Chris Allen gained his Natural Sciences degree from the University of Cambridge then completed a doctorate in the area of Materials Science at the University of Oxford. Following postdoctoral research, he worked for two years as a project leader at Corus in rolled aluminium products, before joining TWI in 2002. Since then, he has specialised in the project management and delivery of laser welding projects for a range of TWI's clients, primarily in the aerospace, shipbuilding, defence, and automotive sectors.
Hybrid laser-arc welding, originally proposed in the late seventies, combines laser and arc welding processes in a single process zone. As Chris Allen reports it offers the benefits of the separate processes, and overcomes some of their respective drawbacks. Reported benefits of the hybrid process, compared to pure laser welding can 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, eg in shipbuilding panel lines. An area of potential growth for the hybrid process is in the automotive sector for thin sheet aluminium joining. 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 robot mounted fibre optic delivered Nd:YAG laser beams. These fabrication methods are flexible, high productivity, and low distortion.
This feature 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 construction.
AC MIG is a low heat input arc process, with demonstrated penetration control and good gap bridging ability when welding thin sheet aluminium. 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 into the base material. In summary, this low heat input, fine penetration control, high deposition rate technique is well suited to welding thin sheet with a demonstrated tolerance to joint gaps.
The AC MIG process has been hybridised with low power density Nd:YAG and diode lasers, and welding speeds of up to 4m/min have been reported, compared to 3m/min using the AC MIG arc without laser, ie a productivity increase of 33%. In this work an Nd:YAG laser of higher power density and capable therefore of keyhole 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 laser welding and more comparable to the AC MIG process. Introduction of such a hybrid process into an existing Nd:YAG laser welding line may allow relaxation of fit up tolerances, and increases in productivity, with minimal additional capital 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 any hydrated aluminium oxides from the region of the weld, which might otherwise improve weld quality ( eg reduce internal porosity). Such steps 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 focused by a robot mounted optic to a 0.6mm diameter spot on the sheet upper surface. A Daihen CPDACR 200 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 MIG torch 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. 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 avoid clipping by the laser beam. In the case of full penetration welds, the underbead was shielded by a flow rate of 5l/min of Ar, supplied through an efflux channel measuring 10mm x 10mm in cross-section, machined into the welding jig.
Welding experiments
Given the number of process parameters and 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.
Table 1 Ranges of principal experimental variables
| Expt. type | Laser power, kW | Travel speed, m/min | Top bead shielding | Arc current, amps | Voltage trim setting* | Penetration control setting** |
Laser melt run
Laser butt weld | 3.0 | 3.0-8.6 | 20l/min Ar or
20l/min He | n/a | n/a | n/a |
| AC MIG melt runs | n/a | 1.0-2.0 | 20l/min Ar or
20l/min He | 40-90 | -2 to +3 | - |
Hybrid melt runs
Hybrid butt welds |
0.5-3.0 |
7.6-10.0 |
20l/min Ar |
80-130 |
0 to +5 |
-5 to +5 |
| Hybrid edge lap welds | | | | | | |
| 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 available arc 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 synergic program 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' by the 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 be selected. 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 been documented to change the penetration characteristics when AC MIG welding. Once again, these settings, when used, have been included as an aid to the practical user wishing to reproduce the results. |
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 metallographic examination 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. As this 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 case basis.
Part 2 Results and Discussion on Chris Allen's work will follow in the next issue of Bulletin.
