Hybrid Nd:YAG Laser-AC MIG welding of thin section automotive aluminium alloy

C M Allen

TWI Ltd., Granta Park, Gt. Abington, Cambridge CB1 6AL, United Kingdom.

Paper presented at Eurojoin 6, Santiago de Compostela, Spain, 28 - 30 June, 2006.

Abstract

Hybrid laser-arc welding processes, in which a laser and an arc are combined in the same process zone, can offer a number of benefits over autogenous laser welding, including increased productivity and a tolerance to fit upcomparable with arc welding. This paper concentrates on the development of welding parameters for an Nd:YAG laser-AC MIG arc hybrid process for thin aluminium sheet, for potential application in the automotive industry. Melt runs, buttwelding and lap welding trials have been carried out on 1.2mm thick 5251-H22 aluminium alloy, with characterisation by metallographic section and radiography. A hybrid condition has been developed which is stable to welding speeds inexcess of 8m/min, representing a four fold increase in productivity compared to MIG welding or a two fold increase in productivity compared to laser welding. This hybrid condition has been applied with particular success to edge lapwelds, where a gap tolerance of ten times that of autogenous laser welding has been demonstrated. Internal weld porosity is outside of BS EN ISO 13919-2:2001, but any application would require a case-by-case assessment of fitness forpurpose, and/or more thorough material preparation to reduce porosity.

Introduction

Hybrid laser-arc welding, originally proposed in the late 1970s, ^[1,2] combines laser and arc welding processes in a single process zone. It offers the benefits of the separate processes, and overcomes some of their respective drawbacks. Reported benefits compared to laser welding ^[3,4,5] include:

• Increased tolerance to joint fit-up.
• Greater welding speed, leading to higher productivity.
• Increased penetration.
• Lower net heat input, leading to reduced distortion.
• Improved weld quality.
• The potential to replace some laser power, for a given depth of penetration, by some less expensive arc power, thereby increasing cost effectiveness.

Hybrid welding has received renewed interest in the past ten years, principally concentrating on laser-MAG welding of C-Mn steels, e.g. in shipbuilding panel lines. ^[6] An area of potential growth for the hybrid process is in the automotive sector for thin sheet aluminium joining. ^[7,8] Aside from hybrid welding operations, the automotive industry is making investments in laser technology, particularly in autogenous laser welding and/or laser brazing of thin sheet steels for car body construction, using robotmounted fibre optic delivered Nd:YAG laser beams. These fabrication methods are flexible, high productivity, and low distortion.

This paper addresses the further development of laser welding, through Nd:YAG laser-AC MIG arc hybrid welding of thin sheet aluminium alloy for future lightweight car body constructions.

AC MIG is a low heat input arc process, with demonstrated penetration control and good gap bridging ability when welding thin sheet aluminium. ^[9,10,11] In AC MIG, each current pulse passed through the consumable electrode consists of both an electrode positive (EP) part and an electrode negative (EN) part. The EN part melts the electrode in preference to the base material,increasing deposition rate and hence gap bridging ability, and also reducing heat input in to the base material. In summary, this low heat input, fine penetration control, high deposition rate technique is well suited to welding ofthin sheet with demonstrated tolerance to joint gaps.

The AC MIG process has been hybridised with low power density Nd:YAG and diode lasers, ^[10,11] and welding speeds of up to 4m/min have been reported, compared to 3m/min using the AC MIG arc without laser, i.e. a productivity increase of 33%. In this work a Nd:YAG laser of higher power density and capable therefore ofkeyhole welding, is combined with AC MIG welding, with the aim of offering welding speeds and productivity better than that of existing autogenous keyhole laser welding processes, and with a gap bridging ability better than laserwelding and more comparable to the AC MIG process. Introduction of such a hybrid process in to an existing Nd:YAG laser welding line would allow relaxation of fit up tolerances, and increases in productivity, with minimal additionalcapital expenditure being incurred.

Experimental Methods

Materials and preparation

5251-H22 (Al-2Mg) sheet aluminium alloy, 1.2mm in thickness, was used. Prior to welding the sheet edges were dry machined, and top and bottom faces of the sheets degreased with acetone. No further steps were taken to remove anyhydrated aluminium oxides from the region of the weld, which might otherwise improve weld quality (e.g. reduce internal porosity). Such approaches would be unlikely in an automotive fabrication environment. Two filler wires were used,both 1.2mm in diameter: AWS ER5356 (Al-5Mg) wire and AWS ER5556 (Al-5Mg-1Mn) wire.

Equipment

A Trumpf continuous wave HL4006D Nd:YAG laser was used, operating at power levels at the workpiece of up to 3kW, whose beam was focussed by a robot mounted optic to a 0.6mm diameter spot on the sheet upper surface. A Daihen CPDACR200 arc power source was used for AC MIG welding with an OTC-Daihen CMWH-147 wire feeding unit. The laser was used with a travel angle (dragging) of 10° off vertical, leading the AC MIG process by a separation of 2mm, with the MIGtorch having a travel angle (pushing) of 15° off vertical. These parameters were chosen on the basis of previous TWI experience with hybrid welding of aluminium and reference to. ^[10,11] Shielding of the weld pool was provided by a flow of 20l/min of either He or Ar down the MIG torch. A slot had to be cut into the MIG gas shroud to avoiding clipping by the laser beam. In the case of full penetration welds, theunderbead was shielded by a flow rate of 5l/min of Ar, supplied through an efflux channel measuring 10mm x 10mm, machined in to the welding jig.

Welding experiments

Given the number of process parameters and hence complexity of the hybrid process, welding parameters were optimised in turn for:

• Full penetration autogenous laser melt runs on sheet
• Full penetration autogenous laser butt welds
• Full penetration AC MIG melt runs on sheet
• Full penetration hybrid melt runs on sheet
• Partial penetration hybrid edge lap welds between two overlapping sheets. In selected cases, the gap between the two sheets was tapered from 0mm to 2mm, in order to assess the gap bridging ability of the hybrid process
• Full penetration hybrid butt welds.

The ranges of principal variables used in each set of experiments are summarised in Table 1.

Weld Examination

Radiography was performed to BS EN 1435:1997, to determine the presence of welding imperfections, eg extent of porosity. Transverse sections were prepared using standard metallographic techniques, with subsequent metallographicexamination determining weld shape and depth of penetration.

In the absence of a broadly accepted standard for fitness for purpose of laser welds for automotive application, as a relative indicator of weld quality, a standard for laser weld workmanship was used, BS EN ISO 13919-2:2001. Asthis is solely a standard for workmanship, failure to meet a given class of weld in this standard does not imply failure to be fit for a given application or purpose. The latter should be assessed independently on a case by casebasis.

Table 1. Ranges of principal experimental variables.

Notes:
- = not varied
* The Daihen AC MIG power source has an operation mode in which the mean arc current can be freely adjusted and set by the operator prior to welding. A feature of this operation mode, in common with other commercially availablearc welding power sources, is that other welding parameters such as the arc length or voltage, and pulse characteristics of the current, are set synergically within the power source. The current set by the operator and the synergicprogram selected, determine the values of these other parameters. In all work a synergic program setting of '42' was used, corresponding to an AC pulsed MIG operation with a 1.2mm diameter Al-Mg wire. Slight adjustments or 'trims' bythe operator to the arc voltage away from this synergic setting are possible, by selecting various positions on a potentiometer dial on the power source. In this manner, deviations of up to +/-5V from the synergic voltage can beselected. The details of the synergic program and trim settings are included as an aid to the practical user wishing to reproduce the results presented, although specific to the power source used.
** the EN ratio of the current setting can be adjusted by the operator away from the synergic setting, through different dial settings of a potentiometer, between arbitrarily denominated values of -5 to +5. The EN ratio has beendocumented to change the penetration characteristics when AC MIG welding. ^[10] Once again, these settings, when used, have been included as an aid to the practical user wishing to reproduce the results.

Results and discussion

Autogenous laser melt runs

With He top bead shielding of 20l/min, at travel speeds of 3 and 4m/min, melt-through occurred in a few positions ('pinholes') along the weld length. At a travel speed of 5m/min, a consistent top and underbead was achieved. Attravel speeds of 6 and 7m/min penetration was lost.

Switching to 20l/min Ar top bead shielding, all welds produced visually exhibited a brighter top bead appearance, being more effectively shielded against oxidation due to the greater density of Ar compared to He. At a travel speedof 3m/min, pinholes occurred. At travel speeds of 4 and 5m/min consistent top beads and underbeads were achieved, with the reduced heat input at 5m/min being preferred. This condition was selected as optimum and repeated three times tocheck consistency. Fig.1 shows a cross-section through one of these melt runs, whose profile was acceptable to the highest class, class B (stringent), according to BS EN ISO 13919-2:2001. Radiography of these melt runs also indicated an internalquality acceptable to class B (stringent) of BS EN ISO 13919-2:2001. At higher travel speeds, of 6, 7 and 8m/min, penetration became intermittent.

The optimum laser melt run condition was transferred to a nominal zero gap butt weld between sheets with machined edges. At 5m/min a narrower top bead was observed than when performing a melt run, and some localised losses inpenetration were observed. Reduction in travel speed by 10%, i.e. to 4.5m/min, increased penetration and top bead width. This condition was again repeated three times to check consistency. Fig.2 shows a cross-section through one of these butt welds. The weld profile was again acceptable to class B (stringent), as was the internal weld quality as determined by radiography.

For AC MIG melt runs, the slotted torch shroud to be used for hybrid experiments was found unsuitable, therefore a conventional shroud improving both melt pool shielding and arc stability, was used. Similarly, arc stability was muchimproved using Ar as a shielding gas, with an electrode stick out and shroud stand off of 10mm, compared to using He as a shielding gas, and an electrode stick out and shroud stand off of 15mm. Varying both arc current setting inincrements of 10A (without voltage trim being applied) and travel speed in increments of 0.25m/min, the most stable condition was found to be 50A at 1m/min. Penetration was less consistent than for the laser melt runs. Faster travelspeeds resulted in loss of arc stability, and higher currents resulted in melt through.

In a second round of trials voltage trim was applied. The condition that gave a stable arc at the highest welding speed achieved was using an arc current set to 80A at 2m/min, with a voltage trim of +2 or +3. This indicates that theapplication of a positive trim stabilises the arc to both higher currents and welding speeds. Top bead and underbead photographs from an AC MIG arc melt run at 2m/min with an arc current set to 80A with a +3 trim arc are shown in Fig.3. Penetration was more consistent than that achieved when using a 50A arc current, untrimmed, at 1m/min, but still not fully consistent, as is shown. In terms of weld profile, and particularly internal quality (porosity),this melt run was not acceptable to BS EN ISO 13919-2:2001. Weldments can tolerate an appreciable amount of porosity, that amount being material and alloy dependant, without significantly affecting static mechanical properties such asyield strength, and tensile strength and elongation being reduced as cross-section area is reduced. ^[12,13] This philosophy, of an acceptance of a certain porosity level, is commonly taken in the automotive industry, where expensive, time-consuming, and difficult to apply pre-welding aluminium cleaning treatments to reduce weldporosity levels are not the norm. That said, reduced porosity levels in both AC MIG and hybrid laser-AC MIG welds can be achieved with more attention to material cleaning. ^[10,11]

Hybrid melt runs

1. Basic conditions: Combining the arc and laser, with the AC MIG melt run conditions (initially, those developed without voltage trim) but at the optimum laser melt run travel speed, successfully resulted in a stablearc at much higher speeds than without the laser. However, the increase in heat input led to melt through. A variety of higher travel speeds and arc current settings (all without voltage trim) were tried to find a stable conditionwhich led to consistent penetration without melt through, resulting in an optimum condition at 7.6m/min with a mean arc current set to 80~90A. This represented a ~50% increase in welding speed compared with autogenous laser welding,and just over three and a half times faster than the fastest condition established when using the AC MIG arc on its own.

2. Effect of penetration control trim: Aside from reducing travel speed down from 8m/min to 7.6m/min to achieve more consistent penetration, the penetration trim control was also adjusted on the arc power set.Penetration trim control settings were chosen between a minimum of arbitrary denomination '-5' to a setting of '+3' (maximum possible setting was '+5'). These adjustments change the EN ratio of the AC current, although the magnitude ofthese changes in EN ratio were not recorded. Penetration control trims were applied to a reference melt run condition of mean arc current of 80A (with zero voltage trim) at a travel speed of 8m/min. Unlike the reported effect ofpenetration control trim on AC MIG welds, ^[11] no such effect was seen in the hybrid melt runs. This was probably due to the dominance of the laser over the arc, in terms of achieving penetration, at the high speeds used in these experiments.

3. Effect of reducing laser power: Melt runs were performed at lower travel speeds with reduced laser powers, to determine the minimum laser power required to stabilise the arc, and up to what speed that arc would bestabilised. All experiments were performed with a mean arc current set to 80A with zero voltage trim. These experiments are summarised in Fig.4. As Fig.4 shows, 1kW of laser power stabilised the arc at 1.5m/min, representing a 50% increase on the welding speed of 1m/min achieved without the laser (shown as the 'arc alone' point in Fig.4). However, a trimmed 80A arc, as reported above, was stable to 2m/min. At these modest welding speeds, voltage trim appears to be a far more 'economic' means of stabilising the arc than adding a low power (1kW) focussedlaser source.

4. Effect of voltage trim:

Hybrid edge lap welds

1. Basic conditions: For edge lap welding, as a start point the same conditions (without voltage trim) and travel angles were used as for hybrid melt runs. Initially, a work angle of 20° off vertical was used butthis led to penetration of the underlying sheet. The work angle was therefore increased to 40° off vertical, the maximum possible given the diameter of MIG shroud used, with the laser focussed on to the top surface of the lowersheet and the MIG wire aimed in to the corner of the joint. This also resulted in penetration of the underlying sheet. To avoid penetration a series of further experiments indicated that it was necessary to increase travel speed to8.6m/min, reduce laser power to 2.8kW, and, to counteract the resulting loss in arc stability, increase the mean arc current set to 110A. Top bead and underbead photographs from a hybrid edge lap weld with these conditions are shown inFig.6. Fig.7 shows a cross-section through this lap weld.

2. Effect of voltage trim:

3. Gap bridging:

Table 2. Gap bridging results for edge lap welds made by the hybrid laser-AC MIG process and the autogenous laser process.

Note:
*Gap bridging for the purposes of this work was defined as physical connection of the two sheets after welding, with the resulting weld bead of a profile not necessarily acceptable to ISO 13919-2.

As Table 2 shows, in the case of the hybrid welds, with a gap increasing from zero, bridging was maintained to a gap size of ~1mm, and with a gap tapering down to zero, gap bridging was first achieved at a gap size also of ~1mm.With larger gap sizes holes appeared in the top bead. In the autogenous laser welding process gap bridging was lost at a value ten times as small at ~0.1mm. The hybrid process is therefore far better in terms of gap bridging in thecase of this joint geometry. This large difference arose from the supply of extra weld metal material from the MIG wire consumable. Laser with cold wire feed would be more tolerant than the autogenous process, but welding speeds wouldhave to be reduced to allow the laser to not only melt the parent material but the wire as well. The hybrid process has the advantage that arc energy, not laser, is effectively used to melt the wire.

Hybrid butt welds

1. Basic conditions: For butt welding, as a start point the same conditions (without voltage trim) were again used as for hybrid melt runs. With these conditions penetration was heavier, and localised melt-throughoccurred in one position. Increasing welding speed to 8.4m/min still led to localised melt through, and higher speeds led to loss of penetration.

2. Effect of voltage trim, heat input, process separation and laser defocus position: Following on from earlier work, a +3 voltage trim was selected. However, this again resulted in localised melt-through. Reducingheat input by increasing speed and/or reducing laser power reduced, but could not entirely eliminate, the occurrence of these localised melt-throughs. Different laser-arc separations of 0mm and 4mm were tried, but without success, andin the case of a 4mm separation arc stability was lost. Different laser defocus positions of +2mm and +4mm were also tried, but led to loss of penetration. The origin of these localised melt throughs, or pinholes, may result from shorttime scale (<20ms) variations in any one of the following:

• Arc power: short term variations in the arc were indeed measured using high frequency monitoring equipment.
• Wire feed rate: consistent high speed feeding of soft aluminium wires during arc welding is a documented problem.
• Laser power arriving at and absorbed by the work: as opposed to variations in output power, these would more likely be fluctuations in power arriving at or absorbed by the work (e.g. due to fluctuations in the laser plume or keyhole).
• A 'random event' occurring in the weld pool (eg sudden localised build up of porosity).

Whatever the cause, these hole features were not seen at the low speeds used when using the arc on its own, where short time instabilities may be better accommodated due to the longer freezing time/slower solidification velocity.Nor were they seen in hybrid melt runs or non fully penetrating butt welds, where the weld pool may be slightly better supported, due to the absence of any abutting edges, or underlying non-melted material respectively.

Conclusions

The hybrid Nd:YAG laser-AC MIG welding process benefits, (and limitations), for joining 1.2mm thick sheets of 5251-H22 aluminium alloy have been quantified for both butt welding and edge lap welding. The main conclusions of thiswork are:

• The Nd:YAG laser and AC MIG welding processes can be successfully combined in a hybrid process suitable for high speed welding of thin sheet aluminium for automotive body construction.
• Hybrid welding at speeds of over 8m/min at 3kW laser power have been achieved, over four times faster than the AC MIG arc on its own, and up to 80% faster than autogenous laser welding.
• The application of a positive voltage trim stabilises the AC MIG arc to currents greater than 50% higher than without trim.
• When hybrid edge lap welding, gaps of up to 1mm between 1.2mm thickness sheets can be bridged, ten times those tolerated by equivalent autogenous laser welding process.
• An assessment of fitness for purpose of the welds made using this process would need to be made on a case by case basis. Attention to material cleanliness may be required to reduce the observed porosity levels, if indeed these prove unacceptable for a given application.
• This hybrid welding process appears sensitive to drop through in full penetration welds, e.g. butt welds, and is therefore better suited to partial penetration weld geometries, such as edge lap welds.

Acknowledgements

This work was funded by the Industrial Members of TWI as part of the TWI Core Research Program.

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