Body-in-white applications

The implementation of laser welding of sheet assemblies as a replacement for resistance spot welding is growing, where welding of the roof to side panel is one of the most common applications. This component is normally a two layer lap joint in zinc coated steel, with periodic three layer thicknesses to be welded, over lengths of 2.5-3m. One of the main challenges for laser welding of these types of joints is the presence of the zinc coating at the interface between the sheets, where the low vapourisation temperature of the zinc (906°C), can cause problems with weld consistency due to the formation of blowholes and porosity, if the sheets are tightly clamped together. Various approaches have been adopted to overcome this problem including: roller clamping systems which create a gap at the interface; and stamping of dimples in the steel pressings of consistent depth.

There are both CO2 and Nd:YAG lasers in production for this kind of application, and it is forecast that Nd:YAG laser applications will grow due to the flexibility of the fibre optic beam delivery as higher powers become available.

Laser welding is also being applied in the production of partial penetration welds, for example, in hem flanges often used in doors, bonnets, boot lids and other closures . Although the majority of work performed has been on steel sheet, there have been a number of programmes conducted on aluminium alloys. The laser welding of aluminium alloys is more difficult than for steel, due to the high reflectivity and thermal conductivity of the material, the low viscosity of liquid aluminium and the tendency for cracking and porosity in welds in certain alloys.

Extensive work over the past 10 years has demonstrated the feasibility of laser welding of aluminium alloys using CO2 and Nd:YAG lasers, where once a threshold power density is exceeded for a particular alloy, keyhole welding can be established and seam welds can be produced with similar welding speeds to steel sheets.