simultaneous sensing of weld pool and keyhole in ... · simultaneous sensing of weld pool and...

WELDING RESEARCH

MARCH 2017 / WELDING JOURNAL 95-s

Introduction Plasma arc welding (PAW), as akind of high-density-beam welding,can be operated in keyhole mode be-cause of the high power of the plasmaarc. This open keyhole moves forwardto melt more metal during the weldingprocess (Ref. 1), which makes PAWachieve deeper penetration than gastungsten arc welding (GTAW) andmore tolerant of joint preparationthan laser beam and electron beamwelding (Ref. 2). It shows great appli-cation potential in manufacturing air-planes, automobiles, and rockets(Refs. 3–6).

The keyhole and weld pool stability,which are critical issues in PAW appli-cations, is sensitive to welding param-eters in conventional keyhole PAW(Refs. 7, 8). During PAW, a smallchange of welding current can disturbthe plasma arc heat and force balance,which leads to keyhole closure or weldpool discontinuity (Ref. 9). To improvethe weld quality and widen the appli-cation of the keyhole PAW process, the“one pulse, one keyhole” strategy wasproposed. The square pulse waveformwas used to weld 4.0–6.7-mm-thickworkpieces (Refs. 10, 11), yet it is onlyfocused on establishment of the key-hole without controlling keyhole clo-sure. Thus, the quasi-keyhole PAW

process was proposed and used to con-trol keyhole behavior. A high-qualityweld bead was achieved on a 3.6-mm-thick workpiece through controllingkeyhole establishment time and key-hole sustaining time (Ref. 12). To con-trol keyhole behavior and improvekeyhole stability in the thicker work-piece, the controlled-pulse keyholingPAW system was developed (Ref. 13). As shown in Fig. 1, the welding cur-rent waveform, which consists of peakcurrent(Ip), base current(Ib), durationof base current(Tb), two differentslopes(f1 and f2), and falling time(Tf1

and Tf2) at the falling stage of weldingcurrent, is specially designed to adjustbehaviors of weld pool and keyhole.Under this control strategy of weldingcurrent, the efflux plasma voltagestarts to be detected by the effluxplasma charge sensor at t2 after the IP

is applied for a certain time. It meansthat an open keyhole appears. At t3,the open keyhole signal reaches itsmaximum value, which is a little high-er than the given voltage (Sp) becauseof a delay of the feedback system, andthen this signal starts to decrease withdecreasing (descending slope f1) weld-ing current. So the duration of peakcurrent (Tp) varies and depends onpeak current, the thickness of the ma-terial, and welding speed. At t4, thecurrent decreases at a steeper descend-ing slope f2 (f2>f1) to make sure thatthe keyhole is closed completely. Afterthe Ib is applied at t5 to sustain theweld pool, the current is switched tothe Ip again to begin a new cycle at

Simultaneous Sensing of Weld Pool and Keyhole in ControlledPulse PAW

The behaviors of the keyhole and the weld pool in plasma arc welding can be used to indicate weld quality

BY G. K. ZHANG, J. CHEN, AND C. S. WU

ABSTRACT The behaviors of the keyhole and the weld pool are dynamicallycoupled incontrolledpulse plasma arc welding and can be used to indicate weld quality. The visionsystem was improved to detect the geometries of both the keyhole and weld pool at thebackside simultaneously during a whole controlledpulse keyholing period by one singleCCD camera without auxiliary illumination. With the assistance of an appropriate opticalfilter system, the unchanged aperture and exposure time of the camera was alsoadopted and can be used under a different current period. An image processing methodwas then proposed to extract the keyhole boundary in open keyhole status and weldpool boundary for the whole welding process. The influences of current waveformparameters on the welding process are studied and discussed. It was found that dimensions of the weld pool at the backside are determined by the heat input of the plasmaarc and keyhole duration. The keyhole moves forward during the welding process whilethe weld pool maximum width at the backside is located behind the keyhole center.

KEYWORDS • Simultaneous Detection • ControlledPulse Keyholing • Plasma Arc Welding • Dynamic Behavior

G. K. ZHANG, J. CHEN ([email protected]), and C. S. WU are with the Key Laboratory for LiquidSolid Structural Evolution and Processing of Materials, Ministryof Education, and Institute of Material Joining, Shandong University, China. G. K. ZHANG is also with the Leibniz Institute for Plasma Science and Technology,Greifswald, Germany.

Chen March 2017.qxp_Layout 1 2/13/17 9:52 AM Page 95

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WELDING JOURNAL / MARCH 2017, VOL. 9696-s

t6 (Tb = t6–t5). So this periodic openkeyhole generated by proper waveformparameters can produce deep penetra-tion welds without melt-through de-fects by PAW. Detecting and analyzing relation-ships among welding current, weld poolbehavior, and keyhole behavior is essen-tial for optimizing current waveformparameters and improving weld quality.Much research has been done to studykeyhole status by back side efflux plas-ma voltage or by front side plasma clouddischarge voltage between the work-piece and detection bar (Refs. 13–16).Photo-transistor can also be used toidentify keyhole status by the light ofthe efflux plasma (Ref. 8) or plasmacloud (Ref. 17). If the environmentalnoise is very low, sound signal is anoth-er effective method to describe evolu-tion of keyhole status, such as nonkey-hole, keyhole, and cutting (Ref. 18).However, these detection methods are

all indirect approach-es to reflect the key-hole behavior andcannot provide di-mension parametersof the keyhole, letalone the informa-tion of the weld pool.

Vision-basedsensing can give suf-ficient informationto directly analyzethe behaviors ofweld pool and key-hole, respectively(Refs. 19–22). Forexample, a vision-based sensing sys-

tem was used in GTAW to measure the3D weld pool surface in real time,which can provide sufficient informa-tion to predict the backside bead width(Refs. 23, 24). The 3D weld pool sur-face characteristic can also be used tocontrol the welding process and build awelding training system (Refs. 25, 26).Because of the big volume of the PAWtorch, it is hard to apply a 3D weldpool surface sensing system from theupside. While the backside keyhole isclose to the front edge of the weldpool, the plasma arc and keyhole willdisturb detection of the weld pool in-formation at the backside. An ultra-high shutter speed vision system wasused to acquire images of the weldpool and keyhole during the conven-tional PAW keyhole period (Ref. 7).This observation system was too ex-pensive to be applied in industry. Alow-cost visual system was developedto mainly monitor the keyhole behav-

ior in conventional PAW and con-trolled-pulsed PAW (Refs. 27, 28).However, only the relationship be-tween keyhole behavior and weldingprocess control quality was studied(Refs. 29, 30). To study the interaction betweenkeyhole and weld pool, an optical sen-sor and an infrared camera were usedto capture the images of the keyholeand its surrounding thermal field dur-ing PAW, respectively (Ref. 31). Thedisadvantage is that it took a lot oftime to finish coordinate transforma-tion and information integration. Alow-cost and time-saving visual sensorwas developed to observe the keyholeand weld pool from the backside of theworkpiece during the open keyhole pe-riod of conventional PAW process inprevious work (Ref. 32). However, thedynamic behaviors of the keyhole andweld pool, especially the weld pool, inblind keyhole period during con-trolled-pulse keyholing PAW process,still lacks research. And the image sys-tem should also be optimized as thelight intensity changes severely duringthis welding process. In this study, a cost-effective vision

system with a single camera was em-ployed to detect and monitor the dy-namic status of both the keyhole andweld pool from the backside of theworkpiece in the controlled-pulse key-holing PAW process. The influences ofwelding parameters on the weldingprocess were studied by analyzing geo-metrical dimensions of keyhole andweld pool in a whole pulse cycle (key-hole formation, expansion, contrac-tion, and closure).

Fig. 1 — Schematic of controlled pulse keyholing PAW.

Fig. 3 — LTE spectrum for Ar (Ref. 27).

Fig. 2 — Schematic of the experimental system for controlledpulse keyholing PAW.


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Experiment Setup and Observation Strategy

Setup

The experimental setup for the con-trolled-pulse keyholing PAW system isshown in Fig. 2. A commercially avail-able PAW machine was used for the in-vestigation. The plasma arc initiatedfrom the welding torch was under thecontrol of a computer. At the sametime, the computer could monitorprocess signals via input/output inter-faces from Hall sensors and a visionsensing camera. A Hall current sensor was used todetect welding current, which was ad-justed in real time according to thefeedback of efflux plasma voltage (Ve)measured by a Hall voltage sensor. The CCD camera (AM1101A), fixedunder the workpiece, was equipped withan infrared filter to simultaneously ob-serve images of both the keyhole andthe weld pool from the backside. Theobservation angle was set to 70 deg,and the distance between the cameralens and the target was 200 mm. The bead-on-plate welding experi-ments were conducted on stainlesssteel (201) plates. The flow rate of ar-gon shielding gas was 20 L/min at top-side and 10 L/min at backside. The ori-fice diameter and throat length of thetorch were 2.8 and 3.0 mm, respective-ly. The torch standoff distance to theworkpiece was 5.0 mm. The tungstenelectrode set back was 2.0 mm. Table 1lists the various process parametersfor different test conditions.

Observation Strategy

Imaging Principle

Plasma arc, welding pool, and basemetal have different radiation intensi-ties because of their different emissivi-ties and temperatures during the con-trolled-pulse keyholing PAW process.The purpose of this vision system is todetect these three kinds of radiationinformation at one observation win-dow, which is emitted from the back-side of the workpiece. For the stainlesssteel, there are two important temper-atures at the backside of the work-piece. Melting point (1800 K) standsfor the boundary between weld pool

Table 1 — Parameters of Welding Current Waveform

Test Ip Tp Tf1 Tf2 Ib Tb Welding Plate No. (A) (ms) (ms) (ms) (A) (ms) Speed Thickness (mm/min) (mm)

1 Varied Varied 250 100 60 100 120 8 2–5 140–170 Varied 40 60 60 100 120 6 6–9 150 Varied 40 60 60 100 100–130 610–13 150 Varied 20–80 60 60 100 120 6

Fig. 4 — Images of keyhole and weld pool with different filters. A — 660nm observationwindow; B — 1080nm observation window.

Fig. 5 — The backside of weld pool and keyhole images in different periods (Test 1). A —Open keyhole period; B — blind keyhole period.

BA

A

B


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and base metal. Boiling point (3000 K)means the boundary between plasmaarc and weld pool, respectively. If theemissivity of the weld pool is assumedto be a constant value at a differenttemperature, the range of workpieceradiation at the backside can be calcu-lated by Plank’s law (Ref. 33),

Under the action of Ip, the work-piece was fully penetrated. Once anopen keyhole was established first inthe weld pool, the plasma effluxflowed out from the keyhole channel,which induced a strong interference inthe visual signals of the camera. Get-ting clear images of the weld pool dur-ing the open keyhole period was a pri-mary consideration. To get rid of the influence of Ar ra-diation on CCD signals, two observa-tion windows (660 and 1080 nm) wereselected as shown in Fig. 3, which wasa LTE spectrum for Ar (Ref. 34). Theblack body infrared radiation ratios atdifferent temperatures (1800 and3000 K) and different wavelengths(660 and 1080 nm) are listed as,

B1800K(660 nm)/B3000K(660 nm) =1/127,

B1800K (1080 nm)/ B3000K (1080 nm) =1/19.

It is clear that there is a significantdifference (1:127) in the weld pool ra-diation between 1800 and 3000 K atthe observation window of 660 nm,which means the radiation differencebetween the unmelted and meltedzone was low for an 8-bit gray scaleimage. So it was hard for the camera toidentify the weld pool boundary. Onthe contrary, the weld pool radiationdifference (1:19) between 1800 and3000 K at an observation window of1080 nm was beneficial to making theweld pool boundary and keyholeboundary detected simultaneously bya single CCD camera sensor as shownin Fig. 4. So the observation windowof 1080 nm was a good choice for de-tection of the whole weld pool duringthe keyhole period. When the welding current wasfalling, the keyhole would close andleave the molten weld pool alone. Asmentioned above, the radiation differ-ence (1:19) between 1800 and 3000 K

at observation window of 1080 nmwas proper to identify the weld poolboundary and the radiation intensityof weld pool was still strong enough tobe detected without illumination.Hence, an observation window of1080 nm was also a good choice forthe detection of whole weld pool dur-ing the blind keyhole period. Mean-while, exposure time and lens apertureof the camera should be optimized tocapture the keyhole and weld pool evo-lution continuously in a completepulse period with infrared filter (cen-tral wavelength was 1080 nm, band-width was 20 nm and transparencywas 90%). Figure 5 was captured whenthe exposure time was 1 ms and aper-ture was F1/8. It shows that not onlycan both keyhole and weld pool be de-tected during the open keyhole period,but also that the weld pool image canbe captured during the blind keyholeperiod.

Image Analysis

According to the principles of cam-era sensors, pixel gray values of imagesare the reflection of objective infraredintensity. For a constant emissivity,

BT (� )= 2hc2

�5 � 1

ehc/�kT �1(1)

Fig. 7 — Weld pool and keyhole boundary. A — Open keyhole; B — blind keyhole.

Fig. 6 — Weld pool gray level distribution. A — Open keyhole; B — blind keyhole.

A

A

B

B


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higher temperature results in strongerinfrared radiation intensity. Therefore,the gray levels of images are indirectreflections of temperature and can beused to analyze backside weld pool images. Figure 6 is the pixel gray graph thatY-direction scanning was conducted inFig. 5 along three lines from the rearpart to the front part (X = 100, 150,200 pixels in Fig. 6A and X = 150, 200,250 pixels in Fig. 6B). During the open keyhole period,the plasma arc penetrated the work-piece and pushed the weld pool away.Accordingly, the gray level distributionof the backside weld pool image wasaffected by efflux plasma arc, whichinduced the central peak (X = 100,150) in Fig. 6A. The influence of effluxplasma arc on gray level gradually di-minishes at the rear part of the weldpool (X = 200). During the blind keyhole period, ef-flux plasma disappeared as the key-hole was closed. The shape of gray lev-els presents a ‘basin’ as shown in Fig.6B. According to Figs. 5B and 6B, theunfused metal reaches its highest tem-perature and peak infrared radianceintensity (peak gray level) at theboundary of the weld pool. Then thegray level obviously decreased in theweld pool though its temperature ishigher than unfused metal. The reasonis that the radiance of gray body wasdetermined by both temperature (T)and emissivity () as shown in Stefan-Boltzmann law (Ref. 33)

j = T4 (2)

was influenced by the surface con-dition. The smoother the surface ap-peared, the lower the became. Theemissivity of the weld pool was muchless than unfused metal since weld pool

can be regarded as a mirror. Thus, theradiation intensity (j) of the weld poolwas relatively lower than solid metal aswell as the gray level. And the gray leveldiscrepancy between melting weld pooland solid metal also made the weld pooledge easily detected. During the blind keyhole period, thepeaks of gray level were selected as weldpool boundary for each Y-directionscanning line, as shown in Fig. 6B.Through image processing, the weldpool boundary can be obtained by link-ing the peak points (dashed lines in Fig.6). The sharp transition area from maxi-mum gray level to ‘basin’ is the mushyzone, which is made of both solid andmolten metal. As shown in Fig. 6A, theefflux plasma can only form peak infront of the weld pool. The samemethod can be applied to detect weldpool boundary during the keyhole peri-od. For each Y-direction scanning line,the first and third peaks were selectedas the weld pool’s front-side boundary,

while the peaks of gray level were select-ed as the weld pool boundary at theweld pool’s rear. As the gray level of the keyhole washigher than 200, it could be dividedfrom the backside image through im-age segmentation, and then a cannyedge detector was used to detect key-hole boundary, which is described inprevious research (Ref. 32). By using the image processingmethods above, the edge and sizes ofkeyhole and weld pool were obtainedoffline after experiments in differentsituations, as shown in Fig. 7.

Results and Discussion

Keyhole and Weld Pool Size Evolution

During controlled-pulse keyholingPAW, the behaviors of keyhole andweld pool can affect the process stabil-ity and weld quality. For Test 1, weld-ing current and arc voltage are shown

Fig. 9 — The sequential images of keyhole and weld pool in one pulse.

Fig. 8 — The sampled electric signals in one pulse. A — Welding current and plasma arc voltage; B — welding current and efflux plasmavoltage.

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in Fig. 8A and efflux plasma voltage isshown in Fig. 8B. The images of bothkeyhole and weld pool in a whole pulsecycle are illustrated simultaneously inFig. 9. The length and width of key-hole and weld pool are defined as geo-metrical dimensions and perpendicu-lar to the welding direction, respec-tively. Figure 10 is the dynamic varia-tion of keyhole and weld pool dimen-sions in one whole cycle. During the base current period(15.5–15.7 s), backside keyhole wasclosed and the temperature of theweld pool dropped. However, the tem-perature of the weld pool at the back-side is still higher than the meltingpoint as heat energy was continuouslytransferred from plasma arc and melt-ed metal in the weld pool. As a result,the dimension of the backside weldpool could keep constant. During the peak current period(15.6–17.1 s), high welding currentprovided sufficient heat input andheat accumulation to the weld pool.

This peak current period can be divid-ed into two parts: blind keyhole periodand open keyhole period. At the firstpart (15.7–16.6 s), the depth of thekeyhole increased under high weldingcurrent and plasma arc force, while thegeometry of the backside weld pool re-mained the same, which means thatmajor heat input mainly acted on theinternal weld pool instead of the back-side of the workpiece. At the second part of the peak cur-rent period (16.6–17.1 s), once an openkeyhole was established after weldingcurrent was exerted for enough time,the plasma arc heated and melted moremetal. As the plasma arc force was big-ger than gravity and surface tension,the size of the keyhole expanded gradu-ally to make a new weld pool generateand grow in the old weld pool thatformed in the last pulse. Meanwhile,the temperature of the old weld pooldecreased step by step because its posi-tion moved far away from the heatsource. At 16.7 s, the old weld pool so-

lidified and the length of the total weldpool decreased sharply. However, thenew weld pool continued growing from16.7 to 17.1 s. Once keyhole size was high enough,the backside keyhole closed to avoidmelt-through defects during the weld-ing current falling period (17.1–17.27s). As plasma arc force is mainly decid-ed by welding current, a slowly de-creasing welding current (slope f1) wasused first to make sure that the size ofthe keyhole decreased smoothly with-out keyhole collapse. Then a steeperslope f2 was utilized to completelyclose the backside keyhole. Figures 9and 10 also show the size of the back-side weld pool reached its highest val-ue at 17.27 s when the backside key-hole closed completely.

Keyhole and Weld Pool Dynamic Position

Figure 9 shows that the relative po-sition of keyhole and weld pool

Fig. 11 — The keyhole center and weld pool front edge positionin one pulse.

Fig. 12 — The keyhole center and weld pool at the widest position.

Fig. 10 — The dimensions of weld pool and keyhole in one pulse. A — Keyhole; B — weld pool.

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changed over time under controlled-pulse keyholing PAW. The dynamic be-haviors of the keyhole center and weldpool front edge on the backside work-piece in one pulse cycle are illustratedin Fig. 11. Position 0 is the intersec-tion of the welding torch axis andbackside workpiece. The weld pool movement in theblind keyhole period determined theposition where the keyhole appears incontrolled-pulse PAW. During the peakwelding current period, the plasmaarc, which flowed to the wokpiecealong the welding torch axis, penetrat-ed the workpiece and appeared on thebackside. However, the keyhole posi-tion was far away from the weldingtorch axis when the backside keyholebegan to appear. Compared with thelow-temperature solid metal, flow re-sistance in a high-temperature meltingweld pool was much smaller. As a re-sult, the plasma arc flowed and pene-trated the workpiece along the frontedge of the weld pool, which was

formed in the last pulse. Under theheat function of plasma arc generatedby peak welding current, the meltingrate of base metal in the front of key-hole was greater than the weldingspeed. Hence, the front edge of theweld pool moved forward and the po-sition of the keyhole center becameclose to 0 position (the axis of thewelding torch) at the keyhole period inFig. 11. The plasma arc in keyhole,which is affected by current, graduallybecame weak if welding current de-creased. If metal melting rate in thefront of the keyhole was less than orequal to the workpiece movementspeed, the keyhole would stop movingforward (welding current falling peri-od), which is shown in Figs. 9 and 11. The behavior of the keyhole affect-ed weld pool behavior and position inthe keyhole period. The keyhole andweld pool were coupled together andthe front edge of the keyhole formedthe weld pool front edge. Plasma arc inthe keyhole heated the base metal and

made the metal transfer from theworkpiece to the weld pool. Therefore,the movement of the keyhole was thedirect reason why the weld pool ex-panded along welding direction. Whenthe open keyhole stopped moving andclosed at the base current period, theweld pool could not move forwardanymore. On the contrary, the weldpool would move backward with theworkpiece, as shown in Fig. 11. During PAW, the keyhole was locat-ed in the front of the weld pool. Themaximum temperature of the backsideworkpiece was located at the positionof the keyhole. However, the keyholewas always located between the solidmetal and the molten pool due to itsmovement. As shown in Fig. 12, thewidest position of weld pool and thelocation of keyhole have a certain lagbecause of thermal hysteresis.

Influences of Welding Parameters

During controlled-pulse keyholing

Fig. 13 — Data under different peak current (tests 2–5): A — Weld pool sizes at the end of the pulse; B — keyhole area in one pulse.

Fig. 14 — Data under different welding speeds (tests 6–9). A — Weld pool sizes at the end of the pulse; B — keyhole area in one pulse.

B

B

A

A


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PAW, the current waveform and weld-ing speed are critical to keyhole andweld pool behaviors. It is essential toanalyze the influence of welding pa-rameters on controlled-pulse keyhol-ing PAW. It is clear that the function ofIb was to keep a proper weld pool at thebackside during Tb and to protect itfrom overheating. Meanwhile, Tb is themajor parameter to guarantee the con-tinuity of backside reinforcement be-tween neighboring pulses during thewelding process. In this research, bothIb and Tb are fixed to constant values.Ip, Tf1, and welding speed were selectedto study the relationship betweenwelding current parameters and key-hole/weld pool behaviors. To decreasethe influence of keyhole forming re-sistance (gravity) and highlight the ef-fect of welding parameters, Experi-ments 2–13 were conducted on 6-mm-thick plate instead of 8-mm-thickplate. The plasma gas flow rate was 2.8L/min. The welding parameters arelisted in Table 1. According to previous analysis, key-hole is the heat source and the forcesource to widen weld pool size anddrive the weld pool moving forward. Ifthe duration of the keyhole is veryshort, weld discontinuities will appearbecause of inadequate melting on thebackside. However, if the duration ofkeyhole is very long, the weld pool willburn down because of overheating. Atthe end of each pulse, the weld poolreaches the largest size, which reflectsthe result of heat and force effects ofplasma arc. To investigate the influ-ence of welding current waveform pa-rameters Ip, Tf1, and welding speed on

the welding process, only one parame-ter was changed in each test based onexperiment design. 1) Influence of peak current In this part, only peak current waschanged to observe the behaviors ofkeyhole and weld pool. In tests 2–5, Ip

was set from 140 to 170 A. Figure 13illustrates the keyhole area and weldpool size at the backside workpiece indifferent tests. With the decrease of peak current, itbecame difficult to produce open key-hole as shown in Fig. 13A. Especiallywhen peak welding current decreased to140 A, the plasma arc could not pene-trate the workpiece because of low heatinput. As shown in Fig. 13B, keyholesize increased with the growth of peakcurrent. However, the duration of key-hole was almost the same at differentpeak currents as it was controlled by Sp

and Tf1. As a result, the size of the weldpool was mainly determined by the heatinput during the open keyhole period. Ahigher peak current will lead to a largerweld pool, which should be controlled incase of burn down. 2) Influence of welding speed Welding speed was one of the mostimportant parameters to control weld-ing heat input. In tests 6–9, only weld-ing speed was changed from 100 to130 mm/min to observe the behaviorsof keyhole and weld pool. Figure 14 il-lustrates the keyhole area and weldpool size at different welding speeds. The keyhole area is large at a lowwelding speed because of high heat in-put. A bigger keyhole brings more heatto melt base metal and increase theweld pool size under decreasing weld-

ing speed. 3) Influence of welding currentfalling time (Tf1) In controlled-pulse keyholing PAW,welding current had two falling slopes.The purpose of the first slope (f1) was todrive the keyhole to close smoothly. Theduration of the keyhole could be con-trolled by Tf1. In tests 10–13, Tf1 was setas 20, 40, 60, and 80 ms, respectively. For a fixed thickness workpiece,there exists the minimum welding cur-rent that makes arc force equal togravity and surface tension to keep thekeyhole open. During Tf1, the weldingcurrent was still higher than the mini-mum welding current to keep the key-hole open. However, Tf1 was much lessthan Tp, so the heat input during Tf1

was much smaller than the total heatinput in one cycle. Increasing Tf1 main-ly increased the duration of keyholetime instead of keyhole size as shownin Fig. 15A. Similarly, though a longerTf1 would give the plasma arc more of achance to increase weld pool size, boththe length and width of the weld poolincreased a little, which is shown inFig. 15B.

Conclusion 1) A vision system combined withan image processing strategy was de-veloped to detect both keyhole andweld pool information during a wholecontrolled-pulse keyholing periodwithout illumination. 2) During the blind keyhole period,heat input mainly acted on the inter-nal weld pool to increase keyhole vol-ume and the weld pool varied little at

Fig. 15 — Data under different current falling time (tests 10–13). A — Keyhole area in one pulse; B — weld pool sizes at the end of pulse.

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the backside. The keyhole position andthe maximum width position of thebackside weld pool were located be-hind the welding torch axis because ofthermal hysteresis. 3) The location of backside weldpool was at rest relative to the work-piece during the blind keyhole period.It only moved forward driven by key-hole under open keyhole status. Thesizes of weld pool and keyhole weremore susceptible to peak current andweld speed than welding currentfalling time(Tf1).

The authors are grateful to the fi-nancial support for this project fromthe National Natural Science Founda-tion of China (Key Program Grant No.50936003).

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