Development of a new hybrid plasma torch for materials processing

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Journal of Materials Processing Technology 212 (2012) 2371– 2379

Contents lists available at SciVerse ScienceDirect

Journal of Materials Processing Technology

jou rna l h om epa g e: www.elsev ier .com/ locate / jmatprotec

evelopment of a new hybrid plasma torch for materials processing

ichard Thomas Lermen ∗, Ivan Guerra Machadoederal University of Rio Grande do Sul – UFRGS, PPGE3M/LS&TC, Porto Alegre, Rio Grande do Sul, Brazil

r t i c l e i n f o

rticle history:eceived 27 April 2012eceived in revised form 30 May 2012ccepted 19 June 2012vailable online 26 June 2012

a b s t r a c t

The main objectives of this study were to construct a plasma generator device and to investigate itsapplications in welding, cutting, and surface hardening. The device was derived from the union oftwo plasma-generating technologies, non-transferred-arc plasma and magnetoplasmadynamic thruster(MPDT), and characterised by the simultaneous formation of two plasma arcs in one device, generatinga plasma jet with high energy density. Initially, trials were conducted to analyse the influence of thephysical variables (gas flow rate and electric current intensities – primary and secondary) on the plasma

eywords:ybrid plasma torchlasma thrusterlasma jet lengthelding

uttingurface hardening

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 shownthat this device can be used for welding, cutting and surface hardening.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

Thermal plasma generated by plasma torches was first usedor technological applications in the 1960s. This technology hasecently become highly promising because of its wide range ofpplications and status as a low-pollution process (Roth, 1995).ithin the field of plasma torches, hybrid plasma generator torches

re characterised by their simultaneous use of two or more pro-esses within a single device (Messler, 2004). Examples of hybridorches include the plasma/MIG welding torch developed by Esserst al. (1981), dual anode plasma torch described by Tu et al. (2008),ybrid non-transferred-arc plasma torch presented by Browning1986) through of a patent, DC/HF hybrid plasma torch have beeneveloped by Professor T. Yoshida at the University of Tokyo in

apan in the late seventy and early eighty (as quoted by Solonenko,003), and others. These torches are generally constructed to gain

particular advantage over conventional manufacturing processesr a different set of attributes.

These torches generate plasma jets that provide high temper-ture gas flows, usually with a power density of approximately08 W/m2. It is possible to reach the melting or vapourisation points

∗ Corresponding author. Tel.: +55 55 3537 6428; fax: +55 55 3537 1614.E-mail addresses: richard@fahor.com.br, richard.lermen@ufrgs.br (R.T. Lermen),

elder@ufrgs.br (I.G. Machado).

924-0136/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jmatprotec.2012.06.009

Pan et al. (2005). Further these applications, the development ofelectric thrusters, which are classified as electrothermal thrusters(Arcjets), electrostatic thrusters (Ion Thrusters) and electromag-netic thrusters (magnetoplasmadynamic thrusters – MPDT), arewell 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 coax-ial geometry constituted by a central cathode and a cylindricalanode, in the form of a chamber, which are electrically isolated.Gas is injected though orifices generating gas flow rate in thechamber, where it is ionised by passing through the electric dis-charge between the electrodes (cathode and anode), generated byan energy source with a high frequency device. Through this plasma(ionised gas) comes a radial current density which crosses the gasin direction to cathode. The electric current in the cathode gen-erates a circumferential magnetic field (B), which interacts withelectric current density (j), originating an electromagnetic force(j∧B), which accelerates the particles and expels ionised gas fromthe chamber.

Physical models are generally used to describe the behavior ofthis electromagnetic device. An analytic model based on the con-tinuous description of electromagnetic propulsion (thruster) wasdeveloped by Maecker (as quoted by Sankaran, 2005), and laterexplained by Jahn (1968). The model, described by Tikhonov et al.(1993), improves Maecker’s formula. It is obtained from a quasi-

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372 R.T. Lermen, I.G. Machado / Journal of Materia

n the electric current and in the type of gas. The last model isemi-empirical, since some experimental data must be consideredor current distribution among the electrodes and the pressureistribution in the internal chamber of the MPDT. According tourton et al. (1983), the electrothermal contribution, which is notescribed by previous models and is typically less than 10% of theotal thrust for electromagnetic device, depends on the type of gas,as flow rate and the electrical current.

Recently, Machado and Lermen (2008) developed, producednd improved a magnetoplasmadynamic thruster for welding andutting of metallic materials. However, this device has not yetubmitted welds and cuts with qualities comparable to conven-ional processes (GTAW, GMAW, etc.). In the absence of sufficientnformation available about the capacity of MPDT, when coupled

ith other plasma generator processes and technologies of powerources, a new hybrid plasma generator device was developed,hich involves the MPDT process and non-transferred-arc plasmarocess (Lermen, 2011).

For this new hybrid plasma torch (“Patent Pending”), the sci-ntific knowledge related to the characteristics of the plasma jetthruster, plasma jet length, etc.) and application in manufacturingrocesses (welding, cutting, surface hardening, etc.) have not beeneported, even in a superficial way. However, the main purpose ofhis study is to describe the design, characterisation and applica-ion of the new hybrid plasma, where the results are fundamentalo the scientific-technological development of this subject.

. Materials and methods

Three stages of materials and methods are presented, i.e., firstlyhe hybrid plasma torch project and operation; after the plasmahruster and jet length measurement, and finally, the hybrid plasmaorch application (materials processing). The goal of this last stageas not to optimise procedures with the new device, but rather to

heck their suitability for welding, cutting and surface hardening.

.1. Hybrid plasma torch – HPT

The new hybrid plasma torch is characterised by the simul-aneous formation of two plasma arcs in just one device, thusenerating a plasma jet of high energy density. This energeticorch jet is derived from the union of two plasma-generating tech-ologies: non-transferred-arc plasma and magnetoplasmadynamic

This torch has a coaxial geometry that consists of two cath-des (primary and secondary) and two cylindrical anodic chambers,hich are electrically isolated through alumina ceramic flanges.

Fig. 1. Schematic drawing with electrical current density li

essing Technology 212 (2012) 2371– 2379

Initially, a gas is injected through holes, generating a flow inthe chamber, where the gas is heated and ionised by passagethrough the electrical discharge between the primary anode andthe primary cathode. This electrical discharge is generated by ahigh-frequency primary energy source. A radial current densityis passed through the resulting plasma (ionised gas), and it runsthrough the gas towards the primary cathode. The current in theprimary cathode generates an induced circumferential magneticfield, which interacts with the current density to produce a Lorentzforce (jP∧BP) interaction, that accelerates the gas particles, formingplasma chamber.

The plasma acts as an electrical conductor, and another elec-trical discharge is generated by secondary energy source betweenthe secondary cathode and the secondary anode in the chamber. Asecond radial current density arises, running through the plasmatowards the secondary cathode. The current in the secondarycathode also generates an induced circumferential magnetic field,which interacts with the secondary current density. The secondaryinteraction produces another electromagnetic force (jS∧BS), whichionises and further accelerates the plasma jet particles producedin the primary ionisation, whilst also expelling the plasma jet fromthe chamber.

Therefore, in this system, the gas is accelerated twice, i.e.,first by the electromagnetic force and thermal expansion betweenthe primary electrodes and then by the resulting electromagneticforce and thermal expansion between the secondary electrodes.A schematic of the hybrid plasma torch with lines representingthe electric current density, circumferential magnetic fields andelectric arcs is shown in Fig. 1.

A sectional view of hybrid plasma torch developed by the Labo-ratory of Welding & Related Techniques (LS&TC) is shown in Fig. 2.This device mainly consists of primary and secondary anodes andcathodes. The secondary cathode is an AWS EWTh-2 tungsten elec-trode with a diameter of 3.2 mm, a length of 150 mm, and an endtapered at 60◦; the primary cathode is a ring-shaped copper elec-trode. The primary and secondary anodes are both copper andchamber-shaped. The electrodes (cathodes and anodes) are fixed inwater cooling chambers by flange fasteners and electrically insu-lated from each other by flange-shaped ceramic insulators. Thecooling chambers are embedded in copper flanges. Teflon bush-ings prevent electrical discharges between the bolts and the flangefasteners. The dimensions of the torch components were presentedby Lermen (2011) in recent Doctoral Thesis.

Argon is used as working gas and injected axially into the torchdischarge chamber via the gas inlets near the primary electrodes,and gas flow rates of up to 25 l/min were possible. The gas flow ratewas controlled by a numerical mass flow meter.

nes, circumferential magnetic fields and electric arcs.

R.T. Lermen, I.G. Machado / Journal of Materials Processing Technology 212 (2012) 2371– 2379 2373

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Two constant current “drooping” power sources were used toun the HPT. These sources are unique because of their electricurrent ranges and application, i.e., the primary power sourcead an electrical current range between 0 and 70 A with an HFenerator (5 kHz); it is used for thermal cutting by plasma. Theecondary power source has an electrical current range between

and 400 A and is used for GTAW welding. The electrical operat-ng conditions are typically 20–30 V/30–60 A for the first arc, and5–25 V/100–250 A for the second arc.

In order to maintain the plasma torch in a stationary state at aeasonable temperature and to limit the electrode wear, four partsf the plasma torch (primary cathode, primary anode, secondaryathode and secondary anode) are water cooled independently.

.2. Design of experiments

Experiments to determine the plasma jet thruster and lengthere developed by the complete factorial statistical method. This

xperiment design depends on three factors with four levels each,.e., the experiments were performed with primary electric currentntensities (IP) of 30, 40, 50, and 60 A; secondary electric currentntensities (IS) of 100, 150, 200, and 250 A; and argon gas flow ratesVg) of 7, 14, 20, and 25 l/min. The levels for each factor were chosenased on previous experiments, which were conducted primarily toetermine the limitations of the operation of the HPT. The executionrder of the trials was randomly determined to avoid systematicrrors. Additionally, each trial was performed in triplicate to reducerror, generating an experimental matrix of 192 trials.

The effects of the physics variables (factors) on the responsesplasma jet thruster and length) were determined by analysis ofariance (ANOVA).

.3. Measurement system

Fig. 3 shows a schematic diagram of the measurement setupor electrical, thruster and length diagnostic. The jet lengths were

easured by taking photographs with a scale, which required aCD camera. The images were processed using “ImageJ” software.

In order to measure thruster, the HPT was rigidly anchored,eeping the nozzle centralised with the dynamometer. To avoideating the electronic dynamometer, thermal isolation was

nserted in it, and a steel sheet, respectively 30 and 12 mm thick.he distance between the nozzle and the steel sheet was 40 mm.or each trial, initially the device was operated without ionizinghe gas, and the thruster of the gas expelled from the chamber was

onstructed in the LS&TC.

measured; later the device was operated (two plasma arcs) ionizingthe gas, and the thruster of the ionised gas (plasma) expelled fromthe chamber was measured. The difference between the thrusters(non-ionised gas and plasma) multiplied by the local gravitationalfield intensity supplies the approximate value of the thruster of thisdevice. The experimental apparatus was in a closed place, isolatingthe system from external forces that could affect the stabilisationof the dynamometer.

The arc voltage and current intensity were measured simul-taneously by using four channels data acquisition board (NI USB6009, 48 MHz, 14 bit) connected to the computer. Each electricalsignal was recorded at 10 MHz acquisition rate and intervals for asampling duration of 20 s.

2.4. Welding, cutting and surface hardening with HPT

A benchtop system with a displacement device was used toconduct the welding, cutting and surface hardening trials. Thisbenchtop consists of the following equipment: power sources, adisplacement system, brackets to secure the torch and cables, a gascylinder, gas regulators, a control system, and other tools. Fig. 4adisplays a schematic of the bench used in the trials, and Fig. 4bdisplays 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). Theplates were fastened and centred under the plasma jet. The follow-ing welding parameters were used: 5 mm distance between thenozzle and the work piece; argon gas flow rates between 7 l/minand 25 l/min; electric current intensity ranges from primary andsecondary power sources from 30 to 60 A and 100 to 250 A, respec-tively; and welding speed between 1.7 mm/s and 8.3 mm/s.

The cutting trials with the hybrid plasma torch were conductedusing various plate thicknesses (0.5–4 mm) and materials (alu-minium and various types of steel). Argon gas was injected into thedevice chamber at a flow rate greater than 20 l/min. A 5 mm dis-tance was maintained between the plasma jet output nozzle andthe work piece. Different torch displacement speeds and electriccurrent intensities (primary and secondary source) were applied.

Surface hardening trials were conducted using cylindrical AISI52100 steel (spheroidised) with 15 mm height and 76.2 mm diame-ter. As in the welding and cutting trials, the surface hardening trials

2374 R.T. Lermen, I.G. Machado / Journal of Materials Processing Technology 212 (2012) 2371– 2379

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alues were obtained in 0.1 mm intervals. The metallographic anal-sis was conducted using an Olympus light microscope, modelX60M. The specimen was sanded, sequentially polished with alu-ina, and chemically attacked with Nital (1% concentration).

. Results and discussion

Three types of results were obtained for the HPT, i.e., one con-erning to its measurement of plasma jet thruster, other referringo its jet length and the last results concerning its application toelding, cutting, and surface hardening.

.1. HPT thruster

The results obtained for HPT thruster checked the influence ofhe following items: primary electric current intensity; secondary

Fig. 4. (a) Bench station used during trials for welding, cutting, an

he experimental setup.

Table 1 displays the analysis of variance results, at a 95% confi-dence level, for the plasma thruster. From this analysis, it was clearthat the P-values were less than 0.05; consequently, it can be saidwith 95% certainty that all factors had a significant influence on theplasma thruster. Additionally, the same factors that had the great-est influence on the plasma thruster can be determined by the Fvalues, where a greater F value indicates greater influence. Thus,in descending order, the most influential factors on the plasmathruster were the secondary electric current intensity, the gas flowrate and primary electric current intensity.

Figs. 5–8 show the thruster as a function of the secondary elec-tric 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, onaverage, increased significantly with the secondary electric currentintensity and gas flow rate as well.

Figs. 9–12 show the thruster as a function of the primary electriccurrent intensity for different gas flow rates. For these trials, sec-ondary electric current intensity was used between the 100, 150,200 and 250 A, respectively. These graphs show that, on average,

d surface hardening. (b) HPT running in the welding trials.

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Table 1Analysis of variance (mean effects ANOVA) for HPT thruster.

Source Sum of square Degree of freedom Mean of square F ratio P-value

IP 128.64 3 42.88 23.005 0.0000IS 2686.05 3 895.35 481.404 0.0000Vg 1675.13 3 558.38 300.223 0.0000Error 338.50 182 1.86 – –

Total 4828.32 191 – – –

Fig. 5. Thruster as a function of secondary electric current intensity for different gasflow rates. Primary electric current intensity of 30 A.

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Fig. 8. Thruster as a function of secondary electric current intensity for different gasflow rates. Primary electric current intensity of 60 A.

ig. 6. Thruster as a function of secondary electric current intensity for different gasow rates. Primary electric current intensity of 40 A.

he thruster increased slightly with the primary electric currentntensity increase, except for trials performed with 7 l/min gas flowate and secondary electric current intensity of 200 A, where thehruster decrease with primary electric current intensity. Possi-ly, some primary electric arc instability provided the exceptionehavior.

The results obtained in the plasma thruster characterisationemonstrated that the thruster was slightly higher than valuesbtained by Burton et al. (1983) model at the same operational

ig. 7. Thruster as a function of secondary electric current intensity for different gasow rates. Primary electric current intensity of 50 A.

Fig. 9. Thruster as a function of primary electric current intensity for different gasflow rates. Secondary electric current intensity of 100 A.

intervals. This occurred because the ionised gas was acceleratedtwice by primary and secondary electric arc, which an electro-magnetic and, mainly, electro thermal contribution was observed.Also, according to the mathematic model described by Tikhonovet al. (1993) and experimental results obtained by Machado andLermen (2008), if the gas flow rate is increased and the current

Fig. 10. Thruster as a function of primary electric current intensity for different gasflow rates. Secondary electric current intensity of 150 A.

2376 R.T. Lermen, I.G. Machado / Journal of Materials Processing Technology 212 (2012) 2371– 2379

Fig. 11. Thruster as a function of primary electric current intensity for different gasflow rates. Secondary electric current intensity of 200 A.

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Fig. 14. Plasma jet length as a function of secondary electric current intensity fordifferent gas flow rates. Primary electric current intensity of 30 A.

Fig. 15. Plasma jet length as a function of secondary electric current intensity fordifferent gas flow rates. Primary electric current intensity of 40 A.

ig. 12. Thruster as a function of primary electric current intensity for different gasow rates. Secondary electric current intensity of 250 A.

.2. Plasma jet length

The plasma jet length was determined by standard distancehown in Fig. 13a. On the other hand, Fig. 13b–e displays a sequencef photos, in which the lengths y1. . .y4 of plasma jet length areepresented. These plasma jets were acquired with the follow-ng parameters: a primary electric current of 30 A; a secondarylectric current of 200 A; and gas flow rates of 7 l/min (Fig. 13b),4 l/min (Fig. 13c), 20 l/min (Fig. 13d), and 25 l/min (Fig. 13e). Inhese images, it is evident that the plasma jet length decreasesith increasing gas flow rate, and that the plasma jet flow under-

oes a transition from laminar to turbulent (Pan et al., 2002). Thisransition was observed in all trials.

Table 2 displays the analysis of variance results, at a 95% con-dence level, for the plasma jet length. From this analysis, it waslear that the P-values were less than 0.05; consequently, it can beaid with 95% certainty that all factors and interactions between

Fig. 13. (a) Standard distance for plasma jet length measurement. (b)–(e)

Fig. 16. Plasma jet length as a function of secondary electric current intensity fordifferent gas flow rates. Primary electric current intensity of 50 A.

most influential to least factors on the plasma jet length were sec-ondary electric current intensity, gas flow rate and primary electric

Figs. 14–17 display the plasma jet length as a function of sec-ondary current intensity for different gas flow rates. For these trials,primary electric current intensity was used between the 30, 40,

Plasma jets from the chamber, indicating their respective lengths.

R.T. Lermen, I.G. Machado / Journal of Materials Processing Technology 212 (2012) 2371– 2379 2377

Table 2Analysis of variance (mean effects ANOVA) for plasma jet length.

Source Sum of square Degree of freedom Mean of square F ratio P-value

IP 639.9 3 213.3 4.248 0.0063IS 18,449.3 3 6149.8 122.466 0.0000Vg 16,315.2 3 5438.4 108.30 0.0000Error 9139.3 182 50.2 – –

Total 44,543.7 191 – – –

Fig. 17. Plasma jet length as a function of secondary electric current intensity fordifferent gas flow rates. Primary electric current intensity of 60 A.

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ig. 18. Plasma jet length as a function of primary electric current intensity forifferent gas flow rates. Secondary electric current intensity of 100 A.

0 and 60 A, respectively. These graphs show that the plasma jetength increased with increasing secondary electric current inten-ity, decreased with increasing gas flow rate.

Figs. 18-21 display the plasma jet length as a function of pri-ary current intensity for different gas flow rates. For these trials,

rimary electric current intensity was used between the 100, 150,00 and 250 A, respectively. These graphs show, on average, thathe plasma jet length varied slightly with primary electric currentntensity.

ig. 19. Plasma jet length as a function of primary electric current intensity forifferent gas flow rates. Secondary electric current intensity of 150 A.

Fig. 20. Plasma jet length as a function of primary electric current intensity fordifferent gas flow rates. Secondary electric current intensity of 200 A.

According to Pan et al. (2002, 2005) the relationship betweenplasma jet length and gas flow rate, keeping constant the electriccurrent intensities (primary and secondary) indicates that therecould exist a critical Reynolds number at which the flow transi-tion between laminar and turbulent state occurs. This phenomenonsuggests the existence of the critical Reynolds number, becauseReynolds number is directly proportional to the mass flow rate andinversely proportional to the viscosity of the flow. That is, increaseof the arc current causes the rise of gas temperature at a given gasflow rate, and thus the rise of viscosity of argon gas, which allowsappropriate increasing of the gas flow rate to keep the Reynoldsnumber lower than the critical transition value and to keep theplasma in laminar flow state.

3.3. Applying the HPT in welding, cutting and hardening surface

Fig. 22 displays the top view of plates subjected to weldingwith the hybrid plasma torch, where the numbers indicated in thephotographs correspond to the welding trial numbers displayed inTable 3, which also shows the welding parameters used. In all tri-

workpiece, and the primary electric current was 60 A. It was pos-sible to use the device in sheet metal manufacturing with gas flowrates equal to or less than 7 l/min because with flows above 7 l/min,

Fig. 21. Plasma jet length as a function of primary electric current intensity fordifferent gas flow rates. Secondary electric current intensity of 250 A.

2378 R.T. Lermen, I.G. Machado / Journal of Materials Processing Technology 212 (2012) 2371– 2379

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 3Experimental data of welding tests using hybrid plasma torch.

Parameters Trials

1 2 3 4 5

IS (A) 200 200 200 200 250Vg (l/min) 7 14 7 7 25

5

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Welding speed (mm/s) 1.7 1.7

Types of steels AISI 304 AISI 304

Sheets dimensions (mm) 100 × 50 × 1.5 100 × 50 × 1.

he plasma jet became turbulent and formed irregular welds (trials and 5). Trial 3 exhibited some fusion defects because of the highelding speed used for this torch.

The cuts made with the device exhibited irregularities becausehe plasma jet did not have enough power to drive the molten

etal, and it was not possible to cut plates thicker than 3 mm.ig. 23 displays a section of a 1.5 mm thick AISI 304 steel sheet cutith the hybrid plasma torch generator. The cut was made with

he following parameters: primary and secondary electric currentntensities of 60 A and 250 A, respectively; gas flow of 25 l/min; cut-ing speed of 8.4 mm/s; and 5 mm distance maintained between the

The results of the surface hardening process with the hybridorch can be observed in Fig. 24, which shows a graph of microardness in relation to the distance from the incidence surface

ig. 24. Graph of microhardness in relation to distance from the incidence surfacef the plasma jet and cross-sectional macrographs of the heat-treated region.

3.4 1.7 1.7AISI 304 AISI 304 SAE 1020100 × 50 × 1.5 100 × 50 × 0.7 200 × 50 × 6.4

to the plasma jet and a cross-sectional macrograph of the heat-treated area. For this trial, the following parameters were used:primary and secondary electric current intensities of 60 A and250 A, respectively; gas flow rate of 7 l/min; torch displacementspeed of 1.7 mm/s; and 5 mm distance between the nozzle and thework piece. The thermally altered region reached a penetration andarea of approximately 0.74 mm and 2.06 mm2, respectively. Evi-dence of surface hardening was obtained by micro hardness testing,in which the base metal and heat-treated region exhibited valuesof approximately 250 Vickers and 700 Vickers, respectively.

4. Conclusions

According to the results obtained in the trials and analyses con-ducted, the following conclusions can be drawn:

• A new hybrid plasma torch was designed, developed andimproved. This torch is characterised by the simultaneous for-mation of two plasma arcs in just one device, which result ofthe synergistic combination of two plasma-generating processes,i.e., non-transferred-arc plasma and magnetoplasmadynamicthruster.

• The results obtained for the application of the hybrid torch tomanufacturing processes were satisfactory; it was possible toperform welding, cutting and surface hardening with the instru-ment. However, optimisation studies of these processes shouldbe conducted with this device.

• The three factors (primary and secondary electric current inten-sities and gas flow rate) had significant influence, with 95%reliability, on the thruster and plasma jet length. The thrusterincreased with increasing electric current intensities (primaryand secondary) and gas flow rate.

• The plasma jet length increased with increasing secondary elec-tric current intensity and varied slightly with primary electriccurrent intensity. As the gas flow was increased from 7 l/min to25 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 transi-tion region for the plasma jet type, i.e., the ionised gas flow shiftsfrom laminar to turbulent.

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