Titanium, Zirconium & Beryllium Brazing

Titanium, Zirconium & Beryllium alloys have similar brazeability. These alloys readily react with oxygen to form stable oxides.

Titanium alloys: Titanium alloys when described are actually metallic materials consisting a mixture of titanium with other chemical elements having an extremely high tensile strength and toughness. These are light weight with extraordinary corrosion resistance having the ability to resist extreme temperatures.

Zirconium alloys: Zirconium alloys, on the other hand, are lustrous, corrosion resistant, gray-white, strong transition metal. It resembles titanium & is obtained from zircon.

Beryllium: Beryllium is a steel gray bivalent element which is strong, light-weight yet brittle, alkaline earth metal used primarily as a hardening agent in alloys as beryllium copper.

Similarities
Titanium, Zirconium & Beryllium are similar in case of brazeability. The two common attributes they share are :

• Formation of stable oxides due to the rapid reaction with oxygen.
• High solubility at an elevated temperature for nitrogen, oxygen & hydrogen increasing the hardness of the metals & reducing toughness & increasing notch sensitivity.

Titanium Alloys
These are generally classified on the basis of the phases in the micro structures into certain groups. These are :

• Commercially Pure
  These are categorized on the basis of minimum mechanical properties & maximum interstitial impurities. Strength variations are produced whenever there is difference in impurities level.
  
  
• Alpha & Near-alpha Alloys
  Alpha alloys are not heat treated to an extent that they increase the strength level. Having a high intake of alpha phase in micro structure, these are commonly used at places requiring moderate elevated temperature strength & creep resistance. In case of near-alpha alloys, the designing is as per the outstanding creep strength & elevated temperature stability.
  
  
• Alpha-beta Alloys
  This contain a fine blend of alpha & beta phases in their micro structures. Solution treating & aging heat treatments help in strengthening the alloys which are been used in annealed condition.
  
  
• Beta Alloys
  These contain high degree of beta phase stabilizing elements where the transformation of beta to alpha phase is quiet sluggish.

Zirconium Alloys
The zirconium base metals meant for commercial purposes actually contain zirconium apart from other alloys such as small percentages of tin, columbium, iron, chromium & nickel. The alloys were mainly developed for corrosion resistance in nuclear applications & pressurized water nuclear power reactors. Zirconium easily reacts with oxygen, nitrogen & hydrogen & its alloying with other metals & alloys lead to the formation of brittle intermetallic compounds.

• Applications
  The alloys are constructively applied in creating components of varied nature in nuclear reactors which are for:
  • Thermal neutrons requiring low absorption cross-section
  • Water, sodium & potassium requiring excellent corrosion resistance
  • Mechanical properties of good quality used as a cladding material for fuel elements

Beryllium Alloys
Mainly applied for nuclear power energy purposes, beryllium is even available as massive hot pressed block, large wrought plate & sheet stock. The metal is atleast 5 times better & efficient as a conventional structural metal.

• Applications
  The varied usages of beryllium are for :
  • Windows in X-ray tubes
  • Nuclear reactor moderators & reflectors
  • Aerospace structures
  
  
• Brazing Conditions
  The metal reacts readily with carbon, oxygen & nitrogen at conventional brazing temperatures. Brazing of beryllium alloy should take place under a condition which minimizes the formation of intermetallic compounds, since, a beryllium reacts with most of the brazing filler metals' constituents. These are :
  • Rapid heating & cooling cycles
  • Low brazing temperatures
  • Minimum brazing temperature time
  • Minimum amounts of filler metals

Filler Metals
For brazing titanium, filler metals in the form of pure silver & certain silver alloys like 95Ag-5A1 & 92.5Ag-7.5Cu are preferred. These impart a vital joint strength to the metal alloy upto about 425°C. In case of zirconium, the brazing filler metal's development has been limited to those with apt corrosion resistance to high water temperature water in a nuclear reactor environment. The commonly used filler metals are 95Zr-5Be having a melting range of 970 to 990°C.

Brazing beryllium requires aluminum silicon filler metals having 7.5 or 12% of silicon which provides high joint strengths upto 150°C. Filler metals like silver & silver-based filler metals are as well used but at high temperature applications.

Flux
Titanium, zirconium & beryllium require brazing procedure with high-purity inert gas atmospheres like argon & helium while extremely low partial pressures of oxygen, nitrogen & water vapor in vacuum. Both vacuum & inert gas atmospheres help protect titanium in case of furnace & induction brazing procedures.

Brazing Processes
The three alloys undergo varied brazing processes, each having a unique specialty of its own.

• Induction Brazing
  Considered apt for brazing titanium, induction heating is used specially with filler metals & alloys readily with titanium, thus, taking full advantage of short brazing cycle & minimizes alloying in the joints.
  
  
• Furnace Brazing
  The brazing process is used for all kinds of joint designs & large assemblies in titanium, zirconium & beryllium. This particular heating is quiet slow. Lot of care is taken while choosing a filler metal since it is vital to see that it does not alloy too much with the base metal. The three metal alloys are easily brazed in a high-purity argon atmosphere either in a sealed retort or bell.
  
  
• Diffusion Brazing
  The brazing process varies for all three. It readily joins the titanium alloys since they consists of much better properties & minimizes atmospheric contamination apart from saving potential weight. The brazing helps reflect the diffusion of the brazing filler metal in the base metal.
  
  Diffusion brazing is perfect for zirconium alloys as well by making use of a thin film plating of either copper or any other metal which helps in forming a situ liquid phase. The diffusion takes place at 1038°C & the subsequent exposure at higher temperatures finally diffuses & forms a solid joint out of zirconium.
  
  
• Torch Brazing
  For brazing titanium, oxyacetyene torch brazing is applied requiring a special flux as well a skilled brazer. Use of preplaced filler metal with some fluxes is made. This particular brazing is comparatively much economical than other brazing processes.

Varied Applications
These three metals have found their usability in varied applications. Some of them are :

• Titanium is used as heat exchangers requiring light weight & high strength.
• Increased usage of titanium in aerospace hardware as jet engine vanes, hydraulic tubing etc.
• Zirconium & beryllium are used in nuclear applications.
• Wrought beryllium as well used for light weight aircraft components & aerospace applications.