Flux free joining of zinc coated steel as well as steel/aluminum through laser and controlled short arc technology

Abstract

The developments in the nineties regarding light weight construction in the automotive industry lead to an increasing set of so called Multi-Material-Mix. The light weight properties of aluminum, magnesium and titanium advantaged the use of these material classes together with new developed high strength steels. Due to the very different metallurgical properties of these materials new joining strategies are necessary. In the case of steel components the corrosion resistance is enhanced through zinc coating, which should remain undamaged during processing. The low evaporation point of zinc as well as the low temperature difference between melting and boiling point requires new rules when joining, as zinc evaporation can occur. In the case of welding of dissimilar materials as steel/aluminum joints intermetallic phases in the melt pool are formed, so that stress cracking can be detected already during processing.In the last years several methods in order to join steels and dissimilar metallic materials were presented. Zinc coated steels can be joined with minimized destruction of zinc coating by brazing with CuSi- or ZnAl-alloys. In order to perform joining of hybrid joints steel/aluminum the set of AlSi or ZnAl and laser processing was outlined as well. In the case of ZnAl- the low processing temperature is of advantage in order to avoid intermetallic phases. Furthermore the conservation of zinc coating on the steel side is possible.Processing of zinc filler materials was for a long time not possible with conventional arc devices. The steady development of the short arc technology offers nowadays the unique chance to join metallic materials with minimized heat input. Due to the high dynamic electronic regulation of the welding current, overheating of the melt pool while the material is transferred can be avoided. Especially for welding of very thin sheets and for joining of coated materials the new process offers several benefits. Moreover this technology allows using filler material with a low melting point, e.g. zinc alloys, as heat input can be limited and eruption of liquid metal from the melt bath due to the vaporization of zinc avoided.Although the low joining speed on the one hand and the low heat input on the other hand represents some disadvantages when joining sheet thicknesses over 1,5 mm or when joining steel/aluminum joints. In these cases the low wetting of the zinc melt on the cold base materials causes high reinforcement on the brazing and negative aspect.In this paper investigation regarding processing with ZnAl-alloys with laser assisted short arc technology will be reported. The combination of a controlled short arc technology with a leading laser beam treatment with the aim to enhance wettability and joining speed is reported. The task of the laser beam, which moves in front of the arc, is the preheating of the materials. Consequently a better wetting of the surface through the controlled arc melt pool is possible.The developments in the nineties regarding light weight construction in the automotive industry lead to an increasing set of so called Multi-Material-Mix. The light weight properties of aluminum, magnesium and titanium advantaged the use of these material classes together with new developed high strength steels. Due to the very different metallurgical properties of these materials new joining strategies are necessary. In the case of steel components the corrosion resistance is enhanced through zinc coating, which should remain undamaged during processing. The low evaporation point of zinc as well as the low temperature difference between melting and boiling point requires new rules when joining, as zinc evaporation can occur. In the case of welding of dissimilar materials as steel/aluminum joints intermetallic phases in the melt pool are formed, so that stress cracking can be detected already during processing.In the last years several methods in order to join steels and dissimilar metallic materials were...