VOLATILIZATION PHENOMENA IN HIGH-TEMPERATURE BRAZING FILLER ALLOYS

Experimental brazing filler alloys were developed containing temperature depressants that were volatilized during the brazing of stainless steel leaving joints of highremelt temperature. The raechanisms by which remelt temperature is increased were studied for the range of alloys investigated which contained nickel, chromium, germanium, iron, lithium, and phosphorus. The main mechanisms are dissolution of the base metal in the filler metal and diffusion of constituents of the filler metal into the base metal and volatilization of the filler metal constituents. It was shown theoretically and experimentally that, in order for remelt temperature to increase, there must be solid solubility of the diffusing or volatile element in the filler alloy. Although compositional changes may result from either mechanism, a remelt temperature change does not necessarily occur. The degree to which a compositional change affects remelt temperature is proportional to the solid solubility. In this program, vacuum pumping was the most efficient vapor-removal method among several discussed and investigated. Furthermore, under vacuum the surface-area-to-volume ratio of the filler alloy became less rate-controlling owing to boiling. Boiling, however, caused porosity in the joint. Of the alloys investigated, a 61% niobium--39% indium alloy exhibited the greatest remelt temperature rise owing to volatilization alone, whereasmore » a 94% niobium--6% phosphorus responded only to dissolution and diffusion to exhibit the largest rise in remelt temperature owing to this mechanism. Nichel-chromium-indium-germanium alloys exhibited a substantial rise in remelt temperature owing to the operation of both mechanisms. This alloy and the binary nickel-indium alloy can be considered practical alloys for high-temperature usage. A direct effect of metallic vapors on wetting and flow of the filler alloy was observed and subsequently discussed with respect to their influence on surface-tension relationships. It was shown that in the presence of metal vapors, discoloration of the base metal, wetting, flow, and skull formation of the filler alloy are functions of the rate and quantity of metal-vapor removal. (auth)« less