The impact of zinc-coating on laser hybrid welding of steel

Abstract

Hybrid Nd:YAG/MAG welding of ultra high strength steel is studied for 1.5 mm thick sheets in I-, T-and overlap joint type configuration. Untreated and zinc-coated steels were compared, causing distinct differences in terms of process stability and welding result.For zinc coating it can be distinguished between domains of trapped zinc layers like in overlap and T-joints and free surface zinc coatings. While the former situation is critical for violent evaporation and in turn spatter in the absence of a gap, in the latter case a zinc domain outside the weld pool area evaporates due to its low boiling point, thus not disturbing the laser welding process. However, during hybrid welding arc instabilities and short circuiting were observed when changing to zinc coated steel, even for butt joints. One hypothesis is an impact of the zinc vapour atoms on the plasma, with an ionisation energy 20% higher than iron, but 40% lower than Ar. The influence was calculated by Saha’s equation.The process window is much narrower than for uncoated steel. Moreover, an enhanced generation of spatter and fume can be observed. A theory for the impact of zinc-coating on hybrid welding is postulated.Hybrid Nd:YAG/MAG welding of ultra high strength steel is studied for 1.5 mm thick sheets in I-, T-and overlap joint type configuration. Untreated and zinc-coated steels were compared, causing distinct differences in terms of process stability and welding result.For zinc coating it can be distinguished between domains of trapped zinc layers like in overlap and T-joints and free surface zinc coatings. While the former situation is critical for violent evaporation and in turn spatter in the absence of a gap, in the latter case a zinc domain outside the weld pool area evaporates due to its low boiling point, thus not disturbing the laser welding process. However, during hybrid welding arc instabilities and short circuiting were observed when changing to zinc coated steel, even for butt joints. One hypothesis is an impact of the zinc vapour atoms on the plasma, with an ionisation energy 20% higher than iron, but 40% lower than Ar. The influence was calculated by Saha’s equation.The process window is much narr...