Abstract: In view of the technical difficulties of aluminum/steel welding, CO2 laser fusion welding-brazing is used to produce lap joint between 5056 aluminum alloy and ST04Z galvanized steel with filler powder. Weld appearance, microstructure and mechanical properties of welding joint are studied. The results show that high quality welding joint can be obtained by using the appropriate welding parameters with preset filler powder. The intermediate layer thickness is less 10 µm, and acicular intermetallic compound is not separated out to melted aluminum alloy. There are two rich Zn zones during fusion joint, which are located at melted edge and welding-brazing root. Si，Zn，Mn and Mg elements constitute compounds or solid solution with Al and Fe elements, which rigid intermetallic compounds are stopped to generate by restraining Al and Fe elements direct contact. Tensile specimen fracture mainly in the aluminum alloy heat affected zone or welds, and Welds fracture surfaces are all at the side of the aluminum melting area. The average mechanical resistance of 190 N/mm is obtained, about 85.2% of the base metal of 5056 alloy. By using this laser fusion welding-brazing method and filler special powder，the thickness of intermetallic compounds layer can be decreased and the fusion welding-brazing joints with excellent mechanical properties can be obtained.

Key words: laser welding;aluminum alloy and steel;fusion welding-brazing;intermetallic compounds