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Current waveform effects on CMT welding of mild steel
Journal of Materials Processing Technology 243 (2017) 395–404
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Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Current waveform effects on CMT welding of mild steel Maoai Chen a,∗ , Dong Zhang b , Chuansong Wu a a MOE Key Laboratory for Liquid-Solid Structure Evolution and Materials Processing, Institute for Materials Joining, Shandong University, Jinan 250061, PR China b Capital Aerospace Machinery Corporation, Beijing 100076, PR China
a r t i c l e
i n f o
Article history: Received 19 July 2016 Received in revised form 27 November 2016 Accepted 6 January 2017 Available online 9 January 2017 Keywords: CMT welding Current waveform Process stability Deposition rate Weld bead formation
a b s t r a c t Distributions of CMT periods and short circuit times are established by analyzing the electrical waveforms and metal transfer images sensed simultaneously under different welding conditions so as to evaluate the process stability for CMT welding of mild steel. Concentrated distributions for both CMT periods and short circuit times are observed in the whole adjustable range of boost current from 250 A to 320 A, indicating that changing the boost current doesn’t disturb the regularity and stability of CMT welding process obviously. With the boost duration ranging from 1.6 ms to 3.6 ms, the welding process is very regular and stable, and beyond this range, the process becomes unstable. The average CMT frequency changes very little with welding parameters, and fall in a narrow ranges from 80.1 Hz to 90.1 Hz. The deposition rate, average wire feed speed and droplet size increase with increasing boost current or boost duration, even the preset wire feed speed is kept constant. As the boost current or boost duration increases, the weld width and penetration increase noticeably, and the reinforcement varies very little. © 2017 Elsevier B.V. All rights reserved.
1. Introduction As an improvement of short circuit GMAW process, cold metal transfer (CMT) welding is characterized by low heat input, spatterfree metal transfer, excellent gap-bridging ability and elegant bead formation. It has attracted a lot of attention from researchers over the years. Extensive experimental studies have been conducted on CMT welding of thin gauge materials and welding of dissimilar metals. Pickin and Young (2006) examined the suitability of CMT welding for aluminum alloy and founded that CMT welding process exhibited a high wire melting coefficient while keeping a low heat input. It was competitive not only for welding of thinsection aluminum alloys but also for thick-section aluminum alloys. Feng et al. (2009) CMT welded pure aluminum sheet and observed that good weld appearance could be ensured due to the no-spatter welding process and low heat input promoted the gap bridging ability and reduced the deflection deformation of the thin sheets. The mechanical properties and microstructure characteristics of the AA6061 joints made by the CMT welding could be enhanced by implementing PWHT, as reported by Ahmad and Bakar (2011). Much work has been done on CMT welding of dissimilar metals.
∗ Corresponding author at: Qianfoshan Campus of Shandong University, 17923 Jingshi Road, Jinan 250061, PR China. E-mail address: [email protected] (M. Chen). http://dx.doi.org/10.1016/j.jmatprotec.2017.01.004 0924-0136/© 2017 Elsevier B.V. All rights reserved.
Zhang et al. (2007, 2009), and Cao et al. (2013a,b, 2014) carried studies on CMT welding of aluminum to galvanized steel and found that the thickness and the composition of the intermetallic compound layer varied with weld heat input. Further studies carried by Yang et al. (2013) showed that intermetallic layer could be kept lower than 10 m thick if the heat input remained low and an appropriate pre-setting gap. Zn coating played an important role not only in improving the wetting of molten aluminum to the steel plate but also in suppressing the formation of brittle intermetallic compound Alx Fey . Furthermore, a pre-setting gap and an appropriate post-weld heat treatment can improve the weld strength. Wang et al. (2008), Shang et al. (2012) and Cao et al. (2013a,b) CMT welded aluminum to magnesium alloy and found that the super low heat input of CMT welding helped to suppress the formation and growth of brittle intermetallic compound in weld metal, although some intermetallic compounds were still observed in the fusion zones. Cao et al. (2014a,b) CMT welded titanium-copper dissimilar joint with ERCuNiAl copper wire and found that the resultant lap and butt joints had fairly high tensile strength and fractured in the heat affected zone (HAZ) of Cu. A layer of intermetallic compounds consisting of Ti2 Cu, TiCu and AlCu2 Ti were observed in titaniumweld interface. Gungor et al. (2014) CMT welded 5083-H111 and 6082-T651 aluminum alloys and found that both the same metal CMT welds and the dissimilar metal CMT welds showed comparable tensile strengths with those of friction stir weld and much higher yield strengths due to low heat input. These studies mainly
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Fig. 2. Schematic of the current waveform of CMT welding. Fig. 1. Schematic of the welding experimental set-up.
focused on microstructures and properties of CMT weld deposited by using synergic welding program directly available in the CMT power source. A few research works have also been reported on the metal transfer behavior of CMT welding. Lorenzin and Rutili (2009), Zhang et al. (2009), Pickin et al. (2011) and Zhang et al. (2012) investigated the basic characteristics of metal transfer behavior in CMT welding of aluminum alloys, and observed that the typical CMT short circuit transfer occurred in lower power range, and hybrid transfer consisting of spray and short circuit transfers was detected in mid to upper power range. Mezrag et al. (2015) investigated the effect of the current waveform on metal transfer and heat transfer in CMT welding of galvanized steel to aluminum. The short circuit frequency was reported to increase with decreasing boost duration of the current waveform, and heat transferred to the base metal was lower for an equivalent deposit rate when the short-circuit frequency was higher. It can be noticed that the reported studies on CMT welding are mainly focused on similar welding of thin gauge aluminum and dissimilar welding of aluminum-steel, aluminum-magnesium and copper-titanium. No works on CMT welding of steel have been reported up to now. In this paper, the process characteristics of CMT welding of mild steel was investigated under different welding conditions to explore the effect of current waveform parameters on characteristics of cold metal transfer, deposition rate, heat input and weld formation. 2. Experimental 2.1. Experimental set-up As presented in Fig. 1, the experimental set-up used in this investigation consists of four parts: (1) the welding system, which includes a Transplus Synergic 2700 CMT power supply with a remote control panel, welding gun, working table and its movingcontrol unit, (2) the sensing system for the welding current and arc voltage, (3) the CMOS camera-based imaging system with a minimum framing rate of 150fps and (4) the industrial PC used as a central control computer. The sensing system consists of a Hall effect current sensor (SA1T50V5V6), a voltage sensor and a PCI data acquisition card. The Hall effect current sensor was used to measure welding current, and the voltage sensor was used to measure the arc voltage between the contact tip of the welding gun and the workpiece. The data acquisition card and the industrial PC were used to acquire and save the transient welding current and voltage data. These data could be on-line displayed and saved for off-line analysis. The imaging system consists of a high-speed CMOS camera (MV-D1024E-160) with an 808 nm interferential filter, an image
acquisition card (Micro Enable IV) and a 808 nm laser (UR808-30) used as background source. The filter installed in front of the camera was used to block the arc lights except those with a wavelength of 808 nm. The background laser beam was projected towards the droplet/wire. Lights blocked by the wire and droplets won’t be viewed by the camera, creating a shadow image on the image screen. The rest of the laser beam reached and illuminated the image plane. The resolution of the high-speed camera used during welding was 197 × 214 and the frame rate used was 3200fps. 2.2. Experimental procedure Bead-on-plate CMT welding was performed on 3 mm thick workpiece. The base material was Q235 mild steel. The workpiece dimensions were 250 × 70 mm. ER50-6 electrode wire with a diameter of 1.2 mm was used with positive polarity. Pure CO2 with a flow rate of 15 L min−1 was used as shielding gas. The tip-to-workpiece distance was kept at 14 mm. All welds were made in the flat position with a stationary welding gun and a welding table moving at a speed of 0.5 m min−1 . This set-up allowed the high-speed video camera to be stationary with respect to the wire, arc and weld pool. The power source can be programmed to produce advanced pulse-like waveforms to generate small molten droplets and detach the droplets at low current with the help of the mechanical retraction motion of the wire. Fig. 2 shows the typical current waveform during CMT welding of mild steel. The fundamental parameters determining the waveform shape are boost current (Ib ), wait current (Iw ), necking current (In ), detachment current (Id ), boost duration (tb ), necking time (tn ), detachment time (td ), current-rise rate, current-drop rate and tan-current drop. The necking time (tn ) and detachment time (td ) make up short circuit time (ts ). Among these waveform parameters, Ib , Iw , Id , tb , current-rise rate, currentdown rate and tan-current drop can be modified through RCU 5000i remote control. From the point of view of controlling the metal transfer and heat input, boost current (Ib ) and boost duration (tb ) are the two main parameters having the greatest effects. Table 1 gives the welding parameters used in this investigation. The CMT welding process is characterized by alternate wire feed and retraction movements with the hot and cold processes alternatively repeated. The instantaneous wire feed/retraction speed was measured with image analysis method. A mark was made on the surface of the exposed wire beyond the contact tip before welding. The wire motion speed was calculated by measuring the displacement of the mark on the successive high speed video images of wire. Before and after each weld deposit, the weight of workpiece was measured using a Denver balance with a precision of 10−3 g to get the deposited metal. The deposition rate is given by D = (W2 − W1 )/Tw
(1)
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Table 1 The current waveform parameters used in the experiment. wfs (m min−1 )
Ib (A)
tb (ms)
Iw (A)
Id (A)
current-rise rate (A/ms)
current-drop rate (A/ms)
tan-current down (ms)
4.0 5.0 6.0
250–320 250–320 250–320
1–5 1–5 1–5
40 50 90
50 80 120
500 500 500
300 300 300
1 1 1
Where D is the average deposition rate, W1 is the weight of the workpiece before welding, W2 is the weight of the workpiece after welding, Tw is the welding time taken to deposit the weld bead on the workpiece. The average droplet weight is given by W = (W2 − W1 )/N
(2)
Where W is the average droplet weight, and N is the number of CMT cycles (or the number of droplets) within the welding time Tw . The number of CMT cycles (N) (the number of droplets) can be expressed as: N = Tw /Tc
(3)
Where Tc is the mean CMT period. The mean CMT period (Tc ) was calculated by averaging out CMT periods for more than 300 CMT cycles from the captured current and voltage waveforms. Three weld cross-sections were cut from every welded sample with spark cutting machine to measure geometrical dimensions of the weld bead (width, penetration and reinforcement). Those cross-sections were polished and final finished with a 1 micrometer diamond suspension, and then etched with a solution of 3% HNO3 in ethanol. The cross-sectional profiles of the weld beads were observed using a Leica Z16 APO macro-scope with Leica LAS software. 3. Experimental results and discussion 3.1. Typical CMT welding cycle Fig. 3 illustrates the typical electrical and wire motion speed waveforms and corresponding metal transfer images taken when welding at a wire feed rate of 5.0 m min−1 , a boost current (Ib ) of 300 A and a boost duration (tb ) of 2 ms. The other welding parameters are listed in Table 1. From Fig. 3, it can be seen that arc ignites and extinguishes periodically accompanied by alternate feed and retraction of the electrode wire during CMT welding. A CMT welding cycle is consisted of four stages: (1) Boost stage: At the beginning of this stage (4550.8 ms), the arc reignites with current rising to the preset value of the boost current at the preset current-rise rate of 500 A/ms. Upon sensing the re-ignited arc, the digital process control unit of the CMT welding system signals the wire operation system to feed the wire forward. The wire speed ramps up from a minus value indicating the retraction of the wire. After about 1.4 ms, the wire feed rate becomes positive, which indicating that the wire start to be fed forward. Due to the inertia of the wire operation system, wire feed rate does not reach its steady-state maximum value at the end of this stage. The arc melts the workpiece and the wire, and a droplet is formed at the end of the wire. The total duration of this stage is 2.0 ms, which is consistent with the preset value (tb = 2 ms). (2) Wait stage: In this stage, the current is drastically reduced in order that the droplet is not detached but remains at the end of the wire. The wire feed rate increases up to its maximum value and this value is maintained until short circuit occurs. The contact between the droplet and the weld pool takes place at such a low current (about 50 A) that twinkle repulsive contact
Fig. 3. Electrical and wire feed rate waveforms for one CMT cycle and corresponding metal transfer images.
of the molten droplet with weld can be successfully avoided. The total duration of this stage is 3.92 ms. The maximum wire feed rate measured is about 40 m min−1 . (3) Necking stage: The droplet spreads into the weld pool smoothly under the combined action of surface tension force and gravitation. A molten metal column bridges the wire and the weld pool. The current rises to the level of necking current (In = 180 A) to enhance the electromagnetic force, speeding up the necking of the molten metal column. The wire feed rate decreases until it become a negative steady-state value, indicating the retraction of the wire. Necking time (tn ) is about 3.84 ms in this case. This stage is peculiar to the CMT cycle for welding of steel. It is not observed in the CMT cycle for welding of aluminum and dissimilar welding of steel to aluminum, as reported by Pickin et al. (2011) and Mezrag et al. (2015). Further research revealed that both the necking current (In ) and necking time (tn ) are constants for a given preset wire feed rate, independent of other settable welding parameters such as Ib and tb , as will be seen in Section 3.2.1. This suggests that In and tn are defined as non-adjustable parameters in the synergic CMT welding program. (4) Detachment stage. In this stage, the welding current decreases to the level of detachment current (Id = 60 A), and the arc voltage decreases to about 3 V in 0.36 ms. The wire feed rate is kept at about negative 22.5 m min−1 , indicating that the wire is mechanically retracted at a speed of 22.5 m min−1 . The retraction motion promotes the pinch-off of the molten metal column and enables the spatter-free droplet detachment at both low
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Fig. 4. Distributions of: (a) CMT periods (Tc ), (b) short circuit times (ts ), and (c) necking times (tn ) as a function of boost current (tb = 2 ms, wfs = 5.0 m min−1 ).
Fig. 5. Distributions of: (a) CMT period (Tc ), (b) short circuit times (ts ), (c) necking times (tn ) as a function of boost duration (Ib = 300 A, wfs = 5.0 m min−1 ).
current and low voltage. Then the arc reignites and the cycle recommences. Detachment time (td ) is 1.88 ms in this case. The boost stage and wait stage make up the arcing phase, while the necking stage and detachment stage constitute the short circuit phase. 3.2. Process stability 3.2.1. Effect of boost current Probability distribution of short circuit periods (or frequencies) and the corresponding standard deviation are appropriate measures to evaluate the process regularity and stability of GMAW-S, as reported by Hermans and Den Ouden (1999), Suban and Tuˇsek (2003). As an improved type of GMAW-S, CMT welding process can also be evaluated with these measures. Fig. 4 shows histograms of CMT period (Tc ), short circuit time (ts ) and necking time (tn ) as functions of boost current. The boost duration (tb ) was kept at 2 ms and the wire feed rate at 5 m min−1 . Over 300 CMT cycles were counted in the statistical calculation for each histogram. From Fig. 4(a), it can be seen that the CMT period distributions for different boost currents are quite similar. The CMT periods fell within a narrow range from about 10.0 ms to 13.0 ms with the probability peak occurring at about 11.5 ms, regardless of the boost current. Furthermore, both the mean periods (Tc ) and its standard deviations were approx-
imately the same for different boost currents. Therefore it can be concluded that boost current (Ib ) had little effect on the process stability when changing in its adjustable range from 250 A to 320 A. The short circuit time (ts ) distributions for different boost currents confirms this conclusion. As indicated in Fig. 4(b), the distributions of boost durations (ts ) were also quite similar for different preset boost currents. In addition, the standard deviation of boost duration varied very little. The necking times at different boost currents were always around 4 ms, as indicated in Fig. 4(c), suggesting that it was fixed as a constant of 4 ms in the CMT synergic welding program. 3.2.2. Effect of boost duration Fig. 5 illustrates the histograms of CMT period (Tc ), short circuit time (ts ) and necking time (tn ) as a function of boost duration (tb ) with the boost current kept at 300 A and the wire feed rate at 5 m min−1 . The distributions of CMT periods and short circuit times changed significantly with boost duration, as indicated in Fig. 5(a) and (b), indicating that tb had great effect on the regularity and stability of CMT process. The necking times at different boost durations distributed around 4 ms, as indicated in Fig. 5(c), confirming that it was defined as a process-independent constant of 4 ms in the CMT synergic welding program. At 1 ms and 3 ms boost duration, the CMT period distributions were much similar to that at 2 ms boost duration. However, similar process stability was only observed for 3 ms boost duration, and not
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Fig. 6. CMT cycle with an abnormally long ts (8.75 ms) recorded when welding with a boost duration of 1 ms.
for 1 ms boost duration during the experiment. This phenomenon can be explained by the distribution range of short circuit times. As seen in Fig. 5(b), the distribution of short circuit time at 3 ms boost duration was also similar to that at 2 ms boost duration, while the distribution of short circuit time at 1 ms boost duration was much wider than that at 2 ms boost duration. CMT cycles with abnormally long short circuit time were observed at 1 ms boost duration. Fig. 6 shows such an abnormal CMT cycle with a short circuit time of 8.75 ms, which is much longer than normal one (6.25 ms). In this cycle, the necked liquid metal column continued to neck down and elongated drastically after necking stage (frame 3601). It took 4.75 ms to break the necked liquid metal column. The liquid metal column seemed to adhere to the retracted electrode wire and be not ready to break. The main reason may be that the viscosity and surface tension of the liquid metal column was much higher due to the low temperature of molten metal column at 1 ms boost duration. At 4 ms and 5 ms boost duration, the CMT periods and short circuit times exhibited much broader distribution ranges and higher standard deviations. The CMT processes were less stable with some spatters, as indicated in Fig. 7. Both abnormal long and abnormal short CMT cycles were observed. Fig. 7(a) shows an abnormal long CMT cycle with a short circuit time of 8.44 ms taken when welding at a boost duration of 5 ms. In this case, the droplet volume was much larger than that for boost duration of 1 ms (compare
399
Fig. 7(a) with Fig. 6). It took much longer time to neck off the larger liquid metal column after necking stage, i.e. the detachment stage was much longer than that of a normal CMT cycle. Fig. 7(b) shows an abnormal short CMT cycle with a short circuit time of only 4.38 ms. The contact of droplet with weld pool occurred at a current of 250A due to the long boost duration (frame 7470.5), which is much higher than the normal waiting current (about 50 A). This made the necking-down of the molten metal column took place at the contacting point between the droplet and weld pool during the necking stage. The droplet separated with the weld pool in just 0.38 ms after the necking stage with no droplet transferred into the weld pool. Upon arrival of the detachment stage, the current fell to 25 A, much lower than the preset value of detachment current (80 A), and the welding voltage rose to an abnormal high value, indicating that the droplet separated from the weld pool. The arc reignited and boosted upon the retraction of the droplet from the weld pool. Thus, the short circuit time (ts ) was abnormally shorter than that of the normal CMT cycle. Further experiments were conducted with boost current kept at 300A and boost duration changing from 1–2 ms and 3–4 ms. The distributions of short circuit times are shown in Fig. 8. From Fig. 5 and Fig. 8, it can be seen that the CMT process was quite regular and stable for any boost duration ranging from 1.6 ms to 3.6 ms. 3.2.3. CMT transfer frequency Fig. 9 shows effects of boost current, boost duration and preset wire feed speed on the average CMT frequency. It can be noted that the CMT frequencies fluctuated very little with the boost duration, boost current and preset wire feed speed, and fell in a very narrow range from 80.1 to 90.1 Hz. This indicated that not only the boost duration and boost current but also the wire feed rate had no direct effect on the transfer frequency during CMT welding of mild steel. This observation was very different from that made in conventional short circuit transfer GMAW. In conventional, the short circuit transfer increased with increasing the preset wire feed rate and then decreased after reaching a maximum, as reported by Hermans and Den Ouden (1999). The explanation of this observation in CMT welding is that the metal transfer process is mainly controlled by the mechanically retraction of the wire. 3.3. Average arc power and current The average welding current I was calculated by time integration of the instantaneous current over the total welding time of the corresponding weld pass (Tw ):
I=
Tw 0
idt
Tw
Fig. 7. Abnormal CMT cycles recorded when welding at tb = 5 ms, wfs = 5.0 m min−1 : (a) with an abnormally long ts (8.44 ms), (b) with an abnormally short ts (4.38 ms).
(4)
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Fig. 8. Distribution of short circuit times as a function of boost duration: (a) tb changing from 1.2 to 1.8 ms, and (b) tb changing from 3.2 to 3.8 ms (Ib = 300 A, wfs = 5.0 m min−1 ).
Fig. 9. Effects of (a) boost current (tb = 2 ms), (b) boost duration (Ib = 300 A) and preset wire feed speed on average CMT frequency.
where i is instantaneous current, and Tw is the total welding time of the weld pass. The average arcing current I a was calculate by counting only arcing phase. It can be expressed as:
m tai idt i=1 0 Ia = m i t i=1 a
(5)
where m is the number of the CMT periods in the total welding time of the weld pass, tai is the duration of ith arcing phase. The average short circuit current I s can be expressed as:
m tsi idt i=1 0 Is = m i t i=1 s
mathematically linked with wire feed rate and the thermal energy for welding is also dependent on the preset wire feed rate. Fig. 11 shows the calculated average welding power, arcing power and short circuit power as functions of boost current or boost duration. Note that the average short circuit power is about 1kw, much higher than expected. Mezrag et al. (2015) calculated the average short circuit power in CMT welding of steel to aluminum and found that it was near zero regardless of the boost current and boost duration. For CMT welding of mild steel, the presence of necking stage in the short circuit phase accounted for the higher short circuit power. The welding power and arcing power increased as boost current or boost duration increased.
(6)
where tsi is the duration of ith short circuit phase. Fig. 10 shows the calculated average welding current, arcing current and short circuit current as functions of boost duration or boost current. It can be noticed that the average short circuit current remained nearly unchanged for different welding conditions. This may be because that the necking stage dominated the short circuit phase from the point of view of energy and the necking current and necking time were kept at constants independent of boost duration or boost current. The average welding current and average arcing current increased slightly with increasing boost current, but increased remarkably with boost duration even though the preset wire feed rate was kept at a constant of 5 m min−1 . Thus it can be concluded that the electrical parameters is partially decoupled with the wire feed rate. This observation is very different from conventional GMAW. In conventional GMAW, the welding current is
3.4. Deposition rate and droplet size Deposition rates at different welding conditions were measured using the method describe in Section 2.2. Fig. 12 shows the measured deposition rate as a function of boost current or boost duration. As can be seen in Fig. 12, the deposition rate increased with increasing boost current or boost duration even though the preset wire feed rate was kept constant. During welding, the melted and deposited wire shall be balanced by the wire pushed out of the contact tip, i.e. the deposition rate shall be equal to the actual wire feed speed because the spatter was low enough to be neglected for CMT welding. Hence it can be concluded that the actual wire feed speed was not equal to preset wire feed speed, representing a great departure from the conventional GMAW and pulsed GMAW. In conventional GMAW, wire is fed at a constant rate, the actual wire feed rate in the welding process is approximately equal to
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Fig. 10. Average current, arcing current and short circuit current as functions of: (a) boost current, (b) boost duration.
Fig. 11. Average welding power, arcing power and short circuit power as functions of (a) boost current, (b) boost duration.
Fig. 12. Effects of (a) boost current, (b) boost duration on deposition rate.
the preset wire feed rate. The wire feed rate and the level of welding current are mathematically linked, and the welding current is always determined by the preset wire feed rate. Thus, it is normal to substitute wire feed control for that of welding current control. This rule is also applied to pulsed GMAW, as reported by Amin (1983), Needham and Carter (1965) and Collard (1988). In CMT welding, the arc periodically reignites and extinguishes, as indicated in Fig. 3. During arcing phase, the wire is feed forward and melted by the arc. While during short circuit phase, the wire is retracted with the welding current reduced. The wire feed rate transient varies with the status of arc, however, the average wire feed rate should be equal to the average melting rate, i.e. the deposition rate because the melt-off must be balanced by the wire fed out of the contact tube. The melting rate depends on the average arcing current. The boost current and boost duration affect the average welding current and average arcing current, thus affect the average melting rate and actual wire feed rate. Fig. 13 shows the relationship between the average arcing current and the actual wire feed rate which is calculated from deposition rate. From Fig. 13, it can be observed that the actual wire feed rate increased approximately linearly with average arcing current no
Fig. 13. Relationship between the average arcing current and the actual wire feed rate.
matter how the average arcing current was adjusted. Similar observation was also made by Mezrag et al. (2015) in CMT welding of steel to aluminum. It can also be noticed that the actual wire feed
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Fig. 14. Effects of (a) boost current, (b) boost duration on droplet weight.
Fig. 15. Weld bead appearances made at (a) tb = 2 ms, (b) Ib = 300 A.
rate increased much faster when the average current was increased by changing the boost current with boost duration kept at 2 ms than when the average current was increased by increasing the boost duration with boost current kept at 300 A. Fig. 14 showed the droplet weight deduced by using the deposition rate and the average CMT periods (Eq. (2)). With boost current or duration increased, the wire melting rate and deposition rate increased and the transfer frequency remains nearly unchanged (see Fig. 9). Therefore, the droplet size increased approximately linearly with the boost current and duration, as indicated in Fig. 14. 3.5. Weld bead formation Fig. 15 shows the weld bead appearances made at different welding conditions. With the boost duration kept at 2 ms, sound
weld beads with uniform width and no surface defects were always obtained at any boost current (Ib ) ranging from 250 A to 320 A, as indicated in Fig. 15(a). As mentioned in Section 3.2, the welding processes were always kept regular and stable when welding at boost duration of 2 ms, regardless of the boost current. Obviously, the effect of the boost current on the weld formation was the consequence of the effect on the welding process. Sound weld appearances were also obtained when welding at different boost durations (tb ) ranging from 1 ms to 5 ms and a constant boost current (Ib ) of 300 A. However, spatters were observed around the weld bead when welding at 4 ms or 5 ms boost duration, indicating that the welding process was less regular and stable at 4 ms or 5 ms than at 2 ms and 3 ms boost duration. This observation was also in accord with that mentioned in Section 3.2.
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Fig. 16. Cross-section profile of welds produced at (a) tb = 2 ms, (b) Ib = 300 A.
Fig. 17. Cross-section dimensions of welds made at (a) tb = 2 ms, (b) Ib = 300 A.
The boost current and duration influence the arcing power, thus affecting the weld profile and size. Fig. 16 illustrates the cross-sectional profiles of welds made at different boost current or duration. It can be seen that both the boost current and boost duration had significant effects on the weld dimension. The measured weld dimensions at different welding conditions are shown in Fig. 17. It can be seen that as the boost current or boost duration increasing, the penetration and weld width increased noticeably, but the reinforcement varied very little.
4. Conclusions (1) The CMT cycle in welding of mild steel consists of four stages: boost stage, wait stage, necking stage and detachment stage. The necking stage is peculiar for the CMT welding of steel. During this stage, the short circuit current rises to a high level (necking current) to enhance the electromagnetic constriction force to facilitate the metal transfer. Both the necking current and necking time are fixed as a non-adjustable constant in the CMT welding program.
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(2) At a preset wire feed rate (5 m min−1 ), changing the boost current from 250 A to 320 A doesn’t affect the regularity and stability of CMT welding process obviously, while the boost duration, ranging from 1 ms to 5 ms, has pronounced effect. Fairly regular and stable processes are achieved in the range from 1.6 ms to 3.6 ms with the boost current kept at 300 A. (3) The deposition rate, average wire feed rate and droplet size increase with increasing boost current or boost duration, although the preset wire feed rate is kept constant. The average CMT frequency changes very little with the boost duration, boost current and wire feed rate. (4) As the boost current or boost duration increases, the weld width and penetration increase noticeably, and the reinforcement varies very little. Acknowledgment This work is supported by Shandong Natural Science Foundation under contract No. ZR2010EZ005. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jmatprotec.2017. 01.004. References Ahmad, R., Bakar, M.A., 2011. Effect of a post-weld heat treatment on the mechanical and microstructure properties of AA6061 joints welded by the gas metal arc welding cold metal transfer method. Mater. Des. 32 (10), 5120–5126. Amin, M., 1983. Pulse current parameters for arc stability and controlled metal transfer in arc welding. Met. Constr. 15 (3), 272–278. Cao, R., Yu, G., Chen, J.H., Wang, P.C., 2013a. Cold metal transfer joining aluminum alloys to galvanized mild steel. J. Mater. Process. Technol. 213 (10), 1753–1763. Cao, R., Wen, B.F., Chen, J.H., Wang, P.C., 2013b. Cold metal transfer joining of magnesium AZ31B-to-aluminum A6061-T6. Mater. Sci. Eng. A560 (1), 256–266. Cao, R., Feng, Z., Lin, Q., Chen, J.H., 2014. Study on cold metal transfer welding—brazing of titanium to copper. Mater. Des. 56 (4), 165–173.
Cao, R., Huang, Q., Chen, J.H., Wang, P.C., 2014a. Cold metal transfer spot plug welding of AA6061-T6-to-galvanized steel for automotive applications. J. Alloys Compd. 585 (2), 622–632. Cao, R., Feng, Z., Chen, J.H., 2014b. Microstructures and properties of titanium–copper lap welded joints by cold metal transfer technology. Mater. Des. 53 (1), 192–201. Collard, J.F., 1988. Adaptive pulsed GMAWcontrol-the digipulse system. Weld. J. 67 (11), 35–38. Feng, J.C., Zhang, H.T., He, P., 2009. The CMT short-circuiting metal transfer process and its use in thin aluminium sheets welding. Mater. Des. 30 (5), 1850–1852. Gungor, B., Kaluc, E., Taban, E., SIK S¸S¸, A., 2014. Mechanical and microstructural properties of robotic cold metal transfer (CMT) welded 5083-H111 and 6082-T651 aluminum alloys. Mater. Des. 54, 207–211. Hermans, M.J.M., Den Ouden, G., 1999. Process behavior and stability in short circuit gas metal arc welding. Weld. J. 77 (4), 137s–141s. Lorenzin, G., Rutili, G., 2009. The innovative use of low heat input in welding: experiences on ‘cladding’ and brazing using the CMT process. Weld. Int. 23 (8), 622–632. Mezrag, B., Deschaux-Beaume, F., Benachour, M., 2015. Control of mass and heat transfer for steel/aluminium joining using cold metal transfer process. Sci. Technol. Weld. Join. 20 (3), 189–198. Needham, J.C., Carter, A.W., 1965. Material transfer characteristics with pulsed current. Br. Weld. J. 12 (5), 229–241. Pickin, C.G., Young, K., 2006. Evaluation of cold metal transfer (CMT) process for welding aluminium alloy. Sci. Technol. Weld. Join. 11 (5), 583–585. Pickin, C.G., Williams, S.W., Lunt, M., 2011. Characterisation of the cold metal transfer (CMT) process and its application for low dilution cladding. J. Mater. Process. Technol. 211 (3), 496–502. Shang, J., Wang, K., Zhou, Q., Zhang, D., Huang, J., Li, G., 2012. Microstructure characteristics and mechanical properties of cold metal transfer welding Mg/Al dissimilar metals. Mater. Des. 34 (2), 559–565. Suban, M., Tuˇsek, J., 2003. Methods for the determination of arc stability. J. Mater. Process. Technol. 143 (12), 430–437. Wang, J., Feng, J.C., Wang, Y.X., 2008. Microstructure of Al-Mg dissimilar weld made by cold metal transfer MIG welding. Mater. Sci. Technol. 24 (7), 827–831. Yang, S., Zhang, J., Lian, J., Lei, Y., 2013. Welding of aluminum alloy to zinc coated steel by cold metal transfer. Mater. Des. 49 (8), 602–612. Zhang, H.T., Feng, J.C., He, P., Hackl, H., 2007. Interfacial microstructure and mechanical properties of aluminium–zinccoated steel joints made by a modified metal inert gas weldingbrazing process. Mater. Charact. 58 (7), 588–592. Zhang, H.T., Feng, J.C., He, P., Zhang, B.B., Chen, J.M., Wang, L., 2009. The arc characteristics and metal transfer behaviour of cold msetal transfer and its use in joining aluminium to zinc-coated steel. Mater. Sci. Eng. A 499 (1), 111–113. Zhang, H.T., Feng, J.C., Hu, L.L., 2012. Energy input and metal transfer behavior of CMT welding process. Mater. Sci. Technol. 20 (2), 128–132.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Current waveform effects on CMT welding of mild steel Maoai Chen a,∗ , Dong Zhang b , Chuansong Wu a a MOE Key Laboratory for Liquid-Solid Structure Evolution and Materials Processing, Institute for Materials Joining, Shandong University, Jinan 250061, PR China b Capital Aerospace Machinery Corporation, Beijing 100076, PR China
a r t i c l e
i n f o
Article history: Received 19 July 2016 Received in revised form 27 November 2016 Accepted 6 January 2017 Available online 9 January 2017 Keywords: CMT welding Current waveform Process stability Deposition rate Weld bead formation
a b s t r a c t Distributions of CMT periods and short circuit times are established by analyzing the electrical waveforms and metal transfer images sensed simultaneously under different welding conditions so as to evaluate the process stability for CMT welding of mild steel. Concentrated distributions for both CMT periods and short circuit times are observed in the whole adjustable range of boost current from 250 A to 320 A, indicating that changing the boost current doesn’t disturb the regularity and stability of CMT welding process obviously. With the boost duration ranging from 1.6 ms to 3.6 ms, the welding process is very regular and stable, and beyond this range, the process becomes unstable. The average CMT frequency changes very little with welding parameters, and fall in a narrow ranges from 80.1 Hz to 90.1 Hz. The deposition rate, average wire feed speed and droplet size increase with increasing boost current or boost duration, even the preset wire feed speed is kept constant. As the boost current or boost duration increases, the weld width and penetration increase noticeably, and the reinforcement varies very little. © 2017 Elsevier B.V. All rights reserved.
1. Introduction As an improvement of short circuit GMAW process, cold metal transfer (CMT) welding is characterized by low heat input, spatterfree metal transfer, excellent gap-bridging ability and elegant bead formation. It has attracted a lot of attention from researchers over the years. Extensive experimental studies have been conducted on CMT welding of thin gauge materials and welding of dissimilar metals. Pickin and Young (2006) examined the suitability of CMT welding for aluminum alloy and founded that CMT welding process exhibited a high wire melting coefficient while keeping a low heat input. It was competitive not only for welding of thinsection aluminum alloys but also for thick-section aluminum alloys. Feng et al. (2009) CMT welded pure aluminum sheet and observed that good weld appearance could be ensured due to the no-spatter welding process and low heat input promoted the gap bridging ability and reduced the deflection deformation of the thin sheets. The mechanical properties and microstructure characteristics of the AA6061 joints made by the CMT welding could be enhanced by implementing PWHT, as reported by Ahmad and Bakar (2011). Much work has been done on CMT welding of dissimilar metals.
∗ Corresponding author at: Qianfoshan Campus of Shandong University, 17923 Jingshi Road, Jinan 250061, PR China. E-mail address: [email protected] (M. Chen). http://dx.doi.org/10.1016/j.jmatprotec.2017.01.004 0924-0136/© 2017 Elsevier B.V. All rights reserved.
Zhang et al. (2007, 2009), and Cao et al. (2013a,b, 2014) carried studies on CMT welding of aluminum to galvanized steel and found that the thickness and the composition of the intermetallic compound layer varied with weld heat input. Further studies carried by Yang et al. (2013) showed that intermetallic layer could be kept lower than 10 m thick if the heat input remained low and an appropriate pre-setting gap. Zn coating played an important role not only in improving the wetting of molten aluminum to the steel plate but also in suppressing the formation of brittle intermetallic compound Alx Fey . Furthermore, a pre-setting gap and an appropriate post-weld heat treatment can improve the weld strength. Wang et al. (2008), Shang et al. (2012) and Cao et al. (2013a,b) CMT welded aluminum to magnesium alloy and found that the super low heat input of CMT welding helped to suppress the formation and growth of brittle intermetallic compound in weld metal, although some intermetallic compounds were still observed in the fusion zones. Cao et al. (2014a,b) CMT welded titanium-copper dissimilar joint with ERCuNiAl copper wire and found that the resultant lap and butt joints had fairly high tensile strength and fractured in the heat affected zone (HAZ) of Cu. A layer of intermetallic compounds consisting of Ti2 Cu, TiCu and AlCu2 Ti were observed in titaniumweld interface. Gungor et al. (2014) CMT welded 5083-H111 and 6082-T651 aluminum alloys and found that both the same metal CMT welds and the dissimilar metal CMT welds showed comparable tensile strengths with those of friction stir weld and much higher yield strengths due to low heat input. These studies mainly
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Fig. 2. Schematic of the current waveform of CMT welding. Fig. 1. Schematic of the welding experimental set-up.
focused on microstructures and properties of CMT weld deposited by using synergic welding program directly available in the CMT power source. A few research works have also been reported on the metal transfer behavior of CMT welding. Lorenzin and Rutili (2009), Zhang et al. (2009), Pickin et al. (2011) and Zhang et al. (2012) investigated the basic characteristics of metal transfer behavior in CMT welding of aluminum alloys, and observed that the typical CMT short circuit transfer occurred in lower power range, and hybrid transfer consisting of spray and short circuit transfers was detected in mid to upper power range. Mezrag et al. (2015) investigated the effect of the current waveform on metal transfer and heat transfer in CMT welding of galvanized steel to aluminum. The short circuit frequency was reported to increase with decreasing boost duration of the current waveform, and heat transferred to the base metal was lower for an equivalent deposit rate when the short-circuit frequency was higher. It can be noticed that the reported studies on CMT welding are mainly focused on similar welding of thin gauge aluminum and dissimilar welding of aluminum-steel, aluminum-magnesium and copper-titanium. No works on CMT welding of steel have been reported up to now. In this paper, the process characteristics of CMT welding of mild steel was investigated under different welding conditions to explore the effect of current waveform parameters on characteristics of cold metal transfer, deposition rate, heat input and weld formation. 2. Experimental 2.1. Experimental set-up As presented in Fig. 1, the experimental set-up used in this investigation consists of four parts: (1) the welding system, which includes a Transplus Synergic 2700 CMT power supply with a remote control panel, welding gun, working table and its movingcontrol unit, (2) the sensing system for the welding current and arc voltage, (3) the CMOS camera-based imaging system with a minimum framing rate of 150fps and (4) the industrial PC used as a central control computer. The sensing system consists of a Hall effect current sensor (SA1T50V5V6), a voltage sensor and a PCI data acquisition card. The Hall effect current sensor was used to measure welding current, and the voltage sensor was used to measure the arc voltage between the contact tip of the welding gun and the workpiece. The data acquisition card and the industrial PC were used to acquire and save the transient welding current and voltage data. These data could be on-line displayed and saved for off-line analysis. The imaging system consists of a high-speed CMOS camera (MV-D1024E-160) with an 808 nm interferential filter, an image
acquisition card (Micro Enable IV) and a 808 nm laser (UR808-30) used as background source. The filter installed in front of the camera was used to block the arc lights except those with a wavelength of 808 nm. The background laser beam was projected towards the droplet/wire. Lights blocked by the wire and droplets won’t be viewed by the camera, creating a shadow image on the image screen. The rest of the laser beam reached and illuminated the image plane. The resolution of the high-speed camera used during welding was 197 × 214 and the frame rate used was 3200fps. 2.2. Experimental procedure Bead-on-plate CMT welding was performed on 3 mm thick workpiece. The base material was Q235 mild steel. The workpiece dimensions were 250 × 70 mm. ER50-6 electrode wire with a diameter of 1.2 mm was used with positive polarity. Pure CO2 with a flow rate of 15 L min−1 was used as shielding gas. The tip-to-workpiece distance was kept at 14 mm. All welds were made in the flat position with a stationary welding gun and a welding table moving at a speed of 0.5 m min−1 . This set-up allowed the high-speed video camera to be stationary with respect to the wire, arc and weld pool. The power source can be programmed to produce advanced pulse-like waveforms to generate small molten droplets and detach the droplets at low current with the help of the mechanical retraction motion of the wire. Fig. 2 shows the typical current waveform during CMT welding of mild steel. The fundamental parameters determining the waveform shape are boost current (Ib ), wait current (Iw ), necking current (In ), detachment current (Id ), boost duration (tb ), necking time (tn ), detachment time (td ), current-rise rate, current-drop rate and tan-current drop. The necking time (tn ) and detachment time (td ) make up short circuit time (ts ). Among these waveform parameters, Ib , Iw , Id , tb , current-rise rate, currentdown rate and tan-current drop can be modified through RCU 5000i remote control. From the point of view of controlling the metal transfer and heat input, boost current (Ib ) and boost duration (tb ) are the two main parameters having the greatest effects. Table 1 gives the welding parameters used in this investigation. The CMT welding process is characterized by alternate wire feed and retraction movements with the hot and cold processes alternatively repeated. The instantaneous wire feed/retraction speed was measured with image analysis method. A mark was made on the surface of the exposed wire beyond the contact tip before welding. The wire motion speed was calculated by measuring the displacement of the mark on the successive high speed video images of wire. Before and after each weld deposit, the weight of workpiece was measured using a Denver balance with a precision of 10−3 g to get the deposited metal. The deposition rate is given by D = (W2 − W1 )/Tw
(1)
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Table 1 The current waveform parameters used in the experiment. wfs (m min−1 )
Ib (A)
tb (ms)
Iw (A)
Id (A)
current-rise rate (A/ms)
current-drop rate (A/ms)
tan-current down (ms)
4.0 5.0 6.0
250–320 250–320 250–320
1–5 1–5 1–5
40 50 90
50 80 120
500 500 500
300 300 300
1 1 1
Where D is the average deposition rate, W1 is the weight of the workpiece before welding, W2 is the weight of the workpiece after welding, Tw is the welding time taken to deposit the weld bead on the workpiece. The average droplet weight is given by W = (W2 − W1 )/N
(2)
Where W is the average droplet weight, and N is the number of CMT cycles (or the number of droplets) within the welding time Tw . The number of CMT cycles (N) (the number of droplets) can be expressed as: N = Tw /Tc
(3)
Where Tc is the mean CMT period. The mean CMT period (Tc ) was calculated by averaging out CMT periods for more than 300 CMT cycles from the captured current and voltage waveforms. Three weld cross-sections were cut from every welded sample with spark cutting machine to measure geometrical dimensions of the weld bead (width, penetration and reinforcement). Those cross-sections were polished and final finished with a 1 micrometer diamond suspension, and then etched with a solution of 3% HNO3 in ethanol. The cross-sectional profiles of the weld beads were observed using a Leica Z16 APO macro-scope with Leica LAS software. 3. Experimental results and discussion 3.1. Typical CMT welding cycle Fig. 3 illustrates the typical electrical and wire motion speed waveforms and corresponding metal transfer images taken when welding at a wire feed rate of 5.0 m min−1 , a boost current (Ib ) of 300 A and a boost duration (tb ) of 2 ms. The other welding parameters are listed in Table 1. From Fig. 3, it can be seen that arc ignites and extinguishes periodically accompanied by alternate feed and retraction of the electrode wire during CMT welding. A CMT welding cycle is consisted of four stages: (1) Boost stage: At the beginning of this stage (4550.8 ms), the arc reignites with current rising to the preset value of the boost current at the preset current-rise rate of 500 A/ms. Upon sensing the re-ignited arc, the digital process control unit of the CMT welding system signals the wire operation system to feed the wire forward. The wire speed ramps up from a minus value indicating the retraction of the wire. After about 1.4 ms, the wire feed rate becomes positive, which indicating that the wire start to be fed forward. Due to the inertia of the wire operation system, wire feed rate does not reach its steady-state maximum value at the end of this stage. The arc melts the workpiece and the wire, and a droplet is formed at the end of the wire. The total duration of this stage is 2.0 ms, which is consistent with the preset value (tb = 2 ms). (2) Wait stage: In this stage, the current is drastically reduced in order that the droplet is not detached but remains at the end of the wire. The wire feed rate increases up to its maximum value and this value is maintained until short circuit occurs. The contact between the droplet and the weld pool takes place at such a low current (about 50 A) that twinkle repulsive contact
Fig. 3. Electrical and wire feed rate waveforms for one CMT cycle and corresponding metal transfer images.
of the molten droplet with weld can be successfully avoided. The total duration of this stage is 3.92 ms. The maximum wire feed rate measured is about 40 m min−1 . (3) Necking stage: The droplet spreads into the weld pool smoothly under the combined action of surface tension force and gravitation. A molten metal column bridges the wire and the weld pool. The current rises to the level of necking current (In = 180 A) to enhance the electromagnetic force, speeding up the necking of the molten metal column. The wire feed rate decreases until it become a negative steady-state value, indicating the retraction of the wire. Necking time (tn ) is about 3.84 ms in this case. This stage is peculiar to the CMT cycle for welding of steel. It is not observed in the CMT cycle for welding of aluminum and dissimilar welding of steel to aluminum, as reported by Pickin et al. (2011) and Mezrag et al. (2015). Further research revealed that both the necking current (In ) and necking time (tn ) are constants for a given preset wire feed rate, independent of other settable welding parameters such as Ib and tb , as will be seen in Section 3.2.1. This suggests that In and tn are defined as non-adjustable parameters in the synergic CMT welding program. (4) Detachment stage. In this stage, the welding current decreases to the level of detachment current (Id = 60 A), and the arc voltage decreases to about 3 V in 0.36 ms. The wire feed rate is kept at about negative 22.5 m min−1 , indicating that the wire is mechanically retracted at a speed of 22.5 m min−1 . The retraction motion promotes the pinch-off of the molten metal column and enables the spatter-free droplet detachment at both low
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Fig. 4. Distributions of: (a) CMT periods (Tc ), (b) short circuit times (ts ), and (c) necking times (tn ) as a function of boost current (tb = 2 ms, wfs = 5.0 m min−1 ).
Fig. 5. Distributions of: (a) CMT period (Tc ), (b) short circuit times (ts ), (c) necking times (tn ) as a function of boost duration (Ib = 300 A, wfs = 5.0 m min−1 ).
current and low voltage. Then the arc reignites and the cycle recommences. Detachment time (td ) is 1.88 ms in this case. The boost stage and wait stage make up the arcing phase, while the necking stage and detachment stage constitute the short circuit phase. 3.2. Process stability 3.2.1. Effect of boost current Probability distribution of short circuit periods (or frequencies) and the corresponding standard deviation are appropriate measures to evaluate the process regularity and stability of GMAW-S, as reported by Hermans and Den Ouden (1999), Suban and Tuˇsek (2003). As an improved type of GMAW-S, CMT welding process can also be evaluated with these measures. Fig. 4 shows histograms of CMT period (Tc ), short circuit time (ts ) and necking time (tn ) as functions of boost current. The boost duration (tb ) was kept at 2 ms and the wire feed rate at 5 m min−1 . Over 300 CMT cycles were counted in the statistical calculation for each histogram. From Fig. 4(a), it can be seen that the CMT period distributions for different boost currents are quite similar. The CMT periods fell within a narrow range from about 10.0 ms to 13.0 ms with the probability peak occurring at about 11.5 ms, regardless of the boost current. Furthermore, both the mean periods (Tc ) and its standard deviations were approx-
imately the same for different boost currents. Therefore it can be concluded that boost current (Ib ) had little effect on the process stability when changing in its adjustable range from 250 A to 320 A. The short circuit time (ts ) distributions for different boost currents confirms this conclusion. As indicated in Fig. 4(b), the distributions of boost durations (ts ) were also quite similar for different preset boost currents. In addition, the standard deviation of boost duration varied very little. The necking times at different boost currents were always around 4 ms, as indicated in Fig. 4(c), suggesting that it was fixed as a constant of 4 ms in the CMT synergic welding program. 3.2.2. Effect of boost duration Fig. 5 illustrates the histograms of CMT period (Tc ), short circuit time (ts ) and necking time (tn ) as a function of boost duration (tb ) with the boost current kept at 300 A and the wire feed rate at 5 m min−1 . The distributions of CMT periods and short circuit times changed significantly with boost duration, as indicated in Fig. 5(a) and (b), indicating that tb had great effect on the regularity and stability of CMT process. The necking times at different boost durations distributed around 4 ms, as indicated in Fig. 5(c), confirming that it was defined as a process-independent constant of 4 ms in the CMT synergic welding program. At 1 ms and 3 ms boost duration, the CMT period distributions were much similar to that at 2 ms boost duration. However, similar process stability was only observed for 3 ms boost duration, and not
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Fig. 6. CMT cycle with an abnormally long ts (8.75 ms) recorded when welding with a boost duration of 1 ms.
for 1 ms boost duration during the experiment. This phenomenon can be explained by the distribution range of short circuit times. As seen in Fig. 5(b), the distribution of short circuit time at 3 ms boost duration was also similar to that at 2 ms boost duration, while the distribution of short circuit time at 1 ms boost duration was much wider than that at 2 ms boost duration. CMT cycles with abnormally long short circuit time were observed at 1 ms boost duration. Fig. 6 shows such an abnormal CMT cycle with a short circuit time of 8.75 ms, which is much longer than normal one (6.25 ms). In this cycle, the necked liquid metal column continued to neck down and elongated drastically after necking stage (frame 3601). It took 4.75 ms to break the necked liquid metal column. The liquid metal column seemed to adhere to the retracted electrode wire and be not ready to break. The main reason may be that the viscosity and surface tension of the liquid metal column was much higher due to the low temperature of molten metal column at 1 ms boost duration. At 4 ms and 5 ms boost duration, the CMT periods and short circuit times exhibited much broader distribution ranges and higher standard deviations. The CMT processes were less stable with some spatters, as indicated in Fig. 7. Both abnormal long and abnormal short CMT cycles were observed. Fig. 7(a) shows an abnormal long CMT cycle with a short circuit time of 8.44 ms taken when welding at a boost duration of 5 ms. In this case, the droplet volume was much larger than that for boost duration of 1 ms (compare
399
Fig. 7(a) with Fig. 6). It took much longer time to neck off the larger liquid metal column after necking stage, i.e. the detachment stage was much longer than that of a normal CMT cycle. Fig. 7(b) shows an abnormal short CMT cycle with a short circuit time of only 4.38 ms. The contact of droplet with weld pool occurred at a current of 250A due to the long boost duration (frame 7470.5), which is much higher than the normal waiting current (about 50 A). This made the necking-down of the molten metal column took place at the contacting point between the droplet and weld pool during the necking stage. The droplet separated with the weld pool in just 0.38 ms after the necking stage with no droplet transferred into the weld pool. Upon arrival of the detachment stage, the current fell to 25 A, much lower than the preset value of detachment current (80 A), and the welding voltage rose to an abnormal high value, indicating that the droplet separated from the weld pool. The arc reignited and boosted upon the retraction of the droplet from the weld pool. Thus, the short circuit time (ts ) was abnormally shorter than that of the normal CMT cycle. Further experiments were conducted with boost current kept at 300A and boost duration changing from 1–2 ms and 3–4 ms. The distributions of short circuit times are shown in Fig. 8. From Fig. 5 and Fig. 8, it can be seen that the CMT process was quite regular and stable for any boost duration ranging from 1.6 ms to 3.6 ms. 3.2.3. CMT transfer frequency Fig. 9 shows effects of boost current, boost duration and preset wire feed speed on the average CMT frequency. It can be noted that the CMT frequencies fluctuated very little with the boost duration, boost current and preset wire feed speed, and fell in a very narrow range from 80.1 to 90.1 Hz. This indicated that not only the boost duration and boost current but also the wire feed rate had no direct effect on the transfer frequency during CMT welding of mild steel. This observation was very different from that made in conventional short circuit transfer GMAW. In conventional, the short circuit transfer increased with increasing the preset wire feed rate and then decreased after reaching a maximum, as reported by Hermans and Den Ouden (1999). The explanation of this observation in CMT welding is that the metal transfer process is mainly controlled by the mechanically retraction of the wire. 3.3. Average arc power and current The average welding current I was calculated by time integration of the instantaneous current over the total welding time of the corresponding weld pass (Tw ):
I=
Tw 0
idt
Tw
Fig. 7. Abnormal CMT cycles recorded when welding at tb = 5 ms, wfs = 5.0 m min−1 : (a) with an abnormally long ts (8.44 ms), (b) with an abnormally short ts (4.38 ms).
(4)
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Fig. 8. Distribution of short circuit times as a function of boost duration: (a) tb changing from 1.2 to 1.8 ms, and (b) tb changing from 3.2 to 3.8 ms (Ib = 300 A, wfs = 5.0 m min−1 ).
Fig. 9. Effects of (a) boost current (tb = 2 ms), (b) boost duration (Ib = 300 A) and preset wire feed speed on average CMT frequency.
where i is instantaneous current, and Tw is the total welding time of the weld pass. The average arcing current I a was calculate by counting only arcing phase. It can be expressed as:
m tai idt i=1 0 Ia = m i t i=1 a
(5)
where m is the number of the CMT periods in the total welding time of the weld pass, tai is the duration of ith arcing phase. The average short circuit current I s can be expressed as:
m tsi idt i=1 0 Is = m i t i=1 s
mathematically linked with wire feed rate and the thermal energy for welding is also dependent on the preset wire feed rate. Fig. 11 shows the calculated average welding power, arcing power and short circuit power as functions of boost current or boost duration. Note that the average short circuit power is about 1kw, much higher than expected. Mezrag et al. (2015) calculated the average short circuit power in CMT welding of steel to aluminum and found that it was near zero regardless of the boost current and boost duration. For CMT welding of mild steel, the presence of necking stage in the short circuit phase accounted for the higher short circuit power. The welding power and arcing power increased as boost current or boost duration increased.
(6)
where tsi is the duration of ith short circuit phase. Fig. 10 shows the calculated average welding current, arcing current and short circuit current as functions of boost duration or boost current. It can be noticed that the average short circuit current remained nearly unchanged for different welding conditions. This may be because that the necking stage dominated the short circuit phase from the point of view of energy and the necking current and necking time were kept at constants independent of boost duration or boost current. The average welding current and average arcing current increased slightly with increasing boost current, but increased remarkably with boost duration even though the preset wire feed rate was kept at a constant of 5 m min−1 . Thus it can be concluded that the electrical parameters is partially decoupled with the wire feed rate. This observation is very different from conventional GMAW. In conventional GMAW, the welding current is
3.4. Deposition rate and droplet size Deposition rates at different welding conditions were measured using the method describe in Section 2.2. Fig. 12 shows the measured deposition rate as a function of boost current or boost duration. As can be seen in Fig. 12, the deposition rate increased with increasing boost current or boost duration even though the preset wire feed rate was kept constant. During welding, the melted and deposited wire shall be balanced by the wire pushed out of the contact tip, i.e. the deposition rate shall be equal to the actual wire feed speed because the spatter was low enough to be neglected for CMT welding. Hence it can be concluded that the actual wire feed speed was not equal to preset wire feed speed, representing a great departure from the conventional GMAW and pulsed GMAW. In conventional GMAW, wire is fed at a constant rate, the actual wire feed rate in the welding process is approximately equal to
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Fig. 10. Average current, arcing current and short circuit current as functions of: (a) boost current, (b) boost duration.
Fig. 11. Average welding power, arcing power and short circuit power as functions of (a) boost current, (b) boost duration.
Fig. 12. Effects of (a) boost current, (b) boost duration on deposition rate.
the preset wire feed rate. The wire feed rate and the level of welding current are mathematically linked, and the welding current is always determined by the preset wire feed rate. Thus, it is normal to substitute wire feed control for that of welding current control. This rule is also applied to pulsed GMAW, as reported by Amin (1983), Needham and Carter (1965) and Collard (1988). In CMT welding, the arc periodically reignites and extinguishes, as indicated in Fig. 3. During arcing phase, the wire is feed forward and melted by the arc. While during short circuit phase, the wire is retracted with the welding current reduced. The wire feed rate transient varies with the status of arc, however, the average wire feed rate should be equal to the average melting rate, i.e. the deposition rate because the melt-off must be balanced by the wire fed out of the contact tube. The melting rate depends on the average arcing current. The boost current and boost duration affect the average welding current and average arcing current, thus affect the average melting rate and actual wire feed rate. Fig. 13 shows the relationship between the average arcing current and the actual wire feed rate which is calculated from deposition rate. From Fig. 13, it can be observed that the actual wire feed rate increased approximately linearly with average arcing current no
Fig. 13. Relationship between the average arcing current and the actual wire feed rate.
matter how the average arcing current was adjusted. Similar observation was also made by Mezrag et al. (2015) in CMT welding of steel to aluminum. It can also be noticed that the actual wire feed
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Fig. 14. Effects of (a) boost current, (b) boost duration on droplet weight.
Fig. 15. Weld bead appearances made at (a) tb = 2 ms, (b) Ib = 300 A.
rate increased much faster when the average current was increased by changing the boost current with boost duration kept at 2 ms than when the average current was increased by increasing the boost duration with boost current kept at 300 A. Fig. 14 showed the droplet weight deduced by using the deposition rate and the average CMT periods (Eq. (2)). With boost current or duration increased, the wire melting rate and deposition rate increased and the transfer frequency remains nearly unchanged (see Fig. 9). Therefore, the droplet size increased approximately linearly with the boost current and duration, as indicated in Fig. 14. 3.5. Weld bead formation Fig. 15 shows the weld bead appearances made at different welding conditions. With the boost duration kept at 2 ms, sound
weld beads with uniform width and no surface defects were always obtained at any boost current (Ib ) ranging from 250 A to 320 A, as indicated in Fig. 15(a). As mentioned in Section 3.2, the welding processes were always kept regular and stable when welding at boost duration of 2 ms, regardless of the boost current. Obviously, the effect of the boost current on the weld formation was the consequence of the effect on the welding process. Sound weld appearances were also obtained when welding at different boost durations (tb ) ranging from 1 ms to 5 ms and a constant boost current (Ib ) of 300 A. However, spatters were observed around the weld bead when welding at 4 ms or 5 ms boost duration, indicating that the welding process was less regular and stable at 4 ms or 5 ms than at 2 ms and 3 ms boost duration. This observation was also in accord with that mentioned in Section 3.2.
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Fig. 16. Cross-section profile of welds produced at (a) tb = 2 ms, (b) Ib = 300 A.
Fig. 17. Cross-section dimensions of welds made at (a) tb = 2 ms, (b) Ib = 300 A.
The boost current and duration influence the arcing power, thus affecting the weld profile and size. Fig. 16 illustrates the cross-sectional profiles of welds made at different boost current or duration. It can be seen that both the boost current and boost duration had significant effects on the weld dimension. The measured weld dimensions at different welding conditions are shown in Fig. 17. It can be seen that as the boost current or boost duration increasing, the penetration and weld width increased noticeably, but the reinforcement varied very little.
4. Conclusions (1) The CMT cycle in welding of mild steel consists of four stages: boost stage, wait stage, necking stage and detachment stage. The necking stage is peculiar for the CMT welding of steel. During this stage, the short circuit current rises to a high level (necking current) to enhance the electromagnetic constriction force to facilitate the metal transfer. Both the necking current and necking time are fixed as a non-adjustable constant in the CMT welding program.
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(2) At a preset wire feed rate (5 m min−1 ), changing the boost current from 250 A to 320 A doesn’t affect the regularity and stability of CMT welding process obviously, while the boost duration, ranging from 1 ms to 5 ms, has pronounced effect. Fairly regular and stable processes are achieved in the range from 1.6 ms to 3.6 ms with the boost current kept at 300 A. (3) The deposition rate, average wire feed rate and droplet size increase with increasing boost current or boost duration, although the preset wire feed rate is kept constant. The average CMT frequency changes very little with the boost duration, boost current and wire feed rate. (4) As the boost current or boost duration increases, the weld width and penetration increase noticeably, and the reinforcement varies very little. Acknowledgment This work is supported by Shandong Natural Science Foundation under contract No. ZR2010EZ005. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jmatprotec.2017. 01.004. References Ahmad, R., Bakar, M.A., 2011. Effect of a post-weld heat treatment on the mechanical and microstructure properties of AA6061 joints welded by the gas metal arc welding cold metal transfer method. Mater. Des. 32 (10), 5120–5126. Amin, M., 1983. Pulse current parameters for arc stability and controlled metal transfer in arc welding. Met. Constr. 15 (3), 272–278. Cao, R., Yu, G., Chen, J.H., Wang, P.C., 2013a. Cold metal transfer joining aluminum alloys to galvanized mild steel. J. Mater. Process. Technol. 213 (10), 1753–1763. Cao, R., Wen, B.F., Chen, J.H., Wang, P.C., 2013b. Cold metal transfer joining of magnesium AZ31B-to-aluminum A6061-T6. Mater. Sci. Eng. A560 (1), 256–266. Cao, R., Feng, Z., Lin, Q., Chen, J.H., 2014. Study on cold metal transfer welding—brazing of titanium to copper. Mater. Des. 56 (4), 165–173.
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