Effects of brazing time and temperature on the microstructure and mechanical properties of aluminum air brazed joints

High-purity aluminum foil was used to join alumina substrates directly in air at temperatures ranging from 800 degrees to 1200 degrees C and soak times of 1-100 h. It was found that the bend strengths of the resulting Al2O3/Al/Al2O3 joints generally increase with increasing brazing temperature and time, with a maximum bend strength of 135 MPa on average achieved in samples joined at 1200 degrees C for 100 h. Additionally it was determined that measurable ductility is retained in the joint even after exposure under extended high-temperature conditions. During joining, an Al2O3 scale forms along the interface between the aluminum and adjacent substrates. An increase in brazing temperature and/or time leads to intergrowth and sintering between this thermally grown oxide layer and the substrate surface, which appears to be the primary source of improved joint strength. Fracture analysis indicates that the Al2O3/Al/Al2O3 joints generally fail via one of three mechanisms, (1) by de-bonding along the foil/substrate interface in specimens that were joined at low temperature or held at temperature for an insufficient period of time; (2) by ductile rupture in specimens that were joined at conditions that promoted sintering between the oxidized foil and adjacent alumina faying surfaces, but left behind a continuous residual aluminum layer within the joint; or (3) by mixed-mode fracture in specimens joined at high temperature and long exposure times, in which the thermally grown alumina that forms between the two substrates is interrupted by dispersed pockets of residual aluminum metal.