Advancement of brazing filler alloy: An Overview

Abstract

The brazing is a special type of joining technique for the complex parts of any engineering components, such as, heat exchangers, turbine engine parts of aircraft, spacecraft etc. In this method, joining area is significantly narrow (~1mm), which demands a specific joining method with optimum heat input to achieve a near net shape fabricated component. A suitable brazing filler alloy in this respect offers specific characteristics like adequate wetting, low thickness, narrow melting zone, avoidance of intermetallic formation and limited extent of stress generation at joint interface. The methodology is applicable for both similar and dissimilar combination of materials depending on particular requirement. Due to minimal thickness constraint, the brazing filler alloy is synthesized in the form of fine powders, paste, thin foils and controlled coating between / over the substrate to be joined. The thin foils (~50 µm thickness) are fabricated by rapid solidification technique. Paste of suitable composition is produced by mechanical alloying to obtain fine grain structure with overall chemical homogeneity. The mixed product is further wetted by chemical fluid, which is non-reactive to the components of mixture, however can provide substantial fluidity of the paste. There are several methods to produce thin foils. Once the composition is achieved by conventional melting and casting, the desired thickness is obtained by repetitive forging or rolling of the stock. Controlled coating of single / multiple metal can be produced by spattering, physical vapour deposition and chemical vapour deposition of pre-determined thickness over the substrate to be joined. The controlled thickness in all the above cases is apposite to bridge narrow crack or join components. This methodology is also lucrative considering ease in process control as the variables are limited to four only i.e. temperature, normal pressure, time and atmosphere. Thus a transition joint with satisfactory efficiency and structural homogeneity can be easily achieved.