BRAZINGFILLER METALS

Reviewers

R. L. PeasleeWall Colmonoy Corporation

D. FortunaSultzer Metco, Incorporated

D. A. FinkThe Lincoln Electric Company

Contents

Introduction 68

Melting of BrazingFiller Metals 68

Brazing FillerMetal–Base MetalInteraction 74

Brazing FillerMetal Selection 74

Brazing FillerMetal Classifications 81

Bibliography 98

Suggested Reading List 99

Photograph courtesy of Prince & Izant Company

AWS BRAZING HANDBOOK 67

CHAPTER 3

INTRODUCTION

68 CHAPTER 3—BRAZING FILLER METALS AWS BRAZING HANDBOOK

Brazing filler metals are metals that are used toproduce a braze. These metals have a liquidus that isabove 840°F (450°C) but below the solidus of thebase metals. With respect to brazing, the term soli-dus denotes the highest temperature under equilib-rium conditions at which the metal is completelysolid, that is, the temperature above which meltingstarts. The term liquidus refers to the lowest temper-ature under equilibrium conditions at which a metalis completely liquid, that is, the temperature belowwhich freezing starts.

It should be noted that the term brazing fillermetal has replaced the terms formerly used to referto this material, including hard solder, silver solder,gold solder, braze filler metal, and brazing alloy.

For satisfactory use as a brazing filler metal,a metal or alloy should possess the followingcharacteristics:

1. The ability to spread and adhere to the basematerials on which it is used. This is referredto as wetting;

2. Suitable melting point or melting range andfluidity to permit its distribution by capillaryaction into properly prepared joints;

3. A composition of sufficient homogeneity andstability to minimize the separation of constit-uents by liquation under the brazing condi-tions to be encountered;

4. The ability to form brazed joints possessingsuitable mechanical and physical propertiesfor the intended service application; and

5. Depending on requirements and specifications,the ability to produce or avoid certain inter-actions between the base metal and brazingfiller metal. Brittle intermetallic compoundsor excessive erosion may be undesirable,

while a higher joint remelt temperature—thetemperature to which a completed brazedjoint must be raised to separate the joint—might be an attribute (see Chapter 17, “Diffu-sion Brazing”).

MELTING OF BRAZING FILLER METALS

Pure metals melt at a single temperature and aregenerally very fluid. For example, pure silver melts at1761°F (961°C), and pure copper melts at 1981°F(1083°C). Combinations of copper and silver havevery different melting characteristics, depending onthe ratio of the two metals.

Figure 3.1 presents the phase diagram for the silver-copper binary system. The solidus temperature line,ADCEB, represents the start of melting for all alloycombinations of silver and copper in the system. Theliquidus temperature line, ACB, represents the tem-peratures above which each of these alloys in thesystem is completely liquid. At Point C, the liquidusand solidus temperature lines meet, indicating that aparticular alloy melts at a constant temperatureinstead of melting over a range of temperatures. Thispoint is known as the eutectic point. The composi-tion of the alloy at this point—in this case, 72% sil-ver and 28% copper—is known as the eutecticcomposition. This composition is essentially as fluidas pure metal.

For temperatures between the solidus and liquidustemperatures, some constituents of noneutectic alloycompositions are in the liquid phase while others arein the solid phase. In general, alloys with melting

BRAZINGFILLER METALS

CHAPTER 3