Geert Verhaeghe and Paul Hilton
Paper presented at 34th International MATADOR Conference, 7th - 9th July 2004, UMIST, Manchester, UK.
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.
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%.  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] 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 [4-8] and this weld imperfection is the subject of the work described in this paper.
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, which can have a detrimental effect on a welded joint's mechanical performance,  is characterised by larger, more irregularly shaped voids, randomly distributed throughout the weld bead. These are generally considered to be the result of low boiling point constituents causing keyhole instabilities and are typically present in partially penetrating welds. [1,7,8]
The work described in this paper, carried out as part of 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, details Nd:YAG laser welding trials aimed at reducing both the fine and coarse porosity in laser welded 3.2mm thickness 2024 aerospace aluminium alloy. 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  but has a very low solubility in solid aluminium. 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.  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 created when using this technique. 
Square-edge butt welds were produced using a 3kW flashlamp-pumped continuous wave (CW) Nd:YAG laser focused into a single 0.45mm diameter spot, on 150mm wide and 300mm long samples. The samples were subjected to a primary cleaning operation, whereby the long edges were cold band sawn and dry machined to ensure a good joint fit-up and the samples degreased with acetone immediately after the dry machining operation. Industrial grade helium (purity grade 5.0 or 99.999% pure) conforming to BS EN 439:1994,  typically containing around 5ppm moisture, was used for shielding both top and bottom of the weld pool. A 1.2mm diameter 2319 filler wire was introduced into the leading edge of the weld pool. 
Process parameters such as welding speed, laser focus position (in relation to the material surface), filler wire position (in relation to the laser-workpiece impingement point) and shielding gas flow rates, were varied to obtain visually acceptable, in accordance with BS EN ISO 13919-2:2001,  fully penetrating, square-edge butt welds. The absence of cracks in the weld metal and heat-affected zone (HAZ) was confirmed through radiographic and macro-metallurgical examination. The presence of micro-cracks in the weld metal or HAZ was not examined. Subsequent welds were produced using the process parameters established earlier, but with different conditions for parent material cleanliness, filler wire cleanliness and condition and delivery of process shielding gas. These factors, widely considered to be the main causes of hydrogen-induced weld metal porosity, were varied in a controlled fashion, as detailed further. In addition, the effect of a twin-spot, i.e. two spots of0.45mm diameter separated in the beam focus, versus a single-spot laser energy distribution on the level of weld metal porosity was also investigated.
A selection of the welds was radiographed and a pore count carried out over a 100mm longitudinal section of the weld, representative of the entire weld length, to quantify the levels of weld metal porosity. These levels and the equivalent pore length/area per given weld length/area calculated from these levels, were compared with the acceptance levels of the stringent weld quality class defined in two international standards, BS EN ISO 13919-2:2001  and AWS D17.1:2001,  and one company internal standard ABP 2-4102.  The European standard was selected because it is specific to laser welding of aluminium, whereas the American standard and the internal standard were chosen because they are specific to fusion welding for aerospace applications. The acceptance criteria that needed to be fulfilled included the diameter of the largest pore, the minimum distance between adjacent pores and the equivalent projected pore area (for the European standard) or pore length (for the American and internal standard) per given length of weld. In this work, the second criterion, i.e. the minimum distance between adjacent pores, was not considered as it was fulfilled for all welds.
2. Results and Discussion
2.1 Parent material cleanliness versus porosity
In addition to the initial dry machining of the sample edges after cutting, they were subjected, just prior to welding, to a secondary cleaning operation. The secondary cleaning methods examined comprised linishing, abrading, scraping, machining or chemical etching (or chemi-etching), each followed by an acetone degrease with a lint-free cloth to remove residual dirt, moisture or lubricant. The acetone degreasing was also investigated on its own as a secondary cleaning operation. For the chemi-etch cleaning, the samples were soaked, at least six times, for two minutes, in anitric and hydrofluoric acid solution, followed by a water rinsing. All cleaning operations, except machining and chemi-etching, were carried out manually and, with the exception of acetone degreasing, performed to remove the porousoxide layer, a potential source of contaminants and moisture, and thus porosity. 
Whether the parent material was cleaned or not prior to welding made little difference visually, but produced an obvious distinction in the resulting level of weld metal porosity, as could be seen by comparing radiographs. Notwithstanding some variability, little difference was apparent in the total level of weld metal porosity, i.e. pore size and distribution (or equivalent pore length/area per given weld length/area) between the different secondary cleaning methods. This is demonstrated by the linished and chemi-etched samples shown on the left of Figure 1. If anything, the chemi-etched samples exhibited less porosity than those linished, presumably because of the uniformity and non-manual operation of the chemi-etching. For the machining and chemi-etch sample preparation, low levels of porosity could only be achieved when cleaning was carried out less than 24 hours before welding, and with the samples kept in a dessicating unit. The spherical nature of the coarse porosity found in some ofthe welds and the fact that the welds were fully penetrating would indicate that this coarse porosity originated from large volumes of entrapped gas rather than the result of keyhole instabilities. 
2.2 Filler wire cleaning versus porosity
To assess the individual effects of parent material and filler material cleaning on weld metal porosity, welding was carried out on samples that were chemi-etched less than four hours prior to welding, with as-received filler wire as well as with filler wire that was chemi-etched immediately before welding. A clear drop in the total level of weld metal porosity, i.e. pore size and distribution, or equivalent pore length/area per given weld length/area, was noticeable for the cleaned filler wire, as demonstrated in Figure 1. The achieved reduction in porosity meant that all welds produced with the chemi-etched filler wire passed both investigated criteria of the European and internal company standard. The equivalent pore length per given weld length of some of the samples however, still remained too high to pass the third criterion (total pore length for given weld length) of the American standard AWS D17.1.
2.3 Twin spot energy distribution versus porosity
Additional trials were carried out to examine if an elongated weld pool created by a twin-spot laser energy profile would delay the rapid solidification of the weld pool, thus allowing more time for hydrogen to escape.  Based on earlier work, [16,17] welds were produced using a twin-spot energy profile with a 0.27mm separation and a 50/50 energy distribution between the two 0.45mm diameter spots.
Figure 1 shows that the levels of coarse porosity for welds produced with a twin-spot energy profile were lower when compared to those produced earlier with a single-spot energy profile. However, a consequence of the twin-spottechnique was that welding was carried out at lower travel speeds, i.e. between 0.8 and 1.0m/min, compared with the single-spot speed techniques where the welding speed was 1.75m/min. With the twin-spot technique, the total laser power is shared between the two spots, creating a lower power density in each of the spots, necessitating a lower travel speed compared with the single-spot technique, to produce fully penetrating welds in a given material thickness. 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 laser welds in 3.2mm thickness6056 aluminium alloy. The presence of coarse porosity in this alloy, however, was less pronounced, indicating that besides energy beam profile, welding speed and spot size, material grade or alloy composition can also have an effect on the level of coarse porosity in laser welds
2.4 Shielding gas moisture content versus porosity
To examine how moisture in the shielding gas contributes 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. The latter comprised the shortest, i.e. maximum 2m, length of polyurethane tubing possible between gas cylinder and coaxial shielding nozzle, and, due to the hygroscopic nature of polyurethane, tubing that was 'acclimatised' several hours prior to welding, purged for several minutes at the onset of the welding trials and 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.
Welds produced using this 'modified' delivery system and a research grade (low-moisture content) helium shielding gas demonstrated a considerable reduction in pore numbers compared with welds made under exactly the same conditions but using industrial grade helium, as can be seen in Figure 1. The resulting equivalent pore length per given weld length of these welds were considerably lower than any of those achieved before. In fact, from all welds produced in this work, these were the only welds thatpassed 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.
A welding procedure was developed for producing fully penetrating, square-edge butt welds in 3.2mm thickness 2024 aluminium 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 AWSD17.1:2001:
- A focus position on or 1mm below the material surface helps achieve full penetration welds 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 condensation-free shielding gas delivery system, produces less weld metal porosity compared with industrial 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, but the elapsed time 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 between two 0.45mm diameter spots helps eliminate coarse porosity in 3.2mm thick 2024 aluminium, but has less of an effect on pores smaller than 0.4-0.5mm diameter.
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