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Development of a new hybrid plasma torch for materials processing
Journal of Materials Processing Technology 212 (2012) 2371–2379
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Development of a new hybrid plasma torch for materials processing Richard Thomas Lermen ∗ , Ivan Guerra Machado Federal University of Rio Grande do Sul – UFRGS, PPGE3M/LS&TC, Porto Alegre, Rio Grande do Sul, Brazil
a r t i c l e
i n f o
Article history: Received 27 April 2012 Received in revised form 30 May 2012 Accepted 19 June 2012 Available online 26 June 2012 Keywords: Hybrid plasma torch Plasma thruster Plasma jet length Welding Cutting Surface hardening
a b s t r a c t The main objectives of this study were to construct a plasma generator device and to investigate its applications in welding, cutting, and surface hardening. The device was derived from the union of two plasma-generating technologies, non-transferred-arc plasma and magnetoplasmadynamic thruster (MPDT), and characterised by the simultaneous formation of two plasma arcs in one device, generating a plasma jet with high energy density. Initially, trials were conducted to analyse the influence of the physical variables (gas flow rate and electric current intensities – primary and secondary) on the plasma jet, for which the thruster and the length of the plasma jet expulsed from the chamber were determined. The relevant parameters for welding, cutting and surface hardening were determined by trial and error, in which the trials were conducted using various plate thicknesses and materials. The results have shown that this device can be used for welding, cutting and surface hardening. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal plasma generated by plasma torches was first used for technological applications in the 1960s. This technology has recently become highly promising because of its wide range of applications and status as a low-pollution process (Roth, 1995). Within the field of plasma torches, hybrid plasma generator torches are characterised by their simultaneous use of two or more processes within a single device (Messler, 2004). Examples of hybrid torches include the plasma/MIG welding torch developed by Essers et al. (1981), dual anode plasma torch described by Tu et al. (2008), hybrid non-transferred-arc plasma torch presented by Browning (1986) through of a patent, DC/HF hybrid plasma torch have been developed by Professor T. Yoshida at the University of Tokyo in Japan in the late seventy and early eighty (as quoted by Solonenko, 2003), and others. These torches are generally constructed to gain a particular advantage over conventional manufacturing processes or a different set of attributes. These torches generate plasma jets that provide high temperature gas flows, usually with a power density of approximately 108 W/m2 . It is possible to reach the melting or vapourisation points of almost all solid materials with these plasma jets, and their applications in welding, cutting, and hardening are well represented in the literature by Welding Handbook (1991), Kou (2002) and
∗ Corresponding author. Tel.: +55 55 3537 6428; fax: +55 55 3537 1614. E-mail addresses: [email protected], [email protected] (R.T. Lermen), [email protected] (I.G. Machado). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.06.009
Pan et al. (2005). Further these applications, the development of electric thrusters, which are classified as electrothermal thrusters (Arcjets), electrostatic thrusters (Ion Thrusters) and electromagnetic thrusters (magnetoplasmadynamic thrusters – MPDT), are well referred by Jahn (1968), Burton et al. (1983), Choueiri (1998) and, recently, by Garrigues and Coche (2011). According to Jahn (1968) the MPDT is characterised by a coaxial geometry constituted by a central cathode and a cylindrical anode, in the form of a chamber, which are electrically isolated. Gas is injected though orifices generating gas flow rate in the chamber, where it is ionised by passing through the electric discharge between the electrodes (cathode and anode), generated by an energy source with a high frequency device. Through this plasma (ionised gas) comes a radial current density which crosses the gas in direction to cathode. The electric current in the cathode generates a circumferential magnetic field (B), which interacts with electric current density (j), originating an electromagnetic force (j∧ B), which accelerates the particles and expels ionised gas from the chamber. Physical models are generally used to describe the behavior of this electromagnetic device. An analytic model based on the continuous description of electromagnetic propulsion (thruster) was developed by Maecker (as quoted by Sankaran, 2005), and later explained by Jahn (1968). The model, described by Tikhonov et al. (1993), improves Maecker’s formula. It is obtained from a quasi1-D MHD (magnetohydrodynamic) analytical model that allows the free boundary of the flow to vary consistently with the flow conditions. Moreover, the model described by Choueiri (1998) also improves Maecker’s formula, which takes into account variations
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in the electric current and in the type of gas. The last model is semi-empirical, since some experimental data must be considered for current distribution among the electrodes and the pressure distribution in the internal chamber of the MPDT. According to Burton et al. (1983), the electrothermal contribution, which is not described by previous models and is typically less than 10% of the total thrust for electromagnetic device, depends on the type of gas, gas flow rate and the electrical current. Recently, Machado and Lermen (2008) developed, produced and improved a magnetoplasmadynamic thruster for welding and cutting of metallic materials. However, this device has not yet submitted welds and cuts with qualities comparable to conventional processes (GTAW, GMAW, etc.). In the absence of sufficient information available about the capacity of MPDT, when coupled with other plasma generator processes and technologies of power sources, a new hybrid plasma generator device was developed, which involves the MPDT process and non-transferred-arc plasma process (Lermen, 2011). For this new hybrid plasma torch (“Patent Pending”), the scientific knowledge related to the characteristics of the plasma jet (thruster, plasma jet length, etc.) and application in manufacturing processes (welding, cutting, surface hardening, etc.) have not been reported, even in a superficial way. However, the main purpose of this study is to describe the design, characterisation and application of the new hybrid plasma, where the results are fundamental to the scientific-technological development of this subject. 2. Materials and methods Three stages of materials and methods are presented, i.e., firstly the hybrid plasma torch project and operation; after the plasma thruster and jet length measurement, and finally, the hybrid plasma torch application (materials processing). The goal of this last stage was not to optimise procedures with the new device, but rather to check their suitability for welding, cutting and surface hardening. 2.1. Hybrid plasma torch – HPT The new hybrid plasma torch is characterised by the simultaneous formation of two plasma arcs in just one device, thus generating a plasma jet of high energy density. This energetic torch jet is derived from the union of two plasma-generating technologies: non-transferred-arc plasma and magnetoplasmadynamic thruster. This torch has a coaxial geometry that consists of two cathodes (primary and secondary) and two cylindrical anodic chambers, which are electrically isolated through alumina ceramic flanges.
Initially, a gas is injected through holes, generating a flow in the chamber, where the gas is heated and ionised by passage through the electrical discharge between the primary anode and the primary cathode. This electrical discharge is generated by a high-frequency primary energy source. A radial current density is passed through the resulting plasma (ionised gas), and it runs through the gas towards the primary cathode. The current in the primary cathode generates an induced circumferential magnetic field, which interacts with the current density to produce a Lorentz force (jP ∧ BP ) interaction, that accelerates the gas particles, forming plasma chamber. The plasma acts as an electrical conductor, and another electrical discharge is generated by secondary energy source between the secondary cathode and the secondary anode in the chamber. A second radial current density arises, running through the plasma towards the secondary cathode. The current in the secondary cathode also generates an induced circumferential magnetic field, which interacts with the secondary current density. The secondary interaction produces another electromagnetic force (jS ∧ BS ), which ionises and further accelerates the plasma jet particles produced in the primary ionisation, whilst also expelling the plasma jet from the chamber. Therefore, in this system, the gas is accelerated twice, i.e., first by the electromagnetic force and thermal expansion between the primary electrodes and then by the resulting electromagnetic force and thermal expansion between the secondary electrodes. A schematic of the hybrid plasma torch with lines representing the electric current density, circumferential magnetic fields and electric arcs is shown in Fig. 1. A sectional view of hybrid plasma torch developed by the Laboratory of Welding & Related Techniques (LS&TC) is shown in Fig. 2. This device mainly consists of primary and secondary anodes and cathodes. The secondary cathode is an AWS EWTh-2 tungsten electrode with a diameter of 3.2 mm, a length of 150 mm, and an end tapered at 60◦ ; the primary cathode is a ring-shaped copper electrode. The primary and secondary anodes are both copper and chamber-shaped. The electrodes (cathodes and anodes) are fixed in water cooling chambers by flange fasteners and electrically insulated from each other by flange-shaped ceramic insulators. The cooling chambers are embedded in copper flanges. Teflon bushings prevent electrical discharges between the bolts and the flange fasteners. The dimensions of the torch components were presented by Lermen (2011) in recent Doctoral Thesis. Argon is used as working gas and injected axially into the torch discharge chamber via the gas inlets near the primary electrodes, and gas flow rates of up to 25 l/min were possible. The gas flow rate was controlled by a numerical mass flow meter.
Fig. 1. Schematic drawing with electrical current density lines, circumferential magnetic fields and electric arcs.
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Fig. 2. Sectional view of HPT constructed in the LS&TC.
Two constant current “drooping” power sources were used to run the HPT. These sources are unique because of their electric current ranges and application, i.e., the primary power source had an electrical current range between 0 and 70 A with an HF generator (5 kHz); it is used for thermal cutting by plasma. The secondary power source has an electrical current range between 0 and 400 A and is used for GTAW welding. The electrical operating conditions are typically 20–30 V/30–60 A for the first arc, and 15–25 V/100–250 A for the second arc. In order to maintain the plasma torch in a stationary state at a reasonable temperature and to limit the electrode wear, four parts of the plasma torch (primary cathode, primary anode, secondary cathode and secondary anode) are water cooled independently.
measured; later the device was operated (two plasma arcs) ionizing the gas, and the thruster of the ionised gas (plasma) expelled from the chamber was measured. The difference between the thrusters (non-ionised gas and plasma) multiplied by the local gravitational field intensity supplies the approximate value of the thruster of this device. The experimental apparatus was in a closed place, isolating the system from external forces that could affect the stabilisation of the dynamometer. The arc voltage and current intensity were measured simultaneously by using four channels data acquisition board (NI USB 6009, 48 MHz, 14 bit) connected to the computer. Each electrical signal was recorded at 10 MHz acquisition rate and intervals for a sampling duration of 20 s.
2.2. Design of experiments
2.4. Welding, cutting and surface hardening with HPT
Experiments to determine the plasma jet thruster and length were developed by the complete factorial statistical method. This experiment design depends on three factors with four levels each, i.e., the experiments were performed with primary electric current intensities (IP ) of 30, 40, 50, and 60 A; secondary electric current intensities (IS ) of 100, 150, 200, and 250 A; and argon gas flow rates (Vg ) of 7, 14, 20, and 25 l/min. The levels for each factor were chosen based on previous experiments, which were conducted primarily to determine the limitations of the operation of the HPT. The execution order of the trials was randomly determined to avoid systematic errors. Additionally, each trial was performed in triplicate to reduce error, generating an experimental matrix of 192 trials. The effects of the physics variables (factors) on the responses (plasma jet thruster and length) were determined by analysis of variance (ANOVA).
A benchtop system with a displacement device was used to conduct the welding, cutting and surface hardening trials. This benchtop consists of the following equipment: power sources, a displacement system, brackets to secure the torch and cables, a gas cylinder, gas regulators, a control system, and other tools. Fig. 4a displays a schematic of the bench used in the trials, and Fig. 4b displays the hybrid plasma torch in operation during a welding trial. The welding trials were performed using two plate materials (SAE 1020 and AISI 304) at various thicknesses (0.7–6.4 mm). The plates were fastened and centred under the plasma jet. The following welding parameters were used: 5 mm distance between the nozzle and the work piece; argon gas flow rates between 7 l/min and 25 l/min; electric current intensity ranges from primary and secondary power sources from 30 to 60 A and 100 to 250 A, respectively; and welding speed between 1.7 mm/s and 8.3 mm/s. The cutting trials with the hybrid plasma torch were conducted using various plate thicknesses (0.5–4 mm) and materials (aluminium and various types of steel). Argon gas was injected into the device chamber at a flow rate greater than 20 l/min. A 5 mm distance was maintained between the plasma jet output nozzle and the work piece. Different torch displacement speeds and electric current intensities (primary and secondary source) were applied. Surface hardening trials were conducted using cylindrical AISI 52100 steel (spheroidised) with 15 mm height and 76.2 mm diameter. As in the welding and cutting trials, the surface hardening trials were conducted with different parameters. The micro hardness profiles were taken using a Shimadzu micro hardness tester, model M-92 080, with a 50 g load and a time of 15 s. These profiles were performed on the surface of the heated region, and micro hardness
2.3. Measurement system Fig. 3 shows a schematic diagram of the measurement setup for electrical, thruster and length diagnostic. The jet lengths were measured by taking photographs with a scale, which required a CCD camera. The images were processed using “ImageJ” software. In order to measure thruster, the HPT was rigidly anchored, keeping the nozzle centralised with the dynamometer. To avoid heating the electronic dynamometer, thermal isolation was inserted in it, and a steel sheet, respectively 30 and 12 mm thick. The distance between the nozzle and the steel sheet was 40 mm. For each trial, initially the device was operated without ionizing the gas, and the thruster of the gas expelled from the chamber was
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Fig. 3. Schematic diagram of the experimental setup.
values were obtained in 0.1 mm intervals. The metallographic analysis was conducted using an Olympus light microscope, model BX60M. The specimen was sanded, sequentially polished with alumina, and chemically attacked with Nital (1% concentration). 3. Results and discussion Three types of results were obtained for the HPT, i.e., one concerning to its measurement of plasma jet thruster, other referring to its jet length and the last results concerning its application to welding, cutting, and surface hardening. 3.1. HPT thruster The results obtained for HPT thruster checked the influence of the following items: primary electric current intensity; secondary electric current intensity and gas flow rate on the exhaust of the plasma jet from the chamber. These results were analyzed using ANOVA and graphs, in which an exponential tendency line (“best fit”) was plotted.
Table 1 displays the analysis of variance results, at a 95% confidence level, for the plasma thruster. From this analysis, it was clear that the P-values were less than 0.05; consequently, it can be said with 95% certainty that all factors had a significant influence on the plasma thruster. Additionally, the same factors that had the greatest influence on the plasma thruster can be determined by the F values, where a greater F value indicates greater influence. Thus, in descending order, the most influential factors on the plasma thruster were the secondary electric current intensity, the gas flow rate and primary electric current intensity. Figs. 5–8 show the thruster as a function of the secondary electric current intensity for different gas flow rates. For these trials, primary electric current intensity was used between the 30, 40, 50 and 60 A, respectively. The graphs show that the thruster, on average, increased significantly with the secondary electric current intensity and gas flow rate as well. Figs. 9–12 show the thruster as a function of the primary electric current intensity for different gas flow rates. For these trials, secondary electric current intensity was used between the 100, 150, 200 and 250 A, respectively. These graphs show that, on average,
Fig. 4. (a) Bench station used during trials for welding, cutting, and surface hardening. (b) HPT running in the welding trials.
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Table 1 Analysis of variance (mean effects ANOVA) for HPT thruster. Source
Sum of square
Degree of freedom
Mean of square
F ratio
P-value
IP IS Vg Error
128.64 2686.05 1675.13 338.50
3 3 3 182
42.88 895.35 558.38 1.86
23.005 481.404 300.223 –
0.0000 0.0000 0.0000 –
Total
4828.32
191
–
–
–
Fig. 5. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 30 A.
Fig. 8. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 60 A.
Fig. 6. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 40 A.
Fig. 9. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 100 A.
the thruster increased slightly with the primary electric current intensity increase, except for trials performed with 7 l/min gas flow rate and secondary electric current intensity of 200 A, where the thruster decrease with primary electric current intensity. Possibly, some primary electric arc instability provided the exception behavior. The results obtained in the plasma thruster characterisation demonstrated that the thruster was slightly higher than values obtained by Burton et al. (1983) model at the same operational
intervals. This occurred because the ionised gas was accelerated twice by primary and secondary electric arc, which an electromagnetic and, mainly, electro thermal contribution was observed. Also, according to the mathematic model described by Tikhonov et al. (1993) and experimental results obtained by Machado and Lermen (2008), if the gas flow rate is increased and the current intensity is kept constant, the thruster will also increase. For the same model, if only current intensity is increased, the thruster will also increase.
Fig. 7. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 50 A.
Fig. 10. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 150 A.
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Fig. 11. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 200 A.
Fig. 14. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 30 A.
Fig. 12. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 250 A.
Fig. 15. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 40 A.
3.2. Plasma jet length The plasma jet length was determined by standard distance shown in Fig. 13a. On the other hand, Fig. 13b–e displays a sequence of photos, in which the lengths y1 . . .y4 of plasma jet length are represented. These plasma jets were acquired with the following parameters: a primary electric current of 30 A; a secondary electric current of 200 A; and gas flow rates of 7 l/min (Fig. 13b), 14 l/min (Fig. 13c), 20 l/min (Fig. 13d), and 25 l/min (Fig. 13e). In these images, it is evident that the plasma jet length decreases with increasing gas flow rate, and that the plasma jet flow undergoes a transition from laminar to turbulent (Pan et al., 2002). This transition was observed in all trials. Table 2 displays the analysis of variance results, at a 95% confidence level, for the plasma jet length. From this analysis, it was clear that the P-values were less than 0.05; consequently, it can be said with 95% certainty that all factors and interactions between factors had a significant influence on the plasma jet length. Additionally, the same factors and interactions that had the greatest influence on the plasma jet length can be determined by the F values, where a greater F value indicates greater influence. Thus, the
Fig. 16. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 50 A.
most influential to least factors on the plasma jet length were secondary electric current intensity, gas flow rate and primary electric current intensity. Figs. 14–17 display the plasma jet length as a function of secondary current intensity for different gas flow rates. For these trials, primary electric current intensity was used between the 30, 40,
Fig. 13. (a) Standard distance for plasma jet length measurement. (b)–(e) Plasma jets from the chamber, indicating their respective lengths.
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Table 2 Analysis of variance (mean effects ANOVA) for plasma jet length. Source
Sum of square
Degree of freedom
IP IS Vg Error
639.9 18,449.3 16,315.2 9139.3
3 3 3 182
Total
44,543.7
191
Fig. 17. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 60 A.
Fig. 18. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 100 A.
Mean of square 213.3 6149.8 5438.4 50.2 –
F ratio
P-value
4.248 122.466 108.30 –
0.0063 0.0000 0.0000 –
–
–
Fig. 20. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 200 A.
According to Pan et al. (2002, 2005) the relationship between plasma jet length and gas flow rate, keeping constant the electric current intensities (primary and secondary) indicates that there could exist a critical Reynolds number at which the flow transition between laminar and turbulent state occurs. This phenomenon suggests the existence of the critical Reynolds number, because Reynolds number is directly proportional to the mass flow rate and inversely proportional to the viscosity of the flow. That is, increase of the arc current causes the rise of gas temperature at a given gas flow rate, and thus the rise of viscosity of argon gas, which allows appropriate increasing of the gas flow rate to keep the Reynolds number lower than the critical transition value and to keep the plasma in laminar flow state. 3.3. Applying the HPT in welding, cutting and hardening surface
50 and 60 A, respectively. These graphs show that the plasma jet length increased with increasing secondary electric current intensity, decreased with increasing gas flow rate. Figs. 18-21 display the plasma jet length as a function of primary current intensity for different gas flow rates. For these trials, primary electric current intensity was used between the 100, 150, 200 and 250 A, respectively. These graphs show, on average, that the plasma jet length varied slightly with primary electric current intensity.
Fig. 22 displays the top view of plates subjected to welding with the hybrid plasma torch, where the numbers indicated in the photographs correspond to the welding trial numbers displayed in Table 3, which also shows the welding parameters used. In all trials, a 5 mm distance was maintained between the nozzle and the workpiece, and the primary electric current was 60 A. It was possible to use the device in sheet metal manufacturing with gas flow rates equal to or less than 7 l/min because with flows above 7 l/min,
Fig. 19. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 150 A.
Fig. 21. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 250 A.
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Fig. 22. Top view of the steel plates subjected to welding.
Fig. 23. Side view of steel plate cut with the hybrid plasma torch.
Table 3 Experimental data of welding tests using hybrid plasma torch. Parameters
IS (A) Vg (l/min) Welding speed (mm/s) Types of steels Sheets dimensions (mm)
Trials 1
2
3
4
5
200 7 1.7 AISI 304 100 × 50 × 1.5
200 14 1.7 AISI 304 100 × 50 × 1.5
200 7 3.4 AISI 304 100 × 50 × 1.5
200 7 1.7 AISI 304 100 × 50 × 0.7
250 25 1.7 SAE 1020 200 × 50 × 6.4
the plasma jet became turbulent and formed irregular welds (trials 2 and 5). Trial 3 exhibited some fusion defects because of the high welding speed used for this torch. The cuts made with the device exhibited irregularities because the plasma jet did not have enough power to drive the molten metal, and it was not possible to cut plates thicker than 3 mm. Fig. 23 displays a section of a 1.5 mm thick AISI 304 steel sheet cut with the hybrid plasma torch generator. The cut was made with the following parameters: primary and secondary electric current intensities of 60 A and 250 A, respectively; gas flow of 25 l/min; cutting speed of 8.4 mm/s; and 5 mm distance maintained between the nozzle and the workpiece. The results of the surface hardening process with the hybrid torch can be observed in Fig. 24, which shows a graph of micro hardness in relation to the distance from the incidence surface
Fig. 24. Graph of microhardness in relation to distance from the incidence surface of the plasma jet and cross-sectional macrographs of the heat-treated region.
to the plasma jet and a cross-sectional macrograph of the heattreated area. For this trial, the following parameters were used: primary and secondary electric current intensities of 60 A and 250 A, respectively; gas flow rate of 7 l/min; torch displacement speed of 1.7 mm/s; and 5 mm distance between the nozzle and the work piece. The thermally altered region reached a penetration and area of approximately 0.74 mm and 2.06 mm2 , respectively. Evidence of surface hardening was obtained by micro hardness testing, in which the base metal and heat-treated region exhibited values of approximately 250 Vickers and 700 Vickers, respectively. 4. Conclusions According to the results obtained in the trials and analyses conducted, the following conclusions can be drawn: • A new hybrid plasma torch was designed, developed and improved. This torch is characterised by the simultaneous formation of two plasma arcs in just one device, which result of the synergistic combination of two plasma-generating processes, i.e., non-transferred-arc plasma and magnetoplasmadynamic thruster. • The results obtained for the application of the hybrid torch to manufacturing processes were satisfactory; it was possible to perform welding, cutting and surface hardening with the instrument. However, optimisation studies of these processes should be conducted with this device. • The three factors (primary and secondary electric current intensities and gas flow rate) had significant influence, with 95% reliability, on the thruster and plasma jet length. The thruster increased with increasing electric current intensities (primary and secondary) and gas flow rate. • The plasma jet length increased with increasing secondary electric current intensity and varied slightly with primary electric current intensity. As the gas flow was increased from 7 l/min to 25 l/min, the plasma jet flow shifted from laminar to turbulent,
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reducing the plasma jet length. This gas flow rate range is a transition region for the plasma jet type, i.e., the ionised gas flow shifts from laminar to turbulent. References Browning, J.A., 1986. Hybrid non-transferred-arc plasma torch system and method of operating same. United States patent 4, 626,648, 2. Burton, R.L., Clark, K.E., Jahn, R.G., 1983. Measured performance of the multimegawatt MPD thruster. Journal Espacecraft and Rockets 20, 299–304. Choueiri, E.Y., 1998. The scaling of thrust in self-field MPD thrusters. Journal of Propulsion and Power 14, 744–753. Essers, W.G., Willems, G.A.M., Buelens, J.J.C., VanGompel, M.R.M., 1981. PlasmaMIG welding – a new torch and arc starting method. Metal Construction 13, 36–42. Garrigues, L., Coche, P., 2011. Electric propulsion: comparisons between different concepts. Plasma Physics and Controlled Fusion 53, 1–11. Jahn, R.G., 1968. Physics of Electric Propulsion. McGraw-Hill, New York, pp. 196–256. Kou, S., 2002. Welding Metallurgy. John Wiley & Sons, New Jersey, pp.16–19. Lermen, R.T., 2011. Development of a hybrid plasma torch for materials processing (in portuguese). Doctoral Thesis. Federal University of Rio Grande do Sul (UFRGS).
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Machado, I.G., Lermen, R.T., 2008. Development and implementation of a magnetoplasmadynamic thruster for welding and cutting. Journal of Materials Processing Technology 200, 398–404. Messler, R.W.J., 2004. What’s next for hybrid welding. Welding Journal 83, 30–34. Pan, W., Zhang, W., Ma, W., Wu, C., 2002. Characteristics of argon laminar DC plasma jet at atmospheric pressure. Plasma Chemistry and Plasma Processing 22, 271–283. Pan, W.X., Li, G., Meng, X., Ma, W., Wu, C.K., 2005. Laminar plasma jets: generation, characterization, and applications for materials surface processing. Pure and Applied Chemistry 77, 373–378. Roth, J.R., 1995. Industrial Plasma Engineering. Volume 1: Principles, Institute of Physics Publishing, Philadelphia, pp. 1–16. Sankaran, K., 2005. Simulation of plasma flows in self-field Lorentz force accelerators. PhD Thesis. Princeton University. Solonenko, O.P., 2003. Thermal Plasma Torches and Technologies. Cambridge International Science Publishing, UK, pp. 42–61. Tikhonov, V.B., Semenihin, S.A., Alexandrov, V.A., Dyakonov, G.A., Popov, G.A., 1993. Research of plasma acceleration processes in self-field and applied magnetic field thrusters. In: 23rd International Electric Propulsion Conference, Seattle, WA, USA, IEPC-93-076. Tu, X., Yu, L., Yan, J., Cen, K., Chéron, B., 2008. Heat flux characteristics in an atmospheric double arc argon plasma jet. Applied Physics Letters 93, 1–3. Welding Handbook, 1991. vol. II – Welding Process. 8th ed., American Welding Society, Miami, 329–350 and 482–489.
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Development of a new hybrid plasma torch for materials processing Richard Thomas Lermen ∗ , Ivan Guerra Machado Federal University of Rio Grande do Sul – UFRGS, PPGE3M/LS&TC, Porto Alegre, Rio Grande do Sul, Brazil
a r t i c l e
i n f o
Article history: Received 27 April 2012 Received in revised form 30 May 2012 Accepted 19 June 2012 Available online 26 June 2012 Keywords: Hybrid plasma torch Plasma thruster Plasma jet length Welding Cutting Surface hardening
a b s t r a c t The main objectives of this study were to construct a plasma generator device and to investigate its applications in welding, cutting, and surface hardening. The device was derived from the union of two plasma-generating technologies, non-transferred-arc plasma and magnetoplasmadynamic thruster (MPDT), and characterised by the simultaneous formation of two plasma arcs in one device, generating a plasma jet with high energy density. Initially, trials were conducted to analyse the influence of the physical variables (gas flow rate and electric current intensities – primary and secondary) on the plasma jet, for which the thruster and the length of the plasma jet expulsed from the chamber were determined. The relevant parameters for welding, cutting and surface hardening were determined by trial and error, in which the trials were conducted using various plate thicknesses and materials. The results have shown that this device can be used for welding, cutting and surface hardening. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermal plasma generated by plasma torches was first used for technological applications in the 1960s. This technology has recently become highly promising because of its wide range of applications and status as a low-pollution process (Roth, 1995). Within the field of plasma torches, hybrid plasma generator torches are characterised by their simultaneous use of two or more processes within a single device (Messler, 2004). Examples of hybrid torches include the plasma/MIG welding torch developed by Essers et al. (1981), dual anode plasma torch described by Tu et al. (2008), hybrid non-transferred-arc plasma torch presented by Browning (1986) through of a patent, DC/HF hybrid plasma torch have been developed by Professor T. Yoshida at the University of Tokyo in Japan in the late seventy and early eighty (as quoted by Solonenko, 2003), and others. These torches are generally constructed to gain a particular advantage over conventional manufacturing processes or a different set of attributes. These torches generate plasma jets that provide high temperature gas flows, usually with a power density of approximately 108 W/m2 . It is possible to reach the melting or vapourisation points of almost all solid materials with these plasma jets, and their applications in welding, cutting, and hardening are well represented in the literature by Welding Handbook (1991), Kou (2002) and
∗ Corresponding author. Tel.: +55 55 3537 6428; fax: +55 55 3537 1614. E-mail addresses: [email protected], [email protected] (R.T. Lermen), [email protected] (I.G. Machado). 0924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2012.06.009
Pan et al. (2005). Further these applications, the development of electric thrusters, which are classified as electrothermal thrusters (Arcjets), electrostatic thrusters (Ion Thrusters) and electromagnetic thrusters (magnetoplasmadynamic thrusters – MPDT), are well referred by Jahn (1968), Burton et al. (1983), Choueiri (1998) and, recently, by Garrigues and Coche (2011). According to Jahn (1968) the MPDT is characterised by a coaxial geometry constituted by a central cathode and a cylindrical anode, in the form of a chamber, which are electrically isolated. Gas is injected though orifices generating gas flow rate in the chamber, where it is ionised by passing through the electric discharge between the electrodes (cathode and anode), generated by an energy source with a high frequency device. Through this plasma (ionised gas) comes a radial current density which crosses the gas in direction to cathode. The electric current in the cathode generates a circumferential magnetic field (B), which interacts with electric current density (j), originating an electromagnetic force (j∧ B), which accelerates the particles and expels ionised gas from the chamber. Physical models are generally used to describe the behavior of this electromagnetic device. An analytic model based on the continuous description of electromagnetic propulsion (thruster) was developed by Maecker (as quoted by Sankaran, 2005), and later explained by Jahn (1968). The model, described by Tikhonov et al. (1993), improves Maecker’s formula. It is obtained from a quasi1-D MHD (magnetohydrodynamic) analytical model that allows the free boundary of the flow to vary consistently with the flow conditions. Moreover, the model described by Choueiri (1998) also improves Maecker’s formula, which takes into account variations
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in the electric current and in the type of gas. The last model is semi-empirical, since some experimental data must be considered for current distribution among the electrodes and the pressure distribution in the internal chamber of the MPDT. According to Burton et al. (1983), the electrothermal contribution, which is not described by previous models and is typically less than 10% of the total thrust for electromagnetic device, depends on the type of gas, gas flow rate and the electrical current. Recently, Machado and Lermen (2008) developed, produced and improved a magnetoplasmadynamic thruster for welding and cutting of metallic materials. However, this device has not yet submitted welds and cuts with qualities comparable to conventional processes (GTAW, GMAW, etc.). In the absence of sufficient information available about the capacity of MPDT, when coupled with other plasma generator processes and technologies of power sources, a new hybrid plasma generator device was developed, which involves the MPDT process and non-transferred-arc plasma process (Lermen, 2011). For this new hybrid plasma torch (“Patent Pending”), the scientific knowledge related to the characteristics of the plasma jet (thruster, plasma jet length, etc.) and application in manufacturing processes (welding, cutting, surface hardening, etc.) have not been reported, even in a superficial way. However, the main purpose of this study is to describe the design, characterisation and application of the new hybrid plasma, where the results are fundamental to the scientific-technological development of this subject. 2. Materials and methods Three stages of materials and methods are presented, i.e., firstly the hybrid plasma torch project and operation; after the plasma thruster and jet length measurement, and finally, the hybrid plasma torch application (materials processing). The goal of this last stage was not to optimise procedures with the new device, but rather to check their suitability for welding, cutting and surface hardening. 2.1. Hybrid plasma torch – HPT The new hybrid plasma torch is characterised by the simultaneous formation of two plasma arcs in just one device, thus generating a plasma jet of high energy density. This energetic torch jet is derived from the union of two plasma-generating technologies: non-transferred-arc plasma and magnetoplasmadynamic thruster. This torch has a coaxial geometry that consists of two cathodes (primary and secondary) and two cylindrical anodic chambers, which are electrically isolated through alumina ceramic flanges.
Initially, a gas is injected through holes, generating a flow in the chamber, where the gas is heated and ionised by passage through the electrical discharge between the primary anode and the primary cathode. This electrical discharge is generated by a high-frequency primary energy source. A radial current density is passed through the resulting plasma (ionised gas), and it runs through the gas towards the primary cathode. The current in the primary cathode generates an induced circumferential magnetic field, which interacts with the current density to produce a Lorentz force (jP ∧ BP ) interaction, that accelerates the gas particles, forming plasma chamber. The plasma acts as an electrical conductor, and another electrical discharge is generated by secondary energy source between the secondary cathode and the secondary anode in the chamber. A second radial current density arises, running through the plasma towards the secondary cathode. The current in the secondary cathode also generates an induced circumferential magnetic field, which interacts with the secondary current density. The secondary interaction produces another electromagnetic force (jS ∧ BS ), which ionises and further accelerates the plasma jet particles produced in the primary ionisation, whilst also expelling the plasma jet from the chamber. Therefore, in this system, the gas is accelerated twice, i.e., first by the electromagnetic force and thermal expansion between the primary electrodes and then by the resulting electromagnetic force and thermal expansion between the secondary electrodes. A schematic of the hybrid plasma torch with lines representing the electric current density, circumferential magnetic fields and electric arcs is shown in Fig. 1. A sectional view of hybrid plasma torch developed by the Laboratory of Welding & Related Techniques (LS&TC) is shown in Fig. 2. This device mainly consists of primary and secondary anodes and cathodes. The secondary cathode is an AWS EWTh-2 tungsten electrode with a diameter of 3.2 mm, a length of 150 mm, and an end tapered at 60◦ ; the primary cathode is a ring-shaped copper electrode. The primary and secondary anodes are both copper and chamber-shaped. The electrodes (cathodes and anodes) are fixed in water cooling chambers by flange fasteners and electrically insulated from each other by flange-shaped ceramic insulators. The cooling chambers are embedded in copper flanges. Teflon bushings prevent electrical discharges between the bolts and the flange fasteners. The dimensions of the torch components were presented by Lermen (2011) in recent Doctoral Thesis. Argon is used as working gas and injected axially into the torch discharge chamber via the gas inlets near the primary electrodes, and gas flow rates of up to 25 l/min were possible. The gas flow rate was controlled by a numerical mass flow meter.
Fig. 1. Schematic drawing with electrical current density lines, circumferential magnetic fields and electric arcs.
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Fig. 2. Sectional view of HPT constructed in the LS&TC.
Two constant current “drooping” power sources were used to run the HPT. These sources are unique because of their electric current ranges and application, i.e., the primary power source had an electrical current range between 0 and 70 A with an HF generator (5 kHz); it is used for thermal cutting by plasma. The secondary power source has an electrical current range between 0 and 400 A and is used for GTAW welding. The electrical operating conditions are typically 20–30 V/30–60 A for the first arc, and 15–25 V/100–250 A for the second arc. In order to maintain the plasma torch in a stationary state at a reasonable temperature and to limit the electrode wear, four parts of the plasma torch (primary cathode, primary anode, secondary cathode and secondary anode) are water cooled independently.
measured; later the device was operated (two plasma arcs) ionizing the gas, and the thruster of the ionised gas (plasma) expelled from the chamber was measured. The difference between the thrusters (non-ionised gas and plasma) multiplied by the local gravitational field intensity supplies the approximate value of the thruster of this device. The experimental apparatus was in a closed place, isolating the system from external forces that could affect the stabilisation of the dynamometer. The arc voltage and current intensity were measured simultaneously by using four channels data acquisition board (NI USB 6009, 48 MHz, 14 bit) connected to the computer. Each electrical signal was recorded at 10 MHz acquisition rate and intervals for a sampling duration of 20 s.
2.2. Design of experiments
2.4. Welding, cutting and surface hardening with HPT
Experiments to determine the plasma jet thruster and length were developed by the complete factorial statistical method. This experiment design depends on three factors with four levels each, i.e., the experiments were performed with primary electric current intensities (IP ) of 30, 40, 50, and 60 A; secondary electric current intensities (IS ) of 100, 150, 200, and 250 A; and argon gas flow rates (Vg ) of 7, 14, 20, and 25 l/min. The levels for each factor were chosen based on previous experiments, which were conducted primarily to determine the limitations of the operation of the HPT. The execution order of the trials was randomly determined to avoid systematic errors. Additionally, each trial was performed in triplicate to reduce error, generating an experimental matrix of 192 trials. The effects of the physics variables (factors) on the responses (plasma jet thruster and length) were determined by analysis of variance (ANOVA).
A benchtop system with a displacement device was used to conduct the welding, cutting and surface hardening trials. This benchtop consists of the following equipment: power sources, a displacement system, brackets to secure the torch and cables, a gas cylinder, gas regulators, a control system, and other tools. Fig. 4a displays a schematic of the bench used in the trials, and Fig. 4b displays the hybrid plasma torch in operation during a welding trial. The welding trials were performed using two plate materials (SAE 1020 and AISI 304) at various thicknesses (0.7–6.4 mm). The plates were fastened and centred under the plasma jet. The following welding parameters were used: 5 mm distance between the nozzle and the work piece; argon gas flow rates between 7 l/min and 25 l/min; electric current intensity ranges from primary and secondary power sources from 30 to 60 A and 100 to 250 A, respectively; and welding speed between 1.7 mm/s and 8.3 mm/s. The cutting trials with the hybrid plasma torch were conducted using various plate thicknesses (0.5–4 mm) and materials (aluminium and various types of steel). Argon gas was injected into the device chamber at a flow rate greater than 20 l/min. A 5 mm distance was maintained between the plasma jet output nozzle and the work piece. Different torch displacement speeds and electric current intensities (primary and secondary source) were applied. Surface hardening trials were conducted using cylindrical AISI 52100 steel (spheroidised) with 15 mm height and 76.2 mm diameter. As in the welding and cutting trials, the surface hardening trials were conducted with different parameters. The micro hardness profiles were taken using a Shimadzu micro hardness tester, model M-92 080, with a 50 g load and a time of 15 s. These profiles were performed on the surface of the heated region, and micro hardness
2.3. Measurement system Fig. 3 shows a schematic diagram of the measurement setup for electrical, thruster and length diagnostic. The jet lengths were measured by taking photographs with a scale, which required a CCD camera. The images were processed using “ImageJ” software. In order to measure thruster, the HPT was rigidly anchored, keeping the nozzle centralised with the dynamometer. To avoid heating the electronic dynamometer, thermal isolation was inserted in it, and a steel sheet, respectively 30 and 12 mm thick. The distance between the nozzle and the steel sheet was 40 mm. For each trial, initially the device was operated without ionizing the gas, and the thruster of the gas expelled from the chamber was
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Fig. 3. Schematic diagram of the experimental setup.
values were obtained in 0.1 mm intervals. The metallographic analysis was conducted using an Olympus light microscope, model BX60M. The specimen was sanded, sequentially polished with alumina, and chemically attacked with Nital (1% concentration). 3. Results and discussion Three types of results were obtained for the HPT, i.e., one concerning to its measurement of plasma jet thruster, other referring to its jet length and the last results concerning its application to welding, cutting, and surface hardening. 3.1. HPT thruster The results obtained for HPT thruster checked the influence of the following items: primary electric current intensity; secondary electric current intensity and gas flow rate on the exhaust of the plasma jet from the chamber. These results were analyzed using ANOVA and graphs, in which an exponential tendency line (“best fit”) was plotted.
Table 1 displays the analysis of variance results, at a 95% confidence level, for the plasma thruster. From this analysis, it was clear that the P-values were less than 0.05; consequently, it can be said with 95% certainty that all factors had a significant influence on the plasma thruster. Additionally, the same factors that had the greatest influence on the plasma thruster can be determined by the F values, where a greater F value indicates greater influence. Thus, in descending order, the most influential factors on the plasma thruster were the secondary electric current intensity, the gas flow rate and primary electric current intensity. Figs. 5–8 show the thruster as a function of the secondary electric current intensity for different gas flow rates. For these trials, primary electric current intensity was used between the 30, 40, 50 and 60 A, respectively. The graphs show that the thruster, on average, increased significantly with the secondary electric current intensity and gas flow rate as well. Figs. 9–12 show the thruster as a function of the primary electric current intensity for different gas flow rates. For these trials, secondary electric current intensity was used between the 100, 150, 200 and 250 A, respectively. These graphs show that, on average,
Fig. 4. (a) Bench station used during trials for welding, cutting, and surface hardening. (b) HPT running in the welding trials.
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Table 1 Analysis of variance (mean effects ANOVA) for HPT thruster. Source
Sum of square
Degree of freedom
Mean of square
F ratio
P-value
IP IS Vg Error
128.64 2686.05 1675.13 338.50
3 3 3 182
42.88 895.35 558.38 1.86
23.005 481.404 300.223 –
0.0000 0.0000 0.0000 –
Total
4828.32
191
–
–
–
Fig. 5. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 30 A.
Fig. 8. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 60 A.
Fig. 6. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 40 A.
Fig. 9. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 100 A.
the thruster increased slightly with the primary electric current intensity increase, except for trials performed with 7 l/min gas flow rate and secondary electric current intensity of 200 A, where the thruster decrease with primary electric current intensity. Possibly, some primary electric arc instability provided the exception behavior. The results obtained in the plasma thruster characterisation demonstrated that the thruster was slightly higher than values obtained by Burton et al. (1983) model at the same operational
intervals. This occurred because the ionised gas was accelerated twice by primary and secondary electric arc, which an electromagnetic and, mainly, electro thermal contribution was observed. Also, according to the mathematic model described by Tikhonov et al. (1993) and experimental results obtained by Machado and Lermen (2008), if the gas flow rate is increased and the current intensity is kept constant, the thruster will also increase. For the same model, if only current intensity is increased, the thruster will also increase.
Fig. 7. Thruster as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 50 A.
Fig. 10. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 150 A.
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Fig. 11. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 200 A.
Fig. 14. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 30 A.
Fig. 12. Thruster as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 250 A.
Fig. 15. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 40 A.
3.2. Plasma jet length The plasma jet length was determined by standard distance shown in Fig. 13a. On the other hand, Fig. 13b–e displays a sequence of photos, in which the lengths y1 . . .y4 of plasma jet length are represented. These plasma jets were acquired with the following parameters: a primary electric current of 30 A; a secondary electric current of 200 A; and gas flow rates of 7 l/min (Fig. 13b), 14 l/min (Fig. 13c), 20 l/min (Fig. 13d), and 25 l/min (Fig. 13e). In these images, it is evident that the plasma jet length decreases with increasing gas flow rate, and that the plasma jet flow undergoes a transition from laminar to turbulent (Pan et al., 2002). This transition was observed in all trials. Table 2 displays the analysis of variance results, at a 95% confidence level, for the plasma jet length. From this analysis, it was clear that the P-values were less than 0.05; consequently, it can be said with 95% certainty that all factors and interactions between factors had a significant influence on the plasma jet length. Additionally, the same factors and interactions that had the greatest influence on the plasma jet length can be determined by the F values, where a greater F value indicates greater influence. Thus, the
Fig. 16. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 50 A.
most influential to least factors on the plasma jet length were secondary electric current intensity, gas flow rate and primary electric current intensity. Figs. 14–17 display the plasma jet length as a function of secondary current intensity for different gas flow rates. For these trials, primary electric current intensity was used between the 30, 40,
Fig. 13. (a) Standard distance for plasma jet length measurement. (b)–(e) Plasma jets from the chamber, indicating their respective lengths.
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Table 2 Analysis of variance (mean effects ANOVA) for plasma jet length. Source
Sum of square
Degree of freedom
IP IS Vg Error
639.9 18,449.3 16,315.2 9139.3
3 3 3 182
Total
44,543.7
191
Fig. 17. Plasma jet length as a function of secondary electric current intensity for different gas flow rates. Primary electric current intensity of 60 A.
Fig. 18. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 100 A.
Mean of square 213.3 6149.8 5438.4 50.2 –
F ratio
P-value
4.248 122.466 108.30 –
0.0063 0.0000 0.0000 –
–
–
Fig. 20. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 200 A.
According to Pan et al. (2002, 2005) the relationship between plasma jet length and gas flow rate, keeping constant the electric current intensities (primary and secondary) indicates that there could exist a critical Reynolds number at which the flow transition between laminar and turbulent state occurs. This phenomenon suggests the existence of the critical Reynolds number, because Reynolds number is directly proportional to the mass flow rate and inversely proportional to the viscosity of the flow. That is, increase of the arc current causes the rise of gas temperature at a given gas flow rate, and thus the rise of viscosity of argon gas, which allows appropriate increasing of the gas flow rate to keep the Reynolds number lower than the critical transition value and to keep the plasma in laminar flow state. 3.3. Applying the HPT in welding, cutting and hardening surface
50 and 60 A, respectively. These graphs show that the plasma jet length increased with increasing secondary electric current intensity, decreased with increasing gas flow rate. Figs. 18-21 display the plasma jet length as a function of primary current intensity for different gas flow rates. For these trials, primary electric current intensity was used between the 100, 150, 200 and 250 A, respectively. These graphs show, on average, that the plasma jet length varied slightly with primary electric current intensity.
Fig. 22 displays the top view of plates subjected to welding with the hybrid plasma torch, where the numbers indicated in the photographs correspond to the welding trial numbers displayed in Table 3, which also shows the welding parameters used. In all trials, a 5 mm distance was maintained between the nozzle and the workpiece, and the primary electric current was 60 A. It was possible to use the device in sheet metal manufacturing with gas flow rates equal to or less than 7 l/min because with flows above 7 l/min,
Fig. 19. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 150 A.
Fig. 21. Plasma jet length as a function of primary electric current intensity for different gas flow rates. Secondary electric current intensity of 250 A.
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Fig. 22. Top view of the steel plates subjected to welding.
Fig. 23. Side view of steel plate cut with the hybrid plasma torch.
Table 3 Experimental data of welding tests using hybrid plasma torch. Parameters
IS (A) Vg (l/min) Welding speed (mm/s) Types of steels Sheets dimensions (mm)
Trials 1
2
3
4
5
200 7 1.7 AISI 304 100 × 50 × 1.5
200 14 1.7 AISI 304 100 × 50 × 1.5
200 7 3.4 AISI 304 100 × 50 × 1.5
200 7 1.7 AISI 304 100 × 50 × 0.7
250 25 1.7 SAE 1020 200 × 50 × 6.4
the plasma jet became turbulent and formed irregular welds (trials 2 and 5). Trial 3 exhibited some fusion defects because of the high welding speed used for this torch. The cuts made with the device exhibited irregularities because the plasma jet did not have enough power to drive the molten metal, and it was not possible to cut plates thicker than 3 mm. Fig. 23 displays a section of a 1.5 mm thick AISI 304 steel sheet cut with the hybrid plasma torch generator. The cut was made with the following parameters: primary and secondary electric current intensities of 60 A and 250 A, respectively; gas flow of 25 l/min; cutting speed of 8.4 mm/s; and 5 mm distance maintained between the nozzle and the workpiece. The results of the surface hardening process with the hybrid torch can be observed in Fig. 24, which shows a graph of micro hardness in relation to the distance from the incidence surface
Fig. 24. Graph of microhardness in relation to distance from the incidence surface of the plasma jet and cross-sectional macrographs of the heat-treated region.
to the plasma jet and a cross-sectional macrograph of the heattreated area. For this trial, the following parameters were used: primary and secondary electric current intensities of 60 A and 250 A, respectively; gas flow rate of 7 l/min; torch displacement speed of 1.7 mm/s; and 5 mm distance between the nozzle and the work piece. The thermally altered region reached a penetration and area of approximately 0.74 mm and 2.06 mm2 , respectively. Evidence of surface hardening was obtained by micro hardness testing, in which the base metal and heat-treated region exhibited values of approximately 250 Vickers and 700 Vickers, respectively. 4. Conclusions According to the results obtained in the trials and analyses conducted, the following conclusions can be drawn: • A new hybrid plasma torch was designed, developed and improved. This torch is characterised by the simultaneous formation of two plasma arcs in just one device, which result of the synergistic combination of two plasma-generating processes, i.e., non-transferred-arc plasma and magnetoplasmadynamic thruster. • The results obtained for the application of the hybrid torch to manufacturing processes were satisfactory; it was possible to perform welding, cutting and surface hardening with the instrument. However, optimisation studies of these processes should be conducted with this device. • The three factors (primary and secondary electric current intensities and gas flow rate) had significant influence, with 95% reliability, on the thruster and plasma jet length. The thruster increased with increasing electric current intensities (primary and secondary) and gas flow rate. • The plasma jet length increased with increasing secondary electric current intensity and varied slightly with primary electric current intensity. As the gas flow was increased from 7 l/min to 25 l/min, the plasma jet flow shifted from laminar to turbulent,
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reducing the plasma jet length. This gas flow rate range is a transition region for the plasma jet type, i.e., the ionised gas flow shifts from laminar to turbulent. References Browning, J.A., 1986. Hybrid non-transferred-arc plasma torch system and method of operating same. United States patent 4, 626,648, 2. Burton, R.L., Clark, K.E., Jahn, R.G., 1983. Measured performance of the multimegawatt MPD thruster. Journal Espacecraft and Rockets 20, 299–304. Choueiri, E.Y., 1998. The scaling of thrust in self-field MPD thrusters. Journal of Propulsion and Power 14, 744–753. Essers, W.G., Willems, G.A.M., Buelens, J.J.C., VanGompel, M.R.M., 1981. PlasmaMIG welding – a new torch and arc starting method. Metal Construction 13, 36–42. Garrigues, L., Coche, P., 2011. Electric propulsion: comparisons between different concepts. Plasma Physics and Controlled Fusion 53, 1–11. Jahn, R.G., 1968. Physics of Electric Propulsion. McGraw-Hill, New York, pp. 196–256. Kou, S., 2002. Welding Metallurgy. John Wiley & Sons, New Jersey, pp.16–19. Lermen, R.T., 2011. Development of a hybrid plasma torch for materials processing (in portuguese). Doctoral Thesis. Federal University of Rio Grande do Sul (UFRGS).
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