Twin spots bridge wider gaps
Paula M. Noaker
ORLANDO, FL--According to one estimate, the annual demand for tailor-welded blanks will reach 40 to 60 million by the year 2000, and that figure is only for the North American automobile industry. Demand is growing because automotive engineers can vary specifications related to thickness, coating, strength, and more in different areas of the tailored blanks, which are formed by joining precut flat metal sheets. One problem is the high cost of scrap for many tailored-blank welding lines. While typical scrap rates are only 3%-4%, costs mount with the high production volumes factored into the equation.
At ICALEO `98, sponsored by the Laser Institute of America (Orlando, FL), Rey Hsu and colleagues at the Fraunhofer Resource Center (Ann Arbor, MI) proposed a simple solution based on carbon dioxide (CO2) laser welding with twin-spot optics. Such an optics configuration splits the laser beam with a mirror that has two surfaces inclined at 0.05° to each other. The split beams are then focused through a parabolic mirror with a 200-mm focal length to deliver twin spots to the workpiece. The goal is to expand the allowable gap tolerance between panels to be joined (see figure on p. 33).
In tests on mild 1008 steel sheets, the researchers prepared blank edges with a laser under high-pressure nitrogen assist to assure an edge roughness of less than 10 µm. They then cut out gaps of specific widths in the test blanks. Welding was done with an off-axis shielding-gas nozzle inclined 45° from the CO2 laser beam and intersecting it at a point 2 mm above the workpiece.
In one example, a laser with a twin-spot optic bridged a 0.45-mm-wide gap when welding at 4.75 m/min and 8 kW power on plates with butt-joint combinations of 3.0-1.9 mm and 3.0-1.2 mm. With the center of the twin spot offset 0.15 mm from the abutted weld line on the thicker blank (3.0 mm), the researchers extended the maximum gap tolerated for a good weld by some 0.2 mm compared to welding tests with the twin-spot center aligned on the abutted line.
Adjustments in both the incidence angle and the twin-spot line angle also impacted the maximum gap width that could be bridged. For in stance, changing the incidence angle from 0° to 45° increased the maximum width of the gap that can be bridged by 0.1 mm for butt-joint com binations of 1.2- 0.6 mm and 1.9- 1.2 mm. Welding speed also in creased 1 m/min. When researchers changed the twin-spot line angle from 0° to 60°, the speed increased by 0.5 m/min, although position accuracy for tolerating a variation in alignment of laser beam and weld seam decreased.
According to Hsu, twin-spot technology produced with beam-splitting optics isn`t perfect. The configuration doesn`t allow varying the distance of the two spots on the workpiece. In addition, the intensity distribution of both spots is always equal, and the focal position is not adjustable for both spots, which means it is not possible to adapt the foci positions to the respective workpieces. The advantage comes with the potential to expand the gap tolerance in tailored-blank welding with a system that costs less than a two-laser scheme.