Prepared by:

C. A. PaponettiExpert Brazing & Heat Treating, Incorporated

M. SappNAVAIR—Cherry Point

Contents

Introduction 2

Historical Perspective 3

Physics of Brazing 6

Factors Controllingthe Properties ofthe Brazement 9

The Five Elementsof Brazing 12

Bibliography 20

Suggested Reading List 20

CHAPTER 1

Photograph courtesy of The Gold Bulletin

BASICS OFBRAZING

AWS BRAZING HANDBOOK 1

INTRODUCTION

2 CHAPTER 1—BASICS OF BRAZING AWS BRAZING HANDBOOK

The process of brazing that we know todaybegan as an ancient art. Through our increasedunderstanding of the nature and behavior of mate-rials, this art evolved into technology and science.In a very general sense, brazing is a joining processthat relies on the melting, flow, and solidification ofa brazing filler metal to form a leak-tight seal, astrong structural bond, or both between materials.The process is unique in that this metallurgicalbond is formed by melting the brazing filler metalonly; the components being joined undergo nomelting.

Brazing is a well-established commercial processcapable of producing strong joints. It is widely usedin industry because, in large part, it is capable ofjoining most metallic and ceramic materials. It isa versatile process that can be performed usingmanual techniques as well as automated productionmodes. Brazing lends itself to the production of largeassemblies and assemblies composed of dissimilarmetals. Brazing produces a tiny, clean fillet in con-trast to the irregular bead made by welding, anadvantage when appearance is critical. One of themain advantages of brazing is usually associatedwith cost savings. High production processes adaptwell to today’s improved processes. Brazing espe-cially adapts to large production quantities as well assingle individual quantities.

The term brazing refers, in fact, to a group of pro-cesses. The American Welding Society (AWS) definesbrazing (B) as a group of joining processes that pro-duce the coalescence of materials by heating them tothe brazing temperature in the presence of a brazingfiller metal that has a liquidus temperature above840ºF (450ºC) and below the solidus temperature ofthe base materials. The brazing filler metal is distrib-uted between the closely fitted faying surfaces of the

joint by capillary action.1, 2 The term brazing temper-ature refers to the temperature to which a material isheated to enable the brazing filler metal to spreadand adhere to, or wet, the base metal and form abrazed joint.3

This definition serves to distinguish brazing fromthe other joining processes of soldering and welding.Brazing and soldering share many important fea-tures, but the term brazing is used to refer to thejoining processes performed above 840ºF (450ºC),while soldering refers to the joining processes per-formed below this temperature. Brazing differs fromwelding in that in brazing the intention is to melt thebrazing filler metal, not the base materials. In weld-ing, both the brazing filler metals and the base metalsare melted to effect the coalescence of materials.

Several factors influence the quality of the brazedjoint. To achieve a good joint using any of the braz-ing processes, the components to be joined must beproperly cleaned and protected from excessive oxi-dation by fluxing or the use of a controlled atmo-sphere. The assembly must be designed so that whenthe components are properly aligned a capillary is

1. American Welding Society (AWS) Committee on Definitions,2001, Standard Welding Terms and Definitions, Including Termsfor Adhesive Bonding, Brazing, Soldering, and Thermal Spraying,AWS A3.0:2001, Miami: American Welding Society, p. 5.2. At the time of the preparation of this chapter, the referencedstandards were valid. If a standard is cited without a date of publi-cation, it is understood that the latest edition of the documentreferred to applies. If a standard is cited with the date of publica-tion, the citation refers to that edition only, and it is understoodthat any future revisions or amendments to the code or standardare not included; however, as standards undergo frequent revision,the reader is encouraged to consult the most recent edition.3. American Welding Society (AWS) Committee on Definitions,2001, Standard Welding Terms and Definitions, Including Termsfor Adhesive Bonding, Brazing, Soldering, and Thermal Spraying,AWS A3.0:2001, Miami: American Welding Society, p. 6.

BASICS OFBRAZING

CHAPTER 1