E. Guimaraens, H. Laukant, U. Glatzel, University of Bayreuth, Bayreuth, Germany
Mixed-materials are important in the automotive industry due to their weight saving potential, weight-optimized construction and good ratio between costs and mass production. Aluminum and steel are difficult to join thermally due to the formation of intermetallic phases and the difference in thermal expansion coefficients of the elements Al and Fe. Therefore the localized heat input and high cooling rates of a laser beam joining process is favorable. We present a laser beam weld-brazing technique for joining aluminum sheets with zinc-coated automotive steel sheets. The joining process is a combination of welding of the aluminum sheet and flux-less brazing of the zinc-coated steel sheet. It can be improved by using a Nd:YAG dual-spot laser beam to promote the wetting behavior of the in-situ forming braze of filler material and molten aluminum sheet. The added filler materials were Zn or Al based. Investigations were carried on to improve commercial fillers by pre-alloying them with elements such as Cu and Ag. The weld-brazing processes were done with different heat inputs and with different energy ratios between the main and the second laser beam. Mechanical as well as dynamical tests and micro hardness distributions of the mixed-materials joints are presented. The microstructure and type of 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. The high hardness and brittleness of the intermetallic phases can strongly affect the mechanical behavior and reliability of the aluminum-steel mixed material joint. Therefore the microhardness of the intermetallic phases was determined. Due to the differences in electrochemical potentials of iron, aluminum and zinc the corrosion behavior of the joints within salt spray tests was investigated as well.
Summary: We present a laser beam weld-brazing technique for joining aluminum sheets with zinc-coated automotive steel sheets. Mechanical tests, corrosion behavior and microhardness distributions are presented. The intermetallic FeAl-phases formed in the contact area of the brazed seam to the steel are analyzed by means of SEM, FIB preparation and TEM.
