Tailored blank welding

The principle of using laser welded tailored blanks in automotive manufacture started with the production of a two piece floor pan for Audi. Since that time, the applications for tailored blanks have grown significantly as it has been gradually realised that tailored blanking is a cost effective method of manufacturing whilst achieving weight savings. The main advantages stem from the ability to join materials of dissimilar thicknesses and grades (thus allowing weight/strength to be placed where required), the improved utilisation of material from steel strip and the high degree of automation possible.

These advantages have enabled a reduction in the number of pressing operations through the elimination of reinforcements, for example, which have more than compensated for the cost of manufacture of the tailored blanks. The types of application for tailored blanks are expanding and include rails, panel rockers, panel skirts, door inners, and body side outers.

In the manufacture of tailored blanks, the two main points of debate centre on the preparation of the edges for welding and the choice of laser type. For the edge preparation, there is a desire to use standard blanked edges but this normally requires special clamping or welding systems to ensure that the blanks can be welded consistently. The clamping systems developed include using side pressure on the fixtures or using a roller system to deform the sheet adjacent to the weld to bring the sheet edges into intimate contact (<0.1mm gap).

The welding systems employed where the gaps between the sheets are >0.1mm include beam weaving or twin spot welding, but this generally reduces the maximum welding speeds that can be achieved. The use of precision re-shearing of the edge prior to welding within a blank welding system is claimed to improve weld consistency as the cross-sectional area of the joint is guaranteed and the welding speed can be maximised.

For the choice of laser type, the established technique is the use of CO2 lasers and automated production systems normally have a 5-6kW laser. With the development of Nd:YAG lasers with workpiece powers of >3kW, a number of production systems have been installed for tailored blank manufacture, with claimed advantages of increased versatility due to the fibre optic beam delivery, higher welding speeds (due to the improved coupling of the laser wavelength), improved tolerances to gaps and negation of the need for gas shielding. In a similar manner to the edge preparation, the economics of manufacture is complex and will have to be resolved on a case by case basis.

Low carbon and high strength steel sheets in thicknesses between 0.7-2mm are used with few reported problems, provided that the welds are located in positions where the weld is not subjected to much movement across the weld line during forming. For chassis and sub-frame components, where thicknesses of steel sheet are generally between 2-4mm and yield strengths are up to 400MPa, there is a similar desire to implement tailored blank technology, but the selection of steel type is more critical to attain the required formability.

There is a growing interest in the exploitation of aluminium alloys for tailored blank application, but the laser welding is more critical as the weld line is a zone of weakness. Developments have focused on the application of wire feed and twin spot welding to improve weld quality and consistency.