Observation of the Keyhole during Arc

Once the keyhole was established, the width of the keyhole did not change with an increasing welding current and a decreasing welding speed

BY Y. M. ZHANG AND S. B. ZHANG ABSTRACT. Keyhole plasma equipment cost slowed this expansion plasma from the underside (Ref. 16). The is a unique arc welding process for deep initially (Ref. 1). During the late 1970s, welding current was changed to establish penetration. To ensure the quality of the remarkable achievements were made different modes associated with plasma welds, the presence of the keyhole is crit- in simplifying the arc welding: stable keyhole, unstable ical. Understanding of the keyhole will process, which originally was just as keyhole and nonkeyhole. They found certainly benefit the improvement of the complex to use as PAW (Ref. 1). As a re- that the photoelectric measurement of process and weld quality. Currently, the sult, the complexity and equipment the light emitted from the underside of size of the keyhole is assumed to be cor- cost of PAW were significantly reduced. the keyhole offered a basis for monitor- relative with the robustness of the key- Now, cost and complexity are no longer ing the keyhole PAW process (Ref. 16). In hole process in maintaining the keyhole. the major factors preventing the expan- more recent efforts, keyhole PAW To verify this assumption, the keyhole sion of PAW to further industrial appli- processes were numerically simulated and the weld pool were simultaneously cations. (Refs. 4, 10, 17–20). However, only monitored from the back side of the The stable state of the keyhole is an Keanini and Rubinsky extensively stud- workpiece. It was found that once the important issue in applying PAW (Ref. 5). ied the correlation between the keyhole keyhole is established, the width of the Although the stability of the keyhole has and the welding parameters during sta- keyhole does not change with an in- been a concern in many research works tionary welding (Ref. 10). creasing welding current and a decreas- (Refs. 5, 11–16), no accurate definition Due to a recent development ing welding speed. This implies the width has been given. Concerning weld quality in imaging technology (Ref. 21), the arc of the keyhole gives no adequate infor- control, the minimum requirement is the welding process can now be clearly mation about the stable state of the key- maintenance of the existence of the key- observed despite the arc light. It is hole and should not be used as an indi- hole. It seems that precise control of key- expected that clear observation of the cation of the robustness of the keyhole hole size could be an effective approach keyhole and the weld pool will provide process in plasma arc welding. to achieving a stable keyhole process and more reliable data for analyzing the quality welds. Thus, studies have been correlation between the geometry of Introduction conducted to correlate the keyhole size the keyhole and its stable state or the to the welding parameters that determine robustness of the process in weld quality (Refs. 10, 11, 16, 17). Keyhole plasma arc welding (PAW) maintaining the keyhole. The findings In a paper by Tomsic and Jackson (Ref. (Ref. 1) offers significant advantages over would be fundamental in guiding the 11), the plasma arc was terminated dur- conventional gas arc welding development of sensing and control ing welding. The workpiece was then (GTAW) in terms of penetration depth, technologies for keyhole PAW, thus sectioned to measure the keyhole. The joint preparation and thermal distortion facilitating the applications of this results were both the back-side width (Ref. 2). Although its energy is less dense unique process. and the weld-face width of the keyhole than (LBW) (Ref. 3) increased as the current increased or the and electron beam welding (EBW) (Ref. Experimentation welding speed decreased (Ref. 11). This 3), keyhole PAW is more cost effective suggested the widths of the keyhole as in- and more tolerant of joint preparation Experimental Setup dicators of the stable state of the keyhole (Ref. 4). Because of these distinct attrib- PAW process. Metcalfe and Quigley utes, keyhole PAW has found applica- The experimental setup is mounted a photo-transistor at the end of tions on the welding of structural steels shown in Fig. 1. The power supply is an the workpiece to monitor the efflux (Ref. 5), automobiles (Ref. 6), airplanes inverter designed for gas tungsten arc (Ref. 7), rockets (Ref. 8), space shuttles welding and plasma arc welding. Its (Ref. 9) and possibly on welding in space current ranges from 10 to 200 A and is (Ref. 10). KEY WORDS precisely controlled by an inner-loop Although keyhole PAW had poten- controller. The host computer adjusts tial to replace GTAW (Ref. 1) in many Bead-on-Plate Welds the welding current through the analog applications as a primary process for Butt Joint Welds output interface to the power supply. precise joining, its complexity and Flow Rate The torch, a regular commercial Keyhole straight-polarity plasma arc welding torch rated at 200 A, and the camera Y. M. ZHANG and S. B. ZHANG are with the RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT Welding Research and Development Labora- Orifice Diameter are attached to a manipulator. The tory, Center for Robotics and Manufacturing Plasma Arc Welding motion of the manipulator is computer Systems, University of Kentucky, Lexington, controlled. Ky. An ultra-high shutter speed vision sys-

WELDING RESEARCH SUPPLEMENT | 53-s Keyhole

Weld pool

Oxidized weld

Fig. 2 — Simultaneous imaging of weld pool and keyhole.

Fig. 1— Experimental Setup.

tem (Ref. 21) was used to simultaneously The flow rate of the image the keyhole and the weld pool was 16.5 from the back side of the workpiece — L/min (35 ft3/h) and 9.4 Fig. 1. The camera system consisted of a L/min (20 ft3/h) for the strobe-illumination unit (pulse laser), weld face and the back camera head and system controller. The side, respectively. The pulse duration of the laser was 3 ns, and flow rate of the plasma the camera was synchronized with the gas, the welding cur- laser pulse. Thus, the intensity of laser il- rent and the welding lumination during the peak was much speed ranged from 1.3 higher than that of the plasma. Using this L/min (2.8 ft3/h) to 2.6 Fig. 3 — Geometrical parameters of weld pool and keyhole. vision system, the weld pool would al- L/min (5.5 ft3/h), from ways be clearly observed and the plasma 55 to 95 A and from 1 from the keyhole completely eliminated to 3.5 mm/s. withstanding the perturbations in welding from the image (Ref. 22). In this study, parameters and conditions for maintain- both the keyhole and the weld pool need Results and Discussion ing a stable keyhole. to be imaged. By increasing the illumi- The geometrical parameters defined in nation area of the laser, the brightness of Assume the keyhole has been estab- Fig. 3 will be used to illustrate the exper- the laser illumination could be reduced lished under certain welding parameters imental results. They include the weld- so that it was close to the brightness of the and conditions. If we decrease the weld- face width of the weld pool (w), the back- plasma. Both the keyhole and the weld ing current or increase the welding speed, side width of the weld pool (wb) and the pool could, therefore, be imaged clearly the keyhole will eventually collapse. Or, back-side width of the keyhole (wh). and simultaneously — Fig. 2. The weld- if we increase the welding current or de- face width of the weld pool was mea- crease the weld speed, burn-through (a Welding Current and Speed sured off-line after the experiment using hole in the root bead) may occur. Of a structured-light 3-D vision sensor and a course, the changes in other welding pa- To verify the suggestion that the size vision algorithm developed in previous rameters or conditions may also cause the of the keyhole is correlative with the sta- work (Ref. 23). collapse and/or burn-through. The mini- ble state of the keyhole process, experi- mum changes in the welding parameters ments were conducted using varying Experimental Procedure or conditions that result in either collapse welding parameters. The method was to or burn-through can quantify the capabil- change the stable state of the keyhole and Bead-on-plate and butt-joint welds ity or the robustness of the process in then examine whether there is a corre- were made on 3-mm-thick stainless steel maintaining a stable keyhole or the stable sponding change in the keyhole size. The RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT (304) plates in the flat position. Pure state of the keyhole process. In the fol- keyhole size was measured by using the argon was used as the shielding gas and lowing discussion, the stable state of the back-side width of the keyhole as defined the plasma gas. The back side of the work- keyhole process will be used as a term to in Fig. 3. The welding speed and welding piece was also shielded using pure argon. describe the robustness of the process current were changed to alter the stable

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CD

Fig. 4 — Weld pool and keyhole under varying welding parameters. Plasma gas flow: 1.3 L/min (2.8 ft3/h), orifice diameter: 1.57 mm (0.062 in.). A — Bead on plate, welding speed 2 mm/s; B — bead on plate, welding speed 2.8 mm/s; C— bead on plate, welding current 65 A; D — butt-joint weld, welding speed 2.2 mm/s. state of the keyhole process. through or a collapse is about to Figure 4 shows the results of four ex- occur. A B periments conducted using an increasing In Fig. 6, the burn-through current or a decreasing speed. Both eventually takes place for all four bead-on-plate and butt-joint welds were experiments. The cause of the made. The flow rate of the plasma gas burn-through was the increase in and the diameter of the orifice were the the welding current or the de- same for the four experiments. As shown crease in the welding speed, and in Fig. 4, when the welding current in- both bead-on-plate and butt joint creased or the welding speed decreased weld experiments were con- to certain levels, the keyhole was estab- ducted. Experimental results C lished. Although the further increase in clearly demonstrated that the D the welding current or the further de- width of the keyhole did not pro- crease in the welding speed increased w vide any information or indication and wb, the width of the keyhole (wh) did about the burn-through that was not change correspondingly: wh re- about to occur. Also, experiments mained at approximately 1 mm. The have been performed using a de- same phenomenon can be observed creasing welding current or an in- from the images given in Fig. 5. creasing welding speed to cause a The experimental results in Figs. 4 and collapse of the keyhole. An exam- 5 showed that the width of the keyhole was ple is illustrated in Fig. 7. It can be Fig. 5 — Back-side view of keyhole and weld pool at dif- not changed by varying the welding cur- seen that the width of the keyhole ferent currents and welding speeds. Plasma gas flow: 1.3 3 rent or the welding speed when no burn- failed to predict the collapse. L/min (2.8 ft /h), orifice diameter: 1.57 mm (0.062 in.). RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT through or collapse occurred. It would be When the welding current in- A — Welding speed 2 mm/s, welding current 60 A; B— interesting to examine whether the width creases or the welding speed de- welding speed 2 mm/s, welding current 65 A; C — weld- ing speed 1.8 mm/s, welding current 70 A; D — weld- of the keyhole will change if a burn- creases, the stable state of the key- ing speed 1.6 mm/s, welding current 75 A.

WELDING RESEARCH SUPPLEMENT | 55-s A B

C D

Fig. 6 — Burn-through experiments. Orifice diameter: 1.57 mm (0.062 in). A — Bead on plate, welding current 55 A, plasma gas flow 1.3 L/min (2.8 ft3/h); B — butt-joint weld, welding current 63 A, plasma gas-flow rate 1.6 L/min (3.4 ft3/h); C — bead on plate, welding speed 2 mm/s, plasma gas- flow rate 1.3 L/min (2.8 ft3/h); D— butt-joint weld, welding speed 1.6 mm/s, plasma gas-flow rate 1.3 L/min (2.8 ft3/h).

Fig. 7 — Keyhole collapse due to decrease in welding current. Butt-joint weld, welding speed: 2.4 mm/s, plasma gas-flow rate: 1.3 L/min (2.8 ft3/h), orifice di- ameter: 1.57 mm (0.062 in).

Fig. 8 — Horizontal forces acting on keyhole wall. RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT

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Fig. 9 — Plasma keyhole arc welding using varied rate of plasma gas flow (bead on plate). A — Welding speed 2.8 mm/s, welding current 65 A, orifice diameter 1.57 mm (0.062 in); B — welding speed 3 mm/s, welding current 95 A, orifice diameter 2.36 mm (0.093 in).

hole changes accordingly. For example, change when the welding parameters the z direction. The thickness of a thin when the keyhole is barely established, a were changed. Therefore, in addition to circular element is ∆z. Assume that the slight decrease in the welding current the stable state of the keyhole process, mass of the plasma gas in any ∆z, de- and/or the flow rate of the plasma gas or the width of the keyhole is also not cor- noted by ∆m, does not change with z. It a slight increase in the welding speed may relative with the weld penetration. is further assumed that ∆m/∆z is only de- result in a collapse of the keyhole. This termined by the flow rate of the plasma Analysis implies that the stable state of the keyhole gas fp and the speed of the plasma gas needs to be improved to prevent the col- flow u. When fp is given, u depends on lapse. If the welding current and the flow The independence of the width of the the diameter of the orifice. From the ideal keyhole on the welding current and the rate of the plasma gas are increased gas law (Ref. 28), the plasma gas stagna- and/or the welding speed is decreased, welding speed is quite understandable. tion pressure Pg can be expressed as the process will withstand larger varia- In fact, once the keyhole is established, tions in the welding parameters, such as the plasma gas passes through the key- the welding current, the flow rate of the hole. Because the majority of the weld- plasma gas and the welding speed, with- ing current earths at the top surface of the out a collapse of the keyhole. The stable weld pool (Ref. 26), the stagnation pres- (3) state of the keyhole is improved. After the sure of the plasma gas flow plays a major welding current and the flow rate of the role in balancing with the surface tension where m˙ is defined as∆m/∆z, and k’ and plasma gas increase and/or the welding pressure plus the hydrostatic head to K are coefficients. When the keyhole is speed decreases to certain levels, further keep the keyhole open (Refs. 10, 16). stationary, the stagnation pressure of the increase in the welding current and the The horizontal forces acting on the plasma gas flow should balance the sur- flow rate of the plasma gas or decrease in keyhole wall are shown in Fig. 8. The sur- face tension pressure and the pressure the welding speed may cause a burn- face tension pressure that tends to close generated by the hydrostatic head. Equa- through. The stable state degrades. the keyhole is (Refs. 16, 27) tions 1–3 show that for the given mater- Hence, when the welding current or the ial and plate thickness h, the back-side welding speed changes, the stable state of Ps=Ts/r (1) width of the keyhole is only controlled by the keyhole process in terms of maintain- Ts, fp and u. ing a stable keyhole changes accordingly. where Ts is the surface tension constant The surface tension constant Ts is tem- Experimental results in Figs. 4, 6 and and r is the radius of the keyhole. (The perature dependent. When the welding 7 demonstrated, when the stable state of keyhole is assumed to be axially sym- current or the welding speed changes, the keyhole varied due to a change in the metric.) The pressure generated by the the temperature of the plasma, and, welding current or in the welding speed, hydrostatic head of the melted weld therefore, the temperature of the keyhole the width of the keyhole remained con- metal is (Ref. 16) surface, should change. However, for a stant. Therefore, the width of the keyhole given material, one may assume that the ρ is not correlative with the stable state of Pm = mgz (2) temperature of the keyhole surface, es- ρ the keyhole process or the robustness of where m is the density of the liquid pecially at the bottom of the keyhole, is the process in maintaining a stable key- metal, g is the gravity acceleration and z not subjected to a severe change. Hence, hole. Also, full penetration is measured is the coordinate along the thickness di- once the keyhole is established and the RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT using the back-side bead width of the rection — Fig. 8. At the bottom surface of heat input is sufficient to melt enough weld pool (Refs. 24, 25). In the experi- the plate, r = wh/2 and z=h. Divide the material, changes in both the welding ments, the back-side bead width wb did keyhole as thin circular elements along current and the welding speed should not

WELDING RESEARCH SUPPLEMENT | 57-s have a substantial influence on the back- the feasibility of using the back-side arc keyhole welding of structural steels. Jour- side width of the keyhole. width of the keyhole as a measurement nal of Materials Processing Technology 52: That the width of the keyhole is not of the stable state of the keyhole process. 68–75. dependent on the welding current has 15. Halmoy, E., Fostervoll, H., and Rams- Experimental results revealed that once land, A. R. 1994. New applications of plasma also been indirectly obtained for station- the keyhole is established, the width of keyhole welding. Welding in the World 34: ary keyhole PAW by Keanini and Rubin- the keyhole does not change with the 285–291. sky (Ref. 11) based on numerical analy- changes in the welding current and the 16. Metcalfe, J. C., and Quigley, M. B. C. sis. In their study, the heat input was welding speed, but it does change with 1975. Keyhole stability in plasma arc welding. measured by the initial plasma tempera- the changes in the flow rate of the plasma Welding Journal 54(11): 401-s to 404-s. ture rather than the welding current and gas and the diameter of the orifice. On 17. Kim, C.-J. 1994. Parametric study of the welding speed. It was found that the the two-dimensional keyhole model for high the other hand, the stable state of the key- power density welding processes. ASME Jour- diameter of the keyhole, thus, the width, hole process and the weld-joint penetra- nal of Heat Transfer 116(1): 209–214. at any given axial position is independent tion vary when either the welding cur- 18. Dowden, J., and Kapadia, P. 1994. of the initial plasma temperature (Ref. rent, the welding speed or the flow rate Plasma arc welding: a mathematical model of 11). Although the study did not attempt of the plasma gas changes. Hence, the the arc. Journal of Physics (D): Applied Physics to correlate the initial plasma tempera- width of the keyhole gives no adequate 27: 902–910. ture with the electric power inputs, they information on the state of the keyhole 19. Nehad, A.-K. 1995. Enthalpy tech- claimed that plasma isotherms for vari- nique for solution of stefan problems: appli- process and the weld-joint penetration. cation to the keyhole plasma arc welding ous arc power inputs nevertheless con- Also, it provides no predictions of the process involving moving heat source. Inter- firmed that the initial plasma conditions burn-through and the collapse that are national Communications in Heat and Mass are comparable to the initial plasma tem- about to occur. Therefore, in general, the Transfer 22(6): 779–790. perature. Keanini and Rubinsky’s numer- width of the keyhole should not be rec- 20. Hung, R. J., and Long, Y. T. 1996. Math- ical analysis supports the fact that the ommended as a critical parameter to ematical model of variable polarity plasma arc welding process. Proceedings of the National back-side width of the keyhole is inde- monitor and control either the keyhole pendent of the welding current during Science Council (A): Physical Science and En- process or the weld-joint penetration. gineering 20(1): 90–109. stationary keyhole PAW. 21. Hoffman, T. 1991. Real-time imaging References for process control. Advanced Material & Flow Rate and Orifice Processes 140(3): 37–43. 1. Craig, E. 1988. The plasma arc welding 22. Kovacevic, R., Zhang, Y. M., and L. Li. — a review. Welding Journal 67(2): 19–25. 1996. Monitoring of weld penetration based Although the back-side width of the 2. Tomsic, M., and Barhorst, S. 1984. Key- on weld pool geometrical appearance. Weld- hole plasma arc welding of aluminum with keyhole is independent of the welding ing Journal 75(10): 317-s to 329-s. variable polarity power. Welding Journal 23. Zhang, Y. M., and Kovacevic, R. 1997. current and the welding speed, the flow 63(2): 25–32. rate of the plasma gas and the diameter of Real-time sensing of sag geometry during GTA 3. Welding Handbook, Vol. 2: Welding welding. ASME Journal of Manufacturing Sci- the orifice both have significant influences Processes. 1991. 8th edition, American Weld- ence and Engineering 119(2): 151–160. on the width of the keyhole. This can be ing Society, Miami, Fla. 24. Zacksenhouse, M., and Hardt, D. E. seen from the dependence of the stagna- 4. Hsu, Y. F., and Rubinsky, B. 1988. Two- 1983. Welding pool impedance identification tion pressure of the plasma gas on the flow dimensional heat transfer study on the keyhole for size measurement and control. ASME Jour- plasma arc welding process. International speed, which can be changed by altering nal of Dynamic Systems, Measurement, and Journal of Heat and Mass Transfer 31(7): Control 105(3): 179–184. the diameter of the orifice and the flow 1409–1421. rate of the plasma gas. As can be observed 25. Zhang, Y. M., Wu, L., Walcott, B. L., 5. Martikainen, J. K., and Moisio, T. J. I. and Chen, D. H. 1993. Determining joint pen- from Fig. 9, for a given orifice, the back- 1993. Investigation of the effect of welding pa- etration in GTAW with vision sensing of weld- side width of the keyhole increases as the rameters on weld quality of plasma arc key- face geometry. Welding Journal 72(10): 463-s flow rate of the plasma gas increases. Also, hole welding of structural steels. Welding to 469-s. a larger width of the keyhole resulted in Journal 72(7): 329-s to 340-s. 26. Dowden, J., Kapadia, P., and Fenn, B. 6. Vilkas, E. P. 1991. Plasma arc welding of Fig. 9B due to the larger diameter of the 1993. Space charge in plasma arc welding and exhaust pipe system components. Welding cutting. Journal of Physics (D): Applied orifice. It is known that the stable state of Journal 70(4): 49–52. the keyhole changes with the flow rate of Physics 26: 1215–1223. 7. Irving, B. 1997. Why aren’t airplanes 27. Kroos, J., Gratzke, U., and Simon, G. the plasma gas and the diameter of the ori- welded? Welding Journal 76(1): 31–41. 1993. Towards a self-consistent model of the fice. Thus, in the case where a change in 8. Irving, B. 1992. Plasma arc welding keyhole in penetration laser beam welding. the stable state is caused by the plasma gas takes on the advanced solid rocket motor. Journal of Physics (D): Applied Physics 26: or the diameter of the orifice, the width of Welding Journal 71(12): 49–50. 474–480. the keyhole gives information about the 9. Nunes, A. C., et al. 1984. Variable po- 28. Evett, J. B., and Liu, C. 1987. Funda- larity plasma arc welding on the space shuttle change in the stable state. However, the mentals of Fluid Mechanics, McGraw-Hill external tank. Welding Journal 63(9): 27–35. Book Company, New York, N.Y. stable state of the keyhole process may be 10. Keanini, R. G., and Rubinsky, B. 1990. caused by different welding parameters, Plasma arc welding under normal and zero including the welding current and the gravity. Welding Journal 69(6): 41–50. welding speed. Therefore, in general, the 11. Tomsic, M. J., and Jackson, C. E. 1974. width of the keyhole and the stable state Energy distribution in keyhole mode plasma arc of the keyhole process are not inherently welds. Welding Journal 53(3): 109-s to 115-s. 12. Bashenko, V. V., and Sosnin, N. A. correlated. 1988. Optimization of the plasma arc welding process. Welding Journal 67(10): 233-s to Summary 237-s. 13. Martinez, L. F., et al. 1992. Front side RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT/RESEARCH/DEVELOPMENT Bead-on-plate and butt-joint weld ex- keyhole detection in aluminum alloys. Weld- periments have been conducted on 3- ing Journal 71(5): 49–52. mm-thick stainless steel plates to verify 14. Martikainen, J. 1995. Conditions for achieving high-quality welds in the plasma-

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