Abstract: 3 mm thickness cold rolled high purity aluminum plate and as-annealed iron plate were selected as the welding base materials, and welding parameters of 300/400/500/600 r/min-100 mm/min and 300 r/min-80/60 mm/min were respectively used to conduct friction stir welding butt welding experiments of aluminum/iron dissimilar metal. The mechanical properties of the joints were tested by microhardness test and tensile test, and the microstructure of the joints was characterized by scanning electron microscope(SEM), energy dispersive spectroscopy(EDS), electron backscattering diffraction (EBSD) and other means. The macroscopic observation of the weld shows that there are holes on the front and back of the weld and burrs on the front of the weld. With the increase of the rotation speed/welding speed ratio, the holes disappear and burrs increase. Tensile test results show that there are three failure modes: defect fracture at the hole location at 300 r/min-100 mm/min, Al/Fe interface fracture at 300 r/min-80/60 mm/min and Al matrix fracture at 400/500/600 r/min-100 mm/min. In this experiment, the maximum welding efficiency of the three modes is 40.1%, 41.0% and 60.4%, respectively, among which the welding efficiency of Al matrix fracture mode is the highest. The hole in the weld zone reduces the joint strength and leads to defect fracture. At the low rotation speed of 300 r/min, the interface fails to achieve effective metallurgical bonding, and the interface strength is lower than the Al matrix strength, leading to the fracture of the Al/Fe interface. The hardness test results show that the hardness of aluminum side decreases first and then increases, and the lowest point of hardness appears in the heat-affected zone 6-7 mm away from the Al-Fe interface, corresponding to the fracture position of Al matrix. With the increase of rotation speed, the interface strength is higher than Al matrix, leading to the fracture of Al matrix.

Key words: aluminum/iron dissimilar metals, friction stir welding, failure mode, metallurgical bonding, mechanical properties