Nickel Brazing

Nickel & nickel alloys make use of brazing for joining process which when applied for a specific application depends upon the chemical & metallurgical properties of the alloy & anticipated service conditions. Lot of care is taken while sulfur & low melting point metals attack the vulnerable part of nickel & high-nickel alloys. Similarly, care is taken when liquid metal embrittlement comes in contact with the molten brazing filler metals.

Categories of Nickel
Nickel & high-nickel alloys have been categorized in many vital parts when seen from brazing point of view. These are :

• Commercially pure nickel
• Nickel copper alloys
• Solid-solution-strengthened pure-alloys
• Precipitation-hardenable superalloys
• Oxide dispersion-strengthened alloys (ODS)

The salient features explaining the above mentioned categories are mentioned here :

• Nickel copper alloys & unalloyed nickel are basically applied in processes requiring corrosion resistance or maintenance of product purity.
  
  
• Whereas dispersion strengthened alloys & superalloys are applied in processes demanding high strength & resistance to corrosion at high temperatures.
  
  
• Special consideration for brazing is given in case of precipitation-hardenable superalloys. Well mixture of aluminum & titanium in such alloys readily oxidizes which forms the film that restrains wetting & brazing filler metal's flow.
  
  
• Brazing is conveniently promoted when strategic use of oxygen & nitrogen getters is made. Nickel plating is strictly recommended in case of complex assemblies since they strongly demand in the vacuum department a rebraze cycle & parts requiring hydrogen brazing.
  
  

Filler Metals
Nickel & high-nickel alloys are easily joined by means of brazing filler metals whose selection depends primarily on the service conditions of the finished assembly. The brazed joints can withstand any temperature involved provided one should be certain about while deciding the heat treatment required for the base metal. In case of nickel & high-nickel alloys, Bag brazing filler metals are mostly used to join them. With the help of proper designing & brazing techniques, brazed joints should be as strong as the base metal. While BAG-1, BAg-1a & BAg-3 low melting filler metals are mostly used, Bag-7 is considered useful in case of liquid metal embrittlement.

Generally, nickel base filler metals which consists of palladium or platinum additions apart from gold base, palladium base & platinum base filler metals are are considered best for brazing nickel alloys, since, they have a good wetting & flow attributes.

Flux
Generally, AWS types FB3-A,-C & -D types of fluxes are applicable for most of nickel alloys whenever making use of Bag filler metals. They are devoid of aluminum elements. Though, FB4-A, another AWS type is considered apt for aluminum containing nickel alloys. The age-hardanable nickel alloys are comparatively difficult to be brazed than the nonheat treatable alloys. The reason being the non-residual nature of elements like aluminum & titanium from refractory oxides during brazing in normal furnace atmosphere.

Certain vital procedures are needed to handle a situation which could prevent the formation of the oxide or which could flux away the oxide formed during heating. These are :

• Harmful oxides can be prevented from forming provided the parts are copper plated.
  
  
• By nickel plating on the parts, the formation of refractory oxides can be avoided.
  
  
• Usage of dry oxygen & nitrogen free atmosphere can further stop the formation of oxides as well reduce them if at all present.
  
  

Brazing Processes
Nickel & high-nickel alloy assemblies mostly make use of commercial brazing processes. Some of the most appropriate processes are :

• Torch Brazing
• Furnace Brazing
• Induction Brazing
• Resistance Brazing
• Salt Bath Brazing
• Metal Bath Dip Brazing