Effects of Brazing Time and Temperature on the Microstructure and Mechanical Properties of Aluminum Air Brazed Joints

Abstract

High‐purity aluminum foil was used to join alumina substrates directly in air at temperatures ranging from 800° to 1200°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°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.

Mechanical behavior of reactive air brazed (RAB) Crofer 22 APU-Al2O3 joints at ambient temperature

“…High-temperature heat exchangers, gas turbines and combustion engines are examples of such advanced power generation devices. There is a growing demand to generate materials appropriate for the industrial use, especially for the operation under oxidizing conditions [2,3]. Ceramics offer totally new capabilities due to their exceptional high-temperature material properties.…”

“…Therefore, in the past 20 years, a new ceramic-ceramic and ceramic-metal joining technique publicized as reactive air brazing (RAB) has been explored exceedingly for high-temperature applications [2,3,[22][23][24][25][26]. In numerous points, RAB is alike to active metal brazing process [9,13].…”

“…The principle of RAB is to use a molten oxide (commonly copper) dissolved in a noble metal solvent (commonly silver) to prewet the ceramic surfaces, in this way shaping a new surface that is more wettable by the remaining molten filler braze metal [3,29]. The wetting of ceramics is possible due to in situ formed oxides in the melted braze during the brazing process [29].The wetting of the ceramic is caused by the partial miscibility gap of the AgCuO-system [29][30][31][32].…”

Effects of Reactive Air Brazing Parameters on the Interfacial Microstructure and Shear Strength of GDC–LSM/Crofer 22 APU Joints

“…Earlier studies reported the effectiveness of RAB for joining a variety of ceramics and metal alloys using Ag-CuO fillers. [1][2][3][4][5][6][7][8][9][10] The Ag-CuO phase diagram has eutectic and monotectic points at 932 and 964°C, respectively. 2) Hence, RAB is generally performed for several minutes at temperatures higher than the melting temperature of the Ag-CuO filler.…”

“…However, joint strength and morphology will vary with changing brazing parameters, which mainly include brazing temperature and holding time. 5,[11][12][13][14] Different researchers have used different brazing protocols for joining various materials. 6,[15][16][17][18][19][20] For instance, LSCF/FeCrAlloy and BCFZ/FeCrAlloy joints were brazed at temperatures of 1020-1100°C for 30 to 180 minutes by Chen et al 6) BSCF/AISI 314 joints were successfully brazed for 20 minutes at a brazing temperature of 955°C.…”