Investigation of fatigue damage in aluminum/stainless steel brazed joints

Abstract

The study deals with the investigation of the fatigue fracture mechanisms in aluminum/stainless steel-brazed joints. The joints are produced by induction brazing using an AlSi10 filler at 600 °C in argon atmosphere. The filler is applied in two different ways: as a conventional filler paste or as a filler cladding on the stainless steel. The brazed joints have to endure a high number of cyclic loads during application. Therefore, in addition to the monotonic mechanical properties, the fatigue behavior must be considered. It is investigated by fatigue tests at ambient and elevated temperatures on a RUMUL resonance pulsator under load controlled condition with a load ratio of R = 0.1. At ambient temperature, a fatigue endurance limit of 107 cycles is reached by joints, brazed with a filler paste, at a stress amplitude of 7 MPa. Joints, brazed with a filler cladding, reach this number of cycles at a stress amplitude of 9 MPa. With an increase of the testing temperature, the fatigue life decreases for both combinations. At elevated temperatures, the joints endure lower stress amplitudes of 5 MPa resp. 6 MPa at 107 cycles. The formation of fatigue cracks leads to a reduction in the resonance frequency. At this point, the fatigue damage of the corresponding samples is investigated by SEM. Especially the crack initiation and crack propagation with regard to the number of cycles is observed using cross sections. The investigations of cross sections of the fracture surfaces show, that the Al7Fe2Si intermetallic layer influences the fatigue behavior of the joints, brazed with a filler paste, predominantly. For joints, brazed with a filler cladding, the Al-Fe-(Cr,Si) intermetallic layers do not have a main influence on the fatigue behavior.