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Brazing - a guide to best practice W B Hanson Section 1. Introduction and Contents Introduction Definition of brazing (BS 499:1983) General principles Advantages and disadvantages of brazing Introduction This Best Practice Guide is written as an introduction to brazing in terms of the different techniques, the types of materials to be joined and the braze (or filler) materials which can be used. It aims to give a broad, but informative overview to the technology and, where possible, provide advice on material and production-route selection Definition of brazing (BS 499:1983) Brazing is: a process of joining generally applied to metals in which, during or after heating, molten filler metal is drawn into or retained in the space between closely adjacent surfaces of the parts to be joined by capillary attraction. In general the melting point of the filler metal is above 450°C, but always below the melting temperature of the parent material. General principles To achieve a sound brazed joint, the filler and parent materials should be metallurgically compatible and the design of the joint should incorporate a gap into which the molten braze filler will be drawn or distributed by capillary action. Where possible, the joint should be designed to be self-aligning, (or self-jigging), since this reduces the cost of the operation. The component should be clean and the joint parts properly fitted prior to brazing. To assist with braze flow, the interfacial parts may be roughened using grit-blasting, and to inhibit unwanted braze flow, a stop-off agent may be used. Flux may also assist with braze flow by forming an oxygen-free protective environment around the joint. Alternatively, a gaseous atmosphere or vacuum may be used (thus removing the need for a fluxing agent) since this inhibits the formation of unwanted surface oxides. Under certain conditions, a self-fluxing filler, such as copper-phosphorus may be used. Inspection and testing are important parts of the brazing procedure since defects may be present in the interface which could affect strength, thermal conductivity or corrosion resistance, for example. All of the above are discussed in detail throughout the guide so that informed decisions may be taken about the suitability of brazing for the required application Advantages of brazing Brazing is a commercially accepted process, used in a wide range of industries, due to its flexibility and the high integrity to which joints may be produced. This makes it reliable in critical and non-critical applications, and it is one of the most widely used joining methods. Brazing is a unique process, since the metallurgical bonds are formed during brazing by melting only the filler metal and not the parts being joined. Its advantages over other joining processes are: many joints can be produced simultaneously parts to be joined are not melted nearly all metals and ceramics can be joined complex geometries can be produced . . . and the disadvantages When compared with other processes, the disadvantages of brazing are: optimum strength is that of the filler metal filler metals can be expensive joint clearance and part cleanliness are critical Contents This Best Practice Guide has six more Sections: Base/parent material combinations - filler metal selection factors - guide to brazing - base metal/filler metal combinations - selection of fillers/base materials/brazing processes Materials issues/selection - brazing filler metals: aluminium, silver, copper, nickel, gold, palladium/platinum-based - active metal brazes - placement of braze filler metals - fluxes and protective atmospheres - flux removal - vacuum brazing Brazing processes - torch brazing - furnace brazing: batch, continuous, retort, hot wall and cold wall vacuum - induction brazing - dip brazing: molten metal and molten chemical (flux) bath - resistance brazing - other brazing processes: infrared, braze welding, diffusion, electron beam, exothermic, laser, microwave Brazing of aluminium alloys - brazeable aluminium alloys - brazing filler metals - brazing fluxes - joint clearances - brazing processes: dip (flux), torch, furnace (fluxless and Nocolok fluxed, inert atmosphere), vacuum (fluxless) Joint design, quality assessment, standards and specifications - butt and lap joints - joint clearance - fluxes - surface finish - base metal/filler metal interactions - joint configurations - discontinuities in brazed joints - destructive inspection - Non-Destructive Evaluation - inspection of joints: failure categories - standards and specifications for aluminium, silver, copper, nickel, gold and palladium braze filler alloys Case studies, health and safety implications, definitions - case studies: C110 copper plate to 51%Ag/49%W sheet using AG7 copper to 304L stainless steel using HTN7 300 series stainless steel tube to copper or brass with silver filler metals installation of retort furnace for copper brazing of AISI 304 stainless steel heat exchangers installation of continuous mesh belt furnace for brazing stainless steel using copper, nickel and silver filler metals naval brass to aluminium bronze with AG7 Ni-Cr-Fe with HTN5 filler using a graphite jig copper to graphite using Ni-Cr-P stainless steel to sintered silicon carbide steel to silicon nitride for automobile engine tappets - health and safety implications: torch, induction, resistance, furnace and immersion/dip brazing - definition of terms used in brazing About the IIW / Mumbai Branch / Other Branches / Coming Events / Technical Lectures Mumbai Weldnet / Trends in welding / Related Websites / IIW Forum / Feedback / Home