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High-productivity multiple-wire submerged-arc welding and cladding with metal-powder addition
Journal of Materials Processing Technology 133 (2003) 207–213
High-productivity multiple-wire submerged-arc welding and cladding with metal-powder addition J. Tusˇeka,*, M. Subanb a
Faculty of Mechanical Engineering, University of Ljubljana, Asˇkercˇeva 6, 1000 Ljubljana, Slovenia b Institut za varilstvo, Ptujska 19, 1000 Ljubljana, Slovenia
Abstract The paper deals with multiple-wire submerged-arc welding and cladding with metal-powder addition. Three different ways of supplying the metal powder to the welding area are described and shown schematically. It was found that the use of metal powder will increase the deposition rate, and the welding-arc efficiency and reduce the shielding-flux consumption. A suitable device permits submerged-arc welding and cladding with metal-powder addition. The process is primarily meant for the cladding of worn surfaces or the production of surfaces with certain characteristics (corrosion or wear resistance). By using the metal-powder addition it is possible to alloy a weld or a cladding with optional chemical elements. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Submerged-arc welding; Multiple wire; Metal powder; Shielding flux; Deposition rate; Melting efficiency
1. Introduction In submerged-arc welding, there are several ways of increasing welding efficiency. The following are known multiple-wire welding, multiple-electrode welding, hot wire welding, cold wire welding, and welding with metal-powder addition. Twin-electrode submerged-arc welding, and lately also twin-electrode gas-shielded welding, are well known and applied in practice. Investigations in this field, however, are continuing [1–3]. The same can be stated about single-wire welding with metal-powder addition [4,5]. In welding literature, no report on multiple-wire welding with metalpowder addition has been traced yet.
2. Short review of welding literature 2.1. Twin-wire welding The first publication of results of an investigation and of an application of twin-electrode welding goes back to 1954 [6]. Ashton in his article gave a lot of data on the practical application of submerged-arc welding with twin electrodes. Twin-electrode welding and multiple-electrode welding were *
Corresponding author. Tel.: þ386-61-136-74-74; fax: þ386-61-136-72-22. E-mail address: [email protected] (J. Tusˇek).
treated in the same period by Knight [7]. In his article a figure of a drive mechanism and of a joint contact nozzle is shown. Heinke et al. [8] carried out extensive investigations on welding with a twin wire arranged in the direction transverse to the welding direction. A metallurgical study of twin-wire welding can be found in an article by Wittke et al. [9]. The effects of welding parameters, powder type, gap size, and quantity of melted material on crack formation in one-side welding were studied. Also Ratzsch [10] was dealing with one-side submergedarc welding with a twin electrode. Two patents granted in the Soviet Union to Shostak [11,12] are of particular interest to present knowledge of twin-electrode welding. In [13] a description is found of an installation for submerged-arc welding with a twin electrode, a research on welding with high velocity and with high welding current density. Plu¨schke [14] applied a twin-electrode welding head in combination with a single-electrode welding head. The deposition rate in several variants of submerged-arc welding was established by Weisselberg [15]. A survey of different variants of twin-electrode welding and multipleelectrode welding was made by Utrachi [16]. He divided multiple-wire welding into five groups. An extensive study on productivity and economic aspects of submerged-arc welding was carried out by a group of researchers [17]. A very practical report on investigations of twin-electrode welding was published by two researchers of the shipbuilding company ESAB [18]. They reported on favourable results obtained in the thin twin-wire (1.6 and 2.0 mm
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diameters) welding of plates (5–12 mm thickness) with higher mechanical properties in the shipbuilding industry. Although twin-electrode welding is put into effect in practice, studies in this field will continue. This is confirmed in an article by the Swiss firm Oerlikon [19] which reports on the technological, mechanical, and economic advantages of the above-mentioned process, and an article of the Welding Institute of Kiev [20] which reports on the general advantages of the welding process concerned. Automation has not avoided submerged-arc welding with a twin electrode either. Only one article in this field will be mentioned [21]. Eichhorn and Nies report on an automatic submerged-arc procedure in twin-electrode narrow-gap welding. Twin-electrode welding is extensively applied in combination with several welding heads and with a single wire. A special variant of the welding system is treated in a document by Lin et al. [22]. It deals with automatic narrow-gap welding with a double welding head. In the first welding head, there is only one wire, and there were two wires in the second one. 2.2. Single-wire welding with metal-powder addition The first report on investigations on welding with metalpowder addition goes back to 1963 [23], but the process itself was not put into practical effect then. Some further articles on welding with metal-powder addition can be found a decade later [24,25]. The process developed quickly towards the end of the eighties and in the beginning of the nineties. Numerous articles and conference papers on the practical application of the process and on its many advantages bear witness of this development [26–29]. The most important advantages of the process are higher welding efficiency, higher welding productivity, and better quality of welded joints.
3. Possible variants of multiple-wire welding and cladding with metal-powder addition In multiple-wire welding, all wires travel simultaneously with the same feed rate through a joint contact nozzle. They have a joint feed rate control, but only one power source. The wires in the contact nozzle can be arranged to be parallel, successively, or to form an optional triangle, with regard to the welding direction. Fig. 1 shows some cases of a possible arrangement of wires in the contact tube. The wire arrangement depends on the purpose of use and the way of supplying the metalpowder addition to the welding area. For example, if it is to be surfaced, the wires will be arranged side by side so that they travel parallel to each other in the welding direction. The distance between the wires is also dependent on the way of supplying the metal powder, the wire diameter, welding parameters and the wire extension. For surfacing, a high arc
Fig. 1. Possible arrangements of wires in the contact tube.
voltage, even up to around 50 V, is recommended. In this way a nice, smooth weld of uniform thickness is obtained. For welding, the wires are arranged one after another in the welding direction. Also in this case the distance between the wires depends on the same factors as in surfacing. The part played by the wires in welding is very different. The first wire will melt the parent metal. The weld pool produced will stay behind the first arc due to high velocity. The remaining wires travelling behind the first one will shape the weld. The first wire, due to a lower resistance, will conduct a higher current than the other wires. The wires in the contact tube may be arranged also in a different manner depending on the purpose. This will be shown in item 5. During welding, it is possible to feed metal powder in several ways. The simplest way is to feed it ahead of the wires. First the metal powder is poured to the welding area and then the flux. An advantage of this method consists first of all in the simple design of the unit concerned. Its disadvantage, however, consists in a less accurate feeding of metal powder to the welding area and a somewhat lower melting efficiency. This occurs, however, only in cladding or welding in wide grooves, i.e. in the case of wide groove angles. A welding device with metal-powder supplied to the welding area ahead of the wires, i.e. arcs, is shown in Fig. 2. The second variant of supplying the metal powder and feeding the wires is shown in Fig. 3. The metal powder is supplied through a tube mounted between the wires. Because of high temperatures, the tube has to be made of heat-resistant ceramics. An advantage of this technique consists in supplying the metal powder directly into the cavity between the arcs. This technique is particularly suitable when two wires are arranged one after another because the first arc is burning between the workpiece and the first wire and not between the metal powder and the wire, as is the case with the first technique, consequently, this permits a deeper penetration of the workpiece. The arc of the second wire is melting the metal powder which then wets the weld and shapes the final layer. In this case, the
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Fig. 4. Comparison of twin-wire submerged-arc welding and twin-wire submerged-arc welding with metal-powder addition.
Fig. 2. Welding device with a double-wire electrode and the metal powder supplied to the welding groove ahead of the welding head.
energy efficiency is very high, while the shielding-flux consumption is very low. In quadruple-wire welding or cladding, the arcs of the first two wires will also melt the parent metal and the arcs of the second two will melt the metal powder and shape the final run. The principle of powder melting between two arcs is shown in Fig. 4(a) whilst Fig. 4(b) shows conventional submerged-arc welding with a twin-wire. A simple comparFig. 5. Schematic representation of the welding head with a quadruple wire and metal powder feeding parallel to all wires.
ison shows that the arc-energy efficiency is much higher if metal powder is supplied between the arcs. In this way the deposition rate can be increased by even 60%. The third possible variant of the welding head with a multiple-wire electrode and metal-powder addition is shown in Fig. 5. The wires in the joint contact tube may be arranged in several ways (see Fig. 1). Fig. 6 shows twin-wire welding with metal powder supplied to the welding area parallel to the wires. The wire
Fig. 3. Schematic representation of the welding head with a quadruple wire and feeding of the metal powder.
Fig. 6. Melting of the metal-powder added around the arcs and partially between them.
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carries the electrical current and a magnetic field attracting the metal powder is produced around the wire. The metal powder is melted around the wire which additionally increases the energy utilization, reduces the consumption of the shielding flux and increases the welding process efficiency. Welding may be carried out with a twin wire (Fig. 6), a quadruple wire (Fig. 3), or any optional number of wires which may be arranged optionally.
4. Study on technological–economic characteristics The following characteristics belong to the technological– economic characteristics of welding processes: deposition rate, energy consumption and consumption of the shielding gas or shielding flux and process productivity. 4.1. Deposition rate It is known that in consumable-electrode welding, melting of the parent metal is produced by the potential energy, which is a product of the welding current intensity and the voltage drop in the wire extension and the arc, i.e. with the electrode positive in the anode area and with the electrode negative in the cathode area of the arc. The energy supplied to the process changes into thermal energy and, consequently, increases the internal energy of the filler material so as to melt it. A droplet of the filler material formed will detach and travel into the weld pool. The droplet transfer is produced mainly by action of natural forces, i.e. gravitational force, and the forces produced due to electric-current conduction along the wire, i.e. electrodynamic force. The energy equation for the deposition rate may, in general, be expressed in a thermodynamic form ðQo þ Qw ÞZ ¼ Mcp DT þ H
(1)
Eq. (1) indicates that the thermal powers developed in the arc Qo (W) and in the wire extension Qw (W) due to resistance heating melt the filler material M (g s1) with the specific heat cp (J g1 K1). Heat losses into the environment, the powder and other losses are contained in the efficiency (Z) and the latent heat (H). In welding with metal-powder addition M is the mass of the wires and the mass of the metal powder melted in unit time. The solution of Eq. (1) requires good knowledge of the welding process, welding parameters and physical properties of the filler material, therefore, in addition to knowledge of the physical properties of the material, it is indispensable to carry out experimental trials. Experimental studies on the deposition rate included a number of combinations of different wire arrangements, different distances between the wires and different metalpowder supply modes. The deposition rate was established in dependence of current intensity, welding speed and arc voltage. The deposition rate was measured experimentally in
Fig. 7. Deposition rate as a function of current intensity per wire, the number of wires and polarity for a 3 mm wire; b ¼ 9 mm, L ¼ 30 mm, U ¼ 2931 V.
two ways, i.e. by weighing the workpiece prior to and after welding and by measuring the length of the wire consumed and weighing the metal powder prior to welding. Fig. 7 shows experimentally obtained values for the deposition rate in welding with a multiple-wire electrode with a wire diameter of 3 mm in dependence of the welding current intensity per wire and the number of wires. A surface evaluation of the results confirms what is already known, i.e. that the deposition rate in welding with the electrode negative, due to specific physical processes in the cathode area, is higher than in welding with the electrode positive. It is, less known, however, that the deposition rate in welding with a twin-wire electrode, and particularly with a triplewire electrode, is higher the greater is the number of wires, the conditions per wire remaining the same. The deposition rate will increase slightly exponentially with the increase in current intensity and exponentially with an increase in the number of wires. For an analysis of the experimentally obtained results a classical statistical method was applied. The number of repetitions of individual experiments under the same conditions was low, i.e. from four to eight; consequently, the ‘‘t’’ test was used to determine the confidence interval. The deposition rate is defined as the weight of filler material deposited in unit time. In the process concerned this involves melting of the filler material and the metal powder. The supply of metal powder to the welding area will strongly increase the deposition rate. Fig. 8 shows the deposition rate as a function of welding current intensity in single-wire and twin-wire welding as well as in twin-wire and triple-wire welding with metal-powder addition. Due to a better arc efficiency in multiple-wire welding, the melting efficiency increases by a factor which is higher than
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Fig. 10. Schematic presentation of input and output elements in arc welding.
Fig. 8. Deposition rate as a function of welding current intensity: U ¼ 31 V, wire diameter ¼ 2 mm, b ¼ 7 mm, vw ¼ 0:6 m/min.
the number of wires. Fig. 9 shows schematically the heat distribution in single-wire and triple-wire welding. Fig. 9 (left) shows three arcs supplied by one current source. The joint current carried through the three arcs is equal to the current carried through the arc shown in Fig. 9 (right). Because of a lower dissipation of heat into the surroundings, the energy efficiency and, consequently, the deposition rate, in multiple-wire welding are increased. 4.2. Melting efficiency of the welding process The efficiencies of the machine used, the working time and the process as a whole, are very important quantities in the establishment of productivity and in an analysis of the costs of the total production. Welding is a production technology which includes machines, and various kinds of energy and materials in different physical conditions. In arc welding the input elements are the electrical energy, the filler material and the parent metal. The output element of the process is a weld or a surfacing weld (Fig. 10). Calculation of the efficiency is a very exacting task due to the complexity of the physical and chemical as well as thermal processes occurring in arc welding. The literature offers various definitions and equations for calculation of the welding process efficiency. In their calculations some authors have taken into account only the arc and energy
transformation into thermal energy. Others have calculated and measured the thermal energy utilized usefully for melting of the parent metal and the filler material, while still others have calculated and measured only the thermal energy distribution in the course of arc welding. In the literature no contribution could be traced dealing with the welding process as a whole and showing calculation of the efficiency of energy transfer from the source and the transformation of this energy into thermal energy, or the calculation of the efficiency of material transfer, i.e. alloying elements through the arc. The melting efficiency of the arc welding process is defined by MEt Zt ¼ R t 0 IU dt
(2)
where M (g s1) is the melting rate, E (J g1) the energy theoretically required for melting the material welded and I (A), U (V) and t (s) are welding parameters. The energy theoretically required for melting 1 g of steel without any loss may be calculated theoretically by E ¼ mcp DTI
(3) 1
1
where m (g) is the mass, cp (J kg K ) the specific heat of steel and DT (K) the difference between the ambient temperature and the melting point of the metal. In heating, the specific heat of steel increases in a nonlinear manner. The specific heat as a function of temperature may be approximated by linear and square functions, but an accurate analytical solution is nevertheless not feasible. The average of functional dependence of the specific heat on temperature was calculated and thus an average actual value was obtained. A mean value of the specific heat may be calculated by the following equation: Z T2 1 cpsr ¼ cp ðTÞ dT (4) T2 T1 T1 where T2 (8C) is the melting point of steel and T1 (8C) is 0 8C. The integral of Eq. (4) falls into a sum of four integrals: 4 Z T2i 1 X cpsr ¼ cp ðTÞ dT (5) T2 T1 i¼1 T1i
Fig. 9. Comparison of heat distributions around one arc and around three arcs with the same welding current intensity.
By entering the above values into Eq. (5) for cpsr, a value of 720 J kg1 K1 is obtained. If the mean value for the specific heat is entered into Eq. (6) and a calculation is made, the
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energy required for the melting of steel without losses will be obtained, i.e. E ¼ mcpsr ðT2 T1 Þ ¼ 1 g 720 J kg1 K1 1500 K ¼ 1080 J g1
(6)
By taking into account the energy required for the transformation of solid steel into liquid steel of 1340 J g1 is obtained, as stated by some other authors. On the basis of the above considerations, Eq. (2) may be changed into Eq. (7) M1340t Z ð%Þ ¼ R t 100 0 IU dt
(7)
The electrode-melting efficiency can be calculated by ME Et ZES ð%Þ ¼ R t 100 0 IUE dt
(8)
where ME (g s1) is the melting efficiency, UE (V) the voltage drop in the cathode area and anode area, and E and I are the same as in Eq. (2). The diagram at the left side of Fig. 11 shows such a comparison. It can be seen that the melting efficiency of the welding process increases by about 15% if metal powder is added. The calculation of the melting efficiency of the electrode shows (Fig. 11) that the difference between the two processes is even greater. The comparison of both processes and both efficiencies, respectively, shows how the energy in the arc is distributed. With metal-powder addition a major part of the arc energy is consumed for melting of the filler material (wire þ metal powder) while the energy consumed for melting the parent metal remains the same. 4.3. Shielding flux Fig. 12 shows the flux consumption per unit time as a function of the welding current intensity. The diagram
Fig. 12. Consumption of shielding flux in submerged-arc welding (classical process) and in submerged-arc welding with metal-powder addition as a function of welding current (U ¼ 25 V, vw ¼ 0:9 m/min, wire diameter ¼ 3 mm).
indicates that the consumption of the shielding flux is lower if metal powder is added. Practical measurements show that arc energy is consumed for melting of the metal powder if it is present and less for melting of the shielding flux. The introduction of metal powder, however, reduces the shielding-flux consumption. In welding with a twin-wire electrode the consumption is lower by 15–25% and in welding with a triple-wire electrode by 12–25%.
5. Conclusions The investigations have shown that it is possible to submerged-arc weld and clad with a multiple-wire electrode and metal-powder addition. In this way the deposition rate and productivity are increased, the consumption of shielding flux is reduced, and the arc efficiency is improved. For industrial application of the process, it will be necessary to design a suitable welding head and to study the most suitable welding parameters. References
Fig. 11. Melting efficiency of the welding process (left) and melting efficiency of the electrode (right) in conventional submerged-arc welding and in submerged-arc welding with metal-powder addition. I ¼ 400 A, U ¼ 30 V, vw ¼ 0:4 m/min, wire diameter ¼ 1:6 mm.
[1] H. Nies, B. Schlatter, Der Praktiker 46 (1994) 311. [2] R. Knoch, A.W.E. Nentwig, Schweißen und Schneiden’94—Vortra¨ ge der Großen Schweißtechnischen Tagung in Bremen vom, September 28–30, 1994, DVS-Verl., Du¨ sseldorf, p. 77. [3] R. Lahnsteiner, Schweißen und Schneiden’94—Vortra¨ ge der gleichnamigen Großen Schweißtechnischen Tagung in Bremen vom, September 28–30, 1994, DVS-Verl., Du¨ sseldorf, p. 82. [4] M. Vilpas, R. Karppi, Weld. Rev. Int. 12 (1993) 87. [5] H. Nies, B. Schlatter, Der Praktiker 46 (1994) 409. [6] T. Ashton, Weld. J. 33 (1954) 350. [7] D.E. Knight, Weld. J. 33 (1954) 303. [8] H. Heinke, H. Schlerfer, U. Franz, H.D. Wendler, Schweißtechnik 17 (1967) 386. [9] K. Wittke, U. Franz, K. Koczelov, A. Synnatscke, H. Hanke, Schweißtechnik 18 (1968) 487. [10] H. Ratzsch, ZIS-Mitteilungen 14 (1981) 1105. [11] I.A. Shostak, No. 381489—Bjulleten izobret No. 22, 1973.
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[18] [19] [20] [21]
I.A. Shostak, No. 397288—Bjulleten izobret No. 37, 1973. J.E. Hinkel, F.W. Forstohoefel, Weld. J. 55 (1976) 175. E.W. Plu¨ schke, ZIS-Mitteilungen 18 (1976) 622. A. Weisselberg, ZIS-Mitteilungen 20 (1978) 528. G.D. Utrachi, Weld. J. 57 (1978) 15. J. Mosny, E. Malinovska, V. Pavelka, Public Session on ‘‘Welding and Allied Processes, Energy and Economy’’ of the IIS/IIW Annual Assembly in Ljubljana, Institut za varilstvo, Ljubljana, 1982, p. B-5. B.A. North, F. Nicolson, Svetsaren 16 (1982) 7. M. Hereth, B. Schlatter, Oerlikon-Schweißmitteilungen 47 (1989) 4. L.I. Oparin, V.I. Malikin, Avtomaticˇ eskaja Svarka 42 (1989) 53. F. Eichhorn, H. Nies, Schweißen und Schneiden 42 (1990) 125.
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[22] S. Lin, S. Yang, B. Tao, Doc. XII-1076-88, International Institute of Welding, Wien, 1988. [23] R.F. Arnoldy, Weld. J. 42 (1963) 885. [24] R.J. De Haeck, Weld. J. 50 (1971) 689. [25] B.G. Cigan, B.D. Lebedev, Svarocˇ noe Proizvodstvo 46 (1975) 19. [26] M. Vilpas, H. Nies, H. Krebs, R. Karppi, Doc. XII-1151-89, International Institute of Welding, Helsinki, 1989. [27] S. Sakaguchi, T. Yamaguchi, C. Shiga, Doc. XII-1156-90, International Institute of Welding, Montreal, 1990. [28] C.E. Thornton, Weld. Met. Fabr. 57 (1988) 342–345. [29] L. Wittung, Doc. XII-1058-88, International Institute of Welding, Wien, 1988.
High-productivity multiple-wire submerged-arc welding and cladding with metal-powder addition J. Tusˇeka,*, M. Subanb a
Faculty of Mechanical Engineering, University of Ljubljana, Asˇkercˇeva 6, 1000 Ljubljana, Slovenia b Institut za varilstvo, Ptujska 19, 1000 Ljubljana, Slovenia
Abstract The paper deals with multiple-wire submerged-arc welding and cladding with metal-powder addition. Three different ways of supplying the metal powder to the welding area are described and shown schematically. It was found that the use of metal powder will increase the deposition rate, and the welding-arc efficiency and reduce the shielding-flux consumption. A suitable device permits submerged-arc welding and cladding with metal-powder addition. The process is primarily meant for the cladding of worn surfaces or the production of surfaces with certain characteristics (corrosion or wear resistance). By using the metal-powder addition it is possible to alloy a weld or a cladding with optional chemical elements. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Submerged-arc welding; Multiple wire; Metal powder; Shielding flux; Deposition rate; Melting efficiency
1. Introduction In submerged-arc welding, there are several ways of increasing welding efficiency. The following are known multiple-wire welding, multiple-electrode welding, hot wire welding, cold wire welding, and welding with metal-powder addition. Twin-electrode submerged-arc welding, and lately also twin-electrode gas-shielded welding, are well known and applied in practice. Investigations in this field, however, are continuing [1–3]. The same can be stated about single-wire welding with metal-powder addition [4,5]. In welding literature, no report on multiple-wire welding with metalpowder addition has been traced yet.
2. Short review of welding literature 2.1. Twin-wire welding The first publication of results of an investigation and of an application of twin-electrode welding goes back to 1954 [6]. Ashton in his article gave a lot of data on the practical application of submerged-arc welding with twin electrodes. Twin-electrode welding and multiple-electrode welding were *
Corresponding author. Tel.: þ386-61-136-74-74; fax: þ386-61-136-72-22. E-mail address: [email protected] (J. Tusˇek).
treated in the same period by Knight [7]. In his article a figure of a drive mechanism and of a joint contact nozzle is shown. Heinke et al. [8] carried out extensive investigations on welding with a twin wire arranged in the direction transverse to the welding direction. A metallurgical study of twin-wire welding can be found in an article by Wittke et al. [9]. The effects of welding parameters, powder type, gap size, and quantity of melted material on crack formation in one-side welding were studied. Also Ratzsch [10] was dealing with one-side submergedarc welding with a twin electrode. Two patents granted in the Soviet Union to Shostak [11,12] are of particular interest to present knowledge of twin-electrode welding. In [13] a description is found of an installation for submerged-arc welding with a twin electrode, a research on welding with high velocity and with high welding current density. Plu¨schke [14] applied a twin-electrode welding head in combination with a single-electrode welding head. The deposition rate in several variants of submerged-arc welding was established by Weisselberg [15]. A survey of different variants of twin-electrode welding and multipleelectrode welding was made by Utrachi [16]. He divided multiple-wire welding into five groups. An extensive study on productivity and economic aspects of submerged-arc welding was carried out by a group of researchers [17]. A very practical report on investigations of twin-electrode welding was published by two researchers of the shipbuilding company ESAB [18]. They reported on favourable results obtained in the thin twin-wire (1.6 and 2.0 mm
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 2 3 5 - 2
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diameters) welding of plates (5–12 mm thickness) with higher mechanical properties in the shipbuilding industry. Although twin-electrode welding is put into effect in practice, studies in this field will continue. This is confirmed in an article by the Swiss firm Oerlikon [19] which reports on the technological, mechanical, and economic advantages of the above-mentioned process, and an article of the Welding Institute of Kiev [20] which reports on the general advantages of the welding process concerned. Automation has not avoided submerged-arc welding with a twin electrode either. Only one article in this field will be mentioned [21]. Eichhorn and Nies report on an automatic submerged-arc procedure in twin-electrode narrow-gap welding. Twin-electrode welding is extensively applied in combination with several welding heads and with a single wire. A special variant of the welding system is treated in a document by Lin et al. [22]. It deals with automatic narrow-gap welding with a double welding head. In the first welding head, there is only one wire, and there were two wires in the second one. 2.2. Single-wire welding with metal-powder addition The first report on investigations on welding with metalpowder addition goes back to 1963 [23], but the process itself was not put into practical effect then. Some further articles on welding with metal-powder addition can be found a decade later [24,25]. The process developed quickly towards the end of the eighties and in the beginning of the nineties. Numerous articles and conference papers on the practical application of the process and on its many advantages bear witness of this development [26–29]. The most important advantages of the process are higher welding efficiency, higher welding productivity, and better quality of welded joints.
3. Possible variants of multiple-wire welding and cladding with metal-powder addition In multiple-wire welding, all wires travel simultaneously with the same feed rate through a joint contact nozzle. They have a joint feed rate control, but only one power source. The wires in the contact nozzle can be arranged to be parallel, successively, or to form an optional triangle, with regard to the welding direction. Fig. 1 shows some cases of a possible arrangement of wires in the contact tube. The wire arrangement depends on the purpose of use and the way of supplying the metalpowder addition to the welding area. For example, if it is to be surfaced, the wires will be arranged side by side so that they travel parallel to each other in the welding direction. The distance between the wires is also dependent on the way of supplying the metal powder, the wire diameter, welding parameters and the wire extension. For surfacing, a high arc
Fig. 1. Possible arrangements of wires in the contact tube.
voltage, even up to around 50 V, is recommended. In this way a nice, smooth weld of uniform thickness is obtained. For welding, the wires are arranged one after another in the welding direction. Also in this case the distance between the wires depends on the same factors as in surfacing. The part played by the wires in welding is very different. The first wire will melt the parent metal. The weld pool produced will stay behind the first arc due to high velocity. The remaining wires travelling behind the first one will shape the weld. The first wire, due to a lower resistance, will conduct a higher current than the other wires. The wires in the contact tube may be arranged also in a different manner depending on the purpose. This will be shown in item 5. During welding, it is possible to feed metal powder in several ways. The simplest way is to feed it ahead of the wires. First the metal powder is poured to the welding area and then the flux. An advantage of this method consists first of all in the simple design of the unit concerned. Its disadvantage, however, consists in a less accurate feeding of metal powder to the welding area and a somewhat lower melting efficiency. This occurs, however, only in cladding or welding in wide grooves, i.e. in the case of wide groove angles. A welding device with metal-powder supplied to the welding area ahead of the wires, i.e. arcs, is shown in Fig. 2. The second variant of supplying the metal powder and feeding the wires is shown in Fig. 3. The metal powder is supplied through a tube mounted between the wires. Because of high temperatures, the tube has to be made of heat-resistant ceramics. An advantage of this technique consists in supplying the metal powder directly into the cavity between the arcs. This technique is particularly suitable when two wires are arranged one after another because the first arc is burning between the workpiece and the first wire and not between the metal powder and the wire, as is the case with the first technique, consequently, this permits a deeper penetration of the workpiece. The arc of the second wire is melting the metal powder which then wets the weld and shapes the final layer. In this case, the
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Fig. 4. Comparison of twin-wire submerged-arc welding and twin-wire submerged-arc welding with metal-powder addition.
Fig. 2. Welding device with a double-wire electrode and the metal powder supplied to the welding groove ahead of the welding head.
energy efficiency is very high, while the shielding-flux consumption is very low. In quadruple-wire welding or cladding, the arcs of the first two wires will also melt the parent metal and the arcs of the second two will melt the metal powder and shape the final run. The principle of powder melting between two arcs is shown in Fig. 4(a) whilst Fig. 4(b) shows conventional submerged-arc welding with a twin-wire. A simple comparFig. 5. Schematic representation of the welding head with a quadruple wire and metal powder feeding parallel to all wires.
ison shows that the arc-energy efficiency is much higher if metal powder is supplied between the arcs. In this way the deposition rate can be increased by even 60%. The third possible variant of the welding head with a multiple-wire electrode and metal-powder addition is shown in Fig. 5. The wires in the joint contact tube may be arranged in several ways (see Fig. 1). Fig. 6 shows twin-wire welding with metal powder supplied to the welding area parallel to the wires. The wire
Fig. 3. Schematic representation of the welding head with a quadruple wire and feeding of the metal powder.
Fig. 6. Melting of the metal-powder added around the arcs and partially between them.
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carries the electrical current and a magnetic field attracting the metal powder is produced around the wire. The metal powder is melted around the wire which additionally increases the energy utilization, reduces the consumption of the shielding flux and increases the welding process efficiency. Welding may be carried out with a twin wire (Fig. 6), a quadruple wire (Fig. 3), or any optional number of wires which may be arranged optionally.
4. Study on technological–economic characteristics The following characteristics belong to the technological– economic characteristics of welding processes: deposition rate, energy consumption and consumption of the shielding gas or shielding flux and process productivity. 4.1. Deposition rate It is known that in consumable-electrode welding, melting of the parent metal is produced by the potential energy, which is a product of the welding current intensity and the voltage drop in the wire extension and the arc, i.e. with the electrode positive in the anode area and with the electrode negative in the cathode area of the arc. The energy supplied to the process changes into thermal energy and, consequently, increases the internal energy of the filler material so as to melt it. A droplet of the filler material formed will detach and travel into the weld pool. The droplet transfer is produced mainly by action of natural forces, i.e. gravitational force, and the forces produced due to electric-current conduction along the wire, i.e. electrodynamic force. The energy equation for the deposition rate may, in general, be expressed in a thermodynamic form ðQo þ Qw ÞZ ¼ Mcp DT þ H
(1)
Eq. (1) indicates that the thermal powers developed in the arc Qo (W) and in the wire extension Qw (W) due to resistance heating melt the filler material M (g s1) with the specific heat cp (J g1 K1). Heat losses into the environment, the powder and other losses are contained in the efficiency (Z) and the latent heat (H). In welding with metal-powder addition M is the mass of the wires and the mass of the metal powder melted in unit time. The solution of Eq. (1) requires good knowledge of the welding process, welding parameters and physical properties of the filler material, therefore, in addition to knowledge of the physical properties of the material, it is indispensable to carry out experimental trials. Experimental studies on the deposition rate included a number of combinations of different wire arrangements, different distances between the wires and different metalpowder supply modes. The deposition rate was established in dependence of current intensity, welding speed and arc voltage. The deposition rate was measured experimentally in
Fig. 7. Deposition rate as a function of current intensity per wire, the number of wires and polarity for a 3 mm wire; b ¼ 9 mm, L ¼ 30 mm, U ¼ 2931 V.
two ways, i.e. by weighing the workpiece prior to and after welding and by measuring the length of the wire consumed and weighing the metal powder prior to welding. Fig. 7 shows experimentally obtained values for the deposition rate in welding with a multiple-wire electrode with a wire diameter of 3 mm in dependence of the welding current intensity per wire and the number of wires. A surface evaluation of the results confirms what is already known, i.e. that the deposition rate in welding with the electrode negative, due to specific physical processes in the cathode area, is higher than in welding with the electrode positive. It is, less known, however, that the deposition rate in welding with a twin-wire electrode, and particularly with a triplewire electrode, is higher the greater is the number of wires, the conditions per wire remaining the same. The deposition rate will increase slightly exponentially with the increase in current intensity and exponentially with an increase in the number of wires. For an analysis of the experimentally obtained results a classical statistical method was applied. The number of repetitions of individual experiments under the same conditions was low, i.e. from four to eight; consequently, the ‘‘t’’ test was used to determine the confidence interval. The deposition rate is defined as the weight of filler material deposited in unit time. In the process concerned this involves melting of the filler material and the metal powder. The supply of metal powder to the welding area will strongly increase the deposition rate. Fig. 8 shows the deposition rate as a function of welding current intensity in single-wire and twin-wire welding as well as in twin-wire and triple-wire welding with metal-powder addition. Due to a better arc efficiency in multiple-wire welding, the melting efficiency increases by a factor which is higher than
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Fig. 10. Schematic presentation of input and output elements in arc welding.
Fig. 8. Deposition rate as a function of welding current intensity: U ¼ 31 V, wire diameter ¼ 2 mm, b ¼ 7 mm, vw ¼ 0:6 m/min.
the number of wires. Fig. 9 shows schematically the heat distribution in single-wire and triple-wire welding. Fig. 9 (left) shows three arcs supplied by one current source. The joint current carried through the three arcs is equal to the current carried through the arc shown in Fig. 9 (right). Because of a lower dissipation of heat into the surroundings, the energy efficiency and, consequently, the deposition rate, in multiple-wire welding are increased. 4.2. Melting efficiency of the welding process The efficiencies of the machine used, the working time and the process as a whole, are very important quantities in the establishment of productivity and in an analysis of the costs of the total production. Welding is a production technology which includes machines, and various kinds of energy and materials in different physical conditions. In arc welding the input elements are the electrical energy, the filler material and the parent metal. The output element of the process is a weld or a surfacing weld (Fig. 10). Calculation of the efficiency is a very exacting task due to the complexity of the physical and chemical as well as thermal processes occurring in arc welding. The literature offers various definitions and equations for calculation of the welding process efficiency. In their calculations some authors have taken into account only the arc and energy
transformation into thermal energy. Others have calculated and measured the thermal energy utilized usefully for melting of the parent metal and the filler material, while still others have calculated and measured only the thermal energy distribution in the course of arc welding. In the literature no contribution could be traced dealing with the welding process as a whole and showing calculation of the efficiency of energy transfer from the source and the transformation of this energy into thermal energy, or the calculation of the efficiency of material transfer, i.e. alloying elements through the arc. The melting efficiency of the arc welding process is defined by MEt Zt ¼ R t 0 IU dt
(2)
where M (g s1) is the melting rate, E (J g1) the energy theoretically required for melting the material welded and I (A), U (V) and t (s) are welding parameters. The energy theoretically required for melting 1 g of steel without any loss may be calculated theoretically by E ¼ mcp DTI
(3) 1
1
where m (g) is the mass, cp (J kg K ) the specific heat of steel and DT (K) the difference between the ambient temperature and the melting point of the metal. In heating, the specific heat of steel increases in a nonlinear manner. The specific heat as a function of temperature may be approximated by linear and square functions, but an accurate analytical solution is nevertheless not feasible. The average of functional dependence of the specific heat on temperature was calculated and thus an average actual value was obtained. A mean value of the specific heat may be calculated by the following equation: Z T2 1 cpsr ¼ cp ðTÞ dT (4) T2 T1 T1 where T2 (8C) is the melting point of steel and T1 (8C) is 0 8C. The integral of Eq. (4) falls into a sum of four integrals: 4 Z T2i 1 X cpsr ¼ cp ðTÞ dT (5) T2 T1 i¼1 T1i
Fig. 9. Comparison of heat distributions around one arc and around three arcs with the same welding current intensity.
By entering the above values into Eq. (5) for cpsr, a value of 720 J kg1 K1 is obtained. If the mean value for the specific heat is entered into Eq. (6) and a calculation is made, the
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energy required for the melting of steel without losses will be obtained, i.e. E ¼ mcpsr ðT2 T1 Þ ¼ 1 g 720 J kg1 K1 1500 K ¼ 1080 J g1
(6)
By taking into account the energy required for the transformation of solid steel into liquid steel of 1340 J g1 is obtained, as stated by some other authors. On the basis of the above considerations, Eq. (2) may be changed into Eq. (7) M1340t Z ð%Þ ¼ R t 100 0 IU dt
(7)
The electrode-melting efficiency can be calculated by ME Et ZES ð%Þ ¼ R t 100 0 IUE dt
(8)
where ME (g s1) is the melting efficiency, UE (V) the voltage drop in the cathode area and anode area, and E and I are the same as in Eq. (2). The diagram at the left side of Fig. 11 shows such a comparison. It can be seen that the melting efficiency of the welding process increases by about 15% if metal powder is added. The calculation of the melting efficiency of the electrode shows (Fig. 11) that the difference between the two processes is even greater. The comparison of both processes and both efficiencies, respectively, shows how the energy in the arc is distributed. With metal-powder addition a major part of the arc energy is consumed for melting of the filler material (wire þ metal powder) while the energy consumed for melting the parent metal remains the same. 4.3. Shielding flux Fig. 12 shows the flux consumption per unit time as a function of the welding current intensity. The diagram
Fig. 12. Consumption of shielding flux in submerged-arc welding (classical process) and in submerged-arc welding with metal-powder addition as a function of welding current (U ¼ 25 V, vw ¼ 0:9 m/min, wire diameter ¼ 3 mm).
indicates that the consumption of the shielding flux is lower if metal powder is added. Practical measurements show that arc energy is consumed for melting of the metal powder if it is present and less for melting of the shielding flux. The introduction of metal powder, however, reduces the shielding-flux consumption. In welding with a twin-wire electrode the consumption is lower by 15–25% and in welding with a triple-wire electrode by 12–25%.
5. Conclusions The investigations have shown that it is possible to submerged-arc weld and clad with a multiple-wire electrode and metal-powder addition. In this way the deposition rate and productivity are increased, the consumption of shielding flux is reduced, and the arc efficiency is improved. For industrial application of the process, it will be necessary to design a suitable welding head and to study the most suitable welding parameters. References
Fig. 11. Melting efficiency of the welding process (left) and melting efficiency of the electrode (right) in conventional submerged-arc welding and in submerged-arc welding with metal-powder addition. I ¼ 400 A, U ¼ 30 V, vw ¼ 0:4 m/min, wire diameter ¼ 1:6 mm.
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