Virtually every car manufacturer now uses laser welding in bodywork construction. Continuous seam welds not only improve stiffness, handling, road noise and crashworthiness, but also enable new joint geometries to be used, leading to material savings. The first applications involved simple butt and lap joints between steel sheets of similar thickness.
Cars manufacturer now uses laser welding in bodywork construction
Today, mass produced cars contain several tens of metres of such welded seams. Trends in recent years have included: an increasing use of lightweight materials, notably aluminium alloys in chassis members and bodywork; the replacement of electron beam welding for powertrain joints requiring penetration less than about 5 mm; the use of Nd:YAG lasers for bodywork welding in three dimensions; the development of automatic process monitoring and control systems; the use of hybrid welding systems comprising a laser beam and an electric arc; and an increase in the use of laser-welded tailored blanks, discussed below.
Two classes of product have been developed for laser welding of automobile bodies: the CO2 laser with articulated mirror delivery, and the Nd:YAG laser with fibreoptic beam delivery. The latter product has a number of important benefits over the former. When setting up, fibreoptic beam delivery is a relatively simple plug-in accessory with no mirrors to align. It can be used with standard relatively cheap welding robots, which are often already being used in a factory.
From an application point of view, welding of mild steel or zinc coated steel can be achieved without the need for shielding gas – only a cross jet of air is required to protect the optics. Compressed air is normally installed in the plant, and so no bottles of gas or high pressure gas lines need to be provided. The focused spot is quite large, at least 0.5 mm, which provides increased tolerance for weld fitup and reduces the required accuracy of the robot.
In comparison with conventional resistance spot welding, laser spot welding is a non-contact process, which eliminates the feature of electrode wear. Continuous seam welds not only improve performance, but also provide opportunities for new joint geometries to be used, which can lead to material savings as well as the use of novel materials. Recent years have seen a noticeable trend towards greater use of Nd:YAG laser welding, particularly for complex geometry joints.
Tailored blanks are made by butt welding steel sheets of differing quality, thickness or coating, to form a composite section. The first tailored blanks were made by EB welding in the 1960s. Toyota introduced the concept of laser-welded tailored blanks in 1985. In August 1985, Thyssen Stahl AG began butt welding two pieces of hot-dipped galvanized steel to make a blank wide enough for the floorpan of the Audi 80. Today, resistance mash seam welding and laser welding are the main fabrication techniques, and the expression ‘tailor-welded blanks’ is now common terminology.
Laser welding has a number of advantages over other joining techniques for sheet materials: a narrow, even and aesthetic weld is produced, which is necessary for subsequent drawing operations, and there are no restrictions on the size of the blanks. They are typically drawn to depths of 16 cm. For example, a 0.8 mm thick galvanized section for the base of a car door can be joined to a section 1.8 mm in thickness for hinge supports, and completed with uncoated steel with a large draw depth to follow the contour of the vehicle. These customized blanks are then ready for drawing or stamping, and contain the desired properties exactly where they are needed.
Laser-welded tailored blanks provide many advantages over bodywork components manufactured in the traditional way, i.e. separate stampings that are subsequently joined using, for example, resistance spot welding. Greater flexibility is offered at the design stage; the designer can bypass limitations normally imposed on part width by available coil widths.
In the various manufacturing phases, fewer parts and stamping dies are needed, scrap can typically be reduced from around 50% to less than 15%, the total number of joining phases is reduced, 100% visual inspection is possible, and reductions in finishing operations can be achieved. Since the cost of material for the bodywork is approximately 50% of the total manufacturing cost, the potential for cost savings through a reduction in material usage is high.
Butt welding enables overlap flanges to be eliminated, giving a reduction in weight of around 10 kg per car, and a reduction in the use of sealing operations. Such factors translate into higher productivity and a lower overall cost. Improvements in quality include a flush surface which is aesthetically pleasing, and increased part stiffness, which allows tighter tolerances to be used, resulting in improved fit. In terms of performance, the weight reduction decreases fuel consumption.
Improved crash performance, fatigue properties and corrosion properties can also be obtained. The full penetration weld bead is clearly visible on both sides of the sheet for inspection. The only requirement is high quality edge preparation, typically to within 0.05 mm over the length of the weld. Tailored blanks are normally welded using CO2 lasers, which are good for two-dimensional applications.
Nd:YAG lasers and fibreoptic beam delivery allow designers to specify blanks of any shape, with three-dimensional curved welds. Laser-welded aluminium blanks are now to be found in cars such as the Lamborghini Gallardo (the front wheel arch), and are finding popularity with many automobile manufacturers.