Generation of aluminium–steel joints with laser-induced reactive wetting

A new mean of assembling steel to aluminium was developed, following previous work by German workers [1]. In this new method, a laser-induced aluminium melt pool spreads and wets a solid steel, to generate, after solidification a sound and resistant interface layer.Joint properties were investigated, in terms of surface aspects, interface microstructures and mechanical resistances under tensile testing, for non-galvanized and galvanized DC04 steels. Thermal and diffusional finite element (FE) simulations were also carried out to calculate temperature history at interfaces, and reaction layer thickness.The 2–20 μm thick reaction layers formed all along the interface were found to be mostly composed of Fe2Al5 intermetallic compound with a high hardness (1200 HV) and rather low ductility (presence of solidification cracks). The presence of a 10 μm thick Zn layer on the steel was shown to have a beneficial influence on the wetting characteristics of the joint, despite the formation of occluded pores in the melt pool due to Zn vaporisation.FE thermal modelling evidenced 760–1020 °C wetting temperatures at the interface between DC04 low carbon steel and 6016 aluminium sheets, with time maintains of the melt pool in the 0.2–0.5 s range, resulting in high-speed reaction kinetics. Using these temperature data, diffusion calculations were shown to provide a rather good prediction of intermetallic thicknesses.Tensile tests were considered on aluminium–steel lap joints and evidenced higher mechanical resistances (220 N/mm linear tensile strength) on galvanized steels, provided that fluxing of the steel surface was carried out prior to welding to avoid zinc vaporisation. Comparatively, non-galvanized assemblies exhibited much lower mechanical resistances (170 N/mm resulting in a 90 MPa interfacial shear strength).It was concluded that the laser-induced wetting technique is a rather effective way for generating Al-steel joints without filler material, and that it should be considered as a competitive technique versus solid assembly modes (friction stir welding …).