Methods and solutions for joining plates made from different metals using voltaic arc welding

摘要

For some years now, the light engineering sector has been in search of welding and/or joining technologies allowing high-speed processing at low temperatures. This way, damage to galvanized coating, for example in the case of steels, could be avoided or significantly reduced. Damage to the galvanized coating represents an outwardly visible phenomenon that can have a negative impact on the component's corrosion resistance. On the other hand, from the metallurgical viewpoint, controlling heat input represents an important way to solve the problems associated with heat joining of metals with incomplete solid-state solubility. The formation of fragile intermetallic phases during cooling, for example, occurs with aluminium/steel or aluminium/titanium. The process and the correct choice of filler materials are key points for the success of the joining method. In the case of the combination of aluminium and steel, zinc represents a suitable filler material, for example for brazing. The melting temperature is around 420 deg C and the wettability, with regard to galvanized steel, is favoured by the zinc layer, while on the aluminium part, it is possible to avoid the use of flux, thanks to the activating action of the voltaic arc. However, the low melting and vaporization temperatures of zinc make even 'short arc' processing rather difficult. Only new developments in electrical sources for welding, and in particular new processes, offer the possibility of reducing the heat input by modifying the 'short arc' process. The difficulties described are also valid for the combination of titanium and aluminium, and are associated with the formation of phases such as Ti_xAl_y,. Again in this case, this paper will present a strategy for joining sheets less than 2 mm thick. In the past years, an increasing number of strategies to join zinc-coated steels can be observed. Brazing represents a profitable way to join zinc-coated steels at low temperatures, reducing or avoiding zinc evaporation. In order to completely avoid damaging zinc coating, the set of ZnAl-fillers was already investigated for laser brazing of zinc-coated steels as well as for joining steel with aluminium. Furthermore, thanks to new developments in arc wire technology, ZnAl-electrodes can be used for MIG-brazing as well. In this case, the conventional short arc, which normally does not allow brazing with zinc wires as eruption-like evaporation occurs, is modified and controlled. The drop formation on the wire tip, the metal transfer, and the heat input can be controlled and modified very precisely, so that processing of filler material can be performed easily for thin sheets (till 0.8-1.5 mm) and at low-processing speed, below 0.3-0.4 m/min. The control of the heat input occurs due to abruptly reducing the current after short arc. The disadvantages connected with a lower heat input in the base material are a restricted wetting behaviour of the zinc pool on the base material and the low-brazing speed due to a very high cooling rate. The focus of the presented investigation is the development of strategies, in order to enhance process conditions for brazing sheets of zinc-coated steels and aluminium.