Braze fillets

Produce much smoother brazed fillets, which is of paramount importance in aero jet engines for aerodynamic efficiency... 

The process depends on a liquid metal flowing over surfaces to form a fillet between components and into the gap between the components, and then solidifying to form a permanent bond. Thus it is essential that the braze experiences high temperature capillary attraction. Without such attraction, solid braze material placed between components will flow out of the gap, sweat , when it melts. Any residue of non-wetting liquid that remains within the gap will not conform to the microscopic features of the component surfaces but form an array of voids, as illustrated schematically in Figure 10.1, that is mechanically deleterious and should be avoided if at all possible. The size of such voids can be decreased if an external pressure is used to confine a non-wetting liquid braze into a gap but cannot eliminate them because the pressure needed to shrink voids increases as they become smaller. 

The condition of the brazed-on wear pads after irradiations in NRX and Halden has been excellent. Pig. 16. The braze metal fillet has a grey appearance and undergoes a little more corrosion than the can, but the actual penetration Is very small. The H concentration in the wear pad is up to 50 greater than in the can, presumably because of the lower temperature. Specimens having brazed wear pads attached have been irradiated In the Dounreay Fast Reactor (DFR) to total neutron doses of 2.5x10 without any noticeable effects on micro-structure. Mechanical tests on similar samples are being carried out. 

Typical Joint designs using brazing lap and scarf in thin Joints with large contact areas or a combination of lap and fillet. Fillets can help to distribute stresses at the Joint. Butt Joints are possible but can cause stress concentrators in bending. 

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