技術解説＞ Adaptive Control of Hybrid Laser-Arc Welding

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技術解説
Technical Review

Adaptive Control of Hybrid Laser-Arc Welding
C.M. Allen, G. Shl, P.A. Hilton and J.G Wylde
TWI Japan/UK Dodwell, 03-5826-7375, dodwell@d1.dion.ne.jp, http://www.twi.co.uk
はじめに不断な産業界の技術革新は溶接の分野も同様で，コスト削減，生産性／品質向上への開発努力は溶接機器，材料，開発，生産技術，自動化など多岐に亘り，その実現の為にはより緻密な制御が求められつつある。
多くの利点を持つ自動溶接は既に一部の産業分野で使われているが，現場の溶接局部はギャップの変化，目違い等により溶接条件が刻々変わり，これ等の変化に瞬時に対応し溶接品質を安定的に確保するには課題も少なくない。従来レーザー／アークハイブリッド溶接は熟練を要するマニュアルシステムが多いが，本稿はレーザーセンサーを用いたハイブリッド溶接におけるアダプティヴ制御について，TWI における近年の研究成果の一部を下記に紹介し自動化の普及に資する。

1 ．Introduction

However, for general fabrication introducing automatedwelding systems is more complex. In addition to thesetrends, developments in equipment, both for arc and power

Technology in every field is continually developing andwelding is no different. In addition to developments inbeam welding, are continuing. Thus providing additional
opportunities for automation.

conventional welding processes such as arc and resistancewelding, newer approaches involving frictional techniques

One area which has attracted a lot of interest is laser

and the use of power beams are finding more applications.

welding. This has been used for many years for high speed,

There are many reasons for this. Industry is continually

low distortion and/or thicker section applications where

seeking to reduce costs while increasing productivity andcost savings can be realized. However, the use of a laser has

maintaining or improving quality. Most traditional welding

certain limitations. Not least of these is the limited ability toprocesses are manual and require skilled operators. In many

cope with variable fit-up between work pieces, which is allmarkets skilled welders are becoming hard to find as fewer

too common in the fabrication environment. As mentioned

young people see welding as an attractive profession.

above, laser power sources are developing rapidly, and therelatively recent Yb-fibre and Yb:YAG disk lasers offer aThis has fuelled a drive towards more semi - or fully

significant step forward in terms of power and penetration

automated welding systems. For certain applications, e.g.

potential, but the very high inherent beam quality exacerbates

circumferential pipe welding, this can be more readily

the issue of improving existing tolerances to variable fit-up.

achieved because the joint geometry is relatively simple.

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One way of overcoming this issue, enabling us to takeIn order to develop the basic data, a range of trials areadvantage of laser power source developments, has been

carried out looking at the optimum welding parameters

hybrid laser-arc welding. Here, an arc welding power source,

for a range of gaps and hi-lo variations. This short paper

typically MIG, can be combined with the laser power sourcesummarises some of the recent work at TWI.

in the same welding head to provide increased fit-up tolerance

2 ．Experimental

while still maintaining good weld profile characteristics andwelding speed. This all represents excellent progress, but
a key issue still facing the application of hybrid laser-arc

2． 1

Equipment and Materials

welding in a production environment remains the ability toAn IPG YLS-5000 Yb fibre laser was used in welding

cope with changes in fit-up. Thus, if we have a fixed gap, wetrials, with a calculated focused spot diameter of 0.28 mm.

can modify the hybrid welding parameters (within certain

The beam focusing optics were mounted on a Kawasaki FS-limits) to cope. However, if the gap varies, perhaps due to060L robot. The optics were protected from fume and spatterdistortion of work pieces, or variation in positioning andusing an air-knife and an appropriate cover slide. All trials

fixturing, this creates a problem.

were performed in the flat position, with the laser beam at 90°
to the workpiece surface. A measured laser power of 4.8 kW

Recent work at TWI on adaptive control of the hybrid
laser-arc welding process1)

at the workpiece was used.

has shown that this approach

offers the potential to cope with variable gaps. In adaptive

When hybrid welding, an ESAB PSF 410 MW metal active

control, a second scanning laser is used ahead of the weldinggas (MAG) welding torch was also used, with an AristoFeed

head. This identifies the position of the joint, the gap and any

30 wire feeding unit, U8 controller and AristoMIG 450 arc

hi-lo mismatch between the work pieces. This information ispower source. All welds were made using a laser-leading

fed back into the welding controller which selects appropriate

configuration. The separation between the laser and the arc

parameters from a database of pre-determined conditions

was set to 2 mm. A contact tip to workpiece distance of 15mm

covering the range of variables within the capacity of thewas used, with a work angle of 60°(pushing) for the MAGprocess. This is shown diagrammatically in Fig. 1.

torch. A synergic pulsed metal transfer mode was also used,with a 1.2 mm diameter A18 grade solid wire consumable.
The weld top bead was shielded by 20 l/min of argon throughthe torch. The weld under bead was shielded by 2 l/min ofargon.
All trials were carried out on 8 mm thick S355 grade
structural steel plates. These were laser cut to size, acetone
degreased, disc ground back to bright metal, and thendegreased again before welding.

2． 2

Experimental Procedure

Initially, laser only butt welding trials were carried out at
a range of welding speeds (0.9-2.2 m/min) to determine asuitable joint preparation and welding speed for the hybrid
trials that followed. Optimum hybrid welding conditions
to produce ISO 13919-1:1997 Class B quality welds (themost stringent weld quality class in this standard) were thenFig. 1. Example of adaptive control algorithm.

determined for close fitting joints.

技術解説＞ Adaptive Control of Hybrid Laser-Arc Welding

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A series of trials were then carried out to establish thetolerance of these hybrid conditions to joint mismatch or joint
gap. Further trials were then carried out on a range of samples
in which a range of gaps were deliberately introduced, as
well as a range of hi-lo mismatch. In each case, the welding
conditions were varied to establish suitable parameters toproduce class B welds.
The above conditions were then used as control points forthe adaptively controlled trials. A Servo-Robot Digi-I/S laser

Fig. 2. Cross-section through hybrid weld made overflush, close fitting V butt joint. 2 mm scale bar.

vision sensor with V300 control unit, communicating with aKawasaki D+ robot controller, was used. Welds were madeThe same conditions were also used to weld joints with upover joints with either varying amounts of joint gap or joint

to 2 mm hi-lo mismatch. Figs. 3a and 3b show sections frommismatch, with welding conditions being changed in realsuch welds with 0.6 and 1.0mm mismatches. This shows thattime.

a Class B root and cap profile were achieved to a mismatch of0.6mm (Fig. 3a), with the root toe blend angle becoming re-Changes in welding (robot) speed and robot position wereeffected by relaying joint fit-up information from the V300

entrant on the higher plate when the mismatch increased to1.0mm (Fig. 3b).

unit to the D+ controller, then referencing a look-up table
written within the D+ controller software. For example,
this table may call for a reduction in robot speed and theintroduction of an offset of the robot tool centre point off ofthe joint line, when a joint mismatch over a given height isdetected. Changes in wire feed rate were effected using anADAP software module in the V300 unit. Thus the wire feed
rate was adjusted in real time as a function of the joint gap. A
suitable time delay between the generation of the signal anda) 0.6 mm mismatchb) 1.0 mm mismatch

Fig. 3. Cross-section s through hybrid weld made overits transmission to the arc welding power source was used, tojoint. with hi-lo mismach. of 0.6 mm a) and 1.0

allow the robot tool centre point to catch up with the point atmm b).

which the laser vision sensor had collected the information on
joint gap.

At higher hi-lo mismatch values, the weld root was both3 ．Results

re-entrant and also undercut. Radiographs of these welds
indicated that class B quality was achieved to mismatchvalues of at least 0.6 mm, with chain or clustered porosity

Initial hybrid trials were carried out on close fitting, flush

and lack of penetration defects present at higher values ofplates, using a V butt joint preparation with a 6mm root facemismatch. These trials showed, therefore, that the conditions

and a 60°included angle. A visually acceptable weld top

suitable for welding flush, close fitting joints, could also bebead appearance was achieved, with slightly convex profile

used to produce Class B welds for a hi-lo mismatch up toand minimal undercut, using a wire feed rate of 7 m/min

0.6 mm.

and a -3V arc voltage trim. Radiography indicated internal
porosity contents to Class B. Fig. 2 shows a cross-section of aTrials were also carried out on joints with various gaps

weld made with these conditions. The weld cap had minimal

between the plates using the same welding parameters. Theseundercut (0.16 mm) and an excess penetration of 0.5 mm, i.e.

trials showed that the base conditions could also be used toconforming to class B.

weld joints with gaps up to 0.3 mm.

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A range of other welding conditions were then examined,
to improve the weld profiles for a range of hi-lo mismatch andgaps. In terms of coping with increased hi-lo mismatch, one
or more of the following changes were introduced:•A reduction in welding speed.

•Deliberately offsetting the laser beam off of the joint line
(by offsetting the welding head).

•Using different laser focus positions.

Fig. 5. Cross-section through hybrid weld made overjoint with 0.5 mm wide gap, welding at 1.2 m/min

These results suggested that a change in laser focus position
was the best way to achieve a modest (<25%) improvement inwith wire feed rate of 9 m/min.

mismatch tolerance (Figure 4).
Adaptive trials were then carried out as described above,
the welding controller selecting the appropriate welding
conditions, for the joint geometry as determined from the laser
scan. Trials were performed using joints with hi-lo mismatch,a tapering joint gap and with a combination of the two. The
welding speed, wire feed rate and laser focus position and/orlaser beam offset were varied in real time during the welding.
In terms of coping with hi-lo mismatch, the most successful
Fig. 4. Cross-section through hybrid weld made overjoint. with 0.8 mm of mismach, focusing laser 2mm below top of root face.

results were achieved by adaptively reducing welding speed.
Figs. 6a and 6b show the weld profiles achieved from thesame weld, over joint mismatches of 1.0 and 1.2 mm, by
adaptive reduction of welding speed from 1.6 to 1.2m/min. AsFigs. 6a and 6b show, an acceptable weld root profile has been

In terms of coping with increased gap, one or more of theachieved with this approach, to a mismatch of at least 1.0 mm.

following changes were introduced:•Increasing the wire feed rate and/or arc voltage trim.

•Reducing the welding speed.

•Focusing the laser 4mm above the top of the root face.
The results from these trials suggested that adaptivechanges in both welding speed and wire feed rate would be
the best option, and allowed for a ～ 200 % improvement ingap tolerance (Fig. 5).

a) 1.0 mm mismatchb) 1.2 mm mismatch

Fig. 6. C ross-section through adaptively controlled
hybrid weld made over joint with hi-lo mismatchof 1.0 mm a) and 1.2 mm b).

技術解説＞ Adaptive Control of Hybrid Laser-Arc Welding

In terms of coping with a tapering joint gap, the most123

4 ．Discussion

successful results were achieved by adaptively reducing thewelding speed and increasing the wire feed rate. Figs. 7aand 7b show the weld profiles achieved at joint gaps of 0.6

In the current work, the gap tolerance of the hybrid process

and 0.9 mm, from the same weld. As Figs. 7a and 7b show,with a high brightness Yb fibre laser without using adaptive

an acceptable weld cap profile is achieved to a gap width ofcontrol is 0.3mm, to produce class B welds over broad rootat least 0.6 mm, twice that achieved without adaptive control.

face V butt joints in 8mm steel plate. This has been doubled

An ISO class C weld was achieved over a 0.9 mm wide gap,

by using adaptive control. However, this maximum is still

but at gaps >1 mm, weld cracks and lack of sidewall fusion

only 0.6mm. This more modest result can be attributed to thedefects were detected.

60 °V butt joint configuration used, necessitating the use ofhigh wire feed rates to achieve a weld profile with acceptably
low levels of underfill. With an alternative choice of joint
configuration, weldable with higher output power levels, agreater maximum gap tolerance for class B welds would beanticipated.
Tolerance to joint mismatch has also been increaseda) 0.6 mm mismatchb) 0.9 mm mismatch

Fig. 7. Cross-section s through adaptively controlled
hybrid weld made over joint with gap of 0.6 mm a)and 0.9 mm b).

through the use of adaptive control and class B welds can beachieved across a hi-lo mismatch value of at least 1 mm.
The individual tolerance limits to either gap or hi-lo
mismatch are reduced when both are present. The limitsare 0.5 and 0.65 mm for class B welds, respectively,compared with 0.6 and 1 mm, if only gap, or mismatch, isThe most effective approach for joints with a combination

present. Nevertheless, class C welds can be achieved overof both hi-lo mismatch and gap was achieved via adaptive

combinations of 0.85, and 1 mm, respectively, with adaptive

control of the welding speed and wire feed rate (Fig. 8)

control.

5 ．Conclusions
Hybrid welding using a 5kW fibre laser can produce
ISO 13919-1 class B butt welds in 8mm thickness steel
with a 60°V butt joint configuration when either joint
gaps of 0.3 mm width, or mismatches of 0.6mm height,a) 0.65 mm mismatch,
0.5 mm gap.

b) 1.0 mm mismatch,

are present.

0.85 mm gap.
With adaptive control of welding parameters, the tolerance

Fig. 8. Cross-section s through adaptively controlled
hybrid weld made over joint with both mismatchand gap.
0.65 mm mismatch and 0.5 mm gap a)
1.0 mm mismatch and 0.85 mm gap b)

limits for class B welds can be extended to 0.6 or 1 mm,respectively.
Hybrid welding can also be adaptively controlled to copewith combinations of gap / hi-lo mismatch, to 0.5/0.65 mm(class B) or 0.85/1 mm (class C).

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Reference1) C.M. Allen, G. Shi and P.A. Hilton:Adaptively controlled
hybrid welding using a high brightness laser, ICPBT, 2010

Appendix

Fig. 9. Difinition of hi-lo mismatch and gap.