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image of An Intro to Brazing

An Intro to Brazing

Publication Date : 2018-09-05

By Harman Nagra

 

Brazing is a welding process that is used to join two pieces of base metal using melted filler which flows across the joint.

 

Brazing Procedure

When the filler metal cools, it creates a solid weld between the two pieces of metal. The process differs from welding in that it does not involve melting the metal pieces and from soldering in using higher temperatures for a similar process, but also requiring closer fitted parts than soldering. Brazing forms very strong welded joint which tends to be stronger than either of the base metal on their own, but also allows user to make the weld without causing change in shape. Overall, these welds can withstand jolting and twisting, which is beneficial in its common uses in creating pipes, rods, metals of a flat nature, and joining unusual combinations that feature linear joints. A brazed joint tends to be smooth and complete and creates an airtight and watertight bond, while also retaining the characteristic of conducting electricity in the same manner as the base metals. Additionally, joining two dissimilar metals is a great advantage of brazing and is seen to be a main reason for its economical nature. It is cost efficient and better for large production because its processes are easier to automate with less skilled labor needed. The level of heat required is less because the filler metal has a lower melting point than the base metals, therefore common gases used are acetylene and hydrogen; with temperature ranges based on various metals:

 

Brazing Temperature for Different Metals

Filler Metal Forms:

  • Tape
    • Made with specific thickness for suitable parts
  • Paste
    • Made of multiple alloy powders, ranging by weight and drying time
  • Powder
    • Uniform and homogeneous mixture
  • Preforms
    • Custom shapes made from cut tape, put in assembly from adhesive backing
  • Foil
    • Flexible sheet material, multiple can be added for greater thickness
  • Rod/Wire
    • From 0.8-9.5mm dia. Commonly used in torch/induction brazing

 

Types of Brazing Techniques:

  • Torch Brazing
    • Most common
    • Used in small production volumes or in specialized operations
    • Three main types of torch brazing
      • Manual
        • Heat applied using a gas flame placed on or near the joint
        • Used when part size makes other methods not usable
        • High cost for skilled labor is only drawback
      • Machine
        • Used commonly when a repetitive braze operation done
        • Mix of automatic and manual operations where mechanism carries out actual braze and operator places material, flux and jigging parts
        • Advantage is that is reduces high labor and skill requirement
        • Still requires flux
      • Automatic (all part preparation done by machine)
        • Only manual operation is loading/unloading of machine
        • High production rate
        • Uniform braze quality
        • Reduced operating cost
      • Cast iron welding
        • Usually a brazing process, using filler metal such as nickel.
      • Braze welding
        • Using a bronze/brass filler coated with flux; needs more heating brazing.
      • Silver brazing
        • Using a sliver alloy-based filler in brazing procedures, joining metals and compensating for the difference in expansion percentage.
      • Furnace brazing
        • Semi-automatic process, ideal for mass production due to lack of skilled labor cost. Various atmospheres prevent part from oxidation; specifically 4 types of furnaces:
          • Batch
            • Can heat each part separately, high flexibility of part type to be brazed.
          • Continuous
            • Best for steady flow parts through furnace, offering almost none manual labor needs. Better for larger scale productions.
          • Retort
            • Using a sealed lining (retort), filled with specific atmosphere and then heated externally.
          • Vacuum
            • Known as an oxide prevention method used to braze metals with balanced oxides. Main types include Sing wall retort, double wall retort and cold wall retort.
  • Induction Brazing

    High frequency induction heating for brazing is clean and rapid, giving close control of temperature and location of heat. Heat is created by a rapidly alternating current which is induced into the workpiece by an adjacent coil.
  • Resistance Brazing

    This is a process in which heat is generated from resistance to an electrical current (as for induction brazing) flowing in a circuit which includes the workpieces. The process is most applicable to relatively simple joints in metals which have high electrical conductivity.