Brazing4

What is brazing and soldering: introduction

Brazing is a kind of welding utilized to join metal parts also in different materials, by means a filler metal, without the fusion of the assembled parts. Braze welding is divided in:

  • Soldering: it is used in joining applications that need preparations similar to the merging welding;
  • Capillary brazing: it makes use of preparations constituting the capillary meatus within which the filler metal or brazing alloy penetrates by capillarity.

Brazing is divided into:

Strong brazing

It is used to welding materials such as cast iron, iron, copper and similar alloys, by means of based copper and zinc filler metals with a high fusion point of about 450° C. The filler metal fusion is obtain by an oxyacetylene torch.

Soft brazing

It is used to welding materials such as iron, copper, zinc and tin, by means a filler metal with a low fusion temperature, about less than 450° C. The melting of brazing alloy is obtained with an electric welder or by a gas torch.

Brazing utilization

Brazing can be applied to a wide range of metals and alloys for jobs such as:

  • joining,
  • hardfacing,
  • rebuilding missing sections, all of which can be performed by brazing and, for joining work only, by soldering, too.

This process has many benefits, such as low brazing temperature, high fluidity and heat input, excellent capillary action, corrosion resistance and ease of use.

How brazing works

Various heating methods can be used to heat the filler material depending, in part, on the material in question: torch brazing, electric heating, vacuum or controlled-atmosphere furnace brazing.

The quality parameters for perfect brazing results are illustrated below, such as the type of joint, preparation, filler material, flux and heat.

The type of joint should be chosen based on the stress it is required to withstand and, consequently, the type of connection will influence the strength of the end result.

It is essential to clean surfaces thoroughly before the process begins so as to achieve greater wettability and penetration of the filler material into the whole joint area.

The choice of flux is key to getting a good result, ensuring a clean surface, protection of the weld pool and wetting, while also serving to indicate temperature.

Type of joint for brazing

Butt joint

butt joint

Overlapping joint

overlapping joint

Overlapping joint

overlapping joint

Overlap L>3t of the thinnest member, but no less than 5 mm. Remove burrs and sharp edges. Melting range, play.

Melting range

< 30 °C
30 – 80 °C
> 80 °C

Play “w”
mm

0,05 – 0,10
0,05 – 0,15
0,10 – 0,25

Overlap L>3t of the thinnest member, but no less than 5 mm. Remove burrs and sharp edges. Melting range, play.

Melting range

< 30 °C
30 – 80 °C
> 80 °C

Play “w”
mm

0,05 – 0,10
0,05 – 0,15
0,10 – 0,25

Brazing: troubleshooting

Defects may occur when executing strong or soft brazing or welding, below the list of the most common ones and their possible causes and solutions.

The brazing alloy does not wet both surfaces oh the joint
  • High surface contamination
  • Ineffective action of the flux
  • Formation of oxides during the heating process
  • Thoroughly analyse the cleaning procedures
  • Check the flux features and increase its amount
  • Contact your customer service
Internal porosity, round and brilliant
  • High surface contamination
  • Flux capture
  • Adjust the flame to neutral or to slightly oxidising
  • Check the heating procedure and the joint play
Poor fluidity of the alloy: rough junction and irregular bead
  • Incorrect assembly
  • Irregular heating
  • Poor ventilation of the coupling
  • Insufficient flux
  • Overheating
  • Presence of liquation
  • Check the joint’s play and its uniformity
  • Bring the entire junction to the brazing temperature simultaneously
  • Ensure there is a proper relief
  • Check the flux features and increase its amount
  • Change the temperature to values closer to the liquidus temperature
  • Increase the speed of heating or use an alloy with a narrow melting range
The brazing alloy does not wet one of the surface of the joint, whilst there is a good flow on the other
  • High surface contamination
  • Formation of oxides during the heating process
  • Heating procedures
  • Incorrect positioning
  • Thoroughly analyse the cleaning procedures
  • Contact your customer service
  • Apply more heat to the thicker component
  • Use a ring spacer to ensure an even coupling clearance
Excessive shrinkage
  • Inadequate local tolerance
  • Localised overheating
  • General overheating
  • High alloy cooling
  • Change the size of the joint
  • Balance the heating
  • Reduce the heat cycle
  • Use an alloy with a narrow melting range
Distorsions
  • Non-uniform heating
  • Adjust the heating procedure to a slower thermal cycle with a more enveloping flame
Gaps, deficiencies
  • Excessive or variable clearance
  • Insufficient or irregular heating
  • Poor ventilation of the coupling
  • Inadequate flux
  • Reduce or adjust the tolerances of the coupling
  • Adjust the heating and the thermal cycle
  • Provide appropriate gas relief
  • Check the flux features and increase its quality
Cracks near the centre of the brazing bed
  • Thermal stress during cooling (shrinkage strain)
  • Make sure that the material with the highest thermal expansion is placed on the outside of the joint
Fading
  • Overheating
  • Insufficient flux
  • Cleaning procedure never performed
  • Components of the material or the flux
  • Adjust the heating cycle
  • Check the flux features, increase its amount
  • Review the cleaning procedures
  • Contact your customer service
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