MAGNESIUM AND MAGNESIUM ALLOYS - American .CHAPTER 27 MAGNESIUM AND MAGNESIUM ALLOYS Prepared by:
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MAGNESIUM AND MAGNESIUM ALLOYS
Prepared by:A. E. ShapiroTitanium Brazing, Incorporated
ContentsIntroduction 498Characterizationand Brazeability ofBase Metals 498Joint Design 502Brazing Filler Metals 502Fluxes 503Brazing Processes 503Precleaning andSurface Preparation 505Assembly andFixturing 506Selection of theBrazing Temperature 506Postbraze Cleaning 506Corrosion Resistance 506Inspection 506Typical Applications 507Safe Practices 508Bibliography 509SuggestedReading List 510
AWS BRAZING HANDBOOK 497
498 CHAPTER 27MAGNESIUM AND MAGNESIUM ALLOYS AWS BRAZING HANDBOOK
The brazing techniques used for the joining ofmagnesium and magnesium alloys are similar tothose used for aluminum (see Chapter 20, Alumi-num Brazing). Furnace, torch, and dip brazing pro-cesses can be employed, although the latter process isthe most widely used.
This chapter examines the physical and mechani-cal properties of magnesium, its alloys, and magne-sium matrix composites, the brazing processes usedto join them, joint design, precleaning and surfacepreparation, fixturing, brazing filler metals andfluxes, postbraze cleaning, corrosion resistance,inspection, and safe practices.1
CHARACTERIZATION AND BRAZEABILITY OF BASE METALS
Magnesium is the lightest and one of the cheapeststructural metals. Magnesium alloys are attractivefor application in various structures in the automo-tive and aerospace industries because of their lowdensity (approximately 0.063 pounds per cubic inch[lb/in.3] 1.75 grams per cubic centimeter [g/cm3]) incombination with relatively high tensile strength33 kips per square inch (ksi) to 42 ksi [228 mega-pascal (MPa) to 290 MPa]heat resistance up to840F (450C); and oxidation resistance up to 930F(500C). They also find extensive use in textile andprinting machines in which lightweight magnesium
1. This chapter was originally prepared by Lloyd Lockwood forthe Brazing Manual, 2nd edition (1963).
components are used to minimize inertial forceswhen they operate at high speed.2
The composition and physical properties of braze-able magnesium alloys are presented in Table 27.1.Because of their low solidus temperatures, othermagnesium alloys cannot be brazed with commercialbrazing filler metal AWS BMg-1; they require theapplication of other brazing filler metals of the Mg-Al-Zn system having a lower brazing temperaturerange.
The typical mechanical properties of brazeablemagnesium alloys are shown in Table 27.2. The tem-peratures involved in brazing reduce the propertiesof work-hardened (tempered) magnesium sheetalloys to the annealed temper level. Torch brazingreduces properties only in those areas heated forbrazing; furnace and dip brazing reduce the proper-ties of the entire structure. The properties of cast orextruded alloys or of annealed sheet alloys are notgreatly affected by the heat of brazing. Table 27.3illustrates the minor reduction in the strength ofannealed sheet alloys after exposure to various braz-ing temperatures.
The brazing of magnesium is a complex processdue to the highest chemical activity among allstructural metals. Complex oxide film containingmagnesium oxide and magnesium hydroxide isformed on the surface of the base metal at heating inair. This chemically stable film is reduced neither inconventional active gaseous atmospheres nor in vac-uum up to 105 torr (105 mm-Hg [103 Pa]). Addi-tionally, magnesium hydroxide is decomposed tohydrogen and water during heating at 572F to
2. Avedesian, M. M., and H. Baker, 1999, Magnesium and Mag-nesium Alloys, Materials Park, Ohio: ASM International.
MAGNESIUM AND MAGNESIUM ALLOYS