1-s2.0-S0030399214001054-main

8/9/2019 1-s2.0-S0030399214001054-main

1/12

A study of narrow gap laser welding for thick plates usingthe multi-layer and multi-pass method

Ruoyang Li, Tianjiao Wang, Chunming Wang n , Fei Yan, Xinyu Shao, Xiyuan Hu, Jianmin LiSchool of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

a r t i c l e i n f o

Article history:Received 4 November 2013

Received in revised form22 February 2014Accepted 18 April 2014Available online 11 June 2014

Keywords:Thick plateNarrow gapHigh speed camera

a b s t r a c t

This paper details a new method that combines laser autogenous welding, laser wire lling welding andhybrid laser-GMAW welding to weld 30 mm thick plate using a multi-layer, multi-pass process. A Y

shaped groove was used to create the joint. Research was also performed to optimize the groove size andthe processing parameters. Laser autogenous welding is rst used to create the backing weld. The lower,narrowest part of the groove is then welded using laser wire lling welding. Finally, the upper part of thegroove is welded using laser-GMAW hybrid welding. Additionally, the wire feeding and droplet transferbehaviors are observed by high speed photography. The two main conclusions from this work are: thewire is often biased towards the side walls, resulting in a lack of fusion at the joint and the creation of other defects for larger groove sizes. Additionally, this results in the droplet transfer behavior becomingunstable, leading to a poor weld appearance for smaller groove sizes.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Thick plate welding technology is very important to the

shipbuilding, pipeline, nuclear, and submarine manufacturingelds. The equipment for these processes is quite large, and for

plate thicknesses of 20 mm, workpieces only can be jointed pieceby piece using traditional methods. However, many problems ariseas a result of the large groove area that is necessary to join twopieces, such as the residual stress and residual deformation thatresults from the large restraints required and poor weld jointmechanical properties because of a lack of plasticity [1] . Presently,laser welding refers mainly to single-run autogenous operations inwhich the weld joint is formed by solidifying the base metalwithout the addition of any other material. This method has manydisadvantages, including the precise t-up requirements prior towelding and the limited weld range for a certain laser outputpower even if high ef ciency lasers are used and small welding

deformations are observed. After 40 years, the most popular twolasers, YAG and CO 2 , are still the two primary lasers that are usedto join most sheet, ( o 5 mm), medium and heavy plate(5 20 mm) thicknesses. Even for the most advanced ber laser,the single-run penetration depth that can be achieved is less than25 mm [2] . Katayama et al. [3] achieved deep-penetration weldbeads to depths of 70 mm in Type 304 stainless steel using 10 kW

and 16 kW high power disk lasers, a welding speed of 0.3 m/minand at pressures of 0.1 kPa. Unfortunately, the cost to control theatmosphere around the workpiece is unrealistic for industrial

production of large and thick plates. Overall, the best method forthick plate ( 4 20 mm) welding is likely through the use of multi-pass non-autogenous welding.

Presently, few studies have been conducted on the use of lasernon-autogenous welding technologies. Hybrid YAG-MIG weldingprocesses have been used to successfully weld 8 mm thick alumi-num alloy plate in two passes at the Huazhong University of Science and Technology [4]. The Beijing University of Technology[5] successfully laser welded a 20 mm thick aluminum alloy platewith a narrow gap using ller wire in six passes with a 3.5 kW CO 2laser. The results from this project have not yet been reported.Wang Baiping [6] et al. reported welding 16 mm thick stainlesssteel plates using laser wire lling welding in three passes.Meanwhile, a number of foreign researchers have made contribu-

tions to this eld. Osaka University welded 24.5 mm thick platesuccessfully using a 10 kW ber laser, at a welding speed of 0.3 m/min [7] . Karhu Miikka [8] et al. reported welding thick AISI 316 L austenitic stainless steel using a 3 kW hybrid YAG laser-GMAW byemploying multiple-layers and multiple passes. Hot cracking wasobserved in this weld. Hayashi, Tomotaka et al. [9] successfullywelded a 22 mm thick steel plate with a square groove and a 4 mmgap using a high power CO2 laser-MIG hybrid welding technique[9] . Choi and Hae Woon [10] welded 15 mm thick A572 Gr50 steelwith a 3.2 mm gap using a hybrid welding process in six passes.MHI has made a great deal of progress in this eld with the help of a self-produced super-power (13.5 kW) YAG laser and a specially

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/optlastec

Optics & Laser Technology

http://dx.doi.org/10.1016/j.optlastec.2014.04.0150030-3992/ & 2014 Elsevier Ltd. All rights reserved.

n Corresponding author at: School of Materials Science and Engineering,Huazhong University of Science and Technology, Wuhan 430074, China.Tel: 86 13871541964; fax: 86 27 87543894.

E-mail addresses: [email protected] , [email protected] (C. Wang).

Optics & Laser Technology 64 (2014) 172 183
http://www.sciencedirect.com/science/journal/00303992http://www.elsevier.com/locate/optlastechttp://dx.doi.org/10.1016/j.optlastec.2014.04.015mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.optlastec.2014.04.015http://dx.doi.org/10.1016/j.optlastec.2014.04.015http://dx.doi.org/10.1016/j.optlastec.2014.04.015http://dx.doi.org/10.1016/j.optlastec.2014.04.015http://dx.doi.org/10.1016/j.optlastec.2014.04.015http://dx.doi.org/10.1016/j.optlastec.2014.04.015mailto:[email protected]:[email protected]://crossmark.crossref.org/dialog/?doi=10.1016/j.optlastec.2014.04.015&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.optlastec.2014.04.015&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1016/j.optlastec.2014.04.015&domain=pdfhttp://dx.doi.org/10.1016/j.optlastec.2014.04.015http://dx.doi.org/10.1016/j.optlastec.2014.04.015http://dx.doi.org/10.1016/j.optlastec.2014.04.015http://www.elsevier.com/locate/optlastechttp://www.sciencedirect.com/science/journal/00303992

8/9/2019 1-s2.0-S0030399214001054-main

2/12

designed external ray, which permitted a 41 mm thick plate to belaser-GMAW hybrid welded in six passes [11] .

In recent years, the Corus in England, the FORCE in Denmark,and the Fraunhofer ILT in Germany have performed research onthick plate hybrid laser arc welding to achieve penetration depthsdown to 30 mm. This work has been funded by the European Coaland Steel Research Foundation [12] . Although western researchers,especially Europeans, have already made many signi cant con-

tributions to this eld, thick plate laser non-autogenous weldinghas not been widely adopted for practical applications. Theexisting studies suffer from dif culties related to process controlstability, technological adaptability, joint appearance and internaldefects (lack of fusion and porosity). Therefore, the advantages of these three methods, laser autogenous welding, laser wire llingwelding and laser-arc hybrid welding, are a good place to start toweld thick plate for practical applications and for differentthicknesses and materials.

2. Experimental details.

2.1. Experimental materials and device.

The material used in this study was Q235 plate steel withdimensions of 150 mm x 75 mm x X mm (where X 16, 18 and 30).A 1.2 mm diameter solid ller wire was used (H08Mn2SiA).

The values X 16, 18, and 30 represent the groove total heightfor the laser wire lling welding, hybrid laser-GMAW welding andthe new welding processes.

The chemical composition of the base metal and the ller wireare given in Tables 1 and 2 .

The laser equipment consisted of an IPG Photonics 4 kW solid-state Yb- ber laser system (YLR-4000) mounted to an ABBIRB4400 special welding robot. The optics consisted of a 250 mmfocal length and a focal diameter of 300 m. A Fronius MAG arcwelding machine was used that consisted of a Fronius TPS4000inverter power supply, a special push and pull wire feeder and aFronius VR2000 MIG/MAG welding torch. A CMOS high speedcamera (Photofocus, Switzerland) was used to observe the weldingprocess at acquisition rates of up to 10000 frames per second. Tosuppress the light bloom associated with the welding process,several optical lenses were employed that matched the wave-length (808 nm) of the laser source. As shown in Figs. 1 and 2 , thehigh speed camera optical lens combinations included bandattenuators to lower the intensity of the light to protect the sensor,a narrow-band lter determined by the backlight conditions andan outer layer glass applied to prevent the inner optical lens frombeing damaged.

2.2. Experimental methods.

Prior to welding, the grooved surface was polished and cleanedwith acetone. Every welding seam was initially rubbed withdifferent types of sandpaper or ground with a grinding wheel,and cleaned by acetone. Laser autogenous welding was used rstto form the backing weld. Pure Ar shielding gas was owed intothe groove bottom to protect the backing weld. A special shieldinggas supply nozzle, capable of entering the narrow gap groove wasdesigned for this purpose, as shown in Fig. 3

The parameters for the laser autogenous backing welding were:a laser power 4 kW, a welding speed 1.0 m/min, and a defocus-ing length 0 mm.

Laser wire lling welding was used to ll the lower portion of the groove. The wire led at a distance of 0.35 mm 0.65 mm and atan angle of 30 1 with respect to the surface. The defocusing length

was kept at 0 mm and the same shielding gas nozzle was used.

Fig. 4 shows the Y shaped groove used. Two different angles( 10 1 /6 1 ) were used to study how the groove size affects thewire feeding process and the weld shape during laser wire llingwelding in a narrow gap.

According to previous experimental results, the most appropri-ate parameters for the welding process are a speed of 0.5 m/minand a wire diameter of 1.2 mm to ll the groove gap. The melt wirewas employed to ll the groove gap entirely. The wire feeding speedis shown in eq. (1) and is determined based on the law of theconservation of mass:

V F V W AG

AF ; 1

Table 1Elemental breakdown of Q235 steel.

element C Mn Si S P Cr Ni Cu

content/% 0.16 0 .61 0 .20 0 .023 0 .019 o 0.30 o 0.30 o 0.30

Table 2

Elemental breakdown of H08Mn2SiA.

element C Mn Si S P

content/% 0.06 0.09 1.80 1.95 0.70 0.85 r 0.020 r 0.015

Fig. 1. High speed camera optical system diagram.

Fig. 2. Image of the high speed camera.

Fig. 3. Narrow gap air supply device.

R. Li et al. / Optics & Laser Technology 64 (2014) 172 183 173

8/9/2019 1-s2.0-S0030399214001054-main

3/12

where V F is wire feeding speed (m/min), A G is the gap cross-sectional area (mm 2 ), V W is the welding speed (m/min), and A F iswire cross-sectional area (mm 2 ).

Using this equation, we can select reasonable wire feedingspeeds to ll the gap effectively based on the different groove sizesfor similar beam powers and welding speeds. Meanwhile, theeffects of different groove sizes on the weld quality for identicalwire feeding rates were studied to adjust the wire feed percen-

tages based on the different passes. We fed wire into a 101

grooveat percentages of 40%, 30%, and 30%. The feed percentages for the6 1 groove were 50% and 50%.

The constant and variable parameters used during multi-passlaser welding with ller wire are shown in Tables 3 and 4

The distance between the laser beam and wire is maintained ina range from 0.35 0.65 mm to form a liquid bridge transition

during the preparation period [13] . The weld bead quality and thegroove fusion play an important role in the later welding processesduring multi-pass welding. Thus the weld bead surface positionshould be re-measured and the laser beam focal point position andwire feed position should be adjusted after each pass. As shown inFig. 5, the mechanism for wire feeding and the stability of thewelding process was monitored by the high speed camera usingan acquisition rate of 2000 frames per second to analyze theexperimental results and optimize the welding parameters.

Laser-GMAW hybrid welding was used to weld the relativelywider groove upper part. The arc led at a distance of 0 5 mm, atan angle of 55 1 from the surface and the wire length past this pointwas between 15 30 mm.

According to J. B. Green s research [14] , the downward force onthe melted metal can be shown by the following formula when themelting metal s lower surface end was bigger than the uppersurface end:

F I 2 log RbRa

; 2

where F is the axial force on the melted metal, I is current, R b is thelower part of the ball shaped melted metal (the current ows from

this area to the radius of the melted metal spherical cap, J. B - thegreen shaded area de nes the effective area), and Ra is wire radius.

The effective arc areas for at plate welding and narrow gapwelding are shown in Figs. 3 and 4 , respectively.

The gap welding pool was relative narrow. Therefore, thespherical cap radius of the effective area is Rb 2 o Rb1 , and the

downward axial force on the melted metal may decrease, leadingto solidi cation of the melted metal as it is transferred to thewelding pool.

As the droplet at the welding wire tip grows, the distance to thewelding pool grows at a smaller rate than the distance to the sidewalls. The droplet grows until it is transferred to the welding pool.In this way, it is easy to form an arc column using the side walls asthe cathode, leading to melted metal being transferred to the side

walls after the distance between them decreases.According to the aforementioned research, Rb 2 o Rb1 may be animportant factor that promotes smooth droplet transfer. A groovewith three different angles ( 40 1 , 60 1 , and 90 1 ), as shown inFig. 6, was designed to meet the requirements and monitoredusing the high speed camera. The technical parameters are shownin Table 5 .

Table 3Multi-pass laser welding with ller wire constant process parameters.

Laserpower P kW

Welding speedV w m/min

Gas ow m 3 /h Defocusingamount (mm)

Wire feed style Wire feed angle

4 0.5 0.1 0 Wire leads 45 1

Table 4Multi-pass laser welding with ller wire variable process parameters.

Vf (m/min) angle

101

61

V f (the rst pass) 4.5 3.3V f (the second pass) 3.4 3.3V f (the third pass) 3.4

Fig. 5. The high speed camera monitor device in the narrow gap laser wire llingwelding.

6mm

18mm

Fig. 6. Laser arc hybrid welding groove size.

6mm

16mm

Fig. 4. Schematic of the groove angle used for multi-pass laser welding usinga ller wire.

R. Li et al. / Optics & Laser Technology 64 (2014) 172 183174

8/9/2019 1-s2.0-S0030399214001054-main

4/12

3. Experimental procedure and analysis

3.1. The in uence of the narrow gap on the wire feeding mechanism for laser welding using ller wire

3.1.1. Experimental results and analysisThe laser welding operations were monitored using the high

speed camera. The following phenomena were identi ed and

described below.

(1) The wire melted into groove smoothly for most of the weldingprocesses.The high speed images of the wire melting into the groove andshowing the behavior of the multi-layer welding process for

the 6 1 and 10 1 grooves are shown in Figs. 7 and 8 . The wire wasfed into the groove, as shown in the top portion of the stillimage and the plasma is the white spray surrounding the wire.The welding pool was beneath the tip of the wire. The wirewas fed into the welding pool smoothly so that dynamicdroplet transfers could be observed in the high speed photos.

(2) A wire de ection phenomenon was observed during thesecond laser welding pass using the ller wire with a 10 1

groove.The high speed images from the second laser pass using wirelling welding are shown in Figs. 9 and 10 . It is easy to

recognize the two sides of the groove from the weld beadposition. In Fig. 9, we can see the wire de ecting during thesecond laser welding pass in the 10 1 groove with the onlylimits being the two sides of the groove. In Fig. 10 , the weldingpool was biased to one side of the groove. The weld bead wasalso biased to the same side during the third laser weldingpass with the 10 1 groove.

(3) The wire melts into the welding pool smoothly and steadily forthe 6 1 groove when monitored with the high speed camera.Additionally, the nal appearance of the weld was generallygood. However, the wire de ection phenomenon appearedduring the second and third passes when using the 10 1 groove.This was mainly because the weld bead surface becameuneven and the wire feeding position during the later weldingpass was greatly affected. This affected the ow and accuracyof wire feeding process and changed the laser-arc distance.Although the wire de ection phenomenon had almost noimpact on the wire liquid bridge transfer mode, the wirefeeding speed could no longer be timely adjusted, resulting in

Fig. 7. Wire feeding high speed images during laser welding for the 10 1 groove size with the ller wire. (a) The rst pass, (b) the second pass and (c) the third pass.

Table 5Laser-arc hybrid welding parameters.

Laser power 4 KW

current 120 ALaser arc distance 3 mmDefocusing amount 2 mmgap 0.5 mmWelding direction Arc leadsWelding speed 0.7 m/minGroove angle 90 1 , 60 1 , 40 1

Shielding gas Ar 20% CO2Gas ow 1.1 m 3 /hStick out 20 mmArc length 2 mmArc mode Continues mode

Fig. 8. Wire feeding high speed images during laser welding for the 6 1 groove size with the ller wire. (a) The rst pass and (b) the second pass.


8/9/2019 1-s2.0-S0030399214001054-main

5/12

8/9/2019 1-s2.0-S0030399214001054-main

6/12

melting the wire was then absorbed by the side walls when thewire shifted to one side. Accordingly, problems associated with thelack of fusion were identi ed on the side wall.The melted metal did not spread completely to the right side of the weld top, as shown in Fig.11 . Meanwhile, even if the wire wasfed into groove steadily, it still caused lack of fusion problems onthe side wall when the groove gap increased to a certain extent.We therefore may conclude that a gap that is too large causes the

wire de ection phenomenon, which were the two major reasons

for problems associated with the lack of fusion on the side walls.Slag and porosity defects were observed at two adjacent weldheating overlap regions and at the fusion center zone. It wasdif cult to remove the oxide slag (SiO 2 and MnO) and the Si andMn present in the base metal and the welding wire. The high-melting temperature slag was dif cult to remelt and rose tosurface, causing slag to be present at the bottom of the weld andcausing porosity defects during the following pass. These defects

were found to always occur at the two adjacent weld overlappingregion.(3) Lack of fusion during laser autogenous backing welding.

The penetration depth for the 10 1 groove was deeper than forthe 6 1 groove, as shown in Figs. 11 and 12 . The smaller thegroove angle was, the shallower the penetration depth wasduring laser autogenous backing welding. This was mainlybecause the side walls of the smaller groove gap absorbedmore laser energy and decreased the amount of energy thatreached the groove bottom.

3.1.3. Welding quality optimization schemes

(1) Optimization schemes to decrease the of lack fusionAccording to the analysis, the root cause of the lack of fusionwas the laser energy not being absorbed adequately by theFig. 12. Micrograph of the weld cross section using a 6

1 groove.

Table 6Droplet transfer behavior on a at plate.

Flat plate

welding

0ms 8ms 16ms

17ms 18ms 19ms

Droplet transfer average frequency:74/sec

Table 7Droplet transfer behavior for the 90 1 groove angle.

Groove

Angle0ms 39ms 46ms

47ms 48ms 49ms

Droplet transfer average frequency 26/sec

( )


8/9/2019 1-s2.0-S0030399214001054-main

7/12

groove side walls, making it dif cult to form a bond betweenthe side walls and the fused wire. The defects associated withthe lack of fusion are almost eliminated for the 6 1 groove,meaning the side walls absorbed the laser energy moreef ciently and that the wire was feed into the groove moreevenly, improved the welding quality and decreasing thenumber of weld defects. In addition, decreasing the groovesize can also reduce the wire lling content and improve

welding ef ciency.

Therefore, the most important conclusion was that the groovesize should be kept less than 2.5 mm for laser wire lledwelding when attempting to weld a 30 mm thick plate.

(2) Optimization schemes to decrease slagThe groove gap was too narrow to clean the weld bead entirelyafter each pass, leading to the formation of slag defects. A1.2 mm thick cleaning plate wrapped in a disposable cottonswab was designed to clean the weld bead and groove side

walls after each welding pass.


Groove

angle

0ms 41ms 46ms

47ms 49ms 50ms

56ms 57ms 58ms

Droplet transfer average frequency 12/sec

( )


Groove

angle

0ms 6ms 9ms

11ms 22ms 55ms

70ms 75ms 81ms

Droplet transfer average frequency -/sec

( )


8/9/2019 1-s2.0-S0030399214001054-main

8/12

3.2. The droplet transfer mechanism in laser-GMAW hybrid welding under narrow gap conditions

3.2.1. Experimental results and analysis

(1) The in uence of the groove angle on laser-GMAW hybridwelding droplet transfer.Droplet transfer high speed images from the laser-GMAW

hybrid welding using different groove angles and averagedroplet transfer frequencies are shown in Table 6 9.The key hole, in the left of the images, was formed by the laserso that welding wires could be fed into groove from the rightside. The wire tips are observed in the droplet transfer neckingdown images. Each set of images shows a complete droplettransfer cycle. From the high speed images, we can see that thearc was shorter, the droplet diameter was no more than thewire diameter, and the transfer mode was incomplete during ashort circuit transfer demonstrating a spray-like feature for atplate conditions. However, the droplet grew bigger and thetransfer mode changed to a drop-like shape transfer duringshaking under narrow gap conditions.The droplet transfer behavior during laser-GMAW hybridwelding of the at plate is shown in Table 6 . The calculateddroplet transfer frequency was 74 times/s. The transfer speed

was relatively faster, resulting in a smaller droplet and a morestable welding process. Compared to the at plate hybridwelding process, the droplet grew bigger if slightly shakenbefore transfer and the transfer frequency decreased to

Table 10The in uence of the groove angle on the weld appearance.

Laser power=4kw current=80A welding speed=0.7m/min

DLA=4mm defocusing distance=-2mm gap=0.5mm

Groove angle()

40 60 90 Flat plate

Weld cross

section

appearance

Surface

appearance

Unfused

depth2.6mm 2mm 1.5mm -

Fig. 14. The weld appearance.

Fig. 15. The weld cross section showing two hybrid welding passes.

Fig. 16. The groove geometry and sizes.Fig. 13. The tendency curve between transfer frequency and groove angle.


8/9/2019 1-s2.0-S0030399214001054-main

9/12

26 times/s for the 90 1 groove angle conditions, as shown inTable 7 .The droplets shaking to the groove side walls and then fallinginto the side walls or welding pool to complete transferbehavior with calculated at a transfer frequency 12 times/sfor the 60 1 groove shown in Table 8 . When the groove angledecreased to 40 1 , the droplet transferred continuously to asingle side wall, as shown in Table 9 . Although the key hole

was blocked by a frozen droplet, we still see the droplettransfer became unstable, causing the transfer position andcontinuity to become uncertain. This resulted in the meltedmetal freezing on the both sides of weld bead groove.

As a result, the groove angle has a great effect on the droplettransfer behavior for a Y shaped groove under narrow gapwelding conditions. As the groove angle decreased, the droplettransfer frequency decreased. When the groove angle reachedits limitation, it caused the droplet to transfer to the grooveside walls, rendering the system unstable. Fig. 13 shows thetendency curve between the transfer frequency and thegroove angle.

(2) The in uence of the groove angle on the weld appearanceduring laser-GMAW hybrid weldingTo study how the groove angle affects the weld quality, adeeper analysis was conducted based on the weld appearanceand cross-sectional macro-morphology. A close relationshipwas observed between the weld appearance and the droplettransfer mode for the same conditions. From the high speedimages, we can see that even if the droplets grew bigger andthe transfer frequency decreased, as for at plate welding, thetransfer process was still stable and the weld appearance wassatisfactory for the 90 1 groove angle conditions. When grooveangle decreased to 60 1 , the weld longitudinal surface uctu-ated due to the droplets shaking before the transfer andresulting in an uncertain transfer cycle. The droplets trans-ferred to a single side wall for the 40 1 groove angle conditions.The sh-scale pattern on the weld surface was uneven, theweld surface was irregular, and a number of droplets frozewithout any pattern on the both sides of groove.Therefore, we conclude that the unstable droplet transfer stagewas the main reason for the poor weld appearance resultingfrom narrow gap laser-GMAW hybrid welding.Additionally, the laser-GMAW hybrid welding penetrationdecreased as the groove angle decreased. As shown inTable 10 , the fusion zone in the weld cross section increasedfrom 1.5 mm to 2.6 mm when groove angle decreased from

90 1 to 40 1 . This result con rmed the fact that narrower sidewalls absorbed more laser energy leading to a lack of fusion atthe root face during laser autogenous backing welding.

3.2.2 Weld metal re-melting during multi-layer, multi-passhybrid laser-GMAW welding of a 18 mm thick plate

As shown in Fig. 14 and 15 , there were no obvious weld defects,

such as porosity or slag. However, two main conclusions werereached. The side wall erosion phenomenon was considered to bea serious issue, showing that the weld width was much wider thanthe groove gap, which meant the welding heat input was muchlarger. The second hybrid welding pass remelted most of the rstpass, which wasted a signi cant amount of energy and reducedthe weld quality. During the narrow gap multi-pass hybrid laserwelding process, the laser energy was mainly used to heat andmelt the former weld, whereas the side wall temeperature waslower, which caused dendrities to rapidly grow from the fusionline to the weld seam center due to supercooling. The narrow gapweld with a taller channel and narrower sizes could not dissipateenough heat, which caused the crystal to be unable to maintain aconsistent orientation and grew larger. Accordingly, the former

weld should not be repeatedly remelted to prevent the crystalfrom growing. As a result, the laser power was decreased to 1 kW.

Table 11The 30 mm thick plate multi-layer, multi-pass laser welding parameters.

Welding passnumber

Laserpower (kW)

Weldingspeed (m/min)

Wire feedingspeed (m/min)

current (A) Laser arcdistance (mm)

Defocusingdistance (mm)

1 4 12 4 0.5 4.53 4 0.5 4.54 1 0.4 7 230 3 05 1 0.4 7 230 3 06 1 0.4 7 230 3 07 1 0.4 7.5 240 3 0

Fig. 18. The 30 mm thick plate laser non-autogenous weld bead appearance.

Fig. 19. The cross-section of the weld bead produced from laser non-autogenouswelding of the 30 mm thick plate.Fig. 17. The wire feeding nozzle.


8/9/2019 1-s2.0-S0030399214001054-main

10/12

8/9/2019 1-s2.0-S0030399214001054-main

11/12

Fig. 23. The HAZ microstructural morphology produced from laser-GMAW hybrid welding: (a) the total HAZ, (b) the fusion zone, (c) the overheat zone, and (d) the

incomplete recrystallization zone.

Fig. 22. The HAZ microstructural morphology produced from laser welding with a ller wire: (a) the total HAZ, (b) the fusion zone, (c) the overheat zone, and (d) theincomplete recrystallization zone.


8/9/2019 1-s2.0-S0030399214001054-main

12/12

The wire consumption was calculated by formula (1). Laser-GMAW hybrid welding laser power was decreased to 1 kW basedon the analysis in section 3.2.2.

3.3.3. Experimental result and analysisThe 30 mm thick plate was successfully welded. The weld

appearance and cross-section are shown as Figs. 18 and 19 . It isclear that the weld was very even and its appearance was

satisfactory.

(1) Weld defects and analysis.Weld defects were found at the junction between rst laserautogenous welding pass and the second laser wire llingwelding pass when the weld cross-section was examined.When the cross-section was examined at 500x, as shown inFig. 20 , holes that resulted from the platform at the bottom of the groove being too narrow are observed, making it dif cultto clean out the residue completely after welding.

(2) The weld heat affected zone (HAZ) microstructural morphol-ogy is shown in Figs. 21 23 .The HAZ produced using laser autogenous backing weldingwas the narrowest, followed by the laser welding with ller

wire HAZ and the HAZ produced using laser-GMAW hybridwelding. The local ne crystalline structure only occurred inthe HAZ from the laser autogenous backing welding. The HAZsproduced from laser welding with ller wire and laser-GMAWexhibited a ne crystalline structure that was mixed betweenthe overheat zone structure and the incomplete recrystalliza-tion structure. These two zones were therefore wider, whichwas the result of being repeatedly heated.

4. Conclusion

(1) During narrow gap, multi-pass laser wire lling welding, thewire was biased to the side walls of the groove if the gap wasincreased past a certain point.

(2) At the top of the weld top, where the groove gap was largest,side wall lack of fusion defects were observed for the narrowgap multi-pass laser welding with ller wire operation.

(3) Slag and porosity defects occurred often and their positionpositions adhered to some rules during narrow gap, multi-passlaser welding with ller wire operation. Most of the slag andporosity was caused by the incompletely cleaned scale thatwas left by the previous welding passes.

(4) A small distance between the side walls and a narrowerwelding pool resulted in changes to the electromagneticcontraction and the plasma jet forces. This small distance also

decreased the droplet transfer frequency or made the dropletsadhere to the side walls during narrow gap, multi-pass, laser-GMAW hybrid welding.

(5) The 30 mm thick plate was satisfactorily welded using onepass of laser autogenous backing welding, two passes of laserwire lling welding, and four passes of laser-GMAW hybridwelding.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant No. 51375191) and the NationalProgram on Key Basic Research Project (973 Program) of China(Grant No. 2014CB46703).

References

[1] Cunyin Hu, Fuju Zhang. Narrow gap welding technology and economicalcharacters analysis. Welding Technology 2001;30(2):47 8.

[2] Xiao Rongshi Kang Li, Wuxiong Yang. Chen Kai. The new technology of high-strength aluminum alloy thick plate narrow gap laser welding. New Tech-nology & New Process 2006;5:63 5.

[3] Katayama S. Deep penetration welding with high power disk lasers in lowvacuum. 30th International Congress on Applications of Lasers and Electro-Optics 2011:669 78 .

[4] Tianjiao Wang. Research on Narrow Gap Non-autogenous Laser Multi-passWelding. Master Dissertation. Wu Han: Huazhong University of Science andTechnology library; 2013 .

[5] Arata,Y, Maruo, H, Miyamoto, I, Nishio, R. High power CO2 laser welding of thickplate-Multi pass welding with ller wire. Transactions of JWRI, 15(2):27-34.

[6] Baiping Wang, Yong Zhao, Jian Huang. Investigation on Microstructure of Thick Plate Stainless Steel Joint Welded by Multi-Pass Laser Welding withFiller Wire. Chinese Journal of Lasers 2013;40(2):1 5.

[7] Roman JM, Kechemair D, Ricaud JP. CO2 laser welding of very large thicknessmaterials with wire ller. Welding International 1994;8(5):376 9.

[8] Ishide, T, Hashimoto, Y, Akada, T, Nagashima, T, Hamada, S. The latest YAG laserwelding system-development of hybrid YAG laser welding technology. Pro-ceedings of the Laser Materials Processing Conference, ICALEO '97. LaserInstitute of America. 1997, 49-156.

[9] Hayashi T, Katayama S. High power CO2 laser MIG hybrid welding process forincreased gap tolerance-Hybrid weldability of thick steel plates with squaregroove. Quarterly Journal of the Japan Welding Society 2003;21(4):522 31 .

[10] Choi Hae Woon. Hybrid welding process for sheet metal and narrow gap llpass. Transactions of Korean Society of Mechanical Engineers 2008;32(11):978 83 .

[11] The communication lecture information of Japanese experts in 2008.[12] Webster S, Kristen JK, Petring D. Joining of thick section steels using hybrid

laser welding. Ironmaking and Steelmaking 2008;7:496 505 .[13] Yangchun Yu. The wire melted into behavior and process research in laser

welding with ller wire. Doctoral dissertation. Wu Han: Huazhong Universityof Science and Technology library; 2010 .

[14] Greene, W.J. An Analysis of Transfer in Gas Shielded Welding Arcs, A.I.E.E.Winter General Meeting, New York (1960) (T.A.I.E.E.).

http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref1http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref1http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref1http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref1http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref1http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref4http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref4http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref4http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref4http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref9http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref9http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref9http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref9http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref9http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref10http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref9http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref9http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref8http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref7http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref6http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref5http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref4http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref4http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref4http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref3http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref2http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref1http://refhub.elsevier.com/S0030-3992(14)00105-4/sbref1

Documents

1-s2.0-S0030399214001054-main