), 7(d), and 7(f).
(a)
(b)
(c)
(d)
(e)
(f)
3.2. EDS Analysis
Copper nozzle material can indeed develop an oxide passive layer upon exposure to cryogenic conditions [9]. Copper has a greater tendency to react with oxygen in the air at cryogenic temperatures, resulting in the formation of an oxide passive layer on its surface. This oxide layer can serve as a barrier against wear. Figures 8(a) and 8(b) clearly show that oxide formation is more in the cryogenically treated nozzle. The EDS values of nontreated and cryogenically treated nozzles are depicted in Tables 6(a) and 6(b).
(a)
(b)
3.3. Grey Relational Analysis
Optimization of the process parameters is carried out using the grey relational analysis technique. The methodology followed in the process is clearly explained with the flowchart shown in Figure 8. The experimental results are collected, and the signal-to-noise ratio for respective results is developed. Furthermore, the SN ratio values are normalized with conditions such that both the responses, i.e., surface roughness and kerf width, are to be minimized. After the deviation sequence is executed, the grey relational coefficient (GRC) and grey relational grade (GRG) are determined as per the equations provided below. Based on GRG, a ranking is made for each parameter combination to identify the optimum setting. The detailed values pertaining to results obtained by using an untreated nozzle are provided in Table 7. The same values for the cryogenic treated nozzle are provided in Table 8. The SN ratio is developed with the objective function of minimizing both SR and Kf. The methodology of the grey relational analysis is illustrated in Figure 9.
Normalization is as follows.
For larger is better,
For smaller is better,
The grey relational coefficient for the normalized values is as follows:where y0(k) = reference sequence and yj(k) = specific comparison sequence.
The value for ξΔmax is taken as 0.5.
Based on the Grey grade, the ranking has been applied for the input parameter combinations chosen during experimentation. The input parameter combination having the least rank is considered to be the most preferable. Hence, the optimized input parameter setting for machining using the untreated nozzle is 6 bar gas pressure, 130 A arc current, and 1800 mm/min cutting speed, respectively (27th trial run). The optimized input setting table for achieving the proposed objective function while machining with the cryogenic treated nozzle is 6 bar gas pressure, 120 A arc current, and 1800 mm/min cutting speed, respectively (26th trial run).
In order to attain the target of reducing SR and Kf, from Table 5, the gas pressure and cutting need to be at higher values of 6 bar and 1800 mm/min, respectively. But the arc current needs to be at a moderate level for treated nozzle machining. The process of treating the nozzle has reduced the requirement of operating current by offering reduced resistance to the plasma arc delivered for machining. This reduces the operating cost as well as increases the lifetime of the nozzle. Hence, downtime of equipment towards frequent nozzle replacement and idle time of teh machine will be reduced. The influential parameter that has shown more impact on the objective function has been presented in Tables 9 and 10 for treated and untreated nozzle machining, respectively. Both the results have shown that the influencing order is as follows: gas pressure, arc current, and cutting speed. It could be inferred that surface roughness and kerf width are mainly decided by the gas pressure.
The optimized values of surface roughness and kerf width in trial runs of 26 and 27 (GRA results) of treated and untreated nozzles were compared. The results of treated nozzles clearly support the plasma arc machining process by significantly reducing surface roughness and kerf width, as shown in Figure 10.
4. Conclusion
The process of plasma arc cutting has been used in numerous industrial applications wherever there is a need for a reduced heat-affected zone. The material S235 played a vital role in fabrication industries in fulfilling their structural demands. The full-factorial design of experiments is followed, and results are recorded. Based on the results, the following conclusions are made:(1)Based on SEM and EDS analyses, the cryogenic treatment of the nozzle improved the erosion resistance and bonding energy of the nozzle material.(2)The wear tracks are higher in the untreated nozzle surface after the machining operation has been carried out.(3)SR was reduced with a simultaneous increase of arc current and cutting speed. The least value of SR is observed for the cryogenic treated nozzle with a value of 0.4670 µm, and the same for the untreated nozzle is about 0.6856 µm.(4)A decrement of 31.88% is observed in SR due to the treatment of the nozzle material.(5)Kf increased with arc current for every value of gas pressure and cutting speed.(6)A minimum Kf of 0.96 mm and 0.9844 mm for a treated and untreated nozzle, respectively, is achieved at 6 bar pressure and 1000 mm/min cutting speed for machining carried out with both the nozzles.(7)The optimized setting for machining with an untreated nozzle was 6 bar gas pressure, 130 A arc current, and 1800 mm/min cutting speed. The optimized setting for machining with the cryogenic treated nozzle was 6 bar gas pressure, 120 A arc current, and 1800 mm/min cutting speed.(8)The cryogenic treated nozzles have resulted in an improvement of surface roughness and kerf width while running with the optimized input parameters by 32 and 17 percentages, respectively.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.