Eva Lindh-Ulmgren, Anna de Try, Johan Löthman

Abstract

Dissimilar joining of aluminium to steel is a complex issue due to the different material properties and also the limited mutual solubility of aluminium and steel which leads to formation of brittle intermetallics in the joint interface.

Weld brazing that allows for welding of the aluminium plate and brazing to the steel plate within the same joint has earlier proved to be a potential method for dissimilar joining. A critical issue is however the delicate balance of heat input to ensure sufficient melting of the aluminium and wetting, but not melting, of the steel plate. New power sources such as CMT and ColdArc open up new possibilities for cold and controlled processes. Trials with CMT andColdArc were performed in the project and could provide sufficient process stability and sound joints.

Choice of filler material is however a critical issue. Filler materials were chosen from findings in literature and experiences from earlier projects. Joints of AA6016 and Dogal F36 using aZnAl4 wire gave the best joint strength with tensile fracture within the heat affected zone of the base material. Tensile strength reached 200MPa.

AlSi12 wires were tested but resulted in embrittlement of the braze metal. AlMg4Mn wires resulted in formation of a thick intermetallic layer (<20 μm)and low strength.

Wetting was sufficient for galvanised samples where removal of molten zinc reveals a clean and oxide free surface. However, wetting of duplex stainless steels were unsatisfactory.

The test matrix contained aluminium plates with different gauge and higher joint strengths were obtained for thin samples. Thick samples often showed lack of penetration in the weld root resulting in lowered strength.

Trials with friction stir welding wasperformed and resulted in joints with high strength (195MPa for LDXAA6016and 165MPa for Dogal F35-AA6016). Process stability was however a problem and tool design should be further optimised.

The project has shown that cold arc processes and FSW can be feasible for dissimilar metals joining although there is still work to be done to ensure sufficient joint integrity.