Active Brazing Alloys
ABOUT ACTIVE BRAZING ALLOYS
Structural ceramics are among the most stable compounds known. As a result of their chemical stability, they are difficult for liquid metals to wet and react with. Because wetting is necessary for the production of useable brazed joints, an essential consideration in ceramic brazing is the need to promote wetting of the ceramic surfaces by the brazing filler metals.
The wetting of ceramic surfaces can be obtained by two general methods: (1) applying wettable coatings prior to brazing or (2) alloying brazing filler metals with elements that activate wetting. The active method is described below:
Brazing filler metals specially alloyed to promote wetting on ceramics are often referred to as active brazing filler metals. Early experimental work on reactions between liquid metals and oxides showed that when a liquid metal contained an element that forms a more stable oxide than the solid oxide on which the liquid metal is held, the wetting or spreading of the liquid occurred. Titanium and zirconium were shown to be particularly effective at wetting and reacting with most oxides. Moreover, since the 1950s, we have known that small additions of Ti to other metals promote their wetting of oxides. At present, the ability and effectiveness of Ti and Zr additions to promote the wetting of Al2O3 and other ceramics like SiC, Si3N4, and sialons are well documented.
APA - Active Precious Alloys
ANA - Active Nickel Alloys
ACTIVE BRAZING ALLOYS RECOMMENDED ATMOSPHERES & TEMPERATURES
ABA’s should be brazed in vacuum or argon. If brazed in argon, the vapor pressure of elements such as silver should be taken into consideration when programing the peak temperature. ABA’s are best suited for furnaces. Induction can cause uneven heating which leads to the cracking of ceramic components. Rapid cool down rates should also be avoided when ceramic is brazed to metal. The fast contraction of metal can put stresses on the ceramic which lead to cracking or residual stress on the braze joint at ambient temperatures.
The soak time at temperature is often extended for ABA’s in order to allow the active agent to react with the surface of the ceramic. ABA’s cannot be brazed in atmosphere with flux or under nitrogen.