REACTIVE METALS: TITANIUM, ZIRCONIUM, AND BERYLLIUM

Reviewers

Y. FlomNASA Goddard Space Flight Center

D. D. KautzLos Alamos National Laboratory

Contents

Introduction 530

ApplicableBrazing Processes 530

Atmospheresand Fluxes 532

Titanium andTitanium Alloys 533

Zirconium andZirconium Alloys 541

Beryllium 543

Safe Practices 545

Bibliography 549

SuggestedReading List 550

CHAPTER 30

Photo courtesy of Linvatec/ConMed. Brazed by Scarrott Metallurgical Company

AWS BRAZING HANDBOOK 529

INTRODUCTION

530 CHAPTER 30—REACTIVE METALS: TITANIUM, ZIRCONIUM, AND BERYLLIUM AWS BRAZING HANDBOOK

Titanium, zirconium, beryllium and their alloys arethree of the several metals that react readily with oxy-gen to form stable oxides. Beryllium, titanium, andzirconium have similar brazeability. That is, becausethey share two common characteristics—(1) rapidreaction with oxygen to form very stable oxides and(2) high solubility for oxygen, nitrogen, and hydrogenat elevated temperatures—these three elements dis-solve interstitially in the metals. Small amounts of dis-solved oxygen and nitrogen significantly increase thehardness of the metals. Dissolved hydrogen reducestoughness and increases notch sensitivity. Therefore,these metals must be brazed in a high-purity inert gasor in high vacuum to avoid embrittlement.

Titanium and zirconium also react with carbon atelevated temperatures to form carbides. Carbon issometimes added intentionally as an alloying elementto increase strength and hardness. However, excesscarbides in these metals lower the ductility. Themetals should be free of oil, grease, and other hydro-carbons before brazing to prevent carburization.Graphite should not be used for fixturing or for damsto control the flow of molten brazing filler metal.

Beryllium differs from titanium and zirconium inthat it has low ductility at room temperature. Beryl-lium mill products are typically fabricated withpowder metallurgy techniques using several consolida-tion methods. Wrought products are produced fromeither cast or powder metallurgy billets. Cold-workedmaterial may have good ductility in only one directionand low ductility perpendicular to that direction(anisotropy). The physical properties of beryllium canvary greatly, depending on the manner of processing.

An adherent refractory oxide film forms rapidlyon exposed beryllium, as with aluminum and mag-nesium. This oxide film inhibits wetting, flow, andfusion during brazing. Therefore, beryllium assem-blies must be adequately cleaned of surface oxidesand protected from reoxidation prior to brazing. Thebrazing process and procedures must prevent oxida-tion during the operation by incorporating the appro-priate shielding medium and techniques, such as theuse of inert gas and vacuum or highly reactive fluxes.

Titanium, zirconium, and beryllium also reactwith nitrogen to form a nitride film that presentsconcerns when wetting with brazing filler metals.The filler metal selection for the brazing of each ofthe three metals is critical to prevent the formation ofundesirable intermetallic compounds.

With respect to applications, commercial titaniumalloys are used in aircraft and corrosion applications.Titanium is often used in heat exchangers in whichboth lightweight and high strength are required. Zir-conium and beryllium have been used primarily innuclear applications. Wrought beryllium is some-times used for lightweight components in aircraftand aerospace applications.

APPLICABLE BRAZING PROCESSES

The high affinity of titanium, zirconium, andberyllium alloys to oxygen and the detrimentaleffects of nitrogen and hydrogen contamination

REACTIVE METALS: TITANIUM, ZIRCONIUM, AND BERYLLIUM

CHAPTER 30