Tech Briefs
Laser-welding aluminum alloys

Weight saving is an ongoing crusade for the motor industry, and use of alumi- num is gaining increasing popularity as a route toward achieving it. Audi showed what could be done using aluminum spaceframe structures with its low-volume A8 sedan, and now it is about to do it again with the high-volume A2.

The A2 spaceframe consists of straight and curved extruded sections with vacuum die-castings at the corners and intersections where the loads are highest. The spaceframe is then clad with body panels. Laser welding is an essential part of the joining technology, allowing more and more aluminum applications. "Laser welding can offer distinct advantages over conventional techniques, such as TIG or MIG, resistance spot welding, mechanical fasteners, and adhesive bonding," says Dr. Mohammed Naeem, Project Manager of GSI Lumonics in the UK. "Its advantages are low input, reduced distortion, high welding speeds, single-sided access, elimination of surface preparation, the potential for automation, and the inherent flexibility of laser systems."

But as almost always in engineering, things are not quite that straightforward. "The main problems associated with laser-welding aluminum alloys in general are the high surface reflectivity, high thermal conductivity, and volatilization of lower boiling point constituents," says Naeem. "These and other material-related problems have led to difficulties with weld and heat-affected zone cracking, degradation in mechanical properties, and inconsistent welding performance."

To help solve these drawbacks, GSI Lumonics has carried out what it terms "extensive research" to develop techniques that will provide consistent Nd: YAG laser welds in various aluminum alloys. "Welding trials have been carried out in a range of aluminum alloys with a Lumonics AM 356 laser fitted with 0.6 mm (0.024 in) fiber," explains Naeem. "This laser has excellent beam quality and is capable of delivering 3.5 kW (4.7 hp) average power at the workpiece. The 0.6 mm (0.024 in) fiber is terminated in an output housing fitted with focusing optics capable of producing spot sizes of 0.3-0.6 mm (0.012 - 0.024 in) diameter." To meet the needs of high-volume manufacturing, this new design incorporates distributed control electronics, allowing a level of monitoring and diagnostics that Naeem has not seen before in an industrial laser. And what he describes as "novel" cooling techniques permit the use of higher-temperature factory cooling water for most requirements, reducing running costs by 20% compared with conventional Nd: YAG laser products.

"These trials have shown that with increased average power and power density at the workpiece, it is possible to produce crack-free, low-porosity welds in 5000 (Al-Mg) and 6000 (Al-Mg-Si) series alloys. A weld bead of consistent geometry and appearance has been obtained at speeds up 13 m/min (43 ft/min) in 1.6-mm-(0.063-in)-thick 5083 (Al-4.5% Mg) series alloy sheet. In overlap joint configurations with the same material at 1.2 mm (0.048 in) thickness, full penetration of 2.4 mm (0.095 in) was achieved at speeds of up to 5.5 m/min (18 ft/min). Results showed that the 5000 series alloy welded sheets had tensile strengths approaching that of the parent material. For the 6000 series, the welds produced generally had tensile properties of 85-90% of parent material. These results are as expected in precipitation-strengthened alloys, which regain some of their original properties on post-welded heat treatment."

However, explains Naeem, in most applications, post-weld heat treatments are not desirable from a production viewpoint, and it remains to be seen whether laser-welded heat-treatable alloys in as-welded condition will have an acceptable performance in structural applications. The use of filler material can also be used to improve mechanical properties and tolerance-to-joint fit-up.

GSI Lumonics, in conjunction with the British government and a consortium of industrial business partners, is leading a new program to develop high-power Nd: YAG laser welding at TWI in the UK. The project's current emphasis is on welding at an average power of 10 kW (13 hp) at the workpiece. This high-power source was created by combining separate beams from three AM 356s via a single optical fiber through a beam combiner. The welding capability of this unique laser source is currently being investigated, not only in thin sections for which welding speed is critical, but also in the thick sections for which the minimal distortion offered by high power will be extremely beneficial.

Detailed results of the work currently being conducted are confidential to the industrial partners involved, but Naeem states that early indications show that it is possible to weld 15-mm-(0.59-in)-thick C-Mn steels at welding speeds of up to 1 m/min (3.28 ft/min).