ALUMINUM AND ALUMINUM ALLOYS

CHAPTER 20

Prepared by:

J. SchusterOmni Technologies Corporation

R. McKinneyPrince & Izant

S. UrbanFusion, Incorporated

Contents

Introduction 370

Materials 370

Material Preparation 373

Joint Types 373

Performance of Joints 375

Corrosion Resistance 376

Applicable BrazingProcesses 376

Dissimilar MetalBrazing 378

Postbraze Cleaning 378

Bibliography 379

SuggestedReading List 379

Photograph © 2006 by Modine Manufacturing Company

AWS BRAZING HANDBOOK 369

INTRODUCTION

370 CHAPTER 20—ALUMINUM AND ALUMINUM ALLOYS AWS BRAZING HANDBOOK

Aluminum and aluminum alloys are brazed usingpractices similar to those used for brazing other metalsexcept that different fluxes, different brazing fillermetals, and generally lower brazing temperatures areused. The brazing of aluminum and aluminum alloysis performed in two modes—with flux and withoutflux.

Aluminum and its alloys can be brazed using moststandard brazing processes. These include the torch,dip, and furnace processes. Furnace brazing can becarried out in air or a controlled atmosphere, includ-ing vacuum. Other processes, including induction,radiant lamp, and resistance brazing may be used forspecific applications. Regardless of the method used,close control of the process parameters is requiredfor successful brazing.

MATERIALS

Aluminum brazing generally involves one alumi-num alloy component being joined to a second dif-ferent aluminum alloy component with a thirdaluminum alloy as the brazing filler metal. This pro-duces an all aluminum structure that can be heattreated and finished using standard practices such asanodizing and conversion coating. The second alloymay be the same as the first, a different alloy, or adissimilar metal, such as steel or nickel. Flux may beused as a consumable.

BASE METALSThe aluminum alloys that can be brazed are listed

in Table 20.1. These include a number of nonheat-

treatable and heat-treatable wrought alloys and cast-ing alloys. The nonheat-treatable alloys include thehigh-purity aluminums and those with low additionsof alloying elements. The mechanical properties ofthese alloys can be increased by cold working, butsoftening by recrystallization will occur due to theheat of brazing. Aluminum Alloy 3003, the mostcommonly brazed alloy, is included in this group.

The heat-treatable alloys, which generally containmagnesium and silicon, are classified as the magne-sium-silicon types and make up the 6000 Series ofaluminum alloys.1 These alloys begin to melt atlower temperatures than those in the 3000 Seriesbecause of their higher total alloy content. Conse-quently, they are generally brazed at lower tempera-tures than the nonheat-treatable alloys.

The heat-treatable alloys, including the commonlyused 6061 alloy, are thermally treated during post-braze fabrication or during the brazing cycle toobtain high mechanical properties. The cast alumi-num alloys, which are most easily brazed, are gener-ally low in silicon and magnesium content, whichmay limit their brazeability.

Not all aluminum alloys can be brazed. The high-strength, high-magnesium 5000 Series wrought alu-minum alloys such as 5052 and certain casting alloyscontain high amounts of alloying ingredients. Thesealloying ingredients often prevent the brazing fillermetal from adequately wetting. These alloys alsomelt at temperatures below those of commerciallyavailable brazing filler metals.2

1. For further information on the aluminum series designationsystem, see the Aluminum Association, www.aluminum.org.2. Refer to Chapter 3 for a listing of brazing filler metals used inthe brazing of aluminum.

ALUMINUM AND ALUMINUM ALLOYS

CHAPTER 20