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Observation of the Keyhole During Plasma Arc Welding

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Observation of the Keyhole During Plasma Arc Welding Observation of the Keyhole during Plasma Arc Welding 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 arc welding 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 plasma cutting 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 tungsten 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 laser beam welding (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 Stainless Steel 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 shielding gas 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 54-s | FEBRUARY 1999 AB 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.).
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