Microstructural Study of Al-Ag-Cu-Si Filler Metal for Brazing High-Strength Aluminum Alloys to Stainless Steel

Abstract

The study deals with the investigation of the microstructural constituents of the brazing filler Al-Ag-Cu-Si and the microstructure of brazed aluminum/stainless steel joints. The low liquidus temperature of the Al-Ag-Cu-Si filler of 497 °C allows the joining of the stainless steel and high-strength, thus far non-brazeable aluminum alloys. Brazing was carried out at a temperature of 520 °C in a vacuum furnace. Due to the lower heat input into the liquid brazing filler, the Fe-Al intermetallic layer in the reaction zone of the brazed joints is thin, which is required for good mechanical properties of the joints. The microstructure was studied by scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) in combination with selected area electron diffraction (SAED). The chemical compositions of the microstructural constituents were analyzed by energy-dispersive X-ray spectroscopy (EDXS). The results have shown that the ternary eutectic microstructure of the brazing filler consists of the α-Al solid solution phase, the θ-Al2Cu phase and a lamelled Ag-Al constituent. During the cooling of the solid filler metal, the Ag2Al phase forms lamellar segregates of μ-Ag3Al with a lamellae thickness of a few nanometers. Thus, the third eutectic constituent is a composition of two phases. The silicon content of the filler metal forms precipitates embedded inside the eutectic cells and in small dimensions inside the cell walls. Moreover, the silicon content prefers the wetting of the stainless steel surface and the formation of the Al7Fe2Si reaction layer with a thickness of 8 µm. The microstructure of the brazing zone is modified in comparison to the solidified pure filler metal. α-Al cells dominate the hypoeutectic structure. Intermetallic phases appear inside the α-cells as well as in the cell walls. Additionally, particles of the reaction phase occur inside the cell walls near the stainless steel. At the interface to the stainless steel in the reaction layer, no cracks or microcracks were detected.