What Causes Porosity in Brazing?

Brazing is the process of joining metal pieces together by melting and flowing filler metal with a lower melting point into the joint.

This is slightly different from welding as brazing doesn’t melt the actual metal pieces.

It’s important to follow proper brazing techniques to avoid porosity, which is when the brazing filler metal doesn’t flow completely, leading to leakage and weaker joints.

Porosity in brazing may be caused by improper cleaning or insufficient filler.

Let’s take a deeper look at the causes of porosity in brazing.

Table of Contents

What is brazing?


Brazing is a form of metalworking where pieces are fixed or joined together with a copper and zinc alloy under extreme heat.

Brazing is used alongside cutting, casting, molding, and welding to make objects, tools, equipment, and buildings.

This joining process also applies to ceramics.

Brazing has been used for thousands of years, as this craft actually dates back to ancient Egyptian civilization.

By 2975 BC, basic brazing techniques were in use, with the Greeks and Romans picking up on this process and making their own improvements.

Throughout the Middle Ages, the Europeans used brazing to build beautiful structures and objects.

The main reason why brazing is so widely used is that the high temperature and closely fitted parts provide a high-quality finish.

The filler metal has a lower melting point, so it fits seamlessly into the adjoining joint. Brazing is known for its advantageous ability to join both similar and different metals with impressive strength.

What does porosity mean?


Porosity describes materials that are full of tiny holes so liquid or air can go right through them.

Porosity impacts the density of objects and the performance of building materials and machinery.

Common examples of porous materials include a sponge, fabric, and wood.

In reference to brazing, porosity describes metal materials with minuscule holes, which may compromise the overall structure and integrity.

When metal pieces are formed or joined with a copper and zinc alloy during the brazing process, it’s important to maintain the proper flow of the filling metal to minimize porosity and structural issues.

The issue with porous metals is that they may experience significant compression and deformation.

If you’ve ever seen a bending or buckled metal sheet, porosity could be the reason why.

What is porosity in brazing?

Porosity in brazing occurs when there is an issue with the flow of filler metal. This creates tiny air holes rather than a completely solid metal surface.

Porosity isn’t ideal for building materials and machinery as it compromises the structural integrity and can raise serious safety concerns.

machinery porosity

In fact, porous metal is considered a manufacturing defect as it makes buildings and machines vulnerable to a wide range of deficiencies.

It’s important to understand what porous metal is and how this porosity affects the overall stability, as replacement materials or more brazing work may be required.

What are the common causes of porosity?

In terms of what causes porosity in brazing, improper cleaning is a big culprit. The formation of pores in die-cast metal may be caused by metal cooling and shrinking, too.

Here are more common causes of porosity:

  • Irregular cleaning and maintenance
  • Insufficient
  • Inaccurate joint clearance
  • Entrapped gas
  • Thermal expansion
  • Shrinking metal

Metal fabricators, builders, engineers, and many other industry professionals are well aware of porosity, which can occur in everything from car parts to city skyscrapers.

Leakage and joint strength problems are likely with porous metals. Porosity impacts the structural integrity and may lead to expensive repairs or even legal troubles if physical injuries or financial damages occur as a result.

What are the benefits of brazing?

Brazing is one of the most common ways to join two or more metal pieces together, and it’s been used around the world since ancient times.

Instead of melting work pieces like with welding, brazing uses a filler metal to combine pieces for heavy-duty performance.

There are many advantages of brazing, including:

  • Lower power input
  • Lower processing temps
  • Minimal residual stresses
  • Minimal thermal distortion
  • Long-lasting joint production
  • No post-processing heat treatment
  • Works with dissimilar base materials
  • Stronger joints compared to soldering
brazing benefit

Keep in mind brazing is viewed as a high-temperature type of soldering, which can be done through several variations like furnace, induction, torch, and vacuum brazing.

What are the downsides of brazing?

While brazing offers numerous advantages for metal fabrication and joining, there are some potential downsides to consider, starting with porosity.

Porous metals are not as strong and may suffer damage and structural issues. Also, the joints won’t be quite as strong as with welding.

Brazing joints may not withstand high temperatures as well as welds, either. With the possibility of porous metals and toxic components in brazed fluxes, improper brazing techniques can cause problems in the integrity of the object or structure.

While this isn’t a major concern, brazed joints may have a slightly different color compared to the base metal.

Also, the metal is joined very closely to support the filler’s capillary action and flow.

How can you avoid porosity in brazing?

pure aluminum

To minimize porosity in brazing, it’s essential to carefully prepare and apply the filler material with the right amount of flow.

If the flow is too fast and forceful, it will lead to a build-up of material and a sloppy joint, while a super slow flow may leave room for air bubbles to become trapped, causing porosity.

The ideal flow is one thing, but choosing pure metals, to begin with, also makes a big difference in avoiding porosity in brazing.

Impure metals are more likely to cool unevenly or too early, which contributes to higher levels of porosity.

That’s why experienced metal fabricators exclusively use pure metals and pure alloys for large castings.

Too much lubricant could also cause porosity in brazing, so it’s best to carefully fill the mold up to its limit only.

Otherwise, the metal could separate prematurely, which is how uneven cooling and bumpy finishes happen.

This would also cause the metal to be overly porous, putting it at risk of structural defects.

What are tips for porosity prevention?

hand brazing

Porosity in brazing isn’t ideal and should be avoided whenever possible to eliminate the risk of defective metal objects, machines, and structures.

You can test for porosity by examining the structural integrity with an x-ray machine.

Here are the top three tips for preventing porosity and ensuring the quality of brazed joints.

  1. Use a high enough temperature. If the metal is not poured into the mold with adequate heat, it could cool early and cause porosity. Always cast at a higher temperature quality, as it doesn’t compromise safety.
  2. Check the die for sharp corners. Jagged edges and sharp corners could cause the metal to cool too soon, particularly in sections with thin pours.
  3. Ensure high-quality materials. It’s better to invest in strong metals from a trusted source than run the risk of porosity and building issues.

Brazing experts who work with different metals all day long recognize the importance of proper flow and positioning for each joint. As long as the metal filler is carefully applied to maintain the right amount of flow, brazed joints can live up to the expectations set by welding and soldering.