The welding of aluminium and its alloys

Laser welding

Laser welding is being used increasingly in both the automotive and aero­space industries for the welding of a range of materials (Fig. 8.2). The laser welding of aluminium and its alloys has, however, presented problems to the welding engineer. Poor coupling of the beam with the parent metal, high thermal conductivity, high reflectivity and low boiling point alloying elements have, until relatively recently, prevented the achievement of con­sistent weld quality.

8.3 Principle of laser welding. Courtesy of TWI Ltd.

The wavelength of the laser light affects the coupling - the absorption of the beam energy by the metal being cut or welded. As the wavelength increases the coupling becomes poorer and this is a particular problem with aluminium and its alloys. The wavelength of light from a CO2 laser is 10.6 pm, that of a Nd-YAG laser 1.06 mm - the solid state laser is therefore better suited to the welding of aluminium. Development work, carried out mostly for the automotive industry on sheet metal, has also been of assis­tance in minimising these problems by improved focusing of the beam with both types of laser. One of the earliest applications for this development is the Audi A2 which has some 30 metres of laser welds in its bodywork. The main reason for the improvement in laser weldability has been the ability to achieve high-power densities, typically above 40kW/mm2, with both the Nd-YAG solid state and CO2 gas lasers. As a process, laser welding offers the advantages of a concentrated, high-energy density heat source. This power density enables the weld to be made in the keyhole mode (Fig. 8.3), improving the absorption of the laser beam due to reflections within the cavity. The deeply penetrating keyhole weld produces very narrow heat affected zones, minimising both distortion and the loss of strength in the HAZ of the work or precipitation-hardened alloys and reducing the loss of low boiling point alloying elements such as magnesium.

The low boiling point elements, however, assist in establishing a stable keyhole. The high-energy beam also enables very fast welding speeds to be achieved, speeds of 2 metres per minute with a 2kW Nd-YAG and 5-6 metres per minute with a 5kW CO2 laser in 2 mm thick sheet being easily attainable. The main welding parameter is the laser power which determines both the depth of penetration and the travel speed. Other variables are the position of the focal point, generally at the upper surface, wire diameter and feed speed and weld gap.

Defects in laser welded joints are similar to those encountered in welds made by other fusion welding processes. Porosity is caused by hydrogen from the environment, dissolved in the parent metal, contained in the oxide film or from an unstable keyhole condition. The solution to this problem is careful surface preparation including pickling and scraping, gas shielding and the use of adequate power to ensure the creation of a stable keyhole. Although most of the non-heat-treatable alloys are capable of being welded successfully, hot cracking may be encountered, particularly in those alloys that are sensitive. This can be reduced or eliminated by the addition of a suitable filler wire. The last difficulty is caused by the low viscosity of the molten weld metal. This causes the problem of ‘drop-through’, where the weld metal falls out of the joint when welds are made in the flat position. This problem can be overcome by welding in the horizontal-vertical (PC) position.

8.3.1 CO2 laser welding

As mentioned above improved focusing has enabled very concentrated beams with energy densities above 40kJ/mm2 to be produced. This has been achieved by using parabolic reflectors or transmissive systems with a focal length of around 150mm. The alloy content affects the energy required to achieve a keyhole with increasing levels of zinc or magnesium requiring less energy. This is attributed to the low vaporisation temperature of these alloy­ing elements assisting in the formation of the keyhole. One corollary of this is that higher welding speeds are possible in those alloys with the higher magnesium contents.

Helium gas shielding of both the root and face of the weld is re­commended for the higher magnesium-containing alloys such as 5083 (Al4.5Mg). Over some 3 mm in thickness a jet of helium, supplementing the shielding gas, directed at the weld pool also gives improved weld appearance. Helium-argon mixture and pure argon gas have also been used with acceptable results although with a reduced parameter tolerance box.

Wire additions may be used to increase the resistance to hot cracking in those alloys that cannot be autogenously welded such as the 6XXX and 7XXX series of alloys. Wire additions are also beneficial in coping with gaps, a 1.2 mm wire can be used to fill gaps of up to 1.2 mm. Wire diameters may be between 0.8 and 1.2mm. Feeding the wire into the leading edge of the

weld pool at an angle of around 45° will improve the bead shape on both root and cap.

The majority of welding has been carried out on butt welded sheets (Fig. 8.4). The joint between the sheets results in an increase in beam absorption and improved keyhole stability compared with bead-on-plate welds. Good fit-up is necessary for autogenous welding if undercut is to be avoided. This requires either square machined or high-quality guillotined edges. Where lower-quality edges are produced, wire additions can be used to cope with any gaps. Where extrusions are welded, a small ‘V’ on the weld face aids in penetration by reflecting laser light within the preparation.

8.3.2 Nd-YAG solid state welding

The wavelength of the light from solid state lasers is only a tenth of the wavelength of light from a gas laser. It is believed that this permits better coupling of the beam with the parent metal. The short wavelength also enables the laser light to be transmitted via fibre optics, rather than by the use of the copper mirrors that are used to manipulate the light from the CO2 laser (Fig. 8.5). This gives greatly improved flexibility, allowing the use of a robot to move and position the beam. Most of the techniques used for CO2 welding apply to the solid state laser and, as with the CO2 laser, there is a critical power density required to achieve keyhole penetration. An average laser power less than 1 kW is regarded as being the lower accept­able limit for the avoidance of lack of penetration or porosity.

Since the early 1990s the power available from the solid state lasers has increased so that a pulsed 3 kW laser is capable of welding speeds of up to

2.3 metres per minute in 1.5 mm thick 5XXX alloys. Gas shielding, similar to that for CO2, can be achieved using either argon, helium or nitrogen fed co-axially with the beam or from a simple side port. Gas shielding of the underbead will also give an improvement in the surface finish.

High-power diode lasers (HPDLs) are also being investigated and it is likely that they will become commercially available in the near future. There are several advantages with this type of laser, price and low maintenance being two. With the recent improvements in optics, HDPLs are capable of achieving power densities of 5 x 104 W/cm2 and, with wavelengths of around 800nm, are producing good results when used to weld aluminium.

8.3.3 Welding defects

CEN standards are being developed to cover the quality assurance and quality control of laser welds in a range of metals, for example EN ISO 13919-1 and EN 288 part 15. ASME IX already has requirements regard­ing procedure approval testing of laser welds. A brief summary of laser welding defects is included in Table 8.2.

8.3.4 Arc augmented laser welding

There have been a number of relatively new developments where the laser has been combined with the arc from a conventional welding power source.

The MIG, TIG and plasma-arc processes have been used, enabling higher welding speeds to be achieved, particularly in thin sheet for the automotive industry. Of these options MIG welding is the preferred fusion welding process, although the plasma/laser process is also being actively developed and is producing good results. Figure 8.6 illustrates a commercially avail­able laser/MIG welding head and Fig. 8.7 the principles of operation.

In addition to higher speeds the enhancement of the laser beam enables greater variations in fit-up to be tolerated. Penetration, it is claimed, is increased and the change in shape of the weld pool assists in allowing hydrogen to diffuse out of the joint, reducing the porosity often encoun­tered in laser welds. At present (2002) these augmented laser processes are in the early stages of development but show great promise in widening the field of applications of the process.