- Email: [email protected]
Study on ceramic cutting by plasma arc
Journal of Materials Processing Technology 129 (2002) 152±156
Study on ceramic cutting by plasma arc W.J. Xua,*, J.C. Fangb, Y.S. Lua a
School of Mechanical Engineering, Dalian University of Technology, Dalian 116023, PR China College of Mechanical Engineering and Automation, Huaqiao University, Quanzhou 362011, PR China
b
Abstract To reduce the kerf width and to improve the kerf quality, the hydro-magnetically con®ned plasma arc was used to cut engineering ceramic plates. By experiments and analyses, the characteristics of the hydro-magnetic con®ned plasma arc were explored and the effects of secondary con®nement on cutting quality, arc properties, and optimal process parameters were determined. By using this new method, the authors achieved better cutting quality and higher cutting speeds. Also, the possibility to reduce the heat load of the nozzle and thus enhance its service life and process stability was studied. When the nozzle diameter is 3 mm, the kerf width of the Al2O3 ceramic plate of 6 mm thickness is less than 4.6 mm, while the cutting speed reaches to 0.9±1.2 m/min. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma cutting; Engineering ceramic; Plasma con®nement
1. Introduction Engineering ceramics are typical dif®cult-to-machine materials because of their high hardness and brittleness [1±4]. Compared with other machining methods, plasma arc cutting (PAC) is a very important thermal cutting process and has been used successfully used in cutting stainless steel, high hardness metal, high melting-point metal, and other dif®cult-to-machine alloys [5±8]. PAC's application to cutting ceramics, however, is still limited because most ceramics are not good electronic conducts, which makes it dif®cult for a transferred plasma arc to be produced between the cathode and the workpiece. Thus the authors presented the method of PAC of engineering ceramics with an additional anode [9] (shown in Fig. 1). The feasibility and effectiveness have been proven by experiments. Its principle is as follows: an additional anode is set up under the ceramic plate so as to enable the plasma arc to be maintained between the cathode and the additional anode. In this way the plasma arc melts and blows away certain material and forms a kerf on the ceramic plate. It is preferential for the plasma arc, as the source of heat, to be of such temperature and velocity that are as high as possible and uniformly distributed in order to minimize the heat difference induced by the thickness of the plate and to
* Corresponding author. Tel.: 86-411-4708410. E-mail address: [email protected] (W.J. Xu).
obtain the corresponding beveling of cuts, high quality cuts and dross-free cutting speeds. Under normal conditions, the plasma arc ejects from the nozzle under constriction by the plasma torch and the organized shielding gas in it, but the intensity of such intorch constriction is restricted by the permitted heat load on the nozzle. Moreover, the exposed part of the plasma arc in the gap between the nozzle and the workpiece is bound to undergo steep divergence due to its blending with air, leading to poor arc shape and considerable drop in the power density of the arc, and consequently unwanted cutting quality and capability [10]. With the aim of reducing or eliminating the diverging of the open section of the arc, and further raising its degree of constriction, techniques that apply additional constriction on the exposed arc column, the so-called ``secondary compression'', have been developed. For the convenience of discussion and study, the authors here term the constriction of the in-torch part of a plasma arc by the nozzle and the working gas stream as ``primary constriction'', with the constriction of the exposed part of arc, usually in non-mechanical ways, termed ``secondary constriction''. The secondary constriction, compared with the primary constriction, is much more important in affecting the arc's shape and cutting quality as well as cutting speeds. One method of secondary constriction that has been applied relatively successfully in industries is water-injection [11,12] which makes use of a conical water-injection to perform further constriction on the open part of the arc.
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 6 0 0 - 3
W.J. Xu et al. / Journal of Materials Processing Technology 129 (2002) 152±156
153
constriction will be increased, the arc shape improved, and higher cutting speeds and better cutting quality obtained. 2. Experimental principle and procedure
Fig. 1. Principle of PAC ceramic plate.
Although it is advantageous over other methods, this approach has certain inherent drawbacks: only a local planar constriction around the focal plane of the water injection can be formed with poor uniformity of the arc column; and with its strong feature of absorbing heat, an improperly large ¯ow-rate of injected water could merely induce instability or even cause extinction of the arc. Therefore, the water-constriction method is far from being perfect as far as precision cutting is concerned. In recent years, preliminary research has been made on cutting with a magnetically constricted plasma arc, of which the feasibility has been proven experimentally [13]. The principle involved is illustrated schematically in Fig. 2. In the 4-pole traverse magnetic ®eld, the plasma arc is subjected to compressive and stretching Lorentz forces in the two perpendicular directions and its acted part changes the round cross-section into an elliptic one, so that along its major axis narrower kerfs can be produced without altering any process variables. Other merits of this method are: no heat is carried away from the arc column; a three-dimensional constriction can be formed about the whole thickness of the pole-piece, and thus the uniformity of the arc column is enhanced; a narrower heated zone, but with more suf®cient heating of material along the cutting path is produced. Nevertheless, its defects are clear as well the incapability of establishing a complete radial compression on the arc column; limitations on the ®eld strength within the arc column by the performance available and possible thermal damage to the ®eld set-up and hence there is limited increase in arc power density. As the two constrictions mentioned above are mutually complimentary, it has been devised in this paper that they are integrated into one plasma torch to form a more effective, three-dimensional constriction so that the degree of arc
Fig. 2. Schematic illustration of magnetic-poles' arrangement.
As shown in Fig. 3, when the water-injection and magnetic ®eld are applied simultaneously, the plasma arc is acted by their joint constriction. Cutting is performed along the major axis of the ®eld-induced elliptical cross-section of the arc column. The hydro-magnetic plasma torch has been made by the authors. The essential conditions of the experiments are listed in Table 1. The secondary constriction in¯uences the arc properties, the cutting quality and speed having been examined by varying the ¯ow-rate of injected water Fw and the magnetizing current If, using nozzles of different diameters DN. 3. Results and discussion 3.1. Characteristics of plasma arc Cutting quality and cutting speed are closely related to the levels and distributions of power density, temperature and velocity of the plasma arc. It is very dif®cult and complicated, however, to measure them in practice. By contrast, the arc voltage U, which is easy to measure, is an indirect but synthetic parameter of arc properties in re¯ecting the input, transformation, transfer and equilibrium of energy within the arc, and thus it is a signi®cant index to analyze the effects of secondary constriction on the arc. Fig. 4 shows the experimental curves of arc voltages under various conditions of secondary constriction. The following notes could be made: 1. The arc voltage exhibits an approximately linearly increasing trend when the flow-rate of injected water, Fw, increases. This trend remains unchanged even if a constant magnetizing current, If, is applied also. 2. As If increases, the arc voltage follows a slow trend of dropping when DN is 3 mm, but a rapid trend of increasing when DN is 4 mm. Such difference observed
Fig. 3. Principle of hydro-magnetically confined PAC ceramic.
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W.J. Xu et al. / Journal of Materials Processing Technology 129 (2002) 152±156
Table 1 Essential experimental conditions Plasma power supply
Water-injection system
Magnetic field system
Workpiece and additional anode
Plasma gas and nozzle diameter
Rated power/60 kW Rated current/500 A Open voltage/240 V Arc current/300 A
Pressure/0.1 MPa Flow-rate/0±1.0 l/min Focal-plane-to-work/1 mm
Pole-circle diameter/15 mm Width of pole piece/3 mm Thickness of pole piece/5 mm Pole-piece-to-work/1.5 mm Pole-piece-to-nozzle/1.5 mm Magnetizing current/0±6 A
Workpiece/Al2O3 plate Thickness/6 mm
When nozzle diameter is 3 mm Flow-rate of N2/2.0 m3/h Flow-rate of Ar/0.8 m3/h When nozzle diameter is 4 mm Flow-rate of N2/2.6 m3/h Flow-rate of Ar/0.8 m3/h
Additional anode/steel plate Thickness/8 mm
Fig. 4. Effects of secondary confinements on arc voltage.
in the arc voltage changes against If stems from the incomplete radial compression of the arc by the magnetic field employed. This is to say, the outcome of the magnetic constriction is the balance between the effects on the arc cross-sectional area of the stretching and compressive forces in the two perpendicular directions (along the major axis and minor axis of the above-mentioned ellipse). Because the arc current I is kept at 300 A, for DN 3 mm, the degree of primary constriction of the arc is already great, and the arc is no longer easily compressed but tends to expand, thus the magnetic field leads to a slightly enlarged cross-section of the arc, meaning a decrease in current density and hence a drop in the arc voltage, while for DN 4 mm, the primary constriction is relatively weak and the arc is easily compressed further, so the results of applying the field are decreased cross-sectional area of arc and a corresponding increase in arc current density, and hence increased arc voltage. Moreover, these trends in U If curves show little change when a fixed Fw is added. 3. The incurred change in arc voltage under the dual constriction of water-injection and magnetic field is greater than the change incurred by either of the two constrictions, yet is not necessarily the linear sum of them as shown in Table 2. This fact can lead to such a generalization that the effect of any secondary constriction on arc depends on both its intensity and the degree of the arc's original constriction (including the primary constriction as well as other secondary constriction already applied). 4. Given appropriate conditions, when DN 4 mm, for example, the critical flow-rate of injected water at the
extinction of arc is approximately 1 l/min for all experiments, which means that the major cause for arc extinction is excessive water injection, but it is significant to note that the arc voltage at which arc-extinction occurs under water constriction is lower than that when magnetic constriction is also applied. This indicates the improvement in arc uniformity by the three-dimensional constriction and better resistance of the arc to disturbance. No similar phenomenon has been observed for a DN of 3 mm probably because it needs larger If for the magnetic ®eld to have a positive effect of compression, instead of the observed negative effect of expansion, on the arc. 3.2. Widths and beveling of cuts Widths and beveling of cuts are signi®cant process indexes which have a close connection to the degree of constriction and the shape of the arc. Figs. 5 and 6 show the relationships between widths and secondary constriction when DN is 3 and 4 mm, respectively. Table 2 Typical values of U (V) and DU (V) under different constrictions DN (mm) U0 jFw 0; If 0 Uw = DUw jFw 0:8 l=min Uf = D Uf jIf 5 A Ufw = DUfw jFw 0:8 l=min; If 5 A DUw DUf
3
4
149 157/8 146/ 3 155/6 5
124 136/12 131/7 140/16 19
W.J. Xu et al. / Journal of Materials Processing Technology 129 (2002) 152±156
155
Fig. 5. Effects of secondary confinements on kerf widths (DN 3 mm).
Fig. 6. Effects of secondary confinements on kerf widths (DN 4 mm).
The major features are as follows: 1. As the intensity of secondary constriction, e.g., If or Fw, or both, increases, the top width of cuts, Wa, decreases, and unlike the voltage, the decrease in the top width, DWa, under hydro-magnetic constriction, is always greater than that under either water-injection or magnetic field at a given intensity. This is because the cross-section of the arc changes from a round one to an elliptical one despite the possible drop in arc voltage due to the magnetic field, e.g., when DN is 4 mm. However, here again, the decrease in the width of cut under joint constriction is not necessarily the sum of those under two single constrictions for the similar reason previously discussed with arc voltage. Table 3 lists typical values and their changes in the top width of cuts. 2. With hydro-magnetic constriction, the difference between the top and bottom widths of a kerf Wa Wb representing the beveling of the kerf, declines significantly with the intensity of secondary constriction, which reflects a considerable improvement in the shape of the Table 3 Typical values of Wa (mm) and DWa (mm) under different constrictions DN (mm) 3 Wa jFw 0; If 0 Waw = DWa w jFw 0:8 l=min Waf = DWa f jIf 5 A Wa fw = DWa fw jFw 0:8 l=min; If 5 A DWa w DWa
4 5.4 4.9/ 0.5 4.9/ 0.5 4.6/ 0.8 1.0
6.3 5.6/ 0.7 5.5/ 0.8 5.0/ 1.3 1.5
plasma arc column, especially in the uniformity of the arc. However, the minimal value of the difference is closely related to the flow-rate of the injected water. When DN is 3 mm, for example, the bottom width of cut, Wb, decreases with Fw slowly when Fw is less than 0.8 l/min, but rapidly when Fw is larger than 0.8 l/min, indicating that exceedingly large Fw results in local over-constricting of the arc and hence a shortened arc column and weak capability of cutting, which in turn leads to increased beveling of the cuts. On the other hand, when DN is 4 mm, Wb increases as Fw is increased up to 0.8 l/min, indicating that, with proper increase in Fw, significant improvement in arc shape can be made, but if Fw exceeds 1 l/min, Wb drops, again indicating unexpected shortening of the arc column due to an excessively large water flow of injection. All in all, a proper match and balance between the waterinjection and the magnetic field is crucial for desirable hydro-magnetic constriction of the plasma arc. 3. As the magnetizing current If increases, the top width Wa decreases faster than the bottom width Wb does, enabling Wa Wb , the beveling of the cuts, to decrease steadily, which represents a positive effect of the threedimensional constriction by the magnetic field on the improvement in arc uniformity. However, if If is too high, then the phenomenon of stagnation occurs, reflecting the maturation of the given magnetic set-up. 3.3. Dross-free cutting speeds Experiments have shown that whether bottom-dross of cuts occur or not in the cutting process is not directly associated with the type of secondary constriction
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Table 4 Dross-free cutting speed V (m/min) under different constrictions DN (mm) Fw Fw Fw Fw
0; If 0 0:8 l=min; If 0 0; If 5 A 0:8 l=min; If 5 A
3
4
0.6±0.8 0.9±1.1 0.6±0.8 0.9±1.2
0.4±0.6 0.6±0.9 0.5±0.8 0.8±1.0
employed, but is the result of coordination of various process parameters. It is, therefore, natural that dross occurs with either water constriction or magnetic constriction, or even hydro-magnetic constriction. On the other hand, however, it must be pointed out that dross-free cutting speeds under hydro-magnetic constriction are usually higher than those under single water constriction or magnetic constriction (with the conditions comparable) as shown in Table 4. 4. Conclusions To reduce the kerf width and to improve the kerf quality, the hydro-magnetically con®ned plasma arc was used to cut Al2O3 ceramic plates. Both theoretical analysis and experimental results have proven the feasibility and validity of the newly advanced hydro-magnetic PAC, and the following conclusions can be drawn: 1. Synthesizing the advantages of both the water constriction and magnetic constriction, the hydro-magnetic constriction of plasma arc forms a three-dimensional constriction with improved shape and uniformity of the arc column, narrower kerfs, minimal beveling of cuts and higher dross-free cutting speeds than those under either water constriction or magnetic constriction alone. The effect of the joint constriction, however, is not necessarily the linear sum of those of the two single constrictions, since it depends also upon the degree of original constriction of the arc: the stronger the original constriction, the more difficult for further constriction to have an effect on the arc. 2. Hydro-magnetic constriction is capable of improving arc stability, which is reflected by the higher arc voltage at arc-extinction, than that under any single constriction.
3. For a given diameter of nozzle, a high quality cut can be produced by using a lower arc current than it is usually required in conventional PAC, while ensuring a fine arc shape and capability of cutting simply by employing hydro-magnetic constriction. At the same time the heat load on the nozzle can be reduced and the service life of the nozzle prolonged. Acknowledgements This work is supported ®nancially by the National Natural Science Foundation of China. The authors would like to take this opportunity to express their sincere appreciation (item numbers: 59805002 and 50175035).
References [1] S. Jahanmir, Ceramic machining research in the United States, Int. Ceram. Rev. 46 (2) (1997) 90±96. [2] Z.X. Jia, Y.F. Guo, Y.H. Liu, State of the machining of ceramic materials in China, J. Mater. Process. Technol. 62 (1±3) (1996) 20± 23. [3] A.B. Sadat, Machining of Advanced Ceramics, American Society of Mechanical Engineers, Petroleum Division, 1999, pp. 4±8. [4] Y.C. Shin, A.J. Shih, H. Neid, Machining and finishing processes for advanced materials, Am. Soc. Mech. Eng., Manuf. Eng. Div. 4 (1996) 221±226. [5] K. Willett, Cutting options for the modern fabricator, Weld. Met. Fabr. 64 (5) (1996) 186±188. [6] W. Lucas, S. Rennie, Cutting processÐthe right choice, Weld. Met. Fabr. 61 (3) (1993) 122±127. [7] S. Ian, Plasma arc cutting takes a slice at competition, Weld. Met. Fabr. 65 (7) (1997) 16±19. [8] Anon., Jet age technology brings new benefits, Mach. Product. Eng. 4 (1996) 3±5. [9] W.J. Xu, X.S. Jiang, Z.J. Jin, Experimental study on plasma arc cutting engineering ceramics, Electro-mach. Mould. 1 (2001) 29±30 (in Chinese). [10] H. Yamamoto, S. Harada, M. Uchida, T. Ueyama, Developments in air plasma cutting machines, Weld. Rev. Int. 13 (3) (1994) 3±6. [11] D. Harris, J. Lowery, High tolerance plasma arc cutting, Weld. World 37 (6) (1996) 283±287. [12] K. Matsuyama, Current status of high tolerance plasma arc cutting in Japan, Weld. World 39 (4) (1997) 165±171. [13] P.S. Zhao, S.Y. Zhu, G.H. Zhao, Plasma arc cutting with cusp magnetic field for secondary compression, J. Mech. Eng. 27 (5) (1991) 23±26 (in Chinese).
Study on ceramic cutting by plasma arc W.J. Xua,*, J.C. Fangb, Y.S. Lua a
School of Mechanical Engineering, Dalian University of Technology, Dalian 116023, PR China College of Mechanical Engineering and Automation, Huaqiao University, Quanzhou 362011, PR China
b
Abstract To reduce the kerf width and to improve the kerf quality, the hydro-magnetically con®ned plasma arc was used to cut engineering ceramic plates. By experiments and analyses, the characteristics of the hydro-magnetic con®ned plasma arc were explored and the effects of secondary con®nement on cutting quality, arc properties, and optimal process parameters were determined. By using this new method, the authors achieved better cutting quality and higher cutting speeds. Also, the possibility to reduce the heat load of the nozzle and thus enhance its service life and process stability was studied. When the nozzle diameter is 3 mm, the kerf width of the Al2O3 ceramic plate of 6 mm thickness is less than 4.6 mm, while the cutting speed reaches to 0.9±1.2 m/min. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Plasma cutting; Engineering ceramic; Plasma con®nement
1. Introduction Engineering ceramics are typical dif®cult-to-machine materials because of their high hardness and brittleness [1±4]. Compared with other machining methods, plasma arc cutting (PAC) is a very important thermal cutting process and has been used successfully used in cutting stainless steel, high hardness metal, high melting-point metal, and other dif®cult-to-machine alloys [5±8]. PAC's application to cutting ceramics, however, is still limited because most ceramics are not good electronic conducts, which makes it dif®cult for a transferred plasma arc to be produced between the cathode and the workpiece. Thus the authors presented the method of PAC of engineering ceramics with an additional anode [9] (shown in Fig. 1). The feasibility and effectiveness have been proven by experiments. Its principle is as follows: an additional anode is set up under the ceramic plate so as to enable the plasma arc to be maintained between the cathode and the additional anode. In this way the plasma arc melts and blows away certain material and forms a kerf on the ceramic plate. It is preferential for the plasma arc, as the source of heat, to be of such temperature and velocity that are as high as possible and uniformly distributed in order to minimize the heat difference induced by the thickness of the plate and to
* Corresponding author. Tel.: 86-411-4708410. E-mail address: [email protected] (W.J. Xu).
obtain the corresponding beveling of cuts, high quality cuts and dross-free cutting speeds. Under normal conditions, the plasma arc ejects from the nozzle under constriction by the plasma torch and the organized shielding gas in it, but the intensity of such intorch constriction is restricted by the permitted heat load on the nozzle. Moreover, the exposed part of the plasma arc in the gap between the nozzle and the workpiece is bound to undergo steep divergence due to its blending with air, leading to poor arc shape and considerable drop in the power density of the arc, and consequently unwanted cutting quality and capability [10]. With the aim of reducing or eliminating the diverging of the open section of the arc, and further raising its degree of constriction, techniques that apply additional constriction on the exposed arc column, the so-called ``secondary compression'', have been developed. For the convenience of discussion and study, the authors here term the constriction of the in-torch part of a plasma arc by the nozzle and the working gas stream as ``primary constriction'', with the constriction of the exposed part of arc, usually in non-mechanical ways, termed ``secondary constriction''. The secondary constriction, compared with the primary constriction, is much more important in affecting the arc's shape and cutting quality as well as cutting speeds. One method of secondary constriction that has been applied relatively successfully in industries is water-injection [11,12] which makes use of a conical water-injection to perform further constriction on the open part of the arc.
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 6 0 0 - 3
W.J. Xu et al. / Journal of Materials Processing Technology 129 (2002) 152±156
153
constriction will be increased, the arc shape improved, and higher cutting speeds and better cutting quality obtained. 2. Experimental principle and procedure
Fig. 1. Principle of PAC ceramic plate.
Although it is advantageous over other methods, this approach has certain inherent drawbacks: only a local planar constriction around the focal plane of the water injection can be formed with poor uniformity of the arc column; and with its strong feature of absorbing heat, an improperly large ¯ow-rate of injected water could merely induce instability or even cause extinction of the arc. Therefore, the water-constriction method is far from being perfect as far as precision cutting is concerned. In recent years, preliminary research has been made on cutting with a magnetically constricted plasma arc, of which the feasibility has been proven experimentally [13]. The principle involved is illustrated schematically in Fig. 2. In the 4-pole traverse magnetic ®eld, the plasma arc is subjected to compressive and stretching Lorentz forces in the two perpendicular directions and its acted part changes the round cross-section into an elliptic one, so that along its major axis narrower kerfs can be produced without altering any process variables. Other merits of this method are: no heat is carried away from the arc column; a three-dimensional constriction can be formed about the whole thickness of the pole-piece, and thus the uniformity of the arc column is enhanced; a narrower heated zone, but with more suf®cient heating of material along the cutting path is produced. Nevertheless, its defects are clear as well the incapability of establishing a complete radial compression on the arc column; limitations on the ®eld strength within the arc column by the performance available and possible thermal damage to the ®eld set-up and hence there is limited increase in arc power density. As the two constrictions mentioned above are mutually complimentary, it has been devised in this paper that they are integrated into one plasma torch to form a more effective, three-dimensional constriction so that the degree of arc
Fig. 2. Schematic illustration of magnetic-poles' arrangement.
As shown in Fig. 3, when the water-injection and magnetic ®eld are applied simultaneously, the plasma arc is acted by their joint constriction. Cutting is performed along the major axis of the ®eld-induced elliptical cross-section of the arc column. The hydro-magnetic plasma torch has been made by the authors. The essential conditions of the experiments are listed in Table 1. The secondary constriction in¯uences the arc properties, the cutting quality and speed having been examined by varying the ¯ow-rate of injected water Fw and the magnetizing current If, using nozzles of different diameters DN. 3. Results and discussion 3.1. Characteristics of plasma arc Cutting quality and cutting speed are closely related to the levels and distributions of power density, temperature and velocity of the plasma arc. It is very dif®cult and complicated, however, to measure them in practice. By contrast, the arc voltage U, which is easy to measure, is an indirect but synthetic parameter of arc properties in re¯ecting the input, transformation, transfer and equilibrium of energy within the arc, and thus it is a signi®cant index to analyze the effects of secondary constriction on the arc. Fig. 4 shows the experimental curves of arc voltages under various conditions of secondary constriction. The following notes could be made: 1. The arc voltage exhibits an approximately linearly increasing trend when the flow-rate of injected water, Fw, increases. This trend remains unchanged even if a constant magnetizing current, If, is applied also. 2. As If increases, the arc voltage follows a slow trend of dropping when DN is 3 mm, but a rapid trend of increasing when DN is 4 mm. Such difference observed
Fig. 3. Principle of hydro-magnetically confined PAC ceramic.
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W.J. Xu et al. / Journal of Materials Processing Technology 129 (2002) 152±156
Table 1 Essential experimental conditions Plasma power supply
Water-injection system
Magnetic field system
Workpiece and additional anode
Plasma gas and nozzle diameter
Rated power/60 kW Rated current/500 A Open voltage/240 V Arc current/300 A
Pressure/0.1 MPa Flow-rate/0±1.0 l/min Focal-plane-to-work/1 mm
Pole-circle diameter/15 mm Width of pole piece/3 mm Thickness of pole piece/5 mm Pole-piece-to-work/1.5 mm Pole-piece-to-nozzle/1.5 mm Magnetizing current/0±6 A
Workpiece/Al2O3 plate Thickness/6 mm
When nozzle diameter is 3 mm Flow-rate of N2/2.0 m3/h Flow-rate of Ar/0.8 m3/h When nozzle diameter is 4 mm Flow-rate of N2/2.6 m3/h Flow-rate of Ar/0.8 m3/h
Additional anode/steel plate Thickness/8 mm
Fig. 4. Effects of secondary confinements on arc voltage.
in the arc voltage changes against If stems from the incomplete radial compression of the arc by the magnetic field employed. This is to say, the outcome of the magnetic constriction is the balance between the effects on the arc cross-sectional area of the stretching and compressive forces in the two perpendicular directions (along the major axis and minor axis of the above-mentioned ellipse). Because the arc current I is kept at 300 A, for DN 3 mm, the degree of primary constriction of the arc is already great, and the arc is no longer easily compressed but tends to expand, thus the magnetic field leads to a slightly enlarged cross-section of the arc, meaning a decrease in current density and hence a drop in the arc voltage, while for DN 4 mm, the primary constriction is relatively weak and the arc is easily compressed further, so the results of applying the field are decreased cross-sectional area of arc and a corresponding increase in arc current density, and hence increased arc voltage. Moreover, these trends in U If curves show little change when a fixed Fw is added. 3. The incurred change in arc voltage under the dual constriction of water-injection and magnetic field is greater than the change incurred by either of the two constrictions, yet is not necessarily the linear sum of them as shown in Table 2. This fact can lead to such a generalization that the effect of any secondary constriction on arc depends on both its intensity and the degree of the arc's original constriction (including the primary constriction as well as other secondary constriction already applied). 4. Given appropriate conditions, when DN 4 mm, for example, the critical flow-rate of injected water at the
extinction of arc is approximately 1 l/min for all experiments, which means that the major cause for arc extinction is excessive water injection, but it is significant to note that the arc voltage at which arc-extinction occurs under water constriction is lower than that when magnetic constriction is also applied. This indicates the improvement in arc uniformity by the three-dimensional constriction and better resistance of the arc to disturbance. No similar phenomenon has been observed for a DN of 3 mm probably because it needs larger If for the magnetic ®eld to have a positive effect of compression, instead of the observed negative effect of expansion, on the arc. 3.2. Widths and beveling of cuts Widths and beveling of cuts are signi®cant process indexes which have a close connection to the degree of constriction and the shape of the arc. Figs. 5 and 6 show the relationships between widths and secondary constriction when DN is 3 and 4 mm, respectively. Table 2 Typical values of U (V) and DU (V) under different constrictions DN (mm) U0 jFw 0; If 0 Uw = DUw jFw 0:8 l=min Uf = D Uf jIf 5 A Ufw = DUfw jFw 0:8 l=min; If 5 A DUw DUf
3
4
149 157/8 146/ 3 155/6 5
124 136/12 131/7 140/16 19
W.J. Xu et al. / Journal of Materials Processing Technology 129 (2002) 152±156
155
Fig. 5. Effects of secondary confinements on kerf widths (DN 3 mm).
Fig. 6. Effects of secondary confinements on kerf widths (DN 4 mm).
The major features are as follows: 1. As the intensity of secondary constriction, e.g., If or Fw, or both, increases, the top width of cuts, Wa, decreases, and unlike the voltage, the decrease in the top width, DWa, under hydro-magnetic constriction, is always greater than that under either water-injection or magnetic field at a given intensity. This is because the cross-section of the arc changes from a round one to an elliptical one despite the possible drop in arc voltage due to the magnetic field, e.g., when DN is 4 mm. However, here again, the decrease in the width of cut under joint constriction is not necessarily the sum of those under two single constrictions for the similar reason previously discussed with arc voltage. Table 3 lists typical values and their changes in the top width of cuts. 2. With hydro-magnetic constriction, the difference between the top and bottom widths of a kerf Wa Wb representing the beveling of the kerf, declines significantly with the intensity of secondary constriction, which reflects a considerable improvement in the shape of the Table 3 Typical values of Wa (mm) and DWa (mm) under different constrictions DN (mm) 3 Wa jFw 0; If 0 Waw = DWa w jFw 0:8 l=min Waf = DWa f jIf 5 A Wa fw = DWa fw jFw 0:8 l=min; If 5 A DWa w DWa
4 5.4 4.9/ 0.5 4.9/ 0.5 4.6/ 0.8 1.0
6.3 5.6/ 0.7 5.5/ 0.8 5.0/ 1.3 1.5
plasma arc column, especially in the uniformity of the arc. However, the minimal value of the difference is closely related to the flow-rate of the injected water. When DN is 3 mm, for example, the bottom width of cut, Wb, decreases with Fw slowly when Fw is less than 0.8 l/min, but rapidly when Fw is larger than 0.8 l/min, indicating that exceedingly large Fw results in local over-constricting of the arc and hence a shortened arc column and weak capability of cutting, which in turn leads to increased beveling of the cuts. On the other hand, when DN is 4 mm, Wb increases as Fw is increased up to 0.8 l/min, indicating that, with proper increase in Fw, significant improvement in arc shape can be made, but if Fw exceeds 1 l/min, Wb drops, again indicating unexpected shortening of the arc column due to an excessively large water flow of injection. All in all, a proper match and balance between the waterinjection and the magnetic field is crucial for desirable hydro-magnetic constriction of the plasma arc. 3. As the magnetizing current If increases, the top width Wa decreases faster than the bottom width Wb does, enabling Wa Wb , the beveling of the cuts, to decrease steadily, which represents a positive effect of the threedimensional constriction by the magnetic field on the improvement in arc uniformity. However, if If is too high, then the phenomenon of stagnation occurs, reflecting the maturation of the given magnetic set-up. 3.3. Dross-free cutting speeds Experiments have shown that whether bottom-dross of cuts occur or not in the cutting process is not directly associated with the type of secondary constriction
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Table 4 Dross-free cutting speed V (m/min) under different constrictions DN (mm) Fw Fw Fw Fw
0; If 0 0:8 l=min; If 0 0; If 5 A 0:8 l=min; If 5 A
3
4
0.6±0.8 0.9±1.1 0.6±0.8 0.9±1.2
0.4±0.6 0.6±0.9 0.5±0.8 0.8±1.0
employed, but is the result of coordination of various process parameters. It is, therefore, natural that dross occurs with either water constriction or magnetic constriction, or even hydro-magnetic constriction. On the other hand, however, it must be pointed out that dross-free cutting speeds under hydro-magnetic constriction are usually higher than those under single water constriction or magnetic constriction (with the conditions comparable) as shown in Table 4. 4. Conclusions To reduce the kerf width and to improve the kerf quality, the hydro-magnetically con®ned plasma arc was used to cut Al2O3 ceramic plates. Both theoretical analysis and experimental results have proven the feasibility and validity of the newly advanced hydro-magnetic PAC, and the following conclusions can be drawn: 1. Synthesizing the advantages of both the water constriction and magnetic constriction, the hydro-magnetic constriction of plasma arc forms a three-dimensional constriction with improved shape and uniformity of the arc column, narrower kerfs, minimal beveling of cuts and higher dross-free cutting speeds than those under either water constriction or magnetic constriction alone. The effect of the joint constriction, however, is not necessarily the linear sum of those of the two single constrictions, since it depends also upon the degree of original constriction of the arc: the stronger the original constriction, the more difficult for further constriction to have an effect on the arc. 2. Hydro-magnetic constriction is capable of improving arc stability, which is reflected by the higher arc voltage at arc-extinction, than that under any single constriction.
3. For a given diameter of nozzle, a high quality cut can be produced by using a lower arc current than it is usually required in conventional PAC, while ensuring a fine arc shape and capability of cutting simply by employing hydro-magnetic constriction. At the same time the heat load on the nozzle can be reduced and the service life of the nozzle prolonged. Acknowledgements This work is supported ®nancially by the National Natural Science Foundation of China. The authors would like to take this opportunity to express their sincere appreciation (item numbers: 59805002 and 50175035).
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