Hard Soldering (Brazing)

Soldering, also known as brazing, is a metallurgical joining method in which a permanent bond is achieved by placing a filler metal with a lower melting point between the materials to be joined and melting that filler metal without melting the base materials. The solidification temperature of the filler metal used in this process must be lower than the melting point of the base material but typically above 450 °C (723 K) to distinguish it from soft soldering.

Process Mechanism

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. The 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.

Heating Methods

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

Filler Metals

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

Fluxes and Atmosphere Control

In brazing fluxes are used to remove surface oxides and prevent reoxidation. However in vacuum or reducing atmospheres flux usage may not be necessary. For example in vacuum environments boron-containing filler metals can act as a deoxidizing agent.

Surface Preparation and Cleaning

Cleaning of the surfaces to be joined is critical. Contaminants such as oil dirt oxides and humidity inhibit wetting and consequently capillary flow. Surfaces may be cleaned mechanically or chemically if needed and protected prior to the process.

Joint Clearance and Mechanical Strength

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

Applications

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 where conventional joining methods such as metal-ceramic bonding are difficult.