Laser beam aluminium-steel joints - Mechanical and dynamical properties and detailed microstructural analysis of intermetallic FeAl-phases

Abstract

Mixed-materials are important in the automotive industry due to their weight saving potential and good ratio between costs and mass production. Aluminium and steel are difficult to join thermally due to the formation of intermetallic phases and differences in thermal expansion coefficients. Therefore the localized heat input of a laser beam is favourable. We present a laser beam weld-brazing technique for joining aluminium sheets with zinc-coated automotive steel sheets. The process is divided into welding of the aluminium sheet and flux-less brazing of the steel sheet. It can be improved by using a Nd:YAG dual-spot laser beam to promote the wetting behaviour of the braze. Mechanical as well as dynamical tests and micro hardness distributions of the mixed-materials joints are presented. Due to the differences in electrochemical potentials of iron, aluminium and zinc the corrosion behaviour of the joints within salt spray tests was investigated. The intermetallic FeAl-phases forming in the contact area of the brazed seam to the steel sheet are analyzed by means of scanning electron microscope (SEM), focused ion-beam (FIB) preparation and transmission electron microscope (TEM) observations. Since their high hardness and brittleness can have a strong effect on the mechanical behaviour and reliability of aluminium-steel joints the micro hardness of the intermetallic phases was determined as well.Mixed-materials are important in the automotive industry due to their weight saving potential and good ratio between costs and mass production. Aluminium and steel are difficult to join thermally due to the formation of intermetallic phases and differences in thermal expansion coefficients. Therefore the localized heat input of a laser beam is favourable. We present a laser beam weld-brazing technique for joining aluminium sheets with zinc-coated automotive steel sheets. The process is divided into welding of the aluminium sheet and flux-less brazing of the steel sheet. It can be improved by using a Nd:YAG dual-spot laser beam to promote the wetting behaviour of the braze. Mechanical as well as dynamical tests and micro hardness distributions of the mixed-materials joints are presented. Due to the differences in electrochemical potentials of iron, aluminium and zinc the corrosion behaviour of the joints within salt spray tests was investigated. The intermetallic FeAl-phases forming in the contact area of ...