Control of Keyhole Exit Position in Plasma Arc Welding Processdial size of the welding arc is...

Introduction Control of the keyhole behavior is of great significancefor practical application of the keyhole plasma arc welding(PAW) process. As the welding arc flows through the rela-tively small-sized water-cooled orifice in the nozzle, the ra-dial size of the welding arc is constricted (Ref. 1). Many im-portant properties of the constricted arc, including arc pres-sure, energy density, and arc temperature, are improvedheavily compared to the free-burning arc. The constrictedarc is called plasma arc for the ionization percentage in thearc column is extremely high (Refs. 1, 2). As the plasma arc

deposits onto the metal surface, the metal is melted and theliquid metal is pressed away to the side, and a keyhole formsin the weld pool. When the penetration ability becomes highenough, the keyhole will fully penetrate through the weldpool. As the welding torch moves along the welding direc-tion, most of the liquid metal is displaced to the rear part ofthe weld pool, cools down and solidifies into the weld (Ref.3). The weld quality mainly depends on the keyhole stabilityduring the welding process (Refs. 4, 5). Because the keyholeis easily affected by many factors (Ref. 6), real-time controlof the keyhole behavior is crucial to improving keyhole sta-bility and to obtaining high-quality welds (Ref. 7). Real-time control of the PAW keyhole process has attract-ed researchers recently. Constant size keyhole weldingprocess is chosen in the beginning period of the keyholePAW application (Ref. 8). However, it is very hard to realizea constant-size diameter keyhole during the welding processbecause the keyhole state can be easily disturbed by manywelding parameters. Pulsed-current PAW was thus devel-oped to overcome the shortcoming of the constant keyhole,and good pipeline welds were obtained (Ref. 9). A “one pulseone keyhole” strategy was first proposed by using the squarepulse waveform to weld 4.0-mm-thick workpieces (Refs. 10,11), yet it is hard to obtain high-quality welds if the work-piece is thicker. In order to improve keyhole stability in thethicker workpiece, two slow-dropping substages of variableslopes were added to the falling edge of the square pulse incontrolled-pulse keyholing PAW process (Refs. 12–14). Key-hole state transformation was smooth, and 8.0-mm-thickstainless steel plates were joined by a controlled-pulse key-holing PAW process (Ref. 15). Unfortunately, during pulsed keyhole PAW controlled byindirect keyhole signals, for example, efflux plasma voltage,the weld surface is not smooth and porosity easily emergesin the resultant welds. At the authors’ research institute, anindustrial camera-based vision system has been developedto observe the keyhole PAW process. It was found that notonly is the keyhole exit size changing dynamically, but thekeyhole exit position is also dynamically varying during the

WELDING RESEARCH

Control of Keyhole Exit Position in Plasma Arc Welding Process

Experiments laid a foundation for the practical application of the keyhole mode PAW process

BY Z. M. LIU, Y. K. LIU, C. S. WU, AND Z. LUO

ABSTRACT Keyholing behavior is controlled by using the keyhole exitimage signal to improve the welding stability in the plasma arcwelding (PAW) process. Keyhole plasma arc welding has multipleadvantages over the traditional gas tungsten arc welding (GTAW)process; however, the keyhole stability may be easily influencedby welding parameters such that obtaining high-quality weldsbecomes difficult, especially in mid-thickness weld metals. Weldsurface quality, porosity, and collapse are closely related to thekeyhole behavior, especially the keyhole exit deviation distance,which is the distance deviated from the keyhole exit center pointto the torch axis. A welding process control system was thus de-signed based on the keyhole exit image in this paper. The pulsewelding current waveform was specially designed, the averagedvalue of the keyhole deviation distance during one pulse waschosen as the controlled variable, and the falling slope wasselected as the controlling variable. A predictive controlalgorithm was designed. Experiments were conducted on 9.5-mm-thick stainless steel plate, and high-quality welds wereobtained.

KEYWORDS • Plasma Arc Welding • Keyhole • Closed-Loop Control • Keyhole Exit Deviation Distance • Controlled Pulse

Z. M. LIU and Z. LUO are with Tianjin Key Laboratory of Advanced Joining Technology & School of Materials Science and Engineering, Tianjin University, China.Z. M. LIU is also with School of Materials Science and Engineering, Shandong University, China, as is C. S. WU ([email protected]). Y. K. LIU is with Center forSustaining Manufacturing and Department of Electrical Engineering, University of Kentucky, Lexington, Ky.

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Liu Supplement June 2015_Layout 1 5/12/15 8:02 AM Page 196

welding process. Keyhole size parameters include keyholewidth and length; keyhole exit deviation distance is definedin Fig. 1. It is the distance of the keyhole exit center pointdeviated against to the welding direction away from thetorch axis (Ref. 16). If the thermal state of the weld pool ischanging, the keyhole exit deviation distance varies by afaster speed and by a wider margin than do the keyhole sizeparameters (Ref. 17). When the keyhole exit deviation dis-tance becomes large, the weld surface is rough and porosityforms easily; if the keyhole exit becomes small, on the otherhand, collapse or even melt-through will take place in theweld pool (Refs. 17, 18). In order to guarantee weld quality, a keyhole PAW processcontrol system was developed based on the keyhole exit de-viation distance. Welding current was pulsed, the waveformwas defined based on the controlled-pulse keyholing strate-gy. The falling slope was chosen as the controlling variable.A predictive control algorithm was derived. High-qualitywelds were obtained in varied speed welding controlprocesses. The results lay a solid foundation for the practicalapplication of the keyhole PAW process.

The Control System As shown in Fig. 2, the innovative keyhole PAW observa-tion and control system contains two subsystems, i.e., thekeyhole PAW system, and the data acquisition and predic-tive control system. In the PAW system, the transferred arcpower source was a Thermal Arc Ultra Flex 350 model andthe nontransferred arc (pilot arc) power source was a MillerElectric Maxstar® 150 STH, the PAW torch was a ThermalArc Dynamics PWM 300, and the coolant box machine wasmade by Miller. In the acquisition and control system, thewelding process electrical signals, including the welding cur-rent and arc voltage, were sensed by a current Hall sensorand a voltage Hall sensor, respectively, and wereacquired/recorded by an NI-6024 DAQ card; the keyholeprocess was imaged by the camera at the ratio of 62frames/s. After calibration and image processing, the key-hole state parameters can be extracted from the keyhole exitimage sequence. In the torch, the tungsten electrode diameter was 4.5mm, the orifice diameter 3.175 mm, throat length 5.0 mm,and the electrode setback 4.0 mm. The torch standoff dis-tance was 6.0 mm. The plasma gas and shielding gas werepure argon, and the flow rates were 2.8 and 20 L/min, re-spectively. The workpiece was stainless steel (SUS 304) witha dimensional size of 300 × 63 × 9.5 mm.

Principles of the Keyhole Behaviors Figure 3 is a sample keyhole exit image captured by theCCD camera mounted at the backside of the workpiece. Key-hole exit size and its distance from the keyhole center pointdeviated to the torch axis (as shown in Fig. 1) were extractedfrom the keyhole image (Refs. 19, 20). The keyhole exit de-viation distance varied by a faster speed and a wider marginthan did the keyhole size parameters as the weld pool ther-mal state was changed. The keyhole exit deviation distancehas a close relationship to the thermal state in the keyholeand weld pool, and can thus be used to reflect the stability

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JUNE 2015 / WELDING JOURNAL 197-s

Fig. 1 — Definition of the keyhole exit deviation distance.

Fig. 2 — Schematic of the control system setup.

Fig. 3 — The capturedkeyhole exit image (-x denotes thewelding direction, *denotes the keyholeexit center point).That means thewelding direction isalong the negative x axis.


of the penetrated keyhole (Ref. 21). If the observed keyholeexit deviation distance is too large, porosity forms easily inthe weld pool as heavy plasma flux pressure blow onto theliquid-gas interface, and more liquid metal distributes in thefront side weld pool to increase the front side weld height; ifthe keyhole exit deviation distance is too small, the weldpool may become particularly large, the weld pool will easilycollapse or even melt-through (Ref. 17). In order to improvethe keyhole stability, a “controlled pulse keyholing strategy”is proposed (Refs. 12, 13), the designed welding currentwaveform is shown in Fig. 4. As observed in the previous experiments, among thewaveform parameters in the current pulse, the Tk1 periodlength is the most critical factor to affect the keyhole-sustaining opening period (Ref. 21), and in turn can be usedto adjust the inclined degree of the leading keyhole wall andthe weld pool volume. Increasing Tk1 prolongs the keyhole-sustaining opening period and decreases the keyhole exit de-viation distance. In the controlled-pulse keyholing weldingprocess, the current waveform is defined in Fig. 4. The wave-form parameters are defined in Table 1, peak current (IP),base current (IB), and base current period (TB), second-stagecurrent falling period (TK2) are fixed to simplify the controlalgorithm; peak current period (TP), i.e., the blind keyholegrowing period is related to the weld pool thermal status, itis a variable and treated as the disturbance factor in the con-trol system; the first stage current falling period (TK1), i.e.,the keyhole-sustaining open period, is set as the controllingfactor, its varying ranges are listed in Table 1. In this research, deviation distance was used to indicatethe keyhole status, the averaged deviation distance in onepulse [averaged deviation distance per pulse (ADSP)] was se-lected as the controlled factor in the control system. Thesystem control process is shown in Fig. 5. During the weld-ing process, the transferred-arc current waveform from themain power source is controlled by the command signalsfrom the host control computer. The keyhole dynamic statevaries following the current waving process. The backside-mounted camera continuously images the keyhole exit be-havior, and keyhole exit deviation distance is calculatedfrom the processed image. As the keyhole opens, the devia-tion distance adds up until the keyhole is closed. During thebase current period, the ADSP is calculated, the difference(e) of the real-time ADSP and its set-point ADSP* is figuredout, and the controller outputs TK1 for the next currentpulse based on the inputted error. The keyhole state will beadjusted to narrow the error.

System Identification The mathematic model between the input factor (TK1) ofthe control system and the output factor (ADSP) should be

obtained by system identification. From test 1 to test 3, theinput factor varies randomly in different ranges; the rangesare listed in Table 1. The response model of the system out-put to the input was obtained by

ADSP(k) = 0.848 × ADSP(k – 1) – 0.27 × TK1(k –1) + 0.59 (1)

where ADSP (mm) is averaged deviation distance in onepulse, Tk1 (ms) is the first stage current falling period, and kis the pulse cycle number. The experimental observed ADSPand the model predictive value are shown in Fig. 6. The rootmean square error (RMSE)

is 0.1788 mm, which is very small, i.e., the error of the mod-el predictive value to the experimental results is very nar-row. In Equation 2, N is the sample number, y(k) is the sam-

∑ ( )( ) ( )= −=RMSE N ŷ k y kkN1 (2)21

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WELDING JOURNAL / JUNE 2015, VOL. 94198-s

Table 1 — Welding Current Waveform Parameters and Welding Speed

Test No. IP (A) TK1 (ms) TK2(ms) IB (A) TB (ms) Welding Speed (mm/s) Set Point (mm)

1 220 10–130 40 60 100 1.8 —2 220 10–100 40 60 100 1.8 —3 220 10–80 40 60 100 1.8 —4 220 10–130 40 60 100 1.8–1.6–1.95 2.3

Fig. 4 — Current waveform in the controlled-pulsed weldingprocess.

Fig. 5 — The closed-loop control process.


ple value, and y (k) is the predication value of the model. According to Equation 1, if the system becomes stable(i.e., reaches its steady state), the controlling factor and thecontrolled factor have the following relationship:

where TK1–s (ms) and ADSP–s (mm) are the inputted TK1 andoutputted ADSP as the system is stable. According to Equa-tion 3, some typical stable input and output values are listedin Table 2.

Control Algorithm

A predictive control algorithm is a robust control methodfor applications where the predictive model is not accurate(Ref. 22), and has been used widely in the welding controlfield (Refs. 23–25). In this research, keyhole behavior wascontrolled by applying the predictive algorithm. At instant k, denote ADSP(k) as y(k), TK1(k) as u(k). Equa-tion 1 can then be written as

y(k) = a × y(k – 1) + b × u(k – 1) + c (4)

where a, b, c are constants whose values are identified as0.848, –0.27, and 0.59 [based on standard least squares al-gorithm (Ref. 23)], respectively. At step (k + 1), (first step prediction)

y(k + 1) = y(k) + y(k + 1)= y(k) + a×y(k) + b×u(k) (5)

where y(k + 1) = y(k + 1) – y(k), u(k) = u(k) – u(k – 1).

At step (k + 2), (second step prediction) y(k + 2) = y(k + 1) + y(k + 2)

= y (k + 1) + a × y (k + 1) + b × u (k + 1)= y (k) + a × y (k) + b × u (k) + a2 × y (k) + ab × u k

= y(k) + a(a + 1) × y(k) + b(a + 1) × u(k) (6) At step (k + 3), (third step prediction) y(k + 3) = y(k + 2) + y(k + 3)

= y(k) + a(a + 1)2 × y(k) + b(a + 1)2 × u(k) (7)

At step (k + j), (j-th step prediction) y(k + j) = y(k + j – 1) + y(k + j)

= y(k) + a(a + 1)j–1 × y(k) + b(a + 1)j–1 × u(k) (8)

Because in Equation 8, a and b are constant, y(k + j) canhence be predicted. Based on the results of modeling and ex-perimental observation, if the predication step number isset to 5, the response time and the predication precision ofthe control system will get an acceptable trade-off. Supposing the control system reaches its steady state,i.e., the input and the output get stable, the system shouldmeet u(k + i) = 0(∀i > 0). The i-step prediction output val-ue is y(k + i) = y(k + i – 1) + y(k + i)

= y(k) + a(a+1)i–1 × y(k) + b(a+1)i–1 × u(k) (9) Suppose,

y(k + i) = y* (10)where y* is the set-point value of the system output. Thus, as the prediction step is set as 5, the control algo-rithm is

The period length of TK1 directly determines the keyholeopen period length in one pulse (Ref. 21). If the keyholeopens too short or too long, the keyhole will be unstable. Ac-cording to Equation 3, if TK1 is less than 10 ms or larger than

( ) ( )( )

( )Δ = −+

− Δu ky * y k

b a

ab

y k1

(11)4

10.27

0.59–0.152 (3)1T ADSPK s s( )=− −

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Table 2 — Predicted Input and Output in the Stable System

TK1–s(ms) ADSP–s(mm)

50 378 2.5

106 2

Fig. 6 — System identification results: A — Test 1; B —Test 2; C — Test 3.

Fig. 7 — Controlled process (Test 4).

A B C


200 ms, the penetrated keyhole may become unstable be-cause the heat input to the weld pool changes too heavily.The system input should be restricted to a range that willguarantee the robustness and the stability of the controlsystem. According to the observation results in tests 1–3, ifTK1�[10, 130] ms, the keyhole should not be so unstable. Inthe control system, the input is set as

Control Experiments

In order to test the robustness of the control system, thewelding speed was varied during test 4. The welded work-piece thickness was 9.5 mm. The set point of the control al-

gorithm (ADSP*) was 2.3 mm. The observed input and out-put of the control system are shown in Fig. 7. At the startingstage of the welding process, the welding speed is zero untila fully penetrated keyhole forms in the weld pool. At the be-ginning stage, the keyhole exit deviation distance was verysmall. TK1 is restricted at 60 ms during the first 10 pulses;the welding torch moves after the first completely penetrat-ed keyhole emerges, and the ADSP is increasing during thisstage. After the control algorithm begins to work, the sys-tem is not so stable; the heat input factor TK1 waves in alarge range. The control system stabilized after about 20pulse cycles. Even though the welding speed was changedduring the welding process, the controlled factor ADSPwaved in a narrow range around the set-point value. Figure 8 shows the keyhole image, current pulse wave-form, and keyhole exit deviation distance behavior duringthe different speed welding stages. The keyhole first pene-trates completely at the instant of 2.292 s. The observedkeyhole exit center point is considered to coincide with thetorch axis for the torch is stationary at this instant. Duringthe first stage, the welding speed is 1.8 mm/s, TK1 keeps in-u k 130 ms, if u k 130 ms;

u k u k , if 10 u k 130 ms;

u k 10 ms, if u k 10 ms.

( ) ( )( ) ( ) ( )( ) ( )

= >= < <= <

⎧

⎨⎪

⎩⎪

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Fig. 8 — Keyhole image, current waveform, and deviation dis-tance in Test 4: A — Welding current and deviation distance (PartI); B — keyhole image (Part I); C — welding current and deviationdistance (Part II); D — keyhole image (Part II); E — welding cur-rent and deviation distance (Part III); F — keyhole image (PartIII).

Fig. 9 — Weld surfaces (Test 4): A — Front side; B — back side.

A

A

B

B

DC

E

F


creasing as the welding point moves toward the middle ofthe workpiece plate, and the current pulse frequency isabout 3.6 Hz during this stage; the keyhole position is stableand the keyhole open period in one pulse is about 157 ms.During the second stage, the welding speed decreases to 1.6mm/s, so less heat should be deposited into the weld pool,TK1 becomes shorter to 30~60 ms to decrease the heat input,and the current pulse frequency increases to 3.8 Hz; keyholeopen period decreases to about 144 ms, and the keyhole exitposition varies steady around the set point. During the thirdstage, the welding speed increases to 1.95 mm/s, TK1 in-creases to 100~120 ms, the keyhole open period increases toabout 194 ms, the welding current pulse frequency decreas-es to about 3.0 Hz, the keyhole exit deviation distance wavessteady around the set point. During the welding process, thecontrol system can overcome the disturbance of the weldingspeed varying, the welding current falling period (TK1) wascontrolled to adjust the heat input into the weld pool; there-fore, the keyhole open period was adjusted and the keyholeposition varied smoothly. The resultant weld was very sound. The weld surfaces areshown in Fig. 9. During different speed stages, the weld sur-faces changed very little except at the arc starting point andextinguishing points. As shown in Fig. 10, the inner weldquality in the three stages was very sound. It was thus con-cluded that the derived predictive control algorithm wasable to control the process with acceptable accuracy.

Conclusion A controlled-pulse keyholing plasma arc welding systemwas designed based on the keyhole exit image. Deviationdistance of the backside keyhole exit was extracted from thekeyhole image captured by a backside-mounted camera. Theaveraged deviation distance in one pulse (ADSP) was chosenas the controlled variable, the first stage falling period (TK1)in the welding current waveform was set as the controllingvariable, the relationship between ADSP and TK1 was identi-fied, and the model based on predictive control algorithmwas applied. During the control experiments, the controlsystem was able to adjust the welding current waveform ac-cording to the keyhole deviation exit distance feedback, thuscontrolling the keyhole exit position. High-quality weldswere obtained in the experiments conducted on the 9.5-mm-thick stainless steel workpiece. The results laid a foun-dation for the practical application of the keyhole modePAW process. The experiments were completed at the University of

Kentucky, Lexington, Ky. The authors are grateful for the fi-nancial support for this project from the National NaturalScience Foundation of China (Grant No. 50936003). Theywould also like to thank Dr. YuMing Zhang and Dr. WeiJieZhang for their help with the experiments.

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Fig. 10 — Crosssections of the weld(Test 4): A — Part I; B — Part II; C — PartIII.

Acknowledgments

References

A B C


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study on PAW butt-welding process with controlled pulse key-holing strategy. Journal of Mechanical Engineering 47(3): 44–50. 16. Liu, Z. M., Wu, C. S., and Chen, M. A. 2014. Experimentalsensing of the keyhole exit deviation from the torch axis in plasmaarc welding. The International Journal of Advanced ManufacturingTechnology 71: 1209–1219. 17. Liu, Z. M., and Wu, C. S. 2012. The influence of keyhole be-haviors to the weld defects in plasma arc welding process. ResearchReport in Shandong University. 18. Liu, Z. M. 2013. Vision sensing and controlling of the back-side keyhole dynamic behaviors in controlled-pulse plasma arcwelding process. Doctoral thesis, Shandong University. 19. Liu, Z. M., Wu, C. S., and Gao, J. Q. 2013. Vision-based ob-servation of keyhole images in plasma arc welding. InternationalJournal of Thermal Sciences 63(1): 38–45. 20. Liu, Z. M., Wu, C. S., and Chen, M. A. 2012. Visualizing theinfluence of the process parameters on the keyhole dimensions in

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