Achieving Low-Porosity Laser Welds in Aerospace Aluminium Alloy

G Verhaeghe and P Hilton
TWI

S Barnes
Warwick Manufacturing Group

Copyright (c) 2003 SAE International.

Paper presented at 2003 Aerospace Manufacturing Technology Conference (AMTC), 8-12 September 2003, Montreal, Canada.

Abstract

Aluminium is currently the preferred material and riveting the preferred joining method for the manufacture of thin-gauge airframe structures. Although the potential of laser welding as a low-distortion alternative for such applications is recognised, questions are still being raised about the weld quality, and in particular the porosity levels, that can be achieved in aluminium. This paper focuses on the cleaning of parent material and filler material prior to welding, the use of a twin-spot energy profile in the laser beam focus and the use of a low-moisture shielding gas and shielding gas delivery, and their individual and combined influence on the presence of weld metal porosity for Nd:YAG laser welds in 3.2mm thickness 2024 aluminium alloy. The paper describes how, through careful selection of processing conditions and aforementioned factors, fully penetrating, square-edge butt welds were achievable with levels of weld metal porosity lower than those specified in the stringent weld quality class of standards relevant to the aerospace industry, including the European BS EN ISO 13919-2:2001 and the American AWS D17.1.

Introduction

Increased international competition has encouraged aerospace companies to investigate new approaches to aircraft design and manufacturing methods to provide reliable and competitively priced products. ^[1] The work described in this paper was carried out as part of an initiative in this field, a programme named CEMWAM (Cost Effective Manufacture: Welding of Aerospace Materials), initiated by a number of leading UK industrial companies, Research and Technology Organisations (RTOs) and Universities.

Currently, the preferred manufacturing route for aircraft fuselage structures is riveting and the principal material for these structures is aluminium. Recent analyses, however, have indicated that a move from riveted to welded airframe structures could lead to manufacturing cost savings in the region of 30%. ^[1] Laser welding is one of the processes currently being considered for this, because of the high processing speeds, low heat input, low distortion, good weld quality and the overall flexibility that the process offers. ^[1-3] Over the past few years, there has been a great effort in the automotive sector to introduce laser welding technology onto the shop floor. A great deal has been learnt from these experiences, but further scrutiny of welding procedures is required to ensure that the weld quality needed for aerospace applications can be achieved reliably, in particular for the laser welding of aluminium.

Although possible, the laser welding of aluminium is generally perceived to be difficult because of the initial high surface reflectivity and the high thermal conductivity of aluminium, both of which contribute to the risk of weld imperfections such as lack of penetration or cracking, in certain alloys. Weld metal porosity is also frequently associated with the laser welding of aluminium and this weld imperfection is the subject of the work described in this paper. Weld metal porosity is always an issue when fusion welding aluminium ^[4,5] and laser welding is no exception. What causes porosity and, in particular, how it is formed in laser welds are issues still being debated, but not considered in detail here. Some suggest that volatilisation of low boiling point constituents in some of the aluminium alloys causes keyhole instabilities, others believe it is simply entrapment of shielding gas in the solidifying weld pool, whilst others attribute porosity to hydrogen entrapment during weld pool cooling and solidification. ^[1,6,7] Irrespective of the cause however, porosity is generally categorised as either fine or coarse, typically differentiated at an average pore diameter of 0.5mm. Fine porosity appears as a distribution of spherical pores and is generally understood to originate from hydrogen or from the rejection of dissolved shielding gases on solidification. Coarse porosity is characterised by larger, more irregularly shaped voids, randomly distributed throughout the weld bead. These are generally considered to be the result of keyhole instabilities and are typically present in partially penetrating welds. ^[1,7] Coarse porosity can have a detrimental effect on a welded joint's mechanical performance. ^[8]

The work described in this paper details Nd:YAG laser welding trials, carried out on 3.2mm thickness 2024 aerospace aluminium alloy, aimed at reducing both the fine and coarse porosity in aluminium weldments. Particular efforts were on reducing the fine porosity, and especially fine porosity resulting from hydrogen-entrapment. Hydrogen dissolves very rapidly into the aluminium weld pool ^[4] but has a very low solubility in solid aluminium, as is shown in Figure 1. With a high-speed process such as laser welding, the time available for diffusion is sufficiently low that a certain amount of hydrogen can become entrapped in the solidifying weld pool. ^[6] Hydrogen can originate from the parent material, the filler wire or the shielding gas and these sources were all investigated. In addition, the performance of twin-spot Nd:YAG laser welding was assessed to establish whether this technique resulted in a reduced level of porosity, because of the resultant elongated weld pool found when using this technique. ^[9]

Figure 6

differs from Figure 5 in that the welds depicted were carried out with a 1:0.75 imaging lens, creating a 0.45mm minimum focus spot diameter instead of the previously used 0.6mm spot distribution. Pores 0.1mm in diameter and smaller are depicted in Figure 6.

Fully penetrating, square-edge butt welds were also produced in the 6056 aluminium alloy using the same single-spot and twin-spot conditions as those used for the 2024 alloy. A similar, but smaller influence of the twin-spot technique and the inherent lower travel speed (compared with the single-spot technique) on reducing the level of coarse weld metal porosity was observed for this alloy ( Figure 7

). For the welds in the 6056 aluminium alloy, produced using the single-spot energy profile however, no pores larger than 0.7mm in diameter were found, indicating that besides energy beam profile, welding speed and spot size, the material grade or alloy composition can have an effect on the level of coarse porosity.

Fig. 7. Weld metal porosity for welds in 6056 aluminium alloy

Shielding gas moisture content versus porosity

To examine how moisture in the shielding gas could contribute to the presence of fine weld metal porosity, welds were also produced using a high purity, low dew-point research grade helium shielding gas and a 'modified' shielding gas delivery system, comprising the shortest (maximum 2m) polyamide tubing lengths possible between gas cylinder and coaxial shielding nozzle. Considering the hygroscopic nature of polyamide, the tubes were 'acclimatised' several hours prior to welding and purged at the onset of the welding trials. The gas delivery system was purged for at least two minutes between subsequent welds. The welds were carried out on chemi-etched samples, with chemi-etched filler wire and using a twin-spot energy profile, based on the thus far accumulated experience.

It can be seen from Figure 6 that the welds carried out using the research grade (low-moisture content) helium shielding gas and the 'modified' shielding gas delivery system, clearly demonstrate a considerable reduction in pore numbers and equivalent pore length/area compared with the welds made under exactly the same conditions but using industrial grade, instead of research grade, helium shielding gas. Moreover, the equivalent pore lengths/areas summarised in Table 3, demonstrate that the welds produced under these shielding gas conditions resulted in a level of weld metal porosity considerably lower than any of those achieved before. In fact, from all welds produced in this work, these were the only set that passed all three porosity criteria for each of the standards, even the third criterion (total pore length per given weld length) of the most stringent standard, AWS D17.1.

Conclusion

A welding procedure was developed for producing fully penetrating, square-edge butt joints in 3.2mm thickness 2024 aluminium alloy using 3kW CW flashlamp-pumped Nd:YAG laser power. By controlling the process conditions, it was possible to achieve a level of weld metal porosity lower than that defined for the stringent quality class in BS EN 13919-1:1997, a typical aerospace industry standard, and even the most rigorous of standards considered, i.e. standard AWS D17.1:2001.

• A focus position on or 1mm below the material surface helps achieve full penetration welds in 3.2mm thickness 2024 aluminium alloy with weld profiles that conform to BS EN ISO 13919-2:2001.
• A high-purity, low dew-point 'research-grade' helium shielding gas, delivered through a moisture and/or condensation-free shielding gas delivery system, should be used, as this produces less weld metal porosity compared withindustrial grade helium gas.
• Removing the porous oxide layer prior to welding contributes to reducing the weld metal porosity in laser welded 2024 aluminium alloy. Linishing, scraping, machining or chemical etching can be used for this purpose, but the elapsedtime between material preparation and subsequent welding needs to be as short as possible (less than 24 hours recommended) to avoid atmospheric moisture pick-up.
• A further reduction in weld metal porosity can be achieved by cleaning the filler wire, for instance with a chemical etching cleaning operation.
• The use of a twin-spot laser energy profile with a 0.27mm spot separation and a 50/50 energy distribution helps eliminate coarse porosity in 3.2mm thickness 2024 aluminium welds. The technique has less of an effect on pores smallerthan 0.4-0.5mm diameter.

At this point, parameter development could be carried out for a given component, to provide a robust and reliable material preparation and welding procedure, to Nd:YAG laser weld typical thin-gauge aerospace aluminium alloys. In addition to porosity-reducing procedures common to all aluminium fusion welding processes, it is possible to mitigate the weld metal porosity to levels acceptable to current aerospace welding standards. However, considering the preparation required, this will be at a cost and it is recommended, that, prior to developing a welding procedure, the maximum allowable levels of weld metal porosity should be established, according to the application and required joint performance. It is unlikely for instance, that some of the smaller, non-surface breaking porosity typically found in laser-welded aluminium will affect static or fatigue performance.

Acknowledgments

This work was funded jointly by Industrial Members of TWI (through its Core Research Programme) and by the UK's Engineering Physics Sciences Research Council (EPSRC) (through the CEMWAM project). The author would like to thank the CEMWAM partners for their help, assistance and provision of materials, and the EPSRC for funding the EngD programme. The assistance of Mr F A S Nolan, R Lombardi and A S Spencer, who carried out the processing trials is gratefully acknowledged. Special thanks also to Dr N C Sekhar for his assistance throughout the work programme.

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Contact

Geert is a Mechanical Engineer and European Welding Engineer who started his career in Belgium, working for steel producer Sidmar (part of Arcelor). He joined TWI Ltd (The Welding Institute) in 1996 where he currently works as a Senior Project Leader in the Laser and Sheet Processes Group. He has particular involvement with projects for the road transport and aerospace industry sector.