Stable keyhole welding process with K-TIG

Stable keyhole welding process with K-TIG

Journal of Materials Processing Technology 238 (2016) 65–72 Contents lists available at ScienceDirect Journal of Materials Processing Technology jou...

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Journal of Materials Processing Technology 238 (2016) 65–72

Contents lists available at ScienceDirect

Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec

Stable keyhole welding process with K-TIG ZuMing Liu a,b,∗ , YueXiao Fang a,b , ShuangLin Cui a,b , Zhen Luo a,b,∗ , WeiDong Liu a,b , ZhiYi Liu a,b , Qu Jiang a,b , Song Yi a,b a b

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, Tianjin 300072, China

a r t i c l e

i n f o

Article history: Received 4 May 2016 Received in revised form 30 June 2016 Accepted 3 July 2016 Available online 4 July 2016 Keywords: Keyhole stability Vision system TIG K-TIG GF-TIG

a b s t r a c t Keyhole welding process was successfully achieved by using a self-made K-TIG torch. High quality welds were produced in 8mm-thick stainless steel plates. To evaluate the keyhole stability in the novel manufacturing process, keyhole behavior was observed by using a vision system from backside of the workpiece. Efflux plasma interference was eliminated by using a filter glass. Keyhole exit behavior was imaged in real time during the welding process, keyhole size and position were extracted from the keyhole image sequence. Keyhole behavior parameters, including keyhole length, width, length/width, and deviation distance were measured. It was found that keyhole exit was deviated away from the torch axis even though the arc current was very high in K-TIG welding process, keyhole exit had an oval shape. Continuous open keyhole welding were easily achieved in K-TIG process, without unstable keyhole stage when the keyhole firstly opens. Keyhole size, shape and position were all related to welding current. The observation results lay solid foundation to understand the thermal-physical behavior in K-TIG, and for the further optimizing of the welding torch. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Tungsten Inert Gas (TIG) welding is a very important manufacturing process because it can produce high-quality welds. But, because the employed heat source has relatively low energy density and arc pressure in traditional TIG, its process efficiency is of low level. To enhance the efficiency of TIG process, the energy density and arc pressure of welding arc are improved by modifying the welding torch to achieve keyhole mode welding. The novel welding process is referred as keyhole TIG (K-TIG) by Jarvis in (Jarvis and Ahmed, 2000) or is referred as cathode focused TIG by Schnick et al. (2010). Basic principle of the K-TIG welding process is shown in Fig. 1. A cooling shoulder is added into the common TIG torch to significantly cool down the tungsten rod. The high temperature area in the tungsten tip, i.e., the cathode region, is restricted into a very narrow area. The current density in the cathode area is hence increased, and the arc constriction is further enhanced by electromagnetic effect.

∗ Corresponding authors at: School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China E-mail addresses: [email protected], [email protected] (Z. Liu), [email protected] (Y. Fang), [email protected] (S. Cui), [email protected] (Z. Luo), [email protected] (W. Liu), [email protected] (Z. Liu), [email protected] (Q. Jiang), [email protected] (S. Yi). http://dx.doi.org/10.1016/j.jmatprotec.2016.07.005 0924-0136/© 2016 Elsevier B.V. All rights reserved.

The whole arc jet diameter is restricted, and consequently, the energy density and arc pressure of the welding arc are increased. When the welding current is high enough (over than 300 A), the arc pressure is power enough to overcome the surface tension of the liquid weld pool, the melted metal is displaced away, a keyhole forms inside the weld pool. If the arc penetration ability is strong enough, the keyhole will fully penetrate the workpiece. Mid-thick workpiece can be welded by K-TIG in a single pass on square butt joints without additional filler. Lathabai et al. (2001) found that using the K-TIG welding not only reduces the manufacturing cost but also increases the process efficiency. K-TIG process has been carried out on the structural metal with relatively low heat conductivity, for example, CP titanium was weld by Lathabai et al. (2003), CP zirconium by Lathabai et al. (2008), stainless steel by Feng et al. (2015), and C–Mn steels by Lohse et al. (2013). Keyhole stability is the most important factor to determine the welding process stability and weld quality, while keyhole behavior involves intricate thermal, electrical, magnetic and fluid dynamics phenomena. In order to understand and determine the K-TIG keyhole stability, keyhole behavior should be firstly monitored. Liu et al. (2016) reported that, in the keyhole mode arc welding process, such as keyhole PAW, a few keyhole behavior sensing methods have been developed. Zhang and Zhang (2001) sensed the electrical potential of the efflux plasma exiting from the keyhole exit. Saad et al. (2006) detected the acoustic signal. Dong et al. (2001) mea-

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Fig. 1. Schematic of K-TIG or CF-TIG (Schnick et al., 2010).

sured the arc light density. Zhang et al. (2001) sensed the plasma cloud and the plasma arc reflection. Zhang and Ma (2001) further built a stochastic modeling to correlate the imaged plasma reflection shape to the keyhole behavior the during keyhole arc welding. These signals can be used to indirectly determine whether the keyhole is fully penetrated (open keyhole) or partially penetrated (close keyhole, blind keyhole). However, they cannot quantitatively characterize the keyhole shape and size. By using a high speed image system with a laser illumination strobe, Zhang and Zhang (1999) monitored the backside keyhole and weld pool behavior during PAW welding process, they found the keyhole width cannot be used to indicate welding process stability evolution. By using a common industrial camera-based observation system, Liu et al. (2013) found that the keyhole exit behavior parameters in PAW were directly referred to the keyhole stability and weld pool thermal state. Liu et al. (2014) further found that the keyhole exit behavior parameters in PAW determined the formation of weld defects. Because the heat source in K-TIG is different to the one in PAW, the keyhole behavior in K-TIG welding process should be determined. In this paper, a self-made cathode focused TIG torch is introduced. Keyhole mode welding testes were successfully carried out in 8mm-thick stainless steel plates. Weld formation quality was evaluated. Keyhole exit dynamic behavior was imaged in real time by a CCD camera equipped at the bottom side of the workpiece to evaluate the keyhole stability. Keyhole size and position were extracted from keyhole image sequence. The influences of the welding current on the keyhole parameters were measured. The observation results lay solid foundation to understand the thermal keyhole process and process control in K-TIG welding process. 2. Experimental procedure 2.1. Welding torch and keyhole mode welding To improve property of the TIG arc, cooling degree of the tungsten electrode is the primary factor to restrict the high temperature area in the tungsten tip. The self-made TIG torch is schematic in Fig. 2(a) and the photograph is shown in Fig. 2(b). The inner body of the torch is full of running water. The tungsten electrode is enfolded into the water-cooled torch body with a copper shoulder. The heat in the tungsten electrode, especially the heat in the tungsten rod tip,

Fig. 2. The self-made K-TIG torch.

Fig. 3. K-TIG welding process.

is conducted through the copper shoulder into the running water. Because the high temperature area in the tungsten tip is restricted, the cathode is hence focused into narrower region. As a result, current density in the cathode region is increased, the magnetic force is in turn enhanced. Heat density and arc force in the core region of the arc is highly strengthened. If the welding current is high enough, keyhole mode welding process may be achieved by using such a cathode-focused TIG torch. A typical image of the keyhole mode TIG welding process is shown in Fig. 3. The arc is free burning between the tungsten tip and the base metal to be welded. The workpiece is fully penetrated by the powerful welding arc, weld pool grows from the front side to the backside of the workpiece. Part of plasma gas spurts out from the keyhole exit with an inclined angle, forms an efflux plasma under the workpiece. In the front side, the plasma arc has very high

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Fig. 4. Experimental set-up.

Fig. 5. Typical keyhole image (w × h: 100pixel × 100pixel, −x denotes welding direction, welding current 575 A).

level illumination, it is not easy to acquire clear front side weld pool image. The backside efflux plasma is not so thick, if the illumination of the efflux plasma is eliminated, one can observe clear keyhole exit profile.

torch are both stationary, while the workpiece travels at the welding speed. A trigger source is used to sync the CCD camera imaging (keyhole image) and the NI-DAQ card capturing (welding current, arc voltage) during the welding.

2.2. System set-up

2.3. Keyhole parameters measuring principle

As shown in Fig. 4, the test system used to observe the K-TIG keyhole behavior includes the K-TIG welding system and the vision system. The K-TIG welding system consists of a constant current (C C) model welding power source (output capacity: 65 A–1000 A), a special designed TIG torch, a cooling water box, and the working table. In the vision system, welding current and arc voltage are sensed by two separated Hall elements, the two electrical signals are acquired by a NI-6002 DAQ card and a LabView-based software system in the host computer. The camera is equipped under the workpiece, aims at the keyhole exit and towards the weld pool with the viewing angle of 70◦ . When the keyhole is fully-penetrated (open), the arc plasma will spurt out from the keyhole exit to form an efflux plasma. If the efflux plasma illumination is filtered, keyhole exit can be imaged by the camera. In the vision system, the filter is a narrow-band glass filter (central wavelength 655 nm, band width 40 nm, and transparency 85%), the camera imaging speed is about 140fps, the distance of the lens to the target is about 200 mm in this paper. During the welding, the CCD camera and welding

Keyhole welding process was carried out by using the self-made K-TIG torch. The typical observed keyhole exit image in K-TIG is shown in Fig. 5(a). Keyhole exit shape is ellipse but not circle. From the leading edge (upper edge in image) to the rear edge (lower edge in image), grey density is not even in the keyhole region, but has a low density (dark) stripe in the middle part. This phenomenon is very similar to the keyhole image in PAW, which was reported by Liu et al. (2013). The keyhole edge line (Fig. 5(b)) was extracted by a processing algorithm introduced in (Liu et al., 2012). From the keyhole line, keyhole dimensional parameters can be calculated in pixels number. Keyhole dimensional parameters are defined in Fig. 6(a). Keyhole length is defined as the keyhole size along −x (welding direction), and keyhole width is the perpendicular dimension. Keyhole center point (x0 vs, y0 ) can be calculated from the edge line. As shown in Fig. 6(b), keyhole deviation distance (DS) is defined as the distance from the keyhole center point to the torch axis along the welding direction. During the zero-speed welding process, keyhole should be straight from top to bottom, the keyhole

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Fig. 6. Keyhole parameters definition.

axis is considered be coincide to the torch axis when the keyhole is stable. The deviation distance (DSpixel ) is calculated as DSpixel = x0 − x0,v=0

(1)

where, x0,v=0 is the keyhole center point position in pixels as the welding speed is zero. The camera coefficients were calibrated by pinhole camera mode. Two calibration coefficients, kx and ky were used in the measuring algorithm. The keyhole parameters in millimeter were calculated as Length (L) = kx (x3 − x1 )

(2)

Width (W ) = ky (y2 − y4 )

(3)

DS = kx x0 − x0,v=0

(4)





Fig. 7. Cross-sections of the K-TIG weld.

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Fig. 8. Keyhole images in different levels current welding process.

The keyhole shape was evaluated by the ratio of keyhole length to width (Length/Width). Details of the camera calibration method were reported in (Liu et al., 2013). The error estimation methods were reported in (Liu et al., 2012). In this research, the calibration coefficients along (x) and perpendicular (y) to the welding direction are kx = 1/30.8 mm/pixel, ky = 1/37.2 mm/pixel, separately. The measurement error is less than 3%. 2.4. Observation experiments Three groups of bead-on plate welding test were carried out on 304 stainless steel plates of dimension 200 mm × 80 mm × 8.0 mm (length × width × thickness). Shielding gas was pure argon, and the flow rate was 15 L/min. The tungsten rod had diameter of 6.0 mm; the tip angle was 60◦ , and electrode tip offsets 3.0 mm from the workpiece. Welding speed was 352.5 mm/min. The welding currents were selected at 485 A, 535 A and 575 A to produce different sizes keyhole. 3. Results and discussion With the K-TIG torch, keyhole welding process was successfully carried out in the 8mm-thich stainless steel plates. During the welding process, the arc voltages are 15.35 V, 15.61 V and 16.23 V in the 485 A, 535 A and 575 A keyhole welding tests. Cross sections of the resultant weld produced in the three test were made, as shown in Fig. 7. Fully penetrated weld was achieved of high quality. The profile of the cross-section of the K-TIG weld is very similar to that produced in keyhole PAW process. The observed keyhole images in three levels welding current are shown in Fig. 8. The torch axis position was obtained from the stationary (welding speed is zero) welding keyhole images. As the welding speed is zero, keyhole channel should be straight. Because the arc jet is an axisymmetric heat source when the welding speed is zero, keyhole and weld pool should be coaxial to the arc jet and welding torch. Hence, a stationary keyhole center point was treated to coincide with the torch axis, and was regarded as the reference to calculate the deviation distance of other keyhole exit. In Fig. 8, the zero-speed keyhole image is circle, while the other three keyhole images are ellipse, i.e., the keyhole dimension along the welding direction is narrower than the perpendicular dimension. As the welding current increasing, keyhole increases dimensional size, and goes forwards along the welding direction and gets closer to the torch axis, but still behind the torch axis even through the welding current is very large. The camera started to image the keyhole process when the welding process starts, and stopped as the arc was extinguished. Keyhole

Fig. 9. Keyhole parameters (welding current 495 A).

dynamic behaviors during the whole welding process were imaged. After camera calibration and image processing, keyhole parameters were calculated. Figs. 9–11 demonstrate the variation of the keyhole size, shape and deviation distance. In K-TIG welding process, keyhole does not yet fully penetrate at the starting stage, no keyhole was observed in the camera, and all the keyhole parameters are zero. This initial preheating period decreases from 2.422 s in 495 A to 1.926 s in 535 A and 1.171 s in 575 A because the heat source power is increased. As the keyhole firstly opens, keyhole has relatively large length during the very

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Fig. 10. Keyhole parameters (welding current 535 A).

Fig. 11. Keyhole parameters (welding current 575 A).

beginning short period (0.02 s–0.05 s), i.e., the ratio of length to width is larger than 1. And the keyhole position deviates very far away from the torch axis. After this initial open keyhole period, the ratio of length to width in keyhole exit decreases. Keyhole deviation distance decreases fast as well. The keyhole process is coupling controlled by multi-physics fields, including the thermal, electrical, magnetic and fluid fields. Keyhole state is easily disturbed. As shown in Fig. 9, welding current is 495 A, keyhole re-closes after the keyhole first opens about 0.5 s. This is an unstable welding process. During the closing stage, keyhole is shrinking in size and enlarg-

ing in deviation distance. The keyhole shape is also changing, and shape ratio transfers from about 0.8–1.6. After about 2.5 s, keyhole re-opens at 5.82 s. Keyhole experiences unstable keyhole period (keyhole opens and closes randomly), and gets stable open keyhole state. In Figs. 10 and 11, welding current was high enough to produce more heat and higher arc force to keep the keyhole continuously open, i.e., keyhole got continuous opening without any unstable process (randomly open and close keyhole as shown in Fig. 9). This

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keyhole experiences size-growing stage and quasi-stable opening stage. During the size-growing stage, both of the keyhole length and width are increasing, keyhole shape ratio is decreasing, and the deviation distance decreases heavily. During the quasi-stable keyhole stage, keyhole length and width, shape ratio, and deviation distance get quasi-stable values. In order to evaluate the influence of the welding current to the keyhole behavior parameters, averaged keyhole values in the quasisteady stage were calculated. Fig. 12(a) shows the effect of the welding current on the keyhole dimensional size. Because the penetration ability is increased with welding current, keyhole length and width are both growing in increasing welding current process. As indicted by the shape ratio in Fig. 12(b), keyhole is oval, i.e., keyhole has smaller length compared with its width. The shape ratio decreases in the increasing welding current K-TIG process. Keyhole exit position is also influenced by welding current in K-TIG process. In higher level current welding process, more heat will be deposited into the weld pool and the keyhole exit deviation distance decreases. 4. Conclusions • By using a self-made TIG torch, keyhole mode welding process was successfully achieved in 8mm-thick stainless steel plates with different levels of welding current. High quality welds were made. • Keyhole exit images are captured in real time during the welding process with a CCD-based vision system. It is found that keyhole exit size, shape and position are all dynamic in K-TIG welding process. During the quasi stable keyhole process, keyhole exit is oval, deviates behind from the torch axis even the welding current is very large. Continuous open keyhole was achieved during the K-TIG welding without any unstable keyhole stage; this is hardly observed in PAW keyhole process. • As the welding current increases, keyhole dimensional size enlarger, ratio of length to width decreased, keyhole exit deviation distance got smaller in K-TIG welding process. Acknowledgements The authors are grateful to the financial support for this research from the National Natural Science Foundation of China (No. 51505329) and Tianjin Research Program of Application Foundation and Advanced Technology (No. 15JCQNJC03400) and Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130032110004). References

Fig. 12. Averaged keyhole parameters versus welding current.

kind of continuous stable open keyhole process in K-TIG welding process has not been observed in PAW process. As reported in (Liu et al., 2012), keyhole repeats open and close random in short period when the keyhole firstly opens even though the welding current was very high or welding speed was very low in PAW. Keyhole stability is controlled by the coupling behavior between the arc and the weld pool. The heat source employed in K-TIG welding is a freeburning arc, while the welding arc is highly constricted in PAW. This may be the causing of the continuous open keyhole in K-TIG welding. During the whole welding, keyhole parameters are oscillating,

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