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 [I]. 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 mu m 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 mu m 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 degrees 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 (170N/mm resulting in a 90MPa 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...) (c) 2006 Elsevier B.V. All rights reserved.