Joining of Aluminum and Steel using AlSi12 Brazing Filler in a Protective Atmosphere Furnace: Microstructure and Mechanical Properties

Abstract

Brazing of pure aluminum (Al) to steel using an Al-12 wt. %Si (AlSi12) filler metal and an Al brazing flux was performed in furnace filled with protective atmosphere. Microstructure characterizations of the full/ fractured joints, tensile shear strength and micro-hardness tests were performed on the samples with holding time from 5-30 min at brazing temperature of 600 °C and additional thermal exposure of 30 min at temperature of 480 °C. It is found that the joint seam for all samples features roughly into four layers, among them, the layer adjacent to steel is an IMC layer and dominantly distributed with η phase. The tensile shear strength of joints is inversely proportional to the thickness of η phase layer and particularly governed by a specific zone which is located in the η phase layer, directly adjacent to the interface between η phase layer and steel and scattered with a lot of visible pores and cracks. Micro-hardness tests show the hardness of the η phase layer remains the highest for each holding time and increases with the increase of holding time. The higher hardness leads to the limited plasticity of the η phase and more fragile of this layer. Furthermore, great differences of hardness exist between the η phase layer and steel may also generate great stresses that induce the crack initiation in the specific zone and finally result in the failure of brazed joints.