Hard Soldering (Brazing)

Brazing is a metallurgical joining method that achieves a permanent bond by placing a filler metal with a lower melting point between the materials to be joined and melting that metal without melting the base materials. In this process, the solidification temperature of the filler metal must be lower than the melting point of the base material but typically above 450 °C (723 K) to distinguish it from soft soldering.

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The brazing process occurs in three fundamental stages:

1. Physical Stage (Melting and Flow): The filler metal melts under heat and spreads into the gaps between the materials through capillary action. These gaps are typically on the order of micrometers. Spreading is influenced by physical parameters such as the surface tension and contact angle of the filler metal.
2. Metallurgical Stage (Diffusion and Reaction): Partial dissolution and diffusion occur between the filler metal and the base material. This interaction may lead to the formation of intermetallic phases and the initiation of microstructure development.
3. Crystallization and Cooling: As the filler metal solidifies, a bond forms in the joined region. The resulting microstructure and cooling rate determine the mechanical properties of the joint.

Brazing can be performed using various heating techniques including furnace brazing, induction, resistance, flame and infrared heating. Furnace brazing in particular provides high dimensional accuracy and repeatability and is widely applied across industries from automotive to aerospace.

Filler metals used in brazing are selected based on the type of base material and the service conditions. Common alloys are based on aluminum, copper, nickel, silver, gold and cobalt. Elements such as phosphorus, boron and silicon in alloy systems improve fluidity reduce surface tension and provide self-fluxing properties. Filler metals are classified according to ISO 17672:2016 and AWS A5.8 standards.

Flux is used in brazing to remove surface oxides and prevent reoxidation. However, in vacuum or reducing atmospheres flux may not be necessary. For example in vacuum environments boron-containing filler metals can act as oxide removers.

Cleaning of the surfaces to be joined is critical. Oil dirt oxides and moisture on the surface inhibit wetting and therefore capillary flow. Surfaces may be cleaned mechanically or chemically if needed and protected prior to the process.

The amount of clearance between the parts to be joined affects both fluidity and the mechanical strength of the joint. Narrower clearances provide better capillary action and higher strength. However excessively narrow gaps may prevent adequate penetration of flux and filler metal.

Brazing is used in numerous fields including automotive aerospace HVAC systems electronics nuclear reactors fusion energy systems radar systems and heat exchangers. It is preferred for components requiring high strength leak tightness and aesthetic joints. Additionally it offers advantages in applications such as metal-ceramic joining that are difficult with conventional methods.