Three-beam laser brazing of zinc-coated steel

This work investigates the use of a trifocal laser intensity distribution to control the applied heat profile for laser brazing of hot-dip galvanized steel. Process analysis of conventional monofocal brazing processes showed that spatter and other process disruptions mainly originate from zinc dissolution and evaporation at the wetting line. In order to increase process stability and seam quality, a trifocal brazing method was developed and applied for brazing experiments. A transient three-dimensional model of the brazing processes shows a shift of the achieved temperature distribution and thus a separated evaporation of the zinc coating prior to the wetting with molten CuSi3 brass. The influence of the adjusted intensity distribution in the process area was analyzed with high-speed video imaging, thermal imaging, and metallographic analysis of the seam properties. The experimental analysis confirms that during trifocal brazing, a local zinc evaporation takes place prior to the spreading of the molten filler metal. As a result, a less turbulent processing zone and a superior seam quality are attained.